Talitha Selby - Academia.edu (original) (raw)

Papers by Talitha Selby

3-benzyl-1,5-hexadiyne attracted our attention because it incorporates an ultraviolet chromophore... more 3-benzyl-1,5-hexadiyne attracted our attention because it incorporates an ultraviolet chromophore attached to a side chain (1,5hexadiyne) that can undergo structural isomerization. The cyclization of 1,5-hexadiyne is known to occur by thermal rearrangement to products such as dimethylenecyclobutene, fulvene, and benzene. The jet-cooled resonance enhanced two photon ionization spectrum of 3-benzyl-1,5-hexadiyne exhibits transitions due to 5 conformations. The S-S¡ origins of these conformers lie within 150 cm ¢ ¡ of the most stable conformation. UV-UV holeburning spectroscopy has been employed to obtain the S-S¡ vibronic spectrum of each conformation. Resonant ion-dip infrared spectroscopy was used to show each conformation has a unique ground state infrared spectrum in the CH stretch region. The acetylenic CH stretch transitions are especially sensitive to the orientation of the two acetylenic groups relative to each other, and have been very useful in assigning anti vs. gauche conformations. The anti or gauche orientation of the gamma methylene group relative to the ring also serves as a sensitive marker by shifting the electronic origins.

The electronic spectroscopy of jet-cooled 5-phenyl-1-pentene was studied by resonant two photon i... more The electronic spectroscopy of jet-cooled 5-phenyl-1-pentene was studied by resonant two photon ionization (R2PI). This molecule is of interest because intramolecular photochemistry occurs from intermediate conformations that bring the vinyl group up over the phenyl ring. a Five conformations were confirmed by UV-UV hole-burning (UVHB) spectroscopy. The

Recently, many groups have suggested that the flexible side chain of alkylated benzene rings may ... more Recently, many groups have suggested that the flexible side chain of alkylated benzene rings may play an important role in the formation of fused ring compounds. Here we present the conformer-specific, vibrationally-resolved electronic spectroscopy of benzylallene along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and

Journal of Physical Chemistry A, 2005

3-Benzyl-1,5-hexadiyne (BHD) was studied by a combination of methods, including resonance-enhance... more 3-Benzyl-1,5-hexadiyne (BHD) was studied by a combination of methods, including resonance-enhanced-two-photon ionization, UV-UV hole-burning spectroscopy, resonant ion-dip infrared spectroscopy, and rotational band contour analysis. There are five conformations of BHD observed in the expansion with their 1<-- S0 origins occurring at 37520, 37565, 37599, 37605, and 37631 cm(-1). DFT calculations predict six low energy conformations. Conformational assignments have been made by comparison of the experimental infrared spectra in the alkyl and acetylenic CH stretch region to DFT vibrational frequency and infrared intensity calculations. Rotational band contours provided further confirmation of these assignments. The electronic origin shifts of BHD compare favorably to the electronic origin shifts of 5-phenyl-1-pentyne with the exception of one conformation. This conformation is unique in that it is the only structure with both acetylenic groups in the gauche position over the ring. This gauche-gauche conformation exhibits a perpendicular (b-type) transition and produces extensive vibronic coupling reminiscent of symmetric monosubstituted benzenes.

The Journal of Physical Chemistry A, 2011

The Journal of Physical Chemistry A, 2007

The energy thresholds to isomerization of the three conformational isomers of m-divinylbenzene (c... more The energy thresholds to isomerization of the three conformational isomers of m-divinylbenzene (cis-cis, cis-trans, and trans-trans) were directly measured by stimulated emission pumping-population transfer (SEP-PT) spectroscopy. The experimentally determined isomerization thresholds are E thresh (cc f ct, tt) ) 1080-1232 cm -1 , E thresh (tt f ct, cc) ) 1130-1175 cm -1 , E thresh (ct f cc) ) 997-1175 cm -1 , and E thresh (ct f tt) ) 997-1232 cm -1 . On the basis of the threshold values for X f Y and Y f X isomerization, the relative energies of the conformational isomers are -102 e E(ct) e +178 cm -1 and -102 e E(cc) e +95 cm -1 relative to E(tt) ) 0. UV-hole-filling (UVHF) spectroscopy was also used to determine the effect of population returning to the ground state via fluorescence. A full set of governing equations for SEP-PT and UVHF spectroscopy is reported that will be generally useful for future studies using these methods. By comparison of these results with the computed stationary points on a calculated surface (DFT B3LYP/6-31+G*), the isomerization pathway was determined to involve sequential isomerization of each vinyl group rather than concerted motion. The energy thresholds were also combined with the ground state torsional vibrational energy levels to obtain a new fitted two-dimensional torsional potential for mDVB.

The Journal of Physical Chemistry A, 2008

Product channels for the self-reaction of the resonance-stabilized allyl radical, C3H5 + C3H5, ha... more Product channels for the self-reaction of the resonance-stabilized allyl radical, C3H5 + C3H5, have been studied with isomeric specificity at temperatures from 300-600 K and pressures from 1-6 Torr using time-resolved multiplexed photoionization mass spectrometry. Under these conditions 1,5-hexadiene was the only C6H10 product isomer detected. The lack of isomerization of the C6H10 product is in marked contrast to the C6H6 product in the related C3H3 + C3H3 reaction, and is due to the more saturated electronic structure of the C6H10 system. The disproportionation product channel, yielding allene + propene, was also detected, with an upper limit on the branching fraction relative to recombination of 0.03. Analysis of the allyl radical decay at 298 K yielded a total rate coefficient of (2.7 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), in good agreement with previous experimental measurements using ultraviolet kinetic absorption spectroscopy and a recent theoretical determination using variable reaction coordinate transition state theory. This result provides independent indirect support for the literature value of the allyl radical ultraviolet absorption cross-section near 223 nm.

The Journal of Physical Chemistry A, 2010

The gas-phase reaction of benzene with O( 3 P) is of considerable interest for modeling of aromat... more The gas-phase reaction of benzene with O( 3 P) is of considerable interest for modeling of aromatic oxidation, and also because there exist fundamental questions concerning the prominence of intersystem crossing in the reaction. While its overall rate constant has been studied extensively, there are still significant uncertainties in the product distribution. The reaction proceeds mainly through the addition of the O atom to benzene, forming an initial triplet diradical adduct, which can either dissociate to form the phenoxy radical and H atom or undergo intersystem crossing onto a singlet surface, followed by a multiplicity of internal isomerizations, leading to several possible reaction products. In this work, we examined the product branching ratios of the reaction between benzene and O( 3 P) over the temperature range 300-1000 K and pressure range 1-10 Torr. The reactions were initiated by pulsed-laser photolysis of NO 2 in the presence of benzene and helium buffer in a slow-flow reactor, and reaction products were identified by using the multiplexed chemical kinetics photoionization mass spectrometer operating at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory. Phenol and phenoxy radical were detected and quantified. Cyclopentadiene and cyclopentadienyl radical were directly identified for the first time. Finally, ab initio calculations and master equation/RRKM modeling were used to reproduce the experimental branching ratios, yielding pressure-dependent rate expressions for the reaction channels, including phenoxy + H, phenol, cyclopentadiene + CO, which are proposed for kinetic modeling of benzene oxidation. † Part of the "Benoît Soep Festschrift". * Corresponding authors.

The Journal of Physical Chemistry A, 2005

4-Phenyl-1-butyne and 5-phenyl-1-pentyne were studied by a combination of methods including reson... more 4-Phenyl-1-butyne and 5-phenyl-1-pentyne were studied by a combination of methods including resonance-enhanced-two-photon ionization, UV-UV hole-burning spectroscopy, and rotational band contour studies. There are two conformations of 4-phenyl-1-butyne observed in the expansion with their S1<-- S0 origins occurring at 37617 and 37620 cm(-1). MP2 and DFT calculations identify these two low energy conformations (with the acetylenic group anti or gauche with respect to the ring) and confirm that these are the only two low energy conformations anticipated to have population in them. The experimental rotational band contours of the origin bands were compared to simulations based on transition moment directions and rotational constants predicted by CIS calculations. This comparison leads to definitive assignments for the bands, with the gauche and anti conformations assigned to the red and blue-shifted conformers, respectively. Three conformations of 5-phenyl-1-pentyne were observed in the expansion with their S1<-- S0 origins occurring at 37538, 37578, and 37601 cm(-1). MP2 and DFT calculations predict four low energy structures arising from gauche or anti conformations about each of the Calpha-Cbeta and Cbeta-Cgamma bonds. Rotational band contour analysis was used to assign the above transitions to gauche-anti (ga), gauche-gauche (gg), and anti-gauche (ag) structures, respectively.

The Journal of Physical Chemistry A, 2008

Photolytic OH-initiated oxidation of cyclopentene, cyclohexene, and 1,4-cyclohexadiene have been ... more Photolytic OH-initiated oxidation of cyclopentene, cyclohexene, and 1,4-cyclohexadiene have been investigated by using tunable synchrotron photoionization mass spectrometry. Electronic structure calculations (CBS-QB3) are employed in Franck-Condon (FC) spectral simulations of the photoionization efficiency curves (PIE) of the observed products. Cyclopentenol (cyclopenten-1-ol, 1-c-C(5)H(7)OH) and its isomers cyclopenten-2-ol (2-c-C(5)H(7)OH) and cyclopentanone (c-C(5)H(8)=O), are detected from OH-initiated cyclopentene oxidation. The measured adiabatic ionization energy (AIE) of 1-c-C(5)H(7)OH is 8.4(+/-0.1) eV, and that of 2-c-C(5)H(7)OH is 9.5(+/-0.1) eV. The calculated AIE of possible cyclopentene oxidation products cis-1,2-epoxycyclopentane and 2,3-epoxycyclopentanol is 9.97 and 9.44 eV, respectively. Product spectra from OH-initiated oxidation of cyclohexene and cyclohexa-1,4-diene show a substantial contribution from linear aldehydes, indicating a prominent role for ring opening. Implications for the oxidation chemistry of cycloalkenes are briefly discussed.

The Journal of Physical Chemistry A, 2007

The ultraviolet spectroscopy of m- and p-divinylbenzene isomers (mDVB and pDVB) was studied by a ... more The ultraviolet spectroscopy of m- and p-divinylbenzene isomers (mDVB and pDVB) was studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), laser-induced fluorescence (LIF), UV-UV hole-burning spectroscopy (UVHB), and single vibronic level fluorescence spectroscopy (SVLF). In mDVB, there are three low-energy conformations, cis-cis, cis-trans, and trans-trans whose S1 <-- S0 origins occur at 31,408, 31,856, and 32,164 cm(-1), respectively, as confirmed by UVHB spectroscopy. There are two possible conformations in pDVB, cis and trans. UVHB studies confirm the S1 <-- S0 origin of trans-pDVB occurs at 32,553 cm(-1), and the corresponding cis-pDVB origin is tentatively assigned to a transition at 32 621 cm(-1). SVLF studies were used to determine several of the vinyl torsional levels of the isomers of mDVB and pDVB. A two-dimensional flexible model was used to fit these levels in mDVB to a potential form and determine the barriers to isomerization.

The Journal of Physical Chemistry A, 2009

The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent... more The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent methods. The rate constant as a function of temperature at 20 Torr has been determined by a laser-photolysis/laser absorption technique. Vinyl iodide is photolyzed at 266 nm, and both the vinyl radical and the iodine atom photolysis products are monitored by laser absorption. The vinyl radical concentration is derived from the initial iodine atom concentration, which is determined by using the known absorption cross section of the iodine atomic transition to relate the observed absorption to concentration. The measured rate constant for the self-reaction at room temperature is approximately a factor of 2 lower than literature recommendations. The reaction displays a slightly negative temperature dependence, which can be represented by a negative activation energy, (E(a)/R) = -400 K. The laser absorption results are supported by independent experiments at 298 K and 4 Torr using time-resolved synchrotron-photoionization mass-spectrometric detection of the products of divinyl ketone and methyl vinyl ketone photolysis. The photoionization mass spectrometry experiments additionally show that methyl + propargyl are formed in the vinyl radical self-reaction, with an estimated branching fraction of 0.5 at 298 K and 4 Torr.

Journal of the American Chemical Society, 2007

The photoionization of 1-alkenylperoxy radicals, which are peroxy radicals where the OO moiety is... more The photoionization of 1-alkenylperoxy radicals, which are peroxy radicals where the OO moiety is bonded to an sp 2 -hybridized carbon, is studied by experimental and computational methods and compared to the similar alkylperoxy systems. Quantum chemical calculations are presented for the ionization energy and cation stability of several alkenylperoxy radicals. Experimental measurements of 1-cyclopentenylperoxy (1-c-C 5H7OO) and propargylperoxy (CH2dCdCHOO) photoionization are presented as examples. These radicals are produced by reaction of an excess of O2 with pulsed-photolytically produced alkenyl radicals. The kinetic behavior of the products confirms the formation of the alkenylperoxy radicals. Electronic structure calculations are employed to give structural parameters and energetics that are used in a Franck-Condon (FC) spectral simulation of the photoionization efficiency (PIE) curves. The calculations also serve to identify the isomeric species probed by the experiment. Adiabatic ionization energies (AIEs) of 1-c-C 5H7OO (8.70 ( 0.05 eV) and CH2dCdCHOO (9.32 ( 0.05 eV) are derived from fits to the experimental PIE curves. From the fitted FC simulation superimposed on the experimental PIE curves, the splitting between the ground state singlet and excited triplet cation electronic states is also derived for 1-c-C 5H7OO (0.76 ( 0.05 eV) and CH2dCdCHOO (0.80 ( 0.15 eV). The combination of the AIE(CH2dCdCHOO) and the propargyl heat of formation provides ∆fH°0 (CH2dCdCHOO + ) of (1162 ( 8) kJ mol -1 . From ∆fH°0 (CH2dCdCHOO + ) and ∆fH°0(C3H3 + ) it is also possible to extract the bond energy D°0(C3H3 + sOO) of 19 kJ mol -1 (0.20 eV). Finally, from consideration of the relevant molecular orbitals, the ionization behavior of alkyl-and alkenylperoxy radicals can be generalized with a simple rule: Alkylperoxy radicals dissociatively ionize, with the exception of methylperoxy, whereas alkenylperoxy radicals have stable singlet ground electronic state cations.

Journal of the American Chemical Society, 2009

The reactions of the methylidyne radical (CH) with ethylene, acetylene, allene, and methylacetyle... more The reactions of the methylidyne radical (CH) with ethylene, acetylene, allene, and methylacetylene are studied at room temperature using tunable vacuum ultraviolet (VUV) photoionization and time-resolved mass spectrometry. The CH radicals are prepared by 248 nm multiphoton photolysis of CHBr3 at 298 K and react with the selected hydrocarbon in a helium gas flow. Analysis of photoionization efficiency versus VUV photon wavelength permits isomerspecific detection of the reaction products and allows estimation of the reaction product branching ratios. The reactions proceed by either CH insertion or addition followed by H atom elimination from the intermediate adduct. In the CH + C2H4 reaction the C3H5 intermediate decays by H atom loss to yield 70(+-8)percent allene, 30(+-8)percent methylacetylene and less than 10percent cyclopropene, in agreement with previous RRKM results. In the CH + acetylene reaction, detection of mainly the cyclic C3H2 isomer is contrary to a previous RRKM calculation that predicted linear triplet propargylene to be 90percent of the total H-atom co-products. High-level CBS-APNO quantum calculations and RRKM calculation for the CH + C2H2 reaction presented in this manuscript predict a higher contribution of the cyclic C3H2 (27.0percent) versus triplet propargylene (63.5percent) than these earlier predictions. Extensive calculations on the C3H3 and C3H2D system combined with experimental isotope ratios for the CD + C2H2 reaction indicate eScholarship provides open access, scholarly publishing services to the University of California and delivers a dynamic research platform to scholars worldwide.

Journal of the American Chemical Society, 2012

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-... more Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S 0 −S 1 origin at 37 483 cm −1 . UV−UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the anti position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump−probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact with the gas pulse in a short tube that extended the collisional region of the expansion. The products were cooled during expansion of the gas mixture into vacuum, before being interrogated using mass-selective resonant two-photon ionization. The UV−vis spectra of the photochemical products were compared to literature spectra for identification. Several wavelengths were chosen for photoexcitation, ranging from the S 0 −S 1 origin transition (266.79 nm) to 193 nm. Comparison of the product spectral intensities as a function of photoexcitation wavelength provides information on the wavelength dependence of the product yields. Photoexcitation at 266.79 nm yielded five products (benzyl radical, benzylallenyl radical, 1-phenyl-1,3-butadiene, 1,2-dihydronaphthalene, and naphthalene), with naphthalene and benzylallenyl radicals dominant. At 193 nm, the benzylallenyl radical signal was greatly reduced in intensity, while three additional C 10 H 8 isomeric products were observed. An extensive set of calculations of key stationary points on the ground state C 10 H 10 and C 10 H 9 potential energy surfaces were carried out at the DFT B3LYP/6-311G(d,p) level of theory. Mechanisms for formation of the observed products are proposed based on these potential energy surfaces, constrained by the results of cursory studies of the photochemistry of 1-phenyl-1,3-butadiene and 4-phenyl-1-butyne. A role for tunneling on the excited state surface in the formation of naphthalene is suggested by studies of partially deuterated benzylallene, which blocked naphthalene formation.

Journal of the American Chemical Society, 2008

Carbonyl oxide species play a key role in tropospheric oxidation of organic molecules and in low-... more Carbonyl oxide species play a key role in tropospheric oxidation of organic molecules and in low-temperature combustion processes. In the late 1940s, Criegee first postulated the participation of carbonyl oxides, now often called "Criegee intermediates," in ozonolysis of alkenes. However, despite decades of effort, no gas phase Criegee intermediate has before been observed. As a result, knowledge of gas phase carbonyl oxide reactions has heretofore been inferred by indirect means, with derived rate coefficients spanning orders of magnitude. We have directly detected the primary Criegee intermediate, formaldehyde oxide (CH2OO), in the chlorine-initiated gas-phase oxidation of dimethyl sulfoxide (DMSO). This work not only establishes that the Criegee intermediate is formed in DMSO oxidation also but opens the possibility for explicit kinetics studies on this critical atmospheric species.

The Journal of Chemical Physics, 2012

Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoioniz... more Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C(3)H(3)) radical, σ(propargyl) (ion)(E), relative to the known absolute cross-section of the methyl (CH(3)) radical. We generated a stoichiometric 1:1 ratio of C(3)H(3):CH(3) from 193 nm photolysis of two different C(4)H(6) isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of σ(propargyl)(ion)(10.213 eV)=(26.1±4.2) Mb and σ(propargyl)(ion)(10.413 eV)=(23.4±3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of σ(propargyl)(ion)(10.213 eV)=(23.6±3.6) Mb and σ(propargyl)(ion)(10.413 eV)=(25.1±3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations.

Journal of Physical Chemistry A, 2010

We report the first isomeric-selective study of the dominant isomeric pathway in the OH-initiated... more We report the first isomeric-selective study of the dominant isomeric pathway in the OH-initiated oxidation of isoprene in the presence of O 2 and NO using the laser photolysis-laser induced fluorescence (LP-LIF) technique. The photolysis of monodeuterated/nondeuterated 2-iodo-2-methylbut-3-en-1-ol results exclusively in the dominant OH-isoprene addition product, providing important insight into the oxidation mechanism. On the basis of kinetic analysis of OH cycling experiments, we have determined the rate constant for O 2 addition to the hydroxyalkyl radical to be 1.0 -0.5 +1.7 × 10 -12 cm 3 s -1 , and we find a value of 8.1 -2.3 +3.4 × 10 -12 cm 3 s -1 for the overall reaction rate constant of the resulting hydroxyperoxy radical with NO. We also report the first clear experimental evidence of the (E) form of the δ-hydroxyalkoxy channel through isotopic labeling experiments and quantify its branching ratio to be (10 ( 3)%. This puts a rigorous upper limit on the branching of the (E)-δ-hydroxyalkoxy radical channel. Since our measured isomeric-selective rate constants for the dominant outer channel in OH-initiated isoprene chemistry are similar to the overall rate constants derived from nonisomeric kinetics, we predict that the remaining outer addition channel will have similar reactivity.

Journal of Physical Chemistry A, 2005

The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and ... more The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and pES) were studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), UV-UV hole-burning spectroscopy (UVHB), resonant ion-dip infrared spectroscopy (RIDIRS), and rotationally resolved fluorescence excitation spectroscopy. In addition, the newly developed method of stimulated emission pumping-population transfer spectroscopy (SEP-PTS) was used to determine the energy threshold to conformational isomerization in m-ethynylstyrene. The S 1 r S 0 origin transitions of oES and pES occur at 32 369 and 33 407 cm -1 , respectively. In mES, the cis and trans conformations are calculated to be close in energy. In the R2PI spectrum of mES, the two most prominent peaks (32672 and 32926 cm -1 ) were confirmed by UVHB spectroscopy to be S 1 r S 0 origins of these two conformers. The red-shifted conformer was identified as the cis structure by least-squares fitting of the rotationally resolved fluorescence excitation spectrum of the origin band. There are also two possible conformations in oES, but transitions due to only one were observed experimentally, as confirmed by UVHB spectroscopy. Density functional theory calculations (B3LYP/6-31+G*) predict that the cis-ortho conformer, in which the substituents point toward each other, is about 8 kJ/mol higher in energy than the trans-ortho isomer, and should only be about 5% of the room temperature population of oES. Ground-state infrared spectra in the C-H stretch region (3000-3300 cm -1 ) of each isomer were obtained with RIDIRS. In all three structural isomers, the acetylenic C-H stretch fundamental was split by Fermi resonance. Infrared spectra were also recorded in the excited electronic state, using a UV-IR-UV version of RIDIR spectroscopy. In all three isomers the acetylenic C-H stretch fundamental was unshifted from the ground state, but no Fermi resonance was seen. The first observed and last unobserved transitions in the SEP-PT spectrum were used to place lower and upper bounds on the barrier to cis f trans isomerization in m-ethynylstyrene of 990-1070 cm -1 . Arguments are given for the lack of a kinetic shift in the measurement.

Journal of Physical Chemistry A, 2009

The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent... more The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent methods. The rate constant as a function of temperature at 20 Torr has been determined by a laser-photolysis/laser absorption technique. Vinyl iodide is photolyzed at 266 nm, and both the vinyl radical and the iodine atom photolysis products are monitored by laser absorption. The vinyl radical concentration is derived from the initial iodine atom concentration, which is determined by using the known absorption cross section of the iodine atomic transition to relate the observed absorption to concentration. The measured rate constant for the self-reaction at room temperature is approximately a factor of 2 lower than literature recommendations. The reaction displays a slightly negative temperature dependence, which can be represented by a negative activation energy, (E(a)/R) = -400 K. The laser absorption results are supported by independent experiments at 298 K and 4 Torr using time-resolved synchrotron-photoionization mass-spectrometric detection of the products of divinyl ketone and methyl vinyl ketone photolysis. The photoionization mass spectrometry experiments additionally show that methyl + propargyl are formed in the vinyl radical self-reaction, with an estimated branching fraction of 0.5 at 298 K and 4 Torr.

3-benzyl-1,5-hexadiyne attracted our attention because it incorporates an ultraviolet chromophore... more 3-benzyl-1,5-hexadiyne attracted our attention because it incorporates an ultraviolet chromophore attached to a side chain (1,5hexadiyne) that can undergo structural isomerization. The cyclization of 1,5-hexadiyne is known to occur by thermal rearrangement to products such as dimethylenecyclobutene, fulvene, and benzene. The jet-cooled resonance enhanced two photon ionization spectrum of 3-benzyl-1,5-hexadiyne exhibits transitions due to 5 conformations. The S-S¡ origins of these conformers lie within 150 cm ¢ ¡ of the most stable conformation. UV-UV holeburning spectroscopy has been employed to obtain the S-S¡ vibronic spectrum of each conformation. Resonant ion-dip infrared spectroscopy was used to show each conformation has a unique ground state infrared spectrum in the CH stretch region. The acetylenic CH stretch transitions are especially sensitive to the orientation of the two acetylenic groups relative to each other, and have been very useful in assigning anti vs. gauche conformations. The anti or gauche orientation of the gamma methylene group relative to the ring also serves as a sensitive marker by shifting the electronic origins.

The electronic spectroscopy of jet-cooled 5-phenyl-1-pentene was studied by resonant two photon i... more The electronic spectroscopy of jet-cooled 5-phenyl-1-pentene was studied by resonant two photon ionization (R2PI). This molecule is of interest because intramolecular photochemistry occurs from intermediate conformations that bring the vinyl group up over the phenyl ring. a Five conformations were confirmed by UV-UV hole-burning (UVHB) spectroscopy. The

Recently, many groups have suggested that the flexible side chain of alkylated benzene rings may ... more Recently, many groups have suggested that the flexible side chain of alkylated benzene rings may play an important role in the formation of fused ring compounds. Here we present the conformer-specific, vibrationally-resolved electronic spectroscopy of benzylallene along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and

Journal of Physical Chemistry A, 2005

3-Benzyl-1,5-hexadiyne (BHD) was studied by a combination of methods, including resonance-enhance... more 3-Benzyl-1,5-hexadiyne (BHD) was studied by a combination of methods, including resonance-enhanced-two-photon ionization, UV-UV hole-burning spectroscopy, resonant ion-dip infrared spectroscopy, and rotational band contour analysis. There are five conformations of BHD observed in the expansion with their 1<-- S0 origins occurring at 37520, 37565, 37599, 37605, and 37631 cm(-1). DFT calculations predict six low energy conformations. Conformational assignments have been made by comparison of the experimental infrared spectra in the alkyl and acetylenic CH stretch region to DFT vibrational frequency and infrared intensity calculations. Rotational band contours provided further confirmation of these assignments. The electronic origin shifts of BHD compare favorably to the electronic origin shifts of 5-phenyl-1-pentyne with the exception of one conformation. This conformation is unique in that it is the only structure with both acetylenic groups in the gauche position over the ring. This gauche-gauche conformation exhibits a perpendicular (b-type) transition and produces extensive vibronic coupling reminiscent of symmetric monosubstituted benzenes.

The Journal of Physical Chemistry A, 2011

The Journal of Physical Chemistry A, 2007

The energy thresholds to isomerization of the three conformational isomers of m-divinylbenzene (c... more The energy thresholds to isomerization of the three conformational isomers of m-divinylbenzene (cis-cis, cis-trans, and trans-trans) were directly measured by stimulated emission pumping-population transfer (SEP-PT) spectroscopy. The experimentally determined isomerization thresholds are E thresh (cc f ct, tt) ) 1080-1232 cm -1 , E thresh (tt f ct, cc) ) 1130-1175 cm -1 , E thresh (ct f cc) ) 997-1175 cm -1 , and E thresh (ct f tt) ) 997-1232 cm -1 . On the basis of the threshold values for X f Y and Y f X isomerization, the relative energies of the conformational isomers are -102 e E(ct) e +178 cm -1 and -102 e E(cc) e +95 cm -1 relative to E(tt) ) 0. UV-hole-filling (UVHF) spectroscopy was also used to determine the effect of population returning to the ground state via fluorescence. A full set of governing equations for SEP-PT and UVHF spectroscopy is reported that will be generally useful for future studies using these methods. By comparison of these results with the computed stationary points on a calculated surface (DFT B3LYP/6-31+G*), the isomerization pathway was determined to involve sequential isomerization of each vinyl group rather than concerted motion. The energy thresholds were also combined with the ground state torsional vibrational energy levels to obtain a new fitted two-dimensional torsional potential for mDVB.

The Journal of Physical Chemistry A, 2008

Product channels for the self-reaction of the resonance-stabilized allyl radical, C3H5 + C3H5, ha... more Product channels for the self-reaction of the resonance-stabilized allyl radical, C3H5 + C3H5, have been studied with isomeric specificity at temperatures from 300-600 K and pressures from 1-6 Torr using time-resolved multiplexed photoionization mass spectrometry. Under these conditions 1,5-hexadiene was the only C6H10 product isomer detected. The lack of isomerization of the C6H10 product is in marked contrast to the C6H6 product in the related C3H3 + C3H3 reaction, and is due to the more saturated electronic structure of the C6H10 system. The disproportionation product channel, yielding allene + propene, was also detected, with an upper limit on the branching fraction relative to recombination of 0.03. Analysis of the allyl radical decay at 298 K yielded a total rate coefficient of (2.7 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), in good agreement with previous experimental measurements using ultraviolet kinetic absorption spectroscopy and a recent theoretical determination using variable reaction coordinate transition state theory. This result provides independent indirect support for the literature value of the allyl radical ultraviolet absorption cross-section near 223 nm.

The Journal of Physical Chemistry A, 2010

The gas-phase reaction of benzene with O( 3 P) is of considerable interest for modeling of aromat... more The gas-phase reaction of benzene with O( 3 P) is of considerable interest for modeling of aromatic oxidation, and also because there exist fundamental questions concerning the prominence of intersystem crossing in the reaction. While its overall rate constant has been studied extensively, there are still significant uncertainties in the product distribution. The reaction proceeds mainly through the addition of the O atom to benzene, forming an initial triplet diradical adduct, which can either dissociate to form the phenoxy radical and H atom or undergo intersystem crossing onto a singlet surface, followed by a multiplicity of internal isomerizations, leading to several possible reaction products. In this work, we examined the product branching ratios of the reaction between benzene and O( 3 P) over the temperature range 300-1000 K and pressure range 1-10 Torr. The reactions were initiated by pulsed-laser photolysis of NO 2 in the presence of benzene and helium buffer in a slow-flow reactor, and reaction products were identified by using the multiplexed chemical kinetics photoionization mass spectrometer operating at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory. Phenol and phenoxy radical were detected and quantified. Cyclopentadiene and cyclopentadienyl radical were directly identified for the first time. Finally, ab initio calculations and master equation/RRKM modeling were used to reproduce the experimental branching ratios, yielding pressure-dependent rate expressions for the reaction channels, including phenoxy + H, phenol, cyclopentadiene + CO, which are proposed for kinetic modeling of benzene oxidation. † Part of the "Benoît Soep Festschrift". * Corresponding authors.

The Journal of Physical Chemistry A, 2005

4-Phenyl-1-butyne and 5-phenyl-1-pentyne were studied by a combination of methods including reson... more 4-Phenyl-1-butyne and 5-phenyl-1-pentyne were studied by a combination of methods including resonance-enhanced-two-photon ionization, UV-UV hole-burning spectroscopy, and rotational band contour studies. There are two conformations of 4-phenyl-1-butyne observed in the expansion with their S1<-- S0 origins occurring at 37617 and 37620 cm(-1). MP2 and DFT calculations identify these two low energy conformations (with the acetylenic group anti or gauche with respect to the ring) and confirm that these are the only two low energy conformations anticipated to have population in them. The experimental rotational band contours of the origin bands were compared to simulations based on transition moment directions and rotational constants predicted by CIS calculations. This comparison leads to definitive assignments for the bands, with the gauche and anti conformations assigned to the red and blue-shifted conformers, respectively. Three conformations of 5-phenyl-1-pentyne were observed in the expansion with their S1<-- S0 origins occurring at 37538, 37578, and 37601 cm(-1). MP2 and DFT calculations predict four low energy structures arising from gauche or anti conformations about each of the Calpha-Cbeta and Cbeta-Cgamma bonds. Rotational band contour analysis was used to assign the above transitions to gauche-anti (ga), gauche-gauche (gg), and anti-gauche (ag) structures, respectively.

The Journal of Physical Chemistry A, 2008

Photolytic OH-initiated oxidation of cyclopentene, cyclohexene, and 1,4-cyclohexadiene have been ... more Photolytic OH-initiated oxidation of cyclopentene, cyclohexene, and 1,4-cyclohexadiene have been investigated by using tunable synchrotron photoionization mass spectrometry. Electronic structure calculations (CBS-QB3) are employed in Franck-Condon (FC) spectral simulations of the photoionization efficiency curves (PIE) of the observed products. Cyclopentenol (cyclopenten-1-ol, 1-c-C(5)H(7)OH) and its isomers cyclopenten-2-ol (2-c-C(5)H(7)OH) and cyclopentanone (c-C(5)H(8)=O), are detected from OH-initiated cyclopentene oxidation. The measured adiabatic ionization energy (AIE) of 1-c-C(5)H(7)OH is 8.4(+/-0.1) eV, and that of 2-c-C(5)H(7)OH is 9.5(+/-0.1) eV. The calculated AIE of possible cyclopentene oxidation products cis-1,2-epoxycyclopentane and 2,3-epoxycyclopentanol is 9.97 and 9.44 eV, respectively. Product spectra from OH-initiated oxidation of cyclohexene and cyclohexa-1,4-diene show a substantial contribution from linear aldehydes, indicating a prominent role for ring opening. Implications for the oxidation chemistry of cycloalkenes are briefly discussed.

The Journal of Physical Chemistry A, 2007

The ultraviolet spectroscopy of m- and p-divinylbenzene isomers (mDVB and pDVB) was studied by a ... more The ultraviolet spectroscopy of m- and p-divinylbenzene isomers (mDVB and pDVB) was studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), laser-induced fluorescence (LIF), UV-UV hole-burning spectroscopy (UVHB), and single vibronic level fluorescence spectroscopy (SVLF). In mDVB, there are three low-energy conformations, cis-cis, cis-trans, and trans-trans whose S1 <-- S0 origins occur at 31,408, 31,856, and 32,164 cm(-1), respectively, as confirmed by UVHB spectroscopy. There are two possible conformations in pDVB, cis and trans. UVHB studies confirm the S1 <-- S0 origin of trans-pDVB occurs at 32,553 cm(-1), and the corresponding cis-pDVB origin is tentatively assigned to a transition at 32 621 cm(-1). SVLF studies were used to determine several of the vinyl torsional levels of the isomers of mDVB and pDVB. A two-dimensional flexible model was used to fit these levels in mDVB to a potential form and determine the barriers to isomerization.

The Journal of Physical Chemistry A, 2009

The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent... more The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent methods. The rate constant as a function of temperature at 20 Torr has been determined by a laser-photolysis/laser absorption technique. Vinyl iodide is photolyzed at 266 nm, and both the vinyl radical and the iodine atom photolysis products are monitored by laser absorption. The vinyl radical concentration is derived from the initial iodine atom concentration, which is determined by using the known absorption cross section of the iodine atomic transition to relate the observed absorption to concentration. The measured rate constant for the self-reaction at room temperature is approximately a factor of 2 lower than literature recommendations. The reaction displays a slightly negative temperature dependence, which can be represented by a negative activation energy, (E(a)/R) = -400 K. The laser absorption results are supported by independent experiments at 298 K and 4 Torr using time-resolved synchrotron-photoionization mass-spectrometric detection of the products of divinyl ketone and methyl vinyl ketone photolysis. The photoionization mass spectrometry experiments additionally show that methyl + propargyl are formed in the vinyl radical self-reaction, with an estimated branching fraction of 0.5 at 298 K and 4 Torr.

Journal of the American Chemical Society, 2007

The photoionization of 1-alkenylperoxy radicals, which are peroxy radicals where the OO moiety is... more The photoionization of 1-alkenylperoxy radicals, which are peroxy radicals where the OO moiety is bonded to an sp 2 -hybridized carbon, is studied by experimental and computational methods and compared to the similar alkylperoxy systems. Quantum chemical calculations are presented for the ionization energy and cation stability of several alkenylperoxy radicals. Experimental measurements of 1-cyclopentenylperoxy (1-c-C 5H7OO) and propargylperoxy (CH2dCdCHOO) photoionization are presented as examples. These radicals are produced by reaction of an excess of O2 with pulsed-photolytically produced alkenyl radicals. The kinetic behavior of the products confirms the formation of the alkenylperoxy radicals. Electronic structure calculations are employed to give structural parameters and energetics that are used in a Franck-Condon (FC) spectral simulation of the photoionization efficiency (PIE) curves. The calculations also serve to identify the isomeric species probed by the experiment. Adiabatic ionization energies (AIEs) of 1-c-C 5H7OO (8.70 ( 0.05 eV) and CH2dCdCHOO (9.32 ( 0.05 eV) are derived from fits to the experimental PIE curves. From the fitted FC simulation superimposed on the experimental PIE curves, the splitting between the ground state singlet and excited triplet cation electronic states is also derived for 1-c-C 5H7OO (0.76 ( 0.05 eV) and CH2dCdCHOO (0.80 ( 0.15 eV). The combination of the AIE(CH2dCdCHOO) and the propargyl heat of formation provides ∆fH°0 (CH2dCdCHOO + ) of (1162 ( 8) kJ mol -1 . From ∆fH°0 (CH2dCdCHOO + ) and ∆fH°0(C3H3 + ) it is also possible to extract the bond energy D°0(C3H3 + sOO) of 19 kJ mol -1 (0.20 eV). Finally, from consideration of the relevant molecular orbitals, the ionization behavior of alkyl-and alkenylperoxy radicals can be generalized with a simple rule: Alkylperoxy radicals dissociatively ionize, with the exception of methylperoxy, whereas alkenylperoxy radicals have stable singlet ground electronic state cations.

Journal of the American Chemical Society, 2009

The reactions of the methylidyne radical (CH) with ethylene, acetylene, allene, and methylacetyle... more The reactions of the methylidyne radical (CH) with ethylene, acetylene, allene, and methylacetylene are studied at room temperature using tunable vacuum ultraviolet (VUV) photoionization and time-resolved mass spectrometry. The CH radicals are prepared by 248 nm multiphoton photolysis of CHBr3 at 298 K and react with the selected hydrocarbon in a helium gas flow. Analysis of photoionization efficiency versus VUV photon wavelength permits isomerspecific detection of the reaction products and allows estimation of the reaction product branching ratios. The reactions proceed by either CH insertion or addition followed by H atom elimination from the intermediate adduct. In the CH + C2H4 reaction the C3H5 intermediate decays by H atom loss to yield 70(+-8)percent allene, 30(+-8)percent methylacetylene and less than 10percent cyclopropene, in agreement with previous RRKM results. In the CH + acetylene reaction, detection of mainly the cyclic C3H2 isomer is contrary to a previous RRKM calculation that predicted linear triplet propargylene to be 90percent of the total H-atom co-products. High-level CBS-APNO quantum calculations and RRKM calculation for the CH + C2H2 reaction presented in this manuscript predict a higher contribution of the cyclic C3H2 (27.0percent) versus triplet propargylene (63.5percent) than these earlier predictions. Extensive calculations on the C3H3 and C3H2D system combined with experimental isotope ratios for the CD + C2H2 reaction indicate eScholarship provides open access, scholarly publishing services to the University of California and delivers a dynamic research platform to scholars worldwide.

Journal of the American Chemical Society, 2012

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-... more Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S 0 −S 1 origin at 37 483 cm −1 . UV−UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the anti position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump−probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact with the gas pulse in a short tube that extended the collisional region of the expansion. The products were cooled during expansion of the gas mixture into vacuum, before being interrogated using mass-selective resonant two-photon ionization. The UV−vis spectra of the photochemical products were compared to literature spectra for identification. Several wavelengths were chosen for photoexcitation, ranging from the S 0 −S 1 origin transition (266.79 nm) to 193 nm. Comparison of the product spectral intensities as a function of photoexcitation wavelength provides information on the wavelength dependence of the product yields. Photoexcitation at 266.79 nm yielded five products (benzyl radical, benzylallenyl radical, 1-phenyl-1,3-butadiene, 1,2-dihydronaphthalene, and naphthalene), with naphthalene and benzylallenyl radicals dominant. At 193 nm, the benzylallenyl radical signal was greatly reduced in intensity, while three additional C 10 H 8 isomeric products were observed. An extensive set of calculations of key stationary points on the ground state C 10 H 10 and C 10 H 9 potential energy surfaces were carried out at the DFT B3LYP/6-311G(d,p) level of theory. Mechanisms for formation of the observed products are proposed based on these potential energy surfaces, constrained by the results of cursory studies of the photochemistry of 1-phenyl-1,3-butadiene and 4-phenyl-1-butyne. A role for tunneling on the excited state surface in the formation of naphthalene is suggested by studies of partially deuterated benzylallene, which blocked naphthalene formation.

Journal of the American Chemical Society, 2008

Carbonyl oxide species play a key role in tropospheric oxidation of organic molecules and in low-... more Carbonyl oxide species play a key role in tropospheric oxidation of organic molecules and in low-temperature combustion processes. In the late 1940s, Criegee first postulated the participation of carbonyl oxides, now often called "Criegee intermediates," in ozonolysis of alkenes. However, despite decades of effort, no gas phase Criegee intermediate has before been observed. As a result, knowledge of gas phase carbonyl oxide reactions has heretofore been inferred by indirect means, with derived rate coefficients spanning orders of magnitude. We have directly detected the primary Criegee intermediate, formaldehyde oxide (CH2OO), in the chlorine-initiated gas-phase oxidation of dimethyl sulfoxide (DMSO). This work not only establishes that the Criegee intermediate is formed in DMSO oxidation also but opens the possibility for explicit kinetics studies on this critical atmospheric species.

The Journal of Chemical Physics, 2012

Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoioniz... more Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C(3)H(3)) radical, σ(propargyl) (ion)(E), relative to the known absolute cross-section of the methyl (CH(3)) radical. We generated a stoichiometric 1:1 ratio of C(3)H(3):CH(3) from 193 nm photolysis of two different C(4)H(6) isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of σ(propargyl)(ion)(10.213 eV)=(26.1±4.2) Mb and σ(propargyl)(ion)(10.413 eV)=(23.4±3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of σ(propargyl)(ion)(10.213 eV)=(23.6±3.6) Mb and σ(propargyl)(ion)(10.413 eV)=(25.1±3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations.

Journal of Physical Chemistry A, 2010

We report the first isomeric-selective study of the dominant isomeric pathway in the OH-initiated... more We report the first isomeric-selective study of the dominant isomeric pathway in the OH-initiated oxidation of isoprene in the presence of O 2 and NO using the laser photolysis-laser induced fluorescence (LP-LIF) technique. The photolysis of monodeuterated/nondeuterated 2-iodo-2-methylbut-3-en-1-ol results exclusively in the dominant OH-isoprene addition product, providing important insight into the oxidation mechanism. On the basis of kinetic analysis of OH cycling experiments, we have determined the rate constant for O 2 addition to the hydroxyalkyl radical to be 1.0 -0.5 +1.7 × 10 -12 cm 3 s -1 , and we find a value of 8.1 -2.3 +3.4 × 10 -12 cm 3 s -1 for the overall reaction rate constant of the resulting hydroxyperoxy radical with NO. We also report the first clear experimental evidence of the (E) form of the δ-hydroxyalkoxy channel through isotopic labeling experiments and quantify its branching ratio to be (10 ( 3)%. This puts a rigorous upper limit on the branching of the (E)-δ-hydroxyalkoxy radical channel. Since our measured isomeric-selective rate constants for the dominant outer channel in OH-initiated isoprene chemistry are similar to the overall rate constants derived from nonisomeric kinetics, we predict that the remaining outer addition channel will have similar reactivity.

Journal of Physical Chemistry A, 2005

The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and ... more The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and pES) were studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), UV-UV hole-burning spectroscopy (UVHB), resonant ion-dip infrared spectroscopy (RIDIRS), and rotationally resolved fluorescence excitation spectroscopy. In addition, the newly developed method of stimulated emission pumping-population transfer spectroscopy (SEP-PTS) was used to determine the energy threshold to conformational isomerization in m-ethynylstyrene. The S 1 r S 0 origin transitions of oES and pES occur at 32 369 and 33 407 cm -1 , respectively. In mES, the cis and trans conformations are calculated to be close in energy. In the R2PI spectrum of mES, the two most prominent peaks (32672 and 32926 cm -1 ) were confirmed by UVHB spectroscopy to be S 1 r S 0 origins of these two conformers. The red-shifted conformer was identified as the cis structure by least-squares fitting of the rotationally resolved fluorescence excitation spectrum of the origin band. There are also two possible conformations in oES, but transitions due to only one were observed experimentally, as confirmed by UVHB spectroscopy. Density functional theory calculations (B3LYP/6-31+G*) predict that the cis-ortho conformer, in which the substituents point toward each other, is about 8 kJ/mol higher in energy than the trans-ortho isomer, and should only be about 5% of the room temperature population of oES. Ground-state infrared spectra in the C-H stretch region (3000-3300 cm -1 ) of each isomer were obtained with RIDIRS. In all three structural isomers, the acetylenic C-H stretch fundamental was split by Fermi resonance. Infrared spectra were also recorded in the excited electronic state, using a UV-IR-UV version of RIDIR spectroscopy. In all three isomers the acetylenic C-H stretch fundamental was unshifted from the ground state, but no Fermi resonance was seen. The first observed and last unobserved transitions in the SEP-PT spectrum were used to place lower and upper bounds on the barrier to cis f trans isomerization in m-ethynylstyrene of 990-1070 cm -1 . Arguments are given for the lack of a kinetic shift in the measurement.

Journal of Physical Chemistry A, 2009

The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent... more The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent methods. The rate constant as a function of temperature at 20 Torr has been determined by a laser-photolysis/laser absorption technique. Vinyl iodide is photolyzed at 266 nm, and both the vinyl radical and the iodine atom photolysis products are monitored by laser absorption. The vinyl radical concentration is derived from the initial iodine atom concentration, which is determined by using the known absorption cross section of the iodine atomic transition to relate the observed absorption to concentration. The measured rate constant for the self-reaction at room temperature is approximately a factor of 2 lower than literature recommendations. The reaction displays a slightly negative temperature dependence, which can be represented by a negative activation energy, (E(a)/R) = -400 K. The laser absorption results are supported by independent experiments at 298 K and 4 Torr using time-resolved synchrotron-photoionization mass-spectrometric detection of the products of divinyl ketone and methyl vinyl ketone photolysis. The photoionization mass spectrometry experiments additionally show that methyl + propargyl are formed in the vinyl radical self-reaction, with an estimated branching fraction of 0.5 at 298 K and 4 Torr.