Determination of absolute photoionization cross-sections of aromatics and aromatic derivatives (original) (raw)
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Chemical Physics Letters, 2004
The photoionization (PI) cross sections of allyl and 2-propenyl radicals to form C 3 H þ 5 were measured using tunable vacuum ultraviolet (VUV) synchrotron radiation coupled with photofragment translational spectroscopy. At 10 eV, the cross sections were found to be 6.2 AE 1.2 and 5.1 AE 1.0 Mb, respectively. Using these values, the PI efficiency curves for each radical were placed on an absolute scale from 7.75 to 10.75 eV.
Determination of absolute photoionization cross sections of the phenyl radical
Physical Chemistry Chemical Physics, 2006
Photofragment translational spectroscopy experiments employing tunable vacuum ultraviolet photoionization yielded absolute photoionization cross sections for vinyl and propargyl radicals at 10 eV of 11.1Ϯ2.2 and 8.3Ϯ1.6 Mb, respectively. From these values, the photoionization efficiency curves from 7.8 -10.8 eV for these radicals were placed on an absolute scale.
Photofragment translational spectroscopy of propargyl radicals at 248 nm
Chemical Physics, 2008
The dissociation dynamics of allene, propyne, and propyne-d 3 at 193 nm were investigated with photofragment translational spectroscopy. Products were either photoionized using tunable VUV synchrotron radiation or ionized with electron impact. Product time-of-flight data were obtained to determine centre-of-mass translational energy (P(E T )) distributions, and photoionization efficiency (PIE) curves were measured for the hydrocarbon products. The two major product channels evident from this study are atomic and molecular hydrogen loss, with a H:H 2 branching ratio of 90:10, regardless of precursor. The P(E T ) distribution for each channel is also largely independent of precursor. Both channels appear to occur following internal conversion to the ground electronic state. The propyne-d 3 results show that there is extensive isotopic scrambling prior to H(D) atom loss, and that the H:D product ratio is approximately unity. The PIE curves for H(D) atom loss from allene, propyne, and propyne-d 3 indicate that the dominant corresponding C 3 H 3 product is the propargyl radical in all cases. There is some evidence from the PIE curves that the dominant C 3 H 2 products from allene and propyne are propadienylidene (H 2 CCC:) and propargylene (HCCCH), respectively.
Absolute Photoionization Cross-Section of the Methyl Radical †
Journal of Physical Chemistry A, 2008
The absolute photoionization cross-section of the methyl radical has been measured using two completely independent methods. The CH 3 photoionization cross-section was determined relative to that of acetone and methyl vinyl ketone at photon energies of 10.2 and 11.0 eV by using a pulsed laser-photolysis/time-resolved synchrotron photoionization mass spectrometry method. The time-resolved depletion of the acetone or methyl vinyl ketone precursor and the production of methyl radicals following 193 nm photolysis are monitored simultaneously by using time-resolved synchrotron photoionization mass spectrometry. Comparison of the initial methyl signal with the decrease in precursor signal, in combination with previously measured absolute photoionization cross-sections of the precursors, yields the absolute photoionization cross-section of the methyl radical; σ CH 3 (10.2 eV) ) (5.7 ( 0.9) × 10 -18 cm 2 and σ CH 3 (11.0 eV) ) (6.0 ( 2.0) × 10 -18 cm 2 . The photoionization cross-section for vinyl radical determined by photolysis of methyl vinyl ketone is in good agreement with previous measurements. The methyl radical photoionization cross-section was also independently measured relative to that of the iodine atom by comparison of ionization signals from CH 3 and I fragments following 266 nm photolysis of methyl iodide in a molecular-beam ion-imaging apparatus. These measurements gave a cross-section of (5.4 ( 2.0) × 10 -18 cm 2 at 10.460 eV, (5.5 ( 2.0) × 10 -18 cm 2 at 10.466 eV, and (4.9 ( 2.0) × 10 -18 cm 2 at 10.471 eV. The measurements allow relative photoionization efficiency spectra of methyl radical to be placed on an absolute scale and will facilitate quantitative measurements of methyl concentrations by photoionization mass spectrometry. † Part of the "Stephen R. Leone Festschrift".
UV photodissociation dynamics of allyl radical by photofragment translational spectroscopy
The Journal of Chemical Physics, 1998
Photodissociation of the allyl radical, CH 2 CHCH 2 , has been studied using the method of molecular beam photo fragment translational spectroscopy following excitation to the C(2 2 Bl) and A(l 2 Bl) states by 248 and 351 nm photons. Two different primary channels have been detected at 248 nm excitation: H-atom loss (84%) and CH 3 elimination (16%). From the product translational energy distribution and polarization dependence studies dissociation processes from the ground state C 3 Hs potential energy surface are inferred for both wavelengths. At 248 nm there may also be a contribution to the H-atom loss channel from predissociation by a higher electronic excited state. RRKM calculations show that the formation of cyclopropene is not important while formation of allene and methylacetylene from 1-and 2-propenyl radicals dissociation are important reaction pathways at both wavelengths. Evidence for CH 3 elimination directly from an allylic structure through a four-member cyclic transition state is present in the translational energy distribution peaked well away from zero.
Ultraviolet Photodissociation Dynamics of the 1-Propenyl Radical
The journal of physical chemistry. A, 2016
Ultraviolet (UV) photodissociation dynamics of jet-cooled 1-propenyl radical (CHCHCH3) were investigated at the photolysis wavelengths from 224 to 248 nm using high-n Rydberg atom time-of-flight (HRTOF) technique. The 1-propenyl radicals were produced from 193 nm photolysis of 1-chloropropene and 1-bromopropene precursors. The photofragment yield (PFY) spectra of the H atom product have a broad peak centered at 230 nm. The H + C3H4 product translational energy P(ET) distribution's peak near ∼8 kcal/mol, and the fraction of average translational energy in the total available energy, ⟨fT⟩, is nearly a constant of ∼0.12 from 224 to 248 nm. The H atom product has an isotropic angular distribution with the anisotropy parameter β ≈ 0. Quasiclassical trajectory calculations were also carried out using an ab initio ground-state potential energy surface for dissociation of 1-propenyl at the excitation energy of 124 kcal/mol (230 nm). The calculated branching ratios are 60% to the methyl ...
Photoionization Efficiencies of Five Polycyclic Aromatic Hydrocarbons
The journal of physical chemistry. A, 2017
We have measured photoionization-efficiency curves for pyrene, fluoranthene, chrysene, perylene, and coronene in the photon energy range of 7.5-10.2 eV and derived their photoionization cross-section curves in this energy range. All measurements were performed using tunable vacuum ultraviolet (VUV) radiation generated at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory. The VUV radiation was used for photoionization, and detection was performed using a time-of-flight mass spectrometer. We measured the photoionization efficiency of 2,5-dimethylfuran simultaneously with those of pyrene, fluoranthene, chrysene, perylene, and coronene to obtain references of the photon flux during each measurement from the known photoionization cross-section curve of 2,5-dimethylfuran.
The journal of physical chemistry. A, 2014
Four molecules were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy: 1-propynylbenzene, indene, ethynylbenzene, and benzocyclobutene. Their threshold photoelectron spectrum was obtained and electronic transitions were assigned by OVGF (outer valence Green's function) calculations. Vibrational progressions observed in the electronic ground and excited states were simulated by calculating Franck-Condon factors based on the neutral as well as the cation ground and excited state geometries. iPEPICO was used to obtain ion dissociation data in threshold photoionization as a function of photon energy, which were modeled with RRKM theory to extract kinetic parameters for the reactions C9H8(+•) (1-propynylbezene) → C9H7(+) + H (R1); C9H8(+•) (indene) → C9H7(+) + H (R2); C8H8(+•) (benzocyclobutene) → C8H7(+) + H (R3); C8H8(+•) (benzocyclobutene) → C6H6(+) + C2H2 (R4); C8H6(+•) (1-ethynylbenzene, aka phenylacetylene) → C6H4(+) + C2H2 (R5). These results we...
Photodissociation Dynamics of the Phenyl Radical via Photofragment Translational Spectroscopy
2010
Photofragment translational spectroscopy was used to study the photodissociation dynamics of the phenyl radical at 193 and 248 nm. Time of flight data collected for the C_6H_4, C_4H_3, and C_2H_2 photofragments show the presence of two decomposition channels. The only C_6H_5 decomposition channel observed at 248 nm corresponds to C-H bond fission from the cyclic radical producing ortho-benzyne. The translational energy distribution peaks at 0 kcal/mol and is consistent with no exit barrier for the H loss process. At 193 nm photodissociation, however, H loss was observed to be the minor channel, while the major decomposition pathway corresponds with decyclization of the C_6H_5 radical and subsequent fragmentation to n-C_4H_3 and C_2H_2. These two momentum matched photofragments have a translational energy distribution that peaks around 9 kcal/mol, indicative of a process that proceeds through a tighter transition state. Previous theoretical work on the unimolecular decomposition of the phenyl radical predicts a second H loss process that occurs after C_6H_5 decyclization resulting in the linear C_6H_4 photofragment. This channel cannot be unambiguously discerned from the C_6H_4^+ time of flight data, but is believed to take place since decyclization is observed. L. K. Madden, L. V. Moskaleva, S. Kristyan, and M. C. Lin J. Phys. Chem. A 1997, 101, 6790.