UV photodissociation dynamics of allyl radical by photofragment translational spectroscopy (original) (raw)
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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.
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.
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 ...
The Journal of …, 2001
This paper presents product translational energy spectroscopy measurements of the primary photofragmentation channels of 2-chloropropene excited at 193 nm and of the unimolecular dissociation of the 2-propenyl radical. Tunable vacuum ultraviolet ͑VUV͒ photoionization of the products allows us to distinguish between the various product isomers formed in these processes. The data show evidence for three significant primary reaction channels in the dissociation of 2-chloropropene: An excited-state C-Cl fission channel producing fast Cl atoms, a C-Cl fission channel producing slow Cl atoms, and HCl elimination. A minor C-CH 3 fission channel contributes as well. The measured branching of the major primary product channels is: ͓fast C-Cl͔:͓slow C-Cl͔:͓HCl elimination͔ϭ62%:23%:15%. The experiments also allow us to resolve selectively the product branching between the unimolecular dissociation channels of the 2-propenyl radical, a high energy C 3 H 5 isomer; we measure how the branching ratio between the two competing C-H fission channels changes as a function of the radical's internal energy. The data resolve the competition between the unimolecular Hϩallene and Hϩpropyne product channels from the radical with internal energies from 0 to 18 kcal/mol above the Hϩpropyne barrier. We find that the barrier to Hϩallene formation from this high-energy C 3 H 5 radical is higher than the barrier to Hϩpropyne formation, in agreement with recent theoretical calculations but in sharp contrast to that predicted for the most stable C 3 H 5 isomer, the allyl radical. The experiments demonstrate a general technique for selectively forming a particular C n H m isomer dispersed by internal energy due to the primary photolysis, thus allowing us to determine the branching between unimolecular dissociation channels as a function of the selected radical isomer's internal energy.
Photodissociation of the propargyl and propynyl (CD) radicals at 248 and 193 nm
The Journal of chemical …, 2009
The photodissociation of perdeuterated propargyl ͑D 2 CCCD͒ and propynyl ͑D 3 CCC͒ radicals was investigated using fast beam photofragment translational spectroscopy. Radicals were produced from their respective anions by photodetachment at 540 and 450 nm ͑below and above the electron affinity of propynyl͒. The radicals were then photodissociated at 248 or 193 nm. The recoiling photofragments were detected in coincidence with a time-and position-sensitive detector. Three channels were observed: D 2 loss, CD+ C 2 D 2 , and CD 3 +C 2 . Observation of the D loss channel was incompatible with this experiment and was not attempted. Our translational energy distributions for D 2 loss peaked at nonzero translational energy, consistent with ground state dissociation over small ͑Ͻ1 eV͒ exit barriers with respect to separated products. Translational energy distributions for the two heavy channels peaked near zero kinetic energy, indicating dissociation on the ground state in the absence of exit barriers.
Time- and frequency-resolved photoionisation of the allyl radical
Faraday Discussions, 2000
We report picosecond time-resolved pumpÈprobe photoelectron spectra of the allyl radical, and the fully deuterated allyl, carried out in order to elucidate the C 3 H 5 , C 3 D 5 , primary photophysical processes upon UV excitation. It is shown that the UV bands of allyl decay in a two-step process : the Ðrst step is an internal conversion to the lower-lying A-state within 20 ps or less, while the second step is a very fast decay from the A-state to the electronic ground state through a conical intersection. In addition we report the Ðrst zero kinetic energy (ZEKE) photoelectron spectrum of allyl, yielding an ionisation energy of 65762 cm~1.
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.
The Journal of Physical Chemistry B, 2002
In the work presented here, we used photofragment translational spectroscopy and H atom Rydberg timeof-flight (HRTOF) spectroscopy to study the primary photofragmentation channels of allyl iodide excited at 193 nm and the ensuing dissociation of the nascent allyl radicals as a function of their internal energy. Two C-I bond fission channels were found to produce the allyl radical, one channel forming I(2 P 3/2) and the other forming I(2 P 1/2). The nascent allyl radicals are dispersed as a function of the translational energy imparted from the photolysis and therefore by their internal energy. Although all of the I(2 P 3/2) and a portion of the I(2 P 1/2) channel allyl radical products have enough internal energy to overcome the 60 kcal/mol barrier to form allene + H, the data showed that a substantial fraction of the allyl radicals from the I(2 P 1/2) channel that formed with internal energies as high as 15 kcal/mol above the 60 kcal/mol barrier were stable to H atom loss. The stability is due to centrifugal effects caused by significant rotational energy imparted to the allyl radical during photolysis and the small impact parameter and reduced mass characterizing the loss of an H atom from an allyl radical to form allene + H. A photoionization efficiency (PIE) curve identified the major C 3 H 4 secondary dissociation products as allene. Comparison of the mass 40 signal in the TOF spectra at two photoionization energies showed that branching to H + propyne does not occur at near-threshold internal energies, indicating that the experimentally determined allyl f 2-propenyl radical isomerization barrier, which is lower than recent ab initio calculations of the barrier by ∼15 kcal/mol, is far too low.
Photofragment translational spectroscopy of allene, propyne, and propyne-d3 at 193 nm
Molecular Physics, 2005
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.
Photofragment translational spectroscopy of 1, 2-butadiene at 193 nm
The Journal of Chemical …, 2001
Photofragment translational spectroscopy has been used to investigate the dissociation dynamics of 1,2-butadiene at 193 nm. Ionization of scattered photoproducts was accomplished using tunable VUV synchrotron radiation at the Advanced Light Source. Two product channels are observed: CH 3 ϩC 3 H 3 and C 4 H 5 ϩH. The C 3 H 3 product can be identified as the propargyl radical through measurement of its photoionization efficiency curve, whereas the C 4 H 5 product cannot be identified definitively. The translational energy P(E T) distributions suggest that both channels result from internal conversion to the ground electronic state followed by dissociation. The P(E T) distribution for the C 4 H 5 product is sharply truncated below 7 kcal/mol, indicating spontaneous decomposition of the slowest C 4 H 5 product.