Ultraviolet Photodissociation Dynamics of the Allyl ... - ACS Publications

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Ultraviolet Photodissociation Dynamics of the Allyl Radical via the B̃ 2A1(3s), C̃ 2B2(3py), and Ẽ2B1(3px) Electronic Excited States Yu Song, Michael Lucas, Maria Alcaraz, and Jingsong Zhang* Department of Chemistry, University of California at Riverside, Riverside, California 92521, United States

Christopher Brazier Department of Chemistry and Biochemistry, California State University, Long Beach, Long Beach, California 90840, United States ABSTRACT: Ultraviolet (UV) photodissociation dynamics of jet-cooled allyl radical via the B̃ 2A1(3s), C̃ 2B2(3py), and Ẽ 2B1(3px) electronically excited states are studied at the photolysis wavelengths from 249 to 216 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance-enhanced multiphoton ionization (REMPI) techniques. The photofragment yield (PFY) spectra of the H atom products are measured using both allyl chloride and 1,5hexadiene as precursors of the allyl radical and show a broad peak centered near 228 nm, whereas the previous UV absorption spectra of the allyl radical peak around 222 nm. This difference suggests that, in addition to the H + C3H4 product channel, another dissociation channel (likely CH3 + C2H2) becomes significant with increasing excitation energy. The product translational energy release of the H + C3H4 products is modest, with the P(ET) distributions peaking near 8.5 kcal/mol and the fraction of the average translational energy in the total excess energy, ⟨f T⟩, in the range 0.22−0.18 from 249 to 216 nm. The P(ET)’s are consistent with production of H + allene and H + propyne, as suggested by previous experimental and theoretical studies. The angular distributions of the H atom products are isotropic, with the anisotropy parameter β ≈ 0. The H atom dissociation rate constant from the pump−probe study gives a lower limit of 1 × 108/s. The dissociation mechanism is consistent with unimolecular decomposition of the hot allyl radical on the ground electronic state after internal conversion of the electronically excited state.

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INTRODUCTION Allyl radical (C3H5) is a prototypical open-shell species in organic chemistry due to its special structure. The three carbon atoms in the allyl radical are sp2 hybridized and lie in a plane. The three unpaired π electrons of the carbon atoms are resonance-delocalized and stabilized. The allyl radical is the smallest π conjugated system with an odd number of π electrons. The allyl radical also plays an important role in combustion chemistry. In addition to propargyl radical, the allyl radical is one of the most important precursors for the formation of benzene and other cyclic compounds in flames.1−3 As a result, the allyl radical has been examined extensively for decades. The first electronic absorption spectrum of the allyl radical was observed at 370−410 nm by Currie and Ramsay4 using flash photolysis and later confirmed by Tonokura and Koshi5 using cavity ring-down spectroscopy. This first absorption band, with an origin at 408.3 nm and sharp structures, was assigned to the transition from the ground electronic state X̃ 2A2 to the first excited electronic state Ã 2B1. An ultraviolet (UV) electronic spectrum of the allyl radical was reported at 210−250 nm by Callear and Lee6 using flash photolysis of eight allyl compounds and reproduced later by van den Bergh and Callear7 and Jenkin et al.8 This UV band is stronger than the first absorption band in the 408.3 nm region and has a diffuse structure with a © 2015 American Chemical Society

maximum absorption around 222 nm. Another UV absorption spectrum in the same region was reported by Bayrakceken et al.,9 but it peaked around 230 nm instead. The UV electronic spectra and vibronic structures of allyl and its isotopologues have been characterized using resonance-enhanced multiphoton ionization (REMPI) and photoelectron spectroscopy (PES) by several research groups.10−16 Four electronic excited states, B̃ 2A1(3s), C̃ 2B2(3py), D̃ 2A1(3pz), and Ẽ 2B1(3px) of allyl have been identified. In a most recent study, the Chen group reinvestigated the vibronic bands of allyl in the UV region using REMPI with jet-cooled allyl radicals generated by pyrolysis of 1,5-hexadiene and photolysis of allyl iodide.14 The difference in cooling in the pyrolytic and photolytic radical sources allowed identification of hot bands, and along with the threshold PES study,15 the Chen group reported some new vibronic peaks and revised the assignments of several previously observed vibronic bands.14 The UV vibronic bands are due to transitions to the Rydberg electronic states, B̃ 2A1(3s), C̃ 2B2(3py), D̃ 2A1(3pz), and Ẽ 2B1(3px). The second electronic excited state of allyl was Special Issue: Dynamics of Molecular Collisions XXV: Fifty Years of Chemical Reaction Dynamics Received: July 12, 2015 Revised: August 29, 2015 Published: September 3, 2015 12318

DOI: 10.1021/acs.jpca.5b06684 J. Phys. Chem. A 2015, 119, 12318−12328

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The Journal of Physical Chemistry A

Figure 1. Potential energy diagram of the C3H5 system. Three possible H atom dissociation channels and two CH3 elimination channels are shown in the figure. The energetics and pathways are based on the theoretical calculations in refs 25 and 26. The energies of the electronic excited states of the allyl radical are based on refs 4 and 14. The bracket next to the electronic states shows the photolysis wavelength region 249−216 nm in this study.

identified in the 2 + 2 REMPI spectrum as the B̃ 2A1(3s) Rydberg state with a planar geometry and band origin at 40 046 cm−1. The B̃ 2A1(3s) state is forbidden in one-photon transition, but several vibronic bands of the B̃ 2A1(3s) state with B1 symmetry are symmetry-allowed in one-photon transition and were identified in the 1 + 1 REMPI spectrum. The next electronic excited state is the C̃ 2B2(3py) Rydberg state, which was also observed in the 1 + 1 REMPI spectrum and has a band origin at 41 556 cm−1. The D̃ 2A1(3pz) Rydberg state was observed as weak peaks in the 2 + 2 REMPI spectrum with its band origin at 42 230 cm−1. At higher energy above 43 300 cm−1, the REMPI signal decreased significantly, whereas the UV/UV-depletion spectrum showed a strong broad absorption band with a few sharp peaks starting from 43 300 cm−1 up to 45 000 cm−1,14 which was consistent with the strong UV absorption spectrum of allyl starting around 231 nm and peaking near 222 nm. This strong UV band was assigned to the Ẽ 2B1(3px) Rydberg state.14 As this band was not observed in the REMPI spectrum in the same wavelength region, the lifetime of the Ẽ 2B1(3px) Rydberg excited state was indicated to be very short due to a fast nonradiative decay. The transition to the Ẽ 2B1(3px) Rydberg state carries most of the oscillator strength among the 3s and 3p Rydberg excited states.14 Theoretical calculations17−21 were also performed to examine the upper electronic excited states. The assignments and energies of the excited electronic states were not in full agreement with the experimental work except for the first excited electronic state Ã 2B1. The ground-state potential energy diagram and dissociation pathways of the allyl radical have been investigated by several theoretical studies (Figure 1).22−26 There are three H-loss dissociation channels and one methyl-loss channel, in addition to a higher energy vinylidene + CH3 channel.22−26

CH 2CHCH 2(X̃ 2A 2) → H + CH3CCH (propyne) ΔH = 54.5 kcal/mol26 CH 2CHCH 2(X̃ 2A 2) → H + c ‐C3H4 ΔH = 77.9 kcal/mol25 CH 2CHCH 2(X̃ 2A 2) → CH3 + C2H 2 ΔH = 47.1 kcal/mol26

CH 2CHCH 2(X̃ 2A 2) → CH3 + CCH 2 ΔH = 88.4 kcal/mol25

Based on the energies of the dissociation transition states, direct dissociation from allyl radical to allene + H has the lowest energy barrier of 60.9 kcal/mol although this product channel is of higher energy than the propyne + H and CH3 + C2H2 channels. The allyl radical could isomerize to the 2propenyl and 1-propenyl radicals with transition-state barriers of 61.2 and 64.2 kcal/mol, respectively. A second pathway to the allene + H products is via dissociation of the 2-propenyl radical (with an overall barrier of 61.2 kcal/mol). Three pathways lead to the propyne + H channel: (1) isomerization of allyl to 2-propenyl and then dissociation to propyne and H, CH2CHCH2 → CH2CCH3 → CH3CCH + H (overall barrier of 61.2 kcal/mol), (2) isomerization of allyl to 1-propenyl and then dissociation to propyne and H, CH2 CHCH 2 → CHCHCH3 → CH3CCH + H (overall barrier of 64.2 kcal/ mol), and (3) isomerization of allyl to 2-propenyl and then to 1-propenyl followed by dissociation, CH 2 CHCH 2 → CH2CCH3 → CHCHCH3 → CH3CCH + H (overall barrier of 64.5 kcal/mol). The allyl radical could also isomerize to the cyclopropyl radical with the lowest isomerization barrier of 50.2 kcal/mol,27,28 which leads to the c-C3H4 + H products at higher energy. The overall reaction barrier of the cyclopropene + H

CH 2CHCH 2(X̃ 2A 2) → H + CH 2CCH 2 (allene) ΔH = 55.6 kcal/mol26 12319


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product energy distribution was statistical with ⟨f T⟩ ∼ 0.24, and the dissociation mechanism of allyl around 248 nm was hot radical dissociation at the ground electronic state following internal conversion from the UV excited electronic state. The lifetimes of several vibronic bands of allyl between 238 and 250 nm were measured using picosecond time-resolved pump− probe PES monitoring the disappearance time of the allyl radical on the excited states by the Fischer group.32,33 The vibronic bands in the 250−238 nm region belong to the B̃ 2A1 and C̃ 2B2 electronic excited states (previously assigned to the B̃ 2 A1, C̃ 2B1, and D̃ 2B2 excited states14), and their lifetimes ranged from 20 to 9 ps,32,33 supporting the fast nonradiative decay of allyl in the previous studies. Stranges et al. also studied the UV photodissociation of allyl at 248 nm via the B̃ 2A1 state (previously assumed to be the C̃ 2B1 state) using mass spectrometry and photofragment translational spectroscopy to monitor the non-hydrogen heavier photofragments (C3Hx, CH3, and C2H2).30 The decay mechanism of the excited B̃ 2A1 state was internal conversion to the ground-state allyl or to the Ã 2B1 state and then to the ground-state allyl, forming hot allyl radical in the ground state with 115 kcal/mol vibrational energy. Both the H atom loss channel and CH3 + C2H2 product channel were observed, with the H-loss channel being dominant and a branching ratio of the H-loss channel to the CH3 + C2H2 channel being 84:16 at 248 nm (later revised to be 95:5).30,35 The P(ET) distribution of the H-loss channel was consistent with a statistical dissociation, peaking near zero and with ⟨f T⟩ ∼ 0.22. The RRKM calculations indicated that ∼50% of the allyl radicals dissociate into H + allene and CH3 + C2H2, whereas the other 50% isomerizes to 1- and 2-propenyl radical and then dissociate into H + allene and H + propyne.30 The main H-loss dissociation products were allene and propyne, with minor c-C3H4 produced, and the observation of the H-loss channel via the heavier counter fragment was consistent with that by direct H detection from Chen group.23,29 The P(ET) distribution of the CH3 + C2H2 channel peaked away from zero at 16 kcal/mol. The dissociation pathway of the CH3 elimination was initially proposed to be through a cyclic fourmembered transition state.30 A more recent study using the same technique and CH2CDCH2 revealed two different P(ET) distributions for the CH3 elimination channel, suggesting two mechanisms via isomerization from allyl to 1-propenyl by a 1,3 H shift or a sequential 1,2 H shift, followed by dissociation to the CH3 + C2H2 products.24,35 The first 1,2 H shift isomerization from allyl to 2-propenyl could also lead to the higher energy CH3 + vinylidene products.24 The ground-state dissociation of the allyl radical at an excitation energy of 115 kcal/mol (equivalent to the 248 nm photoexcitation energy) was also examined by trajectory calculations.25 The overall dissociation rate was 6.3 × 1010/s, giving a ground-state lifetime of 16 ps at 115 kca/mol. The calculations showed that the branching ratio of the H-loss channel and the CH3 elimination channel is 94:6, in good agreement with the experiment.35 The primary dissociation is H atom loss, producing allene and propyne in a ratio of 6:1. The calculated P(ET) distribution of the H-loss channel was in general agreement with the experimental P(ET) distribution from Stranges et al.,30 but the calculated P(ET) peaked at 11 kcal/mol, whereas the experimental P(ET) peaked at ∼3 kcal/mol. The minor CH3 elimination was shown to be from three pathways, 1,3 H shift of allyl to 1-propenyl and dissociation to CH3 + acetylene, sequential 1,2 H shift of allyl to 1-propenyl and dissociation to

channel (79.2 kcal/mol) is higher than those of the other two H dissociation channels and is thus considered as a minor dissociation pathway on the ground electronic state.29,30 Two possible pathways can lead to the CH3 + C2H2 channel: (1) isomerization of allyl to 1-propenyl and then dissociation, CH2CHCH2 → CHCHCH3 → CH3 + C2H2 (overall barrier of 64.2 kcal/mol) and (2) isomerization of allyl to 2-propenyl and then 1-propenyl followed by dissociation, CH2CHCH2 → CH2CCH3 → CHCHCH3 → CH3 + C2H2 (overall barrier of 64.5 kcal/mol). A higher energy vinylidene + CH3 product channel has also been identified via the isomerization of allyl to 2-propenyl, CH2CHCH2 → CH2CCH3 → CH3 + CCH2.24,25 In Figure 1, the energies of the electronically excited states of the allyl radical are also labeled, which are based on the early absorption spectroscopy and the recent REMPI study.4,6−8,14 The photodissociation dynamics of the allyl radical has been investigated in the near-UV region around 351 and 380−420 nm and the UV region around 248 nm,23−25,29−35 whereas the unimolecular dissociation of the hot allyl radical was examined through secondary decomposition of allyl iodide with 193 nm radiation.36,37 Stranges et al. examined the photodissociation of the allyl radical at 351 nm via the Ã 2B1 state using photofragment translational spectroscopy and pyrolysis of allyl iodide.30 The H-loss product channel was observed, with the product translational energy distribution P(ET) peaking near zero, an average translational energy ⟨ET⟩ of 4.5 kcal/mol (corresponding to a fraction of the average translational energy in the total available energy, ⟨f T⟩, of 0.17), and an isotropic product angular distribution. It was proposed that the Ã 2B1 state of allyl decays into the ground-state cyclopropyl radical and then to the ground-state allyl radical via ring opening. The hot allyl radical dissociates to allene + H directly and to propyne + H via isomerization to the 1- and 2-propenyl radicals.30 Chen and co-workers studied the photodissociation of allyl via the Ã 2B1 state in the wavelength range of 380−420 nm using time- and frequency-resolved photoionization of the H atom product and pyrolysis of allyl iodide.34 The unimolecular dissociation rate constants of allyl and partially deuterated allyl (CD2CHCD2) via the vibronic bands Ã 000, Ã 710, and Ã 1410 were measured to be in the range of (1−8) × 107/s and in good agreement with the RRKM theory calculations. The measured dissociation rates and the H/D product ratio in CD2CHCD2 suggested direct loss of the central H atom and formation of allene as the major product channel.34 The average product translational energy release ⟨ET⟩ via Ã 000 was 6.2 kcal/mol (⟨f T⟩ ∼ 0.42), higher than that at the 351 nm photodissociation.30 Chen and co-workers also investigated the UV photodissociation of allyl at 248.15 and 245.85 nm (previously assigned as C̃ 000 and C̃ 710, now revised to be B̃ 1210 and B̃ 7101210 vibronic bands, respectively14).23,29 The allyl radical was also produced from pyrolysis of allyl iodide and the photodissociation was probed using time- and frequencyresolved photoionization of the H products. The dominant dissociation channel was identified to be a fast loss of H atom to generate allene as confirmed by the loss of central H atom from the isotopic labeled CH2CDCH2 and CD2CHCD2. The second H channel was a slower 1,2 shift isomerization of allyl radical to 2-propenyl, followed by H loss to form allene + H and propyne + H. This channel was proposed because the time delay study showed two different H decay rates (∼108/s and 4 × 107/s). The branching ratio of these two channels was between 2:1 and 3:1. The third H channel to generate the cyclopropene and H products was considered to be small. The 12320


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the UV lens ≈100 cm, 216−249 nm, line width ∼0.3 cm−1, 0.25−1.2 mJ/pulse, pulse duration ∼10 ns). The photolysis laser spot size at the interaction region was ∼1−1.5 mm in diameter. The absolute wavelength of the photolysis laser was checked by a Burleigh WA-4500 wavemeter. The polarization of the linearly polarized photolysis radiation was rotated by a Fresnel-Rhomb achromatic λ/2 plate for product angular distribution measurements. The H atom products from the allyl photodissociation were probed by two-color resonant excitation (121.57 nm + 366.24 nm), i.e., from 12S to 22P via the H atom Lyman-α transition and then further to a high-n Rydberg state. These Rydberg H atoms are radiatively metastable; a small fraction of them drifted with their nascent velocities to a microchannel plate (MCP) detector installed perpendicular to the molecular beam and were detected after field ionization in front of the detector. The length of the flight path was 37.1 cm, which was calibrated by photodissociation of HBr at 236 nm [using the spin−orbit splitting energy of Br(2P3/2) and Br(2P1/2) and the bond dissociation energy of HBr]. The ion signals were amplified by a fast preamplifier, and the H atom TOF spectra were accumulated using a multichannel scaler. Typically, the H atom TOF spectra were averaged with ∼105 laser firings. Four types of TOF spectra were recorded to establish the H atom product signals from the allyl radical photodissociation and for background subtraction:41 (1) full spectrum, with both the 193 nm radiation (for allyl radical production) and the UV photolysis radiation on, and the Rydberg atom tagging probe-laser radiations (121.57 + 366.24 nm); (2) precursor background spectrum, with the 193 nm radiation off but the UV photolysis radiation on, plus the probe-laser radiations; (3) radical background spectrum caused by the probe lasers, with the UV photolysis radiation off but the 193 nm radiation and the probe radiations on; (4) probe-laser background spectrum, with only the probe radiations on. The background spectra (3) and (4) had very low intensities. The precursor background spectrum (2) from photolysis of allyl chloride or 1,5-hexadiene by the UV photolysis laser represented the main background. The net TOF spectra of the H atom products in the UV photolysis of allyl radical were obtained after proper background subtractions. The photofragment yield (PFY) spectra of the H atom products (i.e., action spectra) were recorded using two methods. The first one was based on integrated signals of the net HRTOF spectra of the H atom products as a function of photolysis wavelength. The second method involved measuring the H atom REMPI signals at various photolysis wavelengths. The H atom REMPI experiment was carried out in the same HRTOF instrument. Upon photodissociation by the UV photolysis laser, the H atom products were detected by 1 + 1′ REMPI, with the Lyman-α radiation exciting the H atoms from 12S to 22P and the UV radiation at ∼364 nm ionizing the H atoms. The delay time between the photolysis and the REMPI probe lasers was kept at 10 ns for the REMPI action spectra. The H+ ions were extracted and accelerated in the TOF mass spectrometer and monitored by the MCP detector. The ion signals at m/z = 1 were amplified by a fast preamplifier, integrated by a Boxcar averager (SR250), and transferred to the computer. Two types of REMPI spectra were monitored: (i) full spectrum with both the 193 nm radical generation radiation and the UV photolysis radiation on, plus the REMPI probe lasers and (ii) background spectrum with the 193 nm radiation off, but the UV photolysis radiation and the REMPI probes on. The H atom REMPI PFY spectra were obtained from the net H

CH3 + acetylene, and a single 1,2 H shift of allyl to 2-propenyl and dissociation to CH3 + vinylidene.25 In the present study, the high-n Rydberg H atom time-offlight (HRTOF) technique was employed to study the dynamics of the H atom dissociation channel of the allyl radical in the photolysis wavelength region 249−216 nm, which covers the absorption maximum of 222 nm in UV region and spans across the electronic excited B̃ 2A1(3s), C̃ 2B2(3py), D̃ 2 A1(3pz), and Ẽ 2B1(3px) states of the allyl radical.14 The previous studies on the photodissociation of allyl focused on the near-UV region at 420−380 and 351 nm via the first electronic excited state Ã 2B1,30,34 the UV region around 248 nm via the second electronic excited state B̃ 2A1(3s),23,29,30,35 or secondary decomposition of allyl iodide with 193 nm radiation to generate hot allyl radical in the ground electronic state.36,37 Except for the lifetime measurements of the B̃ 2A1 and C̃ 2B2 vibronic bands in the 250−238 nm region,32,33 there have been no studies on the photodissociation dynamics and product distributions of allyl at wavelengths shorter than 245 nm or via the C̃ 2B2(3py), D̃ 2A1(3pz), and Ẽ 2B1(3px) excited states of allyl. The UV absorption spectrum of the allyl radical peaked at ∼220 nm with the large oscillator strength (mostly from Ẽ 2 B1(3px)), whereas the REMPI spectrum had diminishing intensity and the depletion spectrum of allyl had increasing intensity at the wavelengths shorter than 232 nm, indicating a fast nonradiative decay and decomposition of the allyl radical.14 Furthermore, the Rydberg states of the allyl radical, B̃ 2A1(3s), C̃ 2B2(3py), D̃ 2A1(3pz), and Ẽ 2B1(3px), are believed to be strongly coupled, and in particular, a set of three-state same symmetry conical intersections were predicted theoretically in the vicinity of the Ẽ 2B1(3px),20 making it interesting to probe the photochemistry of allyl via these higher excited states. The early photodissociation studies were carried out using pyrolysis of allyl iodide,23,29,30,35 whereas the current study utilized photolytic production of the allyl radical, which tends to generate colder radicals in the beam (with a rotational temperature of ∼10 K from photolysis vs ∼250 K from pyrolysis).14 The HRTOF technique monitoring the H atom product in this study has a higher resolution than the Doppler spectroscopy technique used by Chen group23,29 and can probe lower translational energy products than the studies monitoring the heavier counter fragment of H atom,30,35 thus rendering more reliable product P(ET) distributions. The current work provides the first time study on the photodissociation dynamics of allyl via the C̃ 2B2(3py) and Ẽ 2B1(3px) electronic excited states in the short wavelength region (the D̃ 2A1(3pz) transition is weak and not accessed by one-photon excitation14).

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EXPERIMENTAL SECTION

The HRTOF technique and experimental setup have been described previously.38−41 A pulsed allyl (C3H5) radical beam was produced by photolyzing a ∼5% mixture of allyl chloride (98%, Aldrich) or 1,5-hexadiene (97%, Aldrich) seeded in Ar (at a total pressure of 115 kPa) using 193 nm radiation. The 193 nm radiation from an ArF excimer laser was slightly focused in front of the pulse nozzle. The allyl radicals produced from the photolysis were entrained in the carrier gas and subsequently cooled by supersonic expansion in the molecular beam. The allyl radical beam was collimated by a skimmer (1 mm diameter) at 2.8 cm downstream into a high-vacuum chamber, and it was crossed at 4.6 cm further downstream with a slightly focused UV photolysis laser radiation (focal length of 12321


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The Journal of Physical Chemistry A atom REMPI signals after removing the background (ii) and normalizing the photolysis laser power. The REMPI spectra of the allyl radical were obtained using the 1 + 1 ionization scheme.14 The UV laser radiation was slightly focused with a f.l. ≈ 100 cm lens onto the allyl radical beam, and the m/z = 41 peak signal was collected using an oscilloscope and integrated as a function of photoionization wavelength using the Boxcar averager. The averaged signals from the Boxcar averager were sent to the dye laser data acquisition software to record the allyl REMPI spectra.

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RESULTS To characterize the allyl radical beam, VUV photoionization TOF mass spectra using the Lyman-α radiation (10.2 eV) were obtained with both allyl chloride and 1,5-hexadiene precursors. Figure 2 shows the net TOF mass spectrum of allyl chloride

Figure 3. Lower panel is the 1 + 1 REMPI spectrum of the allyl radical generated from allyl chloride in the region 235−249 nm. The upper panel is the PFY spectrum of the H atom product from photodissociation of the allyl radical using H atom REMPI. The spectra are normalized with the laser power. The onset energies of the electronic excited states of allyl are marked in the figure (from ref 14). The energy of the Ẽ 2B1 state has a large uncertainty.

peak near 42 130 cm−1 is consistent with the C̃ 910 vibronic band. The allyl REMPI peaks diminished at higher photon energy due to the nonradiative decay; the H atom PFY peak at 43 270 cm−1 has no corresponding allyl REMPI peak, but it agrees with the peak in the UV absorption spectrum6 and the depletion spectrum in the same location,14 indicating H-loss following the nonradiative decay of this vibronic band (assigned to the Ẽ 2B1(3px) state). The H atom PFY signals showed an increasing trend as the excitation wavelength decreased, consistent with the UV absorption spectrum. The H atom TOF spectra from the photodissociation of the allyl radical were taken in the photolysis wavelength region 216−249 nm, with the photolysis laser polarization parallel and perpendicular to the flight path, respectively. The net H atom product TOF spectra of the allyl photodissociation were obtained by removing the background spectra (2) (3) (4) from the full spectra (1). Figure 4a,b show the net H atom HRTOF spectra at two different photolysis wavelengths 228 and 222 nm (0.55−0.85 mJ/pulse), respectively, at parallel polarization with two precursors allyl chloride and 1,5-hexadiene. The TOF spectra from the two different precursors at the same wavelength were essentially identical, supporting that the H atom signals were from the allyl photodissociation. The signal intensities of the H atom TOF spectra from the 1,5-hexadiene precursor were checked at three different photolysis laser powers at 230 nm, 1.2, 0.8, and 0.4 mJ/pulse. The signal intensities increased linearly with the photolysis laser power. The shapes of the TOF spectra stayed the same; no fast component was observed in these three TOF spectra, also illustrating the single-photon dissociation process with the photolysis power up to 1.2 mJ/pulse. The photolysis power was maintained below 1.0 mJ/pulse in the present study to minimize multiphoton dissociation process. Figure 5 shows the H atom PFY spectra of both precursors from integrated HRTOF signals at various photolysis wave-

Figure 2. 121.6 nm VUV photoionization mass spectrum of the allyl radical beam using the allyl chloride precursor in Ar carrier gas. This is the net mass spectrum of allyl chloride with the 193 nm photolysis laser on minus off. The main product was allyl radical at m/z = 41 amu, whereas the allyl chloride parent peaks at m/z = 76 and 78 were depleted due to the 193 nm photolysis. The peak between m/z 76 and 78 was an interference signal induced from electronic signal ringing. Photoionization fragmentation was minimum due to the low photon energy at 121.6 nm and was removed in the net mass spectrum.

with the 193 nm photolysis radiation on minus off. The allyl radical peak (m/z = 41) was the main photoproduct in the beam, whereas the parent allyl chloride peaks at m/z = 76 and 78 showed negative net intensities due to photolytic depletion. The TOF mass spectrum using the 1,5-hexadiene precursor was also recorded and the allyl radical peak was observed in the spectrum as well. The REMPI spectra of the allyl radical generated from the allyl chloride precursor in the region 235−249 nm are shown in Figure 3. There are many vibronic bands of allyl in this region,14 and six of them were taken in the REMPI spectrum at around 40 300, 40 630, 40 670, 41 550, 42 130, and 42 500 cm−1. The positions and profiles of the six peaks agree well with the previous REMPI spectra of allyl,14 with the first five peaks assigned to B̃ 1210, B̃ 910, B̃ 7101210, C̃ 000, and C̃ 910, further confirming the allyl production in the radical beam. The PFY spectrum of the H atom product from photodissociation of allyl via a few vibronic bands in this wavelength region were taken using H atom REMPI and is also shown in Figure 3. The recorded H atom product REMPI peak near 40 630−40 670 cm−1 is in agreement with the allyl REMPI peaks of B̃ 910 and B̃ 7101210, corresponding to the H atom photoproducts from these two bands and similar to the study by Chen group.23,29 The PFY 12322


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Figure 5. H atom product yield (PFY) spectra as a function of photolysis excitation energy in the region of 216−249 nm. Full circles (●) represent the integrated HRTOF signals using the allyl chloride precursor. Open circles (○) represent the integrated HRTOF signals using the 1,5-hexadiene precursor. The absorption spectra taken from refs 6−9 are also shown. The photolysis laser power is normalized for the PFY spectra. The PFY spectra are scaled to the UV spectra from Callear and van den Bergh around 232 nm. In addition to the HRTOF PFY spectra (indicated by ● and ○), the locations of the H atom product REMPI PFY spectra and the allyl radical REMPI spectra (shown in Figure 3) are also indicated in the figure. See the text for more information.

Figure 4. H atom TOF spectra of jet-cooled allyl radical (a) at photolysis wavelength 228 nm, produced from 193 nm photolysis of (a1) allyl chloride precursor and (a2) 1,5-hexadiene precursor, and (b) at photolysis wavelength 222 nm, produced from 193 nm photolysis of (b1) allyl chloride precursor and (b2) 1,5-hexadiene precursor. These are the net TOF spectra with the full spectra minus three background spectra. The TOF spectra at each wavelength are scaled to the same maximum intensity. The polarization vector of the photolysis laser radiation was parallel (θ = 0°) to the TOF axis. The photolysis laser power was 0.55−0.85 mJ/pulse.

atom PFY spectra overlap) gives the upper limit of the effective cross sections of the H atom PFY spectra. The net H atom product TOF spectrum of the allyl radical could be transformed to the products’ center-of-mass (CM) translational energy distributions, P(ET)’s. The products’ CM translational energy, ET, is derived from the H atom flight time tH using the following equation:38,42

lengths from 216 to 249 nm. To correct for drift and variation of the experimental conditions, the integrated H atom signals from photolysis at 222 and 228 nm were used as references and taken after every three measurements at other wavelengths. The H atom signals were then normalized to those of the references and with the photolysis laser power. The two HRTOF PFY spectra from both precursors agree well with each other, with the maximum signal at 228 nm. Figure 5 also shows three UV absorption spectra of allyl by Callear and van den Bergh, Jenkin et al., and Bayrakceken et al.6−9 The two absorption spectra from Callear and van den Bergh and Jenkin et al. showed strong and diffuse features, peaking at ∼222 nm,6−8 whereas that from Bayrakceken et al. was weaker and shifted to longer wavelengths.9 There was strong evidence in our experiment supporting that the HRTOF signals and the PFY spectra were from the allyl radical; however, the PFY spectra appear to be different from the two UV spectra by Callear and van den Bergh and Jenkin et al. but agree better with that by Bayrakceken. The more recent REMPI and depletion spectra of allyl by Gasser et al.14 matched the UV absorption features in the spectrum by Callear and van den Bergh,6,7,14 indicating that the UV spectra by Callear and van den Bergh and Jenkin et al. are more reliable. To compare the UV absorption spectra with the PFY spectra from the present study, the PFY spectra are scaled to the UV absorption spectra by Callear and van den Bergh at around 232 nm. As the cross sections in the H atom PFY spectra have to be less or equal to those of the total UV absorption spectra, the scaling at 232 nm (the lowest point on the UV spectra where the UV absorption spectra and the H

⎛ ⎛ m ⎞ mH ⎞ ⎟⎟E H + ⎜⎜ H EC H ⎟⎟ E T = ⎜⎜1 + 3 5 mC3H4 ⎠ ⎝ ⎝ mC3H4 ⎠ =

2 ⎛ m ⎞ mH ⎞⎛ L ⎞ 1 ⎛⎜ ⎟⎟⎜ ⎟ + ⎜⎜ H EC H ⎟⎟ mH⎜1 + 3 5 2 ⎝ mC3H4 ⎠⎝ t H ⎠ ⎝ mC3H4 ⎠

(1)

where EH and EC3H5 are the laboratory translational energy of the H atom photofragment and the parent C3H5 radical, respectively, and L is the flight length. The second term [(mH/ mC3H4)EC3H5] in eq 1 is due to the parent C3H5 motion in the molecular beam that is perpendicular to the TOF path, and it is much smaller compared with the first term (H and C3H4 products’ CM translation) and can be neglected.38,42 By using eq 1, the CM P(ET) distribution can be obtained from the H atom TOF spectrum via direct conversion.38,42 The resulting P(ET) distributions at several sample wavelengths from 246 to 222 nm using allyl chloride or 1,5-hexadiene precursor are shown in Figure 6. All the P(ET) distributions have a similar broad feature that peaks at ∼8.5 kcal/mol and extends to ∼70 kcal/mol, indicating the highly internally excited C3H4 product. The translational energy release in the P(ET)’s in Figure 6 is modest, with the average product translational energy ⟨ET⟩ in the range 13.5−14.0 kcal/mol. The ⟨f T⟩ values for the H-loss 12323


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Figure 7. Fraction of the average translational energy release in the total available energy, ⟨f T⟩, in the UV photodissociation of the allyl radical as a function of photolysis wavelength from 216 to 249 nm. The average translational energies are calculated from the experimental P(ET) distributions using both allyl chloride and 1,5-hexadiene precursors. The total available energy at each photolysis wavelength is derived from the corresponding photon energy and the dissociation of allyl radical to the H + allene products, D0(H−C3H4) = 55.6 kcal/ mol.22,25,26 The error bar represents 95% confident limit from repeated measurements using both precursors.

Figure 6. Center-of-mass product translational energy distributions, P(ET)’s, of the H + C3H4 product channel of C3H5 from 246 to 222 nm with allyl chloride or 1,5-hexadiene precursor. The P(ET) distributions are directly converted from the H atom TOF spectra (such as those in Figure 4). The photolysis laser power was 0.55−0.85 mJ/pulse. The product translational energy onsets for the allene + H and propyne + H channels are indicated for each photolysis wavelength. The electronic states involved are also marked for each wavelength.

product channel at the different photolysis wavelengths from 216 to 246 nm were measured with photolysis energy 0.15−0.9 mJ/pulse and are shown in Figure 7. The ⟨f T⟩’s in this wavelength region are of low values, and they increase slightly with the increasing wavelength, from 0.18 at 216 nm to 0.22 at 246 nm. The ⟨f T⟩ value of 0.22 at 246 nm is in good agreement with that of 0.24 near 248 and 246 nm by Chen group23,29 and 0.22 at 248 nm by Stranges et al.30 Figure 8 shows the H product angular distributions at 228 nm with polarization of photolysis laser perpendicular and parallel to the TOF pathway (upper panel). The two H atom TOF spectra at both polarizations are identical, which indicates an isotropic product angular distribution in the dissociation process. The lower panel in Figure 8 is the anisotropy parameter β at different flight times. Because the linearly

Figure 8. H atom TOF spectra of 228 nm photodissociation of C3H5, with the polarization E vector of the photolysis radiation (a) perpendicular (θ = 90°) and (b) parallel (θ = 0°) to the TOF axis. The 228 nm radiation power intensity was 0.55−0.65 mJ/pulse. The signals have been normalized to the same photolysis power and laser shots. Anisotropy parameter β is plotted as a function of H atom timeof-flight. The β parameter stays close to the limit of an isotropic angular distribution.

polarized light preferentially excites the radicals whose electronic transition dipole moment parallel to the electric vector E of the polarized laser radiation, the product angular 12324


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where kH(t) is the unimolecular dissociation rate constant for H atom production from the allyl radical and a and b are constants that describe the width of the plateau region and the decay of the signal. The fittings are shown in solid lines in Figure 9, giving a dissociation rate constant of kH ≈ 1 × 108/s for allyl at 228 nm. The time profile with the 1,5-hexadiene precursor was also recorded, which gave the similar dissociation rate constant as allyl chloride. The average dissociation rate constant for the allyl radical at 228 nm from both precursors is ∼1 × 108/s. Note that this dissociation rate constant is the lower limit because this number is limited by the laser pulse time width of ∼10 ns. This limit was verified with the UV photodissociation of the SH radical,40 whose dissociation time scale was in subpicoseconds but gave rise a similar H atom product appearance time in the pump−probe time profile.

distribution is important to determine the vector correlations during dissociation. The photofragment angular distribution is given43 by I(θ) = (1/4π)[1 + βP2(cos θ)], where β is the anisotropy parameter (−1 ≤ β ≤ +2), θ is the angle between the electric vector of the linearly polarized laser radiation E and the recoiling velocity vector of the H atom product (direction of detection or the TOF axis), and P2(cos θ) is the second Legendre polynomial. An anisotropy parameter β ≈ 0 is derived using the H atom TOF spectra at both polarizations. The angular distributions at several longer wavelengths were also checked, showing similar isotropic distributions. The H atom TOF spectra of allyl at 228 nm with both precursors were recorded at different delay times between photolysis pump laser and probe lasers. The integrated H signals at each delay time were plotted as a function of the pump−probe delay time, as shown in Figure 9 with allyl

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DISCUSSION The UV photodissociation of the allyl radical via its electronic excited B̃ 2A1(3s), C̃ 2B2(3py), and Ẽ 2B1(3px) states in the region 249−216 nm is studied; in particular, the photodissociation dynamics of the C̃ 2B2(3py) and Ẽ 2B1(3px) states of allyl are investigated for the first time. The H + C3H4 product channel is directly observed in the UV photodissociation of the allyl radical, supporting the nonradiative decay mechanism of the allyl radical at wavelengths shorter than 238 nm, which was suggested in the previous REMPI studies.11−15 The two HRTOF PFY spectra of allyl from the two different allyl precursors at 216−249 nm were in good agreement with each other, with the maximum at 228 nm (Figure 5). However, these two H atom PFY spectra do not match the previously reported UV absorption spectra by Callear and van den Bergh,6,7 with the PFY spectra being a fraction of the UV absorption spectra, especially in the shorter wavelength region. The previous REMPI and depletion spectroscopy study of the allyl radical indicated a fast nonradiative decay in the UV region, with increasing importance at the shorter wavelengths.14 The difference between the H atom PFY spectra and the UV absorption spectra is likely due to the increasing participation of another dissociation channel as the wavelength is decreased. Based on the report by Stranges et al.,30 there were two dissociation pathways for the allyl radical at 248 nm: the H dissociation channel and CH3 elimination channel with a branching ratio of 84:16 (later revised to 95:5). In 2010, Stranges and co-workers 24 also reported that the CH 3 elimination from the allyl radical resulted in two dissociation channels, which were CH3 + HCCH and CH3 + CCH2 (vinylidene). Because the H atom loss channel is one of the above dissociation channels, the difference between the H atom PFY spectra and the UV spectra in Figure 5 is likely due to the CH3 + C2H2 channel (the pathway for CH3 + C2H2 is illustrated in Figure 1), which apparently becomes more important with the increasing excitation energy. Further experimental and theoretical work is required to confirm this assumption. The CM product translational energy distributions of the H + C3H4 channel from excitation wavelengths 249−216 nm are not repulsive and show a modest translational energy release, with ⟨f T⟩ in the range 0.22−0.18 (Figures 6 and 7). At the longer photolysis wavelengths, two previous studies examined the photodissociation of allyl via the vibronic bands of the B̃ state around 248 and 246 nm using photoionization of hydrogen products and photofragment translational spectroscopy.23,29,30 Chen’s group23,29 reported ⟨ET⟩ = 14 ± 1 kcal/mol

Figure 9. H atom product signals as a function of photolysis and probe laser delay time. The photolysis wavelength was 228 nm and power was ∼0.5 mJ/pulse. The signals were obtained by integrating the HRTOF spectra with the allyl chloride precursor at various photolysisprobe delay times. Different time windows of integration, 18−100, 40−100, and 50−100 μs, were used. The solid lines represent exponential fits that give the unimolecular decay rate constants.

chloride as the allyl radical precursor. This measurement is to obtain the microcanonical rate for the dissociation of allyl radical. The initial rise of the time profiles in Figure 9 represents the H production rate from the photodissociation of allyl radical, whereas the following decay is caused by the H atoms flying out of the photolysis-probe interaction region. Different integral areas on the TOF spectra were selected to check the bias of the flying-out H atoms and the possibility of multiphoton process, as shown by different symbols in Figure 9. The time profile of the slow H atoms should be least affected by the flight-out bias. The three time profiles give the same initial rise time of 10−20 ns, indicating similar dissociation rates of the allyl dissociation. The slower decay of the profile with the 50−100 μs integration window compared to those of the 18− 100 and 40−100 μs windows is due to the smaller speed of the H atoms that flew out of the detection region more slowly. To obtain the unimolecular dissociation rate constants of the allyl radical, the time profiles of the H atom signals, SH(t), are fitted using an expression based on the previous work from Chen’s group:44 ⎤ ⎡ 1 SH(t ) = N[1 − exp(−kHt )]·⎢ ⎥ ⎣ exp[(t − a)/b] + 1 ⎦

(2) 12325


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The Journal of Physical Chemistry A and ⟨f T⟩ = 0.24 at 248.15 nm (B̃ 1210) and 245.85 nm (B̃ 7101210), whereas Stranges et al.30 reported ⟨ET⟩ = 13.2 kcal/mol and ⟨f T⟩ = 0.22 at 248 nm.30 The study by Stranges et al. provided the only translational energy distribution P(ET), with a monotonic decreasing curve starting at ET ∼ 3 kcal/mol and extending to the maximum available energy (Figure 10). In the

possible dissociation mechanisms for the allyl radical at 248 nm. The first mechanism is the internal conversion for the allyl radical from the excited electronic state to the ground electronic state followed by the dissociation of the allyl radical on the ground electronic state. The second mechanism is the internal conversion into the first electronic excited Ã state followed by the electrocyclic transformation into the cyclopropyl radical which goes through the isomerization to form allyl radical on the ground electronic state. Chen’s group also proposed the internal conversion of the B̃ state and dissociation of the hot allyl radical on the ground electronic state.23,29 The dissociation mechanism of the C̃ and Ẽ state are proposed here to be similar to that of the B̃ state, which is internal conversion of the allyl radical from the electronic excited states to the lower or ground electronic state followed by the dissociation of the hot allyl radical on the ground electronic state. The similar P(ET) distributions and dissociation mechanisms from 249 to 216 nm for the B̃ 2A1(3s), C̃ 2B2(3py), and Ẽ 2B1(3px) states are consistent with the strong mixing of these Rydberg states, as proposed by Matsika and Yarkony.20 The P(ET) distributions at the different photolysis wavelengths, 245.85, 240.6, 232, 228, and 222 nm from the three electronic excited states B̃ 2A1(3s), C̃ 2B2(3py), and Ẽ 2B1(3px) extend to the maximum available energy (labeled in Figure 6). The ET energy onsets of the allene + H and propyne + H product channels are very close, whereas that of the c-C3H4 + H channel is ∼23 kcal/mol lower in ET.25,26 The P(ET) distributions (in Figure 6) extending beyond the c-C3H4 + H onset are consistent with the productions of the allene + H and propyne + H products. As shown in Figure 1, the theoretical calculations have identified three H-loss product channels, allene + H, propyne + H, and cyclopropene + H, and two CH3loss product channels, CH3 + C2H2 and CH3 + CCH2, in the dissociation of the allyl radical on the ground electronic state X̃ 2A2. Because the direct dissociation pathway of allyl to allene + H has the lowest overall dissociation energy barrier (60.9 kcal/mol), slightly smaller than that of the lowest propyne + H pathway (61.2 kcal/mol), the allene + H dissociation channel is considered to be more important than the propyne + H channel. The c-C3H4 + H channel is expected to be unimportant due to its higher energetics and dissociation barrier. This is also consistent with the previous experimental result and the RRKM calculations at 248 nm (115 kcal/ mol),23,30 which indicated that allene + H and propyne + H are the main H-loss channels whereas the cyclopropene + H channel makes a minor contribution. The recent trajectory calculations on the ground-state dissociation of allyl at 115 kcal/mol internal energy also supported these results (with the H-loss channels dominant and a 6.4:1 ratio of allene to propyne).25 It is interesting to compare the P(ET) distributions from the current study with those from Stranges et al. at 248 nm and the trajectory calculations by Chen et al. at 115 kcal/mol (Figure 10).25,30 The comparison of the P(ET) at 245.85 nm (B̃ 7101210), closest in energy to 248 nm (115 kcal/mol), shows that although all the three distributions extend to the maximum available energy, there are significant differences in the lower energy region. The P(ET) from Stranges et al. had a maximum at a low energy of ∼3 kcal/mol and did not decrease toward zero. The P(ET) from the trajectory calculations included the contributions from both the allene + H and propyne + H product channels with a branching ratio of 6.4:1. The experimental P(ET) distribution at 245.85 nm from the current

Figure 10. Center-of-mass product translational energy distributions, P(ET)’s, of the H + C3H4 product channel at photolysis wavelength 228 nm (Ẽ state), 240.6 nm (C̃ 000), and 245.85 nm (B̃ 7101210), in comparison with the P(ET) at 248 nm by Stranges et al.30 and the trajectory calculations at 115 kcal/mol by Chen et al.25

current study, the P(ET) distribution at 245.85 nm (B̃ 7101210) shows a peak around 8.5 kcal/mol with ⟨ET⟩ = 13.6 kcal/mol and ⟨f T⟩ = 0.22 (Figures 6 and 7). Although our ⟨f T⟩ value at 245.85 nm is close to that of 0.24 by Chen’s group and 0.22 by Stranges et al. at the nearby 248 nm, the shape of the P(ET) is somewhat different from that by Stranges et al. at 248 nm (Figure 10). The P(ET) from the photodissociation via the C̃ 000 state at 240.6 nm in this study also peaks around 8.5 kcal/mol, with ⟨ET⟩ = 13.8 kcal/mol and ⟨f T⟩ = 0.21. The weak signals in the H atom TOF spectra and the noisier P(ET) distributions at these longer photolysis wavelengths were due to the small absorption cross sections of allyl in this region. The C̃ state was also probed at a higher excitation wavelength of 232 nm, showing similar results. The P(ET) distributions at the photolysis wavelength 228 and 222 nm (Figure 6, corresponding to the Ẽ 2B1 (3px) Rydberg state14) also peak around 8.5 kcal/mol, with ⟨ET⟩ = 13.5 kcal/mol and ⟨f T⟩ = 0.19 at 228 nm and ⟨ET⟩ = 13.7 kcal/mol and ⟨f T⟩ = 0.18 at 222 nm, respectively. When the P(ET) distributions in Figure 6 are compared, it is found that the P(ET) distributions at the different photolysis wavelengths, 245.85, 240.6, 232, 228, and 222 nm, are essentially identical, suggesting the similar dissociation mechanism for the allyl radical at all the three electronic excited states B̃ 2A1(3s), C̃ 2B2(3py), and Ẽ 2B1(3px). On the basis of the translational energy distribution and polarization dependence, Stranges et al.30 proposed two 12326


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249 to 216 nm. In particular, the photodissociation of allyl via the C̃ 2B2(3py) and Ẽ 2B1(3px) excited states were investigated for the first time. The PFY spectra of the H atom product using both the allyl chloride and 1,5-hexadiene precursors peaked at 228 nm, which did not match the previous UV absorption spectra that peak at 222 nm, indicating another possible dissociation channel in this region (e.g., CH3 elimination channel). The H atom TOF spectra at different wavelengths with both precursors confirmed that the H atom product signals were from the allyl dissociation. The product translational energy release was modest, with the P(ET) distributions peaking at around 8.5 kcal/mol. The ⟨f T⟩ value decreased gradually from 0.22 to 0.18 from 246 to 216 nm. The P(ET) distributions supported the allene and propyne production mechanisms. The product angular distribution was isotropic. The dissociation rate constant from the H atom time profile had the lower limit of 1 × 108/s. The dissociation mechanism was consistent with internal conversion of the allyl radical from the B̃ 2A1(3s), C̃ 2B2(3py), and Ẽ 2B1(3px) excited electronic states to the ground electronic state, followed by unimolecular dissociation on the ground electronic state to the products (allene + H and propyne + H).

study is in better agreement with that from the trajectory calculations than with that from Stranges et al. This general agreement with the trajectory calculations supports that the main H-loss product channels are allene + H and propyne + H. The P(ET) from the trajectory calculations peaks at 11 kcal/ mol, higher than the 8.5 kcal/mol value in our experimental P(ET). The peaking in the P(ET) is generally associated with the reverse energy barrier that could channel the reverse activation energy into the products’ translation. The peak in our P(ET) is closer to the 5−6 kcal/mol reverse barrier for the allene + H product channel (Figure 1). Assuming the same statistical dissociation mechanism via the ground electronic state of allyl, the P(ET)’s at the higher excitation energies are also compared with that from Stranges et al. at 248 nm and the trajectory calculations by Chen et al. at 115 kcal/mol (Figure 10), similar to that at 245.85 nm. In 1997 and 1998, Fischer group32,33 measured the decay time of the allyl radical from 250 to 238 nm, corresponding to the electronic states B̃ 2A1(3s) and C̃ 2B2(3py). The lifetime of the allyl radical on these electronic excited states monotonically decreased with the deceasing wavelengths, with values ranging from 20 to 9 ps. Later, Chen’s group studied the photodissociation of the allyl radical around 248 nm,23 which showed the unimolecular dissociation rate constants of the allene channel (experimental result: >108/s, RRKM calculation: 4.2 × 108/s) and the propyne channel (experimental result: 4 × 107/ s, RRKM calculation: 1.9 × 108/s). At the same time, Stranges et al.30 calculated the RRKM microcanonical rate constants for the different dissociation channels (4.4 × 1010/s for allene and 13.0 × 1010/s for propyne) at the 115 kcal/mol (248 nm) excitation energy. In the recent trajectory calculations, the dissociation rate of the ground-state allyl radical at the excitation energy of 115 kcal/mol (248 nm) was estimated to be 6.3 × 1010/s,25 corresponding to a ground-state lifetime of 16 ps at 115 kca/mol. In the present study, the angular distribution of the H atom product from the UV photodissociation of allyl is isotropic (the 228 nm example is shown Figure 8), suggesting that the time scale of the UV photodissociation of the allyl radical in the wavelength range from 216 to 249 nm is longer than one rotational period of the allyl radical (>ps). The average rotational period of the allyl radical parent is estimated to be in the range of 5−15 ps for its three rotational axes on the basis of the observed rotational temperature of ∼15 K in the REMPI spectrum. The pump− probe experiment at 228 nm in this study (Figure 9) shows that the lower limit of dissociation rate constant of the allyl radical at 228 nm is in the order of 108/s. These experimental results indicate that the dissociation time scale of the allyl radical at 228 nm excitation is longer than approximately picoseconds, which is consistent with the previous lifetime and dissociation time investigations as mentioned above. The dissociation time scale of > ps from the excited B̃ 2A1(3s), C̃ 2B2(3py), and Ẽ 2B1(3px) electronic states allows statistical distribution of the excess energy to each degree of freedom during the unimolecular dissociation of the allyl radical, which is consistent with the low-fraction translational energy release ⟨f T⟩ values (0.18−0.22 from photolysis wavelength 216−246 nm) in the present study.

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AUTHOR INFORMATION

Corresponding Author

*J. Zhang. E-mail: [email protected]. Also at Air Pollution Research Center, University of California, Riverside, CA 92521. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Profs. Paul L. Houston and Joel M. Bowman for sharing their trajectory calculation results. This work is supported by the National Science Foundation (CHE1214157).

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REFERENCES

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CONCLUSION The H atom photodissociation channels of the allyl radical via the B̃ 2A1(3s), C̃ 2B2(3py), and Ẽ 2B1(3px) electronic excited states were investigated in the photolysis wavelength region of 12327


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