Photodissociation dynamics of the tert-butyl radical via photofragment translational spectroscopy at 248 nm

The photodissociation dynamics of the tert-butyl radical (t-C(4)H(9)) were investigated using photofragment translational spectroscopy. The tert-butyl radical was produced from flash pyrolysis of azo-tert-butane and dissociated at 248 nm. Two distinct channels of approximately equal importance were identified: dissociation to H + 2-methylpropene, and CH(3) + dimethylcarbene. Neither the translational energy distributions that describe these two channels nor the product branching ratio are consistent with statistical dissociation on the ground state, and instead favor a mechanism taking place on excited state surfaces.     

Photodissociation dynamics of dichlorocarbene at 248 nm

The dynamics of the 248 nm photodissociation of the CCl(2) molecule have been investigated in a molecular beam experiment. The CCl(2) parent molecule was generated in a molecular beam by pyrolysis of CHCl(3), and both CCl(2) and the CCl photofragment were detected by laser fluorescence excitation. The 248 nm attenuation cross sections was estimated from the reduction of the CCl(2) signal as a function of the photolysis laser fluence. The internal state distribution of the CCl photofragment was derived from analysis of laser fluorescence excitation spectra in the A (2)Delta- X (2)Pi band system. The CCl(X (2)Pi, nu = 0) rotational state distribution was found to be bimodal, with maximum populations at N approximately 10 and 85, and was dependent upon the source backing pressure, and hence upon the internal state distribution of the CCl(2) precursor. The 248 nm photodissociation dynamics appears to involve two separate channels, namely nearly impulsive rotational energy release and predissociation with little rotational energy imparted to the CCl fragment.     

Photodissociation dynamics of ketene at 157.6 nm

Photodissociation dynamics of ketene at 157.6 nm has been investigated using the photofragment translational spectroscopic technique based on photoionization detection using vacuum-ultraviolet synchrotron radiation. Three dissociation channels have been observed: CH2+CO, CH+HCO, and HCCO+H. The product translational energy distributions and angular anisotropy parameters were measured for all three observed dissociation channels, and the relative branching ratios for different channels were also estimated. The experimental results show that the direct C-C bond cleavage (CH2+CO) is the dominant channel, while H migration and elimination channels are very minor. The results in this work show that direct dissociation on excited electronic state is much more significant than the indirect dissociation via the ground state in the ketene photodissociation at 157.6 nm.     

Photodissociation dynamics of 1,2-butadiene at 157 nm

Photodissociation dynamics of 1,2-butadiene at 157 nm has been investigated using a molecular beam apparatus based on photoionization using vacuum ultraviolet synchrotron radiation. Six dissociation pathways have been observed. The observed channels are C4H5+H, C4H4+H2, C3H3+CH3, C2H3+C2H3, C2H4+C2H2, and C4H4+H+H. Among all the dissociation channels, the C3H3+CH3 channel is found to be the dominant process. The product kinetic energy distributions of all dissociation channels have been determined from simulating the experimental time-of-flight spectra. Relative branching ratios for all observed dissociation channels were also estimated based on all detected products.     

Photodissociation dynamics of ClN3 at 193 nm

Photofragment translational spectroscopy was used to identify the primary and secondary reaction pathways in 193 nm photodissociation of chlorine azide (ClN(3)) under collision-free conditions. Both the molecular elimination (NCl+N(2)) and the radical bond rupture channel (Cl+N(3)) were investigated and compared with earlier results at 248 nm. The radical channel strongly dominates, just as at 248 nm. At 193 nm, the ClN(3) (C (1)A(")) state is excited, rather than the B (1)A(') state that is accessed at 248 nm, resulting in different photofragment angular distributions. The chlorine translational energy distribution probing the dynamics of the radical bond rupture channel shows three distinct peaks, with the two fastest peaks occurring at the same translational energies as the two peaks seen at 248 nm that were previously assigned to linear and "high energy" N(3). Hence, nearly all the additional photon energy relative to 248 nm appears as N(3) internal excitation rather than as translational energy, resulting in considerably more spontaneous dissociation of N(3) to N(2)+N.     

Photodissociation dynamics of benzyl alcohol at 193 nm

Photodissociation dynamics of benzyl alcohol, C(6)H(5)CH(2)OH and C(6)H(5)CD(2)OH, in a molecular beam was investigated at 193 nm using multimass ion imaging techniques. Four dissociation channels were observed, including OH elimination and H(2)O elimination from the ground electronic state, H atom elimination (from OH functional group), and CH(2)OH elimination from the triplet state. The dissociation rate on the ground state was found to be 7.7 × 10(6) s(-1). Comparison to the potential energy surfaces from ab initio calculations, dissociation rate, and branching ratio from Rice-Ramsperger-Kassel-Marcus calculations were made.     

Photodissociation dynamics of methyl nitrate at 193 nm: energy disposal in methoxy and nitrogen dioxide products

The photodissociation dynamics of methyl nitrate, CH(3)ONO(2), has been investigated at 193 nm by examining the products from the primary dissociation channel, namely CH(3)O and NO(2). The CH(3)O (X (2)E) photoproducts were probed by laser-induced fluorescence (LIF) on the A (2)A(1)-X (2)E transition under both nascent and jet-cooled conditions. The 3 and 3 bands originating from the vibrationless and C-O stretch (nu(3)) levels, respectively, were characterized to obtain the internal energy distribution of the CH(3)O products. Only a small fraction of the CH(3)O products (< or =10%) were produced with one quantum of C-O stretch excitation as determined from the relative intensities of the bands in combination with transition probabilities derived from dispersed fluorescence measurements and/or calculated Franck-Condon factors. The CH(3)O products also had minimal rotational excitation: those produced in the ground vibrational state had a rotational temperature of 238 +/- 7 K, corresponding to less than 1% of the available energy. Products with C-O stretch excitation were found to have a higher rotational temperature, but still a small fraction of the total energy. Combining the CH(3)O internal energy findings with previous photofragment translational energy measurements [X. Yang, P. Felder and J. R. Huber, J. Phys. Chem., 1993, 97, 10903] indicates that most of the available energy is deposited in the NO(2) fragment. This is verified through dispersed fluorescence measurements which show that the NO(2) fragment is produced electronically excited with internal energies extending to the NO + O dissociation limit. Ab initio calculations confirm that the dominant initial excitation is strongly localized on the NO(2) moiety. The calculations are also used to reveal the forces that give rise to internal excitation of the CH(3)O fragment upon electronic excitation.     

Photodissociation dynamics of ClOOCl at 248.4 and 308.4 nm

The dynamics of ClOOCl photodissociation at 248.4 and 308.4 nm was studied with photofragment translational spectroscopy. At 248.4 nm photoexcitation, the observed products are Cl, O(2), ClO and O. Product translational energy distributions P(E) and anisotropy parameters β were deduced from the measured time-of-flight spectra of the Cl, O(2), and ClO photoproducts. The photodissociation mechanisms have been discussed and compared with available theoretical results. Synchronous and fast sequential breaking of the two Cl-O bonds may both contribute to the dissociation. The relative product yields for [ClO]:?[Cl] was measured to be 0.15 ± 0.04:1. The relative amounts of [O]:[O(2)] products were estimated to be 0.12:1. The branching ratios among the Cl + O(2) + Cl:ClO + ClO:ClO + Cl + O product channels were estimated to be 0.82:0.08:0.10. At 308.4 nm excitation, time-of-flight spectra of the O(2) and ClO photoproducts were recorded while there was interference from Cl(2) impurity in detecting the Cl product. Nonetheless, the observed ClO yield relative to the O(2) yield at 308.4 nm is 1.5 times that at 248.4 nm. The branching ratio between the Cl + O(2) + Cl:ClO + ClO product channels was estimated to be 0.81:0.19 at 308.4 nm. This result suggests that the ClO product may contribute a noticeable yield in the photolysis of ClOOCl at the atmospherically important wavelengths above 300 nm.     

Photodissociation dynamics of the HCNN radical

The photodissociation dynamics of the diazomethyl (HCNN) radical have been studied using fast radical beam photofragment translational spectroscopy. A photofragment yield spectrum was obtained for the range of 25,510-40,820 cm(-1), and photodissociation was shown to occur for energies above 25,600 cm(-1). The only product channel observed was the formation of CH and N2. Fragment translational energy and angular distributions were obtained at several energies in the range covered by the photofragment yield spectrum. The fragment translational energy distributions showed at least two distinct features at energies up to 4.59 eV, and were not well fit by phase space theory at any of the excitation energies studied. A revised C-N bond dissociation energy and heat of formation for HCNN, D0(HC-NN)=1.139+/-0.019 eV and DeltafH0(HCNN)=5.010+/-0.023 eV, were determined.     

Photodissociation dynamics of ethyltoluene and p-fluoroethylbenzene at 193 and 248 nm

Photodissociation of jet-cooled o-, m-, and p-ethyltoluene and p-fluoroethylbenzene at both 193 and 248 nm was studied separately using vacuum ultraviolet photoionization/multimass ion imaging techniques. Dissociation occurs exclusively through alkyl chain C-C bond cleavage. The measured photofragment translational energy distributions at 193 nm decrease monotonically with increasing translational energy. The distributions indicate that dissociation occurs from the ground electronic state after internal conversion. However, the photofragment translational energy distributions from o-, m-, and p-ethyltoluene obtained at 248 nm contain a slow and a fast component; the ratios between these components are 1:4, 1:1.3, and 1:6, respectively. On the other hand, only the slow component was observed from p-fluoroethylbenzene at 248 nm. The fast components are attributed to the dissociation from the triplet state after intersystem crossing, and the slow components result from the dissociation in the ground electronic state. Comparison with the photodissociation of benzene and toluene and ab initio calculation has been made.     

Photodissociation dynamics of HN3 and DN3 at 157 nm

Photodissociation dynamics of HN 3 at 157.6 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been measured. From these distributions, three H-atom channels are observed. The vibrational structure in the fast-H channel could be assigned to a progression in the N 3 symmetric stretching mode (nu 100), together with a progression of the symmetric stretching mode with one quantum of bending motion (nu 110). The broad translational energy distribution of the slow-H channel is energetically consistent with the cyclic-N 3 formation process or a triple product dissociation channel. Photodissociation of DN 3 was also investigated using the same technique. Isotope effect on the product translational energy distribution has been observed, in which the slow H-atom is clearly more pronounced.     

Photodissociation dynamics of the 2-methylallyl radical

Time- and frequency-resolved photoionization of the hydrogen atom product from a jet-cooled electronically excited 2-methylallyl radical, C4H7, provides information on the dissociation dynamics. The measured dissociation rates and kinetic energy release of 2-methylallyl and its isotopologue CD3C3H4 combined with high level ab initio calculations suggests unimolecular dissociation with methylenecyclopropane and hydrogen as the major C-H bond fission channel with no evidence for nonstatistical behavior in dissociation. Other possible dissociation and isomerization pathways are discussed based on the results of the coupled-cluster ab initio calculations.     

Photodissociation of isobutene at 193 nm

The collisionless photodissociation dynamics of isobutene (i-C(4)H(8)) at 193 nm via photofragment translational spectroscopy are reported. Two major photodissociation channels were identified: H + C(4)H(7) and CH(3) + CH(3)CCH(2). Translational energy distributions indicate that both channels result from statistical decay on the ground state surface. Although the CH(3) loss channel lies 13 kcal mol(-1) higher in energy, the CH(3):H branching ratio was found to be 1.7 (5), in reasonable agreement with RRKM calculations.     

Photodissociation dynamics of allyl bromide at 234, 265, and 267 nm

The photodissociation dynamics of allyl bromide was investigated at 234, 265, and 267 nm. A two-dimensional photofragment ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization scheme was utilized to obtain the angular and translational energy distributions of the nascent Br* (2P1/2) and Br (2P3/2) atoms. The Br fragments show a bimodal translational energy distribution, while the Br* fragments reveal one translational energy distribution. The vertical excited energies and the mixed electronic character of excited states were calculated at ab initio configuration interaction method. It is presumed that the high kinetic energy bromine atoms are attributed to the predissociation from 1(pipi*) or 1(pisigma*) state to the repulsive 1(nsigma*) state, and to the direct dissociation from 3(nsigma*) and 3(pisigma*) states, while the low kinetic energy bromine atoms stem from internal conversion from the lowest 3(pipi*) state to 3(pisigma*) state.     

Photodissociation Dynamics of Benzene at 193 nm

Photodissociation of benzene at 193 nm has been investigated using the photofragment translational spectroscopy (PTS) technique. H atom elimination channel for benzene at 193 nm is from a one‐photon dissociation process, while H₂ and CH₃ elimination channels come from a two‐photon excitation process.     

Photodissociation Dynamics of OCS at 217 nm

The S(¹D₂)+CO(X¹Σ⁺) product channel from photodissociation of OCS at 217 nm has been measured using the DC slice velocity map imaging (VMI) technique in combination with resonance enhanced multiphoton ionization (REMPI). Two diflerent REMPI intermediate states (¹F₃ and ¹P₁) and several pump-probe laser polarization geometries are used to detect the angular momentum polarization of the photofragmented S(¹D₂). The molecular- frame polarization parameters, as well as the laboratory-frame anisotropy parameters, for individual rotational states of co-fragment CO, are determined using two diflerent full quantum theories. The measured total kinetic energy release spectrum from photodissociation of OCS indicates two dissociation channels, corresponding to the fast and slow recoiling velocities of S(¹D₂), respectively. The slow channel is concluded to originate from an initial photoexcitation to the A(¹A') state, followed by a non-adiabatic transition to the ground state. The fast channel is found to follow a coherent excitation to A(¹A') and B(¹A') states, where contributions of the two states are almost equal at 217 nm. The determined alignment and anisotropy parameters further indicate that the slow channel follows an incoherent excitation, while the fast channel follows a coherent excitation to A(¹A') and B(¹A') states with a phase di erence of π/2.     

Photodissociation of propargyl chloride at 193 nm

The photodissociation of propargyl chloride (C3H3Cl) has been studied at 193 nm. Ion imaging experiments with state-selective detection of the Cl atoms and single-photon ionization of the C3H3 radicals were performed, along with measurements of the Cl + C3H3 and HCl + C3H2 recoil kinetic energy distributions, using a scattering apparatus with electron bombardment ionization detection to resolve the competing Cl and HCl elimination channels. The experiments allow the determination of the Cl (2P3/2) and Cl (2P1/2) (hereafter Cl) branching fractions associated with the C-Cl bond fission, which are determined to be 0.5 +/- 0.1 for both channels. Although prior translational spectroscopy studies by others had concluded that the low velocity signal at the Cl+ mass was due to daughter fragments of the HCl elimination products, the present work shows that Cl atoms are produced with a bimodal recoil kinetic energy distribution. The major C-Cl bond fission channel, with a narrow recoil kinetic energy distribution peaking near 40 kcal/mol, produces both Cl and Cl, whereas the minor (5%) channel, partitioning much less energy to relative kinetic energy, produces only ground spin-orbit state Cl atoms. The maximum internal energy of the radicals produced in the low-recoil-kinetic-energy channel is consistent with this channel producing electronically excited propargyl radicals. Finally, in contrast to previous studies, the present work determines the HCl recoil kinetic energy distribution and identifies the possible contribution to this spectrum from propargyl radicals cracking to C3+ ions in the mass spectrometer.     

Photodissociation dynamics of CBr4 at 267 nm by means of ion velocity imaging

The photodissociation dynamics of CBr4 at 267 nm has been studied using time of flight (TOF) mass spectrometry and ion velocity imaging techniques. The photochemical products are detected with resonance enhanced multiphoton ionization (REMPI) as well as single-photon vacuum ultraviolet ionization at 118 nm. REMPI at 266.65 and 266.71 nm was used to detect the ground Br(2P32) and spin-orbit excited Br(2P12) atoms, respectively. The translational energy and angular distributions are consistent with direct dissociation from an excited triplet state and indirect dissociation from high vibrational levels on the singlet ground state surface. Br2+ ions are also observed in the TOF spectra with a focused 267 nm laser. The counter fragment, CBr2+, is observed when this photolysis laser is unfocused, and photons at 118 nm are used to ionize the radical products. The translational energy distributions of the CBr2+ and Br2+ products can be momentum matched, which indicates that molecular Br2 elimination is one of the primary dissociation channels.     

Photodissociation spectroscopy and dynamics of the CH2CFO radical

The photodissociation spectroscopy and dynamics resulting from excitation of the B (2)A(")<--X (2)A(") transition of CH(2)CFO have been examined using fast beam photofragment translational spectroscopy. The photofragment yield spectrum reveals vibrationally resolved structure between 29 870 and 38 800 cm(-1), extending approximately 6000 cm(-1) higher in energy than previously reported in a laser-induced fluorescence excitation spectrum. At all photon energies investigated, only the CH(2)F+CO and HCCO+HF fragment channels are observed. Both product channels yield photofragment translational energy distributions that are characteristic of a decay mechanism with a barrier to dissociation. Using the barrier impulsive model, it is shown that fragmentation to CH(2)F+CO products occurs on the ground state potential energy surface with the isomerization barrier between CH(2)CFO and CH(2)FCO governing the observed translational energy distributions.