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The photodissociation dynamics of the methyl perthiyl radical (CH3SS) have been investigated using fast-beam coincidence translational spectroscopy. Methyl perthiyl radicals were produced by photodetachment of the CH3SS- anion followed by photodissociation at 248 nm (5.0 eV) and 193 nm (6.4 eV). Photofragment mass distributions and translational energy distributions were measured at each dissociation wavelength. Experimental results show S atom loss as the dominant (96%) dissociation channel at 248 nm with a near parallel, anisotropic angular distribution and translational energy peaking near the maximal energy available to ground state CH3S and S fragments, indicating that the dissociation occurs along a repulsive excited state. At 193 nm, S atom loss remains the major fragmentation channel, although S2 loss becomes more competitive and constitutes 32% of the fragmentation. The translational energy distributions for both channels are very broad at this wavelength, suggesting the formation of the S2 and S atom products in several excited electronic states. [ABSTRACT FROM AUTHOR]

Photofragment translational spectroscopy was used to study the photodissociation of the methyl perthiyl radical CH3SS at 248 nm. The radical was produced by flash pyrolysis of dimethyl disulfide (CH3SSCH3). Two channels were observed: CH3 + S2 and CH2S + SH. Photofragment translational energy distributions indicate that CH3 + S2 results from C-S bond fission on the ground state surface. The CH2S + SH channel can proceed through isomerization to CH2SSH on the ground state surface but also may involve production of electronically excited CH2S. [ABSTRACT FROM AUTHOR]

Photofragment translational spectroscopy was used to study the photodissociation dynamics of the phenyl radical C6H5 at 248 and 193 nm. At 248 nm, the only dissociation products observed were from H atom loss, attributed primarily to H+o-C6H4 (ortho-benzyne). The observed translational energy distribution was consistent with statistical decay on the ground state surface. At 193 nm, dissociation to H+C6H4 and C4H3+C2H2 was observed. The C6H4 fragment can be either o-C6H4 or l-C6H4 resulting from decyclization of the phenyl ring. The C4H3+C2H2 products dominate over the two H loss channels. Attempts to reproduce the observed branching ratio by assuming ground state dynamics were unsuccessful. However, these calculations assumed that the C4H3 fragment was n-C4H3, and better agreement would be expected if the lower energy i-C4H3+C2H2 channel were included. [ABSTRACT FROM AUTHOR]

The photodissociation of propargyl radical,C3H3, and its perdeuterated isotopolog was investigated using photofragment translational spectroscopy. Propargyl radicals were produced by 193 nm photolysis of allene entrained in a molecular beam expansion and then photodissociated at 248 nm. Photofragment time-of-flight spectra were measured at a series of laboratory angles using electron impact ionization coupled to a mass spectrometer. Data for ion masses corresponding to C3H2+, C3H+, C3+, and the analogous deuterated species show that both H and H2 loss occur. The translational energy distributions for these processes have average values ≤ET≥=5.7 and 15.3 kcal/mol, respectively, and are consistent with dissociation on the ground state following internal conversion, with no exit barrier for H loss but a tight transition state for H2 loss. Our translational energy distribution for H atom loss is similar to that in a previous work on propargyl in which the H atom, rather than the heavy fragment, was detected. The branching ratio for H loss/H2 loss was determined to be 97.6/2.4±1.2, in good agreement with previous calculations. [ABSTRACT FROM AUTHOR]

The fragmentation dynamics of imidazole molecules following excitation at 193.3 nm and at many wavelengths in the range of 210<=λphot<=240 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Long wavelength excitation within this range results in population of the 1 1A″(1πσ*) excited state, but the 2 1A′←X 1A′(π*←π) transition becomes the dominant absorption once λphot<=220 nm. The measured energy disposals show parallels with those found in recent studies of the UV photolysis of pyrrole [Cronin et al., Phys Chem. Chem. Phys. 6, 5031 (2004)]. The total kinetic energy release (TKER) spectra display a “fast” feature, centred at TKER ∼9200 cm-1. The analysis of the structure evident in the fast feature reveals the selective population of specific in-plane stretching vibrational levels of the imidazolyl cofragment; these fragments are deduced to carry only modest amounts of rotational excitation. Comparison with calculated normal mode vibrational frequencies allows the assignment of the populated levels and a precise determination of the N–H bond strength in imidazole: D0=33 240±40 cm-1. The observed energy disposal can be rationalized using Franck-Condon arguments, assuming that the potential energy surface (PES) for the 1 1A″(1πσ*) state has a topology similar to that of the corresponding 1πσ* state of pyrrole. As in pyrrole, photoexcitation populates skeletal motions in the S1 state (in-plane motions in the present case) that are only weakly coupled to the N–H dissociation coordinate and thus map through into the corresponding product vibrations. A second, “slow” feature is increasingly evident in TKER spectra recorded at shorter λphot. This component, which exhibits no recoil anisotropy, is attributed to H atoms formed by the “statistical” decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at short λphot is rationalized by assuming two possible decay routes for imidazole molecules excited to the 2 1A′(1ππ*) state. One involves fast 2 1A′(1ππ*)[functional_relationship]1 1A″(1πσ*) radiationless transfer and subsequent fragmentation on the 1 1A′(1πσ*) PES, yielding fast H atoms (and imidazolyl cofragments)—reminiscent of behavior seen at longer excitation wavelengths where the 1 1A″(1πσ*) PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, most probably by successive 2 1A′[functional_relationship]1 1A″[functional_relationship]X 1A′ couplings, mediated by conical intersections between the relevant PESs and the subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding slow H atoms. [ABSTRACT FROM AUTHOR]

The fragmentation dynamics of gas phase phenol molecules following excitation at many wavelengths in the range 279.145>=λphot>=206.00 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Many of the total kinetic energy release (TKER) spectra so derived show structure, the analysis of which confirms the importance of O–H bond fission and reveals that the resulting phenoxyl cofragments are formed in a very limited subset of their available vibrational state density. Spectra recorded at λphot>=248 nm show a feature centered at TKER ∼6500 cm-1. These H atom fragments, which show no recoil anisotropy, are rationalized in terms of initial S1←S0 (π*←π) excitation, and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S0) state levels carrying sufficient O–H stretch vibrational energy to allow efficient transfer towards, and passage around, the conical intersection (CI) between the S0 and S2(1πσ*) potential energy surfaces (PESs) at larger RO–H, en route to ground state phenoxyl products. The observed phenoxyl product vibrations indicate that parent modes ν16a and ν11 can both promote nonadiabatic coupling in the vicinity of the S0/S2 CI. Spectra recorded at λphot≤=248 nm reveal a faster, anisotropic distribution of recoiling H atoms, centered at TKER ∼12 000 cm-1. These we attribute to H+phenoxyl products formed by direct coupling between the optically excited S1(1ππ*) and repulsive S2(1πσ*) PESs. Parent mode ν16b is identified as the dominant coupling mode at the S1/S2 CI, and the resulting phenoxyl radical cofragments display a long progression in ν18b, the C–O in-plane wagging mode. Analysis of all structured TKER spectra yields D0(H–OC6H5)=30 015±40 cm-1. The present findings serve to emphasize two points of wider relevance in contemporary organic photochemistry: (i) The importance of 1πσ* states in the fragmentation of gas phase heteroaromatic hydride molecules, even in cases where the 1πσ* state is optically dark. (ii) The probability of observing strikingly mode-specific product formation, even in “indirect” predissociations, if the fragmentation is driven by ultrafast nonadiabatic couplings via CIs between excited (and ground) state PESs. [ABSTRACT FROM AUTHOR]

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[sub 3]+C[sub 3]H[sub 3] and C[sub 4]H[sub 5]+H. The C[sub 3]H[sub 3] product can be identified as the propargyl radical through measurement of its photoionization efficiency curve, whereas the C[sub 4]H[sub 5] product cannot be identified definitively. The translational energy P(E[sub T]) distributions suggest that both channels result from internal conversion to the ground electronic state followed by dissociation. The P(E[sub T]) distribution for the C[sub 4]H[sub 5] product is sharply truncated below 7 kcal/mol, indicating spontaneous decomposition of the slowest C[sub 4]H[sub 5] product. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of CH3I at 225 nm is studied on our high resolution mini-TOF photofragment translational spectrometer. The photofragment translational spectra of the I* and the I channels via parallel (∥) and perpendicular (⊥) transitions, i.e., of the four pathways 3Q0, 3Q0 ← 1Q1, 1Q1, and 1Q1 ← 3Q0, are obtained with both the symmetric stretch (ν1) and the umbrella (ν2) vibrational modes of the CH3 fragments partially resolved. The strong excitation of the symmetric stretch mode (ν1) is revealed in both the I and the I* channels. The branching fractions for the four pathways (0.09 for 3Q0, 0.03 for 3Q0 ← 1Q1, 0.34 for 1Q1, and 0.54 for 1Q1 ← 3Q0) show that the parallel transition (3Q0 ← X) is the major and the I channel is dominant in the photodissociation of CH3I at 225 nm. The curve-crossing probability is found to be 0.86 for 1Q1 ← 3Q0 but 0.08 for 3Q0 ← 1Q1. [ABSTRACT FROM AUTHOR]

The ultraviolet photolysis of jet-cooled mass-selected ketenyl radicals has been investigated using the technique of fast radical beam photofragment translational spectroscopy. The C2Π(2A″)–X2A″ photofragment yield cross section spans 33 400–48 000 cm-1 and exhibits resolved resonances and broad continua. Dissociation produces both ground and excited state CH radicals in association with ground state CO fragments; there is no evidence for H atom elimination. Analysis of the photofragment kinetic energy release spectra yield a value for the C–C bond dissociation energy and heat of formation of HCCO: D0(HC–CO)=3.14±0.03 eV (72.4±0.7 kcal/mol) and ΔHf,00(HCCO)=1.82±0.03 eV (42.0±0.7 kcal/mol). © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of IBr has been studied at 304 nm by state-selective photofragment translational spectroscopy. Velocity distributions, anisotropy parameters, and relative quantum yields are obtained for the ground I(2P3/2) and spin–orbit excited state I*(2P1/2) iodine atoms, which are produced from photodissociation of IBr at this wavelength. Two sharp velocity distributions observed for the I channel suggest the two dissociation pathways that correlate with ground-state iodine formation. Based on the expected translational energy release and the energy separation between the peaks, the two distributions have been assigned to dissociation of IBr to form I(2P3/2)+Br(2P3/2) and I(2P3/2)+Br*(2P1/2) with the former channel appearing at higher translational energy. The I* distribution shows one strong peak indicating that there is one dominant channel for formation of I* atoms at this wavelength which has been assigned to dissociation of IBr to form I*(2P1/2)+Br(2P3/2) with a quantum yield of 0.1. The I* signal formed from the I*(2P1/2)+Br*(2P1/2) dissociation channel is observed very weakly.The observed anisotropy parameter indicates that the I+Br* product (β=-0.7) is formed mainly from the perpendicular 1Π1(2341)←X transition while the I*+Br channel (β=1.8) is formed predominantly from the parallel 3Π0+(2341)←X transition followed by curve crossing to the 3Σ0+-(2422) state. The recoil energy dependence of the anisotropy parameter in the I atom produced in the I+Br channel shows a positive β value above maximum of the peak recoil energy and a negative value below the peak maximum of the recoil energy distribution. These results are interpreted in terms of the presence of more than one path for the formation of I+Br photoproduct with opposite polarization for their absorbing transitions, most likely the 3Π0+(2341)←X and the 3Π1(2341)←X transitions. The possible excited state... [ABSTRACT FROM AUTHOR]

The photodissociation of ozone has been studied at 193 nm using high resolution photofragment translational spectroscopy. The results show six distinct peaks in the time-of-flight spectra for the O2 product and its momentum-matched O atom counterpart. The translational energy distributions determined from the time-of-flight spectra reveal the production of a range of electronic states of the photofragments. The product electronic states were identified based on the translational energy distributions, with the aid of state-resolved imaging experiments by Houston and co-workers. The results reveal the production of a substantial yield of highly excited triplet states of O2, recently suggested to play an important role in the stratospheric ozone balance. In addition, peaks corresponding to O2(a 1Δg) and O2(b 1Σg+) were observed, the latter confirming a previous report [A. A. Turnipseed et al., J. Chem. Phys. 95, 3244 (1991)]. Evidence was seen for a small contribution from the triple dissociation O3→3O(3P), and insight into the dissociation dynamics for this process was inferred from the translational energy distributions. Branching fractions and angular distributions were measured for all channels. The latter were found in general to yield negative β parameters, in contrast to what is seen at longer wavelengths. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The technique of H Rydberg atom photofragment translational spectroscopy has been applied to a high resolution study of the primary photochemistry of methanethiol (CH3SH) following excitation at a wide range of wavelengths in the near ultraviolet. In accord with previous studies of this molecule, excitation within its first (1 1A‘-X 1A’) absorption continuum is shown to result in S–H bond fission. Spectral analysis yields a refined value for the bond dissociation energy: D00(CH3S–H)=30 250±100 cm-1. The resulting CH3S(X) fragments are deduced to carry only modest vibrational excitation, distributed specifically in the ν3 (C–S stretching) mode and in one other mode having a wave number of ∼1040 cm-1. We associate this latter mode with bending of the CH3 moiety in the plane containing the C and S nuclei and the lobe of the unpaired electron which was originally involved in the S–H bond. Decreasing the excitation wavelength (while remaining within the first absorption continuum) results in an increase in both the vibrational and rotational excitation of the CH3S(X) fragments, but a decrease in the relative yield of the upper (2E1/2) spin–orbit component. Excitation at still shorter excitation wavelengths accesses the second (2 1A‘-X 1A’) absorption band of CH3SH. The CH3S fragments resulting from S–H bond fission at these excitation wavelengths are observed to carry very much higher levels of vibrational excitation in the above two modes. The observation of H atoms attributable to secondary photolysis of SH(X) fragments indicates increased competition from the alternative C–S bond fission channel at these shorter excitation wavelengths. Additional peaks in the H atom time-of-flight spectrum, most clearly evident following excitation at wavelengths in the range 213–220 nm, are interpretable in terms of secondary photolysis of the primary CH3S(X) fragments yielding... [ABSTRACT FROM AUTHOR]

The photodissociation of CO2 at 157 nm was studied by the photofragment-translational spectroscopy technique. Product time-of-flight spectra were recorded and center-of-mass translational energy distributions were determined. Two electronic channels were observed—one forming O(1D) and the other O(3P). With previously determined anisotropy parameters of β=2 for the O(3P) channel and β=0 for the O(1D) channel, an electronic branching ratio of 6%±2%O(3P) was obtained, consistent with previous results. The translational energy distribution for the CO(v)+O(3P) channel was very broad (over 30 kcal/mol) and appeared to peak near CO(v=0). The value of β=2 for the O(3P) channel was confirmed by comparing Doppler profiles, derived from our measured translational energy distribution, with previously measured Doppler profiles. This suggests that the O(3P) channel arises from a direct transition to an excited triplet state. The O(1D) channel had a structured time-of-flight which related to rovibrational distributions of the CO product. The influence of the excitation of the CO2(ν2) bending mode was investigated and shown to have a small but not negligible contribution. Based upon a comparison of our data with a previous vacuum-ultraviolet (VUV) laser induced fluorescence study, we obtain as our best estimate of the vibrational branching ratio, CO(v=0)/CO(v=1)=1.9, for the CO(v)+O(1D) channel. [ABSTRACT FROM AUTHOR]

Photodissociation of the phenoxy radical (C6H5O) is investigated using fast beam photofragment translational spectroscopy. Phenoxy radicals are generated through photodetachment of phenoxide anions (C6H5O-) at 532 nm. Following photoexcitation of the radicals at 225 nm (5.51 eV), 290 nm (4.27 eV), or 533 nm (2.33 eV), photofragments are collected in coincidence to determine their masses, translational energy, and scattering angle for each dissociation event. Two-body dissociation yields exclusively CO + C5H5, and three-body dissociation to CO + C2H2 + C3H3 and CO + C5H4 + H is also seen at the two higher energies. The translational energy distributions for two-body dissociation suggest that dissociation occurs via internal conversion to the ground electronic state followed by statistical dissociation. The absorption of an additional 532 nm photon in the photodetachment region provides some C6H5O radicals with an additional 2.33 eV of energy, leading to much of the two-body dissociation observed at 533 nm and the three-body dissociation at the two higher excitation energies.

Femtosecond time-resolved phototfragment translational spectroscopy was used for examining the photodissociation dynamics of acetone S2 (n, 3s) Rydberg state excited at 195nm. Average total kinetic energy releases of 31 kJ/mol for the primary dissociation and 50 kJ/mol for the secondary dissociation at 120ps were observed by the three-body kinematic analyses of the methyl kinetic energy distributions (KEDs).

Photophysics and photochemistry of conjugated polyenes like 1,3-butadiene are studied, along with its electronic spectrum as well as short term dynamics, using different experimental and theoretical methods. The results of photodissociation dynamics of 1,3-butadiene at 193nm show a complex photochemistry, and the observations of ground state dynamics is found to be in line with theoretical studies as well as spectroscopic studies.