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We characterize the 157 nm photodissociation of dichloromethane (CH2Cl2) using photofragment translational spectroscopy, detecting the products with photoionization mass spectrometry. The major photodissociation products are Cl + CH2Cl and HCl + CHCl. Comparing the integrated ion signal at m/e = 35 (Cl+) and m/e = 49 (CH2Cl+) gives the partial photoionization cross section of CH2Cl to parent ion to be σ = 26.6 Mb at 13.7 eV. This value is a quantitative anchor point of the energy-dependent photoionization cross section. In recent work we used this result to quantify the product branching to HCO + CH2Cl in the photodissociation of 2-chloroacetaldehyde.

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.

The 304 nm photodissociation of the C-H symmetric stretch excited CH3I [v 1 = 1, v 2 = 0] (v 1 denotes the C-H symmetric stretch mode, and v 2 denotes the umbrella mode) is studied with our simple photofragment translational spectrometer. An IR laser is used to excite the ground state CH3I [0, 0] to the C-H symmetric stretch excited CH3I [1, 0]. With IR laser OFF and ON, the fractions of photofragments CH3 (ν 1, ν 2) from the 304 nm photodissociation of CH3I [1, 0] have been determined through the photofragment translational spectra (PTS) from measuring I and I* and also through the PTS from measuring CH3 (0, 0) (1, 0) (0, 1) and (1, 1). The experimental results show that the C-H symmetric stretch vibration (v 1 = 1) in parent molecules is about 66% retained in the photofragments in the I channel, but only 24% in the I* channel. The populations of photofragments CH3 (0, 2) and (0, 3) are higher than CH3 (0, 0) and (0, 1), showing strong inverted population both in I and I* channels.

The photodissociation dynamics of the i-methylvinoxy (CH3COCH2) radical have been studied by means of fast beam coincidence translational spectroscopy. The radical was produced by photodetachment of the i-methylvinoxide anion at 700 nm, followed by dissociation at 225 nm (5.51 eV), 248 nm (5.00 eV), and 308 nm (4.03 eV). At all three dissociation energies, the major products were found to be CH3 + CH2CO, with a small amount of CO + C2H5 produced at the higher dissociation energies. Photofragment mass distributions and translational energy distributions were recorded for each wavelength. Comparison of the mass distributions with dissociation of fully deuterated i-methylvinoxy aided the assignment of the observed channels. Electronic structure calculations were performed to determine the relative energies of minima and transition states involved in the dissociation and to aid interpretation of the experimental results. The proposed dissociation mechanism involves internal conversion from the initially excit...

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.

The photodissociation dynamics of ICl has been studied near 304 and 280 nm on a simple miniature time of flight (mini-TOF) photofragment translational spectrometer with a short pulse of a weak acceleration field. An intense hot band effect was observed. Many small peaks were resolved in each photofragment translational spectrum (PTS). Based on simulations, the principal peaks were assigned not only to the different photodissociation channels (1) I + Cl, (2) I + Cl*, (3) I* + Cl, or (4) I* + Cl*, but also to the different chlorine isotopes (35Cl and 37Cl). Moreover, some extra peaks showed the existence of an intense hot band effect from vibrationally excited ICl molecules, though only a few percent of ICl molecules remained in the vibrationally excited states in our supersonic molecular beam. Based on the spectra near 304 nm, the quantum yield Φ of each channel, the curve crossing, and the branching fraction σ from each transition state were determined.

The photodissociation dynamics of CH 3 I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH 3 I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH 3 fragment. There are some extra peaks showing the existence of the hot-band states of CH 3 I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH 3 fragment is determined. The fraction σ of photofragments produced from the hot-band CH 3 I varies from 0.07 at 277.38 nm to 0.40 at 304.02 nm in the I * channel and from 0.05 at 277.87 nm to 0.16 at 304.67 nm in the I channel. E int /E avl of photofragments from ground-state CH 3 I remains at about 0.03 in the I * channel for all four wavelengths, but E int /E avl decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel. From the ground-state CH 3 I, the quantum yield Φ(I * ) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability P cc from the hot-band CH 3 I is lower than that from the ground-state CH 3 I. The potential energy at the curve-crossing point is determined to be 32 740 cm ―1 .

The photodissociation of the clusters (HI) m (H2O) n in the supersonic molecular beam has been investigated at 281.73 nm and 280.99 nm by photofragment translational spectroscopy (PTS). The translational energy distributions of the photofragment I*(2 P 1/2) show that the photodissociated species is mainly HI(H2O) n (n ≥ 4) clusters. In addition, the structures and the relative stability of (HI) m , (H2O) n and (HI) m (H2O) n (m = 1 ∼ 4, n = 1 ∼ 6) clusters have been obtained by ab initio calculation (the Density Functional Theory). The conclusion of the theoretical analysis is agreeable with the experimental results. So these results indicate that PTS combining theoretical calculation can be used as a new, indirect way to judge the composition of the clusters which are easy to be dissociated and difficult to be detected by mass spectroscopy.

The fragmentation dynamics of indole molecules following excitation at 193.3 nm, and at a number of different wavelengths in the range 240or = lambda(phot)or = 286 nm, have been investigated by H Rydberg atom photofragment translational spectroscopy. The longer wavelength measurements have been complemented by measurements of excitation spectra for forming parent and fragment ions by two (or more) photon ionisation processes. Analysis identifies at least three distinct contributions to the observed H atom yield, two of which are attributable to dissociation of indole following radiationless transfer from the 1pi pi* excited states (traditionally labelled 1L(b) and 1L(a)) prepared by UV single photon absorption. The structured channel evident in total kinetic energy release (TKER) spectra recorded at lambda(phot)or = 263 nm is rationalised in terms of N-H bond fission following initial pi*-- pi excitation and subsequent coupling to the 1pi sigma* potential energy surface via a conical intersection between the respective surfaces--thereby validating recent theoretical predictions regarding the importance of this process (Sobolewski et al., Phys. Chem. Chem. Phys., 2002, 4, 1093). Analysis provides an upper limit for the N-H bond strength in indole: D0(H-indolyl)or = 31,900 cm(-1). Unimolecular decay of highly vibrationally excited ground state molecules formed by internal conversion from the initially prepared 1pi pi* states is a source of (slow) H atoms but their contribution to the TKER spectra measured in the present work is dwarfed by that from H atoms generated by one or more (unintended but unavoidable) multiphoton processes.

The photodissociation dynamics of 2,5-dimethylpyrrole (2,5-DMP) has been investigated following excitation at 193.3 nm and at many near ultraviolet (UV) wavelengths in the range 244lambda(phot)282 nm using H Rydberg atom photofragment translational spectroscopy (PTS). Complementary UV absorption and, at the longest excitation wavelengths, one photon resonant multiphoton ionisation spectra of 2,5-DMP are reported also; analysis of the latter highlights the role of methyl torsional motions in promoting the parent absorption. The deduced fragmentation dynamics show parallels with that reported recently (B. Cronin, M. G. D. Nix, R. H. Qadiri and M. N. R. Ashfold, Phys. Chem. Chem. Phys., 2004, 6, 5031) for the bare pyrrole molecule. Excitation at the longer wavelengths leads to (vibronically induced) population of the 1(1)A(2)(pisigma*) excited state of 2,5-DMP, but once lambda(phot) decreases to approximately 250 nm stronger, dipole allowed transitions start to become apparent in the parent absorption. All total kinetic energy release (TKER) spectra of the H + 2,5-dimethylpyrrolyl (2,5-DMPyl) fragments measured at lambda(phot)or=244 nm show a structured fast component, many of which are dominated by a peak with TKER approximately 5100 cm(-1); analysis of this structure reveals lambda(phot) dependent population of selected vibrational levels of 2,5-DMPyl, and enables determination of the N-H bond strength in 2,5-DMP: D(0) = 30 530 +/- 100 cm(-1). Two classes of behaviour are proposed to account for details of the observed energy partitioning. Both assume that N-H bond fission involves passage over (or tunnelling through) a small exit channel barrier on the 1(1)A(2) potential energy surface, but differ according to the vibrational energy content of the photo-prepared molecules. Specific parent out-of-plane skeletal modes that promote the 1(1)A(2)-X(1)A(1) absorption appear to evolve adiabatically into the corresponding vibrations of the 2,5-DMPyl products. Methyl torsions can also promote the 1(1)A(2)-- X(1)A(1) absorption in 2,5-DMP, and provide a means of populating a much higher density of excited vibrational levels than in pyrrole. Such excited levels are deduced to dissociate by redistributing the minimum amount of internal energy necessary to overcome the exit channel barrier in the N-H dissociation coordinate. Coupling with the ground state surface via a conical intersection at extended N-H bond lengths is proposed as a further mechanism for modest translational --vibrational energy transfer within the separating products. The parent absorption cross-section increases considerably at wavelengths approximately 250 nm, and PTS spectra recorded at lambda(phot)or = 254 nm display a second, unstructured, peak at lower TKER. As in pyrrole, this slower component is attributed to H atoms from the unimolecular decay of highly vibrationally excited ground state molecules formed via radiationless decay from photo-excited states lying above the 1(1)A(2) state.

As a continuation of the preceding paper in this issue (J. Phys. Chem. A 2005, 109, 6805), we studied photodissociation dynamics of the acetone S2 (n, 3s) Rydberg state excited at 195 nm using femtosecond time-resolved photofragment translational spectroscopy. The technique, which is implemented by the combination of fs pump-probe ionization spectroscopy and kinetic energy resolved time-of-flight mass spectrometry (KETOF), measured temporal evolutions of the product kinetic energy distributions (KEDs) with a time resolution limited only by the laser pulse widths. Two methyl product KED components were resolved and assigned to the primary and secondary methyl products on the basis of their temporal behaviors. The results support the mechanism in which the primary dissociation occurs on the acetone S1 surface and provide complementary dynamical information to that discussed in the preceding paper.

The dissociation dynamics of allene, propyne, and propyne-d3 at 193 nm were investigated with photofragment translational spectroscopy. Products were either photoionized using tunable VUV synchrotron radiation or ionized with electron impact. Product time-of-flight data were obtained to determine centre-of-mass translational energy (P(ET)) distributions, and photoionization efficiency (PIE) curves were measured for the hydrocarbon products. The two major product channels evident from this study are atomic and molecular hydrogen loss, with a H:H2 branching ratio of 90:10, regardless of precursor. The P(ET) distribution for each channel is also largely independent of precursor. Both channels appear to occur following internal conversion to the ground electronic state. The propyne-d3 results show that there is extensive isotopic scrambling prior to H(D) atom loss, and that the H:D product ratio is approximately unity. The PIE curves for H(D) atom loss from allene, propyne, and propyne-d3 indicate that the do...

This study examines the 248-nm photodissociation of nitric acid (HNO 3 ) and characterizes the translational energy distribution of the nascent photofragments. Photofragment translational spectroscopy with VUV photoionization detection evidenced one product channel, cleavage of HNO 3 to form OH + NO 2 , and established an upper limit on the contribution from the O + HONO formation channel of 3%. These data contribute an independent measurement to a literature debate regarding the branching between these two channels.

The photodissociation dynamics of ICH2Cl → CH2Cl + I*/I at 304 and 277 nm has been investigated with our mini-TOF photofragment translational spectrometer with a weak acceleration field of1 V cm(-1). Many peaks are resolved or partially resolved in the TOF spectra and the photofragment translational spectra (PTS) of both the I*((2)P1/2) channel and the I((2)P3/2) channel. These resolved peaks are assigned to the C-Cl stretch vibrational states of the CH2Cl fragment. The rotational energy ER of the CH2Cl fragment is highly excited due to its asymmetric structure. The value of ER/ET is measured to be about 0.71. In the I* channel, the partitioning of the available energy Eavl into the translational energy ET, the rotational energy ER, and the vibrational energy EV for each resolved vibrational state has been calculated.

The photodissociation of ethyl iodide at 279.71, 281.73, 304.02 and 304.67 nm has been studied on our new mini-photofragment translational spectrometer with a total flight path of only 5 cm. Some vibrational peaks are firstly resolved in the TOF spectra of I*(2P1/2) and I(2P3/2) channels. These vibrational peaks are assigned to the excitation states (v2 = 0, 1, 2, …) of the umbrella mode (v2, 540 cm−1) of the photofragment C2H5, and the distribution of the vibrational states is obtained. The dissociation energy has been determined to be D0(C-I)=2.314 ± 0.03 eV. The energy partitioning of the available energy (Eavl=ET+Eint=ET+EV,R) calculated from our experimental data Ēint / Eavl = 22.1% at 281.73 nm, 22.4% at 304.02 nm for the I* channel, and Ēint / Eavl = 25.2% at 279.71 nm, 25.9% at 304.67 nm for the I channel, seem to be more reliable.

Photodissociation of propylene sulfide at 193 nm has been studied using photofragment translational spectroscopy. Five primary dissociation channels were observed: S(1D) + C3H6, S(3P) + C3H6, HS +...

The photodissociation dynamics of 1,3-butadiene at 193 nm have been investigated with photofragment translational spectroscopy coupled with product photoionization using tunable VUV synchrotron radiation. Five product channels are evident from this study: C(4)H(5) + H, C(3)H(3) + CH(3), C(2)H(3) + C(2)H(3), C(4)H(4) + H(2), and C(2)H(4) + C(2)H(2). The translational energy (P(E(T))) distributions suggest that these channels result from internal conversion to the ground electronic state followed by dissociation. To investigate the dissociation dynamics in more detail, further studies were carried out using 1,3-butadiene-1,1,4,4-d(4). Branching ratios were determined for the channels listed above, as well as relative branching ratios for the isotopomeric species produced from 1,3-butadiene-1,1,4,4-d(4) dissociation. C(3)H(3) + CH(3) is found to be the dominant channel, followed by C(4)H(5) + H and C(2)H(4) + C(2)H(2), for which the yields are approximately equal. The dominance of the C(3)H(3) + CH(3) channel shows that isomerization to 1,2-butadiene followed by dissociation is facile.

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: CH3+C3H3 and C4H5+H. The C3H3 product can be identified as the propargyl radical through measurement of its photoionization efficiency curve, whereas the C4H5 product cannot be identified definitively. The translational energy P(ET) distributions suggest that both channels result from internal conversion to the ground electronic state followed by dissociation. The P(ET) distribution for the C4H5 product is sharply truncated below 7 kcal/mol, indicating spontaneous decomposition of the slowest C4H5 product.

Photofragment translational spectroscopy of CBrClF2 at 157.6 nm was carried out using a crossed laser-molecular beams technique. Detected species are Br, Cl, and CF2. From the analyses of time-of-flight (TOF) spectra for these three species, the molecules were found to dissociate competitively through CBrF2+Cl, CClF2+Br, and CF2+Br+Cl channels with the branching ratio of 1.0:1.6−0.22+0.13:0.87−0.18+0.13. All of the CClF2 and CBrF2 radicals were found to dissociate spontaneously to produce Cl or Br+CF2, respectively. The angular distributions of these secondary products were found to be anisotropic. These fast secondary reactions are discussed on the basis of the calculated dissociation rates and rotational frequencies.

The ultraviolet photodissociation of 1-bromo-3-fluorobenzene and 1-bromo-4-fluorobenzene at 266 nm has been performed on a universal crossed molecular beams machine combined with the photofragment translational spectroscopy (PTS) detection technique. The time-of-flight (TOF) spectra of Br and FC6H4 photofragments have been measured. The observed translational energy distributions P(E-t) of photofragments revealed that about 46.8 +/- 3.3% and 41.7 +/- 3.0% of the available energy are partitioned into translational energy and the anisotropy parameter beta is 0.7 +/- 0.1 and -0.4 +/- 0.1 for 1-bromo-3-fluorobenzene and 1-bromo-4-fluorobenzene, in accord with that of bromobenzene (-0.7), indicating significant change caused by the fluorine atom substitution. To better interpret the experimental results, ab initio calculations have been performed and the calculation results are in good agreement with the experimental results. A plausible photodissociation mechanism is suggested, and the substitution effect of the fluorine atom is discussed.

Photodissociation of ethylene sulfide at 193 nm has been studied using photofragment translational spectroscopy and ab initio theoretical calculations. Tunable synchrotron radiation was used as a universal but selective probe of the reaction products to reveal new aspects of the photodissociation dynamics. The channel giving S + C2H4 was found to be dominated by production of ground-state sulfur atoms (S(3P):S(1D) = 1.44:1), mostly through a spin-forbidden process. The results also suggest the presence of a channel giving S(3P) in conjunction with triplet ethylene C2H4 (3B(1u)) and allow insight into the energy of the latter species near its equilibrium geometry, in which the two methylene groups occupy perpendicular planes. In addition, a channel leading to the production of H2S with C2H2 also has been observed. Our experimental results are supported and elaborated by theoretical calculations.

We have investigated the photodissociation of CS2 at 193 nm using the technique of photofragment translational spectroscopy. The utilization of vacuum ultraviolet synchrotron radiation for product photoionization has permitted a determination of the vibrationally resolved translational energy distribution for the CS+S(1D) channel and the translational energy distribution for the CS+S(3P) channel. A simulation of the coincident S(1D) translational energy distribution is consistent with a CS vibrational distribution of 0.02:0.17:0.19:0.46:0.15 in ν=0:1:2:3:4 and an average rotational energy of ∼1–3 kcal/mol. We find that the S(3P)/S(1D) branching ratio is 3.0±0.2, in good agreement with previous reports. Both asymptotic channels exhibit similar velocity dependent anisotropy parameters that decrease with decreasing translational energy release. The results extend earlier reports and provide further insight into the dissociation dynamics at 193 nm.

We have investigated the photodissociation of acrylonitrile (H2CCHCN) at 193 nm using the technique of photofragment translational spectroscopy. The experiments were performed at the Chemical Dynamics Beamline at the Advanced Light Source and used tunable vacuum ultraviolet synchrotron radiation for product photoionization. We have identified four primary dissociation channels including atomic and molecular hydrogen elimination, HCN elimination, and CN elimination. There is significant evidence that all of the dissociation channels occur on the ground electronic surface following internal conversion from the initially optically prepared state. The product translational energy distributions reflect near statistical simple bond rupture for the radical dissociation channels, while substantial recombination barriers mediate the translational energy release for the two molecular elimination channels. Photoionization onsets have provided additional insight into the chemical identities of the products and their ...

Photofragment translational spectroscopy (PTS) is a general and versatile technique for exploring the dynamics of gas-phase molecular photodissociation processes, through measurement of the speeds and directions of the resulting atomic and molecular products. This article describes recent enhancements in the achievable kinetic energy resolution and then focuses on applications employing one particular high-resolution variant of the technique, wherein H(D) atom photofragments are monitored by Rydberg tagging methods. Systems described range from small hydrides such as H2O, NH3, and C2H2 to larger heteroaromatic molecules such as pyrrole and phenol. In the former cases, the achievable resolution allows observation of individual rovibrational quantum states of the radical partner, providing detailed insights into the forces acting during the dissociation process. This allows intimate contact with complementary ab initio and wave packet modeling studies, and accurate determination of bond strengths and related thermodynamic quantities. The density of possible product states increases rapidly for larger radical cofragments. PTS studies of several heteroaromatics point to the generality of prompt dissociation from 1πσ* excited states, which manifests itself in remarkably mode specific fragmentation dynamics. The article concludes with a summary of complementary photofragmentation studies of hydride radicals and an anticipation of future activities in this field. Keywords: molecular photodissociation; photofragmentation dynamics; photofragment translational spectroscopy; bond dissociation energies; product's kinetic and internal energies; hydride molecules; H atoms; Rydberg atoms

The photofragmentation of C6H5] at 266 nm is investigated on the nuiversal crossed molecular beam ma-chine, and the translational spectroscopy as well as the angular distribution of I atom is measured. The results reveal that under the laser intensity of 108 W/cm2 the single-photon dissociation competes with multi-photon processes. In singlephoton dissociation the anisotropy parameter β is 0.4 and the average translational energy is only 1.04 kcal/mol, which indicates that this process is a slow predissociation. In two-photon photofragmentation the average translational energy is 51.64 kcal/mol, which accounts for about 35% of the available energy. Another photofragmentation channel is even more faster, whose peak in time-of-flight spectra corresponds to four or five photon absorptions. The branching ratio of these three channels is determined to be about 3:3:4.

Photodissociation of p -bromotoluene at 266 nm has been investigated on the universal crossed molecular beam machine, and translational energy distribution P ( E t ) as well as the anisotropy parameter β have been obtained. Photofragment translational energy distribution P ( E t ) reveals that ∼38.5% of the available energy is partitioned into translational energy. The anisotropy parameter β is determined to be −0.4±0.2. From P ( E t ) and β , we deduce that p -bromotoluene photodissociation is a fast process and the perpendicular transition plays a central role at this wavelength. The possible mechanism has been discussed and comparison of p -bromotoluene with bromobenzene, o -bromotoluene has also been made.

State-selective photofragment translational spectroscopy is used to probe the detailed nature of the photodissociation dynamics of iodobenzene at 304 nm. Simultaneous determination of the recoil speed, the spatial anisotropy, and the final state of the iodine fragment reveals that three dissociation channels with different dynamical characteristics compete in the photodissociation of iodobenzene at 304 nm. Based on the observed energy partitioning between the internal and translational modes and the dissociation time t d determined from the spatial anisotropy by using a rotational depolarization model, the three dissociation channels are assigned as follows. Two fast dissociation channels, which result in formation of I * ( 2 P 1/2 ) (t d = 0.4 ps, quantum yield Φ = 0.005 ± 0.002) and high velocity 1( 2 P 3/2 ) (t d = 0.3 ps, Φ=0.70 ± 0.04), are due to a parallel transition to the repulsive 3 Q 0 (n,σ * ) state in the C-I bond, followed by dissociation along the same state or curve crossing to the 1 Q 1 state respectively. A slow dissociation channel (t d = 0.5 - 1.4 ps, Φ = 0.30 ± 0.04) which produces low velocity I( 2 P 3/2 ) is due to a parallel transition to the triplet π,π * state(s) in the phenyl ring that is predissociated by the repulsive ν,σ * state(s). The dissociation times determined in the present work are in excellent agreement with those of the recent femtosecond real-time measurements by Cheng et al. at 278 nm (P. Y. Cheng, D. Zhong and A. H. Zewail, Chem. Phys. Lett. 237 (1995) 399).

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).

The photodissociation of jet-cooled C2H2, C2HD, and C2D2 molecules following excitation within their respective predissociated absorption band systems has been investigated using the technique of H(D) Rydberg-atom photofragment translational spectroscopy. Analyses of the rotational structure apparent in the resulting total kinetic-energy release spectra yields precise values for the DCC–H, HCC–D, and DCC–D bond dissociation energies: (DCC–H) = 46144 ± 50 cm−1, (HCC–D) = 46687 ± 50 cm−1, and (DCC–D) = 46754 ± 50 cm−1, respectively. The average internal excitation of bending vibration in the C2H(C2D) fragments formed from these photolyses of all three parent isotopomers is shown to scale linearly with the excess energy over and above that required for bond fission by ca. 540 cm−1.

The predissociation dynamics of CS2 in the (B- 1(2)) state have been investigated by means of high resolution photofragment translational spectroscopy at 193 nm. The photofragment translational energy distributions derived from the time-of-night (TOF) distributions of the CS fragment are qualitatively similar to the ones obtained in previous investigations. Detailed measurements carried out with a linearly polarised photolysis laser reveal a pronounced anisotropy of the fragment recoil which is dependent on the translational energy and on the molecular beam carrier gas. It is possible to partition each TOF distribution of the CS fragment into two components corresponding to the formation of S(P-3) and S(D-1) partner fragments, respectively, in a manner consistent with recent branching ratio determinations. In the case of the Ne beam this partitioning leads to the anisotropy parameters beta(S D-1)approximate to 0.2 and beta(S 3P)approximate to 1.0. These results indicate that the overall photodissociation process occurs on a time scale of similar to 1.5 ps and that the formation of CS + S(D-1) proceeds through a strongly bent transition state.

Polarized photofragment translational spectroscopy was used to investigate the photodissociation process CF3I → CF3 + I(2P3/2) and CF3I → CF3 + I*(2P1/2) in the A absorption band between 275 and 30...

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....

We have generated a cold beam of the highly corrosive sulfur trioxide SO 3 and have studied its photodissociation at 193 nm by photofragment translational spectroscopy. The spin-allowed process SO 3 ( X 1 A ′ 1 ) + hv → SO 2 ( X 1 A 1 ) + O ( 1 D ) was found to be the primary dissociation channel. A part of the hot SO 2 fragments emerging from this decay undergoes secondary photodissociation SO 2 ( X 1 A 1 ) + hv → SO ( X 3 ∑ − ) + O ( 3 P ) . No evidence was found for the occurrence of the energetically feasible pathway SO 3 ( X 1 A ′ 1 ) + hv → SO ( 3 ∑ − ) + O 2 ( 3 ∑ g − ) .

The dissociation dynamics of the methoxy radical (CH 3 O) following excitation of the A ( 2 A 1 ) ← X ( 2 E ) transition are examined using photofragment translational spectroscopy. The radicals are produced from photodetachement of a fast, cold negative ion beam. We find that levels of the Ā state are strongly predissociated starting with ν 3 = 6 at 35419 cm −1 excitation energy. Three product channels are identified: (1) CH 3 + O, (2) CH 2 + OH, and (3) CH 2 O + H, with channel (1) by far the dominant channel. In channel (1), resolved vibrational structure of the methyl fragment is seen in the translational energy distribution, providing insight into the dissociation dynamics of CH 3 O. Several thermodynamics quantities are also obtained: D 0 (CH 3 O) = 87.8 ± 0.3 kcal/mol, ΔH f,0 0 (CH 3 O) = 6.8 ± 0.4, D 0 (CH 3 OH) = 104.0 ± 0.5, ΔH acid,0 (CH 3 OH) = 381.4 ± 0.7, and D 0 (HCH 2 O) = 19.8 ± 0.4.

The photodissociation of jet-cooled NO:! at 355 and 351 nm has been investigated by polarized high-resolution photofragment translational spectroscopy. The translational energy distributions P(&) of the nascent photofragment pairs NO + 0 were derived from the measured time-of-flight (TOF) distributions. By comparison of P(ET) with the available energy, the population of vibrationally excited NO was determined to be P(v=O) = 62 f 3% and P(v=l) = 38 F 3% at 355 nm and P(v=O) = 57 f 3% and P(v=l) = 43 F 3% at 351 nm. These findings are consistent with the trend predicted by statistical models of the dissociation process. Nonstatistical decay dynamics are, however, indicated for the rotational degrees of freedom as manifested by bimodal (or multimodal) rotational distributions of NO(v=O, 1). The recoil anisotropy parameter p as a function of fragment translational energy was obtained from the polarized TOF spectra and was found to depend on the photolysis wavelength and on the vibrational state of the NO product: p(v=O) = 1.42 and p(v=l) = 1.25 at 355 nm, whereas p(v=O) = 1.77 and p(v=l) = 1.48 at 351 nm. The NO rotational alignment Af) measured by the laser-induced fluorescence method is at high J values close to the theoretical (perpendicular-type) limit of -0.4 at both photolysis wavelengths. This result, in conjunction with the values of p, implies that the photon absorption occurs via the :!B:! - electronic transition and that the photodissociation takes place essentially without anisotropy loss and hence on a subpicosecond time scale. The observed dependence of p on the product vibrational state suggests two different decay pathways with quantum mechanical effects playing an important role.

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 b...

Photodissociation of n-C 4 H 9 I at excitation wavelengths of 277 and 304 nm has been studied utilizing state-selective photofragment translational spectroscopy. The quantum yields of I * ( 2 P 1/2 ) at these wavelengths are determined to be 0.61 and 0.14, respectively. The anisotropy parameters observed at 277 nm are β(1)=1.6±0.1 for the ground-state I( 2 P 3/2 ) and β(I * )=0.9±0.1 for the excited-state I * ( 2 P 1/2 ) which substantially differs from the limiting value

The primary and secondary dissociation processes ofCF 2Br2 and CF 2BrI have been studied by photofragment translational spectroscopy in a pulsed molecular beam. The photolysis of CF 2Br2 at 193 nm produces mainly the fragment pair CF2Br + Br and, in a competing primary process, CF 2 + Br r The CF 2Br radicals are formed with sufficient internal energy to overcome the barrier of the unimolecular secondary dissociation to CF 2 + Br. The photodissociation of CF 2BrI at 248 nm also proceeds along two decay pathways involving the fission of the weak C-I bond and the stronger C-Brbond, respectively. The former reaction produces spin-orbit excited iodine atoms 1(21\/2) and stable CF 2Br radicals, whereas the latter reaction produces unstable CF 2I radicals. The results of this study are compared with those of other workers, and an account of the ultraviolet photochemistry of fluorinated halomethanes is given. CHPN0 2 , 11 and HNO/2 which are found in the atmo­ sphere, by investigating the primary and secondary de­ cays after irradiation at 193 and 248 nm using PTS. Some pertinent results of the present study are briefly summa­ rized. CF 2CI2 and CFCI 3 belong to the two Halon com­ poundsmostwidely usedduring the pastdecades, and the worldwide emission of CF 2BrCi used in fire extinguish­ ers is considerable. The photolysis of these molecules in the upper atmosphere may release chlorine and bromine atoms and thereby contribute via the CIOx and BrO x catalytic cycles to ozonedestruction. The photodissocia­ tion of CF 2CI2 and CFCl 3 at 193 nm studied in this laboratory is shown to produce chlorine atoms exclu­ sively and the corresponding stable CFFI or CFCl 2 fragments, respectively. In contrast, two primary decay routes are operative for the photodissociation of CF 2BrCi at 193 nm

I*( 2 P1/2) + Cl( 2 P3/2) and I*( 2 P1/2) + Cl*( 2 P1/2), respectively. The quantum yield of I*( 2 P1/2) is determined to be 0.30, indicating that the formation of ground state iodine is clearly the favored dissociation channel at 304 nm. The observed anisotropy in the angular distribution of dissociation products ( ‚) indicates that the I* + Cl and I* + Cl* channels are formed predominantly from the parallel transition (‚ ) 1.7 for both channels) while the I + Cl and I + Cl* products are formed mainly from perpendicular transitions (‚ )- 0.5). The decrease in the anisotropy parameter of the I formation channels from their limiting value of -1 is attributed to the presence of more than one path for the formation of I + Cl/Cl* photoproducts with opposite polarization for their absorbing transitions. The possible excited state dynamics, which give the observed results, are discussed in terms of the previously proposed energy correlation diagram for ICl.

The decomposition of poly(methyl methacrylate) (PMMA) under the action of 248 nm, 16 ns laser pulses at 250 mJ/cm2 was studied in vacuum. A sensitive electron impact ionizer/quadrupole mass spectrometer system was used to measure mass spectra and time-of-flight (TOF) distributions of the neutral photoproducts. The incubation and photoablation reactions were unambiguously resolved in the TOF distributions. The incubation reaction produces slow-moving species with mass peaks at m/e = 15, 28, 29, 30, 31, 32, 59, and 60 which can be assigned to methanol, CO, and methyl formate. These products are consistent with the incubation mechanism proposed by Stuke and co-workers, namely, single-photon absorption at the carbonyl side-chain chromophore followed by side-chain scission. The incubation reaction produces a partially unsaturated and more strongly UV-absorbing polymer backbone. The ablation of this degraded material at 250 mJ/cm2 produces mainly species with molecular weights < 150 and translational energies a...

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 beta=2 for the O(3P) channel and beta=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 beta=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(nu2) 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.

Photofragment translational spectroscopy was used to study the photodissociation dynamics of the phenyl radical C(6)H(5) at 248 and 193 nm. At 248 nm, the only dissociation products observed were from H atom loss, attributed primarily to H+o-C(6)H(4) (ortho-benzyne). The observed translational energy distribution was consistent with statistical decay on the ground state surface. At 193 nm, dissociation to H+C(6)H(4) and C(4)H(3)+C(2)H(2) was observed. The C(6)H(4) fragment can be either o-C(6)H(4) or l-C(6)H(4) resulting from decyclization of the phenyl ring. The C(4)H(3)+C(2)H(2) 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 C(4)H(3) fragment was n-C(4)H(3), and better agreement would be expected if the lower energy i-C(4)H(3)+C(2)H(2) channel were included.

The dissociation dynamics of gas phase phenol-d(5) molecules (C(6)D(5)OH) following excitation at numerous wavelengths in the range 275or = lambda(phot)or = 193.3 nm have been investigated using the techniques of H (Rydberg) atom photofragment translational spectroscopy and resonance enhanced multiphoton ionization spectroscopy. The results are compared with those from recent studies of the fully hydrogenated and fully deuterated isotopologues (C(6)H(5)OH and C(6)D(5)OD), and various halo- and methyl-substituted phenols. Analysis of the vibrational energy disposal within the phenoxyl-d(5) dissociation products identifies three distinct O-H bond fission pathways, involving nonadiabatic coupling to dissociative states of (1)pisigma* character, following initial pi*-- pi excitation. Dissociation at lambda(phot)248 nm involves internal conversion (IC) to high vibrational levels of the electronic ground ((1)pipi) state and subsequent coupling to the lowest (1)pisigma* potential energy surface (PES) via a conical intersection (CI) between the (1)pipi/(1)pisigma* PESs at extended O-H bond lengths (R(O-H)). Once lambda(phot)or = 248 nm, dissociation proceeds directly, via a (1)pipi*/(1)pisigma* CI. Both pathways yield phenoxyl-d(5) products in selected vibrational levels of the ground (X(2)B(1)) electronic state. The detailed energy disposal within the phenoxyl-d(5)(X) products shows many parallels with that deduced from companion studies of other phenol isotopologues and various substituted phenols, but a notable isotope effect is identified, thus providing yet greater insights into the factors controlling the vibrational energy disposal in the phenoxyl products. A hitherto unobserved O-H bond fission channel yielding phenoxyl-d(5) fragments in the electronically excited B(2)A(2) state is identified at the shortest excitation wavelength (lambda(phot) = 193.3 nm) and rationalized in terms of nonadiabatic coupling to, and subsequent dissociation on, the second excited (1)pisigma* PES. Selective deuteration as in phenol-d(5) causes little reduction in the intensity of the "slower" H atom products that are observed from all phenol systems, suggesting that C-H/D bond fission makes at most a minor contribution to this feature.

The translational energies of different fragments produced in laser induced unimolecular reactions are investigated in a molecular beam experiment. The time‐of‐flight of the photoproducts is measured in the forward direction with a mass spectrometer. By operating the ion source at low electron impact energies it is possible to nearly eliminate fragmentation and observe the nascent products at their parent mass. With this technique the primary and secondary reaction channels can be identified and branching ratios and product translational energy distribution down to small kinetic energies can be measured. The method is illustrated in a study of the photodissociation of toluene (methylbenzene) at three excitation energies (Eex=51 590, 52 240, and 63 240 cm−1). From the measured product time‐of‐flight spectra the branching ratio as well as the translational energy distributions of the product molecules benzyl+H and phenyl+CH3 in the two reaction channels were determined. The measured product energy distribut...

Iodocyclohexane (C6H11I) is photodissociated and studied with state-selective photofragment translational spectroscopy. Angular, velocity, and translational energy distributions are obtained for the ground state I(2P 3 2 ) and the excited state I*(2P 1 2 ) photodissociated iodine atoms. The kinetic energy released in the photodissociation of the axial conformer is 6.0 ± 0.8 kcal/mol greater than that produced from the equatorial conformer. This suggests that the cyclohexyl radical produced from the axial iodocyclohexane has less internal energy than that formed from the equatorial conformer. The conclusion is thus reached that the ground state of the cyclohexyl radical has the hydrogen atom at its radical center in the equatorial position.

The photodissociation of propargyl radical, C{sub 3}H{sub 3}, 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 C{sub 3}H{sub 2}{sup +}, C{sub 3}H{sup +}, C{sub 3}{sup +}, and the analogous deuterated species show that both H and H{sub 2} loss occur. The translational energy distributions for these processes have average values = 5.7 and 15.9 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 H{sub 2} loss. The translational energy distribution for H atom loss is similar to that in previous work on propargyl in which the H atom, rather than the heavy fragment, was detected. The branching ratio for H loss/H{sub 2} loss was determined to be 97.6/2.4 {+-} 1.2, in good agreement with RRKM results.

A simple method is developed for the measurement of the most probable recoil velocity of photofragments produced from direct (fast, nonstatistical) photodissociation of neutral molecules. The time-of-flight (TOF) distribution of photoions, produced by a linearly polarized laser, is measured in a pulsed-acceleration TOF mass spectrometer with a small detection solid angle, in which a delay time is introduced between the photolysis laser and the acceleration voltage pulses. From the delay time dependence of the TOF peak positions of I + , the most probable recoil velocity of the ground electronic state of the iodine produced in 266 nm photodissociation of C 2 H 5 I is determined, which agrees with the value determined by high resolution TOF photofragment translational spectroscopy within 2%.

The collisionless photodissociation of CFCl 3 at 193 nm has been studied with the method of crossed laser and molecular beams. The primary dissociation is shown to produce exclusively the fragment pairs CFCl 2 + Cl with a non-thermal translational energy distribution. The average kinetic energy of 32 kcal/mol corresponds to 42% of the available energy and implies an average internal energy of CFCl 2 of 43 kcal/mol. From the recoil anisotropy parameter β = +0.74 ± 0.15 the photoexcitation is inferred to proceed via an E ← A 1 transition polarized perpendicular to the C–F bond. At higher laser fluence a fraction of the nascent CFCl 2 radicals absorbs a further photon and dissociates into CFCl + Cl; the cross section for this secondary process is estimated to be approximately twice as large as that of the primary photodissociation.

Some of the most detailed insight into the dynamics of molecular photofragmenation processes is provided by measurements of the translational energy distributions of the recoiling fragments. In this article we survey recent progress in the extraction of such fragment kinetic energy distributions from careful measurement and analysis of (a) their Doppler broadened spectral line shapes and/or (b) their time-of-flight spectra