Photofragment Translational Spectroscopy Research Articles

Near ultraviolet photolysis of ammonia and methylamine studied by H Rydberg atom photofragment translational spectroscopy

H(D) Rydberg atom photofragment translational spectroscopy has been used to provide new insights into the primary photochemistry of methylamine, ammonia and various of their respective isotopomers following excitation at wavelengths in the near ultraviolet (UV). The bimodal appearance of the total kinetic energy release (TKER) spectra associated with H atom production in the near UV photolysis of methylamine is consistent with there being both ‘dynamical’ (high TKER) and ‘statistical’ (slower) contributions to the total H + CH3NH dissociation yield. Both contributions arise as a result of one H atom tunnelling through (or passing over) an earlier barrier in the N{H dissociation coordinate of the ~ A state potential energy surface and then evolving into the region of the conical intersection connecting the ~ A state and ground-state surfaces. ‘Dynamical’ energy disposal is associated with those molecules which pass directly through this conical intersection en route to the ground-state (H + CH3NH( ~ X)) asymptote, whilst the ‘statistical’ contribution is attributed to those molecules that ‘miss’ the conical intersection on the rst traversal and only make the ~ A! ~ X transfer on a later encounter. This interpretation has inspired further consideration of the form of the TKER spectra derived from TOF measurements of the H/D atom products arising in the dissociation of various isotopomers of ammonia following excitation to the 0 0 and 2 1 levels of their respective ~ A states. A similar model which associates ‘dynamical’ energy disposal with those molecules that pass through the ~ A= ~ X conical intersection during bond extension, and ‘statistical’ kinetic energy release with those that transfer during N{H(D) bond compression, appears to provide a qualitative explanation for the way the observed H and/or D atom yields and their associated TKER spectra vary with excitation wavelength (0 0 versus 2 1 band excitation) and isotopic composition.

Photofragmentation of OClO Clusters in a Supersonic Jet at 360 and 275 nm

Using photofragment translational energy spectroscopy, we studied the photochemical decay of chlorine dioxide (OClO) clusters in a cold beam with Ne as carrier gas. The products ClO, OClO, Cl2O3, and Cl2O4 generated by 360 nm excitation wereexcept for the fast ClO fragments which arise from monomer photodissociationfound to be slow moving and with broad time-of-flight distributions. They show an approximately thermal translational energy distribution of 300−500 K and a recoil anisotropy β ≈ 0, as measured for the representative OClO product. Furthermore with excitation at 275 nm where the OClO monomer has no absorption, the same slow moving product species were created, indicating that they are evaporation products of excited cluster species. A reaction scheme is proposed which includes unimolecular photochemical decays and bimolecular cage reactions of initially formed fragments among themselves and with cluster constituents.

Photodissociation spectroscopy and dynamics of the HCCO free radical

The photodissociation spectroscopy and dynamics of the HCCO radical have been investigated using fast radical beam photofragment translational spectroscopy. An electronic band with origin at 33 424 cm−1 has been identified. This band exhibits rotational resolution near the band origin, but the well-defined rovibronic structure is homogeneously broadened at higher photon energies. Based on the rotational structure this band is assigned to the B̃ 2Π←X̃ 2A′′ transition. Photofragment translational energy and angular distributions were obtained at several excitation energies. At excitation energies close to the origin, the excited, spin-forbidden CH(a 4Σ−)+CO channel dominates, while the ground state CH(X 2Π)+CO channel is the major channel at higher photon energies. The translational energy distributions provide evidence of competition between intersystem crossing and internal conversion dissociation mechanisms, with some evidence for nonstatistical dynamics in the CH(X 2Π)+CO channel. This work yields an improved heat of formation for HCCO, ΔHf,2980=1.83±0.03 eV.

Translational energy distributions of the products of the 193 and 157 nm photodissociation of chloroethylenes

The 193 and 157 nm photodissociations of three isomers of dichloroethylene (DCE) and trichloroethylene (TCE) were investigated using a technique of photofragmentation translational spectroscopy. The photofragmentation mechanisms were constructed by analyzing the time-of-flight spectra of C2H2+, Cl+, HCl+, C2HCl+, and C2Cl2+ produced by electron impact of neutral photofragments. In the 193 nm photodissociation, both the HCl elimination and the C–Cl bond rupture were important for all the compounds examined. It was concluded that secondary dissociation of the vibrationally excited chlorinated vinyl radical produced by the C–Cl bond rupture was important even at 193 nm. In the 157 nm photodissociation, the mechanisms were similar to those at 193 nm for cis-DCE, 1,1-DCE, and TCE, while only the C–Cl bond rupture occurred for trans-DCE. This result suggests that the 157 nm photodissociation of trans-DCE proceeds via the direct photodissociation following the photoexcitation to the repulsive nσ*1 state. A minor C–H bond rupture was also found in the 157 nm photodissociations of cis-DCE and TCE. On the basis of the present mechanisms, the translational energy distributions and the branching ratios were estimated for all the possible processes.

The photodissociation dynamics of hexafluoropropene examined at 193 nm

The photodissociation of hexafluoropropene at 193 nm was studied using photofragment translational spectroscopy. One primary channel results in loss of a fluorine atom; from the maximum translational energy release, a value of 121 kcal/mol was determined to be the upper limit for the CF bond dissociation energy. In addition, elimination of CF 2 and CF 3 compeetes with C-F bond cleavage. A branching fraction, CF 2:F:CF 3, of 0.42:0.41:0.17 was found among these three primary channels. A comparison of UV photolysis and infrared multiphoton dissociation experiments suggests that excitation and dissociation on an excited state surface competes with internal conversion and dissociation from the ground state.

The photodissociation dynamics of OClO between 306 and 370 nm: Fragment translational energy release and recoil anisotropy

The photodissociation OClO(à 2A2)→ClO(X̃ 2Π)+O(3P) was studied at wavelengths between 306 and 370 nm using photofragment translational energy spectroscopy. The flight time distributions and anisotropies of the recoiling fragments were measured with the photolysis wavelength tuned to 10 maxima of the structured absorption spectrum, corresponding to a vibronic excitation of the parent molecule with 9–18 quanta in the symmetric stretching coordinate on the à 2A2 surface. The translational energy distributions show that the ClO fragments are created in highly inverted vibrational state distributions which become extremely broad [v(Cl–O)∼1–15] with increasing excitation energy. The large fraction of vibrationally hot ClO fragments produced–particularly at λ<325 nm–could enhance various thermodynamically unfavorable atmospheric reactions in connection with ozone depletion. The main mechanistic features of the dissociation process, which account for the almost constant average translational energy and linearly increasing vibrational energy of ClO as a function of the excitation energy, can be interpreted, to a first approximation, as vibrational predissociation on the à 2A2 potential energy surface involving a relatively late exit barrier. From the measured translational energies the barrier height is estimated to be about 48 kJ/mol.

Near ultraviolet photolysis of HFCO: The H+FCO channel

The technique of H (Rydberg) atom photofragment translational spectroscopy has been used to study the process HFCO(S1)→H+FCO(X̃), near its appearance threshold, at excitation wavelengths ca. 247 nm. Analyses of the resulting total kinetic energy release spectra lead to an accurate determination of the C–H bond strength: D0(H–FCO)=34950±20 cm−1. The resulting FCO fragments are observed to be formed with little internal energy, distributed mainly in the form of a-axis rotation. Fragmentation is shown to involve S1–T1 intersystem crossing, followed by rapid passage along the minimum energy path to the eventual H+FCO products. This minimum energy path involves passage over (or H atom tunnelling through) a saddle point, the height of which is ⩾4740 cm−1 above the dissociation asymptote. The observed propensity for a-axis rotation in the FCO product reflects changes in the parent geometry as it evolves along the C–H dissociation coordinate on the T1 surface; past the saddle point, the barrier energy is released largely in the form of product recoil.

Fast beam photodissociation spectroscopy and dynamics of the vinoxy radical

The spectroscopy and photodissociation dynamics of the vinoxy (CH2CHO) radical B̃(2A″) ←X̃(2A″) transition have been investigated by fast beam photofragment translational spectroscopy. We show conclusively that excitation to the B̃ state is followed by predissociation, even for the origin transition. Two photodissociation channels are observed: (1) CH3+CO, and (2) H+CH2CO, with a branching ratio of ≈1:4. The form of the translational energy distributions imply a significant exit barrier to formation of CH3+CO, and a considerably smaller barrier for H+CH2CO formation. Dissociation ultimately proceeds by internal conversion to the ground electronic state; the internal conversion rate appears to be significantly enhanced by a curve crossing with either the Ã(2A) or C̃(2A) states. Ab initio calculations of critical points on the global potential energy surfaces aid in determining the dissociation mechanism. We present a simple model for dissociation over a barrier, the statistical adiabatic impulsive model, which satisfactorily reproduces the translational energy distributions.

Diode laser measurements of CD3 quantum yields and internal energy for the dissociation of dimethyl sulfoxide-d6

Infrared diode laser absorption spectroscopy has been used to measure the CD3 radical photoproducts formed in the 193 and 222 nm photodissociation of dimethyl sulfoxide-d6. Quantum yields of CD3 have been determined to be 1.4±0.1 at 193 nm and 1.2±0.2 at 222 nm, compared to 2.0 for acetone-d6 at 193 nm. An analysis of transient waveforms reflecting the vibrational relaxation and radical recombination kinetics also yields an estimate of the nascent CD3 vibrational energy content by determining the fraction of total CD3 initially produced in the vibrationless state. The nascent CD3 population in the vibrational ground state decreases in order for the following photodissociation systems: CD3I at 248 nm, DMSO-d6 at 193 and 222 nm and acetone-d6 at 193 nm. The DMSO results are in good agreement with recent photofragment translational spectroscopy results and support a stepwise mechanism for the dissociation of DMSO at both wavelengths.

Unraveling the dissociation of dimethyl sulfoxide following absorption at 193 nm

We have studied the photodissociation of dimethyl sulfoxide, DMSO-h6 and DMSO-d6, at 193 nm using the technique of photofragment translational spectroscopy with a tunable vacuum ultraviolet product probe provided by undulator radiation on the Chemical Dynamics Beamline at the Advanced Light Source. In contrast to previous investigations we have found the dissociation to proceed via a stepwise mechanism involving multiple reaction channels. The primary dissociation, S–C bond cleavage to eliminate a methyl radical, was found to have two competing channels with distinct translational energy distributions. The translational energy distribution for the major primary dissociation channel suggests that it proceeds in a statistical manner on the ground electronic surface following internal conversion. In competition with this channel is a primary dissociation that exhibits a translational energy distribution suggestive of dissociation on an excited electronic surface with most of the available energy partitioned into translational and electronic degrees of freedom. Secondary decomposition of the CD3SO intermediate was found to proceed exclusively via C–S bond cleavage, CD3SO→CD3+SO. However, secondary decomposition of the CH3SO intermediate was found to exhibit competition between CH3SO→CH3+SO and CH3SO→CH2SO+H. The dissociation to CH3 and SO was the major secondary decomposition channel with the translational energy distribution indicating a barrier to recombination of >8 kcal/mol. While a minor hydrogen atom elimination channel was found to play a role in secondary decomposition of CH3SO intermediates, no analogous secondary C–D bond cleavage was detected from the CD3SO intermediates indicating the importance of tunneling in the secondary decomposition of CH3SO.

Competing Pathways in the Infrared Multiphoton Dissociation of Hexafluoropropene

The infrared multiphoton dissociation of hexafluoropropene was studied by photofragment translational spectroscopy. Two primary channels and one secondary channel were identified. The predominant primary channel produces CF3CF or C2F4 and CF2, with the heavier species undergoing further dissociation to two CF2 fragments. A number of dissociation mechanisms are proposed for the elimination of CF2, including direct cleavage of the carbon−carbon double bond. In the second primary channel, a simple bond rupture reaction produces CF3 and C2F3. As expected, the translational energy distribution for this channel peaks near zero, indicating no exit barrier is present. The activation energy for this simple bond rupture is estimated to be 100−105 kcal/mol. The branching ratio, [CF2]/[CF3], between the two primary pathways is 4.0 ± 1.0.

Excited state dynamics in clusters of oxygen

Considerable insights into the dynamics of both ionic (photodissociation) and neutral (dissociative photodetachment) decomposition pathways of O4- and O6- have been gained using photoelectron and photofragment translational spectroscopy in a fast-ion beam. The O4- data at 532 nm reveal a novel process involving sequential photodetachment of an electron with a near-zero binding energy from photodissociating O4-. Studies of O6- at 532 nm reveal that addition of a third O2 to the O4- core leads to a dramatic change in the photodissociation dynamics, producing highly vibrationally excited O2- photofragments not observed in the case of O4-. At 355 nm, both O4- and O6- yield vibrationally excited O2- photofragments, as observed by autodetachment of the nascent O2- (v5)→O2+e-. At 266 nm, photofragment time-of-flight (TOF) measurements on O6- and O4- show that the dynamics of dissociative photodetachment in O6- are only slightly perturbed relative to O4-. The anisotropic product angular distribution previously observed in O4- is observed to persist in the three-body neutral decomposition O6-+hν→O2+O2+O2+e-. The origins of these diverse phenomena in O4- and O6- are discussed.

Coherent control of high- n Rydberg lifetimes using wavepacket technology

We describe a simple, experimentally feasible scheme for enhancing lifetimes of high- n Rydberg states. We propose a new control scheme, based upon a ‘wavepacket technology’ which uses time-dependent, crossed electric and magnetic fields. Model (hydrogenic) calculations illustrating the control over l and m quantum numbers are presented and experimental parameters are suggested. The effects of core multipole moments on the decay of the target state (high l, high m) are discussed. Coherent lifetime enhancement should be applicable to small signal zero-kinetic-energy (ZEKE) photoelectron spectroscopy experiments and to Rydberg photofragment translational energy spectroscopy.

Near-UV photodissociation dynamics of formic acid

H (Rydberg) atom photofragment translational spectroscopy has been used to study the photodissociation dynamics of jet-cooled formic acid molecules following excitation to their first excited singlet (S1) state at numerous wavelengths in the range 216–241 nm. Analysis of the resulting H-atom time-of-flight spectra indicates contributions from three H-atom formation channels, which we identify as the primary C–H and O–H bond fission processes and the secondary photolysis of HCO() fragments resulting from primary C–O bond fission. It also allows determination of the bond dissociation energies: D0(H–CO2H)≈30000 cm-1 and D0(HCOO–H)=39080±100 cm-1. The former bond fission is deduced to occur after intersystem crossing to the neighbouring a3A″ state, and to involve passage over (or tunnelling through) a barrier in the C–H dissociation coordinate on the triplet potential-energy surface. O–H bond fission, in contrast, is shown to occur predominantly on the S1 surface but it, too, must overcome an activation barrier, the magnitude of which we can estimate at ca. 5400 cm-1, measured relative to the asymptotic products H+HCOO(and/or A). The latter assignment affords a refined value for the 0 K heat of formation of the formyloxyl radical: ΔfH0o (HCOO)=-119.5±3 kJ mol-1.

Ultraviolet photodissociation dynamics of formyl fluoride Part 2Energy disposal in the H+FCO product channel

C. L. Reed, M. Kono, S. R. Langford, R. N. Dixon and M. N. R. Ashfold, J. Chem. Soc., Faraday Trans., 1997, 93, 2721 DOI: 10.1039/A701632H

Translational spectroscopy studies of the photodissociation dynamics of O−4

An investigation of the photodissociation dynamics of the dimer anion O−4 at 523.6, 349.0, and 261.8 nm is reported. Product translational energy and angular distributions have been obtained using photofragment translational spectroscopy in a fast ion beam. At all wavelengths photodissociation (O−4+hν→O2+O−2) is observed to proceed via a rapid parallel electronic transition, with the photofragment angular distribution strongly peaked along the laser electric vector. The lowest energy photodissociation channel produces O2(a1Δg) and ground state O−2(X2Πg), indicating that O−4 is a doublet anion. The partitioning of energy in the dissociation reveals a complicated wavelength dependence.

The photodissociation of ClO2: Potential energy surfaces of OClO→Cl+O2

Using large multireference configuration interaction wave functions, potential energy surfaces involved in the photodissociation of symmetric ClO2 to Cl+O2 are investigated. The production of atomic chlorine from OClO, which may have important implications for stratospheric ozone chemistry, is predicted to occur via the excited 1 2B2 electronic state after initial excitation to the A 2A2 state. A calculated C2v transition state connecting 1 2B2 OClO to Cl+O2 is strongly bent and has a barrier height relative to the X 2B1 ground state of 2.86 eV (2.75 eV with zero-point vibrational corrections). However, this is only a 2nd-order transition state with imaginary vibrational frequencies along both the OClO→Cl+O2 and OClO→ClO+O reaction paths (symmetric bending and asymmetric stretching modes, respectively). Thus, the present theoretical work suggests that only a small amount of Cl+O2 will be formed in the photodissociation of ClO2 due to the dominance of the ClO+O channel. Much of the O2 that is produced is predicted to be in the a 1Δg state, since the 1 2B2 potential energy surface in C2v symmetry correlates with this state of O2. However, other nearby electronic states of OClO, namely the 1 2A1 and 2 2B2, interact in the exit channel and will facilitate the production of especially X 3Σ−g and perhaps b 1Σ+g O2, respectively. The present results are in very good accord with the recent photofragment translational energy spectroscopy experiments of Davis and Lee [J. Chem. Phys. 105, 8142 (1996)].

Photodissociation dynamics of iodobenzene by state-selective photofragment translational spectroscopy

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 I( 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 n,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).

Photodissociation dynamics of OClO

Photofragment translational energy spectroscopy was used to study the dissociation dynamics of a range of electronically excited OClO(A 2A2) vibrational states. For all levels studied, corresponding to OClO(A 2A2←X 2B1) excitation wavelengths between 350 and 475 nm, the dominant product (≳96%) was ClO(2Π)+O(3P). We also observed production of Cl+O2 with a quantum yield of up to 3.9±0.8% near 404 nm, decreasing at longer and shorter wavelengths. The branching ratios between the two channels were dependent on the OClO(A 2A2) excited state vibrational mode. The Cl+O2 yield was enhanced slightly by exciting A 2A2 levels having symmetric stretching+bending, but diminished by as much as a factor of 10 for neighboring peaks associated with symmetric stretching+asymmetric stretching. Mode specificity was also observed in the vibrationally state resolved translational energy distributions for the dominant ClO(2Π)+O(3P) channel. The photochemical dynamics of OClO possesses two energy regimes with distinctly different dynamics observed for excitation energies above and below ∼3.1 eV (λ∼400 nm). At excitation energies below 3.1 eV (λ≳400 nm), nearly all energetically accessible ClO vibrational energy levels were populated, and the minor Cl+O2 channel was observed. Although at least 20% of the O2 product is formed in the ground (X 3Σ−g) state, most O2 is electronically excited (a 1Δg). At E<3.1 eV, both dissociation channels occur by an indirect mechanism involving two nearby excited states, 2A1 and 2B2. Long dissociation time scales and significant parent bending before dissociation led to nearly isotropic polarization angular distributions (β∼0). At excitation energies above 3.1 eV (λ<400 nm), the Cl+O2 yield began to decrease sharply, with this channel becoming negligible at λ<370 nm. At these higher excitation energies, the ClO product was formed with relatively little vibrational energy and a large fraction of the excess energy was channeled into ClO+O translational energy. The photofragment anisotropy parameter (β) also increased, implying shorter dissociation time scales. The sharp change in the disposal of excess energy into the ClO products, the decrease of Cl+O2 production, and more anisotropic product angular distributions at E≳3.1 eV signify the opening of a new ClO+O channel. From our experimental results and recent ab initio calculations, dissociation at wavelengths shorter than 380 nm to ClO+O proceeds via a direct mechanism on the optically prepared A 2A2 surface over a large potential energy barrier. From the ClO(2Π)+O(3P) translational energy distributions, D0(O–ClO) was found to be less than or equal to 59.0±0.2 kcal/mol.

Ultraviolet photodissociation of the HCCO radical studied by fast radical beam photofragment translational spectroscopy

The ultraviolet photolysis of jet-cooled mass-selected ketenyl radicals has been investigated using the technique of fast radical beam photofragment translational spectroscopy. The C̃2Π(2A″)–X̃2A″ 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).