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The dynamics of N-methylpyrrole following excitation at wavelengths in the range 241.5-217.0 nm were studied using a combination of time-resolved photoelectron spectroscopy (TRPES), ab initio quantum dynamics calculations using the multi-layer multi-configurational time-dependent Hartree method, as well as high-level photoionization cross section calculations. Excitation at 241.5 and 236.2 nm results in population of the A2(πσ*) state, in agreement with previous studies. Excitation at 217.0 nm prepares the previously neglected B1(π3py) Rydberg state, followed by prompt internal conversion to the A2(πσ*) state. In contrast with the photoinduced dynamics of pyrrole, the lifetime of the wavepacket in the A2(πσ*) state was found to vary with excitation wavelength, decreasing by one order of magnitude upon tuning from 241.5 nm to 236.2 nm and by more than three orders of magnitude when excited at 217.0 nm. The order of magnitude difference in lifetimes measured at the longer excitation wavelengths is attributed to vibrational excitation in the A2(πσ*) state, facilitating wavepacket motion around the potential barrier in the N-CH3 dissociation coordinate. [ABSTRACT FROM AUTHOR]

The production of H2 in highly excited vibrational and rotational states (v=0–5, J=0–17) from the 157 nm photodissociation of amorphous solid water ice films at 100 K was observed directly using resonance-enhanced multiphoton ionization. Weaker signals from H2(v=2,3 and 4) were obtained from 157 nm photolysis of polycrystalline ice, but H2(v=0 and 1) populations in this case were below the detection limit. The H2 products show two distinct formation mechanisms. Endothermic abstraction of a hydrogen atom from H2O by a photolytically produced H atom yields vibrationally cold H2 products, whereas exothermic recombination of two H-atom photoproducts yields H2 molecules with a highly excited vibrational distribution and non-Boltzmann rotational population distributions as has been predicted previously by both quantum-mechanical and molecular dynamics calculations. [ABSTRACT FROM AUTHOR]

The laser-induced photodissociation of formaldehyde in the wavelength range 309≤λ<330 nm has been investigated using H (Rydberg) atom photofragment translational spectroscopy. Photolysis wavelengths corresponding to specific rovibronic transitions in the à 1A2←X 1A1 201403, 202401, 202403, 203401, and 201501 bands of H2CO were studied. The total kinetic energy release spectra so derived can be used to determine partial rotational state population distributions of the HCO cofragment. HCO product state distributions have been derived following the population of various different NKa levels in the à 1A2 2243 and 2341 states. Two distinct spectral signatures are identified, suggesting competition between dissociation pathways involving the X 1A1 and the ã 3A2 potential energy surfaces. Most rovibrational states of H2CO(à 1A2) investigated in this work produceH+HCO(X 2A′) photofragments with a broad kinetic energy distribution and significant population in high energy rotational states of HCO. Photodissociation via the à 1A2 2243 11,1 (and 11,0) rovibronic states yields predominantly HCO fragments with low internal energy, a signature that these rovibronic levels are perturbed by the ã 3A2 state. The results also suggest the need for further careful measurements of the H+HCO quantum yield from H2CO photolysis at energies approaching, and above, the barrier to C–H bond fission on the ã 3A2 potential energy surface. [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]

Rydberg excited states of the CS2 molecule in the energy range 56 000–81 000 cm-1 have been further investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet-cooled sample of the parent molecule. Spectral interpretation has been aided by parallel measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. Thus we have been able to extend, and clarify, previous analyses of the tangled spin–orbit split vibronic structure associated with the 3Πu and 1Πu states derived from the configuration [2Πg]4pσu and the 3Δu, 1Δu, and 1Σ+u states resulting from the configuration [2Πg]4pπu, and to deduce an approximate wave number for the origin of the hitherto unidentified 3Σ+u state derived from this same configuration. Moving to higher energies we are able to locate, unambiguously, the origins of the next (n=5) members of four of these [2Πg]np Rydberg series, and to identify extensive series based on the presumed Rydberg configurations [2Πg]nsσg and [2Πg]nfλu with, in both cases, n≤10. We also identify MPI resonances attributable to CS(a 3Π) fragments, to ground state C atoms, and to S atoms in both their ground (3P) and excited (1S) electronic states. Analysis of the former resonances indicates that the CS(a 3Π) fragments resulting from two photon dissociation of CS2 at excitation wavelengths around 300 nm are formed with substantial rovibrational excitation. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of HCN(DCN) at 121.6 nm. Analyses of the H(D) atom time-of-flight spectra reveal the partner CN fragment to be formed predominantly in its A 2Π excited electronic state. Branching into the H/D+CN(B 2∑+) product channel accounts for a few percent of the total fragment yield, but we discern no evidence for any contribution from the product channel yielding H/D atoms in conjunction with ground state CN(X 2∑+) fragments. The majority of the CN (A) fragments are formed in their v’=0 level but with a markedly bimodal rotational state population distribution. This bimodality has been rationalized in the light of the available information regarding the form of the potential energy surface of the excited 1Π state of HCN(DCN) populated following photoexcitation at 121.6 nm. [ABSTRACT FROM AUTHOR]

The near ultraviolet photolysis of jet-cooled acetylene molecules has been further investigated using the technique of H atom photofragment translational spectroscopy. Analysis of the rotational structure evident in the total fragment kinetic energy spectrum yields a precise value for the C–H bond dissociation energy: D0(HCC–H)=46 074±8 cm-1 (551.2±0.1kJ mol-1). [ABSTRACT FROM AUTHOR]