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Determining the product velocities offers one of the most direct and penetrating experimental probes of the dynamics of gas phase molecular photodissociation and bimolecular collision processes and provides an obvious point of contact with theoretical molecular dynamics simulations, potential energy surfaces, and non-adiabatic couplings between such surfaces. This perspective traces the development of the H Rydberg atom translational spectroscopy technique from a serendipitous first encounter through to the present, highlights the advances that make it the method of choice for studying many benchmark photofragmentation and photoinduced collision processes that yield H (or D) atoms amongst the products, and anticipates some future opportunities afforded by the technique. [ABSTRACT FROM AUTHOR]

The detailed features of molecular photochemistry are key to understanding chemical processes enabled by non-adiabatic transitions between potential energy surfaces. But even in a small molecule like hydrogen sulphide (H2S), the influence of non-adiabatic transitions is not yet well understood. Here we report high resolution translational spectroscopy measurements of the H and S(1D) photoproducts formed following excitation of H2S to selected quantum levels of a Rydberg state with 1B1 electronic symmetry at wavelengths λ ~ 139.1 nm, revealing rich photofragmentation dynamics. Analysis reveals formation of SH(X), SH(A), S(3P) and H2 co-fragments, and in the diatomic products, inverted internal state population distributions. These nuclear dynamics are rationalised in terms of vibronic and rotational dependent predissociations, with relative probabilities depending on the parent quantum level. The study suggests likely formation routes for the S atoms attributed to solar photolysis of H2S in the coma of comets like C/1995 O1 and C/2014 Q2. The photodissociation dynamics of small molecules in the vacuum ultraviolet range can have key implications for astrochemical modelling, but revealing such dynamical details is a challenging task. Here the authors, combining high resolution experimental techniques, provide a detailed description of the fragmentation dynamics of selected rotational levels of a predissociated Rydberg state of H2S. [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]

Five new triplet excited states of the ND radical (three in the case of NH) in the wave-number range 85 000–91 000 cm-1 have been identified through analysis of the two-photon resonance enhancements they provide to the wavelength-resolved multiphoton ionization spectrum of X 3Σ- state NH(ND) radicals. The lowest energy of these, the B 3Π state, is found to be a ‘‘regular’’ Rydberg state which, on the basis of its observed quantum defect and its deduced rotational and spin–orbit coupling constant, is surmised to be the 3Π state derived from a 3pσ electron built on the 2Π ground-state ion core. Perturbations are evident in the B 3Π–X 3Σ- origin bands of both NH and ND. In the case of ND the perturbing state provides its own resonance enhancements, the analysis of which enables its definitive identification as the C 3Σ- state. The very small spin–orbit splitting found for the D 3Π state is taken to indicate that (at least in the Franck–Condon region) its wave function is dominated by the configuration involving one 3pπ Rydberg electron and a 4Σ- ion core. To still higher wave number we identify two more 3Σ- excited states, the upper of which (the F 3Σ- state) has a very small rotational constant which we take to imply that it has substantial valence character. Further indications that the F 3Σ- (and C 3Σ-) states possess significant valence character is provided by the observation that both parent and daughter (N+) ions contribute to the overall ion yield when the multiphoton ionization proceeds via these two states. Daughter-ion formation is considered to occur via an overall four-photon excitation process in which the coherent two-photon excitation to the 3Σ- state of interest is followed by a one-photon excitation to a ‘‘superexcited’’ state of the neutral. This is then presumed to absorb a... [ABSTRACT FROM AUTHOR]

Rydberg excited states of the OCS molecule in the energy range 70500–86000 cm-1 have been 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 assisted by companion measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. The present study supports the earlier conclusions of Weinkauf and Boesl [J. Chem. Phys. 98, 4459 (1993)] regarding five Rydberg origins in the 70500–73000 cm-1 energy range, attributable to, respectively, states of 3Π, 1Π, 3Δ, 1Δ and 1Σ+ symmetry arising from the 4pλ←3π orbital promotion. We also identify a further 21 Rydberg origins at higher energies. These partition into clumps with quantum defects ca. 3.5 and 4.5, which we associate with the orbital promotions npλ←3π (n=5,6), and others with near integer quantum defect which are interpretable in terms of excitation to s,d and (possibly) f Rydberg orbitals. We also identify MPI resonances attributable to CO(X 1Σ+) fragments and to S atoms in both their ground (3P) and excited (1D) electronic states. Analysis of the former resonances confirms that the CO(X) fragments resulting from one photon dissociation of OCS at excitation wavelengths ca. 230 nm are formed with a highly inverted, bimodal rotational state population distribution, whilst the latter are consistent with previous reports of the wavelength dependence for forming ground and excited state S atoms in the near uv photolysis of OCS. © 1996 American Institute of Physics. [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 photochemistries of HCN and DCN at the H(D) Lyman-α wavelength have been reinvestigated using the technique of H(D) Rydberg atom time-of-flight spectroscopy, with angular resolution of the H/D atom signal about the polarization vector of the photolysis radiation. In the case of HCN photodissociation, the previous assignment of substantial branching to H+CN(A [sup 2]Π)[sub v=0] products is confirmed. Analysis of the profile taken under parallel polarization of the Lyman-α radiation relative to the time of flight axis reveals additional structure attributable both to a progression in CN(A) products with high rovibrational excitation (v=4-9, with N∼26-41, for all v), and to various rotationally excited levels associated with CN(B [sup 2]Σ[sup +])[sub v=0,1]. From these various assignments an improved value for the dissociation energy, D[sub 0](H-CN)=43 710±70 cm[sup -1], is obtained. The determined β parameter, which is a measure of the angular part of the photofragment velocity distribution about the polarization vector of the photolysis radiation, shows an increasingly parallel distribution of H atom velocities with increasing CN internal energy. DCN photolysis at the D Lyman-α wavelength yields both CN(A)[sub v=0] and a range of rovibrationally excited CN(A) products but, in contrast to HCN, no significant branching to CN(B) products is observed. The corresponding β parameter, which is found to be relatively invariant with CN internal energy, indicates a near limiting perpendicular distribution of D atom velocities about the photolysis radiation. These results are interpreted with reference to the available ab initio calculated potential energy surfaces of A[sup ′] and A[sup ″] symmetry, and the relative propensities for excitation to, and the likely dynamics on, these various excited states is discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]