Search

The total kinetic energy release spectra of H atoms from the photolysis at 121.6 nm of water entrained in an argon jet show a number of peaks with widths of 100-150 cm[sup -1]. The relative intensity of these peaks varies with the physical conditions, which suggests that they should be attributed to molecular clusters. Calculation of the energy levels of Ar-OH with a known potential shows that there exists a wide range of states in which the OH moiety is excited to levels of the A[sup 2]Σ[sup +] state with high internal angular momentum, but which are remarkably stable to dissociation to Ar + OH. Indeed their stability increases with increasing rotation, even though their internal energy is many times the binding energy of the complex. The lowest frequency vibrational mode of the complex (ν < 10 cm[sup -1]) is a nutation of the OH rotational axis in the potential field of the argon atom. On this basis the broad peaks in the observed spectra have been simulated as unresolved bands involving these metastable levels. The calculated lifetimes for these levels are up to a nanosecond and more, so that in principle transitions involving them may give rise to resolvable spectra. It is also concluded from a comparison of spectra with and without clustering that photodissociation of Ar-H[sub 2]O leads exclusively to levels of Ar-OH(A). This is attributed to the blocking by the argon atom of the conical intersections which would be the pathways to Ar-OH(X), and which dominate in the dissociation of bare H[sub 2]O to give OH(X). [ABSTRACT FROM AUTHOR]

The dissociation dynamics for NH3(A) H+NH2(X) have been calculated using time-dependent quantum theory and an ab initio potential in the three dimensions of NH stretching, the out-of-plane motion and HNH bending. These calculations use a Hamiltonian for a combination of internal coordinates for NH2 and Jacobi coordinates for the relative motion of the H atom.Inclusion of HNH bending removes most of the gross inaccuracies present in earlier two-dimensional calculations. The calculated dissociation rates are, in general, lower than observed experimentally, indicating that the effective potential barrier to dissociation is too high. This is attributed to the influence of neglected vibrations. Excited state vibronic resonance energies, resulting product-state population distributions and state-dependent product recoil anisotropy factors compare favourably with experiment, permitting a detailed interpretation of the dissociation dynamics. Dissociation through the 00 and 21 levels of the A state proceeds by quantum tunnelling through the barrier. Scattering of the outgoing waves ata conical intersection of the A and X adiabatic surfaces then accelerates the out-of-plane motion to generate high rotation of NH2 about its a inertial axis. Forces at this intersection also generate modest excitation of NH2 bending. For all higher levels, dissociation is mediated by intramolecular vibrational energy redistribution, two quanta of nu2 being converted to NH stretching. A local mode calculation of the NH stretching resonance confirms a proposed assignment of broad bands in C' A emission spectra to the nu1 modes of A state NH3 and ND3. The recoil anisotropy calculated for each product state gives a satisfactory account of the extreme range of values observed experimentally. Comparison of the calculations with a semi-classical model supports an earlier assumption that the conical intersection of adiabatic surfaces acts as a defining point for most trajectories leading to dissociation. [ABSTRACT FROM AUTHOR]

The near-threshold photofragmentation dynamics of state-selected H2O+ cations have been investigated using velocity map ion imaging methods. The cations were prepared in the v = 0 level of the ground ( [image omitted] 2B1) electronic state, by 2 + 1 resonance enhanced multiphoton ionization via selected rotational levels of the H2O, [image omitted] 1B1, v = 0 state. Subsequent two photon excitation of the resulting H2O+ cations to [image omitted] 2B2 state levels lying above the lowest dissociation limit (i.e. at total energies in the range 46000-50600 cm-1) results in O-H bond fission and OH+ fragment ion formation. These fragments display isotropic recoil velocity distributions, which peak at low kinetic energy but extend to the highest speeds allowed by energy conservation. Ab initio calculations of key sections through the potential energy surfaces (PESs) for the ground and first few excited states of H2O+ suggest two possible mechanisms for the observed rotational and (when energetically allowed) vibrational excitation of the OH+ fragments. Both require initial non-adiabatic (vibronic coupling) from the photo-prepared [image omitted] state level to high levels of the [image omitted] 2A1 state, but involve different subsequent HO+-H bond fission mechanisms. One involves Renner-Teller coupling to the ground state PES, while the alternative requires spin-orbit induced coupling to the repulsive [image omitted] 4B1(4A'') state PES. [ABSTRACT FROM AUTHOR]

The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the dissociation of jet-cooled HOD molecules following excitation to individual rovibrational levels of its [image omitted](1B1) Rydberg state near 124 nm. Spectra have been recorded for both the D + OH and H + OD dissociation channels. The branching ratios between OH/OD(X [image omitted], high v, low N), OH/OD(X [image omitted], low v, high N), and OH/OD(A [image omitted]) channels vary between different parent rotational levels, and between the OH and OD products. The variation between the OH and OD channels can be attributed to an isotopic mass effect on the rotational axes which influences non-adiabatic coupling strengths. In addition, there is a population alternation with product rotational quantum number N for the OH/OD(X, high v, low N) product, most striking for the OH case, the sense of which is correlated with the value of the parent pseudo-quantum number [image omitted]. It is shown that this is a consequence of recoil forces which lead to a closing of the HOD angle to near 90° accompanying non-adiabatic transfer from the [image omitted] state to the [image omitted](1B1) state. The oscillation in population then derives from the symmetry properties around this angle of the product rotational wavefunctions. This suggests that the symmetry-induced population oscillation can only occur for a narrowly defined angular dissociation path. Model calculations also explain the different structure of the spin-orbit doubled low N band heads for each v(OH/OD) which also alternates for even and odd [image omitted]. The necessary conditions to observe these phenomena are discussed. [ABSTRACT FROM AUTHOR]