Search

Comparing the recoil energy distributions of the fragments from one-photon dissociation of phenol-d5 with those from vibrationally mediated photodissociation shows that initial vibrational excitation strongly influences the disposal of energy into relative translation. The measurements use velocity map ion imaging to detect the H-atom fragments and determine the distribution of recoil energies. Dissociation of phenol-d5 molecules with an initially excited O–H stretching vibration produces significantly more fragments with low recoil energies than does one-photon dissociation at the same total energy. The difference appears to come from the increased probability of adiabatic dissociation in which a vibrationally excited molecule passes around the conical intersection between the dissociative state and the ground state to produce electronically excited phenoxyl-d5 radicals. The additional energy deposited in electronic excitation of the radical reduces the energy available for relative translation. [ABSTRACT FROM AUTHOR]

Orientation and alignment parameters have been computed from first principles for the photodissociation of the HF and DF diatomic molecules. The calculations are entirely ab initio and the break-up dynamics of the molecule is treated rigorously taking account of the electronically nonadiabatic dynamics on three coupled adiabatic electronic potential energy curves. The potential energy curves and spin-orbit interactions, which have been previously reported [J. Chem. Phys. 113, 1870 (2000)], are computed using ab initio molecular electronic structure computer codes. These are then used to compute photofragmentation T matrix elements using a time-dependent quantum mechanical wave packet treatment and from these a complete set of anisotropy parameters with rank up to K = 3 is computed. The predicted vector correlations and alignment parameters are presented as a function of energy for HF and DF initially in both their ground and first excited vibrational states. The parameters predicted for the molecules which are initially in their excited vibrational states display a pronounced sharp energy dependence arising from the nodal structure of the initial vibrational wavefunction. The theoretical results are analyzed using a simple model of the dynamics and it is demonstrated how the magnitude and relative phases of the photofragrnentation T matrix elements can be deduced from the experimentally measured alignment parameters. No experimental measurements have yet been made of alignment parameters for hydrogen halide diatomics and the present results provide the first predictions of these quantities which may be compared with future experimental observations. [ABSTRACT FROM AUTHOR]

The H(D) Rydberg atom photofragment translational spectroscopy technique has been applied to a further detailed investigation of the photodissociation dynamics of NH3 and ND3 molecules following excitation to the lowest two (v2=0 and 1) vibrational levels of the first excited (A 1A2″) singlet electronic state. Analysis of the respective total kinetic energy release spectra, recorded at a number of scattering angles Θ [where Θ is the angle between the ε vector of the photolysis photon and the time-of-flight (TOF) axis], enables quantification of a striking, quantum state dependent, μ-v correlation in the NH2(ND2) products. The spatial distribution of the total flux of H(D) atom photofragments is rather isotropic (βlab∼0). However, more careful analysis of the way in which the TOF spectra of the H(D) atom photofragments vary with Θ reveals that each H+NH2(D+ND2) product channel has a different ‘‘partial’’ anisotropy parameter, βlab(v2,N), associated with it: The H(D) atom ejected by those molecules that dissociate to yield NH2(ND2) fragments with little rotational excitation largely appear in the plane of the excited molecule (i.e., perpendicular to the transition moment and the C3 axis of the parent, with β tending towards -1). Conversely, the H(D) atoms formed in association with the most highly rotationally excited partner NH2(ND2) fragments tend to recoil almost parallel to this C3 axis (i.e., β→+2). Such behavior is rationalized in the context of the known potential energy surfaces of the A and X states of ammonia using a classical, energy and angular momentum conserving impact parameter model in which we assume that all of the product angular momentum is established at the ‘‘point’’ of the conical intersection in the H–NH2(D–ND2) dissociation coordinate. We conclude by reemphasizing the level of care needed in interpreting... [ABSTRACT FROM AUTHOR]

The results of time-dependent quantum mechanical calculations on the photodissociation of initially nonrotating HOCl and DOCl molecules are presented. Two photodissociation processes, 1 1A‘←X 1A’ and 2 1A’←X 1A’, are considered. The dynamics are treated in a two dimensional model with a fixed O–H distance and the dissociation process is followed in a rotating body-fixed frame with total angular momentum of J’=1. The total photodissociation spectra and product rotational distributions resulting from dissociation of HOCl and DOCl originating in a number of different vibrational states are presented over a continuous range of incident photon wavelengths in the range 140–440 nm. It is found that, for HOCl, the physically more correct treatment with J’=1 leads to a qualitatively different OH product rotational distribution than that resulting from the more usual model treatment in which both initial and excited states are assumed to possess zero total angular momentum. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of ICN to CN+I(2P3/2) are investigated by state selective one-dimensional photofragmentation translation spectroscopy at 304.67 nm. Translational energy release, laboratory anisotropy factors, and energy distributions are obtained from analysis of the velocity and spatial distributions of the photodissociated iodine atoms. Two velocity distributions peaks are deconvoluted which are found to be separated by 2000 cm-1, which is the CN stretching vibration of the CN radical. The high intensity velocity peak is assigned to dissociation to I+CN(X 2Σ+) in v=0 (channel I), while the weak lower velocity peak is attributed to dissociation to I+CN(X 2Σ+) in v=1 (channel II). More than 80% of the iodine are produced from channel I and are found to have a relatively small anisotropy parameter, β, that is independent of velocity, suggesting a mixed absorption polarization leading to rapid dissociation. The weak shoulder, representing less than 20% of the photodissociated iodine, is formed via channel II and found to have a β value that decreases with velocity and produces CN with more of the available excess energy appearing in rotation, suggesting longer dissociation time that allows for more energy redistribution prior to dissociation. The dissociation mechanisms involved in these two channels are discussed in terms of these results, the theoretically predicted properties of the 3Π+0 and 3Π1 surfaces of ICN, our previous conclusion that suggests that ICN bends prior to dissociation via channel II, the laser wavelength used, and curve crossing between the 3Π+0 and 1Π1 surfaces. [ABSTRACT FROM AUTHOR]

Photodissociation dynamics of a model diatomic molecule adsorbed on an MgO (001) surface is studied using Hamiltonians having two actively treated mathematical dimensions. Dissociation/desorption at three orientations, i.e., θ=0°, 90°, and 180° (where θ is the angle between the molecular axis and the surface normal), is investigated. The dynamics is represented by quantum wave packets which provide detailed information about the time evolution of the system. Trapping of one or two dissociated atoms is observed and the trapped vibrational state distributions are calculated. When the light atom is between the surface and the heavy one, oscillation of this atom between the two heavy partners can cause energy transfer to the outer atom. This results in a broadened and upward shifted translational energy distribution of the desorbed heavy atom, compared to the gas phase result. [ABSTRACT FROM AUTHOR]