Richard N. Dixon

The determination of the vector correlation between photofragment rotational and translational motions from the analysis of Doppler‐broadened spectral line profiles

A theoretical framework is presented to describe the possible correlation between the translational and rotational motion of a molecular photofragment, and to relate this to experimental observables. The correlation is defined by the values of a number of bipolar moments of the translational and rotational angular distributions which reflect the dynamics of the dissociation process. Detailed equations are presented for the polarization dependence of the profiles of recoil Doppler–broadened spectral lines of the photoproduct for two distinct classes of system: (i) molecular photodissociation, followed by spontaneous fluorescence by a molecular product with dispersion of its emission spectrum, and (ii) LIF excitation of a photoproduct, with or without dispersion of the resulting emission. The application of the theory is illustrated by calculating the line profiles for two model systems, each with several excitation–detection geometries. It is pointed out that there may still be detectable correlation betwe...

The Role of πσ* Excited States in the Photodissociation of Heteroaromatic Molecules

High-resolution measurements of the kinetic energies of hydrogen atom fragments formed during ultraviolet photolysis of imidazole, pyrrole, and phenol in the gas phase confirm that N(O)–H bond fission is an important nonradiative decay process from their respective 1πσ* excited states. The measurements also reveal that the respective cofragments (imidazolyl, pyrrolyl, and phenoxyl) are formed in very limited subsets of their available vibrational states. Identification of these product states yields uniquely detailed insights into the vibronic couplings involved in the photoinduced evolution from parent molecule to ultimate fragments.

A nomenclature for Λ‐doublet levels in rotating linear molecules

It is proposed that the two Λ‐doublet levels of linear molecules with nonzero electronic orbital angular momentum be labeled Λ(A’) and Λ(A‘), e.g., Π(A’) and Π(A‘) for Π states, etc., according to the following prescription: All series of levels in which the electronic wave function at high  J is symmetric with reflection of the spatial coordinates of the electrons in the plane of rotation will be designated Λ(A’) for all values of J, and all those for which the electronic wave function is antisymmetric with respect to reflection will be denoted Λ(A‘). It is emphasized that this notation is meant to supplement, and not replace, the accepted spectroscopic e/f labeling and the parity quantum number. The utility of the Λ(A’)/Λ(A‘) notation is that it is of most relevance in the mechanistic interpretation of reactive or photodissociative processes involving open‐shell molecules.

Grid methods for solving the Schrödinger equation and time dependent quantum dynamics of molecular photofragmentation and reactive scattering processes

Abstract This review will concentrate on new theoretical methods for solving the quantum dynamics of molecular systems. The approach will be that of solving the time dependent Schrodinger equation, and extracting from it the measurable quantities of experimental interest. Applications to the modelling of molecular photodissociation processes and to the theory of reactive molecular collisions will be discussed.

Dynamics of H2O2 photodissociation: OH product state and momentum distribution characterized by sub‐Doppler and polarization spectroscopy

Hydrogen peroxide has been optically excited at a wavelength of 266 nm and the OH photofragment completely characterized by Doppler and polarization spectroscopy using the laser‐induced fluorescence technique. The entire internal state distribution (vibration, rotation, spin, and Λ components), translational energy, angular distribution, rotational alignment, and vector correlations between rotational and translation motions of OH products is measured. The hydroxyl radicals are formed in the X 2Π3/2,1/2 ground state with 90% of the available energy (248 kJ/mol) being released as OH recoil translation. The angular distribution is nearly a sin2 θ distribution about the electric vector of the photolysis laser. The internal motion of OH is vibrationally cold (no vibrationally excited OH was found) while the rotational excitation in v‘=0 can be described by a Boltzmann distribution with a temperature parameter of Trot=(1530±150) K. The two spin states are found to be populated nearly statistically, in contrast...

State selective photodissociation dynamics of à state ammonia. II

The photofragmentation dynamics of ammonia molecules following pulsed laser excitation to the two lowest levels (v’2 =0 and 1) of their A 1A‘2 excited state has been investigated by monitoring the time‐of‐flight spectra of the nascent H‐atom products. These spectra show well resolved structure. Analysis of this structure confirms recent revised estimates of the quantity D00 (H–NH2) (4.645±0.01 eV) and reveals that the majority of the accompanying NH2 (X 2B1) fragments are formed vibrationally unexcited, but with high levels of rotational excitation specifically concentrated about the a‐inertial axis. The detailed energy disposal is sensitive to the initially excited parent vibronic (and even rovibronic) level: the NH2 (X) fragments resulting from photodissociation via the v’2 =1 level of NH3 (A) carry a higher level of excitation of the N=Ka rotational levels, which show an inverted population distribution. We also describe the results of trajectory calculations employing the recently reported [M. I. ...

The Renner effect in a bent triatomic molecule executing a large amplitude bending vibration

The theory is given for a model hamiltonian which combines the effects of large amplitude bending for a bent triatomic molecule with those arising from the coupling of vibrational, electronic and spin angular momenta characteristic of linear molecules. Two numerical examples are given for the application of the theory. (i) The vibronic and spin-orbit coupling between the A, 2 A 1 and [Xtilde], 2 B 1 states of PH2 is discussed in detail in terms of the 2Π u state of linear PH2 with which they correlate. The v 2 and K dependence of the doublet splitting in the A state is well represented in terms of a single spin coupling parameter for the 2Π u state. (ii) The calculations give a quantitative account of the inversion for high v 2 of the K = 0 and K = 1 levels of the bent B 2 component of the a, 3 A 2 state of CS2 arising from its correlation with a 3Δ2u state of linear CS2.

Photodissociation of D2O at 121.6 nm: A state-to-state dynamical picture

Photodissociation dynamics of H2O at 121.6 nm have been studied using the H atom Rydberg “tagging” time-of-flight technique and by quasiclassical trajectory (QCT) calculations. Product kinetic energy distributions and angular distributions have been measured. From these distributions, rovibronic distributions of the OH radical product as well as the state resolved angular anisotropy parameters were determined. The dissociation energy D00(H–OH) is determined to be 41151±5 cm−1. Two clear alternations in the OH(X,v=0) rotational distribution have been observed, with each alternation corresponding to an oscillation in the anisotropy distribution. These oscillations had been attributed to the dynamical interference between the two conical intersection pathways. Further theoretical modeling in this work strongly supports this argument. Very highly vibrationally excited OH(X) products (up to v=9) have also been observed. These are ascribed to interconversion of H–O–H bending (H–H vibration) and O–H vibration in...

Exploring nuclear motion through conical intersections in the UV photodissociation of phenols and thiophenol.

High-resolution time-of-flight measurements of H atom products from photolysis of phenol, 4-methylphenol, 4-fluorophenol, and thiophenol, at many UV wavelengths (λphot), have allowed systematic study of the influence of ring substituents and the heteroatom on the fragmentation dynamics. All dissociate by XH (X = O, S) bond fission after excitation at their respective S1(1ππ*)–S0 origins and at all shorter wavelengths. The achieved kinetic energy resolution reveals population of selected vibrational levels of the various phenoxyl and thiophenoxyl coproducts, providing uniquely detailed insights into the fragmentation dynamics. Dissociation in all cases is deduced to involve nuclear motion on the 1πσ* potential energy surface (PES). The route to accessing this PES, and the subsequent dynamics, is seen to be very sensitive to λphot and substitution of the heteroatom. In the case of the phenols, dissociation after excitation at long λphot is rationalized in terms of radiationless transfer from S1 to S0 levels carrying sufficient OH stretch vibrational energy to allow coupling via the conical intersection between the S0 and 1πσ* PESs at longer OH bond lengths. In contrast, H + C6H5O(X2B1) products formed after excitation at short λphot exhibit anisotropic recoil-velocity distributions, consistent with prompt dissociation induced by coupling between the photoprepared 1ππ* excited state and the 1πσ* PES. The fragmentation dynamics of thiophenol at all λphot matches the latter behavior more closely, reflecting the different relative dispositions of the 1ππ* and 1πσ* PESs. Additional insights are provided by the observed branching into the ground (X2B1) and first excited (2B2) states of the resulting C6H5S radicals.

On the Jahn-Teller distortion of CH4 +

Ab initio calculations of the vibronic potential surface of the 2 T 2 ground state of CH4 + have been carried out, using a contracted gaussian approximation to a double-zeta basis of Slater orbitals. The minimum energy is found for a tetragonal distortion to the point group D 2d , with two HCH angles of 141° and four of 96°, and all r ch equal. All calculations for distorted CH4 + were carried out with the mean r ch held at 1·147 a, which minimises the energy for Td symmetry. The distortion from Td symmetry lowers the electronic energy by 1·41 ev. Distortions from Td to C 3v lead to a maximum stabilization of 1·22 ev. Vibronic perturbation parameters derived from this surface are employed in discussions of the 2 T 2-1 A 1 photoelectron transition of CH4 and a possible 2 E←2 B 2 absorption transition of CH4 +. A vibrational interval of ∼ 1200 cm-1 observed near the origin of the 2 T 2-1 A 1 photoelectron transition is assigned to one component of v 2 (calculated frequency, 1300 cm-1).