Theoretical study of the rotationally and vibrationally inelastic collision dynamics of small molecules

Rotationally and vibrationally inelastic collision dynamics of several small molecules are investigated through ab initio calculations of potential energy surfaces (PESs) and time-independent close-coupling scattering calculations. The scattering resonances in the collision energy dependent rotationally inelastic cross sections of OH in collisions with He and Ne, and NH3 in collisions with H2 were computed and analyzed. Both shape and Feshbach resonances were identified and the prospects for experimentally observing scattering resonances using Stark decelerated beams of OH radicals were discussed. A new PES for the interaction between CH3 with different umbrella displacements and a He atom were computed and the collisional vibrational relaxation of the $\nu_2$ mode of CH3 were studied. The vibrational relaxation rate constant was found to be two orders of magnitude smaller than the pure-rotational relaxation between two lower levels. Differential cross sections for the rotationally inelastic scattering of CH3 and CD3 with He, Ar, and H2 were computed and compared with results of velocity map imaging experiments conducted by Orr-Ewing and coworkers. In general, good agreement was found between theory and experiment, confirming the accuracy of our theoretical approach. Also, new sets of PESs describing the interaction between OH and H2 were computed, and bound-state calculations and scattering calculations were performed for this system. The computed dissociation energy of OH--\emph{ortho}-H2 complex and state-to-state cross sections of OH in collisions with H2 are in excellent agreement with earlier experimental results

Constructing velocity distributions in crossed-molecular beam studies using fourier transform doppler spectroscopy

The goal of our scattering experiments is to derive the distribution the differential cross-section and elucidate the dynamics of a bimolecular collision via pure rotational spectroscopy. We have explored the use of a data reduction model to directly transform rotational line shapes into the differential cross section and speed distribution of a reactive bimolecular collision. This inversion technique, known as Fourier Transform Doppler Spectroscopy (FTDS), initially developed by James Kinsey [1], deconvolves the velocity information contained in one-dimensional Doppler Profiles to construct the non-thermal, state-selective three-dimensional velocity distribution. By employing an expansion in classical orthogonal polynomials, the integral transform in FTDS can be simplified into a set of purely algebraic expressions technique; i.e. the Taatjes method [2]. In this investigation, we extend the Taatjes method for general use in recovering asymmetric velocity distributions. We have also constructed a hypothet- ical asymmetric distribution from adiabatic scattering in Argon-Argon to test the general method. The angle- and speed-components of the sample distribution were derived classically from a Lennard-Jones 6-12 potential, with collisions at 60 meV, and mapped onto Radon space to generate a set of discrete Doppler profiles. The sample distribution was reconstructed from these profiles using FTDS. Both distributions were compared along with derived total cross sections for the Argon-Argon system. This study serves as a template for constructing velocity distributions from bimolecular scattering experiments using the FTDS inversion technique

Angular distribution, kinetic energy distributions, and excitation functions of fast metastable oxygen fragments following electron impact of CO2

Dissociative excitation of CO2 by electron impact was studied using the methods of translational spectroscopy and angular distribution analysis. Earlier time of flight studies revealed two overlapping spectra, the slower of which was attributed to metastable CO(a3 pi) fragments. The fast peak is the focus of this study. Threshold energy, angular distribution, and improve time of flight measurements indicate that the fast peak actually consists of five overlapping features. The slowest of the five features is found to consist of metastable 0(5S) produced by predissociation of a sigma u + state of CO2 into 0(5S) + CO(a3 pi). Oxygen Rydberg fragments originating directly from a different sigma u + state are believed to make up the next fastest feature. Mechanisms for producing the three remaining features are discussed

An investigation of polarized atomic photofragments using the ion imaging technique

This thesis describes measurement and analysis of the recoil angle dependence of atomic photofragment polarization (atomic v-J correlation). This property provides information on the electronic rearrangement which occurs during molecular photodissociation. Chapter 1 introduces concepts of photofragment vector correlations and reviews experimental and theoretical progress in this area. Chapter 2 described the photofragment ion imaging technique, which the author has used to study the atomic v-J correlation in chlorine and ozone dissociation. Chapter 3 outlines a method for isolating and describing the contribution to the image signal which is due exclusively to angular momentum alignment. Ion imaging results are presented and discussed in Chapter 4. Chapter 5 discusses a different set of experiments on the three-fragment dissociation of azomethane. 122 refs