Spectroscopic investigations of molecular dynamics

Bell, Andrew John (1990) Spectroscopic investigations of molecular dynamics. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

A laser-molecular beam pump-probe experiment is used to investigate the photodissociation dynamics of HOCl by determining the internal state distributions of the nascent OH radical using laser induced fluorescence. The HOCl is carried in a molecular beam and spectra are recorded with various backing pressures of argon. Two theoretical models for photodissociation are compared. The mean rotational energies observed experimentally are far less than those predicted by the impulsive model. The rotational temperature of the OH fragment is found to correlate with the cooling of the parent HOCl achieved using different backing pressures of argon. The mean rotational energy of the OH fragment obtained with an expansion of HOCl/250 torr Ar is less than the model which indicates that for a rotationless parent the fragment rotation arises from the parent molecule zero point bending vibration. The alignment parameter, β0⁽²⁾(20), is estimated to be negative and almost limiting (im-0.4). This implies that the intense absorption at 219 nm dominates the photodissociation processes both at 248 nm and 266 nm. After the photolysis of HOCl the OH radical is shown to leave with a rotation in the original plane of the parent molecule. The resonance Raman technique is applied to the photodissociation of NOCl and Cl₂ at a number of visible and near uv wavelengths. The investigation of NOCl yielded only the ν2 and ν3 fundamentals at wavelengths longer than and including 355 nm. This may imply that the photodissociation at these wavelengths is not as fast as previously thought. At an excitation wavelength of 266 nm a short but significant progression comprising the fundamentals, overtones and combinations of ν2 and ν3 is observed. At an excitation wavelength of 355 nm, Cl₂ shows a progression up to ν= 12. The splittings observed in the progression are attributed to transitions from naturally occurring isotopic species. A broad underlying band is observed in this spectrum and is thought to be due to fluorescence from the neutral species Cl₃. The reactions of electronically excited barium atoms, Ba(¹P₁), with van der Waals clusters of CO₂ and H₂O are investigated using a crossed molecular beam apparatus. No chemiluminescence is observed from either of the reactions. This is regarded as evidence of the formation of a reaction product solvated in the van der Waals cluster. The total cross-sections for the inelastic processes Ba(¹P₁) + Ar → Ba(^3P_1) and Ba(^1P_1) + Ar → Ba(³P₂) are obtained by a crossed beam technique. Sub-Doppler spectra of the Ba(³P₂) state provide information on the differential cross-section for the Ba(¹P₁) + Ar → Ba(^3P_2). This data is deconvoluted to give an angular distribution. The distribution is predominantly back scattered with an overlying oscillation. The inelastic collision process is understood in terms of a potential energy curve crossing mechanism. The oscillation may be interpreted as originating from the quantum mechanical interference of almost identical trajectories.