A quantum mechanical derivation of Beer's Law is presented. The expression includes a term for the probability of radiationless energy transfer during gas-kinetic collisions. The theory indicates that this effect is the major contributor to the pressure intensity enhancement observed in the infrared spectra of small gaseous molecules in the presence of a foreign (non-absorbing) gas. Further investigation of the phenomenon shows that it is due to deactivation of the 1st excited vibrational state by inter-molecular collisions. The derived equation is: ln(Io/I)= Bmn C'd-2 ln [(z+(1-Tt) Amn)/(Z+(1-To)Amn)] where Bmn is the Einstein coefficient of induced absorption, C' is the concentration, d is the pathlength, Z is the probability of radiationless energy transfer, Amn is the Einstein coefficient of spontaneous emission, andTt and To are the fractional transmissions of the transmitted and incident light respectively. The effect is studied by reference to a number of small gaseous molecules (CH4, C2H6, C2H4, C2H2, CO, CO2 NO, HCN, HC1 and CH3Br) in the foreign gases N2, 02, He and CO2 using a grating spectrometer equipped with a pair of 40 metre long path gas cells. The investigation involves the following stages: (a) measurement of the observed pressure enhancement factors (b) determination of the absolute intensity of individual vibration-rotation bands. (c) calculation of the Einstein probability coefficients Bmn and Amn (d) computation of the collisional deactivation probabilities (Z), relaxation times (Y) and Napier numbers (Z10) for each transition. The mechanism of vibration - translation and rotation - translation energy conversion is discussed, the relationships between various molecular parameters and the observed spectra are examined, and comparison is made between measured and literature values obtained by acoustical experiments. Some areas of interest for future research are discussed.

We developed and tested a new approach to treat low energy electron collisions with molecular clusters, called Multiple Scattering, which simplifies the scattering process by dividing the target cluster into molecular sub-units; ab initio methods are employed to calculate collisional data for the electron - sub-unit scattering process, which is later combined by the Multiple Scattering method to account for the interference between sub-units. We applied the novel method to the scattering from water and formic acid clusters; the results (cross sections) were compared to other theoretical and experimental results, showing good agreement. The ab initio R-matrix method was employed both for producing the collisional information on the sub-units and also for calculating comparison cross sections where previous results were not available.

The characterisation and commissioning of a new molecular beam apparatus is described. This equipment was designed to have the capability of observing coincidence signals between photon emission detection and scattered particle detection resulting from ion-molecular and ion- atom collisions. Early experiments involving the observation of photon emission, which resulted from non adiabatic charge exchange processes in He<SUP>+</SUP>/Ar collisions, are detailed, A mechanism is proposed for the excitation of the <SUP>2</SUP>p <SUP>o</SUP> and <SUP>2</SUP>D <SUP>o</SUP> states of Ar<SUP>+</SUP>. in those collisions. Excitation resulting from charge exchange processes in the He<SUP>+</SUP>/HC1 collision system are also described. In particular a strong non Franck-Condon effect is observed in the excitation of the HC1<SUP>+</SUP> A <SUP>2Σ+</SUP> state. The unexpectedvibrational population of the HC1<SUP>+</SUP> A <SUP>2Σ+</SUP> state is examinedunder a variety of conditions and is contrasted to the vibrational distribution of that state resulting from collisions between Ar<SUP>+</SUP> and HC1. A mechanism is proposed which accounts for these findings. Finally the principles and execution of the first coincidence experiment are described. This experiment involved the observation of emission from the N<SUB>2</SUB>C<SUP>3πu</SUP>state, excited in collision with He<SUP>+</SUP>, and recording the time differential between the detection of those photons and the detection of the associated scattered ions. The results of this experiment are detailed and suggestions made as to the reasons for our failure to observe a clear result.

No description available.

No description available.

This thesis describes studies of collisional energy transfer in two small, combustion relevant free radicals. Collision-induced electronic energy transfer (EET) between the B2r and A2 fj. states of the CH and CD radicals was investigated with the collision partners He, Ar, H2, N2, CO and C02 using a dispersed laser induced fluorescence technique. CH or CD radicals were prepared photolytically, and excited into a single rotational level in either of the B2::E-, v ==°or A211, v =1 levels. Wavelength dispersed, time-resolved emission was then recorded from the initial and product states. Microscopic rate constants for vibronically resolved transfer between the two electronic states were determined for each collision partner, as well as those for vibrational energy transfer in the A state and total removal to other, unobserved states. EET was demonstrated to be ubiquitous, occurring with all of the collision partners used in the study, with varying efficiencies. These relative efficiencies were found to correlate well with long range attractive forces exerted by the collider. No special enhanced efficiency was observed with those colliders chemically reactive towards CHID. Transfer from B2::E- to A2 1::,. was not greatly affected by the significant differences to the vibronic energy level structure -between CH and CD, demonstrating the inapplicability of empirical energy gap scaling laws to this process. The collisional evolution of alignment and orientation moments in the OH radical was investigated using polarisation spectroscopy (PS). One-Colour PS measurements were made using the first five P branch lines in the A2::E+ - x2n (0,0) band of OH, in the presence of fixed pressures of He, Ar or N2. The time delay between pump and probe laser pulses was varied to inspect the remaining alignment or orientation in the sample as a function of time after the pump pulse. The results of these experiments were analysed using a detailed theoretical treatment developed previously in the laboratory, producing rate constants and cross-sections for the collisional decay of alignment and orientation over the range of quantum states and collision partners studied. While some uncertainty exists in the numerical values of the results, due to the observation of an apparently pressure-independent depolarisation process, clear phenomenological distinctions were found between the collision partners He, Ar and N2, over the range of studied quantum states.

No description available.

No description available.

No description available.

No description available.