The UV adsorption and photodissociation dynamics have been extensively studied by spectroscopic analysis of both the atomic and diatomic photofragments. Central to the study is the UV absorption which consists of the extremely intense Hartley band (200 - 310nm) and the less intense structured tail to lower energy called the Huggins bands (310 - 360 nm). The dissociation channels along with their thermodynamic thresholds for the dissociation of vibrationally relaxed ground state ozone are as follows: (A) O₃ + h<i>n</i> (r) O(³<i>P_J</i>) + O₂ (<i>X</i> ³ <i>S</i>^-<i>_g</i>) (1178 nm) (B) (r) O(³<i>P_J</i>) + O₂ (a¹ <i>D_g</i>) (612 nm) (C) (r) O(³<i>P_J</i>) + O₂ (<i>b</i>¹ <i>S</i>⁺<i>_g</i>) (463 nm) (D) (r) O(¹<i>D</i>) + O₂ (<i>X</i> ³ <i>S</i>^-<i>_g</i>) (411 nm) (E) (r) O(¹<i>D</i>) + O₂ (a¹ <i>D_g</i>) (310 nm) The contribution of the spin-forbidden channels to Huggins band photodissociation was quantified using Resonance Enhanced Multiphoton Ionisation (REMPI) detection of the O (³<i>P₀</i>) fragments combined with a delayed pulsed field extraction technique. The translational angular properties of the photofragments were measured as part of this technique. The observed trimodal velocity distribution of the O (³ <i>P₀</i>) fragments was suggested to be a result of the different amounts of internal energy taken from the dissociation event by the diatomic co-fragments of channels (A), (B) and (C). However, a bimodal vibrational distribution of one of the channels could not be excluded at this stage as the slowest velocity peak of the O (³<i>P₀</i>) fragments may be due to formation of high vibrational levels of the diatomic fragments of channels (A) or (B). REMPI spectroscopy was used to detect the diatomic photofragments in a rotationally and vibrationally selective manner. In these experiments ozone was photolysed with tunable UV radiation and the fragments were state selectivity ionised using (2 + 1) REMPI <i>via</i> the O₂ (<i>d</i> 3<i>s</i><i>s_g </i>¹<i>II_g</i>) state using a second probe laser. This Rydberg state was found to be heavily perturbed by nearby valence state, yet it was possible to identify O₂ (<i>b</i> ¹ <i>S</i>⁺<i>_g</i>, <i>v</i> = 0) fragments produced <i>via</i> spin-forbidden dissociation from a singlet state of ozone excited by absorption in the Huggins band region.

This thesis is concerned with spectroscopic and dynamical studies of transient species in a flow reactor. For the SiCl radical various studies using Laser Induced Fluorescence were performed. The excitation spectra detailed the band positions of the lower lying vibrational levels of the B<SUP>2Σ+</SUP> and B'<SUP>2Δ states, with the bands appearing to be unperturbed but suffering from spectral congestion. Dispersed fluorescence studies of these bands enabled measurement of the vibrational transition probabilities between the ground and excited states. By comparing these probabilities with calculated values, estimates of the transition dipole moment functions for the B-X and B'-X transition were made: with the conclusion that the B-X moment was nearly constant with internuclear separation, while the B'-X moment was a strongly decreasing function with increasing internuclear separation. A simple, yet consistent, molecular orbital argument was presented as an explanation of the form of the transition moment functions. Because of the longer radiative lifetime of the B' state (</SUP>im 1.0μs compared to im 10ns for the B state) collision induced energy transfer of the B' state was readily observed experimentally. Total quenching cross sections for removal of v' = 0 and 1 of the B'-state are reported for a variety of nonpolar molecular colliders, with the magnitude of the cross sections appearing to be correlated with long range attractive forces between the collision pair. Partial cross sections for transfer from initially populated B' state to the B states are also reported, in this case, the cross sections may be influenced by specific resonant energy transfer processes. However, for transfer to v' = 2 of the B state (for which no resonant processes were obvious) the magnitude of the partial cross sections again appeared to be correlated with long range attractive forces. Resonance Enhanced Multiphoton Ionisation studies were performed on the HI molecule, formed as a result of the reaction:H+ I<SUB>2 → HI</SUB>+ I. Spectroscopic observations on this molecule are reported at selected positions between 67500 and 73500 cm^-1. Wherever possible effective molecular constants were calculated from the observed spectra. While various transitions were evident from ground state hot bands, observation of nascent rotational or vibrational product state distributions, of the reaction, were not made in these relatively slow flow conditions. The complementary nature of the single photon (vuv) and two photon absorption techniques was highlighted by the apparent inversion of intensities with which bands were observed by the two techniques. Bands previously unobserved in the single photon absorption work are reported. The bands are assigned as transitions to the i^3Δ(2) and d<SUP>3Πo</SUP>(0±) states.

No description available.

No description available.

The techniques of supersonic molecular beam cooling and laser multiphoton ionisation (MPI) spectroscopy have been combined to give a potentially powerful analytical technique. Two methods of sample introduction into the molecular beam have been employed, namely resistive heating and laser desorption. The resistive heating method allowed 2-colour MPI spectra of naphthalene, anthracene and perylene to be recorded in a simple free jet apparatus. A sensitivity for anthracene of 600 ppb is estimated. A time-of-flight (TOF) mass spectrometer has been developed, incorporating both linear and reflecting-geometry (reflectron) flight tubes, to allow laser desorption MPI (LD-MPI) mass spectra to be recorded for a number of involatile and thermally unstable compounds. Mass resolutions of 300 (linear) and 850 (reflectron) have been obtained for aniline. The constraints affecting the mass resolving power of both spectrometer designs are discussed. Finally, the potential of LD-MPI mass spectrometry as a sensitive, selective analytical technique is evaluated. The mass spectra of a number of polynuclear aromatic hydrocarbons, porphyrins and amino acids are presented, along with those of a simple mixture of the three aromatic amino acids tryptophan, tyrosine and phenylalanine. A sub-nanomole detection limit is estimated for tryptophan. Means to improve the sensitivity of the technique are discussed. The wider analytical applications of LD-MPI mass spectometry are considered.

New results for the threshold ionisation of the molecules, HF, DF, HCl, DCl, HBr, DBr, HI and F<SUB>2</SUB> have been recorded using threshold photoelectron spectroscopy (TPES) at resolutions down to 3 meV. The entire outer valence ionisation region has been studied for all of these molecules, and additionally, the inner valence region of the molecules not containing fluorine has been recorded. This represents a significant volume of consistent spectroscopic data for the halogen containing diatomic systems. The completeness of the study utilising both hydrogen isotopes has proven very useful for the in-depth analysis of the TPE spectra, allowing each system to be analysed by combining the data for both hydrogen isotopes. This has permitted the production of improved spectroscopic constants compared to previous TPE work. In all study molecules, the TPE spectrum of the lowest electronic (X) state shows extended vibrational progressions due to the autoionisation of super-excited neutral (Rydberg) states, producing the molecular ion and a threshold electron. The symmetry of the autoionising Rydberg states can be tentatively assigned from the position of enhancements in the TPE spectra. The assignments suggest that the Rydberg states that autoionise to produce threshold electrons do not have the same symmetry as the Rydberg states seen most strongly in photoabsorption and photoionisation spectra. The similarity between the recorded TPE spectra and fluorescence excitation spectra in the literature suggests the predominant autoionisation route leading to threshold electron production could be via repulsive intermediate states. The first excited (A) ion state for HX and DX species is well known to be predissociated for all systems except X=F. The TPE spectra of the predissociated vibrational levels in the HCl, DCl, HBr, and DBr systems show evidence of an additional ionisation mechanism that is not seen for non-resonant ionisation. These additional mechanisms have been modeled as being due to a propensity for a large loss of rotational angular momentum from the ion core during the ionisation process. It is suggested that these mechanisms involve the rotational autoionisation of dissociative Rydberg states into dissociative ion states accompanied by the ejection of a near threshold electron.

No description available.

No description available.

No description available.

A series of experiments was undertaken to establish the spectroscopic basis of subsequent dynamical studies of elementary bimolecular reactions in the gas phase, in which laser methods are used both for the preparation of reagents and the state-specific detection of products. An apparatus was constructed which allowed gas phase molecules to be detected spectroscopically at very low densities in a state-specific fashion by either laser-induced fluorescence (LIF) or resonance-enhanced multiphoton ionization (REMPI). In REMPI experiments, further species discrimination was achieved by time-of-flight detection of the ions produced. An attempt was made to investigate the dynamics of reactions of the type Cl(<SUP>2</SUP>P) + HX (where X=Br, I) by combining REMPI detection of the products with laser photolytic production of the Cl(<SUP>2</SUP>P) atoms. It was successfully demonstrated, by direct REMPI detection, that Cl(<SUP>2</SUP>P) atoms were produced with an anisotropic spatial distribution. Due to heterogeneous reactions occurring on the surfaces of the inlets necessary to introduce the reagent precursor gases, it was, however, not possible to unambiguously identify any nascent HCl product molecules. Some preliminary measurements were also made of coherent anti-Stokes Raman spectroscopy (CARS) of HCl, with the intention of establishing the viability of the intimately related stimulated Raman pumping process for the state-selective preparation of molecular reagents in dynamical studies. In particular, the polarization properties of different spectroscopic branches were examined, allowing the feasibility of the preparation of a spatially anisotropic sample to be assessed.