Conventional supersonic jet spectroscopy is limited by the thermal stability of the molecules under investigation. The molecules of interest are heated and the resultant vapour mixed with the carrier gas before cooling. This method has the drawback that only volatile and/or thermally stable molecules may be studied. Laser desorption, in contrast, utilises an infra-red laser pulse to desorb molecules directly from the solid state into the gas phase, and enables thermally labile molecules to be vaporised without decomposition. Thus, the use of laser desorption allows a greater range of molecules to be investigated using the technique of jet cooling. A specialised desorption 'faceplate' was used in a supersonic jet apparatus to interface the laser desorption and collisional cooling processes. Molecules were thus desorbed directly into the path of the pulsed supersonic jet, such that they were entrained in the jet carrier gas and cooled. These cold and isolated molecules were then investigated as in a conventional free jet, using laser-induced fluorescence (LIF) and resonant two-photon ionisation (R<SUP>2</SUP>PI) spectroscopy. The S<SUB>1</SUB>←S<SUB>0</SUB> excitation spectra of various molecules have been investigated and well-resolved vibronic spectra obtained. The efficacy of this technique has been validated using carbazole, benzoic acid and para-amino benzoic acid. The potential of laser desorption supersonic jet spectroscopy as an analytical tool was demonstrated by the detection of carbazole desorbed directly from real-world samples such as pollution filters and contaminated soils. A nonresonant model of pulsed IR laser desorption using the 10.6μm line of a CO<SUB>2 </SUB>laser is proposed for a sample of several mm thickness. Molecules are desorbed from the surface of the sample via absorption of the laser radiation into the near-surface bulk phonons. This energy is transferred to the bonds holding the surface molecules to the bulk leading to desorption of the molecule.

No description available.

A novel tandem time-of-flight (TOF) mass spectrometer has been developed for studying the photo-induced dissociation of large molecules and elemental clusters. It consists of a linear first stage TOF analyser for primary mass separation and precursor ion selection, and a second orthogonal reflecting field TOF analyser for product ion analysis. The instrument is equipped with a large volume throughput molecular beam source chamber allowing the production of jet-cooled molecules and molecular clusters, as well as elemental clusters, using either a pulsed laser vaporisation source (LVS) or a pulsed arc cluster ion source (PACIS). A second differentially pumped chamber can be used with effusive sources, or for infrared laser desorption of large molecules, followed by laser ionisation. These primary ions can then be irradiated with a second, high energy laser to induce photodissociation. Detailed information about the fragmentation mechanisms can be deduced from the product ion mass spectra. A theoretical overview of the technique of tandem time-of-flight mass spectrometry is presented, together with a detailed description of the experimental procedures and equipment used. In order to assist with the design and optimisation of the instrument a "virtual" mass spectrometer was drawn to scale using the SIMION software program, in order to simulate ion trajectories for differing voltage, geometry's and dimensions of the ion optics. An ion gate was designed and manufactured to provide primary mass selection following the first time-of-flight mass analyser. The device consisted of four layers of interleaved wires; primary ions could be selectively transmitted by application of a fast rising high voltage pulse to the middle set of wires. The mass gate was measured to have a mass resolving power, m/Dm = 30.

No description available.

The results of studies of the higher electronic states of small halogenated molecules using resonance enhanced multiphoton ionisation (REMPI) spectroscopy are presented. A substantial part of this work concerned the Rydberg states of some methyl halides (methyl bromide, methyl iodide and methyl chloride). The assignment of these states was considerably added by polarisation studies. Also, the propensity rules apparent in the VUV and two-photon spectra of the diatomic halogens and interhalogens were found to be useful. Thus, features comprising the two-photon spectra of the methyl halides were assigned to Ω=0 and Ω=2 components of <i>nd</i> and <i>np</i> states. In the three-photon spectra, transitions to the above states were observed along with transitions to <i>ns, np, nd</i> and <i>nf</i> states, all with Ω=1. In fact, three-photon spectra are seen to be dominated by <i>nf</i>;1 series while <i>nd</i> series dominate the two-photon spectra. This study involves a considerable reappraisal of the identity of the states underlying Rydberg transitions in these methyl halides, apart from the first <i>ns</i> cluster. In a similar vein, Rydberg transitions in trifluoromethyl iodide were also investigated but their vibrational envelopes were found to depart from those seen for the methyl halides. This can be attributed to two causes: first, the increased amount of valence character of the orbital from which an electron is being promoted to a Rydberg orbital, as suggested by the appearance of the photoelectron spectrum, and second, the possibility of Rydberg-ion-pair interactions. This increased amount of valence character in the orbital from which an electron is promoted in a Rydberg transition also complicates the analysis of the two-photon spectrum of dichloromethane, especially in the energy region just below the first ionisation energy. Even so, the observed band systems are assginable to <i>nd</i> states of A₁ symmetry. Given that propensity rules in the spectra of the halogens are used to aid the spectral assignment of the above polyatomic molecules, an examination of the VUV spectroscopy of molecular chlorine in the 72000-77000 cm^-1 region, using (1+1') REMPI, is included as an example of the operation of these propensity rules.

Vacuum-UV emission spectra were obtained in a discharge flow system for reactions of bromine-, chlorine- and iodine containing reagents with metastable argon atoms (Ar(3P2~0). Halogen atomic lines were observed for many of these spectra. Noble gas halide spectra were identified for the reactions of the bromine- and chlorine- containing reagents (ArBr- and ArCl- spectra). HC1 and DC1 spectra were observed for the reactions of these reagents. The pressure- and temperature-dependence behaviour of some of the argon halide spectra as well as their structure was studied, and the spectra were analysed by simulation calculations. Detailed studies of the effect of the various parameters on the calculated spectra were carried out for the simulation of the ArBr spectra. This showed that important information on the various parameters could be derived from the simulation calculations. Spectral contributions due to three different bound-free transitions (B-X, C-A(3/2)) and B-A(1/2)) and the spectral ranges of these were identified for some ArBr spectra. Only the very lowest vibrational levels in the excited states were found to emit due to an efficient predissociation of high vibrational levels. The occurrence of a collision induced crossing between the emitting states (B and C) and a vibrational relaxation in these states was observed. Estimates were made of relative rates of collisional deactivation processes and population distributions in the emitting states (B and C) for some of the spectra. Approximate potential curves (X, A(3/2), B and C) were obtained and the transition moments for the transitions B-X' and C-A(3/2) were found to vary rapidly with internuclear distance. Analyses of some ArCl MC spectra made it possible to estimate the population distribution in the ArCl B-state and the occurrence of a rapid vibrational relaxation in the B-state was observed. Approximate potential curves (X, B) were also obtained.

No description available.

The technique of time-resolved resonance absorption spectroscopy was used to monitor the decay of photolytically produced CH radicals. Care was taken to ensure that any vibrationally excited CH, produced in the primary photolytic process, was relaxed co the ground state. Thus rate constants given in this work apply strictly for the removal of CN radicals in the absence of feeding by vibrational relaxation of higher levels, in contrast to previous work where this was not taken into account. For the first time, rate constants are reported for the reactions of CH radicals with the molecules N20, D200 NCI, DCl, CO2, H20, OCg, CO and HCN. No isotope effects were observed for the dauterated species. Rate data for the reactions of CN with 02, N0, H2, CH4 and C2H6 were also obtained and compared with previous data. The data show that recombination of CN radicals in the presence of both NO and CO occurs by a mechanism analogous to that for catalysed recombination of halogen atoms. A small negative activation energy (-10±4 kJ mol 1), characteristic of such processes, was calculated for the NO case. The reaction of CN with H2 vas also studied and compared with the CH case. However, it was not possible to state categorically that rate enhancement occurred with vibrational excitation, as vibrational to rotational energy transfer nay have bean important. Hence, only an upper limit of 1.7 is given for the rate constant ratio. A value of AN of (CH) - 417.6±1.3 W moll was determined using the equilibrium 293 content for the reaction CN + HCN C2N2 + H. This is in agreement with several recent results, reinforcing the need for a change in the currently accepted value. Work on the study of CA radicals in flames at Thornton Research Contra (Shell Research Ltd.), using the technique of resonance fluorescence, led to several practical suggestions for improving this technique, especially the use of a tunable dye laser to excite the CH radicals.

Matrix assisted laser desorption/ionisation time-of-flight mass spectrometry was used to mass analyse a range of synthetic polymers. Synthetic polymers with average molecular weights of up to 20000 Da were investigated. The polymers studied included polyglycols, polystyrene and poly(methyl methacrylate). Information on the repeat units, endgroups and average molecular weights was obtained. The stability of metal ion/polymer adducts was studied for poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG) and poly(methyl methacrylate) (PMMA) adducts of lithium, sodium, potassium and rubidium, as well as the silver ion adduct of polystyrene. No post source decay (PSD) was seen for the lithium and sodium adducts of PEG, PPG and PMMA, however potassium and rubidium adducts of these polymers did undergo PSD. Rubidium adducts were seen to decay more readily than the potassium adducts. Pulsed-field delayed ion extraction experiments were carried out. These experiments suggest that gas-phase reactions contribute comparatively little to the cation adduct formation of synthetic polymers. Further experiments showed that the ratio between salt and matrix in the sample did not affect the ionisation behaviour. During the course of this work several improvements were made to the mass spectrometer used. The length of the time-of-flight mass analyser was increased and an inline detector was fitted to the existing instrument. The addition of the second detector made it possible to operate the instrument in a linear mode. A mass gate was added to make it possible to avoid detector saturation by low-mass ions.

No description available.