The tilden lecture. Fundamental collisional processes of alkali and alkaline earth atoms studied by spectroscopic methods
Recent developments for characterising absolute rate data for fundamental reactions of ground-state alkali-metal atoms by direct spectroscopic monitoring in the time-domain are presented against the historical background of diffusion flames. Time-resolved atomic resonance absorption and fluorescence spectroscopy, together with the use of atomic resonance fluorescence techniques in fast-flow systems have led to an extensive body of fundamental rate data that may be compared, in some instances, with measurements derived from molecular beams and some which are of particular relevance to flame chemistry and to the chemistry of alkali-metal atoms in the mesos-phere. Emphasis is given to the kinetic study of the groups of the recombination processes between (i) Li, Na, K, Cs + O2+ M, (ii) Na, K, Rb, Cs + OH + M and (iii), K, Rb, Cs + I + M with detailed consideration of the role of ionic surfaces and the dynamics on those surfaces. Whilst limited consideration is directed towards the study of oxidation reactions of ground state alkaline-earth-metal atoms in diffusion flames and in molecular beams, the kinetic study of electronically excited alkaline-earth-metal atoms which are optically metastable to varying degrees and where collisional removal may compete significantly with radiative loss is dealt with in detail. The general nature of kinetic data for electronically excited alkaline-earth-metal atoms in 3PJ and 1D2 states derived from direct monitoring on low-pressure flames, discharge-flow systems, optical modulation methods and time-resolved emission following pulsed dye-laser excitation is presented. Detailed consideration is principally directed towards the study of diatomic oxidation products in specific vibronic states resulting from collisions between the electronically excited alkaline-earth-metal atoms and simple oxidants using the quantitative combination of time-resolved molecular and atomic emission, on the one hand, and both chemiluminescence and laser-induced fluorescence from molecular beams on the other. The kinetic study of product vibronic states, including the measurements of branching ratios, resulting from direct reaction and energy transfer is described. This quantitative, complementary combination of the time-domain and of the single-collision condition to investigate the details of such product states is emphasised as a major modern development in the study of atomic collisions in specific electronic states.