Collisionally induced intramultiplet mixing of Sr(5 3P J) metastable states by He, Ar and Sr ground state atoms

Abstract

Intramultiplet collisionally induced mixing within the Sr[5s5p( 3P 0,1,2)] manifold is investigated by time-resolved laser-induced fluorescence (LIF) following the pulsed dye-laser generation of Sr( 3P 1) of the electronic ground state {Sr[5s5p( 3P 0,1,2)]←Sr[5s 2( 1S 0)], λ=689.26 nm} at elevated temperatures. The population profiles of the three spin–orbit states were individually monitored by LIF as well as that of Sr( 3P 1) by spontaneous emission at the resonance wavelength. A kinetic model is employed that enables the process of spontaneous emission from Sr( 3P 1) to be isolated initially and characterised by experiment. Particular emphasis is placed on the modelling procedure itself in which the separate kinetic component due to spontaneous emission and the positions of the maxima in the 3P 0 and 3P 2 population profiles constitute severe constraints on the model. The collisional components within the model are reduced to three rate constants where pairs of J states are connected in this context by detailed balance. Thus k 10 and k 12, and, by detailed balance, k 01 and k 21 are quantified at various temperatures and pressures to yield the absolute value of these collision properties for Sr( 3P J ) with He and Ar. Rate data for collisionally induced intramultiplet mixing in Sr( 3P J ) by Sr( 1S 0) itself is also reported found to proceed at close to unit collisional efficiencies in all cases. Thus, at elevated temperatures, variations in atomic profiles are dominated by the differing vapour pressures of atomic strontium. Estimates of the activation energies associated with k 10 and k 12 for the noble gases observed against this large competing background are found to be of the order of the spin–orbit splittings. The model overall is found to be insensitive to k 02 and k 20 whose magnitudes are small by comparison with those for the collisional rate data connecting adjacent J states. Whilst collisional processes for He are some two orders of magnitude more efficient than those for Ar, all of these are seen to be ‘adiabatic’, in contrast with the gas kinetic rate constants of ground Sr, considered to be ‘sudden’ in character. The results are compared with analogous data derived by atomic resonance absorption spectroscopy following pulsed generation of Sr( 3P 1) and are considered in the context of theoretical calculations employing quantum close coupling calculations.