Cross sections of excitation transfer between the fine-structure sublevels of Cs(7 2 P), Rb(6 2 P) and Rb(7 2 P) induced by collisions with rare-gas atoms have been determined from the time dependence of the sensitised fluorescence. The Cs and Rb atoms, contained in absorption cells filled with inert gases at different pressures, were excited using a pulsed dye laser pumped by a nitrogen laser. The fluorescence was observed time resolved employing a photon counting technique. The use of a two-level approximation allowed the Maxwellian (velocity) averaged transfer cross sections to be calculated. Comparison of the cross section values determined in this work with the values quoted by other authors shows good consistency in the case of Rb 6 2 P inert-gas collisions while in the case of Cs 7 2 P inert-gas collisions there are some discrepancies. Rb 7 2 P excitation transfer has not been dealt with before.
The dependence of energy pooling cross sections on the alignment of atomic angular momentum with respect to the collision axis is investigated. In a vapor cell, excitation with counter-propagating polarized laser beams that define the collision axis prepares aligned strontium atoms in the
5s5p3P1 level. Excited-state energy pooling collision products from the endothermic $5s5p{}^{3}{P}_{1}+5s5p{}^{3}{P}_{1}\ensuremath{\rightarrow}5s6s{}^{1}{S}_{0}{+5s}^{2}{}^{1}{S}_{0}$ reaction and the exothermic $5s5p{}^{3}{P}_{1}+5s5p{}^{3}{P}_{1}\ensuremath{\rightarrow}5s5p{}^{1}{P}_{1}{+5s}^{2}{}^{1}{S}_{0}$ reaction are studied. Alignment effects are studied as a function of collision energy, using pulsed laser excitation followed by delayed fluorescence detection to lower the transverse collision velocity and laser detuning to vary the collision velocity in the laser-beam direction. The rate coefficient for energy transfer to the
61S0 state is enhanced by $38\ifmmode\pm\else\textpm\fi{}13%$ when the
MJ alignment of the collision partners is perpendicular to the collision axis and parallel to each other compared to when there is no alignment of the collision partners. No alignment dependence is observed for energy transfer to the
51P1 state. This experiment offers the unique capability of simultaneously and independently controlling collision velocity and angular momentum alignment.