VELOCITY DEPENDENCE OF ENERGY POOLING COLLISIONS IN STRONTIUM

The threshold behavior of the cross section of an endothermic energy p ooling reaction is investigated. Zn a vapor cell experiment, pulsed; e xcitation followed by delayed fluorescence detection narrows the trans verse velocity distribution df colliding pairs of laser-excited stront ium atoms in the 5s5p P-3(1) level, and laser detuning establishes the collision velocity in the direction of counterpropagating laser-beams . A probe laser detects the excited-state collision product of the end othermic 5s5p 3P1 + 5s5p P-3(1) --> 5s6s S-1(0) + 5s(2 1)S(0) reaction . The cross section rises steeply at threshold and is nearly flat for 1.2-2 times the threshold energy. This behavior can be partly explaine d by the shape of the molecular potential curve of the collisionally p roduced state. From the measured 1.1 x 10(-12) cm(3) S-1 thermally ave raged rate coefficient, the above-threshold cross section is found to be approximately 6 x 10(-17) cm(2). The thermally averaged rate coeffi cient for the highly exothermic energy pooling reaction 5s5p P-3(1) 5s5p P-3(1) --> 5s5p P-1(1) + 5s(2 1)S(0) is also reported and measure d to be 1.1 x 10(-10) cm(3) s(-1). The cross section for this exotherm ic reaction is found to be nearly independent of collision velocity.