Velocity dependence of energy pooling collisions in strontium

The threshold behavior of the cross section of an endothermic energy pooling reaction is investigated. In a vapor cell experiment, pulsed excitation followed by delayed fluorescence detection narrows the transverse velocity distribution of colliding pairs of laser-excited strontium atoms in the 5s5p³P₁ 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 endothermic 5s5p³P₁+5s5p³P₁→5s6s¹S₀+5s²¹S₀ 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 explained by the shape of the molecular potential curve of the collisionally produced state. From the measured 1.1×10^-12 cm³ s^-1 thermally averaged rate coefficient, the above-threshold cross section is found to be approximately 6×10^-17 cm². The thermally averaged rate coefficient for the highly exothermic energy pooling reaction 5s5p³P₁+5s5p³P₁→5s5p¹P₁+5s²¹S₀ is also reported and measured to be 1.1×10^-10 cm³ s^-1. The cross section for this exothermic reaction is found to be nearly independent of collision velocity.