Collision induced dephasing in fluorescence quantum beat of SO2((C)over-tilde1B2)

The collision-induced pure dephasing rate constant of a pair of coherently excited eigenlevels of SO2 is measured. This represents the first measurement of a collision-induced pure dephasing constant for a molecule in gaseous phase. The pair of eigenlevels arises from the coupling of the 1 (210)716 level and an isoenergetic high vibrational level, 44877.52 cm(-1) above the zero point, of the (X) over tilde state. The pure dephasing cross sections measured for the three colliding partners He, Ar, and SO2 at the temperature 4.8 K are respectively 26, 40, and 720 Angstrom(2). All three pure dephasing cross sections are about a factor of 2 smaller than the corresponding depopulation cross sections. Comparison with the hard sphere and Lennard-Jones cross sections suggests that collision-induced dephasing involves more than hard sphere, repulsive forces.