Rotational relaxation within the 1B2u(S1) state of benzene

Abstract

Relaxation from nonequilibrium rotational distributions to a Boltzmann‐like distribution within the zero point level of S 1 benzene vapor is studied using ground state benzene and argon as collision partners. The initial distributions are established by excitation with a narrow‐band (0.3 cm−1) laser tuned to various positions within the rotational contour of the 60 1 absorption band. The course of rotational relaxation is then followed by observations of the dependence of rotational contours in fluorescence bands upon gas pressure. Arguments are given to suggest that relaxation approximates a single‐step process, with collisions taking the initial nonequilibrium distribution directly to a broad distribution of J′K′ levels reminiscent of the Boltzmann distribution. The rate constant for this relaxation is about 9×107 torr−1 sec−1 for benzene as a collision partner, which is about eight times the gas kinetic value. This constant is observed for each of two different initial distributions. It is likely that switching of electronic energy in collisions between S 1 benzene and rotationally equilibrated S 0 benzene makes a substantial contribution to this relaxation. The rate constant for relaxation in argon collisions is much smaller and depends on the initial distribution, being 1.5×107 and 3.5×107 torr−1 sec−1 for the two cases measured. Some applications of these data to rotational effects in radiationless transitions are discussed.