Dynamics of collisional relaxation of the optically induced anisotropy of para-quaterphenyl in the gas phase

The decay kinetics of the optically induced anisotropy of para-quaterphenyl molecules is theoretically and experimentally studied in a wide range of pressures of buffer gases, pentane and argon. The anisotropy of absorption is measured in real time using a femtosecond spectrometer. Comparison with the theoretical models of orientational relaxation shows that the collisional interruption of the angular momentum corresponds to a situation that is intermediate between the situations described by the J-diffusion model and by the model of the Fokker-Planck equation and is closer to the latter situation. In terms of the Keilson-Storer model, such a regime corresponds to the collision efficiency parameter γ ≈ 0.78 for argon and 0.66 for pentane; i.e., 4.5 collisions with argon or 2.9 collisions with pentane are necessary to completely randomize the angular momentum of para-quaterphenyl molecules.