Rotational Relaxation ofBenzophenone in the VapourPhase

The excited singlet state lifetimes of benzophenone vapour have been measured at shorter wavelengths than previous studies and with picosecond time resolution. Excitation was at a series of wavelengths from 313.5 nm to 284 nm, which gave S2( Π Π *) decay times of tens of picoseconds. We also report measurements of the polarisation anisotropy decay due to free rotation of excited state benzophenone. Time-dependent anisotropies calculated by both quantum mechanical and classical formulae are in good agreement with the experimental data and demonstrate benzophenone to behave as a rigid rotor. From the residual anisotropy, at long times, it is concluded that no rotation to vibrational energy transfer occurs during the excited state lifetime.