Excited State Relaxation Dynamics in Benzophenone and meta-Methyl Benzophenone Revealed by Time-Resolved...

The ultrafast electronic relaxation dynamics of benzophenone and meta-methyl benzophenone following photoexcitation in the ultraviolet range is investigated using the femtosecond time-resolved photoelectron spectroscopy method. At pump wavelengths of 267.6 and 241.0 nm, the optically bright S₂(¹ππ*) state is directly excited and decays in 50 ± 10 fs via an efficient depopulation channel of internal conversion from the S₂(¹ππ*) state to the S₁(¹nπ*) state. Here, the subsequently populated S₁(¹nπ*) state contains substantial excess vibrational energy, and its decay mechanism is tentatively proposed as the following: the wavepacket decays rapidly (in ∼0.7 ps for benzophenone and ∼1.4 ps for meta-methyl benzophenone) out of the Franck-Condon region on the S₁(¹nπ*) state potential energy surface and probably bifurcates into two parts somewhere. One part undergoes ultrafast intersystem crossing to the T₂(³ππ*) state (and/or the T₁(³nπ*/³ππ*) state), while the other further decays to the S₁ state minima. The remaining wavepacket evolves out of the S₁ state minima over a time constant of 6-20 ps and finally funnels down to the triplet manifold via intersystem crossing. In addition, methyl substitution effects and excitation energy dependence on the excited-state dynamics of benzophenone are also discussed. This experimental study provides valuable insights into the decay dynamics of photoexcited benzophenone.