Experimental evidence of laser cooling of room temperature trans-stilbene upon excitation to the S1 state

Energy relaxation of a dissipative quantum oscillator

The dissipative harmonic oscillator is studied as a model for vibrational relaxation in a liquid environment. Continuum limit expressions are derived for the time-dependent average energy, average width of the population, and the vibrational population itself. The effect of the magnitude of the solute-solvent interaction, expressed in terms of a friction coefficient, solvent temperature, and initial energy of the oscillator on the relaxation has been studied. These results shed light on the recent femtosecond stimulated Raman scattering probe of the 1570 cm(-1) -C=C- stretching mode of trans-Stilbene in the first (S1) excited electronic state. When the oscillator is initially cold with respect to the bath temperature, its average energy and width increase in time. When it is initially hot, the average energy and width decrease with time in qualitative agreement with the experimental observations.

Photoisomerization of Stilbene: The Detailed XMCQDPT2 Treatment

We report the detailed XMCQDPT2/cc-pVTZ study of trans-cis photoisomerization in one of the core systems of both experimental and computational photochemistry-the stilbene molecule. For the first time, the potential energy surface (PES) of the S1 state has been directly optimized and scanned using a multistate multiconfiguration second-order perturbation theory. We characterize the trans-stilbene, pyramidalized (phantom), and DHP-cis-stilbene geometric domains of the S1 state and describe their stationary points including the transition states between them, as well as S1/S0 intersections. Also reported are the minima and the activation barriers in the ground state. Our calculations correctly predict the kinetic isotope effect due to H/D exchange at ethylenic hydrogens, the dynamic behavior of excited cis-stilbene, and trans-cis branching ratio after relaxation to S0 through a rather unsymmetric conical intersection. In general, the XMCQDPT2 results confirm the qualitative adequacy of the TDDFT (especially SF-TDDFT) picture of the excited stilbene but also reveal quantitative discrepancies that deserve further exploration.

Harmonic Theory of Thermal Two-Photon Absorption in Benzene

A correlation function formalism is applied to compute the two-photon absorption spectrum of benzene. Using harmonic Hamiltonians for the ground and excited electronic states, we find that the theory agrees qualitatively with the experimentally observed sparsity of the thermal two-photon absorption spectrum as compared with the single-photon absorption spectrum. An expression for the average vibrational energy in the excited state is derived. We find that cooling of the nascent vibrational energy in the electronically excited state is not as extensive in the two-photon absorption process as compared to the single-photon case.