Ultrafast quantum dynamics and resonance Raman spectroscopy of photoexcited I₂(B) in large argon and xenon clusters

The early quantum dynamics following the B(³Π_0u+)→-X photoexcitation of I₂ in large rare gas clusters is studied and the resonance Raman spectrum of these systems is calculated by a novel time-dependent quantum mechanical simulation approach. The method used is the classically based separable potential (CSP) approximation, in which classical molecular dynamics simulations are used in a first step to determine an effective time-dependent separable potential for each mode, then followed by quantum wavepacket calculations using these potentials. In the simulations for I₂(Ar)_n and I₂(Xe)_n, with n = 17, 47, all the modes are treated quantum mechanically. The Raman overtone intensities are computed from the multidimensional time-dependent wavepacket for each system, and the results are compared with experimental data on I₂ in Ar matrices and in liquid Xe. The main findings include: (i) Due to wavepacket dephasing effects the Raman spectra are determined well before the iodine atoms hit the rare gas "wall" at about 80 fs after photoexcitation. (ii) No recurrencies are found in the correlation functions for I2(Ar)n. A very weak recurrence event is found for I₂(Xe)_n. (iii) The simulations for I₂(Ar)₁₇ (first solvation layer) and for I₂(Ar)₄₇ (second solvation shell) show differences corresponding to moderate cluster size effects on the Raman spectra, (iv) It is estimated that coupling to the B″(1Π_1u) state or to the a(1g) state have a small effect on the Raman intensities, (v) For I₂(Ar)₄₇, the results are in very good quantitative agreement with I₂/Ar matrix experiments. The I₂(Xe)_n results are in qualitative agreement with experiments on I₂ in liquid Xe. The reported calculations represent a first modeling of resonance Raman spectra by quantum dynamical simulations that include all degrees of freedom in large systems, and they demonstrate the power of the CSP method in this respect.