Vibrational energy relaxation dynamics of diatomic molecules inside superfluid helium nanodroplets. The case of the I molecule.

The vibrational energy relaxation (VER) of a homonuclear diatomic molecule (X) in a He superfluid nanodroplet (HeND; T = 0.37 K) was studied adapting appropriately a hybrid theoretical quantum approach recently proposed by us. In the first application the interesting case of the I(X) molecule was examined and, as far as we know, this corresponds to the first theoretical investigation on the VER of molecules embedded in HeND. Vibrational relaxation of I takes place according to a cascade mechanism [sequential transitions between two consecutive vibrational levels (ν → ν - 1; ν - 1 → ν - 2; …; 2 → 1; 1 → 0), where an arbitrary relaxation, e.g., ν - 1 → ν - 2, can only occur once the previous relaxation has taken place, and so on]. The global relaxation from the initial excited state ν down to the ground state (ν = 0) happens on the nanosecond scale. Data on the VER of molecules in HeND are very scarce and a vibrational lifetime not far from the I one for ν = 1 has been estimated experimentally for Na(2E') on HeND (a qualitatively similar 0 → 1 vibrational energy gap occurs in both species), but metastability has not been reported in the second case. The cascade mechanism was understood once the values of the coupling matrix elements were examined, and the time evolution of the populations of two consecutive vibrational levels was adequately described using a non-linear two-state Hamiltonian model for I in the HeND. According to the calculations, superfluid liquid helium is very efficient in releasing the excess of energy arising from the I vibrational de-excitation, as it should be. However, the number of He atoms evaporated is small compared to what is expected. We hope that this first theoretical work on the molecular VER dynamics in HeND will encourage researchers to investigate this important process about which we still know very little.