Vibrational Relaxation of N2 Aggregates in Rare Gas Matrices

Abstract

In an experimental investigation of the conversion of electronic energy into vibrational degrees of freedom of a cluster followed by dissipation into matrix phonons it is essential that all intermediate steps of the energy flow can be monitored. For this purpose each vibrational level v'=1 up to v'=6 of the lowest excited electronic state A 3 Σ u + of N2 in a solid Xe matrix has been excited selectively by monochromatized synchrotron radiation. The stepwise relaxation to lower A 3 Σ u + vibrational levels has been derived from luminescence spectra. The large vibrational energy of 174 meV corresponding to 35 matrix phonons inhibits vibrational relaxation in completely isolated N2 molecules within the radiative lifetime of ≈ 10-3s. For N2 pairs and larger clusters a complex relaxation cascade is observed which leads to different steady state distributions for each initial level. The first relaxation steps involve only changes of v' by two quanta. For excitation of v′ ≥ 4 states also intermediate vibrational levels are populated in the end of the cascade. The experimental results indicate the following two basic processes: Radiationless electronic energy transfer between two N2 molecules based on exchange interaction leads to a deexcitation of molecule I from the initially (v')A 3 Σ u + state to the groundstate ( v ' ' = 1 ) × 1 Σ g + and the neighbouring molecule II is excited from ( v ' ' = 0 ) x 1 Σ g + to ( v' − 2 ) A 3 Σ u + corresponding to a reduction of v' by two quanta.