Fluorescence spectrum of Mo2 in argon and krypton matrices

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Abstract
The time resolved fluorescence spectra of Mo2 isolated in Ar and Kr matrices at 14 °K are presented. Depending on excitation wavelength, either of two separate vibrational progressions with 475.7 (±4.5) cm−1 spacings can be observed in emission. Similarities between several features in the two emission spectra lead to the conclusion that the progressions arise from Mo2 trapped in two distinct sites, each characterized by its own absorption spectrum. Excitation spectra elucidate the overlapping absorption spectra of the two sites. The measured emission lifetimes are long and not single exponential. In krypton matrices, lifetimes of 0.85 msec from one site and a biexponential emission decay of 0.70 and 1.94 msec from a second site are observed. In argon matrices, 2.1 msec emission decay times in one site and a triexponential decay with components of 2.1, 3.93, and 6.3 msec were measured in a second site. Phonon structure in the well resolved spectra show that the matrix coupling is different for the two matrix sites. Various empirical correlation rules are used to test the validity of ωe as well as to compare it to other experimental quantities for Mo2. These rules indicate a 1.9 Å bond length for Mo2. The fluorescence lifetimes and the large spectral shift of the emission relative to the excitation energy lead to the conclusion that the emission does not emanate from the initially excited level. Instead, the emission appears to come from one of the higher spin multiplicity levels in a dense manifold of states lying at somewhat lower energies than the 1Σu+ level coupled by laser radiation to the 1Σg+ ground state.

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