IR spectra and vibrational dephasing of the CO stretching mode in W(CO)6 doped cryogenic matrices

Environment effects on the vibrational dynamics of tungsten hexacarbonyl in cryogenic matrices are investigated with the use of the CLIO infrared free-electron laser by measuring the population relaxation time T1 in the pump-probe configuration and the dephasing time T2 in the two-pulse photon-echo arrangement. The probe molecule W(CO)6 is selected because of its exceptionally large vibrational transition dipole moment on the CO stretching mode. Results in cryogenic matrices are obtained for the first time. In these solids (Ar, N2, CH4) in the 6-30 K temperature range, the degeneracy of the T1u IR-active mode is removed. In all matrices, the T1 time remains of the same order of magnitude, also as measured in other systems, suggesting that T1 is only weakly influenced by the nature of the host. The W(CO)6 absorption band is inhomogeneously broadened and the dephasing time T2, which is long in the argon host (T2 = 2T1 = 400 ps) is considerably shortened in the molecular matrices, nitrogen or methane (T2 = 40-70 ps at 7 K). Interestingly also, the absorption spectrum of W(CO)6 in the methane host becomes narrower upon a temperature increase. In these two systems, specific dephasing mechanisms exist while these are absent in the case of argon. They can be correlated to the existence of additional orientational degree of freedom of the host, libration in the case of nitrogen, and nearly free rotation in the case of methane.