Relaxation process of CO stretching mode of metal carbonyls adsorbed on Y zeolites studied by pump-probe experiment

The vibrational relaxation lifetimes of Mo(CO)₆ and Cr(CO)₆, adsorbed in a cage of HY, DY, and NaY zeolites, were measured at various temperatures by the pump-probe method using picosecond infrared laser pulses. The lifetimes at 110 K of the υ = 1 level of the CO stretching mode were 70±4, 53±5, and 150±9 ps for Mo(CO)₆ on HY, DY, and NaY zeolite, respectively, and 87 ±6, 74±7, and 140±12 ps for Cr(CO)₆, respectively. It was shown by comparison of the lifetimes at 110 K, as well as the temperature dependence, that the rate of vibrational relaxation is determined by the coupling of the CO stretching mode with four quanta of the accepting modes comprised of: (i) modes with displacement of central metals Cr or Mo in all of the three zeolite systems, (ii) modes associated with the OH and OD groups on zeolite surface in the HY and DY zeolite systems, and probably (iii) the frame vibrational modes of zeolite with energies at 300-800 cm^-1.