Ultrafast vibrational relaxation dynamics of carbonyl stretching modes in Os₃(CO)₁₂

The vibrational relaxation dynamics of the four infrared active carbonyl (CO) stretching normal modes of Os₃(CO)₁₂ at 2068 cm ^-1, 2034 cm^-1, 2014 cm^-1, and 2002 cm ^-1 were measured using broad-band frequency resolved pump-probe spectroscopy. Transient absorption spectra of these modes were collected, and the fundamental, overtone, and combination bands were assigned. The frequency resolved pump-probe traces measured at the fundamental frequencies for the four stretching normal modes exhibited marked differences: the two axial modes at frequencies of 2068 cm^-1 and 2034 cm^-1 yielded similar bi-exponential decay traces, while the two equatorial modes at 2014 cm ^-1 and 2002 cm^-1 showed a rising component, in addition to a bi-exponential decay. Due to the independence of the axial and equatorial stretching modes, it is shown that the axial-equatorial combination anharmonicity constants are near zero. This results in the appearance of the pump-probe signals of these combination bands at the same frequencies as the fundamental transitions, thus leading to interference and the resultant anomalous rising features. If unaccounted for, these interferences may lead to erroneous conclusions about the dynamics of these vibrational stretches. To avoid such pitfalls, it is therefore imperative to resolve such ambiguities. A corrected dynamical picture of the equatorial modes can be obtained by varying the center frequency of the pump pulse. The four modes have a slow vibrational excited population decay time of between 400 to 600 ps. We observe no obvious direct vibrational energy transfer between the axial and equatorial CO stretching modes.