Temperature-dependent vibrational relaxation in polyatomic liquids: Picosecond infrared pump–probe experiments

Abstract

Vibrational lifetimes of metal carbonyl solutes in two liquids were studied as a function of temperature from the melting points to the boiling points. Picosecond infrared pump–probe experiments were performed at the absorption maxima of the T1u CO stretching vibration (∼1980 cm−1) of Cr(CO)6 and W(CO)6 dissolved in carbon tetrachloride (CCl4) and chloroform (CHCl3). The temperature dependencies of the lifetimes in CCl4 are dramatically different from those in CHCl3. A decrease in the vibrational lifetime with temperature is seen for both metal carbonyls in CCl4. However, the vibrational lifetime of W(CO)6 in CHCl3 becomes longer as the temperature is increased, while the lifetime of Cr(CO)6 decreases only slightly. To understand the vibrational dynamics in these systems it is necessary to consider the temperature dependencies of the thermal populations of low frequency phonons (instantaneous normal modes) of the liquids, the phonon density of states, and the anharmonic coupling matrix elements.