TRANSIENT INFRARED-SPECTRUM OF VIBRATIONALLY EXCITED RH-6(CO)(16)

The transient IR spectrum of Rh-6(CO)(16) in CCl4 with one quanta of e xcitation in the GO-stretch modes has been measured by picosecond time -resolved IR saturation spectroscopy as a function of pump-probe time delay and polarization. ht short time delays (<10 ps), the spectra can be consistently deconvoluted into a bleach of the nu(6)F(2) fundament al at 2076.0 cm(-1), and two hot-band transitions at 2068.6+/-0.4 and 2073.8+/-0.3 cm(-1), which we tentatively assign to the A(1) and E sym metry overtones of the nu(6)F(2) fundamental. The broader hot-band fea ture observed at long lime delay (t(D) = 600 ps) is assigned to an inh omogeneous superposition of transitions to overtone and combination mo des from an equilibrated population of v = 1 CO stretch modes, A mean field model of coupled anharmonic oscillators substantially overestima tes the magnitude of average frequency shift of the hot band in this i ntermediate size molecular system, The rapid (<10 ps) decay of the pum p-induced dichroism in the sample suggests that vibrational coupling o f the degenerate nu(6)F(2) modes proceeds on a time scale similar to o r faster than physical rotation of the molecule. (C) 1997 American Ins titute of Physics.