Vibrational dephasing mechanisms in liquids and glasses: Vibrational echo experiments

Picosecond vibrational echo studies of the asymmetric stretching mode $\text{(2010\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}})$ of (acetylacetonato)dicarbonylrhodium(I) $[{\mathrm{Rh(CO)}}_{\mathrm{2}}\mathrm{acac}]$ in liquid and glassy dibutyl phthalate (DBP) (3.5 K to 250 K) are reported and compared to previous measurements of a similar mode of tungsten hexacarbonyl $[{\mathrm{W(CO)}}_{\mathrm{6}}\mathrm{].}$ The ${\mathrm{Rh(CO)}}_{\mathrm{2}}\mathrm{acac}$ pure dephasing shows a $T^1$ dependence on temperature at very low temperature with a change to an exponentially activated process $\text{(ΔE≅400\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}})$ above $\text{∼20\hspace{0.17em}}\mathrm{K}.$ There is no change in the functional form of the temperature dependence in passing from the glass to the liquid. It is proposed that the $T^1$ dependence arises from coupling of the vibration to the glass’s tunneling two level systems. The activated process arises from coupling of the high-frequency CO stretch to the $\text{405\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ Rh–C stretch. Excitation of the Rh–C stretch produces changes in the back donation of electron density from the rhodium $d_{\pi }$ orbital to the CO ${\pi }^{*}$ antibonding orbital, shifting the CO stretching transition frequency and causing dephasing. In contrast, ${\mathrm{W(CO)}}_{\mathrm{6}}$ displays a $T^2$ dependence below $T_g$ in DBP and two other solvents. Above $T_g,$ there is a distinct change in the functional form of the temperature dependence. In 2-methylpentane, a Vogel–Tammann–Fulcher-type temperature dependence is observed above $T_g.$ It is proposed that the triple degeneracy of the $T_{\text{1u}}$ mode of ${\mathrm{W(CO)}}_{\mathrm{6}}$ is broken in the glassy and liquid solvents. The closely spaced levels that result give rise to unique dephasing mechanisms not available in the nondegenerate ${\mathrm{Rh(CO)}}_{\mathrm{2}}\mathrm{acac}$ system.