Orientational relaxation and vibrational excitation transfer in methanol–carbon tetrachloride solutions

Time and polarization resolved ultrafast infrared vibrational spectroscopy of the hydroxyl stretch of methanol dissolved in carbon tetrachloride has been utilized to investigate orientational relaxation and vibrational excitation transfer. The anisotropy decay of the deuterated hydroxyl stretch of methanol-d was measured in two solutions: Isotopically mixed 0.8 mol % methanol-d 23 mol % methanol-h in ${\mathrm{CCl}}_4$ and isotopically pure methanol-d at 26 mol % in ${\mathrm{CCl}}_4.$ The anisotropy decay in the isotopically mixed methanol solution is a biexponential characterized by 1.7±0.7 ps and 17±3 ps time constants, with 40±10% of the decay occurring with the slower time constant. The biexponential anisotropy decay has been analyzed with a restricted orientational diffusion model that involves fast orientational diffusion within a cone of semi-angle ${\theta }_c,$ followed by slower, full orientational relaxation. The fast orientational relaxation occurs within a cone semi-angle of ${\theta }_c\text{=45°±5°,}$ with a diffusion coefficient of $D_c^{\text{−1}}\text{=13±5\hspace{0.17em}}\mathrm{ps}.$ The slower anisotropy decay results from the full orientational diffusion and occurs with a diffusion coefficient of $D_{\theta }^{\text{−1}}\text{=100±20\hspace{0.17em}}\mathrm{ps}.$ The anisotropy decay for isotopically pure methanol-d in ${\mathrm{CCl}}_4$ is much faster because of vibrational excitation transfer in addition to the orientational relaxation. The excitation transfer has been successfully analyzed as transition dipole–transition dipole mediated transfer using a theory developed for randomly distributed chromophores.