Dynamics of solvation in supercooled liquids confined to the pores of sol-gel glasses

We have measured the solvation dynamics of the probe molecule quinoxaline in a glass-forming solvent, 2-methyltetrahydrofuran, geometrically confined to the pores of sol-gel glasses having nominal pore diameters $\phi =2.5, 5.0, \mathrm{and} 7.5$ nm. Within the time range $1\mathrm{ms}<~t<~1\mathrm{s}$ the $\alpha$ process and a second extremely slow process are observed. On the basis of emission spectra, the latter relaxation is assigned to a surface layer with strongly frustrated dynamics, opposed to the picture of a certain fraction of pores entirely filled with liquid of reduced molecular mobility. According to the Stokes-shift results for porous samples, the thickness of this surface layer increases significantly with $T_g\leftarrow T$ for the system under study. The results are compared with solvation dynamic experiments for the bulk solvent and with dielectric relaxation measurements.