Vibrational deactivation of surface OH chemisorbed on SiO2: Solvent effects

Abstract

Picosecond infrared transmission spectroscopy was used to directly measure the vibrational energy relaxation time T₁ of hydroxyl groups chemisorbed on the surface of colloidal silica (SiO₂). T₁ was obtained for OH(ν_stretch=1) in the strongly bound ‘‘isolated sites’’ of fumed silica particles in vacuum and dispersed in several liquids at T=293 K. At the SiO₂/vacuum interface, T₁=204±20 ps. When the SiO₂ particles are surrounded by solvents, the relaxation time of the surface OH(v=1) groups decreases: for the liquids CCl₄, CF₂Br₂, CH₂Cl₂, and C₆H₆, T₁(ps)=159±16, 140±30, 102±20, and 87±30, respectively. T₁ does not depend on the size of the SiO₂ particles for the range 70 Å≤ diameter ≤150 Å, or on the surface OH coverage up to an average density of 4 OH/100 Ų. Significant amounts of physisorbed water (5 H₂O/100 Ų) decreased T₁ for the isolated OH(v=1) to T₁=56±10 ps. For comparison to the surface hydroxyls, the vibrational deactivation time for OH(v=1) groups in the bulk of fused silica (OH/SiO₂≊130 ppm by weight) was determined to be T₁=109±11 ps. These observations are discussed in terms of the possible mechanisms of vibrational energy flow in these systems. The observed T₁ values demonstrate that the spectral linewidths (e.g., IR and Raman) observed for these surface vibrations are too large (by factors of 200–2000) to be caused solely by T₁ uncertainty broadening. The slow transfer of vibrational energy between surface and lattice vibrations may have important implications for surface chemistry.