Vibrational overtone absorption of interfacial hydrogen in porous Vycor glass

The first measurements of the overtone absorption spectrum of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ in samples of porous Vycor glass are reported. The single-molecule vibrational overtone and the combination of the intramolecular vibrational fundamental with the surface hydroxyl fundamental are observed. The measured frequencies can be explained, with use of the results for bulk ${\mathrm{H}}_2$ as a guide, by taking into account appropriate surface parameters. It is also found that the overtone absorption of interfacial ${\mathrm{H}}_2$ is enhanced by a factor of 150 from that of the bulk liquid. The observation of these otherwise weak spectral features provides important data for the extension of vibrational spectroscopic treatments to molecular hydrogen on surfaces, particularly in regard to induced dipole moments, electrical and mechanical anharmonicities, and dissociation energies.