Sum-frequency spectroscopy of physisorbed and chemisorbed molecules at liquid and solid surfaces using band-width-limited picosecond pulses

We studied the vibrational Sum-Frequency (SF) spectra of long chain fatty alcohols and amines physisorbed at liquid/air interfaces and of OctadecylTrichlorSilan (OTS) chemisorbed on glass/air interfaces in situ, i. e., in normal laboratory environment. The intense, band-width-limited IR pulses generated by our laser system are tunable from 2600 to 4000 cm^–1 with a constant pulse duration of 3 ps and a band width of 5 cm^–1 (FWHM) over the entire tuning range, thus covering the CH-, NH-, and OH-stretching regions.Using suitable polarization geometries, information on the molecular orientation is obtained from the amplitudes of the symmetric and degenerate methyl-stretching modes. Opposite phase of adjacent vibrational modes can lead to destructive interference in the SF signal, as analyzed theoretically. This interference effect is observed experimentally for the first time, due to the superior spectral resolution and signal-to-noise ratio of our spectra.