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. Varying the pH value of the aqueous subphase of a Langmuir film formed by octadecylamin, characteristic changes in the CH- and NH-spectral region are observed. For bulk pH≤10 the symmetric CH₂ mode appears, which is a strong hint for trans-gauche deformations dependent on pH. In this way, SF spectroscopy is demonstrated to be an independent method for determinining the surface pK of surfactants. Chemisorbed films of OTS have been prepared by polymerization of OTS on a glass substrate in anhydrous solution. Vanishing CH₂ resonances indicate straight chains of the adsorbed molecules, whereas from the line strength of residual Si-OH vibrations information on the degree of polymerization may be obtained.