VIBRATION-INDUCED ORDER-DISORDER TRANSITIONS IN A LANGMUIR-BLODGETT-FILM AS INVESTIGATED VIBRATIONAL SUM-FREQUENCY GENERATION SPECTROSCOPY

It was observed by vibrational sum-frequency generation (VSFG) spectroscopy that the excitation of CH stretching bands by IR pulses induced the destruction of the in-plane orientation of cadmium arachidate molecules on a nine-layer Langmuir-Blodgett (L-B) film. The resonant excitation of the antisymmetric CH stretch band, v(a)(CH)(CH2), of methylene groups by an infrared beam with an energy exceeding 100 mu J/pulse gave basically the same result as was obtained by thermally heating the sample to above 110 degrees C; the in-plane orientation of the alkyl frame was destroyed, but the terminal methyl groups stayed pointing outward. However, irradiation by pulses with lower energy gave results which varied significantly according to the vibrational mode excited.