Coherent Vibrational Quantum Beats as a Probe of Langmuir-Blodgett Monolayers

We combine frequency- and femtosecond time-domain measurements of vibrational coherences for spectroscopic characterization of surface monolayer films, utilizing 3-wave mixing as the surface-selective technique. Frequency-domain spectra in the CH-stretch region are obtained by infrared + visible sum frequency generation (SFG). Time-domain coherences are measured using SFG free induction decay (SFG-FID), where a 75 fs IR pulse excites several vibrational modes and a delayed 40 fs visible pulse probes the oscillating surface polarization. A unified framework based on optical Bloch equations is used to simultaneously analyze the time- and frequency-domain data. We compare molecular organization of monolayers in different twodimensional phases. Highly ordered films transferred at high surface pressure are dominated by two transitions in the frequency domain, CH₃ symmetric stretch (2875 cm^-1) and CH₃-Fermi resonance with bend overtone (2935 cm^-1), and a coherent quantum beat in the time-domain at the difference frequency (~540 fs period). At lower surface pressure, relative amplitudes change and additional transitions emerge (CH₃ asymmetric stretch and CH₂ modes), indicating changes in molecular orientation and onset of disorder. Information redundancy in the combined frequency- and time-domain data allows more accurate determination of the spectral parameters than purely frequency- or time-domain techniques.