Sum-frequency generation of the symmetric CH3-stretching vibration in organized molecular ultrathin films: ultrafast coupling interaction effect

Langmuir-Blodgett films (LB) of arachidic acid (AA, CH3(CH2)18COOH) at the glass/air interface are investigated by sum-frequency generation (SFG) vibrational spectroscopy for all the possible polarization combination of input and output radiation. The newly identified band at 2818 cm−1 in LB films has been attributed to a soft mode of the CH vibration induced by the interaction (coupling) with the rest of molecules near the glass substrate. The coherent response of the CH3-stretching vibration in LB films and self-assembled monolayer (SAM) films of octadecanethiol on gold are also studied as a function of the delay between the visible and infrared radiation, i.e. the free-induction delay (FID). The pure dephasing time T2* obtained for the first time for the LB film is ∼3.2 ps, which is fairly long, reflecting the methyl group coupled with the rest of the molecules near the glass substrate. The short dephasing time of ∼1.0 ps obtained for the SAM film mainly represents the relaxation time of the lattice phonon of gold film.