Ultrafast coupling interaction in organized molecular ultrathin films

Langmuir-Blodgett (LB) films of arachidic acid (AA, CH3(CH2)18COOH) at the glass/air interface are investigated by sum-frequency generation (SFG) vibrational spectroscopy. The newly identified band at 2818 cm−1 in LB films has been attributed to a soft mode of the C-H vibration induced by the interaction (coupling) with the rest of the molecule near the glass substrate. The coherent responses of the CH3 stretching vibration in LB films and self-assembled monolayer (SAM) films of octadecanethiol (ODT, CH3(CH2)17SH) on gold are also studied by the free-induction decay (FID) method. 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 molecule near the glass substrate. The short dephasing time of ∼1 ps obtained for the SAM film mainly represents the relaxation time of the lattice phonon of gold film.