Vibrational Energy Redistribution between CH Stretching Modes in Alkyl Chain Monolayers Revealed by Time-Resolved Two-Color Pump–Probe Sum Frequency Spectroscopy

摘要： The vibrational dynamics of the various CH stretching modes in a fatty acid Langmuir?Blodgett ?lm was studied using a resonant narrowband infrared (IR) laser pulse for pumping and a broadband femtosecond IR visible pulse pair for detection in a sum frequency spectroscopy setup. The resulting two-dimensional spectra indicate that pumping either the antisymmetric methyl or methylene stretch results in the transfer of energy to the other modes on a time scale faster than 2 ps. This rapid process is followed by energy redistribution to other modes, presumably the bending and internal rotational modes, with a time constant of approximately 85 ps. The formation of gauche defects is not observed within the ?rst 250 ps. The whole spectrum recovers on a time scale of several nanoseconds, indicating dissipation of the excitation energy into the substrate.