Development of a setup for interferometric two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) measurements to study liquid interfaces with sub-100 fs time resolution

We constructed an apparatus for interferometric 2D heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy, which is applicable to the study of ultrafast dynamics at liquid interfaces. By using a multiple-plate pulse compressor and Michelson interferometer, we generated IR pump pulse pairs having a broad bandwidth (∼400 cm-1 FWHM) and sufficient energy (∼12 μJ/pulse maximum) to observe transient responses of molecules at a liquid interface. The instrumental responses of the apparatus are ∼120 fs at ∼2900 cm-1 (CH stretch region) and ∼90 fs at ∼3350 cm-1 (OH stretch region) of the IR frequencies. Using this setup, we performed 2D HD-VSFG measurements of the n-octadecanol (n-OD) Langmuir monolayer formed on the heavy water (D2O) surface. With the 1.2 ps stroke of the interferometric scan, the 2D ΔImχ(2) spectrum was obtained at the waiting time of T = 50 fs, which shows well-separated diagonal and cross-peaks of the CH stretch vibrations. This result demonstrates the high capability of the interferometric 2D HD-VSFG setup for revealing the vibrational dynamics of the liquid interface under various conditions, including ultrafast dynamics of interfacial water.