Puzzling spectral structures of molecular vibration observed in ultrafast pump–probe experiment of transparent material

A sub-5-fs ultrafast pump–probe experiment was applied to neat liquid tetrahydrofuran (THF). The observed periodic modulation of transmittance change (ΔT), ∼5 ×10−3, is assigned to the ground-state vibrational mode of C–H stretching. Although the sample has no stationary absorption in the wavelength region under study, the modulation amplitude shows strong dependence on the probe photon energy. A model is proposed to explain this feature based on the coherent Raman coupling between the fundamental and Stokes-shifted spectral components. The results demonstrate that a special attention is required in distinguishing the ground- and excited-state vibrational signals in ultrafast transient absorption spectroscopy.