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

Abstract

A sub-5-fs ultrafast pump-probe experiment was applied to neat liquid tetrahydrofuran (THF). The observed periodic modulation of transmittance change (Delta T), similar to 5 x 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. (C) 2009 Elsevier B. V. All rights reserved.