Probing the π−π* transitions in conjugated compounds with an infrared femtosecond laser

Abstract

We show that the processes of $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ transitions imprint themselves on the high-harmonic spectra when conjugated molecules interact with intense femtosecond laser pulses. It is found that a noninteger order peak appears in the harmonic spectrum with the photon energy equaling the excitation energy of the $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ excitation. Further studies prove that this radiation is caused by the $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ transition. The transition signals are prominent and can be easily measured as the corresponding radiation intensities are comparable to those of integer order harmonics. Our results pave the way for the study of excited-state electron-ion dynamics using high-harmonic spectroscopy. In comparison to the traditional absorption spectroscopy method relying on the synchrotron radiation source, the present approach is easily accessible for the use of a tabletop laser-based source. Furthermore, our study also provides a potential tool to probe the $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ transition processes in femtosecond resolution.