Understanding the modulation mechanism in resonance-enhanced multiphoton probing of molecular dynamics

Time-resolved spectroscopy on isolated molecules gives fundamental insight into the conversion of light energy to other degrees of freedom. Probing of the photoinduced dynamics can be accomplished by ionization, via a single-photon or multiphoton transition. In this Rapid Communication we directly contrast transient spectra on the molecule perylene obtained with multiphoton ionization (MPI) to single-photon ionization (SPI). The photoinduced nuclear geometry relaxation modulates the MPI transient with a decay time constant of 0.9 +/- 0.2 ps. In contrast, the SPI transient completely lacks any indication for relaxation. We attribute this difference to a change in resonance enhancement of the MPI probe as the molecular geometry changes. Our results underline the importance of a detailed knowledge about these resonances for a proper interpretation of transient signals of molecular dynamics subject to nuclear and electronic relaxation effects. At the same time, the direct comparison to SPI directly demonstrates the higher sensitivity of resonance-enhanced MPI as a probe in time-resolved dynamical studies.