Speaker: Thomas Jacob Wolf, SLAC
Program Description
Ultrafast dynamics in photoexcited molecules are based on the interplay of electronic and nuclear degrees of freedom under violation of the Born-Oppenheimer approximation. Comprehensive understanding of the underlying processes can only be achieved with detailed knowledge of both electronic and nuclear dynamics. I want to show at two examples, what we can learn from experimental methods with selective sensitivity to ultrafast changes in either the electronic or the nuclear structure of molecules. In the first part of the talk, I will focus on our investigation of the ultrafast photoprotection mechanism in the nucleobase thymine with time-resolved near-edge soft x-ray absorption spectroscopy at LCLS. The site specificity of soft x-ray spectroscopy can here be transformed into selective sensitivity to changes in the electronic structure of thymine. The experimental data unambiguously show that the molecule undergoes internal conversion through a conical intersection between two excited states within 60 fs after photoexcitation.
In the second part of my talk, I want to present first results from an investigation of the photoinduced ring opening of 1,3-cyclohexadiene at SLAC's ultrafast electron diffraction (UED) facility. The large accessible momentum transfer range in the diffraction data permits us to directly and selectively follow transient changes of the nuclear structure in time. The experimental observable is ideal for comparisons with quantum chemical simulations of the ultrafast processes.
