Non-linear Spectroscopy of Conical Intersections

Abstract: Conical Intersections (CIs) in a molecule originate from the breakdown of the Born-Oppenheimer approximation when electronic states come close in energy. Ultrafast non-radiative decay channels open up in the vicinity of a CI, thus funneling a molecule from one electronic state to another. Observing such non-adiabatic processes can be challenging since they take place at the timescale of femtoseconds. Therefore, we use non-linear spectroscopy to probe the existence of CIs and the non-adiabatic processes in their vicinity. In this thesis, we discuss non-linear spectroscopic methods that may probe CIs in a molecule. The CI in the pyrrole molecule is probed using transient X-ray absorption, time-resolved X-ray spontaneous emission, and off-resonant Ra-man spectroscopies. Time-resolved photoelectron spectroscopy is found useful in observing the separation between the electronic states and the electronic coherence near a CI simultaneously.