Description
Photoexcitation is an inherent part of any photochemical or spectroscopic experiment, yet its role in excited-state dynamics is often overlooked. In practice, however, it is the photoexcited molecular state, built upon photoexcitation and shaped by the characteristics of the light source, that ultimately determines the fate of the excited molecule and its subsequent photochemical behavior. In this talk, I will discuss how different laser pulses build excited molecular states, using two complementary representations of the molecular wave function: the Born–Huang expansion and exact factorization. I will consider two limiting cases: the preparation of a stationary molecular state by a long 100-fs laser pulse and the generation of an electronic wave packet by an ultrashort (attosecond) pulse. I will further show that standard concepts such as population transfer between electronic states, resonance conditions, and sudden vertical excitation—concepts naturally associated with the Born–Huang representation and widely used in chemical thinking about photoexcitation—are fundamentally challenged when viewed through the framework of exact factorization. Finally, I will comment on suitable strategies for modelling photoexcitation in both femto- and attochemistry based on the presented results.
