On the interpretation of molecular photoexcitation with long and ultrashort laser pulses

TL;DR

This paper investigates how different laser pulse durations influence the formation of excited molecular states, challenging traditional concepts using advanced wave function representations to better understand photochemical reaction dynamics.

Contribution

It introduces a novel analysis of molecular excitation using exact factorization, contrasting it with traditional Born-Huang approaches, to better understand laser pulse effects.

Findings

Long pulses produce stationary molecular states

Ultrashort pulses generate electronic wave packets

Exact factorization challenges traditional excitation concepts

Abstract

Photoexcitation is an inherent part of any photochemical or spectroscopic experiment, yet its impact on the excited-state dynamics is often overlooked. However, it is the excited molecular state, built upon photoexcitation and shaped by the characteristics of the light source, that determines the fate of the excited molecule and its subsequent photochemical reactions. In this work, we investigate how excited molecular states are built by different laser pulses, leveraging two representations of the molecular wave function: Born-Huang expansion and exact factorization. We explore the generation of two limiting cases: a stationary molecular state with a long laser pulse and an electronic wave packet by an ultrashort (attosecond) laser pulse. The standard concepts of population transfer between electronic states, resonance condition, or sudden vertical excitation, inherent to the Born-Huang representation and used by chemists to approximate the impact of photoexcitation on molecular systems, are challenged by the exact factorization.