Modeling laser pulses as $\delta$-kicks: reevaluating the impulsive limit in molecular rotational dynamics

TL;DR

This paper reevaluates the impulsive limit in molecular rotational dynamics, revealing its dependence on pulse intensity and duration, and proposes a rescaling method for more accurate modeling of laser-molecule interactions.

Contribution

It develops an effective theory for ultrashort laser pulses, identifying a critical angular momentum where the impulsive limit fails, and introduces a rescaling method to improve interaction models.

Findings

Impulsive limit validity depends on pulse intensity and duration.

A critical angular momentum $l_{crit}$ marks the breakdown of the sudden approximation.

Rescaling matrix elements enhances the accuracy of laser-molecule interaction models.

Abstract

The impulsive limit (the "sudden approximation") has been widely employed to describe the interaction between molecules and short, far-off-resonant laser pulses. This approximation assumes that the timescale of the laser--molecule interaction is significantly shorter than the internal rotational period of the molecule, resulting in the rotational motion being instantaneously "frozen" during the interaction. This simplified description of laser-molecule interaction is incorporated in various theoretical models predicting rotational dynamics of molecules driven by short laser pulses. In this theoretical work, we develop an effective theory for ultrashort laser pulses by examining the full time-evolution operator and solving the time-dependent Schr\"odinger equation at the operator level. Our findings reveal a critical angular momentum, $l_\mathrm{crit}$, at which the impulsive limit breaks down. In other words, the validity of the sudden approximation depends not only on the pulse duration, but also on its intensity, since the latter determines how many angular momentum states are populated. We explore both ultrashort multi-cycle (Gaussian) pulses and the somewhat less studied half-cycle pulses, which produce distinct effective potentials. We discuss the limitations of the impulsive limit and propose a new method that rescales the effective matrix elements, enabling an improved and more accurate description of laser-molecule interactions.