PULSE-PySpawn: Time-Resolved Transient Absorption Simulation through <i>Ab Initio</i> Multiple Spawning Dynamics with Explicit Pulses

We report a new workflow to model time-resolved transient absorption spectra based on propagating non-adiabatic dynamics with <i>ab initio</i> multiple spawning. By explicitly including a laser field in the molecular Hamiltonian, we can model both the effect of a pump pulse in depopulating the ground state and a series of probe pulses to monitor the evolution of the nuclear density. We developed a flexible and efficient workflow that enables the inclusion of probes of different energies at different time delays, matching real experimental conditions. In this way, we are able to reconstruct the spectroscopic signal from a single set of trajectories. We included our workflow in the open-source code PySpawn to exploit all of the implementations recently added to this software and to ensure wider usability of our method. Finally, we showed the applicability and strength of our development in simulating and interpreting the time-resolved transient absorption spectrum of azomethane.