# Time-resolved X-ray spectroscopy of phenanthridine: elucidating the photodynamics of a nitrogen-containing polycyclic aromatic hydrocarbon

## TL;DR

This study uses advanced X-ray techniques to track how phenanthridine, a nitrogen-containing hydrocarbon, behaves when excited by light, revealing its decay and dissociation processes.

## Contribution

The paper provides new insights into the excited-state dynamics and dissociation mechanisms of phenanthridine using time-resolved X-ray spectroscopy.

## Key findings

- Phenanthridine's excited-state decay occurs via internal conversion with time constants of ~0.3 ps and ~3 ps.
- Dissociation of phenanthridine occurs on a timescale of several hundred picoseconds.
- Trajectory surface hopping simulations support the experimental observations of electronic transitions.

## Abstract

The photophysics and photochemistry of isolated phenanthridine have been investigated by time-resolved UV pump/X-ray probe spectroscopy at the SwissFEL free-electron laser combined with computations. Phenanthridine serves as the example for a polycyclic aromatic nitrogen-containing hydrocarbon (PANH), a class of molecules of considerable interest in material science and astrochemistry. It was excited at 268 nm into the bright 2ππ* state. The dynamics was subsequently probed by time-resolved X-ray photoelectron (TR-XPS) and X-ray absorption (TR-XAS) spectroscopy at the nitrogen 1s edge. Two time constants of τ1 ≈ 0.3 ps and τ2 ≈ 3 ps were determined. The excited-state dynamics was simulated using the trajectory surface hopping method and computed TR-XAS to support the band assignments. The study reveals a sequential decay to the electronic ground state via internal conversion. Spectra recorded over longer delay times indicate a dissociation on a time scale of several hundred picoseconds.

Phenanthridine serves as a model for the photochemistry of nitrogen-containing polycyclic aromatic hydrocarbons (PANH). Here, it has been investigated by time-resolved X-ray spectroscopy using free electron laser radiation.