# Femtosecond Time- and Spectrally Resolved Ion Photofragmentation Spectroscopy: Case Studies of Two Alkylbenzene Cations

## TL;DR

This study uses ultrafast laser techniques to observe how alkylbenzene cations relax after being ionized, showing they are good models for studying more complex ionic systems.

## Contribution

The paper introduces alkylbenzene cations as benchmark systems for ultrafast ionic relaxation dynamics due to their minimal postionization changes.

## Key findings

- Parent ion depletion signals are solely from intact cation relaxation, not fragmentation.
- Visible resonance absorption in alkylbenzene cations remains unchanged over time after ionization.
- These cations are ideal for benchmarking ultrafast dynamics in complex ionic systems.

## Abstract

Ultrafast photoionization-induced
ionic relaxation dynamics
in n-propylbenzene and 2,2-dimethylpropylbenzene
cations were
investigated using time- and spectrally resolved ion photofragmentation
spectroscopy with a femtosecond photoionization–photofragmentation
(PI–PF) detection scheme. Photoionization was initiated via
1 + 1 REMPI using femtosecond UV pump pulses well below the strong-field
ionization regime, and the evolving ionic systems were probed by delayed
visible-wavelength probe pulses to induce photofragmentation. Despite
substantial ionic fragmentation induced by UV photoionization, the
observed parent ion depletion transient signals can be attributed
exclusively to the relaxation dynamics of intact parent cations generated
at the two-photon level. Highly excited cationic states initially
accessed by absorption of additional UV photons within the pump pulse
do not contribute to the parent ion depletion transient signals, although
their dynamics may appear in fragment ion formation transients. Ultrafast
time-resolved photofragmentation spectra, obtained by measuring probe-induced
parent ion depletion yield as a function of wavelength at a fixed
delay time, reveal prominent ionic resonance absorption in the visible
region. The observed time invariance of ion depletion signals across
this spectral range indicates the absence of substantial postionization
relaxation processes capable of altering the nature or resonance absorption
characteristic of the cation. These findings establish alkylbenzene
cations as ideal reference systems for benchmarking ultrafast dynamics
in more complex ionic systems that do undergo substantial structural
or electronic transformations following photoionization.