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

Ultrafast photoionization-induced ionic relaxation dynamics in -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.