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Ultrafast decay of superexcited states of O2 probed with femtosecond photoelectron spectroscopy
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Image of FIG. 1.
FIG. 1.

The relevant superexcited states converging to the ion core are sketched here along with the atomic and molecular states observed in this work. The high-harmonic spectrum is plotted along the vertical energy axis to illustrate the resonances that are excited within the pump pulse spectral bandwidth. The superexcited states can autoionize to various ion states. Dashed lines represent the adiabatic ionization energy to form the molecular ions; the numbers associated with these dashed lines are the adiabatic ionization energies. The superexcited states can also predissociate (shown as dotted lines), to form neutral fragments (electron binding energies shown as dashed-dotted lines). The superexcited states and neutral fragments are probed with a delayed 805 nm ionization pulse to eject photoelectrons that are measured here.

Image of FIG. 2.
FIG. 2.

The time-resolved photoelectron spectrum is plotted here for all photoelectron kinetic energies measured. The positive signals are superexcited states and neutral atomic products that are probed with the time delayed 805 nm probe pulse. Negative signals (or depletions) are a result of probing a superexcited state before it can autoionize. These features are described individually in the text. The ROIs used in the analysis of the time-resolved photoelectron signals are indicated along the top of the figure.

Image of FIG. 3.
FIG. 3.

The time-integrated photoelectron spectrum plotted here as a function of binding energy is generated by integration over time delays spanning 441 fs to 105.09 ps. Photoelectron lines from excited atomic fragments are observed at binding energies of 0.87 eV, 1.34 eV, and 1.52 eV, which correspond to , , and fragments, respectively. The regions of interest used in analysis of the time-dependent signals are also indicated in the figure as shaded and labeled columns. The inset shows the entire photoelectron spectrum plotted as a function of electron kinetic energy.

Image of FIG. 4.
FIG. 4.

The transient photoelectron signals and fit kinetic traces (solid lines) are plotted here in the range of −750 fs to 3500 fs; time delays up to +105.09 ps were measured with no change in signal. The assignment of each region, the kinetic energy range integrated to produce the transient signals, the fit functions used to quantify the dynamics, and the resulting timescales are summarized in Table I. The vertical dashed line represents t = 0 fs.


Generic image for table
Table I.

Tabulated here for each region of interest (ROI) are the spectral assignments, electron kinetic energy ranges used to make each ROI, the applied fit function(s), and a summary of the associated lifetimes.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Ultrafast decay of superexcited cΣu−4nlσgv=0,1 states of O2 probed with femtosecond photoelectron spectroscopy