Molecular Orbital Imaging of the Acetone $S_2$ Excited State Using Time-Resolved ($e$, $2e$) Electron Momentum Spectroscopy

We report a time-resolved ($e$, $2e$) experiment on the deuterated acetone molecule in the $S_2$ Rydberg state with a lifetime of 13.5 ps. The acetone $S_2$ state was prepared by a 195 nm pump laser and probed with electron momentum spectroscopy using a 1.2 keV incident electron beam of 1 ps temporal width. In spite of the low data statistics as well as of the limited time resolution ($\pm 35\mathrm{ps}$) due to velocity mismatch, the experimental results clearly demonstrate that electron momentum spectroscopy measurements of short-lived transient species are feasible, opening the door to time-resolved orbital imaging in momentum space.

Figure 1

A schematic representation of the experimental setup for time-resolved symmetric-noncoplanar ($e$, $2e$) experiments.

Figure 2

Experimental laser-on and laser-off EMS binding energy spectra of the acetone-$d6$ molecule obtained at a delay time $t_{\text{delay}}$ of 0 ps. The broken and solid lines represent associated theoretical calculations for the laser-off spectrum. See text for details.

Figure 3

Experimental time-resolved EMS binding energy spectra of the acetone-$d6$ $S_2$ state obtained at delay times $t_{\text{delay}}$ of (a) 0 ps and (b) 100 ps. The solid line represents a band due to ionization from the outermost $3s$ Rydberg orbital.

Figure 4

Comparison of spherically averaged electron-momentum-density distributions between experiment and theory for the $3s$ Rydberg orbital of the acetone-$d6$ $S_2$ state. A theoretical distribution for the outermost orbital of the acetone-$d6$ $S_0$ state is also shown by the broken curve as a reference to highlight the behavior of the $3s$ Rydberg orbital.