Observation of a Molecular Frame $(e,2e)$ Cross Section: An $(e,2e+M)$ Triple Coincidence Study on ${\mathrm{H}}_2$

We report the first experimental results showing transition-specific anisotropy of molecular frame $(e,2e)$ cross sections. Vector correlations between the two outgoing electrons and the fragment ion have been measured for specific ionization-excitation processes of ${\mathrm{H}}_2$. The results enable us to obtain molecular frame $(e,2e)$ cross sections for transitions to the $2s{\sigma }_g$ and $2p{\sigma }_u$ excited states of $\mathrm{H}_2{}^{+}$, thereby making stereodynamics of the electron-molecule collisions directly visible.

Figure 1

Schematic of an $(e,2e+M)$ experimental setup based on the binary ($e,2e$) scattering in the symmetric noncoplanar geometry, showing seven channel-electron multipliers (CEMs), and a pair of entrance apertures (EAs) of a spherical analyzer followed by a pair of position-sensitive detectors.

Figure 2

Calculated kinetic energy distributions of ${\mathrm{H}}^{+}$ (a) and observed one (b). In the lower panel, results of a least-squares fit using the theoretical predictions for the $2s{\sigma }_g$, $2p{\sigma }_u$, and $2p{\pi }_u$ transitions are shown by broken lines. The solid line represents their sum.

Figure 3

$(e,2e+M)$ binding energy spectra of ${\mathrm{H}}_2$ associated with the ${\mathrm{H}}^{+}$ kinetic energies corresponding to the regions (a) I, (b) II, (c) III, and (d) the entire range defined in Fig. 2b. The broken and solid lines in (d) represent the deconvoluted curves and their sum.

Figure 4

Experimental molecular frame $(e,2e)$ cross sections of ${\mathrm{H}}_2$ for the transitions to the (a) $2s{\sigma }_g$ and (b) $2p{\sigma }_u$ excited ion states, obtained at an impact energy of 1200 eV. The molecular axis is in the vertical direction. The solid lines represent associated PWIA calculations.