Experimental investigation of electron impact on $\text{Si}_2{}^{-}$

A merged beams technique has been used to investigate collisions between electrons and $\text{Si}_2{}^{-}$ ions over a relative kinetic energy range of 0–210 eV. Absolute cross sections for pure electron detachment, detachment plus dissociation, and dissociation involving atomic and ionic products were measured. The dominant process over the energy range studied is pure electron detachment. A search for a resonance associated with a ${\text{Si}}_2$ dianion was made but none was observed.

Figure 1

A schematic of the electron-negative ion interaction region and the downstream surface barrier detector used to monitor neutral fragments produced in the interaction region. Mass 56 and 28 a.m.u. fragments were detected in the present experiment.

Figure 2

The ramping of the cathode voltage in the electron gun is shown in Fig. 2a. The modulation of the electron beam is indicated by Fig. 2b. Figure 2c shows a typical output from the surface barrier detector. The signal is situated on top of an exponentially decaying background of neutrals produced in collisions between ions and residual gas. The modulation of the signal arises from the chopping of the electron beam.

Figure 3

Absolute cross sections for electron impact induced fragmentation of the $\text{Si}_2{}^{-}$ ion via the pure electron detachment channel (filled squares) and detachment plus dissociation channel (open circles). Error bars are not shown if the statistical uncertainties are smaller than the data points. Scans over the entire energy range are shown in Fig. 3a. Figure 3b shows the threshold behaviors, together with the classical fits given by Eq. (7).

Figure 4

Energy dependence of the ratio of the cross sections for pure detachment $\left({\sigma }_1\right)$ and detachment plus dissociation $\left({\sigma }_3\right)$ in the $e^{-}+\text{Si}_2{}^{-}$ collision system.

Figure 5

Absolute cross section for electron impact induced fragmentation of the $\text{Si}_2{}^{-}$ ion via two dissociation channels with heavy particle products $\text{Si}+{\text{Si}}^{-}$ and $\text{Si}+{\text{Si}}^{+}$. Some representative statistical error bars are shown. In Fig. 5a we show how the cross section varies over the entire energy range used in the experiment. The threshold behavior, together with the classical fit, is shown in Fig. 5b.