Experimental determination of ${\mathrm{H}}_2$ mass stopping powers for low-energy electrons

We present experimental mass stopping powers of electrons in gaseous ${\mathrm{H}}_2$ obtained with an electron time-of-flight spectrometer, for the incident electron energy range of 11 to 25 eV. In our procedure, the average energy loss is derived from conversion of the measured electron time-of-flight spectra into equivalent electron energy-loss spectra. Our present results are compared with the only available experimental measurement and with theoretical models. The measurements are a significant improvement to the available experimental data to date.

Figure 1

A simplified schematic diagram (not to scale) of the TOF spectrometer. The collimated target gas ($•$) emanates out of the page. The nanosecond pulsed electron gun crosses the gas at a right angle. The TOF tube (heated to about $80{}^{\circ }\mathrm{C}$) is placed on a precise rotation platform and is able to detect the scattered electron at scattering angles ($\theta$) of ${20}^{\circ }$ to ${135}^{\circ }$. Electrons are detected by a triple-sandwich Z-stack microchannel plate (MCP) detector. The pulsed electrons are produced by capacitively pulsing (p) a reversed biased lens ${\mathrm{L}}_{\mathrm{P}}$. ${\mathrm{L}}_{\mathrm{A}}$, ${\mathrm{L}}_1$, ${\mathrm{L}}_{\mathrm{c}}$, ${\mathrm{L}}_{\mathrm{F}}$ are the anode lens, anode coupling lens, collimating lens, and focusing lens, respectively. The a1 to a4 are the collimating apertures in the TOF tube. For detailed setup and discussion, see Ref. [2].

Figure 2

Background-subtracted energy-loss spectra of 15, 20, and 25 eV electrons for ${\mathrm{H}}_2$ at scattering angle of $\theta ={90}^{\circ }$. For clarity, the inelastic signal is magnified.

Figure 3

$\langle E_{\mathrm{L}}\rangle$ values as a function of $E_0$. Shown are the most recent theoretical values of Fursa et al. [18] and the only available experimental data of Munoz et al. [25]. See text for discussion.

Figure 4

MSP for electron scattering from the ground state of ${\mathrm{H}}_2$. Experiment: Munoz et al. [25]; theory: Fursa et al. [18], Gumus [29], Dalgarno et al. [34], Miles et al. [35], Takayanagi and Nakata [33], Peterson and Green [32], Sugiyama [28], and Spencer and Pol [31]. See text for discussion.