Laboratory Measurements of Electron Impact with Cometary Neutrals

Around cometary comae and other small bodies, electron impact competes with photoionization for producing both atomic and ionic species at low energies. Electron impact provides a unique spectral signature and allows for remote diagnostics of the plasma environment (see [1] and references therein). The required laboratory data to fully understand the role of electron impact in these tenuous plasma environments is still incomplete, though several reviews have highlighted these discrepancies (c.f. [2] and [3]).

In general, the laboratory studies of electron impact may be broken into two categories. First, total cross sections of common cometary neutrals (H₂O, CO, CO₂) are typically well constrained (see e.g. [3]), and partial cross sections are often normalized to these total cross sections. However, these partial cross sections may be uncertain as some laboratory experiments suffered from energy discrimination issues wherein higher energy product ions (due to the gas temperature and/or the dissociation dynamics) escape the collection angle of the apparatus; for an example, see the discussion in [4]. Second, measurements of the emission cross sections are typically only available at a select few energies, some of which are outside the ~0–~300 eV range of cometary electrons, with significant discrepancies [1]. For example, while the cross sections for the production of atomic O from H₂O dissociation are reasonably well known, there are up to a factor of 4 difference in the O I 121.7 nm emission cross section [5].

We discuss the modification and preparation of a Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) apparatus for studying the full dissociation dynamics of cometary molecules. In this COLTRIMS system, a pulsed electron beam is crossed with a cold (~mK) gas jet of density n ~ 10¹² cm^-3. A pulsed extraction field, modeled in SIMION, ensures complete detection of all product ions on a position-sensitive multichannel plate. Partial cross sections, normalized to total cross sections in the literature, will be measured for dissociative ionization of common cometary neutrals such as CO. Planning is underway for future inclusion of a custom FUV spectrometer coupled to the COLTRIMS timing scheme. Together, these two detection schemes will enable the simultaneous measurement of both ionization and emission cross sections within an expanded energy range up to many hundreds of electrovolts.

1. Bodewits et al. 2018, ApJ, 885, 167

2. Itikawa, Y., & Mason, N. 2005, JPCRD, 34, 1

3. McConkey et al. 2008, PhR, 466, 1

4. Tian, C., & Vidal, C. R., 1997, J. Chem. Phys., 108, 927

5. Van de Burgt et al. 1989, JPCRD, 18, 1757