Cynthia S. Trevisan

Calculated rates for the electron impact dissociation of molecular hydrogen, deuterium and tritium

At temperatures below 15 000 K, the major pathway for the electron impact dissociation of H2 is through excitation to the b 3Σu+ excited electronic state. Total cross sections and energy differential cross sections for threshold energies as a function of vibrational states (v) for H2 v = 0–4, D2 v = 0–6 and T2 v = 0–7 are calculated. The rates of dissociation as a function of electron temperature for each state are parametrized. Near-threshold rates are shown to be so critically dependent on the vibrational level that dissociation from very high-lying vibrational levels must be included in calculations of the rate at local thermal equilibrium (LTE) even at low temperature. An adapted version of the extrapolation procedure of Stibbe and Tennyson (1999 Astrophys. J. 513 L147) is used to approximate the rates for all of the higher vibrational levels, which are then used to calculate the LTE rate. The LTE rate is an order of magnitude greater than the v = 0 rate. Calculations of energy differential cross sections suggest that impact dissociation of vibrationally excited molecules could be the source of low energy H atoms observed in tokamak plasmas.

Differential cross sections for near-threshold electron impact dissociation of molecular hydrogen

At low energies, the major pathway for the electron impact dissociation of H2 is through excitation to b 3 � + . Ab initio calculations using the adiabatic nuclei, energy balance model of Stibbe and Tennyson (1998 New J. Phys. 1 2.1) of total cross sections, angular differential cross sections, energy differential cross sections and double differential cross sections for the electronic ground state initial vibrational v = 0 level, dissociating into continuum states are presented. The formal expressions needed for such calculations, which involve three fragments in the exit channel, are derived.

Electron molecule collisions calculations using the R-matrix method

The R-matrix method provides a complete theoretical framework for the treatment of low energy electron collisions. Recent results obtained with the UK R-matrix codes are presented focusing on electron impact electronic excitation of water and the CF radical, electron impact dissociation of molecular hydrogen and its isotopomers, and the dissociative recombination of the CO2+ dication. Examples of other processes studied in recent calculations are also given.

Calculated rates for the electron impact dissociation of molecular hydrogen: mixed isotopomers and scaling laws

Following our calculations of electron impact dissociation rates of molecular H2, D2 and T2 (Trevisan and Tennyson 2002 Plasma Phys. Control. Fusion 44 1263–76), calculations for the mixed isotopomers HD, HT and DT have been made. Total cross sections and energy differential cross sections at threshold energies as a function of vibrational states (v) for HD: v = 0–5, for HT: v = 0–5 and for DT: v = 0–7 are calculated for the electron impact dissociation through excitation to the b 3Σu+ excited electronic state, which is the dominant dissociation process at such energies. The rates of dissociation as a function of electron temperature for each state are parametrized. Coinciding with our previous results, near-threshold rates are shown to be so critically dependent on the vibrational level that dissociation from very high-lying vibrational levels must be included in calculations of the rate at local thermal equilibrium (LTE) even at low temperature. Rates for the higher vibrational levels are obtained by extrapolation following the procedure discussed in our previous publication, and are then used to calculate the LTE rate. The LTE rate is an order of magnitude greater than the v = 0 rate. A scaling law for the electron impact dissociation cross sections of vibrationally excited H2 and its isotopomers has been derived.

Low Energy Electron Collisions with Molecular Hydrogen

We discuss the theory of electron-molecule excitation processes of importance in fusion plasmas. We consider elastic scattering, rotational, vibrational, and electronic excitation in turn and also dissociative attachment and impact dissociation.