Faure, A

Tennyson, J

English

UCL Discovery

SF2A 2003F. Combes, D. Barret and T. Contini (eds)ELECTRON COLLISIONS IN HARSH ENVIRONMENTSFaure, A.1 and Tennyson, J.2Abstract.We present our most recent R-matrix calculations on the electron-impactrotational excitation of astronomically important molecules. We show thatelectron collisions are very eﬃcient in producing rotationally ’hot’ moleculesin harsh environments.1 IntroductionRate coeﬃcients for the collisional excitation of molecules by electrons are crucialparameters for modelling the physical conditions of harsh environments such asdiﬀuse interstellar clouds or photo-dissocation regions (PDRs), where electrons areabundant (Black 1998). In fact, even at modest electron fractions, n(e)=n(H) ¼10¡5, electron collisions can dominate the molecular excitation because electron-impact collisional rates exceed that for excitation by neutral species (mainly H,He and H2) by typically 5 orders of magnitude.The reference methods for obtaining electron-impact excitation rates have beenthe Coulomb-Born approximation for molecular ions and the Born approximationfor neutral species. These methods assume that the collisional excitation ratescan be simply determined by the dominant long-range interaction. Within thisapproach, these theories predict that only single jumps in rotational quanta areallowed for polar species. Recent R-matrix studies have shown that this predictionis incorrect (Faure & Tennyson 2003 and references therein). Indeed, R-matrixcalculations have shown that the inclusion of short-range interactions can lead torotational transitions with ∆J up to 6.2 ResultsIn environments below the critical density, it has often been assumed that molecu-lar ions rotationally excited by electron impacts will emit j = 1 ! 0 photons only.1 Laboratoire d’Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble C´ edex 09,France2 Department of Physics and Astronomy, University College London, Gower Street, WC1E6BT, United Kingdomc ° EDP Sciences 2003238 SF2A 2003Our recent R-matrix calculations have shown that in all cases considered so far asigniﬁcant ﬂux from higher rotational states should also be emitted. In particular,Faure & Tennyson (2003) have shown that electron collisions might contribute tothe pumping of a maser in the (4, 4)! (3, 1) transition of H+3 at 217 GHz.In the case of H2O, Faure et al. (2003) have found that the dominant transitionsare those for which ∆J = 0;§1, as predicted by the dipolar Born approximation.However, a pure Born treatment was found to overestimate the cross sectionsclose to threshold energies (see ﬁgure) and to neglect important dipole-forbiddentransitions. In the context of cometary water, the contribution of electron collisionsmight thus explain the need for large H2O-H2O collisional excitation rates inpopulation models which neglect electrons.0,01 0,1 1Collision Energy (eV)01000200030004000Integral Cross Section (Å2)Born calculationPresentFig. 1. Rotational excitation cross section for the 000¡111 transition in H2O. Our resultsare represented by the solid line. The dashed line gives the pure Born calculation.3 ConclusionWe have shown that electron-impact excitation is a very eﬃcient process to rota-tionally excite molecules in harsh environments. An important prediction concernsthe contribution of electron collisions to the pumping of cosmic masers.ReferencesBlack, J., 1998, Faraday. Discuss., 109, 257Faure, A., Tennyson, J., 2003, MNRAS, 340, 468Faure, A., Gorﬁnkiel J. D., Tennyson, J., MNRAS, submitted

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