Visible to near infrared emission spectra of electron-excited H2

H2 is the most abundant molecule in the universe and is an active component of star formation. Inside dense molecular clouds, heating and ionization occur by cosmic rays, X-rays and shock waves, generating energetic electrons. Collisional excitation by electrons is the source of both UV and Visible-optical-IR (VOIR) H2 fluorescence in the ISM, circumstellar disks and certain classes of stars [1]. The importance of collisional excitation processes has been verified with analysis of HST and IUE observations of Herbig-Haro (HH) Objects, T Tauri stars and reflection nebulae [2,3]. In particular, intense H2 transitions in the VOIR from various vibrational levels have been observed in highly-collimated jets of matter from young stellar objects [4]. These observed lines trace the colder molecular part of the post-shocked gas [5]. In recent work, we have demonstrated [6] that the gerade series (EFΣg , GKΣg , H Σg , I Πg, J ∆g...) makes a significant contribution to the UV spectrum of H2 via its cascade spectrum in the visible/near IR to the n = 2pσB and 2pπC states, the upper states of the Lyman and Werner bands, respectively. Here, we have measured the electron-impact-induced emission spectrum of H2 in the VOIR wavelength region 700 nm to 950 nm at a spectral resolution of 2 nm (FWHM). A model spectrum of H2, based on newly calculated transition probabilities and line positions including rovibrational coupling for the strongest band systems is in excellent agreement with observed intensities. The VOIR emission spectra of H2 and HD have never been studied before in optically-thin single-scattering conditions. This work will complete analytic models for use in electron transport codes of the two most fundamental sets of electronic cross sections in UV astronomy: the Lyman and Werner band systems (BΣu 1sσ2pσ – X Σu and C Πu 1sσ2pπ – XΣu ) of H2 and HD [7]. [1] Delgarno et al., Ap. J. Supp., 125, 237, (1999) ∗Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 †LUTH and UMR 8102 du CNRS, Observatoire de Paris, 92195 Meudon Cedex, France