Charge exchange and dissociative processes in collisions of slowHe2+ions withH2Omolecules

Abstract

Experimental and theoretical studies of one-electron capture in collisions of He 2+ ions with H 2 O molecules have been carried out in the range 0.025-12 keV amu −1 corresponding to typical solar wind velocities of 70-1523 km s −1 . Translational energy spectroscopy ͑TES͒, photon emission spectroscopy ͑PES͒, and fragment ion spectroscopy were employed to identify and quantify the collision mechanisms involved. Cross sections for selective single electron capture into n = 1, 2, and 3 states of the He + ion were obtained using TES while PES provided cross sections for capture into the He + ͑2p͒ and He + ͑3p͒ states. Our model calculations show that He + ͑n =2͒ and He + ͑n =3͒ formation proceeds via a single-electron process governed by the nucleus-electron interaction. In contrast, the He + ͑1s͒ formation mechanism involves an exothermic two-electron process driven by the electron-electron interaction, where the potential energy released by the electron capture is used to remove a second electron thereby resulting in fragmentation of the H 2 O molecule. This process is found to become increasingly important as the collision energy decreases. The experimental cross sections are found to be in reasonable agreement with cross sections calculated using the Demkov and Landau-Zener models.

EBIT charge-exchange measurements and astrophysical applications

“…6. The measurement was carried out using 50 keV He 2+ ions at the Kernfysisch Versneller Instituut in Groningen [96,97]. While most of the emission is the result of single-electron capture, a small feature corresponding to the 1s2p 1 P 1 → 1s 2 1 S 0 transition in neutral He is seen.…”

“…The latter transition is observed in first order and is attributed to double-electron capture during CX. The spectrum was obtained at the Kernfysisch Versneller Instituut in Groningen [96,97] at a collision energy of 48 keV.…”

Division Xii / Commission 14 / Working Group Collision Processes

“…In these environments charge transfer plays an important role. Recent studies of ionmolecule charge transfer include He 2+ (Dubois et al 2005, Kusakabe et al 2006, C 4+ (Zarour et al 2005), O + , O 5+ and Ar 5+ (Dubois et al 2005), and F 7+ (Zouros et al 2008) with H 2 ; H + (Lindsay et al 2005a, Wells et al 2005, Kumar et al 2006, Lin et al 2007 and He 2+ (Kusakabe et al 2006) , He 2+ (Bodewits, et al 2005, Seredyuk et al 2005c, O 6+ (Seredyuk et al 2005b, Bodewits & Hoekstra 2007, and a range of ions (Otranto & Olson 2008) (Seredyuk et al 2005a), and C 4+ and O 6+ (Seredyuk et al 2005b) with CH 4 ; H + (Suzuki et al 2005) and He 2+ (Seredyuk et al 2005a) with C 2 H 4 ; and He 2+ (Seredyuk et al 2005a) For applications to x-ray emission from comets and planetary atmospheres, charge exchange has been considered for molecular targets including C (3−6)+ , N (4−7)+ , and O (5−7)+ with methane (Djurić et al 2008); highly charged L-shell Fe ions with various neutrals (Wargelin et al 2005, Beiersdorfer et al 2008; and for a range of ions and molecules (Otranto et al 2006). …”

Dissociative ionization of the H2O molecule induced by medium-energy singly charged projectiles

“…In most of the experiments, the dissociation pattern of water was studied thoroughly only for the low-charge-state transient molecular ion (H 2 O q+ , where q 3) [4,5,9,10,14,15,22,23]. In a recent work Pedersen et al [2] studied the dissociation of the H 2 O 2+ molecular ion in detail, induced by XUV photons from H 2 O + ions.…”

“…From the spectra we determined absolute cross sections for the individual fragmentation channels. The latter procedure is based on extensive earlier studies performed by several research groups [2,[10][11][12][13]15,[22][23][24][25][26], in which the overwhelming majority of the fragmentation channels have been identified and their KER data have been determined, dominantly for H + , He q+ , and HCI projectiles. From the cross sections determined for the individual fragmentation channels we deduced the multipleionization cross sections for the target molecule.…”