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Reactive molecular ions

AuthorsGoicoechea, Javier R.
Issue Date24-Jun-2019
CitationAstrochemistry: From nanometers to megaparsecs (2019)
AbstractReactive ions are transient species for which the timescale of reactive collisions (leading to a chemical reaction) with H2, H, or e¿ is comparable to, or shorter than, that of inelastic collisions (Black et al. 1998). The formation of reactive ions such as CH+ and SH+ depends on the availability of C+ and S+ (i.e., of UV pho- tons, thus high ionization fractions xe = n(e¿ ) / nH ) and on the presence of excited H2 (either UV-pumped or hot and thermally excited). This allows overcoming the high endothermicity (and sometimes barrier) of their formation (Sternberg & Dalgargo 1995; Agu ¿ndez et al. 2010). The reaction C+ + H2(v) ¿ CH+ + H (1), for example, is endothermic by ¿E/k ¿ 4300 K if v=0, but exothermic and fast for v¿1 (Hierl et al. 1997). The reaction S+ + H2(v) ¿ SH+ + H (endothermic by ¿E/k¿9860 K when v=0) becomes exothermic when v¿2 (Zanchet et al. 2019). CH+ was one of the first molecules detected in the 1930s. Due to the high endothermicity of reaction (1), explaining the presence of CH+ absorption lines in diffuse clouds has been a long standing problem in astrochemistry (e.g. Godard, Falgarone et al. 2012). Herschel and now ALMA, have allowed us to image the emission from CH+ and SH+ rotational lines toward interstellar and circumstellar sources irradiated by strong stellar UV fields (see Gerin et al. 2016 for a review). In dense gas, nH ¿¿ 105 cm¿3, the lifetime of CH+ is so short, a few hours, that the molecule may form and be de- stroyed without experiencing non-reactive collisions with other species. Contrary to most interstellar molecules, this implies that CH+ does not have time to thermalize, by elastic collisions, its translational motions to a ve- locity distribution at Tk. Hence, CH+ rotational lines are expected to show broad line shapes related to the energy excess upon formation (thousands of K) and not to the actual Tk nor to an enhanced gas turbulence. Hence, CH+ can be excited by radiation many times during its its mean-free-time for non-reactive collisions, so that it remains kinetically hot (large velocity dispersion) and rotationally warm (high Trot) while it emits. Reac- tive ions can thus ¿retain some memory of the energetics of the formation process¿. This ¿formation pumping¿ was anticipated by John Black in his eloquent Faraday Discussion¿s paper (1998) and explicitly modelled af- ter Herschel¿s detections (Nagy et al. 2013; Godard & Cernicharo 2013). Today, CH+ (J = 1¿0) emission has mapped at parsec scales along Orion star-forming region (Goicoechea et al. 2019) and SH+ emission has been spatially resolved at the edge of the Orion Bar PDR with ALMA at ¿¿ 10¿2 pc resolution (Goicoechea et al. 2017). Research on reactive ions is active and offers a common ground to study fundamental processes by as- tronomers and chemists (both in the lab and by ab initio quantum calculations).
DescriptionConferencia invitada. -- A symposium in honour of John H. Black, Gothenburg, June 24 - 28, 2019
Appears in Collections:(CFMAC-IFF) Comunicaciones congresos
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