Dissociative recombination in the interstellar medium and planetary ionospheres

Abstract

Dissociative recombination is a corner-stone reaction in the synthesis of interstellar molecules and plays an important role in the ionised layers of the atmospheres of the Earth, other planets, and their satellites. The studies of dissociative recombination reactions was for a long time dominated by afterglow techniques, and these techniques are used in improved versions also today, but the bulk of the relevant data have in recent years been produced at ion storage rings. We review briefly these different experimental techniques. As examples of recent developments in the field, we discuss in particular the dissociative recombination of H 3 + , the role of dissociative recombination in the chemistry of hydrocarbons and sulphur compounds in the interstellar medium and planetary ionospheres, and the synthesis of interstellar methanol. Keywords: Dissociative Recombination, Ion-electron interaction, interstellar medium, planetary ionospheres AMS Subject Classification: 85-05 IntroductionHalf a century ago, when this journal was founded, interstellar clouds were thought to be essentially atomic with a small component of diatomic molecules. Now the situation is radically different. The realization that the Universe is highly molecular has led to new areas of astronomy such as astrochemistry and molecular astrophysics. Prestigious conferences, such as several Faraday Discussions [1,2], International Astronomical Union meetings [3, 4, 5, 6, 7, 8], and the Nobel Symposium No 133 [9] have been devoted to these new areas. Astrochemistry was also a theme in the 5 th International Chemical Congress of Pacific Basin Societies (PACIFICHEM) [10] and a special feature issue on the subject has been published in the Proceedings of the National Academy of Sciences [11]. Molecules are ubiquitous almost anywhere in the Universe cooler than sunspots.Dissociative recombination (DR) of molecular ions with electrons was first discussed in connection with the Earth's ionosphere, with photoionisation being the source of free electrons and DR the sink [12]. When synthetic schemes based on ion-molecule reactions were introduced to explain molecular abundances [13, 14], they scored an immediate success with the prediction of HCO + [15] as the carrier of the interstellar 89.190 GHz line [16], later confirmed by laboratory molecular spectroscopy [17], dissociative recombination became an important part of the scheme for interstellar molecule formation. In this article we 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Recent reviews of dissociative recombination have been written by , Adams, Poterya and Babcock [19], and Thomas [20], and a research monograph by Larsson and Orel [21] has just been published. It would by far exceed the scope of this article to cover all dissociative recombination processes that are relevant in the interstellar medium and p...

Experimental studies of the dissociative recombination of CD3CDOD+and CH3CH2OH$_{2}^+$Unknown node mtable found in MathML fragment.

“…Subsequently, heavy-ion storage rings like ASTRID (Denmark), TSR (Heidelberg) and CRYRING (Stockholm, Sweden) have been successfully used as tools for investigations of astrophysically relevant DR reactions (see e.g. Larsson et al 1993Larsson et al , 1995Al-Khalili et al 1998;Neau et al 2000;McCall et al 2003;Hamberg et al 2005;Geppert et al 2005;Geppert & Larsson 2008;Thomas 2008;Andersen et al 1996;Jensen et al 2000;Kreckel et al 2005;Nevo et al 2007). However, such time-consuming experiments are restricted to a few places over the world and investigators are A&A 522, A90 (2010) therefore often struggling to meet the dire need of experimental data from astrochemical modelers.…”

“…Although DR reactions are omnipresent in plasmas and thus crucial processses in these environments, product branching ratios of DR reactions have proven to be very unpredictable and present one of the great remaining challenges for theoreticians (see e.g. Geppert & Larsson 2008). To elucidate these completely by experimental methods is also far from trivial and it was as late as in the 1990s when Datz et al (1995) ingeniously applied the technique of merged ion electron beams using a storage ring in conjunction with a previously invented grid technique (Mitchell et al 1982) to obtain solid information about these distributions.…”

Planetary Magnetism as a Parameter in Exoplanet Habitability

“…Water is lost through a number of non-thermal processes such as dissociation and dissociative recombination (Geppert & Larsson 2008), ion pickup (Lammer et al 2006), charge exchange (Dong et al 2017), sputtering (Terada et al 2009), and atmospheric escape (Zahnle & Catling 2017). Future research should be conducted to more carefully investigate which water-loss processes dominate in different planetary scenarios.…”

SUISEI—A Versatile Global Model of Comets with Applications to Small Solar System Bodies

“…It is seen in Figure 3 state, leading to prompt emission of "hot" rotational-vibrational bands. In addition, the recombination process converts a fraction of molecules to the isomer, HNC, also observed in comets [13]. HCN molecules observed in the coma can consist of those initially released from the nucleus (native) and those that are freshly formed at different locations in the coma via these protonation/recombination reactions.…”