Upper Limits to the Reaction Rate Coefficients of C_n^– and C_nH^– (n = 2, 4, 6) with Molecular Hydrogen

Abstract

In the interstellar medium (ISM) ion–molecule reactions play a key role in forming complex molecules. Since 2006, after the radioastronomical discovery of the first of by now six interstellar anions, interest has grown in understanding the formation and destruction pathways of negative ions in the ISM. Experiments have focused on reactions and photodetachment of the identified negatively charged ions. Hints were found that the reactions of CnH(–) with H2 may proceed with a low (<10(–13) cm(3) s(–1)), but finite rate [Eichelberger, B.; et al. Astrophys. J. 2007, 667, 1283]. Because of the high abundance of molecular hydrogen in the ISM, a precise knowledge of the reaction rate is needed for a better understanding of the low-temperature chemistry in the ISM. A suitable tool to analyze rare reactions is the 22-pole radiofrequency ion trap. Here, we report on reaction rates for Cn(–) and CnH(–) (n = 2, 4, 6) with buffer gas temperatures of H2 at 12 and 300 K. Our experiments show the absence of these reactions with an upper limit to the rate coefficients between 4 × 10(–16) and 5 × 10(–15) cm(3) s(–1), except for the case of C2(–), which does react with a finite rate with H2 at low temperatures. For the cases of C2H(–) and C4H(–), the experimental results were confirmed with quantum chemical calculations. In addition, the possible influence of a residual reactivity on the abundance of C4H(–) and C6H(–) in the ISM were estimated on the basis of a gas-phase chemical model based on the KIDA database. We found that the simulated ion abundances are already unaffected if reaction rate coefficients with H2 were below 10(–14) cm(3) s(–1).

Complex formation and internal proton-transfer of hydroxyl-hydrogen anion complexes at low temperature

“…The experiments are carried out in a cryogenic rf ion trap experiment that has been described previously [21,32,47]. A schematic drawing of the setup is shown in figure 1.…”

“…The field of cold molecules has caught increasing attention during the last decades, as it creates new opportunties for studies of the dipole moment of the electron [1,2] or possible change of the fine-structure constant [3][4][5]. It also made it possible to perform new precision spectroscopy experiments [6][7][8][9][10][11], and study collision [12][13][14] and reaction dynamics [15][16][17][18][19][20][21] of molecules at low temperatures. Optical pumping and sympathetic and buffer gas cooling are the most common approaches to create cold molecules, spanning a temperature range from the ultracold regime to several kelvin.…”

“…Several ion trap experiments have been carried out to study the cold chemistry of anions with neutrals at low temperatures down to about 10 K [16,21,42]. Recently chemical reactions and also inelastic collisions of the hydroxyl negative ion at temperatures below 20 K have been performed [33,43].…”

Incomplete rotational cooling in a 22-pole ion trap

“…This has opened up a diverse field of research on rate coefficients of ion-molecule reactions of astrochemical relevance [2,3], cross sections for photodetachment of interstellar ions [4,5] and high resolution molecular spectroscopy [6,7,8,9]. As sensitive spectroscopic probes laser-induced reactions [10], desorption of a tagged atom [11,12], and photodetachment [9] have been implemented.…”

H/D exchange in reactions of OH− with D2 and of OD− with H2 at low temperatures

“…1 However, the interest in anions diminished because, due to a lack of spectroscopic data, they could not be detected. This has changed recently when the first anions were observed in the interstellar medium, [2][3][4] reactivating the interest in theory 5,6 and experiments [7][8][9][10][11][12][13] with anion-molecule reactions, including associative detachment reactions. Also photodetachment of electrons from anions including interstellar anions has been reported.…”