No abstract

No abstract

Complexes of the triatomic hydrogen ion with helium were synthesised in a low-temperature 22-pole rf ion trap at He number densities of up to 10 16 cm −3 . Absolute ternary rate coefficients for sequentially attaching He atoms have been determined from the growth of complexes with increasing storage time. The number of helium-tagged ions is significantly reduced when increasing the nominal temperature from 4 to 25 K. Competition between attachment and dissociation via collisions leads to stationary He n -H + 3 (n up to 9) distributions. State-specific excitation of the trapped H + 3 ions via IR transitions significantly reduces the formation of complexes. Tuning the laser to v 2 = 1 transitions in the range of 2726 cm −1 leads to LIICG lines, i.e., to spectra caused by laser-induced inhibition of complex growth. In addition, almost 100 lines have been found between 2700 and 2765 cm −1 , which are attributed to laser-induced dissociation of the in situ formed He-H + 3 complex ions. These lines are not yet assigned; however, their absorption strength, statistics and predissociation lifetimes provide interesting information on both the stable complexes as well as on scattering resonances in low-energy H + 3 + He collisions. New calculations of the potential energy surface will help to analyse the dissociation spectrum. There are some indications that para-H + 3 is enriched under the conditions of the present experiment. IntroductionStudies of reactions between ions and neutrals under conditions relevant for astrochemistry are important for understanding different processes in the interstellar medium (ISM). Reactions involving helium and hydrogen significantly influence the overall evolution of our universe including the early universe chemistry and especially the 'Dark Age'. For modelling such environments, one needs reliable rate coefficients. A detailed discussion of primordial chemistry can be found in [1] and references therein. According to [2], helium became neutral at a red shift of z ∼ 2500. At those times, the density was already very low and the only way to form molecules was via radiative association. Because hydrogen was still ionised, models predict that the first molecule formed in space was HeH + . The coexistence of He and He + also lead to the formation of some He + 2 . Molecules were needed as coolants during the formation of the first galaxies. Also after these early times, helium and hydrogen play an important role. In all models of interstellar ion chemistry, the first steps are ionisation of He and H 2 by cosmic rays.

Hydrogenation and deuteration of C3+, C3H+, C3H2+ in collisions with H2 and HD has been studied from room temperature down to 10 K using a 22-pole ion trap. Although exothermic, hydrogenation of C3+ is rather slow at room temperature but becomes faster with decreasing temperature. In addition to the increasing lifetime of the collision complex this behavior may be caused by the floppy structure of C3+ and the freezing of soft bending modes below 50 K. For C3(+) + HD it has been shown that production of C3D+ is slightly favored over C3H+ formation. The controversy over which products are really formed in C3H(+) + H2 collisions and deuterated variants has a long history. Previous and new ion trap results prove that formation of C3H2(+) + H is not endothermic but rather fast, in contradiction to erroneous conclusions from flow tube experiments and ab initio calculations. In addition the reaction shows a complicated isotope dependence, most probably caused by the influence of zero point energies in entrance and exit transition states. For example hydrogen abstraction with HD is faster than with H2 while radiative association is slower. The most surprising result has been obtained for C3H(+) + HD. Here C3HD+ formation is over one hundred times faster than C3H2+. In addition to the details of the potential energy surface it may be that in this case an H-HD exchange reaction takes place via an open-chain propargyl cation intermediate (H2CCCH+). Reactions of C3H2+ and C3H3+ with H2 are very slow but, due to the unique sensitivity of the trapping technique, significant rate coefficients have been determined. The presented results are of fundamental importance for understanding the energetics, structures and reaction dynamics of the deuterated variant of the C3Hn+ collision system. They indicate that the previous quantum chemical calculations are not accurate enough for understanding the low energy behavior of the Cn,Hm+ reaction systems. The laboratory experiments are of essential relevance for the carbon chemistry of dense interstellar clouds, both for formation of small hydrocarbons and deuterium fractionation.

No abstract