Illia Zymak

ION TRAP STUDIES OF H– + H → H2 + e – BETWEEN 10 AND 135 K

Thermal rate coefficients for forming H2 via associative detachment in H– + H collisions were determined using the combination of a 22-pole ion trap (22PT) with a skimmed effusive beam of atomic hydrogen penetrating the ion cloud. The temperature of both reactants have been varied independently (ion trap: T 22PT = 10-150 K, neutral beam accommodator T ACC = 10, 50, 120 K). Using various combinations, the temperature range between 10 and 135 K has been accessed for the first time experimentally. The effective number density of H (typically some 108 cm–3) is determined in situ via chemical probing with CO+ 2 ions. With decreasing temperature, the measured thermal rate coefficients decrease slowly from 5.5 × 10–9 cm3 s–1 at 135 K to 4.1 × 10–9 cm3 s–1 at 10 K. The relative error is 10%, while the absolute values may deviate systematically by up to 40%, due to uncertainties in the calibration reaction. Significant improvements of the versatile and sensitive experiment are possible, e.g., by using electron transfer from H to D+ as calibration standard.

State specific stabilization of H+ + H2(j) collision complexes.

Stabilization of H3(+) collision complexes has been studied at nominal temperatures between 11 and 33 K using a 22-pole radio frequency (rf) ion trap. Apparent binary rate coefficients, k(*) = kr + k3[H2], have been measured for para- and normal-hydrogen at number densities between some 10(11) and 10(14) cm(-3). The state specific rate coefficients extracted for radiative stabilization, kr(T;j), are all below 2 × 10(-16) cm(3) s(-1). There is a slight tendency to decrease with increasing temperature. In contrast to simple expectations, kr(11 K;j) is for j = 0 a factor of 2 smaller than for j = 1. The ternary rate coefficients for p-H2 show a rather steep T-dependence; however, they are increasing with temperature. The state specific ternary rate coefficients, k3(T;j), measured for j = 0 and derived for j = 1 from measurements with n-H2, differ by an order of magnitude. Most of these surprising observations are in disagreement with predictions from standard association models, which are based on statistical assumptions and the separation of complex formation and competition between stabilization and decay. Most probably, the unexpected collision dynamics are due to the fact that, at the low translational energies of the present experiment, only a small number of partial waves participate. This should make exact quantum mechanical calculations of kr feasible. More complex is three-body stabilization, because it occurs on the H5(+) potential energy surface.

Stabilization of H+–H2 collision complexes between 11 and 28K

Formation of via association of H+ with H2 has been studied at low temperatures using a 22-pole radiofrequency trap. Operating at hydrogen number densities from 1011 to 1014 cm−3, the contributions of radiative, kr, and ternary, k3, association have been extracted from the measured apparent binary rate coefficients, k*=kr+k3[H2]. Surprisingly, k3 is constant between 11 and 22 K, (2.6±0.8)×10−29 cm6 s−1, while radiative association decreases from kr(11 K)=(1.6±0.3)×10−16 cm3 s−1 to kr(28 K)=(5±2)×10−17 cm3 s−1. These results are in conflict with simple association models in which formation and stabilization of the complex are treated separately. Tentative explanations are based on the fact that, at low temperatures, only few partial waves contribute to the formation of the collision complex and that ternary association with H2 may be quite inefficient because of the ‘shared proton’ structure of .

Interaction of O− and H2 at low temperatures

Reactive collisions between O(-) and H2 have been studied experimentally at temperatures ranging from 10 K to 300 K using a cryogenic radiofrequency 22-pole ion trap. The rate coefficients for associative detachment, leading to H2O + e(-), increase with decreasing temperature and reach a flat maximum of 1.8 × 10(-9) cm(3) s(-1) at temperatures between 20 K and 80 K. There, the overall reaction probability is in good agreement with a capture model indicating efficient non-adiabatic couplings between the entrance potential energy surfaces. Classical trajectory calculations on newly calculated potential energy surfaces as well as the topology of the conical intersection seam leading to the neutral surface corroborate this. The formation of OH(-) + H via hydrogen transfer, although occurring with a probability of a few percent only (about 5 × 10(-11) cm(3) s(-1) at temperatures 10-300 K), indicates that there are reaction paths, where electron detachment is avoided.

Anion Chemistry on Titan: systematic studies of the growth and stability of large negative ions

The reactions of C2n+1N−anions and their complexes with cyanoacetylene, HC3N, and hydrogen cyanide, HCN, were studied experimentally using methods of tandem mass spectrometry. The reactions described in detail in this communication may represent a possible way of formation of large anions found in the ionosphere of Titan.

Reactions of O− with D2 at temperatures below 300 K

The reaction of O− anions with molecular deuterium D2 has been studied experimentally using a cryogenic 22-pole radiofrequency ion trap. Two reaction channels were observed. In the associative detachment D2O and an electron are formed and for atom transfer formation OD− + D was observed. The rate coefficients of the reactions have been determined at temperatures below 300 K. The reaction rate coefficient k 1 of the associative detachment increases with decreasing temperature from k 1(300 K) = 0.5 × 10−9 cm3 s−1 at 300 K up to k 1(70 K) = 1.2 × 10−9 cm3 s−1 at 70 K both with 30 % overall uncertainty.

Reactions of O− with D2 at low temperatures 10 − 300 K

Energetically possible are also reactions leading to the formation of metastable D2O anion in three body or radiative association. However these processes have not been detected experimentally. Studies of gas-phase processes involving water and especially those leading to isotopic fractionation [1] is essential for understanding of the water formation in the Universe. The reactions were studied using a 22-pole radiofrequency ion trap. It was mounted on a cryo-cooler in an ultra-high vacuum system. The measuring procedure was based on iterative filling of the trap with a well-defined number of primary ions O where they react with molecular deuterium. The content of the trap were analyzed after chosen times using a quadrupole mass spectrometer and a micro-channel plate detector. Further detail may be found in [2] and references therein. We studied previously analogous reactions involving O and H2. At first we measured electron energy spectrum originating from associative detachment [3] at 300 K. Later on we studied both reactions using the 22-pole ion trap [4]. Very interesting dependence on temperature is shown on Figure 1. Up to now the associative detachment has been measured mostly at temperatures higher than 300 K and the observed reaction rate coefficient was about 1⁄3 of the Langevin rate. It is explained by the fact that trajectories on only one of three electronic surfaces of the collision system can lead directly to autodetachment. At lower temperatures we observed unexpectedly high value of the associative detachment rate coefficient. Theoretical model in our paper [4] for the reaction O + H2 explained this behavior by rearrangement of the collisional system in a shallow local minimum. The increase of the reaction rate coefficient at low temperatures is in qualitative agreement with the theoretical predictions. The presented data will be compared with classical trajectory simulation model. An isotopic effect will be evaluated.

Ion-induced ionization and fragmentation of organic molecules

Processes in high-energy collisions of positive and negative ions with selected thermal neutral molecules were studied. It was shown that internal energy deposited in nascent positive ions strongly depends on the nature of incident ions.

Formation of NH+ in collisions of N+ with para- or ortho-H2 at low temperatures – an experimental study

Using a radiofrequency ion trap at low temperatures, an experimental study of collisions of N+ with or-tho/para-H2 is reported.

Radiative association of H+ and H2 - experimental study

The radiative association of H+ and H2 was studied at low temperature in 22-pole radiofrequency trap. The rate coefficient measured at 11 K is (1.4±0.7)×l0−16 cm3 s−1.