Dieter Gerlich

Ion trap studies of association processes in collisions of CH3(+) and CD3(+) with n-H2, p-H2, D2, and He at 80 K

Formation of stable CH5(+) and CD5(+) has been studied in a liquid nitrogen cooled radio-frequency (RF) ion trap. Ternary rate coefficients, k3, have been measured for He, H2, and D2 as the stabilizing collision partner. The results for He are in satisfying agreement with SIFT data. The experiments have been performed at very low densities permitting information to be extracted on the radiative association process. Because both k3 and the radiative rate coefficient k(r) are observed under the same experimental conditions, comparison leads to a reliable upper limit of the radiative stabilizing rate of the intermediate CH5(+) collision complex. The results are significantly smaller than the values currently discussed in the literature, but they are in accord with infrared relaxation rates measured directly for highly excited CH5(+) ions. Using two different mixtures of p-H2 and o-H2 at 80 K, the influence of the H2 rotational energy was determined. The derived ratio is four times smaller than predicted from theoretical work. 30 refs.

REACTIONS OF COLD TRAPPED CH+ IONS WITH SLOW H ATOMS

The destruction of CH + ions in collisions with H atoms has been studied in a temperature-variable 22 pole ion trap (22PT) combined with a cold effusive H-atom beam. The stored ions are relaxed to temperatures of T22PT 12 K. The hydrogen atoms, produced in a radio frequency discharge, are slowed down to various temperatures of TACC 7 K. They are formed into an effusive beam. The effective density of the hydrogen atoms in the trap as well as the H2 background are determined in situ using chemical probing with CO2 + . The experimental arrangement allows us not only to measure thermal rate coefficients (T22PT = TACC), but also to extract state-specific rate coefficients k(J,Tt )a t selected translational temperatures Tt and for the CH + rotational states J = 0, 1, and 2. The measured thermal rate coefficients have a maximum at 60 K, k = (1.2 ± 0.5)×10 −9 cm 3 s −1 . Toward higher temperatures, they fall off in accordance with previous measurements and the trend predicted by phase space theory. Toward lower temperatures, the rate coefficients decrease significantly, especially if the rotation of the ions is cooled. At the coldest conditions achieved (beam: 7.3 K; trap: 12.2 K), a value as low as (5 ± 4) × 10 −11 cm 3 s −1 has been measured. This leads to the conclusion that non-rotating CH + is protected against attacks of H atoms. This surprising result is not yet

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.

Action spectroscopy of H3+ and D2H+ using overtone excitation.

The ion and its deuterated isotopologues H2D+, D2H+ and play an important role in astrophysical and laboratory plasmas. The main challenge for understanding these ions and their interaction at low temperatures are state-specific experiments. This requires manipulation and a simple but efficient in situ characterization of their low-lying rotational states. In this contribution we report measurements of near infrared (NIR) absorption spectra. Required high sensitivity is achieved by combining liquid nitrogen cooled plasma with the technique of NIR cavity ringdown absorption spectroscopy. The measured transition frequencies are then used for exciting cold ions stored in a low-temperature 22-pole radiofrequency ion trap. Absorption of a photon by the stored ion is detected by using the laser-induced reactions technique. As a monitor reaction, the endothermic proton (or deuteron) transfer to Ar is used in our studies. Since the formed ArH+ (or ArD+) ions are detected with near unit efficiency, the stored ions can be characterized very efficiently, even if there are just a few of them.

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 .

Cold and ultracold reactive collisions between FeO+ and H2

The nominal temperature range of cryogenic radio-frequency ion traps has recently been extended down to T=2.3 K. Whereas in situ He tagging of mass-selected ions embedded in dense helium buffer gas is becoming common for recording IR spectra through photofragmentation of small and large ions, much less activity is devoted to the field of cold chemistry, which in this contribution means the two orders of magnitude extending from 300 to below 3 K. The importance of this temperature range for understanding the dynamics of bi- and termolecular reactions is illustrated with new results for the time-honored reaction of FeO+ with H2 obtained with the cryogenic ion trap ISORI in Prague. The rate coefficient for forming Fe+ +H2 O increases steeply with decreasing temperature. In addition more product channels open up, such as the stabilized reaction-intermediate complexes H2 FeO+ and Hen -FeO+ formed by ternary association with He. For the FeOH+ +H channel only a minor signal is observed. The rate coefficients provide deep insight into lifetimes, bottlenecks, and barriers impeding almost completely the exothermic, but spin-forbidden, reaction at room temperature. For some of the He-tagged ions, IR predissociation spectra are recorded. A breakthrough is obtaining the first spectrum of [(H2 )FeO]+ , synthesized and tagged in situ with He. These results pave the way to study the structures of reaction intermediates stabilized in the gas phase by means of collisions with helium.

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.

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.

Interactions of H? Anions with Atomic Hydrogen?Ion Trap study at 10?100 K

Results of an experimental study of the associative detachment reaction H + H− → H2 + e− and a description of the ion trap apparatus combined with a variable temperature beam of atomic hydrogen are presented.