Radek Plašil

Dynamical constraints and nuclear spin caused restrictions in collision systems

This contribution summarizes a variety of results and ongoing activities, which contribute to our understanding of inelastic and reactive collisions involving hydrogen ions. In an overview of our present theoretical knowledge of various collision systems (m+n≤5), it is emphasized that although the required potential energy surfaces are well characterized, no detailed treatments of the collision dynamics are available to date, especially at the low energies required for astrochemistry. Instead of treating state-to-state dynamics with state of the art methods, predictions are still based on: (i) simple thermodynamical arguments, (ii) crude reaction models such as H atom exchange or proton jump, or (iii) statistical considerations used for describing processes proceeding via long-lived or strongly interacting collision complexes. A central problem is to properly account for the consequences of the fact that H and D are fermions and bosons, respectively. In the experimental and results sections, it is emphasized that although a variety of innovative techniques are available and have been used for measuring rate coefficients, cross-sections or state-to-state transition probabilities, the definitive experiments are still pending. In the centre of this contribution are our activities on various m+n=5 systems. We report a few selected additional results for collisions of hydrogen ions with p-H2, o-H2, HD, D2 or well-defined mixtures of these neutrals. Most of the recent experiments are based on temperature variable multipole ion traps and their combination with pulsed gas inlets, molecular beams, laser probing or electron beams. Based on the state-specific model calculations, it is concluded that for completely understanding the gas phase formation and destruction of in a trap, an in situ characterization of all the experimental parameters is required with unprecedented accuracy. Finally, the need to understand the hydrogen chemistry relevant for dense pre-stellar cores is discussed.

The recombination of H3+ ions with electrons: dependence on partial pressure of H2

Abstract We observed an increase in the effective recombination coefficient ( α eff ) of the recombination of H 3 + ions with electrons with a number density of H 2 , n( H 2 ) . With increasing n (H 2 ) from 1×10 11 to 2×10 12 cm −3 the α eff increased from 1.3×10 −8 to 1.5×10 −7 cm 3 s −1 . The dependence of α eff on n (H 2 ) indicates that the recombination of H 3 + ions in an afterglow plasma is a complex process in which collisions with H 2 take place. It is stressed that at conditions corresponding to plasma in interstellar space the dissociative recombination of H 3 + ions is very slow with a rate coefficient α⩽1.3×10 −8 cm 3 s −1 .

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.

Binary and ternary recombination of para-H3+ and ortho-H3+ with electrons: State selective study at 77–200 K

Measurements in H(3)(+) afterglow plasmas with spectroscopically determined relative abundances of H(3)(+) ions in the para-nuclear and ortho-nuclear spin states provide clear evidence that at low temperatures (77-200 K) para-H(3)(+) ions recombine significantly faster with electrons than ions in the ortho state, in agreement with a recent theoretical prediction. The cavity ring-down absorption spectroscopy used here provides an in situ determination of the para/ortho abundance ratio and yields additional information on the translational and rotational temperatures of the recombining ions. The results show that H(3)(+) recombination with electrons occurs by both binary recombination and third-body (helium) assisted recombination, and that both the two-body and three-body rate coefficients depend on the nuclear spin states. Electron-stabilized (collisional-radiative) recombination appears to make only a small contribution.

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.

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.

Binary recombination of para- and ortho-H3+ with electrons at low temperatures

Results of an experimental study of binary recombination of para- and ortho- ions with electrons are presented. Near-infrared cavity-ring-down absorption spectroscopy was used to probe the lowest rotational states of ions in the temperature range of 77–200 K in an -dominated afterglow plasma. By changing the para/ortho abundance ratio, we were able to obtain the binary recombination rate coefficients for pure and . The results are in good agreement with previous theoretical predictions.

Electron collisions and rovibrational action spectroscopy of cold H3+ molecules

Electron recombination of H3+ has found a lot of attention due to its outstanding relevance for the chemistry of the interstellar medium (ISM) and its role as a benchmark for the treatment of dissociative recombination (DR) of polyatomic ions. We report DR measurements performed at the TSR storage ring utilizing a cryogenic ion trap injector. Furthermore, a chemical probing spectroscopy technique is described that allows for a very sensitive monitoring of the populated states inside the ion injector. Since H3+ exists in two different nuclear spin modifications, a controlled manipulation of the ortho/para fraction is needed in order to perform state-selective measurements.

The recombination rate coefficient of a protonated acetone dimer with electrons: indication of a temperature dependence

The formation and recombination of a protonated acetone dimer with electrons was studied in the flowing afterglow. H3O+ ions were formed in the early post-discharge region. The subsequent addition of acetone leads to the formation of the protonated ions CH3COCH3H+. These ions further associate with acetone forming the cluster ions H+(CH3COCH3)2 that react further with acetone, but very slowly. This facilitates the creation of an afterglow plasma with a dominant population of H+(CH3COCH3)2. The evolution of electron number density (ne) and electron temperature (Te) is measured using the Langmuir probe. The recombination rate coefficients that are obtained indicate a negative temperature dependence: α = (3.4±1)×10-6 cm3 s-1 at Te = (580±150) K and α = (7±2.5)×10-6 cm3 s-1 at Te = (450±100) K.

Combined Langmuir probe, electrical and hybrid modelling characterization of helium glow discharges

We present a systematic study of electrical characteristics and Langmuir probe measurements on plane-parallel electrode helium glow discharges operated in normal and abnormal glow regimes. The experimental studies are complemented with numerical modelling based on a hybrid approach combining fluid and particle descriptions of the discharge plasma. A comparison between the experimental data and the two-dimensional modelling results is presented. The temperature of slow Maxwellian electrons is found to be ≈0.055 eV in the negative-glow region nearly independent of the discharge conditions. This temperature value is used as an input parameter of the hybrid model. Good agreement is obtained between the calculated spatial density distribution of electron impact excitation events and the CCD image of the light emission from the discharge. The calculated electron densities are 1.3–2 times higher than the corresponding Langmuir probe data; the possible reason for these differences is discussed.