Štěpán Roučka

PHOTODETACHMENT AS A DESTRUCTION MECHANISM FOR CN- AND C3N- ANIONS IN CIRCUMSTELLAR ENVELOPES

Absolute photodetachment cross sections of two anions of astrophysical importance CN- and C3N- were measured to be (1.18 +- (0.03)_stat (0.17)_sys) * 10^-17 cm^2 and (1.43 +- (0.14)_stat (0.37)_sys) * 10^-17 cm^2 respectively at the ultraviolet wavelength of 266 nm (4.66 eV). These relatively large values of the cross sections imply that photodetachment can play a major role in the destruction mechanisms of these anions particularly in photon-dominated regions. We have therefore carried out model calculations using the newly measured cross sections to investigate the abundance of these molecular anions in the cirumstellar envelope of the carbon-rich star IRC+10216. The model predicts the relative importance of the various mechanisms of formation and destruction of these species in different regions of the envelope. UV photodetachment was found to be the major destruction mechanism for both CN- and C3N- anions in those regions of the envelope, where they occur in peak abundance. It was also found that photodetachment plays a crucial role in the degradation of these anions throughout the circumstellar envelope.

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.

Binary and ternary recombination of and ions with electrons in low temperature plasma

Measurements of recombination rate coefficients of binary and ternary recombination of and ions with electrons in a low temperature plasma are described. The experiments were carried out in the afterglow plasma in helium with a small admixture of Ar and parent gas (H2 or D2). For both ions a linear increase of measured apparent binary recombination rate coefficients (αeff) with increasing helium density was observed: αeff = αBIN + K He[He]. From the measured dependencies, we have obtained for both ions the binary (αBIN) and the ternary (K He) rate coefficients and their temperature dependence. For the description of observed ternary recombination a mechanism with two subsequent rate determining steps is proposed. In the first step, in + e− (or + e−) collision, a rotationally excited long-lived Rydberg molecule (or ) is formed. In the following step (or ) collides with a He atom of the buffer gas and this collision prevents autoionization of (or ). Lifetimes of the formed (or ) and corresponding ternary recombination rate coefficients have been calculated. The theoretical and measured binary and ternary recombination rate coefficients obtained for and ions are in good agreement.

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.

Temperature dependence of binary and ternary recombination of D3 + ions with electrons

Flowing and stationary afterglow experiments were performed to study the recombination of D(3)(+) ions with electrons at temperatures from 77 to 300 K. A linear dependence of apparent (effective) binary recombination rate coefficients on the pressure of the helium buffer gas was observed. Binary (D(3)(+)+e(-)) and ternary (D(3)(+)+e(-)+He) recombination rate coefficients were derived. The obtained binary rate coefficient agrees with recent theoretical values for dissociative recombination of D(3)(+). We describe the observed ternary process by a mechanism with two rate determining steps. In the first step, a rotationally excited long-lived neutral D(3)* is formed in D(3)(+)-e(-) collisions. As the second step, the D(3)* collides with a helium atom that prevents autoionization of D(3)*. We calculate lifetimes of D(3)* formed from ortho-, para-, or metastates of D(3)(+) and use the lifetimes to calculate ternary recombination rate coefficients.

Binary and ternary recombination of D3+ ions at 80–130 K: Application of laser absorption spectroscopy

Recombination of D(3)(+) ions with electrons at low temperatures (80-130 K) was studied using spectroscopic determination of D(3)(+) ions density in afterglow plasmas. The use of cavity ring-down absorption spectroscopy enabled an in situ determination of the abundances of the ions in plasma and the translational and the rotational temperatures of the recombining ions. Two near infrared transitions at (5792.70 ± 0.01) cm(-1) and at (5793.90 ± 0.01) cm(-1) were used to probe the number densities of the lowest ortho state and of one higher lying rotational state of the vibrational ground state of D(3)(+) ion. The results show that D(3)(+) recombination with electrons consists of the binary and the third-body (helium) assisted process. The obtained binary recombination rate coefficients are in agreement with a recent theoretical prediction for electron-ion plasma in thermodynamic equilibrium with α(bin)(80 K) = (9.2 ± 2.0) × 10(-8) cm(3) s(-1). The measured helium assisted ternary rate coefficients K(He) are in agreement with our previously measured flowing afterglow data giving a value of K(He)(80 K) = (1.2 ± 0.3) × 10(-25) cm(6) s(-1).

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.

Application of NIR – CRDS for state selective study of recombination of para and ortho H3+ ions with electrons in low temperature plasma

We present a study of H3+ recombination performed at 77 K on the two lowest rotational levels of this ion, which belong to its two different nuclear spin states of the studied ion. A near infrared cavity ring-down spectrometer (~1381 nm, CRDS arrangement) has been used to obtain the time evolution of concentration of both states. From the overall ion density decay during the afterglow we obtained the binary recombination rate coefficient αbin (77 K) = 1.2×10−7 cm3s−1. We have also observed ternary helium assisted recombination of both para and ortho H3+. The process is very slow (at 77 K) and the obtained ternary recombination rate coefficient is in contradiction with the theoretical prediction. It is the first time that the binary and ternary H3+ recombination rate coefficient was measured at a known population of para and ortho H3+ ions in decaying plasma.

Extending PIC Models to Higher Pressures—Enhanced Model of Collisions

This paper treats the problems of the particle-in-cell (PIC) models at higher pressures. We discuss the influence of the time discretization on the results of a model of a highly collisional plasma using a simplified analytical model. A new algorithm is proposed that calculates the collisions independently of the simulation time step, thus eliminating the errors introduced by the time discretization. The benchmarks at the pressure of the neutral gas of 665 Pa show that the presented algorithm is approximately seven times faster than a classical PIC model with the same accuracy.