Edouard Hugo

Chemical modeling of L183 (= L134N) : an estimate of the ortho/para H2 ratio

Context: The high degree of deuteration observed in some prestellar cores depends on the ortho-to-para H2 ratio through the H3+ fractionation. Aims: We want to constrain the ortho/para H2 ratio across the L183 prestellar core. This is required to correctly describe the deuteration amplification phenomenon in depleted cores such as L183 and to relate the total (ortho+para) H2D+ abundance to the sole ortho-H2D+ column density measurement. Methods: To constrain this ortho/para H2 ratio and derive its profile, we make use of the N2D^+/N2H+ ratio and of the ortho-H2D+ observations performed across the prestellar core. We use two simple chemical models limited to an almost totally depleted core description. New dissociative recombination and trihydrogen cation-dihydrogen reaction rates (including all isotopologues) are presented in this paper and included in our models. Results: We estimate the H2D+ ortho/para ratio in the L183 cloud, and constrain the H2 ortho/para ratio: we show that it varies across the prestellar core by at least an order of magnitude, being still very high (≈0.1) in most of the cloud. Our time-dependent model indicates that the prestellar core is presumably older than 1.5-2 × 105 years but that it may not be much older. We also show that it has reached its present density only recently and that its contraction from a uniform density cloud can be constrained. Conclusions: A proper understanding of deuteration chemistry cannot be attained without taking into account the whole ortho/para family of molecular hydrogen and trihydrogen cation isotopologues as their relations are of utmost importance in the global scheme. Tracing the ortho/para H2 ratio should also place useful constraints on the dynamical evolution of prestellar cores. Appendices A and B are only available in electronic form at http://www.aanda.org

H3++H2 isotopic system at low temperatures: Microcanonical model and experimental study

State-to-state thermal rate coefficients for reactions of all H(3)(+) + H(2) isotopic variants are derived and compared to new experimental data. The theoretical data are also sought for astrochemical modeling of cold environments (<50 K). The rates are calculated on the basis of a microcanonical approach using the Langevin model and the conservation laws of mass, energy, angular momentum, and nuclear spin. Full scrambling of all five nuclei during the collision is assumed for the calculations and alternatively partial dynamical restrictions are considered. The ergodic principle of the collision is employed in two limiting cases, neglecting (weak ergodic limit) or accounting for explicit degeneracies of the reaction mechanisms (strong ergodic limit). The resulting sets of rate coefficients are shown to be consistent with the detailed balance and thermodynamical equilibrium constants. Rate coefficients, k(T), for the deuteration chain of H(3)(+) with HD as well as H(2)D(+)/H(3)(+) equilibrium ratios have been measured in a variable temperature 22-pole ion trap. In particular, the D(2)H(+) + HD --> D(3)(+) + H(2) rate coefficient indicates a change in reaction mechanism when going to higher temperatures. The good overall agreement between experiment and theory encourages the use of the theoretical predictions for astrophysical modeling.

Overtone spectroscopy of H2D+ and D2H+ using laser induced reactions.

The method of laser induced reaction is used to obtain high-resolution IR spectra of H2D+ and D2H+ in collision with n-H2 at a nominal temperature of 17 K. For this purpose three cw-laser systems have been coupled to a 22-pole ion trap apparatus, two commercial diode laser systems in the ranges of 6100-6600 cm(-1) and 6760-7300 cm(-1), respectively, and a high-power optical parametric oscillator tunable in the range of 2600-3200 cm(-1). In total, 27 new overtone and combination transitions have been detected for H2D+ and D2H+, as well as a weak line in the nu1 vibrational band of H2D+ (2(20)<--1(01)) at 3164.118 cm(-1). The line positions are compared to high accuracy ab initio calculations, showing small but mode-dependent differences, being largest for three vibrational quanta in the nu2 symmetric bending of H2D+. Within the experimental accuracy, the relative values of the ab initio predicted Einstein B coefficients are confirmed.

Modelling line emission of deuterated H3+ from prestellar cores

Context. The depletion of heavy elements in cold cores of interstellar molecular clouds can lead to a situation where deuterated forms of H + are the most useful spectroscopic probes of the physical conditions. Aims. The aim is to predict the observability of the rotational lines of H2D + and D2H + from prestellar cores. Methods. Recently derived rate coefficients for the H + + H2 isotopic system were applied to the “complete depletion” reaction scheme to calculate abundance profiles in hydrostatic core models. The ground-state lines of H2D + (o) (372 GHz) and D2H + (p) (692 GHz) arising from these cores were simulated. The excitation of the rotational levels of these molecules was approximated by using the state-to-state coefficients for collisions with H2. We also predicted line profiles from cores with a power-law density distribution advocated in some previous studies. Results. The new rate coefficients introduce some changes to the complete depletion model, but do not alter the general tendencies. One of the modifications with respect to the previous results is the increase of the D + abundance at the cost of other isotopologues. Furthermore, the present model predicts a lower H2D + (o/p) ratio, and a slightly higher D2H + (p/o) ratio in very cold, dense cores, as compared with previous modelling results. These nuclear spin ratios affect the detectability of the submm lines of H2D + (o) and D2H + (p). The previously detected H2D + and D2H + lines towards the core I16293E, and the H2D + line observed towards Oph D can be reproduced using the present excitation model and the physical models suggested in the original papers.

CF16 differentially pumped window for ultrahigh vacuum applications

For the spectroscopy of gas phase molecules, ions, and surfaces in UHV setups, leak-tight infrared window assemblies are mandatory. While flange-sealed windows are commercially available for materials such as ZnSe, CaF2, or even chemical vapor deposited CVD diamond, they are either too expensive, not available, or simply just do not work for alakali halides, KRS-5, HDPE, or TPX. Latter three window materials are especially interesting for far infrared FIR applications, as is also diamond. Therefore, differentially pumped O-ring sealed window assemblies based on CF35 flanges have been proposed in the literature. Their advantages are good vacuum performance and easy interchangeability of the window material. For the spectroscopy of ions in an ion trap setup, we needed a “simple as possible” differentially pumped window assembly which fits on a small CF16 flange, and the resulting construction is shown in Fig. 1. It can house an IR window of 25 mm in diameter thickness arbitrary for transmission or even a 1 in. mirror for reflection of a laser beam. The clear aperture in this construction is only 8 mm, although this size can be easily increased. In comparison to the earlier designs of differentially pumped windows, it is small and uses only two sealing O-rings of 3 mm thickness whose interspace is pumped via a tube welded into the outer rim of the assembly. A third O-ring contained in a circular cap can be used to hold the window/mirror and press it slightly against the sealing O-rings if required, although the construction works as well without it. Relatively thick O-rings have been chosen to guarantee a good sealing and to prevent metal parts from touching the window. Furthermore, to avoid air pockets on the UHV side, instead of using a retainer for the inner O-ring, the bed of the inner O-ring is curved as seen in Fig. 1. This curved bed offers a bigger sealing surface for the O-ring and prevents it from being sucked inside the vacuum. As a final advantage, the window is easily accessible also at the sides in case it would stick to the O-rings after baking.

Probing the low-temperature rotational population of H2D+

The gas phase exchange reactions of isotopologues of H+3 with isotopologues of H2 are responsible for the observed deuteration in low-temperature interstellar clouds. At the prevailing cryogenic temperatures many quantum effects, as zero-point vibrational energies, large rotational level spacings and nuclear spin effects become important. In order to understand the processes on a level-to-level basis, experiments in a 22-pole ion trap are carried out accompanied by microcanonical simulations. In particular, the method of laser induced reaction (LIR) is applied to probe the four lowest rotational levels of H2D+.

Deuterium Fractionation and Ion-Molecule Reactions at Low Temperatures

Understanding deuterium fractionation is currently one of the greatest challenges in astrochemistry. In this contribution deuteration experiments of the series CH +, n = 2{5, in a low temperature 22-pole ion trap are used to systematically test a simple chemical rule predicting which molecular ion undergoes deuterium exchange in collisions with HD. CH + turns out to be a problem case, where prediction fails. The method of laser induced reaction (LIR) is used to determine the population ratio of the lowest ortho-to-para states of H2D + relaxed in collisions with H2. Preliminary results indicate that the ortho-to-para ratio of H2D + is substantially reduced in para-H2. This points at the important role of nuclear spin in deuterium fractionation, in particular at the destruction of ortho-H2D + in collisions with ortho-H2. More systematic LIR experiments are needed for a chemical model of deuterium fractionation including state-to-state modiflcations of the species involved.