Ternary Recombination of H3+ and D3+ with Electrons in He–H2 (D2) Plasmas at Temperatures from 50 to 300 K

Rydberg States of H3 and HeH as Potential Coolants for Primordial Star Formation

Current theory and measurements establish the age of the universe as ca. 13.8 billion years. For the first several hundred million years of its existence, it was a dark, opaque void. After that, the hydrogen atoms comprising most of the "ordinary" matter began to condense and ionize, eventually forming the first stars that would illuminate the sky. Details of how these "primordial" stars formed have been widely debated, but remain elusive. A central issue in this process is the mechanism by which the primordial gas (mainly hydrogen and helium atoms) collected via the action of dark matter cooled and further accreted to fusion densities. Current models invoke collisional excitation of H₂ molecular rotations and subsequent radiative rotational transitions allowed by the weak molecular quadrupole moment. In this work, we review the salient considerations and present some new ideas, based on recent spectroscopic observations of neutral H₃ Rydberg electronic state emission in the mid-infrared region.

Towards state selective recombination of H3+ under astrophysically relevant conditions

We present studies on the thermalisation of H3+ ions in a cold He/Ar/H2 plasma at temperatures 30-70 K. We show that we are able to generate a rotationally thermalised H3+ ensemble with a population of rotational and nuclear spin states corresponding to a particular ion translational temperature. By varying the para-H2 fraction used in the experiment we are able to produce para-H3+ ions with fractional populations higher than those corresponding to thermodynamic values. At 35 K, only the lowest rotational states of para and ortho H3+ are populated. This is the first step towards experimental studies of electron-molecular ion recombination processes with precisely specified quantum states at astrophysically relevant temperatures.

Low temperature rates for key steps of interstellar gas-phase water formation

The gas-phase formation of water molecules in the diffuse interstellar medium (ISM) proceeds mainly via a series of reactions involving the molecular ions OH⁺, H₂O⁺, and H₃O⁺ and molecular hydrogen. These reactions form the backbone for the chemistry leading to the formation of several complex molecular species in space. A comprehensive understanding of the mechanisms involved in these reactions in the ISM necessitates an accurate knowledge of the rate coefficients at the relevant temperatures (10 to 100 K). We present measurements of the rate coefficients for two key reactions below 100 K, which, in both cases, are significantly higher than the values used in astronomical models thus far. The experimental rate coefficients show excellent agreement with dedicated theoretical calculations using a novel ring-polymer molecular dynamics approach that offers a first-principles treatment of low-temperature barrierless gas-phase reactions, which are prevalent in interstellar chemical networks.

Stationary afterglow apparatus with CRDS for study of processes in plasmas from 300 K down to 30 K

A cryogenic stationary afterglow apparatus equipped with a near-infrared cavity-ring-down-spectrometer (Cryo-SA-CRDS) for studies of electron-ion recombination processes in the plasma at temperatures 30-300 K has been designed, constructed, tested, and put into operation. The plasma is generated in a sapphire discharge tube that is contained in a microwave cavity. The cavity and the tube are attached to the second stage of the cold head of the cryocooler system, and they are inserted to an UHV chamber with mirrors for CRDS and vacuum windows on both ends of the tube. The temperature of the discharge tube can be made as low as 25 K. In initial test measurements, the discharge was ignited in He/Ar/H₂ or He/H₂ gas mixtures and the density of H₃⁺ ions and their kinetic and rotational temperatures were measured during the discharge and afterglow. From the measured decrease in the ion density, during the afterglow, effective recombination rate coefficients were determined. Plasma relaxation was studied in He/Ar gas mixtures by monitoring the presence of highly excited argon atoms. The spectroscopic measurements demonstrated that the kinetic temperature of the ions is equal to the gas temperature and that it can be varied from 300 K down to 30 K.

Binary and ternary recombination of H2D+ and HD2+ ions with electrons at 80 K

The recombination of deuterated trihydrogen cations with electrons was studied at 80 K using stationary afterglow with cavity ring-down spectroscopy.