Nuclear spin effect on recombination of H₃⁺ ions with electrons at 77 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.

Global tectonic evolution of Venus, from exogenic to endogenic over time, and implications for early Earth processes

Venus provides a rich arena in which to stretch one's tectonic imagination with respect to non-plate tectonic processes of heat transfer on an Earth-like planet. Venus is similar to Earth in density, size, inferred composition and heat budget. However, Venus' lack of plate tectonics and terrestrial surficial processes results in the preservation of a unique surface geologic record of non-plate tectonomagmatic processes. In this paper, I explore three global tectonic domains that represent changes in global conditions and tectonic regimes through time, divided respectively into temporal eras. Impactors played a prominent role in the ancient era, characterized by thin global lithosphere. The Artemis superstructure era highlights sublithospheric flow processes related to a uniquely large super plume. The fracture zone complex era, marked by broad zones of tectonomagmatic activity, witnessed coupled spreading and underthrusting, since arrested. These three tectonic regimes provide possible analogue models for terrestrial Archaean craton formation, continent formation without plate tectonics, and mechanisms underlying the emergence of plate tectonics. A bolide impact model for craton formation addresses the apparent paradox of both undepleted mantle and growth of Archaean crust, and recycling of significant Archaean crust to the mantle.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

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.

Storage ring measurements of the dissociative recombination of H3+

The dissociative recombination (DR) of H(3)(+) is a key process in interstellar chemistry. More than 30 experimental studies of the DR process have been published in the literature. The H(3)(+) DR rate coefficient results obtained from these measurements, however, have not always been consistent. The outcome seems to depend on the experimental method, on the exact measurement procedure and sometimes even on the interpretation of the experimental data. In the past two decades, heavy-ion storage rings have become the working horse for DR measurements, as they provide a direct measurement of the DR products. Furthermore, storage ring measurements yield energy-resolved rate coefficients with unprecedented resolution that allow for detailed comparison with theory. DR results from different storage ring facilities have shown a remarkable consistency throughout the years and they provide additional information on break-up dynamics and internal excitation. In this study, we will review the storage ring DR measurements that have been carried out for H(3)(+).

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

Results of an experimental study of binary recombination of para- and ortho-H(3)(+) ions with electrons are presented. Near-infrared cavity-ring-down absorption spectroscopy was used to probe the lowest rotational states of H(3)(+) ions in the temperature range of 77-200 K in an H(3)(+)-dominated afterglow plasma. By changing the para/ortho abundance ratio, we were able to obtain the binary recombination rate coefficients for pure and para-H(3)(+) and ortho-H(3)(+). The results are in good agreement with previous theoretical predictions.

The ortho : para ratio of H3+ in laboratory and astrophysical plasmas

In diffuse molecular clouds, the nuclear spin temperature of H(3)(+) (approx. 30 K) is much lower than the cloud kinetic temperature (approx. 70 K). To understand this temperature discrepancy, we have measured the ratio of the hop to exchange pathways (α) in the H(3)(+) + H(2) --> H(2) + H(3)(+) reaction (which interconverts ortho- and para-H(3)(+)) using high-resolution spectroscopy of the ν(2) fundamental band of H(3)(+) in a hydrogenic plasma. We find that α decreases from 1.6±0.1 at 350 K to its statistical value of 0.5±0.1 at 135 K. We use this result to model the steady-state chemistry of diffuse molecular clouds, finding good agreement with astronomical data provided the dissociative recombination rates of ortho- and para-H(3)(+) are equal and the identity branching fraction for the H(3)(+) + H(2) reaction is large. Our results highlight the need for further studies of the H(3)(+) + H(2) reaction as well as state-selective measurements of H(3)(+) dissociative recombination.

Dissociative recombination of H3+: 10 years in retrospect

The dissociative recombination of H(3)(+) has been an intriguing problem for more than half a century. The early experiments on H(3)(+) during the first 20 years were carried out without mass analysis in decaying plasma afterglows, and thus the measured rates pertained to an uncontrolled mixture of H(3)(+) and impurity ions. When mass analysis was used, the rate coefficient was determined to be an uneventful value of about 10(-7) cm(3) s(-1), a very common rate coefficient for many molecular ions. But this was not the end of the story, not even the beginning of the end; it marked only the end of the beginning. The story I will tell in this article started about 10 years ago, when the dissociative recombination of H(3)(+) was approaching its deepest crisis. Today, owing to an extensive experimental and theoretical effort, the state of affairs has reached a historically unique level of harmony, although there still remains many things to sort out.

Chemistry, astronomy and physics of H3+

The great developments in the chemistry, astronomy and physics of H(3)(+) since 2006, which have led to this Royal Society Theo Murphy Meeting, are reviewed.