Laboratory Study of Rate Coefficients for H₂:H₂ Inelastic Collisions between 295 and 20 K

Abstract

A laboratory study of state-to-state rate coefficients (STS rates) for H 2 :H 2 inelastic collisions in the v=0 state is reported. The study, which spans the 295-20 K thermal range, is based on the use of a kinetic master equation. It describes the time-space evolution of populations of H 2 rotational levels as induced by inelastic collisions. It is applied here to a supersonic jet of natural H 2 . This medium bears a large amount of relevant data that allows for the establishment of best values and confidence margins for the dominant STS rates of H 2 :H 2 inelastic collisions on an experimental basis. The primary experimental data derived from the supersonic jet are the local number density, the populations of the H 2 rotational levels, and their gradients along the jet by means of high-sensitivity Raman spectroscopy with superb space resolution. First, two sets of theoretical STS rates from the literature have been tested against the experiment. The set that shows a better agreement with the experiment has then been scaled to derive an improved set of experiment-scaled STS rates (ES rates). They allow the reproduction of more than 50 experimental population gradient data within a standard deviation <1.4% along the 295-20 K thermal range. The estimated uncertainty for the ES rates ranges from ≈3% near 300 K to ≈6% near 20 K. ES rates and uncertainties for H 2 :H 2 ground-state inelastic collisions between 300 and 20 K are presented in machine-readable format. Other (incomplete) sets of theoretical rates from the literature are discussed. 100. 1 5 J J J J CC exptl exptl = dP dz F T dP dz . 1 9 J J J ES CC ( ) ( )( ) ( ) Since dP dz J ES ( ) are expected to reproduce the dP dz J exptl ( ) gradients as close as possible, i.e., » dP dz dP dz J J ES exptl 100 23 J J J J calc exptl exptl I z C J J J J J J J J J P z 0 1 7 45 1 2 1 2 3 0 1 , 26 J J 2 2

Collisional cooling of primordial and interstellar media by H2

“…1. We note that Montero et al (2020) have made a limited comparison, for four rotationally-inelastic transitions, of the rate coefficients computed by Wan et al (2018) with empirically-derived values and found the theoretical rate coefficients to be lower than their measurements for temperatures 20 < T < 295 K. In a more recent study, Hernández et al (2021) compared experimental and theoretical rate coefficients for three rotationally inelastic transitions, induced by either H2(J = 0) or H2(J = 1), and found that the data adopted in the present calculations are generally in better accord with their measurements than those of Wan et al (2018).…”

Review of Optical Thermometry Techniques for Flows at the Microscale towards Their Applicability to Gas Microflows

“…So far it has been successfully applied to free (i.e., unconfined) flows such as miniflames [149] or supersonic gas microjets of H 2 , N 2 , O 2 , CO 2 , H 2 O (gas) and some of their mixtures [150][151][152][153][154]. In N 2 [155], O 2 [156], and CO 2 [157,158] the temperature was determined through the rotational populations from the rotational Raman spectra, while in H 2 the rotational populations were measured from the rotationallyresolved vibrational Q-branch [159,160]. For H 2 O (gas), separate rotational lines in the vibrational Q-branch cannot be fully resolved, and the temperature was retrieved from the simulation of the Q-branch profile [161].…”