Nuclear spin state-resolved cavity ring-down spectroscopy diagnostics of a low-temperature ${\rm H}_3^+$ -dominated plasma

Abstract

We have applied a continuous-wave near-infrared cavity ring-down spectroscopy method to study the parameters of a H + 3 -dominated plasma at temperatures in the range 77-200 K. We monitor populations of three rotational states of the ground vibrational state corresponding to para and ortho nuclear spin states in the discharge and the afterglow plasma in time and conclude that abundances of para and ortho states and rotational temperatures are well defined and stable. The non-trivial dependence of a relative population of para-H + 3 on a relative population of para-H 2 in a source H 2 gas is described. The results described in this paper are valuable for studies of state-selective dissociative recombination of H + 3 ions with electrons in the afterglow plasma and for the design of sources of H + 3 ions in a specific nuclear spin state.

Binary recombination of para- and ortho-H 3 + with electrons at low temperatures

“…We have proved experimentally [25,26] that the assumption of TDE under our experimental conditions is correct. To obtain a eff from the measured electron density decay curves, we used direct fits to the data and, in addition, the more advanced 'integral analysis' technique [31].…”

“…The experimental apparatus is the same as that in our previous studies [20,26]. A pulsed microwave discharge generates a plasma in a tube (inner diameter of approx.…”

“…The enrichment was measured by nuclear magnetic resonance spectroscopy (RNDr Jan Lang PhD 2010, personal communication). NIR-CRDS in the continuous wave modification (based on the configuration described by Romanini et al [28]) was developed in our laboratory for time-resolved studies [26,29,30]. The light source is a fibre-coupled distributed feedback laser diode with a central wavelength of 1381.55 nm, line-width less than 2 MHz, and maximum output optical power of 20 mW.…”

“…A measurement of a eff for two or more different values of p f 3 , but under otherwise identical conditions (temperature, and density of He and H 2 ), then permits a determination of the individual recombination rate coefficients p a eff and o a eff . The fraction p f 3 can be enhanced from about 0.5 to 0.8 by using para-enriched hydrogen instead of normal hydrogen (for details, see [20,26]). …”

State selective study of H3+recombination in Cryo-FALP and SA-CRDS experiments at 77 K

“…[8][9][10][11]. The advantage of the afterglow experiments is that H + 3 ions are rotationally thermalized by multiple collisions with ambient neutral gas molecules/atoms at properly chosen conditions [5,12]. In the recent afterglow type experiments the evolution of densities of H + 3 ions in the particular spin state and the thermalisation (rotational temperature within para and ortho nuclear spin manifolds and kinetic temperature) are monitored by a laser absorption spectroscopy (SA-CRDS -Stationary Afterglow with Cavity Ring-Down absorption Spectrometer [6,7,13]).…”