A combined experimental and theoretical study of rotational energy transfer in collisions between NO(X 2Π1/2, v=3,J) and He, Ar and N2 at temperatures down to 7 K

Abstract

Infrared-ultraviolet double resonance (IRUVDR) experiments have been implemented in the ultra-cold environment provided by a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus. With this technique rate coefficients of two kinds have been measured for rotational energy transfer in collisions between NO and He, Ar and N2: (a) rate coefficients for total removal from specific states of NO(X 2Π1/2; v=3; J=0.5, 3.5 or 6.5) and (b) state-to-state rate coefficients for rotational energy transfer from these levels to specific final states. Using different Laval nozzles, results have been obtained at several different temperatures: for He as collision partner, 295, 149, 63, 27, 15 and 7 K; for Ar, 139, 53, 44 and 27 K; and for N2, 86 and 47 K. The thermally averaged cross-sections for total removal show remarkably little variation, either with temperature or with initial rotational state. The variation of state-to-state rate coefficients with ΔJ shows three general features: (i) a decrease with increasing ΔJ; (ii) a propensity to favor even ΔJ transitions over odd ΔJ changes; and (iii) at lower temperatures, decreases in J are increasingly favored over increases in J and the distribution of rate coefficients against ΔJ becomes narrower. The experimental rate coefficients for collisions with He and Ar are compared with those from both close coupled and coupled states calculations based on potential energy surfaces determined within the coupled electron pair approximation (CEPA) with a large atomic orbital basis set. The agreement between theory and experiment of both the total and the state-to-state rate coefficients is excellent over the complete range of temperatures covered in the experiments.

Pulsed Laval nozzle study of the kinetics of OH with unsaturated hydrocarbons at very low temperatures

“…The gas flow did not encounter the walls of the 115 chamber, avoiding condensation problems, and the residence time in the flow was long enough to monitor a chemical reaction. The technique, known as CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme), was initally used to measure rate coefficients for ion-molecule reactions 14 , 120 but its application was extended to the study of neutral-neutral reactions by Rowe, Sims and Smith, who generated freeradicals using laser flash photolysis (LFP) and monitored their temporal decay using laser-induced fluorscence (LIF) spectroscopy 1, 15,16 or by chemiluminescence. 17,18 The method 125 has enjoyed considerable success, and has been extensively reviewed.…”

Experimental Investigation of Neutral-Neutral Reactions and Energy Transfer at Low Temperatures

“…The resultant exponential decay is fitted to yield a pseudo-first-order rate constant, and the experiment repeated at various co-reagent concentrations to yield a second-order plot, the gradient of which yields the rate constant of interest at the temperature of the supersonic flow. Full details of the technique may be found in Sims et al (1994) and James et al (1998). An example of the signal and results obtained for the reaction (vibrational relaxation) of CH (v = 1) with CO may be seen in Figure 4.…”

Rotational energy transfer in collisions between CO and Ar at temperatures from 293 to 30 K

“…The CRESU technique was first developed by Rowe and co-workers [34,35] for the study of ion-molecule reaction kinetics, and has subsequently been adapted for investigations of both neutral-neutral kinetics [36] and energy transfer at temperatures as low as 7 K [18,[37][38][39]. The CRESU and IRVUVDR techniques have been described in detail in James et al [39] and Carty et al [18].…”

“…The CRESU and IRVUVDR techniques have been described in detail in James et al [39] and Carty et al [18]. VUV laser induced fluorescence (LIF) provides a sensitive detection method, allowing us to work with very low [CO] (<0.5%).…”