Rotational energy transfer in collisions between CO(X 1Σ+, v=2, J=0, 1, 4, and 6) and He at temperatures from 294 to 15 K

Abstract

Infrared-vacuum ultraviolet double resonance experiments have been implemented in the ultracold environment provided by a 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 CO and He: (a) those for total removal from the selected rotational states J=0, 1, 4, and 6 in the vibronic state X 1Σ+, v=2, and (b) those for transfer between selected initial and specific final states. Using different Laval nozzles, results have been obtained at several different temperatures: 294, 149, 63, 27, and 15 K. The thermally averaged cross sections for total removal by collisions with He show only slight variations both with initial rotational state and with temperature. The variation of state-to-state rate coefficients with ΔJ show several general features: (i) a decrease with increasing ΔJ; (ii) a propensity to favor odd ΔJ over even ΔJ; and (iii) at lower temperatures, the distribution of rate coefficients against ΔJ becomes narrower, and decreases in J are increasingly favored over increases in J, a preference which is most strongly seen for higher initial values of J. The results are shown to be in remarkably good agreement with those obtained in ab initio scattering calculations by Dalgarno and co-workers [Astrophys. J. 571, 1015 (2002)].

Improved low-temperature rate constants for rotational excitation of CO by H$_\mathsf{2}$

“…The propensity to favor even ∆ j 1 over odd ∆ j 1 has been explained semiclassically by McCurdy & Miller (1977) in terms of an interference effect related to the even anisotropy of the PES. The reverse propensity can also occur if the odd anisotropy is sufficiently large, as observed experimentally in the case of CO-He (Carty et al 2004). Here, the propensity depends on the quantum state of the projectile because in the para case ( j 2 = 0), some terms of the PES expansion (those associated with the quadrupole of H 2 ) vanish identically.…”

Rotational excitation of SiO by collisions with helium

“…In the present case the propensity rule σ(odd ∆ j) > σ(even ∆ j) arises from the large asymmetry of the ab initio SiO-He interaction potential, leading to odd V 1 and V 3 anisotropic terms characterised by deep well depths. It is worth noticing that such an unusual propensity rule for rotational excitation cross sections has been already discussed by McCurdy & Miller (1977) in terms of model potentials, and that it has been observed experimentally (Carty et al 2004) as well as from theoretical calculations (Cecchi-Pestellini et al 2002) for the CO-He collision system.…”

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%).…”

“…They are also needed to predict the lifetimes of energetically excited molecules -" " ff y g w non-thermal product distributions [17]. RET, particularly at low temperatures, is of great importance in deriving column densities in the interstellar medium [18,19].…”