Rotationally Inelastic Collisions of CN(A²Π) with Small Molecules

Abstract

An optical-optical double resonance technique has been utilized to investigate rotational energy transfer of selected rotational/fine-structure levels of CN(A(2)Pi, v = 3) in collisions with the molecular partners CO(2) and CH(4). Total removal rate constants for several rotational/fine-structure levels were measured for both collision partners. State-to-state relative rate constants were determined for several initial levels. These show a strikingly strong collisional propensity to conserve the fine-structure/Lambda-doublet label, akin to our previous observations with the N(2) collision partner. The possible origin of this propensity is discussed.

Parity-Dependent Rotational Energy Transfer in CN(A2Π, ν = 4, j F1ε) + N2, O2, and CO2 Collisions

“…10,11 Similarly, we do not believe that the small change in the initial state rotational energy and angular momentum, relative to the available collision energy and orbital angular momentum, will lead to substantial changes in relative state-to-state propensities when comparing the two studies. The N 2 and CO 2 relative state-to-state cross sections reported here show marked disagreements with the experimental results previously reported by Khachatrian et al 13,14 For F ε-conserving relative cross sections, while the general decrease with increasing Δ j is approximately the same as that observed by Khachatrian et al, they do not report the characteristic even–odd alternation corresponding to conservation of total parity that we observe and is predicted by theory for N 2 . Further, we observe F ε-changing cross sections that have the same general Δ j dependence as those observed with Ar.…”

“…We can compare the general form of our state-to-state cross sections for N 2 and CO 2 to those previously reported from experiments by Khachatrian et al 13,14 They prepared different initial states, specifically ν = 3 j = 7.5 F 1 f and ν = 3 j = 9.5 F 2 f . We do not expect the difference in initial vibrational state to have a significant effect on the relative state-to-state rotational cross sections, although it may affect the overall absolute removal cross section.…”

“…Studies of state-to-state RET have been limited to collisions with H 2 , N 2 , CO 2 , and CH 4 . 13,14,30,31 An open question with these molecular colliders is what effect, if any, the presence of the EET or reactive channels has on the observed RET within the excited state.…”

“…13,14 In each case, they used optical–optical double resonance (OODR) to measure fully state-resolved j F ε to j ′ F ′ε′ RET rate constants of low- j CN(A 2 Π, ν = 3). In addition, for CN(A)–N 2 , they calculated ab initio PESs for three limiting orientations of the CN and N 2 and subsequently performed QS calculations on spherically averaged PESs constructed from these limiting cases.…”

“…The separate experimental measurements of RET with CO 2 and CH 4 as colliders also showed a very strong preference for spin–orbit and Λ-doublet conservation. 14 …”