Abstract
We report state-resolved total removal cross sections and state-to-state rotational energy transfer (RET) cross sections for collisions of CN(A2Π, ν = 4, jF1ε) with N2, O2, and CO2. CN(X2Σ+) was produced by 266 nm photolysis of ICN in a thermal bath (296 K) of the collider gas. A circularly polarized pulse from a dye laser prepared CN(A2Π, ν = 4) in a range of F1e rotational states, j = 2.5, 3.5, 6.5, 11.5, 13.5, and 18.5. These prepared states were monitored using the circularly polarized output of an external cavity diode laser by frequency-modulated (FM) spectroscopy on the CN(A–X)(4,2) band. The FM Doppler profiles were analyzed as a function of pump–probe delay to determine the time dependence of the population of the initially prepared states. Kinetic analysis of the resulting time dependences was used to determine total removal cross sections from the initially prepared levels. In addition, a range of j′ F1e and j′ F2f product states resulting from rotational energy transfer out of the j = 6.5 F1e initial state were probed, from which state-to-state RET cross sections were measured. The total removal cross sections lie in the order CO2 > N2 > O2, with evidence for substantial cross sections for electronic and/or reactive quenching of CN(A, ν = 4) to unobserved products with CO2 and O2. This is supported by the magnitude of the state-to-state RET cross sections, where a deficit of transferred population is apparent for CO2 and O2. A strong propensity for conservation of rotational parity in RET is observed for all three colliders. Spin–orbit-changing cross sections are approximately half of those of the respective conserving cross sections. These results are in marked disagreement with previous experimental observations with N2 as a collider but are in good agreement with quantum scattering calculations from the same study (Khachatrian et al. J. Phys. Chem. A2009, 113, 392219215110). Our results with CO2 as a collider are similarly in strong disagreement with a related experimental study (Khachatrian et al. J. Phys. Chem. A2009, 113, 1339019405498). We therefore propose that the previous experiments substantially underestimated the spin–orbit-changing cross sections for collisions with both N2 and CO2, suggesting that even approximate quantum scattering calculations may be more successful for such molecule–molecule systems than was previously concluded.
Highlights
Rotational energy transfer (RET) is a fundamental collisional process of importance in many gas-phase chemical environments, such as plasmas, combustion, and the atmosphere
Consistent with the observed differences in the long-time behavior of the kinetic traces for the different colliders and the second-order plots of the individual transfer rates shown in Figure 5, we find that for Ar and N2, the total removal cross sections are completely dominated by transfer to states from which return is possible, σ(j→0)j′
We have presented state-resolved removal cross sections and state-to-state RET cross sections for the collisions of CN(A, ν = 4) with N2, O2, and CO2 at thermal collision energies
Summary
Rotational energy transfer (RET) is a fundamental collisional process of importance in many gas-phase chemical environments, such as plasmas, combustion, and the atmosphere. Rotational state-to-state transfer cross sections or rate constants are crucial quantities for predictive modeling in such environments and as such have been the subject of numerous experimental and theoretical studies.[1,2] Radical species, those possessing nonzero electronic orbital angular momentum, have been the subject of particular interest, notably NO(X2Π),[3−7] OH(X2Π),[8,9] and the subject of this work, CN(A2Π).[10−19] These radicals are among the most important chemical species in many of the environments of interest and present challenges to experiment and theory arising from their electronic structure. The nonzero electronic orbital angular momentum of these 2Π states results in both spin−orbit and Λ-doublet fine structure splitting of the rotational levels. RET rate constants may be strongly dependent on the conservation or changing of either the spin− orbit state or the rotational parity
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