Collisional depolarization of OH(A) studied by Zeeman quantum beat spectroscopy

Abstract

The depolarization of the rotational angular momentum of electronically excited OH( 2 AE) radicals through collisions with water molecules has been measured using Zeeman quantum beat spectroscopy. The new data have permitted the evaluation of OH(A) state-specific quenching and angular momentum depolarization cross-sections for superthermal OH(A) radicals with mean relative velocities centred around 3500 m s À1 . The quenching cross-sections are compared both with values available in the literature, and with predictions based on a simple harpoon model, and are found to be in good qualitative accord with previous findings. For the lowest rotational levels studied, the depolarization cross-sections (which include contributions from both elastic and inelastic processes in OH(A)) are found to approach s100 Å 2 , only slightly below the high-temperature cross-sections for rotational energy transfer determined elsewhere. The data suggest that under the present conditions rotational energy transfer is accompanied by significant depolarization. The cross-section for the translational moderation of superthermal OH(X) by water is also determined in the present study.

Tensor cross sections and the collisional evolution of state multipoles: OH(XΠ2)–Ar

“…Brouard and co-workers 24,25 employed Zeeman quantum beat spectroscopy to investigate the collisional depolarization of specific rotational/fine-structure levels of electronically excited OH͑A 2 ͚ + ͒ with Ar. McKendrick and co-workers 26,27 used two-color polarization spectroscopy to measure orientation and alignment depolarization rates in collisions of OH͑X 2 ⌸͒ with Ar and He.…”

Elastic depolarization and polarization transfer in CN(A2Π,v= 4)+Ar collisions

“…[23][24][25] Although the large low-j cross sections are again reminiscent of CN(A) + Ar, the lack of a significant decline with j is not. It appears that in this more extreme case, the deep, anisotropic, well is able to efficiently depolarize even rapidly rotating OH(A) molecules.…”

“…17,[19][20][21][22][23]33 Comparison with full quantum scattering calculations on accurate ab-initio PESs has helped to unravel the influence of different parts of the PES on RET and elastic depolarization. Brouard and co-workers have used an independent technique, Zeeman quantum beat spectroscopy, to study elastic and inelastic depolarization in the OH(A)-Ar system, [23][24][25]34 and most recently NO(A 2 Σ + )-Ar/He. 35 For OH(A), excellent processes.…”

“…Experiments have used various techniques under both controlled collision conditions [15][16][17][18][19][20][21][22][23][24][25][26] and in atmospheric pressure flames. [27][28][29][30][31] We have exploited Polarization Spectroscopy, a 3 rd -order non-linear spectroscopic method, 16,32 to measure rate constants for elastic depolarization of both orientation and alignment of OH(X 2 Π or A 2 Σ + ) in collisions with rare gases and simple molecular colliders.…”

Sliced fluorescence imaging: a versatile method to study photo-induced dynamic processes

“…There are six distinctly different experimental approaches based on RET to investigate potential energy surfaces (PES). (109−111) They are: velocity-map imaging, (112,113) hexapole field selection, (114,115) additional electrostatic field selection, (116,117) Zeeman quantum beat spectroscopy, (118) polarization spectroscopy (119) and frequency-modulated spectroscopy. (120,121) However, none of them is universally applicable.…”