Grant Paterson

Orientation and alignment depolarization in OH(X Π2)+Ar/He collisions

The depolarization of OH(X (2)Pi(3/2),v=0,J=1.5-6.5,e) rotational angular momentum (RAM) in collisions with He and Ar under thermal conditions (298 K) has been studied using two-color polarization spectroscopy (PS). Orientation or alignment of the OH RAM was achieved using circularly or linearly polarized pulsed excitation, respectively, on the off-diagonal OH A (2)Sigma(+)-X (2)Pi(1,0) band. The evolution of the ground-state OH(X) RAM polarization, exclusively, was probed using an independent, linearly polarized pulse tuned to the diagonal OH A (2)Sigma(+)-X (2)Pi(0,0) band. The PS signal decay rate constant k(PS) decreases with increasing rotational quantum number for OH(X)+Ar but does not vary monotonically for OH(X)+He. The measured k(PS) equals the sum k(RET)+k(Lambda)+k(dep), where k(RET), k(Lambda), and k(dep) are the rate constants for rotational energy transfer, Lambda-doublet changing collisions, and rotationally elastic depolarization (of orientation or alignment of the OH(X) angular momentum, as specified), respectively. Values of k(dep) can be extracted from the measured k(PS) with prior knowledge of k(RET) and k(Lambda). Because k(RET) and k(Lambda) were not previously available for collisions of Ar with OH(X, v=0), we performed exact, fully quantum-mechanical scattering calculations on a new potential energy surface (PES) presented here for the first time. The raw experimental results show that k(dep) is systematically markedly higher for alignment than for orientation for OH(X)+Ar but much more weakly so for OH(X)+He. Calculated k(RET) and k(Lambda) values at 298.15 K are consistent with a substantial contribution from k(dep) for OH(X)+Ar but not for OH(X)+He. This may point to the role of attractive forces in elastic depolarization. The experimental results provide a very sensitive test of the ability of the most recent ab initio OH(X)-He PES of Lee et al. [J. Chem. Phys. 113, 5736 (2000)] to reproduce k(RET)+k(Lambda) accurately.

Rotational angular momentum polarization: The influence of stray magnetic fields

We show that weak residual magnetic fields can significantly affect the preparation and measurement of molecular rotational angular momentum alignment in a typical gas-phase stereodynamics apparatus. Specifically, polarization spectroscopy, a third-order nonlinear spectroscopic technique, is used to prepare and probe the collisional and noncollisional losses of rotational angular momentum alignment of OH X (2)Pi. Residual magnetic fields of the order of the geomagnetic field are shown to have a significant effect on the prepared polarization on a submicrosecond timescale. This can be expected to be a significant effect for many gas-phase free radicals, such as those of interest in combustion, atmospheric chemistry, and the burgeoning field of cold molecules. We demonstrate a simple experimental remedy for this problem.

Collisional depolarisation of rotational angular momentum: influence of the potential energy surface on the collision dynamics?

We review recent progress in the loss or transfer of the polarisation of angular momentum in collisions of small free radicals with a thermal bath of closed-shell partners. Our primary theme is the connection between the observed behaviour and the nature of the underlying potential energy surfaces (PESs). We consider the systems NO(A2Σ+) + He and Ar; OH(A2Σ+) + He and Ar; OH(X2Π) + He, Ar and Xe; NO(X2Π) + Ar and CN(A2Π) + Ar, which vary both kinematically and in the strength of any attractive interaction. They are chosen because reliable theoretical PESs have been predicted; QS or QCT scattering calculations have been carried out; and they have been studied by recently developed experimental methods sensitive to polarisation. The efficiency of elastic depolarisation depends intimately on the competition with other, inelastic outcomes. It is generally found to be inefficient for systems dominated by impulsive forces, which promote instead changes of state. This is exacerbated by kinematic effects, in particular for the light He collision partner. Moderately attractive PESs support more efficient elastic depolarisation for low rotational levels, but this drops off rapidly with increasing rotation. Persistent elastic depolarisation across all rotational levels is a feature of deeply attractive, strongly anisotropic PESs. The 2Σ+-rare gas systems are characterised by a single controlling PES and are well described by a spin-spectator model. The 2Π-rare gas systems require two PESs, expressible as 2 A′ and 2 A′′ adiabatic surfaces or their diabatic sum and difference, V sum and V diff. Propensities rules reflecting the symmetries of these surfaces help to explain much of the detailed behaviour.

Elastic depolarization of OH(A) by He and Ar: A comparative study

Two color polarization spectroscopy has been employed to measure the collisional depolarization of OH(A(2)Sigma(+), v = 1) by He and Ar. Complementary experiments using Zeeman quantum beat spectroscopy have also been performed to determine separately the cross sections for rotational energy transfer (RET) out of selected rotational levels of OH(A, v = 0) + Ar, as well as those for elastic depolarization. This has been achieved by dispersing the emission, so as to observe a single fluorescence transition. Elastic depolarization of OH(A) by Ar is found to be significant with that for loss of rotational alignment exceeding that for loss of orientation. In the case of OH(A) + He, the polarization spectroscopy measurements suggest that elastic depolarization plays a relatively minor role in the loss of the polarization signal compared with RET. The experimental data for OH(A) + Ar are compared in detail with the results of quasi-classical trajectory calculations that accommodate the effects of electron spin. These classical calculations are assessed against the results obtained using full close-coupled open shell quantum mechanical scattering methods. Overall the level of agreement between the two experiments, and between experiment and theory, is very reasonable. Surprisingly, at low N the elastic depolarization cross sections for OH(A) + Ar are found to be quite similar in magnitude to those observed for OH(X) + Ar despite the fact that the well depth in the latter system is considerably smaller than that for OH(A)-Ar. However, for OH(A) + Ar the depolarization cross sections are insensitive to N in the range 1-14. It is proposed that this behavior partly reflects the relatively anisotropic nature of the potential energy surface, which exhibits deep wells of different depths at the two linear configurations OH(A)-Ar and Ar-OH(A), and partly the nature of elastic depolarizing collisions, which must occur with a velocity component perpendicular to the plane of rotation of the diatomic molecule.

Collisions of electronically excited molecules: differential cross-sections for rotationally inelastic scattering of NO(A2Σ+) with Ar and He

The paper reports experimental measurements and theoretical calculations of rotational-state-resolved differential scattering cross-sections (DCS) for collisions between electronically excited NO(A2Σ+) molecules and rare gas atoms. The experimental NO(A2Σ+) + Ar and NO(A2Σ+) + He state-resolved product scattering distributions are determined using velocity-mapped ion imaging. The ion images are analysed to determine the state-resolved DCS, which are compared with new theoretical DCS calculated using quantum scattering methods on ab initio electronic potential energy surfaces. Both collision systems are imaged simultaneously; this constraint on the collision energies of the two systems aids the comparison to theory. The experimental and calculated DCS agree well for the NO(A2Σ+)/He system. For the NO(A2Σ+)/Ar scattering system, the experiments do not recover the degree of forward-scattering theoretically predicted and significant differences in the positions of the observed and predicted rotational rainbow features are apparent at large scattering angles, particularly for the most rotationally inelastic collision channels investigated: ΔN = 12,14.

Depolarisation of rotational orientation and alignment of OH (X2Π) in collisions with molecular partners: N2 and O2

The depolarisation of selected OH (X(2)Pi(3/2)v = 0, J = 1.5 and 4.5, e) levels in collisions with the molecular partners N(2) and O(2) at room temperature (nominally 298 K) has been studied using the polarisation spectroscopy (PS) technique. We obtain total depolarisation rate constants, k, which are the combination of population transfer out of the initial level and elastic depolarisation of the tensor moment of respective rank K = 1 (orientation) or K = 2 (alignment) of its angular momentum distribution. N(2) causes more rapid decay of PS signals than O(2). There are no clear dependences of k on J for either partner. The K-dependence for N(2) mirrors that determined previously for the noble gases, but is less regular for O(2), warranting further investigation. Comparison with independent line-broadening data suggests that there may be an additional, pure-elastic-dephasing contribution to collisional broadening for N(2) that is not apparent for O(2). The presence of an independently established deeper HO-OO attractive minimum at shorter range clearly does not outweigh other factors that favour k for N(2).The most obvious explanation is stronger, longer-range attractive interactions due to the larger quadrupole moment of N(2). However, this appears to be contradicted by the rigorous ab initio calculations currently available on OH-O(2).

Rotationally elastic and inelastic dynamics of NO(X2Π, v = 0) in collisions with Ar

A combined theoretical and experimental study of the depolarization of selected NO(X(2)Π, v = 0, j, F, ɛ) levels in collisions with a thermal bath of Ar has been carried out. Rate constants for elastic depolarization of rank K = 1 (orientation) and K = 2 (alignment) were extracted from collision-energy-dependent quantum scattering calculations, along with those for inelastic population transfer to discrete product levels. The rate constants for total loss of polarization of selected initial levels, which are the sum of elastic depolarization and population transfer contributions, were measured using a two-color polarization spectroscopy technique. Theory and experiment agree qualitatively that the rate constants for total loss of polarization decline modestly with j, but the absolute values differ by significantly more than the statistical uncertainties in the measurements. The reasons for this discrepancy are as yet unclear. The lack of a significant K dependence in the experimental data is, however, consistent with the theoretical prediction that elastic depolarization makes only a modest contribution to the total loss of polarization. This supports a previous conclusion that elastic depolarization for NO(X(2)Π) + Ar is significantly less efficient than for the electronically closely related system OH(X(2)Π) + Ar [P. J. Dagdigian and M. H. Alexander, J. Chem. Phys. 130, 204304 (2009)].

Inelastic scattering of OH radicals from organic liquids: isolating the thermal desorption channel

Inelastic scattering of OH radicals from liquid surfaces has been investigated experimentally. An initially translationally and rotationally hot distribution of OH was generated by 193 nm photolysis of allyl alcohol. These radicals were scattered from an inert reference liquid, perfluorinated polyether (PFPE), and from the potentially reactive hydrocarbon liquids squalane (C30H62, 2,6,10,15,19,23-hexamethyltetracosane) and squalene (C30H50, trans-2,6,10,15,19,23-hexamethyltetracosa-2,6,10,14,18,22-hexaene). The scattered OH v = 0 products were detected by laser-induced fluorescence. Strong correlations were observed between the translational and rotational energies of the products. The high-N levels are translationally hot, consistent with a predominantly direct, impulsive scattering mechanism. Impulsive scattering also populates the lower-N levels, but a component of translationally relaxed OH, with thermal-desorption characteristics, can also be seen clearly for all three liquids. More of this translationally and rotationally relaxed OH survives from squalane than from squalene. Realistic molecular dynamics simulations confirm that double-bond sites are accessible at the squalene surface. This supports the proposition that relaxed OH may be lost on squalene via an addition mechanism.

Depolarization of rotational angular momentum in open-shell collisions: OH+rare gases

This paper addresses recent progress in studies of collisional depolarization of rotational angular momentum in open-shell molecules. The application of the polarization spectroscopy (PS) technique to the OH radical in collisions with the rare gases He and Ar is described. The capacity of quantum mechanical scattering calculations on ab initio potential energy surfaces to reproduce the experimental results is considered. We explain how the very recent progress in theoretical methodology has enhanced the value of this comparison, reflect more generally on the relationship to other experimental methods and molecular systems, and speculate on some promising future directions for this research field.