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The depolarization of OH(X 2Π3/2,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 2Σ+-X 2Π(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 2Σ+-X 2Π(0,0) band. The PS signal decay rate constant kPS decreases with increasing rotational quantum number for OH(X)+Ar but does not vary monotonically for OH(X)+He. The measured kPS equals the sum kRET+kΛ+kdep, where kRET, kΛ, and kdep are the rate constants for rotational energy transfer, Λ-doublet changing collisions, and rotationally elastic depolarization (of orientation or alignment of the OH(X) angular momentum, as specified), respectively. Values of kdep can be extracted from the measured kPS with prior knowledge of kRET and kΛ. Because kRET and kΛ 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 kdep is systematically markedly higher for alignment than for orientation for OH(X)+Ar but much more weakly so for OH(X)+He. Calculated kRET and kΛ values at 298.15 K are consistent with a substantial contribution from kdep 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 kRET+kΛ accurately. [ABSTRACT FROM AUTHOR]

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 2Π. 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. [ABSTRACT FROM AUTHOR]

We report direct doubly differential (quantum state and angle-resolved) scattering measurements involving short-lived electronically excited molecules using crossed molecular beams. In our experiment, supersonic beams of nitric oxide and argon atoms collide at 90°. In the crossing region, NO molecules are excited to the A2Σ+state by a pulsed nanosecond laser, undergo rotationally inelastic collisions with Ar atoms, and are then detected 400 ns later (approximately twice the radiative lifetime of the A2Σ+state) by 1 + 1′ multiphoton ionization via the E2Σ+ state. The velocity distributions of the scattered molecules are recorded using velocity-mapped ion imaging. The resulting images provide a direct measurement of the state-to-state differential scattering cross sections. These results demonstrate that sufficient scattering events occur during the short lifetimes typical of molecular excited states (∼200 ns, in this case) to allow spectroscopically detected quantum-state-resolved measurements of products of excited-state collisions. [ABSTRACT FROM AUTHOR]

The dynamics of the interfacial reactions of O(3P) with the 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) have been studied experimentally. Laser-induced fluorescence (LIF) was used to detect the nascent gas-phase OH products. The O(3P) atoms are acutely sensitive to the chemical differences of the squalane and squalene surfaces. The larger exothermicity of abstraction from allylic C–H sites in squalene is reflected in markedly hotter OH rotational and vibrational distributions. There is a more modest increase in translational energy release. A larger fraction of the available energy is deposited in the liquid for squalene than for squalane, consistent with a more extensive geometry change on formation of the allylic radical co-product. Although the dominant reaction mechanism is direct, impulsive scattering, there is some evidence for OH being accommodated at both liquid surfaces, resulting in thermalised translation and rotational distributions. Despite the H-abstraction reaction being strongly favoured energetically for squalene, the yield of OH is substantially lower than for squalane. This is very likely due to competitive addition of O(3P) to the unsaturated sites in squalene, implying that double bonds are extensively exposed at the liquid surface. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The destruction and transfer of polarization of the rotational angular momentum of small molecules in an isotropic collisional environment is reviewed. Several recent independent treatments are drawn together, including unpublished details from the authors' own work, of the formal kinetics in terms of moments of the density matrix. The final results are shown to be equivalent and directly amenable to comparison with results of exact quantum scattering calculations. In passing, some differences in nomenclature are noted and a self-consistent version is presented that might usefully be adopted. The existing experimental approaches are surveyed, within the common theme of laser-based creation of the initial rotational anisotropy of relatively low rank, K, combined with a spectroscopic probe, also sensitive to restricted K. Those formally defined quantities that may be measured are identified, either individually or in some combination, with each method. In particular, an attempt is made to distinguish between measurements of individual tensor moments of the density matrix, or ‘bulk polarizations’, and alignment moments, which are normalized to the population. Some cases are noted where experimental procedures have compromised the results, or where the analysis has been similarly approximate or carried out on a less rigorous empirical basis. [ABSTRACT FROM PUBLISHER]

The depolarisation of selected OH (X2Π3/2v= 0, J= 1.5 and 4.5, e) levels in collisions with the molecular partners N2and O2at room temperature (nominally 298 K) has been studied using the polarisation spectroscopy (PS) technique. We obtain total depolarisation rate constants, k(K)PS, 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. N2causes more rapid decay of PS signals than O2. There are no clear dependences of k(K)PSon Jfor either partner. The K-dependence for N2mirrors that determined previously for the noble gases, but is less regular for O2, warranting further investigation. Comparison with independent line-broadening data suggests that there may be an additional, pure-elastic-dephasing contribution to collisional broadening for N2that is not apparent for O2. The presence of an independently established deeper HO–OO attractive minimum at shorter range clearly does not outweigh other factors that favour k(K)PSfor N2.The most obvious explanation is stronger, longer-range attractive interactions due to the larger quadrupole moment of N2. However, this appears to be contradicted by the rigorous ab initiocalculations currently available on OH–O2. [ABSTRACT FROM AUTHOR]

We have applied the polarisation spectroscopy (PS) technique to the collisional depolarisation of selected OH (X2Π, v= 0, F1, J= 1.5 and 4.5, e) levels by Xe at room temperature (nominally 298 K). The measured total depolarisation rate constants, k(K)PS, 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. Neither k(K)PSis strongly J-dependent. k(2)PSis consistently larger than k(1)PS, as expected for |J,mJ〉→ |J,m′J〉 propensities that decline with |ΔmJ|. We have predicted the population transfer rate constants, kPOP, viaquantum scattering calculations on a recent ab initioOH(X)–Xe potential energy surface. Elastic depolarisation rate constants, k(K)DEP, have been inferred by difference, k(K)DEP= k(K)PS−kPOP. The results imply that elastic depolarisation is not substantially more rapid for Xe than for Ar, despite the corresponding increase from He to Ar. The dominant effect of the deeper attractive potential for OH(X)–Xe appears to be enhanced Λ-doublet transfer. This may speculatively be explained by the respective changes in odd and even terms in Legendre expansions of the potentials. [ABSTRACT FROM AUTHOR]