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Inelastic scattering of hydroxyl radicals with helium and argon by velocity-map imaging


The hydroxyl radical (OH) is one of the most interesting molecules in molecular dynamics. In particular, inelastic collisions of free radicals such as OH are profoundly important in environments ranging from combustion to astrochemistry. However, measuring the velocities of OH molecules in specific internal quantum states has proven to be very difficult. A method that can provide this important information is velocity-map imaging. Although this technique is very widely applicable in principle, it does require a sensitive and selective laser-ionization scheme. Here we show that, under the right conditions, velocity-map imaging can be applied to the study of the inelastic scattering of OH using crossed-molecular-beam methods. We measure fully quantum-state-specified product angular distributions for OH collisions with helium and argon. The agreement between exact close-coupling quantum scattering calculations on ab initio potential energy surfaces and experimental data is generally very satisfactory, except for scattering in the most forward directions.

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Figure 1: Schematic of the VMI setup together with the geometry of the crossed molecular beams.
Figure 2: Velocity-map images and DCSs for inelastic scattering of OH(j = 1.5f,F1) with He and Ar.
Figure 3: Three-dimensional contour plots of OH radical angular distributions.


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Research in Nijmegen was supported by the Netherlands NWO-CW project 700.58.029, the Dutch Astrochemistry Network, and the EU-ITN Network ‘ICONIC’ 238671. S. Marinakis was supported by EPSRC (UK) and the British Council–Platform Bèta Techniek Programme in Science. K.G.M. was supported by EPSRC (UK). S. Marinakis thanks S. Cybulski and J. Kłos for providing the HeOH/ArOH PES, J. Kłos for many useful discussions and suggestions, B.J. Howard (Oxford) and F.J. Aoiz (Madrid) for encouraging discussions, and the use of the Oxford Supercomputing Centre in carrying out this work.

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D.H.P., J.J.tM. and K.G.M. supervised the project and designed the experiments. G.S. performed the VMI experiments and collected and analysed the data with assistance from S.M., who performed the quantum calculations with assistance in interpretation from K.G.M. G.S. and D.H.P. wrote the manuscript with significant input from J.J.tM., S.M. and K.G.M.

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Correspondence to David H. Parker.

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The authors declare no competing financial interests.

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Sarma, G., Marinakis, S., ter Meulen, J. et al. Inelastic scattering of hydroxyl radicals with helium and argon by velocity-map imaging. Nature Chem 4, 985–989 (2012).

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