We have employed crossed molecular beam and velocity map imaging methods to study the dynamics of collisions between symmetric top molecules (ND3) or radicals (CD3) and various colliders such as He, Ar, H2, D2 and N2. The dependence of the scattering on the final rotational levels of the symmetric top have been the subject of recent publications. In the case of scattering with He atoms, comparisons have been made with quantum mechanical scattering calculations on accurate potential energy surfaces, and agreement is excellent. The experiments provide a rigorous test of all features of the potential energy surface for interaction of the two colliding species. The computational studies are carried out by our collaborators Prof P.J. Dagdigian (Johns Hopkins University), Prof M.H. Alexander (University of Maryland) and Prof A. van der Avoird (Radboud University Nijmegen). Experiments on ND3 scattering were carried out with Prof D.H. Parker (Nijmegen) as part of the ICONIC EU Marie Curie network.
Velocity map images of CD3 following inelastic scattering with He
Images such as those shown above for CD3 scattered in collisions with He atoms were obtained using a mini crossed molecular beam apparatus in our laboratory. The apparatus was designed by Dr Stuart Greaves (now at Heriot-Watt University) and uses velocity map imaging to observe the scattering dynamics in the centre-of-mass frame with final quantum state resolution. In the figures below, we compare the angular scattering of the CD3 radicals about the relative velocity vector with the outcomes of quantum scattering calculations performed by Prof Paul Dagdigian and Prof Millard Alexander. The excellent agreement indicates that the ab initio potential energy surface employed for the calculations is very accurate.
Differential cross sections for scattering of CD3 in collisions with He atoms. The panels show outcomes for CD3 probed in different final rotational levels, as indicated by the spectroscopic transition labels. Red curves are experimental data and black curves are from theoretical calculations.
The photo below shows the mini crossed molecular beam machine, with the molecular beams crossing in the horizontal plane and the vertical arm of the instrument being a time-of-flight mass spectrometer and imaging detector.
Mini crossed molecular beam machine used for inelastic scattering experiments in Bristol.
This research has benefited greatly from consistent support from EPSRC (e.g. via Programme Grant EP/G00224X/1). Our VMI work has further benefited from collaborations with the groups Kitsopoulos (FORTH, Heraklion), Parker (Nijmegen), Whitaker (Leeds), and Brouard and Vallance (Oxford) via the successive EU networks IMAGINE, PICNIC and ICONIC.
Representative papers:
Rotationally inelastic scattering of CD3 and CH3 with helium: comparison of velocity map-imaging data with quantum scattering calculations, O. Tkáč, A.G. Sage, S.J. Greaves, A.J. Orr-Ewing, P.J. Dagdigian, Q. Ma and M.H. Alexander, Chemical Science, 4, 4199-4211 (2013). DOI: 10.1039/C3SC52002A
State-to-state resolved differential cross sections for rotationally inelastic scattering of ND3 with He, O. Tkáč, A. Saha, J. Onvlee, C.-H. Yang, G. Sarma, C. Bishwarkarma, S.Y.T. van de Meerakker, A. van der Avoird, D.H. Parker, and A.J. Orr-Ewing, Phys. Chem. Chem. Phys. PCCP 16, 477-488 (2014). DOI 10.1039/C3CP53550A
