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Start the Clock

Preparation of a single rotational quantum state of NO(A2+) in a crossed molecular beam apparatus is used to investigate the stereodynamics of rotational energy transfer in encounters with a range of collision partners.

FM spectroscopy project image FM spectroscopy project image FM spectroscopy project image

FM spectroscopy image

Laser preparation of a collision species precisely defines a zero-point in the time period over which collisions may occur, and the resulting images may be straightforwardly interpreted to resolve the detailed spatial properties of the collision process.

Start the Clock: A New Direct Method to Study the Collisions of Electronically Excited Molecules

This EPSRC-funded project employs a crossed molecular beam velocity map ion imaging (CMB-VMI) apparatus to investigate the dynamics of rotationally inelastic collisions of electronically excited NO(A2+) in the gas phase. Owing to the relative difficulty of preparing electronically excited species for use in scattering experiments, their collision dynamics have not received anything like the same level of attention paid to ground-state species, despite the important role that collisional energy transfer for such highly excited species plays in high energy environments such as combustion systems and plasmas.

These experiments employ pulsed laser radiation to prepare NO(A) in the region of intersection between molecular beams containing NO and a collider gas, and to probe the NO(A) molecules using Resonance Enhanced Multiphoton Ionization after collisions have been allowed to occur. Velocity Map Imaging of the ionized molecules produces an image which is a 2D projection of the laboratory frame velocity distribution. The state preparation and probe steps are rotationally state selective, meaning that collision-induced transitions between individual pairs of rotational states can be examined in isolation. The well-defined start and finish times imposed by the pump and probe steps mean that the experiment directly measures the scattered flux, rather than just the instantaneous density. This in contrast to conventional CMB scattering experiments, where collisions occur throughout the molecular beam pulse, typically 100 microseconds or more. Differential loss of fast and slow moving scattered molecules in conventional experiments requires a tricky density to flux transformation in the data analysis, something that the 'Start the Clock' method avoids. This is apparent in the excellent degree of symmetry about the relative collision velocity displayed in our images.

DCS differences for the above NO(A) + Ne scattering ion-images

Images acquired for NO(A)+Ne scattering, in real time. Each spot is the result of an single ion hitting the detector. The initial direction of the NO containing molecular beam is towards the centre bottom of the image, where the most intense scattering is observed.

The resulting images may be analysed to extract the differential cross sections (DCSs), the distribution describing the direction into which the products scatter, relative to the initial collision velocity. The product rotational angular momentum may also be polarized, i.e. the products may be preferentially rotating in a particular plane, or even with a particular handedness, as a function of the scattering angle. By controlling the polarization of the probe laser beam and taking images with different polarizations relative to the scattering plane we can also determine this scattering angle-dependent product rotational angular momentum polarization.

NO(A) + Ne scattering ion-images

3-d plots of images for NO(A, v = 0, N = 0)) + Ar scattering to form individual final quantum states N'. The images are sums of images taken with probe laser polarizations both horizontal (H) and vertical (V) in the laboratory frame. This V+H sum is less sensitive to polarization of the product rotational angular momentum than either of specific (V or H) polarizations. As such, the images are dominated by the differential cross sections. The first column shows experimental data, the centre column contains the results of fitting to extract the DCS, and the final column shows simulations assuming the results of quantum scattering calculations on an ab initio potential energy surface.

After initial experiments carried out in collaboration with Dr David Chandler of the Combustion Research Facility at Sandia National Laboratories, we have designed, constructed and commissioned a new CMB-VMI apparatus at Heriot-Watt specifically for collisions of electronically excited molecules. We have performed initial experiments on collisions of NO(A) with the rare gases He, Ne, Ar. High-level ab initio PESs are available for NO(A) with the rare gases, and we are collaborating with Prof Millard Alexander (University of Maryland) and Prof Paul Dagdigian (Johns Hopkins University) to perform close-coupled quantum scattering calculations to compare to our experimental results, and hence to test the accuracy of the PESs.

DCS differences for the above NO(A) + Ne scattering ion-images

Difference images, V-H for NO(A, v = 0, N = 0)) + Ar scattering. These are strongly dependent on the rotatational angular momentum polarization. The top row is the experimental results, the centre row are the results of a fitting procedure, and the bottom row are the results of QS calculations. The red-blue alternation around the edge of many of the images is the result of oscillations in the sign of the rotational angular momentum polarization, which is in general well-reproduced by the theoretical predictions. These oscillations are in marked contrast to the smoothly varying polarization observed in collisions of NO(X) + Ar.

We are currently extending the programme to collisions with molecules, including D2, and a selection of those significant in combustion and atmospheric environments, such as N2, O2 and CO. These diatomic colliders introduce additional complexity, as they also have rotational degrees of freedom that may be excited in the collision. Some of them also quench the electronic excitation of NO(A), providing an additional inelastic or reactive channel that can compete with rotational energy transfer.

Journal of physical chemistry A stereodynamics front cover

Our collaboration with Sandia on this project featured on the front cover of the Journal of Physical Chemistry A Special Issue from the Stereodynamics Symposium in 2013.

  1. Pair-correlated stereodynamics for diatom-diatom rotational energy transfer: NO(A2Σ+) + N2

    Thomas F. M. Luxford, Thomas R. Sharples, Kenneth G. McKendrick and Matthew L. Costen

    Journal of Chemical Physics (2017) 147, 013912

    doi: 10.1063/1.4979487

  2. Experimental testing of ab initio potential energy surfaces: Stereodynamics of NO(A2Σ+) + Ne inelastic scattering at multiple collision energies

    Thomas F. M. Luxford, Thomas R. Sharples, Kenneth G. McKendrick and Matthew L. Costen

    Journal of Chemical Physics (2016) 145, 174304

    doi: 10.1063/1.4966688

  3. Comparative stereodynamics in molecule-atom and molecule-molecule rotational energy transfer: NO(A2Σ+) + He and D2

    Thomas F. M. Luxford, Thomas R. Sharples, Dave Townsend, Kenneth G. McKendrick and Matthew L. Costen

    Journal of Chemical Physics (2016) 145, 084312

    doi: 10.1063/1.4961258

  4. Rotationally Inelastic Scattering of NO(A2Σ+) + Ar:Differential Cross Sections and Rotational Angular Momentum Polarization

    Thomas R. Sharples, Thomas F. M. Luxford, Dave Townsend, Kenneth G. McKendrick and Matthew L. Costen

    Journal of Chemical Physics (2015) 143, 204301

    doi: 10.1021/1.4935962

  5. Rotational Alignment of NO (A2Σ+) from Collisions with Ne

    Jeffery K. Steill, Jeffery J. Kay, Grant Paterson, Thomas R. Sharples, Jacek Kłos, Matthew L. Costen, Kevin E. Strecker, Kenneth G. McKendrick, Millard H. Alexander and David W. Chandler

    Journal of Physical Chemistry A (2013), 117, 8163

    doi: 10.1021/jp402019s

  6. Collisions of Electronically Excited Molecules: differential cross-sections for rotationally inelastic scattering of NO(A2Σ+) with Ar and He

    Jeffery J. Kay, Jeffery K. Steill, Jacek Kłos, Grant Paterson, Matthew L. Costen, Kevin E. Strecker, Kenneth G. McKendrick, Millard H. Alexander and David W. Chandler

    Molecular Physics (2012), 110, 1693

    doi: 10.1080/00268976.2012.670283

  7. Direct angle-resolved measurements of collision dynamics with electronically excited molecules: NO(A2Σ+) + Ar

    Jeffery J. Kay, Grant Paterson, Matthew L. Costen, Kevin E. Strecker, Kenneth G. McKendrick and David W. Chandler

    Journal of Chemical Physics (2011), 134, 091101

    doi: 10.1063/1.3563016

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