Start the clock: a new direct method to study collisions of electronically excited molecules

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of Engineering and Physical Science

Abstract

Understanding the chemistry of the atmosphere, combustion systems and technological plasmas is of great importance in the modern world. A very significant process in these gas-phase environments is the transfer of energy between molecules during collisions. The total amount of internal energy that a molecule possesses is important, but equally if not more important is what form that energy takes. It may be in the form of translational, rotational or vibrational motion of the molecule, or even in the arrangement of the electrons. A large amount of energy may be stored in a rearrangement of the electronic structure of a molecule, producing an electronically excited state. Such excited states occur in energetic environments such as combustion or plasmas, and their decay is what is responsible for the typically observed light emission. The excited states are often also created when molecules are probed in experiments using optical methods. The electronic energy may be re-radiated, but generally not before the molecule has undergone collisions with other gas-phase molecules. This may result in translational, rotational or vibrational energy transfer within the excited electronic state. Significantly, it may also involve either a reaction that removes the excited molecule altogether, or a quenching collision in which the electronic energy is lost to the collision partner.

Despite their importance, surprisingly little is known about the fundamental forces involved in collisions of electronically excited molecules. The best way to determine the detailed dynamics of the collisions of such molecules is using a technique called crossed molecular beam (CMB) scattering. In this method, defined beams of different molecules are collided in a vacuum chamber, and the details of their directions of travel and internal energies are observed in some fashion. We will extend this technique previously used for electronic ground state molecules to the collisions of electronically excited molecules, specifically NO, which is an important molecule in the atmosphere and combustion. We will build a new state-of-the-art CMB scattering apparatus, and by using a combination of laser pulses of different wavelengths will prepare NO molecules in their excited state. After these have collided with the target molecules we will probe the scattered molecules with a laser-based detection technique called velocity map ion-imaging, which is capable of accurately measuring their velocities. The laser pulse that prepares the excited NO defines a zero-time for the experiment, and so 'starts the clock' ticking. This will give us much better definition of the experiment than is usual in CMB experiments, and hence higher sensitivity to the scattering dynamics. We will use this sensitivity to study the collisions of NO with simple molecules relevant to combustion and the atmosphere, namely N2, O2 and CO. These are known to show very different quenching and reactive behaviours with excited NO, but little or nothing is known about the forces involved in these interactions.

The results from our experiments will be used to deepen our understanding of what is important is directing energy transfer in electronically excited molecules, and will help to drive the development of better theoretical models and calculations, as well as providing information of direct relevance to scientist working in the combustion and atmospheric probing of NO.

Planned Impact

The proposed work is fundamental in nature and therefore it is expected that the societal and economic benefits deriving from the results will primarily be realised in the longer term. These specifically project-related benefits will follow from the improved understanding and capacity to model inelastic energy transfer and electronic quenching effects. These collisional processes are central to the behaviour of any gas-phase systems where energetic free radicals are present, initiated either by exothermic reactions or photolysis. The principal practical examples are the atmosphere, combustion, and technological plasmas. More accurate modelling and improved predictive capability will help to guide actions to mitigate the effects of undesirable atmospheric phenomena such as global warming or ozone destruction. They will similarly enable improved efficiency of combustion processes, with consequential economic benefits and reductions in environmental impact. The specific molecular target of the current research, NO, is one of the key species in both cases. Equally, one of the principal methods to determine trace-species concentrations and temperatures in the atmosphere or in combustion is optical probing based on various kinds of laser spectroscopy. Under the prevailing relatively high-pressure conditions, the relationship between signal strengths and species concentration is intimately dependent on collisional energy-transfer processes of the type whose mechanisms are to be investigated here. The improved understanding will enhance the accuracy and hence the value of the information derived from such optically based diagnostics.

There will, in addition, be more immediate impact from the output of highly trained personnel with technical skills in the use of modern laser, vacuum, electronic-data capture and data processing technologies. They will also have well-developed generic and transferable communication, presentation and problem-solving skills. They will be ideally suited to contribute to the growth or creation of high-technology companies, enhancing innovative capacity and consequently increasing business revenues.

We will also continue to enhance public awareness of science by disseminating the aims and results of this work to the wider public, at a suitable level, through available media including lectures, on-line video presentations and websites.
 
Description Molecules with unpaired electrons, free radicals, are in general highly reactive, and are important in the chemistry of many gas-phase environments, including the atmosphere, combustion, technological plasmas. Understanding what happens in their collisions with other molecules and atoms is a vital part of being able to predict and model the chemical and physical evolution of these environments. This includes inelastic collisions, ones in which no chemical reaction occurs, but in which energy is transferred e.g. from translational motion of the molecules into rotational motion. This project has developed a new experimental apparatus to make precise and highly detailed measurements of inelastic collisions. In a unique step, the radical studied, nitric oxide (NO), is electronically excited, by absorption of a ultraviolet photon, before the collision occurs. The excited NO is produced in a molecular beam, travelling in a well-defined initial direction, and collided with another molecular beam of a chosen atom (e.g. Helium, Neon or Argon) or molecule (e.g. Nitrogen, Oxygen, Carbon Monoxide). The NO is rotationally excited in the collision, and these products are probed using high-resolution laser spectroscopy, in which the NO is ionized. The NO+ cations resulting can be accelerated by electric fields to a detector, in a process called velocity-map imaging. This enables us to measure their velocity (speed and direction of travel) after collision By choosing the polarization of the probe light, we can also measure how the plane of rotation of the NO relates to its velocity. Both of these properties are very sensitive probes of the forces acting between the NO and its collision partner, which may be independently calculated by theoretical methods.
Using this newly designed and constructed apparatus, we have made the first-ever systematic measurements of the dynamics of the collisions of electronically excited NO with both rare gases (He, Ne and Ar), and molecules (D2 and N2). We have been able to compare these measurements to the results predicted by theory, and have been able to confirm where theory is right, and to show where it is currently inaccurate. In addition, we have been able to make predictions about the specific failings of the current theoretical models (e.g. with Ne), and to provide predictions for theory to test for systems which have not yet been the subject of calculations (e.g. D2 and N2). These results will stimulate new theoretical studies, and should help to improve not just the calculation of these particular systems, but also related systems featuring other small radical species, e.g. OH, NH, CH. During the course of the project, we have trained a PDRA from post-PhD to an experienced position, one PhD student has been supervised to thesis submission. We have developed and maintained an active collaboration with two world-leading research groupings in experiment and theory in the USA.
Exploitation Route This is a pure research project, and as such, our findings from it are generally of benefit to the academic community. Our findings are relevant to both experimentalists and theoreticians. They are directly relevant to those working in the same field on inelastic and reactive collisions of small molecules. They demonstrate for the first time that it is possible to perform such measurements on electronically excited molecules, opening up a wide range of new molecular systems for study. Our results have also directly challenged some of the more widely used models of product rotational angular momentum, demonstrating that these models are not universally applicable. The results test specific details of the energy of interaction of excited NO with rare gases, previously calculated by ab initio electronic structure calculations. This provides new challenges for ab initio theory on this system, and will inform calculations on related excited state systems. We also hope that the availability of high quality experimental information on the interaction of excited NO with small molecules (e.g. D2) will encourage high-level electronic structure calculations on these systems. More widely, our measurements and their tests of theory are relevant to anybody modelling electronically excited NO in gas-phase environments, e.g. combustion, the atmosphere, technological plasmas.
Sectors Chemicals,Education,Energy,Environment

URL http://dynamics.eps.hw.ac.uk/
 
Description EPSRC DTP Award
Amount £65,000 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 09/2016 
End 03/2020
 
Description James-Watt PhD Scholarship
Amount £57,300 (GBP)
Organisation Heriot-Watt University 
Sector Academic/University
Country United Kingdom
Start 09/2013 
End 08/2016
 
Description Platform Grant
Amount £1,277,251 (GBP)
Funding ID EP/P001459/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 08/2016 
End 08/2021
 
Description Experiment - Dave Chandler 
Organisation Sandia Laboratories
Department Combustion Research Facility
Country United States 
Sector Public 
PI Contribution The PDRA, Dr T. R. Sharples, employed on the project visited the laboratory of Dr David Chandler at Sandia National Lab, Livermore for a 4 week period in Jan/Feb 2014. The primary aim of the visit was to learn about the technique of velocity-map imaging and in particular its application in minature crossed-molecular beam systems. Dr Sharples contributed to on-going experiments in Dr Chandler's laboratory during the visit. Dr Chandler has subsequently visited Heriot-Watt for 1 week (Sept 2016) during which we performed the first proof of concept measurements for a 4-vector correlation in inelastic rotational energy transfer, this lead to a publication in Nature Chemistry.
Collaborator Contribution Dr David Chandler is a world-expert on the application of imaging techniques in molecular dynamics, having introduced the idea of ion-imaging itself in the 1980s. He is also an expert in crossed-molecular beam experiments, and designed the first miniature apparatus to include velocity-map imaging. Previous collaboration with him lead to the proof-of-concept publications that were instrumental in the funding of this project. In this particular collaboration, he has provided training for the PDRA in crossed-beam VMI techniques, and provided valuable technical insight into the design of the new apparatus at Heriot-Watt.
Impact Steill et al. Journal of Physical Chemistry A, 117, 8163 (2013) Sharples et al. Journal of Chemical Physics 143 204301(2015) Luxford et al. Journal of Chemical Physics 145 084312 (2016) Luxford et al. Journal of Chemical Physics 145 174304 (2016) Luxford et al. Journal of Chemical Physics 147 013912 (2017) Sharples et al. Nature Chemistry 10 1148 (2018)
Start Year 2013
 
Description Stark deflection for molecular cluster separation 
Organisation University of Hamburg
Country Germany 
Sector Academic/University 
PI Contribution Secondment of Dr Thomas Sharples to the group of Prof Jochen Kupper at DESY/U. of Hamburg to develop the application of Stark deflection methods to the separation of weakly bound molecular clusters. Dr Sharples has proposed the experiments to Prof Kupper, and performed experiments at U. of Hamburg with Prof Kupper's research group. Dr Sharples has subsequently helped develop the theoretical modelling of the deflection.
Collaborator Contribution Prof Kupper provided the experimental apparatus and hosted Dr Sharples' visit. He has subsequently provided computing resources and manpower towards the interpretation of the experimental results.
Impact None to date.
Start Year 2018
 
Description Theory - Millard Alexander 
Organisation Johns Hopkins University
Country United States 
Sector Academic/University 
PI Contribution We provided experimental data and results, and provided time of PDRA on the research team, who visited our collaborators in the USA.
Collaborator Contribution Our collaborators provided training in theoretical methods to the PDRA employed on the project, and subsequently provided advance copies of the latest versions of the relevant computer codes (not yet publicly released). They also provided the results of theoretical calculations in support of our experimental results, and associated interpretation.
Impact Training of the PDRA on the project in state-of-the-art time-independent quantum scattering calculations and associated theory. Sharples et al. Journal of Chemical Physics 143 204301 (2015) Steill et al. Journal of Physical Chemistry A 117 8163 (2013)
Start Year 2012
 
Description Theory - Millard Alexander 
Organisation University of Maryland
Country United States 
Sector Academic/University 
PI Contribution We provided experimental data and results, and provided time of PDRA on the research team, who visited our collaborators in the USA.
Collaborator Contribution Our collaborators provided training in theoretical methods to the PDRA employed on the project, and subsequently provided advance copies of the latest versions of the relevant computer codes (not yet publicly released). They also provided the results of theoretical calculations in support of our experimental results, and associated interpretation.
Impact Training of the PDRA on the project in state-of-the-art time-independent quantum scattering calculations and associated theory. Sharples et al. Journal of Chemical Physics 143 204301 (2015) Steill et al. Journal of Physical Chemistry A 117 8163 (2013)
Start Year 2012
 
Description van de Meerakker (Nijmegen) 
Organisation Radboud University Nijmegen
Department Institute for Molecules and Materials
Country Netherlands 
Sector Academic/University 
PI Contribution We have sent a PDRA (Dr T. Sharples) to Prof. Meekakker's laboratory to learn about (1) their work on electric discharge radical molecular beam sources and (2) hexapole state selection.
Collaborator Contribution Prof Meerakker hosted the visit of Dr T Sharples, and has provided information related to the design of (1) electric discharge radical molecular beam sources and (2) hexapole state selectors.
Impact None so far.
Start Year 2016
 
Description School Visits 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Multiple visits to schools across Northern England and Scotland, as part of the Public Understanding Outreach programme of the School of Engineering and Physical Sciences. Presentation to penultimate/final year school pupils, called 'How do Chemical Reactions Go?' which incorporates information on and results from our EPSRC funded projects. Question and answer sessions included, as well as discussion with the teaching staff present. Teaching staff generally report subsequent enthusiastic discussion about the topics raised.
Year(s) Of Engagement Activity 2012,2013,2014,2015,2016