Infrared emission from the quenching of electronically excited states

Lead Research Organisation: University of Oxford
Department Name: Oxford Chemistry


When a molecule absorbs light to form an excited state, its total energy increases, it is no longer in equilibrium with its surroundings, and processes will naturally occur to counteract this and thereby restore equilibrium. For an electronically excited bound state (where the absorption is in the visible or ultraviolet region of the spectrum), the process of energy loss can take place through emission of light (fluorescence) or by collisional processes (quenching). Fluorescence is well understood, as are the rates of quenching, but what is far less understood are the specific fates of the quenched species - where does the (considerable) energy contained in the excited state go - does it appear in kinetic or internal energy of the ground state product and if chemical reaction is possible, what are the products and are they formed with internal energy?

The purpose of this study is to investigate the products of the quenching of two important gas phase free radical species, OH and NO. They will be formed in their electronically excited states by absorption of ultraviolet photons (at 308 nm and 226 nm respectively), and the products observed by the technique of Time Resolved Fourier Transform InfraRed Emission (TRFTIRE) using an apparatus unique to the UK. The first system to be studied will be the quenching of the OH(A) state in collisions with molecular hydrogen. Here the main result of quenching is the formation of water, and preliminary results have shown that the water is hot, with a great deal of the available energy appearing as vibration, and resulting in emission in the mid-infrared region of the spectrum. The emission spectrum of H2O will be compared with the complementary results already obtained for this reaction by Lester's group on the H atom cofragment kinetic energy distribution, and the internal energy distribution in the OH ground state product of inelastic quenching. State of the art calculations of the quenching processes will be carried out by our theoretical partners.

Other systems to be studied include the quenching of OH(A) by O2, CO and CO2, where atomic reaction products have been observed - our studies will complete the picture by looking for the first time at the molecular reaction products. Similar studies on the NO(A) state will also be carried out. The results are of importance in interpreting the laser induced fluorescence measurements in combustion systems, where quenching processes can result in a serious overestimation of the radical concentrations.

Planned Impact

1. The main industrial beneficiaries will be those in the combustion related industries (power, automotive, energy) where an understanding of the behaviour of important intermediates such as OH and NO in the gas phase requires information upon the fates of electronically excited states of these species when their detection and quantification is carried out by laser induced fluorescence.

2. Part of the pathways to impact mechanisms suggested is to provide technical notes for major laser suppliers indicating how laser measurements need to take the behaviour of these excited states into account.

3. The track record of the applicants in Impact Activities is high: one successful spin out company and several patents have emerged from previous EPSRC supported research in fundamental physical chemistry. None of the commercial applications relating to these activities were foreseen at the time of submission of these proposals.


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Description Molecules can exchange energy with one another by collisions and there are well developed rules for predicting which energy states will be formed. In this work we have carried out a carefully conducted and analyzed study of the collisional reactive and non-reactive quenching of electronically excited NO employing time resolved step scan FTIR spectroscopy in emission. NO is key player in atmospheric and combustion chemistry. In kinetic models used for simulations of atmospheric systems the chemistry of electronically excited species is often not well represented due to lack of understanding and thus reliable kinetic data. Our work sheds some new light on one of these important processes, adding complementary data to previous work, significantly improving our understanding of the excited NO* + O2 reaction and this work has now been published. One of the findings - that the low vibrational levels of ground state NO are formed in conjunction with electronically excited molecular oxygen - has now been corroborated by work in another group who used direct measurements of the translational energy of the products to confirm this result. In addition, we have discovered an important factor in the collisional quenching of NO*. The electronically excited doublet state has been previously assumed to be quenched directly to the ground electronic state, but by carrying out the first time-resolved observations of the formation of the vibrational levels of the ground state we have proved that the mechanism also involves the initial formation of the spectroscopically elusive lowest excited quartet state. In addition, we have found a remarkable effect when the rare gas, Xenon, is added to the system. Xenon also quenches the quartet state to give highly vibrationally excited ground state NO formed with a population inversion. Rate constants and quantum yields of the quenching processes have been measured, and two papers are being prepared for publication.This may seem to be an esoteric quirk of collisional behaviour but is has an initially unexpected consequence. In the atmosphere (particularly the stratosphere) there is NO* present, and its major route for quenching is by molecular oxygen. We have recently found hints that this could take place through the quartet state and if this is corroborated it may well open up the necessity of studying the kinetic behaviour of this state for both atmospheric and combustion interest - so far it has been neglected.
Major findings have thus been the measurement of the fraction of excited molecules which pass through the quartet state in collisions with NO and Xe. Experiments are planned for later this year (with a summer visitor) to study the behaviour in collisions with molecular oxygen.
Exploitation Route The findings may help to improve kinetic modelling in combustion and plasmas, and be of interest in atmospheric chemistry
Sectors Chemicals,Energy,Environment,Other

Description Nitric oxide NO is an important atmospheric constituent, in both normal and polluted atmospheres. The molecule absorbs uv light at a wavelengths of 226 nm and below, and in the atmosphere is it usually collisionally quenched. In the past the assumptions are made that the end products of the quenching are the ground state of the molecule, and we have recently shown that these can produce vibrationally excited ground state species. The present work has in the last year also shown that a little studied first excited state of NO, a quartet state which is not produced in absorption, is formed when excited NO is self quenched. This state acts as an energy reservoir as it is optically metastable: the question now arises as to whether or not this state takes any part in atmospheric chemistry. Collisional formation in quenching by NO is irrelevant, but a characteristic we have observed is that spin allowed processes may produce the quartet state, and if this is widespread, then it could be formed by quenching with molecular oxygen in the atmosphere. We have preliminary date, taken over the past year, that this is the case, and a series of experiments has been devised to put this conjecture onto a quantitative footing. We had hoped to do this with a summer visitor in 2018, but another equipment failure has meant a delay. This is now solved, and plans are in place to finish the work in 2020. If we find that appreciable quartet NO is produced by quenching then the behaviour of the this state will need to be assessed in atmospheric modelling, particularly in the stratosphere. In addition, quenching by molecular oxygen will contribute to corrections needed for the interpretation of methods used to detect NO by laser absorption, methods which normally assume that the quenched molecule returns rapidly to the ground state and is not affected by ground state depletion.
Sector Environment
Impact Types Policy & public services

Description Complex Chemistry and Chemical Activation
Amount £1,122,825 (GBP)
Funding ID EP/V029630/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 06/2021 
End 12/2023
Organisation University of Cordoba
Country Spain 
Sector Academic/University 
PI Contribution The collaboration resulted in several visits to our department by a very gifted graduate student Miss Lucia Lanfri fron the University of Cordoba. We have had contacts and visitors from Argentina for may years, and this is another in the sequence. Miss Lanfri was awarded a Researcher Mobility Grant from the Royal Society of Chemistry, and was able to work in the department on this project in 2017 and 2018. She has now completed her doctorate at the University of Cordoba.
Collaborator Contribution Miss Lanfri joined in experiments which probed the collisional behaviour of the A state of nitric oxide in collisions with various added gases. She made a substantial contribution to the results, which showed unequivocally that self quenching occurred over 50% of the time by transfer to the elusive quartet state, the first time that this has been clearly invoked in such collisional studies, and worked on the quenching of this state in collisions with Xe. She produced many results for her thesis, and these results are now being writted up for publication.
Impact Two papers in preparation. They have not been completed yet, as we await a repair to the equipment to gether somne final data
Start Year 2016