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Professor Stephen Price

Research Overview

Ions and electronically excited molecules can posses a considerable amount of excess electronic energy. These energized species display a very different reactivity to unexcited molecules, and play an important role in the chemistry of media such as planetary atmospheres. Our research concentrates on developing new experimental techniques to probe the chemistry, structure and reaction dynamics of these energized species.

Chemistry of multi-ionized molecules

Multi-ionized molecules (e.g. CF32+, CS23+) are short-lived species with lifetimes which range from seconds to femtoseconds. These ions are extremely energy rich and have remarkably different properties from singly-charged ions. In fact, it has been proposed that doubly-charged molecular ions play a part in the chemistry of a variety of energized media, such as interstellar clouds and planetary ionospheres. We are using coupled mass spectrometry to study the unimolecular and bimolecular reactivity of a wide variety of multi-ionized species and our experiments indicate a rich and varied chemistry. For example, as shown in the figure below, CF22+ reacts with HD to preferentially form DCF2+ rather than HCF2+.

Such experiments allow a detailed insight into the dynamics of these new chemical reactions. For example, we conclude that in this reaction the abundance of the products is influenced by statistical effects in the dissociation of a collision complex.

The right mass spectrum in figure 1 shows CF2+ (m/z =50), HCF2+ (m/z =51) and DCF2+ (m/z =52) formed when CF22+ reacts with HD.

We have also found dication chemical reactions which do not involve the separation of the dipositive charge. For example, SF2+ + Ar -> ArS2+ + F (see figure 1-right). We have developed models involving the electrostatic interactions between the reactants and products to explain when the chemical products of dication reactions will be a pair of monocations and when the products will be a dication and a neutral.

Mass spectrum showing signals for dication reactions not involving separation of the dipositive chargeMass spectrum obtained when CF2 reacts with HD

Figure 1. (left) Mass spectrum showing products of SF2+ + Ar -> ArS2+ + F. (right) Mass spectrum showing CF2+ (m/z =50), HCF2+ (m/z =51) and DCF2+ (m/z =52) formed when CF22+ reacts with HD. 

To probe the dynamics of these dication reactions in more detail we have developed a new experimental technique for studying the relative motions and energetics of the reactions products. This Position-Sensitive COincidence (PSCO) experiment involves detecting the pairs of product monocations formed in dication reactions in a specially designed time of flight mass spectrometer equipped with a position-sensitive detector. These measurements reveal the complete reactivity in a dication-neutral collision system allowing us, for example, to show that N22+ exhibits an extensive bond-forming chemistry, with relevance to the terrestrial ionosphere, and that CO22+ has a similarly extensive chemistry of relevance in the ionosphere of Mars. Our PSCO measurements also allow us to determine the motion (velocity vectors) of the nascent monocation products of each of the bond-forming reactions we detect. From these velocity vectors, via conservation of momentum, we can device the velocity of the neutral species which often accompany such dication chemical reactions:

N22+ + O2 -> NO+ + O+ + N

The relationships between these velocity vectors provides a powerful probe of the mechanism of the bond-forming process. Our investigations have shown that many, but not all, dication reactions proceed via formation of a collision complex which lives for markedly longer than it's rotational period and then dissociates via either charge-separation or neutral loss:

N22+ + O2 -> N2O22+

NO+ + O+ + N <- N2O22+-> NO+ + N+ + O


PCSO spectrum showing ions formed following collisions of N2 2+ with water

Figure 2. Section of a two-dimensional PSCO spectrum showing some of the ion pairs formed following collisions of N22+ with water. The bond-forming reactions generating NO+ together with H+ + NH and H2+ + N are clearly seen.

Centre of mass scattering diagram derived from PSCO data for collision between N2 2+ and O2

Figure 3. Centre-of mass scattering diagram derived from PSCO data for the reaction: N22+ + O2 -> NO++ O+ + N.  The scattering clearly shows the product ions are scattered effectively isotropically about the original velocity of the dication, a strong signature of the formation of a "long-lived" collision complex. 

Internal frame scattering diagram showing motion of N+ and N products from dissociative electron transfer reaction of N2 with Ne2+

Figure 4. "Internal-frame" scattering diagram showing the motion of the N+ and N products from the dissociative electron transfer reaction of Ne2+ with N2. The diagram shows how the velocity vectors of the N+ and N products are angularly distributed with respect to the velocity of the Ne+ product. The extremely strong angular correlations are clearly due to the break up of a collision complex [NeN2]2+

We are currently upgrading the PSCO experiment to generate data with even higher energy resolution to allow us to resolve the reaction dynamics of individual dicationic states. This will involve the installation of a molecular beam source, a larger position sensitive detector and a new design of time-of-flight mass spectrometer to allow velocity-imaging with high mass resolution from extended ion sources.

Consequences of electron-molecule collisions.

Reactions of ions dominate the chemistry of many media, such as planetary ionospheres, plasma etchers and interstellar space. In these energized media primary ions are generated from neutral molecules by energetic photons and electrons. These primary ions then react with neutral species to create secondary ions.

2D mass spectrum recorded following ionisation of C2F6

Figure 5. Two-dimensional mass spectrum recorded following ionization of C2F6. A huge number of charge-separating dissociation reactions of the di- and tri-cation are observed.

In order to model the chemistry of such ionized media it is important to know (i) which primary ions are formed when the available neutral molecules are ionized and (ii) how those primary ions go on to react. Dealing with the first point seems simple, especially for the relatively small molecules that often predominate in gas-phase media. To this end, partial ionization cross sections (numbers which quantify the different ions formed in ionizing events) have been measured for many years, following electron-impact ionization of molecules, using conventional mass spectrometers. Due in part to our recent work, it has become apparent that even for simple molecules (e.g. N2O, Cl2) the existing partial ionization cross sections are often in error. These errors arise because the mass spectrometric experiments used to identify and quantify the ions did not detect the translationally energetic ions formed from multiple (and to some extent single) ionization. We have developed an experiment involving two-dimensional mass spectrometry which can efficiently detect and identify all the ions formed following an ionization event. This procedure produces the reliable relative partial ionization cross sections which are required for an accurate representation of the primary ion distribution in an energized medium. In addition, our experimental data, which contains the intensities of ion pairs and triples, as well as single ions, also quantifies the ionization level which is responsible for any fragment ions. For example, we can determine how many NO+ ions from the ionization of N2O come from fragmentation of N2O+ and how many from N2O2+. That is, we measure charge-state specific partial ionization cross sections. The data produced by our experiments can be used to upgrade models for the ionic abundances in energized media. In addition the experiment is a powerful probe of the properties of the multiply charged molecules generated by electron ionization. For example , we can see the multitude of different dissociation pathways that occur when you doubly ionize C2F6 and have also studied the ionization of a variety of reactive molecules.

Graph showing the contribution of single double and triple ionization to the formation of F+ following electron ionization of C2F6.

Figure 6. Graph showing the contribution of single double and triple ionization to the formation of F+ following electron ionization of C2F6. Above 60 eV multiple ionization is the main source of F+ ions

Studies of the recombination of H and D atoms on graphite - "Laboratory Astrochemistry".

Molecules in interstellar gas play a fundamental role in astronomy. However, the formation of the simplest molecule, molecular hydrogen, is still not fully understood. At UCL (as part of the Centre for Cosmic Chemistry and Physics [CCCP]) we have developed an experiment to address these issues and explore the following questions: how is H2 formed on dust grain surfaces? What is the budget between internal, kinetic and surface energies in the formation process? What are the astronomical consequences of these results?

Image of star forming region IC1396

Figure 7. The star forming region IC1396. Image taken by taken by Nick Wright of UCL for the IPHAS consortium. This image shows extensive regions of low density of gas (red), rich in atomic hydrogen, and high density (black) interstellar gas, rich in molecular hydrogen. Star formation is occurring in the high density gas, but is shielded from view by the dust mixed in the gas. Scattered light from the newly forming stars emerges around the edges of the dark cloud.

H2 is the most abundant molecule in the Universe and is the key partner in almost all reactions of cosmic chemistry; therefore it is fair to say that the most important reaction involving cosmic dust grains is the formation of molecular hydrogen from incident atomic hydrogen. It has now become possible to investigate in the laboratory reactions on surfaces at low temperature and low pressure that mimic those occurring on cosmic dust. Our aim at the UCL-CCCP is to understand the nature of those reactions, to determine their efficiency in the interstellar medium, their energy budget, and their consequences in the interstellar medium.

Schematic of principles of cosmic dust experiment

Figure 8. Schematic of the principles of the Cosmic Dust Experiment. Beams of H and D atoms are incident on a cold graphite surface and the HD molecules formed are probed by laser spectroscopy.


In 1997 we began a combined programme of experimental and theoretical work to study the formation of H2 and HD from atomic hydrogen and deuterium on astrophysically relevant surfaces. The experiments focus on measuring the internal energy content of newly-formed hydrogen. Our experimental technique involves continuously irradiating a graphite (HOPG) surface, held at 13K, with beam(s) of incident H (and D) atoms in a UHV environment. These atomic beams are generated by microwave dissociation of H2 (D2). The nascent product molecules desorbing from the surface are detected by state-selective laser ionization and the resulting signals can be transformed into the relative populations of the nascent ro-vibrational states formed at the surface.

Photograph of the cosmic dust apparatus

Figure 9. Photograph of the cosmic dust apparatus. The H and D atom sources are on the left and the UHV chamber is on the right. The target cold head is hidden behind the laser on the extreme right.

Photograph of the hydrogen discharge used to generate the beam of atomic hydrogen for the experiments.


Figure 10. Photograph of the hydrogen discharge used to generate the beam of atomic hydrogen for the experiments.Photograph of the hydrogen discharge used to generate the beam of atomic hydrogen for the experiments.

We have detected H2 molecules in their first (v=1) and second (v=2) excited vibrational states and HD molecules formed in v=1-4 when the target temperature is 15 K. Qualitative indications are that the relative number density of nascent HD increases with vibrational excitation over the states we have detected and that a considerable amount of the energy released on forming the H-H bond flows into the surface. We are about to implement an experimental extension to determine the translational energy of the nascent molecules as well as their ro-vibrational excitation.

Bar chart showing reliminary ro-vibrational distribution for HD formed on a 15K graphite surface.

Figure 11. Preliminary ro-vibrational distribution for HD formed on a 15K graphite surface. Some uncertainties are introduced in the intercomparison between the populations of different vibrational levels. However, it seems clear the population increases with increasing vibrational quantum number for v=1-4.

In collaboration with colleagues in the Physics Department, we are incorporating our results into chemical models of H2 formation in the interstellar medium and are planning an observational campaign to search for infrared emission from these vibrationally excited nascent molecules.

Further information on the Cosmic Dust experiment is available.

Final reports on recent grants are available.

Studies of the reaction of molecules with O and H atoms on interstellar dust grains at low temperatures.

The elemental composition of the known universe comprises almost exclusively light atoms (~99.9% hydrogen and helium). However, to date, over 160 different molecules have been detected in the interstellar medium. The vast majority of these interstellar molecules contain hydrogen. The distribution of these molecules is far from uniform across the interstellar medium (ISM), where dense interstellar clouds are observed to harbour relatively large densities and varieties of complex molecules that have been proposed as precursors to biologically relevant species. In these interstellar clouds molecular lifetimes are extended with respect to other more diffuse parts of the ISM. Specifically, the relatively opacity of these dense interstellar clouds shields the cloud’s centre from high energy photons, allowing relatively fragile molecules to survive for extended periods.

In dense interstellar clouds molecules are detected in abundances that cannot be completely accounted for by known gas-phase kinetics. Hence, it is now widely accepted that there is a contribution to these molecular abundances from reactions on and within the molecular ices on the surfaces of interstellar dust. This interstellar dust comprises predominantly silicate or carbonaceous particles, of typical diameter of 100 nm,7 making up typically 1% of the mass of an interstellar cloud.

Recently we have been investigating the reactions of prototypical interstellar molecules (alkenes, alkynes, sulfur containing molecules) with H and O atoms on laboratory analogues of interstellar dust at low temperatures representative of the interstellar medium. We observe significant reactivity in these experiments and the kinetic data we obtain from our experiments has begin to be incorporated into models of the chemistry in interstellar clouds. Our work shows that reactions on surfaces can have a significant influence on the molecular abundances observed in interstellar clouds.

Representative mass spectra


Sections of representative mass spectra recorded following the reaction of propyne with oxygen atoms on a cold graphite surface. The peaks at m/z = 56 and m/z = 72 correspond to single and double addition products.

Selected Publications

[1] J.D. Fletcher, M.A. Parkes and S.D. Price Chem-Eur.J. 19 (2013) 10965

[2] M.D. Ward, I.A. Hogg and S.D. Price Mon. Not. Roy. Astron. Soc. 425 (2012) 1264

[3] M.D. Ward and S.D. Price Astrophys. J. 741 (2011) 121

[4] J.F. Lockyear, C.L. Ricketts, M.A. Parkes and S.D. Price Chemical Science 2 (2011) 150

[5] J.F. Lockyear, M.A. Parkes and S.D. Price Angewandte Chemie International Edition 50 (2011) 1322