John Thrower

• PhD
• Laboratory Scientist at Aarhus University

The interaction of a curved polycyclic aromatic hydrocarbon (PAH), corannulene (C20H10) with H-atoms leading to the formation of highly superhydrogenated species was studied. In particular, we determined the H-atom addition sequence to a monolayer of corannulene deposited on a graphite surface in order to understand the effect of curvature on the i...

Aims. We investigate the interaction of a linear, catacondensed polycyclic aromatic hydrocarbon (PAH), pentacene (C 22 H 14 ), and its oxygen-functionalised form 6, 13 pentacenequinone (C 22 H 12 O 2 ) with atomic hydrogen (H) under interstellar conditions. We compare their reaction cross-sections and reaction products to elucidate the possible rol...

A combined high resolution X-ray photoelectron spectroscopy and X-ray standing wave study into the adsorption structure of hydrogenated graphene on Ir(111) is presented. By exploiting the unique absorption profiles and significant modulations in signal intensity found within the X-ray standing wave results, we refine the fitting of the C 1s X-ray p...

We present the results of an experimental study of the interaction of Lyα photons with superhydrogenated coronene films. The effects of ultraviolet (UV) irradiation have been analyzed with infrared spectroscopy. The spectral changes provide evidence for UV photodestruction of the C–D bonds of the superhydrogenated coronene with a cross section of 8...

Interesting electronic properties arise in vertically stacked graphene sheets, some of which can be controlled by mutual orientation of the adjacent layers. In this study, we investigate the MBE grown multilayer graphene on Ir(111) by means of STM, LEED and XPS and we examine the influence of the substrate on the geometric and electronic properties...

Experimental measurements on the thermal and non-thermal behaviour of water and other simple molecules, including organic compounds such as methanol and benzene, on model interstellar dust grain surfaces and on solid water surfaces using surface science techniques and methodologies are reviewed. A simple qualitative model of the early stages mantle...

Superhydrogenated polycyclic aromatic hydrocarbon (PAH) molecules have been demonstrated to act as catalysts for molecular hydrogen formation under interstellar conditions. Here we present combined thermal desorption mass spectrometry measurements and density functional theory calculations that reveal the most stable configurations in the superhydr...

Experimental evidence for the formation of hydrogenated fullerene molecules is presented. Films of C 60 were grown on a highly oriented pyrolytic graphite (substrate) and exposed to a beam of deuterium atoms. Thermal desorption combined with mass spectrometry was used to determine the deuterated fullerene products formed, revealing a maximum degree...

Reactions on carbonaceous surfaces play an important role in processes such as H 2 formation in the interstellar medium. We have investigated the adsorption of C 2 molecules on a highly oriented pyrolytic graphite (HOPG) surface and then exposed them to a beam of deuterium atoms in order to investigate the formation of deuterated fullerenes. Scanni...

Experimental data showing superhydrogenation of neutral polycyclic aromatic hydrocarbons (PAHs) coronene, pentacene and pentacenequinone is presented. PAH monolayers were prepared on a highly oriented pyrolytic graphite surface and subsequently exposed to a beam of atomic hydrogen. The superhydrogenated PAH species were examined via temperature pro...

A synthesis observations from laboratory studies on the thermal and non-thermal physics and chemistry of dust grains as they evolve from diffuse to dense environments in the interstellar medium will be presented. We will specifically report on (1) thermal desorption and the impact of surface heterogeneity; (2) agglomeration of water (H2O) on an amo...

The photochemical processing of a CH4:D2O 1:3.3 ice mixture adsorbed on a HOPG surface in the XUV regime has been investigated using pulses obtained from the Free-electron LASer in Hamburg (FLASH) facility. The ice films were exposed to femtosecond pulses with a photon energy of hν = 40.8 eV, consistent with the HeII resonance line. Cationic specie...

We present temperature programmed desorption (TPD) measurements of CO, CH$_4$, O$_2$ and CO$_2$ from the forsterite(010) surface in the sub-monolayer and multilayer coverage regimes. In the case of CO, CH$_4$ and O$_2$, multilayer growth begins prior to saturation of the monolayer peak, resulting in two clearly distinguishable desorption peaks. On...

We present temperature programmed desorption (TPD) measurements of CO, CH$_4$, O$_2$ and CO$_2$ from the forsterite(010) surface in the sub-monolayer and multilayer coverage regimes. In the case of CO, CH$_4$ and O$_2$, multilayer growth begins prior to saturation of the monolayer peak, resulting in two clearly distinguishable desorption peaks. On...

Desorption from solid water surfaces resulting from interaction with electromagnetic and particle radiation is reviewed in the context of the role of non-thermal desorption in astrophysical environments. Experimental observations are interpreted in terms of mechanisms sharing a common basis in the Menzel-Gomer-Redhead (MGR) framework underpinned by...

Electron-promoted desorption (EPD) from compact amorphous solid water (c-ASW) has been studied. Low-energy electron bombardment with 200 to 300 eV electrons leads to H2O depletion as monitored by reflection-absorption infrared spectroscopy (RAIRS) of the remaining c-ASW film. Cross-sections for H2O depletion were calculated to be in the range 1.6±1...

The changes in the strength of the interaction between the polycyclic aromatic hydrocarbon, coronene, and graphite as a function of the degree of super-hydrogenation of the coronene molecule are investigated using temperature programmed desorption. A decrease in binding energy is observed for increasing degrees of super-hydrogenation, from 1.78 eV...

This paper will describe recent laboratory studies of relevence to understanding the structure and behaviour of icy grains in a variety of astrophysical environments. We will report on two sets of experimental observations; (1) diffusion of water on silica nanoparticle films, and (2) highly efficient desorption of benzene promoted by electronic exc...

Desorption of benzene (C6H6) from solid water surfaces [compact amorphous solid water (c-ASW) and crystalline ice (CI)] during irradiation of ultrathin solid films with low energy (250-300 eV) electrons has been investigated. The observed desorption behaviour is complex but typically two desorption components, with particularly large cross-sections...

The thermal desorption of ammonia (NH3) from single crystal forsterite (010) has been investigated using temperature-programmed desorption. The effect of defects on the desorption process has been probed by the use of a rough cut forsterite surface prepared from the cleaved forsterite sample. Several approaches have been used to extract the desorpt...

The morning of the 7th April 2014 saw the arrival of around 120 astrochemists from around the globe in the quiet South Holland town of Noordwijkerhout, just outside Leiden. The delegates for Faraday Discussion 168 were treated to a premier view of Holland’s famous tulip fields en route via coach from Schiphol airport (Fig. 1). Hosted at the NH Conf...

Polycyclic aromatic hydrocarbons (PAHs) have been shown to catalyse molecular hydrogen formation. The process occurs via atomic hydrogen addition reactions leading to the formation of super-hydrogenated PAH species, followed by molecular hydrogen forming abstraction reactions. Here, we combine quadrupole mass spectrometry data with kinetic simulati...

Carbonaceous materials contribute to a significant proportion of the interstellar dust inventory. Reactions on such grain surfaces are thought to play important roles in interstellar chemical networks. Of particular importance are reactions involving hydrogen atoms, and pathways to the formation of the most abundant molecular species, H2. Polycycli...

The structure and bonding of solid acetonitrile (CH3CN) films on amorphous silica are studied, and chemical and physical processes under irradiation with 200 keV protons and 250-400 eV electrons are quantified using transmission infrared spectroscopy, reflection-absorption infrared spectroscopy and temperature-programmed desorption, with the assist...

The adsorption of polycyclic aromatic hydrocarbon (PAH) molecules on graphitic surfaces provides a model system with which to investigate weak van der Waals (vdW) interactions. There are few experimental investigations of either the interaction between large PAH molecules and graphite or the binding between graphite layers. Determining the adsorpti...

Electronic solid state spectra are recorded for C60 embedded in 40 K water ice using broad band direct absorption spectroscopy, and assigned with reference to existing matrix data. The results are interesting in view of the recent gas phase detection of fullerenes in the interstellar medium and provide a realistic solid state signature to search fo...

Mass spectrometry measurements show the formation of highly superhydrogenated derivatives of the polycyclic aromatic hydrocarbon molecule coronene through H atom addition reactions. The observed product mass distribution provides evidence also for abstraction reactions resulting in H2 formation, in agreement with recent IR measurements. Complementa...

We present the results of an experimental study on the interaction of atomic deuterium with coronene films. The effects of D atom irradiation have been analyzed with infrared spectroscopy. The spectral changes provide evidence for deuteration of the outer edge coronene C sites via a D addition reaction. A cross section of 1.1 Å2 is estimated for th...

Laboratory studies of the interaction of H atoms with nanosized aliphatic carbon grains under simulated interstellar conditions have shown the key role of this interaction for the formation of H2 molecules at high grain temperatures in the interstellar medium. New results of H atom irradiation experiments, showing the formation of molecular hydroge...

The interaction between thin films of polycyclic aromatic hydrocarbons (PAHs) and atomic H has been studied using scanning tunneling microscopy (STM). Observational evidence suggests that hydrogenated PAHs are located in regions of the interstellar medium (ISM) where there are high concentrations of molecular hydrogen (H2)1. It has previously been...

The possible role of neutral PAHs as catalysts for H2 formation in the interstellar medium is investigated by a combined experimental and density function theory study of the superhydrogenation of coronene (C24H12). The calculations suggest efficient hydrogenation of both edge and centre sites, along with competing abstraction reactions to form H2...

We present experimental investigations of the formation of superhydrogenated polycyclic aromatic hydrocarbons (PAHs) through exposure of neutral PAHs to atomic hydrogen. Post irradiation thermal desorption measurements demonstrate that it is possible to form superhydrogenated coronene molecules with surprisingly high numbers of additional H atoms....

i-system due to rehybridization of carbon centers from sp^2 to sp^3. Hydrogenated coronene species have also been observed after adsorption of coronene on a D-saturated HOPG surface.

The nonthermal desorption of water from ice films induced by photon and low energy electron irradiation has been studied under conditions mimicking those found in dense interstellar clouds. Water desorption following photon irradiation at 250 nm relies on the presence of an absorbing species within the H 2 O ice, in this case benzene. Desorption cr...

The thermal desorption of C(6)H(6) from two astrophysically relevant surfaces has been studied using temperature programmed desorption. Desorption from an amorphous SiO(2) substrate was used as a mimic for bare interstellar grains, while multilayer films of amorphous solid water (ASW) were used to study the adsorption of C(6)H(6) on grains surround...

We present experimental measurements of photodesorption from ices of astrophysical relevance. Layers of benzene and water ice were irradiated with a laser tuned to an electronic transition in the benzene molecule. The translational energy of desorbed molecules was measured by time-of-flight (ToF) mass spectrometry. Three distinct photodesorption pr...

Experimental results on the thermal desorption of benzene (C6H6) from amorphous silica (SiO2) are presented. The amorphous SiO2 substrate was imaged using atomic force microscopy (AFM), revealing a surface morphology reminiscent of that of interplanetary dust particles (IDPs). Temperature programmed desorption (TPD) experiments were conducted for a...

The kinetic energy of benzene and water molecules photodesorbed from astrophysically relevant ices on a sapphire substrate under irradiation by a UV laser tuned to the S 1 ← S 0 → * transition of benzene has been measured using time-of-flight mass spectrometry. Three distinct photodesorption mechanisms have been identified—a direct adsorbate-mediat...

We present experimental measurements of photodesorption from ices of astrophysical relevance. Layers of benzene and water ice were irradiated with a laser tuned to an electronic transition in the benzene molecule. The translational energy of desorbed molecules was measured by time-of-flight (ToF) mass spectrometry. Three distinct photodesorption pr...

Dust particles and their interaction with gases play important roles in star formation and in solar nebulae. Appropriate model dust grains are needed for the laboratory simulation of gas-grain interactions. Nanoparticles formed from carbonaceous meteorites may be particularly suitable, as these particles are formed from materials that were formed o...

The photoelectron spectrum of hydrogen selenide has been recorded using synchrotron radiation in the photon energy range 15–110 eV and the inner valence region has been studied in detail for the first time. Green’s function methods have been employed to evaluate the ionisation energies and spectral intensities of all valence states and the results...

Theoretical and experimental studies have revealed that the properties of graphene can be changed substantially by hydrogenation. Theoretical calculations show that fully hydrogenated graphene, referred to as graphane, is an insulator [1] and that hydrogen line structures can induce graphene nanoribbon – like band gaps in graphene [2]. Experimental...