John D. Thrower

The Catalytic Role of Coronene for Molecular Hydrogen Formation

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 A2 is estimated for the deuteration process of coronene. HD and D2 molecules form, through abstraction reactions, on deuterated coronene sites with a cross section of 0.06 A2. The magnitude of both cross sections is in line with an Eley-Rideal type process. The results show that hydrogenated neutral polycyclic aromatic hydrocarbon molecules act as catalysts for the formation of molecular hydrogen.

EXPERIMENTAL EVIDENCE FOR THE FORMATION OF HIGHLY SUPERHYDROGENATED POLYCYCLIC AROMATIC HYDROCARBONS THROUGH H ATOM ADDITION AND THEIR CATALYTIC ROLE IN H2 FORMATION

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. Complementary density functional theory calculations confirm the stability of the observed superhydrogenated species toward spontaneous H and H2 loss indicating that abstraction reactions may be the dominant route to H2 formation involving neutral polycyclic aromatic hydrocarbons (PAHs). The results indicate that highly superhydrogenated PAHs could well be formed and could act as efficient catalysts for H2 formation in the interstellar medium in low UV flux regions.

Polycyclic aromatic hydrocarbons – catalysts for molecular hydrogen formation

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 simulations to follow the addition of deuterium atoms to the PAH molecule coronene. When exposed to sufficiently large D atom fluences, coronene is observed to be driven towards the completely deuterated state (C24D36) with the mass distribution peaking at 358 amu, just below the peak mass of 360 amu. Kinetic models reproduce the experimental observations for an abstraction cross-section of sigma(abs) = 0.01 angstroms2 per excess H/D atom, and addition cross-sections in the range of sigma(add) = 0.55-2.0 angstroms2 for all degrees of hydrogenation. These findings indicate that the cross-section for addition does not scale with the number of sites available for addition on the molecule, but rather has a fairly constant value over a large interval of super-hydrogenation levels.

The influence of coronene super-hydrogenation on the coronene-graphite interaction

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 with no additional hydrogenation to 1.43 eV for the fully super-hydrogenated molecule. Density functional theory calculations using the optB88-vdW functional suggest that the decrease in binding energy is mostly due to an increased buckling of the molecule rather than the associated decrease in the number of π-electrons.