A TPD and RAIRS comparison of the low temperature surface behavior of benzene, toluene, and xylene on graphite

Abstract

The first comparative study of the surface behaviour of four small aromatic molecules, benzene, toluene, p-xylene and o-xylene, adsorbed on graphite at temperatures ≤ 30 K, is presented. Intermolecular interactions are shown to be important in determining the growth of the molecules on the graphite surface at low (monolayer) exposures. Repulsive intermolecular interactions dominate the behaviour of benzene and toluene. In contrast, stronger interactions with the graphite surface are observed for the xylene isomers, with islanding observed for o-xylene. Multilayer desorption temperatures and energies increase with the size of the molecule, ranging from 45.5 to 59.5 kJ mol-1 for benzene and p-xylene respectively. Reflection absorption infrared spectroscopy gives insight into the effects of thermal processing on the ordering of the molecules. Multilayer benzene, p-xylene and o-xylene form crystalline structures following annealing of the ice. However, we do not observe an ordered structure for toluene in this study. The ordering of p-xylene shows a complex relationship dependent on both the annealing temperature and exposure.

A new technique for determining the refractive index of ices at cryogenic temperatures

“…deposited on HOPG at 27 K. At this deposition temperature, benzene is in its amorphous form. 44,45 The bands in the amorphous C 6 H 6 spectrum at 3090 cm -1 and 3032 cm -1 ( Figure 4A) and those at 1479 cm -1 and 1036 cm -1 ( Figure 4B) have been observed previously. 42 They are assigned to the C-H stretch (ν(CH) aromatic ), ν(CH) combination mode, aromatic C-C stretch (ν(CC) aromatic ) and C-H in plane deformation (δ(CH) in-plane ) mode of C 6 H 6 respectively 68,69,79 .…”

“…These are assigned to ν(CC) aromatic and δ(CH) modes respectively. The appearance of additional bands compared to previous work is explained by the fact that the ices examined by Salter et al 44 are estimated to be approximately 10 nm thick , whereas those in this work are of the order of hundreds of nm thick. 26 C 6 H 6 ice is known to undergo a thermally induced phase change.…”

“…Initially, RAIRS was used to characterise the ices grown on HOPG. Infrared spectra of C 6 H 6 42,44,45,48,52,68,69 , MF [70][71][72][73][74][75] and H 2 O 42,43,56,[76][77][78] ices have all been recorded previously and are used as a benchmark for the spectra presented here. Figure 4 shows several RAIR spectra of C 6 H 6…”

“…C 6 H 6 has well-characterised surface behaviour relevant to astrochemistry [42][43][44] and has well defined UV/visible absorption behaviour that can be used as a benchmark. [45][46][47] In previous work, 44 multilayers of C 6 H 6 deposited on HOPG at 25 K have been shown to undergo a phase change after annealing to 90 K. This phase change was shown by changes in the reflectionabsorption infrared (RAIR) spectrum of C 6 H 6 upon annealing. Other authors have also observed this thermally induced phase change using vibrational spectroscopy methods.…”

Influence of Dispersion Interactions on the Thermal Desorption of Nonplanar Polycyclic Aromatic Hydrocarbons on HOPG

“…As already stated, lateral interactions between flat PAHs on HOPG are negligibly small. By contrast, lateral interactions between alkyl side chains of functionalized PAHs can be strong enough to dominate self‐organization of the corresponding films . Rubrene–rubrene dispersion interactions contributed to mainly by the phenyl functional groups on their respective tetracene backbones fall somewhere in between.…”