Abstract
It has been known for some time that the properties of a material are changed in nanometer size. Since it is nontrivial that polycyclic aromatic hydrocarbon (PAH) grains at the nanometer scale show similar properties with that of macroscopic scale, PAH grains were synthesized by a gas evaporation method and were analyzed using a transmission electron microscope and their UV-Vis and mid-IR spectra were measured. Results from these experiments showed that condensed anthracene grains from the gas phase had a planar form and showed similar infrared peaks compared to anthracene molecules, but with somewhat wider bands and different relative intensities. Some new features were also observed. The anthracene molecules formed photodimer in their grain and showed different infrared features after UV irradiation. This would be the first report concerning nanometersized PAH grains.
Highlights
After the discovery of unidentified infrared (UIR) bands (Gillett et al, 1973; Russell et al, 1977a; Willner et al, 1977), which are characterized by the main features at 3.3, 6.2, 7.7, 8.6 and 11.2 μms, many minor bands and the underlying broad plateaus, many candidate materials of the origin have been proposed, such as hydrogenated amorphous carbon (Duley and Williams, 1981), quenched carbonaceous composite (Sakata et al, 1984) and polycyclic aromatic hydrocarbons (PAHs) (Leger and Puget, 1984; Allamandola et al, 1985)
Since the UIR bands are observed in a wide variety of objects in various stages of stellar evolution, such as Herbig Ae/Be stars, HII regions, reflection nebulae, diffuse interstellar medium, post-asymptotic giant branch (AGB) stars and planetary nebulae (e.g. Russell et al, 1977b; Roche and Aitken, 1985; Cohen et al, 1986; Onaka et al, 1996), it appears that the carriers of the UIR bands should be stable materials
Since the 12-μm band has never appeared in the spectra of molecules and commercial bulk powder, nor in that of anthracene grains excluding film, we considered that the 12-μm band was caused by the anthracene film
Summary
After the discovery of unidentified infrared (UIR) bands (Gillett et al, 1973; Russell et al, 1977a; Willner et al, 1977), which are characterized by the main features at 3.3, 6.2, 7.7, 8.6 and 11.2 μms, many minor bands and the underlying broad plateaus, many candidate materials of the origin have been proposed, such as hydrogenated amorphous carbon (Duley and Williams, 1981), quenched carbonaceous composite (Sakata et al, 1984) and polycyclic aromatic hydrocarbons (PAHs) (Leger and Puget, 1984; Allamandola et al, 1985). We performed the formation and growth of PAH grains at the nanometer scale, which are part of the life cycle of PAHs, and performed alteration by UV irradiation, using mainly anthracene in the laboratory. This is the first report on IR spectra for nanometersized PAH grains. Anthracene grains were produced by the gas evaporation method, which is known as the smoke experiment and has been used to produce nanometer-sized metal, oxide or semiconductor particles (Kimoto et al, 1963; Kaito, 1978). The evaporated vapor subsequently cooled and became condensed in the gas atmosphere
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