Photodarkening and photobleaching in a-C:H films

Iida, Shinpei; Ohtaki, T.; Seki, Tomofumi

doi:10.1063/1.34750

Cited by 14 publications

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“…This structural difference can lower the destruction cross section as discussed in Paper I. Moreover, UV irradiation measurements of hydrogenated amorphous carbon films (a-C:H) have been performed by Iida et al (1984). They observed a film photodarkening (i.e.…”

Section: Experiments and Resultsmentioning

confidence: 99%

“…Recently, Mennella et al (1999) have shown that hydrogenation by H atoms of hydrogenfree nano-sized carbon grains takes place under simulated diffuse interstellar medium conditions. On the other hand, it is known that UV irradiation decreases the intensity of the CH stretching feature in hydrogenated carbon materials (Iida et al 1984). This trend is confirmed by the systematic study on several hydrocarbon molecules reported in Muñoz Caro et al (2001), hearinafter Paper I.…”

Section: Introductionmentioning

confidence: 99%

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UV photodestruction of CH bonds and the evolution of the 3.4 $\mathsf{\mu}$m feature carrier

Mennella

Muñoz

Ruiterkamp

et al. 2001

A&A

101

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Abstract. We present the results of a laboratory program aimed at studying the effects induced by energetic UV photons on hydrogenated carbon particles. Experiments have been performed under simulated diffuse and dense interstellar medium conditions. To monitor the effects of UV irradiation on grains IR spectroscopy has been used. In both circumstances UV photons lead to a reduction of the aliphatic 3.4 µm band. An estimation of the destruction cross section by UV photons for the hydrogenated carbon particles has been derived from the reduction of the 3.4 µm intensity band as a function of the UV fluence. The results of the present work, together with previous laboratory data, can shed light on the enigmatic difference observed for the 3.4 µm band between dense and diffuse interstellar medium clouds. This difference is compatible with the transformation of hydrogenated carbon particles produced by UV photons and hydrogen atoms and with the changes of the grain properties in the two environments.

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Section: Experiments and Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

UV photodestruction of CH bonds and the evolution of the 3.4 $\mathsf{\mu}$m feature carrier

Mennella

Muñoz

Ruiterkamp

et al. 2001

A&A

101

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“…Laboratory amorphous hydrocarbon solids (a-C:H) are known to darken upon exposure to ultraviolet (UV) light and in response to thermal annealing (e.g., Iida et al 1985;Smith 1984). This photo-darkening of a-C:H materials leads to a decrease in the band gap or optical gap energy, E g .…”

Section: Introductionmentioning

confidence: 99%

Variations on a theme – the evolution of hydrocarbon solids

Jones

2012

A&A

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Context. The properties of hydrogenated amorphous carbon (a-C:H) dust are known to evolve in response to the local conditions. Aims. We present an adaptable model for the determination of the optical properties of low-temperature, interstellar a-C:H grains that is based on the fundamental physics of their composition. Methods. The imaginary part of the refractive index, k, for a-C:H materials, from 50 eV to cm wavelengths, is derived and the real part, n, of the refractive index is then calculated using the Kramers-Kronig relations. Results. The formulated optEC (s) model allows a determination of the complex dielectric function, , and refractive index, m(n, k), for a-C:H materials as a continuous function the band gap, E g , which is shown to lie in the range −0.1 to 2.7 eV. We provide expressions that enable a determination of their optical constants and tabulate m(n, k, E g ) for 14 different values of E g . We explore the evolution of the likely extinction and emission behaviours of a-C:H grains and estimate the relevant transformation time-scales. Conclusions. With the optEC (s) model we are able to predict how the optical properties of an a-C:H dust component in the interstellar medium will evolve in response to, principally, the local interstellar radiation field. The evolution of a-C:H materials appears to be consistent with many dust extinction, absorption, scattering and emission properties, and also with H 2 molecule, daughter "PAH" and hydrocarbon molecule formation resulting from its photo-driven decomposition.

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“…Hydrocarbon solids darken with ultraviolet (UV) irradiation and thermal annealing (e.g., Iida et al 1985;Smith 1984) due to the evolution of the band gap or optical gap energy, E g , of the material in response to the local conditions. The result is a loss of hydrogen atoms from the structure and an evolution towards more aromatic materials.…”

Section: Introductionmentioning

confidence: 99%

Variations on a theme – the evolution of hydrocarbon solids

Jones

2012

A&A

102

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Context. The properties of hydrogenated amorphous carbon (a-C:H) dust evolve in response to the local radiation field in the interstellar medium (ISM) and the evolution of these properties is particularly dependent upon the particle size. Aims. A model for finite-sized, low-temperature amorphous hydrocarbon particles, based on the microphysical properties of random and defected networks of carbon and hydrogen atoms, with surfaces passivated by hydrogen atoms, has been developed. Methods. The eRCN/DG and the optEC (s) models have been combined, adapted and extended into a new optEC (s) (a) model that is used to calculate the optical properties of hydrocarbon grain materials down into the sub-nanometre size regime, where the particles contain only a few tens of carbon atoms. Results. The optEC (s) (a) model predicts a continuity in properties from large to small (sub-nm) carbonaceous grains. Tabulated data of the size-dependent optical constants (from EUV to cm wavelengths) for a-C:H (nano-)particles as a function of the bulk material band gap [E g (bulk)], or equivalently the hydrogen content, are provided. The effective band gap [E g (eff.)] is found to be significantly larger than E g (bulk) for hydrogen-poor a-C(:H) nano-particles and their predicted long-wavelength (λ > 30 μm) optical properties differ from those derived for interstellar polycyclic aromatic hydrocarbons (PAHs). Conclusions. The optEC (s) (a) model is used to investigate the size-dependent structural and spectral evolution of a-C(:H) materials under ISM conditions, including: the IR-FUV extinction, the 217 nm bump and the infrared emission bands. The model makes several predictions that can be tested against observations.

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Photodarkening and photobleaching in a-C:H films

Cited by 14 publications

References 0 publications

UV photodestruction of CH bonds and the evolution of the 3.4 $\mathsf{\mu}$m feature carrier

UV photodestruction of CH bonds and the evolution of the 3.4 $\mathsf{\mu}$m feature carrier

Variations on a theme – the evolution of hydrocarbon solids

Variations on a theme – the evolution of hydrocarbon solids

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