Photoluminescence of hydrogenated amorphous carbons Wavelength-dependent yield and implications for the extended red emission

Context. Hydrogenated amorphous carbons (a-C: H or HAC) have proved to be excellent analogs of interstellar dust observed in galaxies diffuse interstellar medium (DISM) through infrared vibrational absorption bands (3.4 mu m, 6.8 mu m, and 7.2 mu m bands). They exhibit photoluminescence (PL) after excitation by UV-visible photons, and are possible carriers for the extended red emission (ERE), a broad red emission band observed in various interstellar environments. Aims. As many candidate materials/molecules can photoluminesce in the visible, along with the carrier abundance, the PL efficiency represents one of the strongest constraints set by such ERE observations. We wish to precisely characterize the PL behavior of a-C: H as a family of materials. Methods. The a-C: H samples are produced in the form of films deposited on substrates by plasma-enhanced chemical vapor deposition. The produced films were analyzed in transmission by UV-visible and IR spectroscopy, and the wavelength dependent PL spectra were recorded. The intrinsic absolute quantum yield eta was then rigorously calculated taking self-absorption of the PL by the film and interfaces effects into account. Results. A wide range of different laboratory synthesized a-C: H were analyzed. Their PL properties are dependent on the optical gap E(04): when E(04) decreases from 4.3 eV to 2.8 eV, the a-C: H vary from highly (eta similar to 1%) yellow photoluminescent soft materials to hard materials that emit a wider PL band in the red spectral range, with a lower efficiency (eta similar to 0.01-0.1%). For any given a-C: H, the PL characteristics (central wavelength, band width and efficiency) are found to be essentially constant over the explored excitation range (lambda(exc) greater than or similar to 250 nm). We compared the characteristics of the produced interstellar dust analog to the constraints imposed by the ERE observations. Conclusions. As for ERE observations, PL efficiencies and band widths of a-C: H are both correlated to the PL central wavelengths. The excitation responsible for the a-C: H emission is efficient over a wide spectral range that matches the ERE excitation. The present a-C: H encounter difficulties for the diffuse ISM ERE observations (eta >= 10%) in simultaneously satisfying the high quantum yield criteria and PL spectral characteristics. We still need to investigate the role of a small number of residual oxygen atoms in the laboratory-produced a-C: H network in quenching the PL yield, as well as to consider the interstellar temperature effect for our analogs.