Observation of Inter-rydberg Transitions in H2 Coinciding with Diffuse Interstellar Bands

In a laboratory study inter-Rydberg transitions originating from the lowest rotational levels of the C P v 5 5 and v 5 6 states of molecular hydrogen are found to coincide with frequency positions and widths of diffuse interstellar bands. It is the first time that diffuse interstellar bands are assigned to transitions measured in gas-phase molecules that are known to exist in interstellar space. Subject headings: ISM: molecules—molecular processes—methods: laboratory The problem of the diffuse interstellar bands (DIBs) has challenged spectroscopists and astrophysicists ever since the first accounts of their observation in the 1930s. By now some 200 unidentified absorption features of various strengths and line shapes throughout the entire visible wavelength range are observed in the line of sight of stars of different types (Jenniskens & Désert 1994; Herbig 1995). Despite extensive laboratory studies, the carriers of the DIBs still have to be identified. Even the debate whether to assign the DIBs to gas-phase molecules or to solid particles has not yet been fully resolved. In recent years the gas-phase hypothesis has become favored, and classes of polycyclic aromatic hydrocarbon (PAH) molecules (Salama et al. 1996) and unsaturated hydrocarbon chain molecules (Fulara et al. 1993) were proposed as DIB carriers. Furthermore, two DIB features at wavelengths in the near-infrared were assigned to the C60 1 ion (Foing & Ehrenfreund 1994). The assignment of the proposed carriers is based on a comparison of DIB positions with laboratory spectroscopic information derived from matrix studies. The evidence is ambiguous because in matrices spectral lines are severely broadened and shifted by unknown amounts from their true gas-phase position. A coincidence between a strong DIB feature and a gas-phase molecule was observed for the chlorin molecule (C20H10N4) (Miles & Sarre 1992), but it is unlikely for this porphyrin molecule to exist in large enough abundance in interstellar space to cause the DIB. Sorokin & Glownia (1995) proposed a model in which inter-Rydberg transitions in gas-phase molecular hydrogen cause the DIBs. The H2 hypothesis is appealing because H2 is known to be omnipresent in interstellar space, in abundances of some 10 orders of magnitude higher than the abovementioned species. The difficulty with the H2 hypothesis is to devise a pumping scheme that can explain the population of selected rovibronic levels of electronically excited states of hydrogen involved in DIB absorptions. Sorokin and Glownia proposed such schemes, initially based on resonance-enhanced two-photon excitation (Sorokin & Glownia 1995), and later on near-resonance stimulated Raman pumping induced by stellar vacuum-ultraviolet (VUV) radiation (Sorokin & Glownia 1996). Their work was criticized by Snow (1995). Models for the DIBs relying on gas-phase molecules as carriers have two key requirements. One is to provide an explanation for the molecular abundances along the lines of sight of observation, involving a chemical, as well as a radiative transfer model. The second requirement is to establish firm spectroscopic evidence by showing that the DIB features coincide with transitions in gas-phase molecules. Inspired by the work of Sorokin and Glownia, we have employed a XUV-visible double-resonance laser setup to investigate spectroscopic aspects of the H2 hypothesis. Initially, we focused on recording transitions via the C P v 5 9 intermediate state in H2, first proposed as a DIB-carrying state. Although some coincidences with DIBs were found, the observed spectral features in excitation from C P v 5 9 J states do not match the DIBs in specific ranges 766–788 nm (Hinnen & Ubachs 1995) and 700–740 nm (Hinnen & Ubachs 1996). Particularly, the observation of many strong resonances in H2 at wavelengths where no DIBs are documented could be considered as evidence against the H2 hypothesis. However, the observed spectral patterns and regularities in the spectra resembled DIB features, and this encouraged us to proceed in our search for coincidences between DIBs and strong inter-Rydberg transitions in molecular hydrogen. Particularly, the well-understood concept that transitions to autoionizing and predissociating states in H2 can account for a wide variety of line shapes, including asymmetries, is stimulating. During the course of laboratory investigations we found that in excitation from C P rovibronic states in the energy range near the ionization potential of H2, only a single strong resonance exists. In Figures 1 and 2 spectral recordings from C P v 5 5 and v 5 6 intermediate states into this energy range are displayed. Here only a limited energy range is shown, but it is noted that no other strong features are observed in a range spanning 1000 cm. At the temperatures prevailing in interstellar clouds, only J 5 0 (para-hydrogen) and J 5 1 (ortho-hydrogen) ground states are populated. Via R(0), R(1), and Q(1) transitions, respectively, C P J 5 1(2), 2(1), and 1(1) intermediate states are excited. These states for vibrational quantum numbers v 5 5 and v 5 6 were used as intermediates and further excited in a XUVvisible double-resonance experiment as described previously (Hinnen & Ubachs 1995). In fact, only two different final states are probed by the six transitions listed in Table 1. For para-hydrogen, probed via J 5 1(2), a resonance at 124701.9 cm is excited, via either C P v 5 5 or v 5 6. The spectroscopy of the C P state was performed in a high-resolution experiment, resulting in energy levels determined with an accuracy of 0.03 cm (Reinhold, Hogervorst, & Ubachs 1996). THE ASTROPHYSICAL JOURNAL, 476 :L93–L96, 1997 February 20 q 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.