Absorption lines of H 3 + have been detected in the spectra of two infrared sources in the Galactic center and also toward the heavily reddened star Cygnus OB2 No. 12, whose line of sight is believed to include only diffuse interstellar gas. The absorptions toward the Galactic center sources (IRS 3 and GCS 3-2) probably are due to H 3 + both in diffuse gas and in molecular clouds. The ratios of H 3 + line equivalent width to extinction toward these three sources are much greater than those toward dense clouds where H 3 + has been detected previously. Analysis of the spectra coupled with a simple model for the abundance of H 3 + in the diffuse interstellar medium implies that the observed H 3 + is present at low densities along long path lengths. These are the first detections of H 3 + in the diffuse interstellar medium.
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Geballe et al. (1999) studied this question.
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