Diffuse interstellar clouds have long been thought to be relatively devoid of molecules, because of their low densities and high radiation fields. However, in the past ten years or so, a plethora of polyatomic molecules have been observed in diffuse clouds, via their rotational, vibrational, and electronic transitions. In this review, we propose a new systematic classification method for the different types of interstellar clouds: diffuse atomic, diffuse molecular, translucent, and dense. We review the observations of molecules (both diatomic and polyatomic) in diffuse clouds and discuss how molecules can be utilized as indicators of the physical and chemical conditions within these clouds. We review the progress made in the modeling of the chemistry in these clouds, and the (significant) challenges that remain in this endeavor. We also review the evidence for the existence of very large molecules in diffuse clouds, and discuss a few specific clouds of particular interest.

Echelle spectra of the double-lined spectroscopic binary HD 204827 were obtained on five nights, at a resolving power R ¼ 38;000 and with a S/N ¼ 750 near 6000 8 in the final, combined spectrum. The stars show E(B À V ) ¼ 1:11 and spectral types near O9.5 V and B0.5 III. A catalog is presented of 380 diffuse interstellar bands (DIBs) measured between 3900 and 8100 8 in the stars' spectrum. The central wavelengths, the widths (FWHM ), and the equivalent widths of nearly all of the bands are tabulated, along with the minimum uncertainties in the latter. The reliable removal of very weak stellar lines from the catalog, and of some stellar lines from the less severe blends with DIBs, is made generally easy by the highly variable radial velocities of both stars. The principal result of this investigation is that the great majority of the bands in the catalog are very weak and relatively narrow. Typical equivalent widths amount to a few m8, and the bandwidths (FWHM ) are most often near 0.55 8. Therefore, most of these DIBs can be detected only in spectra obtained at a resolving power and a S/ N at least comparable to those used here. In addition, the anomalous interstellar reddening and the very high value of the ratio N(C 2 )/E(B À V ) seen toward HD 204827 indicate that the physical conditions in one or more of the several interstellar clouds seen in this direction differ significantly from those found toward the prototypical DIB target HD 183143, for example. Probably primarily for these reasons, 113 of the 380 bands (30%) were not detected in four previous modern surveys of the DIBs seen in the spectra of stars other than HD 204827. No preferred wavenumber spacings among the 380 bands are reliably identified which could provide clues to the identities of the large molecules thought to cause the DIBs.

Observations of H + 3 in the Galactic diffuse interstellar medium (ISM) have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (ζ 2 ) is about 1 order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H + 3 observations to 50, with detections in 21 of those. Ionization rates inferred from these observations are in the range (Upper limits (3σ) derived from non-detections of H + 3 are as low as ζ 2 < 0.4 × 10 −16 s −1 . These low upperlimits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in ζ 2 between different diffuse cloud sight lines. A study of ζ 2 versus N H (total hydrogen column density) shows that the two parameters are not correlated for diffuse molecular cloud sight lines, but that the ionization rate decreases when N H increases to values typical of dense molecular clouds. Both the difference in ionization rates between diffuse and dense clouds and the variation of ζ 2 among diffuse cloud sight lines are likely the result of particle propagation effects. The lower ionization rate in dense clouds is due to the inability of low-energy (few MeV) protons to penetrate such regions, while the ionization rate in diffuse clouds is controlled by the proximity of the observed cloud to a site of particle acceleration.

We establish correlations between equivalent widths of eight diffuse interstellar bands (DIBs), and examine their correlations with atomic hydrogen, molecular hydrogen, and E B−V . The DIBs are centered at λλ 5780. 5, 6204.5, 6283.8, 6196.0, 6613.6, 5705.1, 5797.1, and 5487.7, in decreasing order of Pearson's correlation coefficient with N(H) (here defined as the column density of neutral hydrogen), ranging from 0.96 to 0.82. We find the equivalent width of λ5780.5 is better correlated with column densities of H than with E B−V or H 2 , confirming earlier results based on smaller datasets. We show the same is true for six of the seven other DIBs presented here. Despite this similarity, the eight strong DIBs chosen are not well enough correlated with each other to suggest they come from the same carrier. We further conclude that these eight DIBs are more likely to be associated with H than with H 2 , and hence are not preferentially located in the densest, most UV shielded parts of interstellar clouds. We suggest they arise from different molecules found in diffuse H regions with very little H 2 (molecular