2017ApJ...850..194F

We study the behavior of eight diffuse interstellar bands (DIBs) in different interstellar environments, as characterized by the fraction of hydrogen in molecular form (f_H2), with comparisons to the corresponding behavior of various known atomic and molecular species. The equivalent widths of the five "normal" DIBs (λλ5780.5, 5797.1, 6196.0, 6283.8, and 6613.6), normalized to E_B–V, show a "lambda-shaped" behavior: they increase at low f_H2, peak at f_H2 ∼ 0.3, and then decrease. The similarly normalized column densities of Ca, Ca⁺, Ti⁺, and CH⁺ also decline for f_H2 > 0.3. In contrast, the normalized column densities of Na, K, CH, CN, and CO increase monotonically with f_H2, and the trends exhibited by the three C₂ DIBs (λλ4726.8, 4963.9, and 4984.8) lie between those two general behaviors. These trends with f_H2 are accompanied by cosmic scatter, the dispersion at any given f_H2 being significantly larger than the individual errors of measurement. The lambda-shaped trends suggest the balance between creation and destruction of the DIB carriers differs dramatically between diffuse atomic and diffuse molecular clouds; additional processes aside from ionization and shielding are needed to explain those observed trends. Except for several special cases, the highest W_λ(5780)/W_λ(5797) ratios, characterizing the so-called "sigma-zeta effect," occur only at f_H2 < 0.2. We propose a sequence of DIBs based on trends in their pair-wise strength ratios with increasing f_H2. In order of increasing environmental density, we find the λ6283.8 and λ5780.5 DIBs, the λ6196.0 DIB, the λ6613.6 DIB, the λ5797.1 DIB, and the C₂ DIBs.