Interstellar absorption diffuse bands

Interstellar absorption diffuse bands are spectral bands due to the absorption of light emitted by stars from the interstellar medium. The word diffuse in their name reflects the blurred nature of these bands, which indicates the molecular structure of the absorbing substance. To date, more than 500 diffuse bands have been discovered in the infrared and visible spectral ranges of various stars, but the origin of most of these bands is still not known. ^{[1] [2]} Under terrestrial conditions, the registration of diffuse bands is significantly complicated by their screening by the earth's atmosphere. ^[3] Therefore, the spectra obtained outside the Earth, for example, by the Hubble Space Telescope , are considered the most reliable. ^[4] However, even these spectra need to be corrected due to the overlapping of the diffuse bands with the emission bands of the corresponding star.

Despite active attempts to determine the molecules that form diffuse bands in the astronomical spectra, to date, only one such compound is reliably known - the Buckminsterfullerene cation, C ₆₀ ⁺ . ^[5] Due to the high instability of the cation, for a long time it was not possible to obtain a reliable absorption spectrum of C ₆₀ ⁺ in the infrared range. Only in 2015, the Meyer group for the first time managed to obtain the C ₆₀ ⁺ spectrum at a temperature of less than 10 K, which is comparable to the temperature in interstellar space. The extremely exact coincidence of two previously known diffuse bands at 9632 Å and 9577 Å and two spectral lines of C ₆₀ ⁺ allowed us to reliably confirm the presence of C ₆₀ ⁺ in interstellar space. ^[6] Later, three more bands in the C ₆₀ ⁺ spectrum were detected among diffuse bands with a wavelength of 9428 Å, 9365 Å and 9348 Å. ^[7] In 2018, the methods of quantum chemistry determined the cause of the appearance of such closely spaced absorption bands. Due to the Jahn-Teller effect in C ₆₀ ⁺ ( D _{5 d} ), the excited state of symmetry E _{1 g} decays into electronic levels A _g and B _g , which forms two progression bands in the absorption spectrum. Thus, two bright diffuse bands at 9632 Å and 9577 Å were assigned to “cold” electronic transitions to excited symmetry levels B _g and A _g , while weak diffuse bands at 9428 Å, 9365 Å and 9348 Å to “hot” vibronic transitions . ^[8]