Cyclic C₄ ⁺ as the Carrier of the Diffuse Interstellar Band at 503.9 nm?

Abstract The diffuse interstellar bands (DIBs) have remained a mystery in astronomy since their discovery over a century ago. The only currently known carrier is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi mathvariant="normal">C</mml:mi> <mml:mn>60</mml:mn> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> , responsible for five DIBs, while more than 550 are yet to be interpreted. The spectra of short carbon chain cations <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> <mml:mi>n</mml:mi> <mml:mo>+</mml:mo> </mml:msubsup> <mml:mo stretchy="false">)</mml:mo> </mml:math> , which are considered one of the most promising classes of species for the role of carriers of DIBs, are successfully recorded using He-tagging spectroscopy. The comparison of laboratory spectra with observations demonstrates a close match of two absorption bands of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi mathvariant="normal">C</mml:mi> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> with the broad DIB at 503.9 nm. This defines a high abundance of these ions in the interstellar medium (ISM), which should exceed those of other similar-sized carbon chain cations. It is anticipated that all other short carbon chain cations will exhibit linear geometry and, as a consequence, will have a long vibrational progression. However, the distinctive cyclic geometry of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi mathvariant="normal">C</mml:mi> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> is postulated to underpin the elevated abundance of these ions in the ISM, as well as the distinctive spectrum of this ion, which displays a single, strong, relatively narrow absorption band that exceeds in intensity all other absorption bands in the visible range.