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I am studying electron collision with molecules, and came across with notation like this $\left(\mathrm{B}^{1} \Sigma_{u}^{+}, \mathrm{c}^{3} \Pi_{u}, \mathrm{a}^{3} \Sigma_{g}^{+}, \mathrm{C}^{1} \Pi_{u}\right)$ for the $H_2$ molecules and I have no idea what they are. Is there any textbook that explains what this symbols are?

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    $\begingroup$ The question has been answered on SE (the first two links), but also please refer to the Resources post (also linked): the books by Atkins and Levine will probably have more information. Consider also Hollas's Modern Spectroscopy, which is referenced in my answer to the second linked question. In a comment, porphyrin also suggests Herzberg's Molecular Spectra and Molecular Structure series. $\endgroup$ – orthocresol Nov 29 '20 at 21:00