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I am studying gas-phase Uv-Vis spectra of radical molecules. If one electron is removed/captured from the neutral even electron molecule, what can I expect from the absorption spectra? Will new peaks appear? Will they be blue/red-shifted?

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    $\begingroup$ A cheeky answer is 'yes and yes '. Why not look in the literature for example spectra, there are bound to be many examples. $\endgroup$
    – porphyrin
    Commented Oct 18, 2019 at 17:31

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I have not found a general answer to how the ground state absorption would shift. Here are two publications 1, 2 that observe shifts, but both times a redshift, be it anion or cation. So as of now I dont know how to explain that.

But, I can certainly say that new peaks will apear and they will most likely be in the red part of the spectrum. In the image below you see that the HOMO/LUMO gap is the biggest. Assume now an radical anion, i.e. an additional electron which occupies the LUMO, then transitions to LUMO+i (i=1,2,3,...) are possible which likely have lower energies than the HOMO/LUMO gap. Image taken from here.

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