Vibrational Feshbach resonances in electron attachment to carbon dioxide clusters
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Using a high resolution (\(\) meV) laser photoelectron attachment method, we have studied the formation of (CO 2) q − ions (q = 4−22) in collisions of low energy electrons (1−180 meV) with (CO2) N (\(\)) clusters. The previously reported “zero energy resonance”, observed at much larger electron bandwidths, actually consists of several narrow vibrational Feshbach resonances of the type [(CO 2) N −1CO \(\) which involve a vibrationally-excited molecular constituent (\(\) denotes vibrational mode) and a diffuse electron weakly bound to the cluster by long range forces. The resonances occur at energies below those of the vibrational excitation energies of the neutral clusters [(CO 2) N −1CO \(\)]; the redshift rises with increasing cluster ion size q by about 12 meV per unit; these findings are recovered by a simple model calculation for the size dependent binding energies. The size distribution in the cluster anion mass spectrum, resulting from attachment of very slow electrons, mainly reflects the amount of overlap of solvation-shifted vibrational resonances with zero energy; the cluster anion size q is identical with or close to that of the attaching neutral cluster.
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