Unimolecular processes in diatomic carbon anions at high rotational excitation

On the millisecond to second time scale, stored beams of diatomic carbon anions <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi mathvariant="normal">C</a:mi><a:mn>2</a:mn></a:msub><a:msup><a:mrow/><a:mo>−</a:mo></a:msup></a:mrow></a:math> from a sputter ion source feature unimolecular decay of yet unexplained origin by electron emission and fragmentation. To account for the magnitude and time dependence of the experimental rates, levels with high rotational and vibrational excitation are modeled for the lowest electronic states of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:msub><c:mi mathvariant="normal">C</c:mi><c:mn>2</c:mn></c:msub><c:msup><c:mrow/><c:mo>−</c:mo></c:msup></c:mrow></c:math>, also including the lowest quartet potential. Energies, spontaneous radiative decay rates (including spin-forbidden quartet-level decay), and tunneling dissociation rates are determined for a large number of highly excited <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msub><e:mi mathvariant="normal">C</e:mi><e:mn>2</e:mn></e:msub><e:msup><e:mrow/><e:mo>−</e:mo></e:msup></e:mrow></e:math> levels and their population in sputter-type ion sources is considered. For the quartet levels, the stability against autodetachment is addressed and recently calculated rates of rotationally assisted autodetachment are applied. Nonadiabatic vibrational autodetachment rates of high vibrational levels in the doublet <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:msub><g:mi mathvariant="normal">C</g:mi><g:mn>2</g:mn></g:msub><g:msup><g:mrow/><g:mo>−</g:mo></g:msup></g:mrow></g:math> ground potential are also calculated. The results are combined to model the experimental unimolecular decay signals. Comparison of the modeled to the experimental rates measured at the cryogenic storage ring facility CSR gives strong evidence that <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:msub><i:mi mathvariant="normal">C</i:mi><i:mn>2</i:mn></i:msub><i:msup><i:mrow/><i:mo>−</i:mo></i:msup></i:mrow></i:math> ions in quasistable levels of the quartet electronic states are the so far unidentified source of unimolecular decay. Published by the American Physical Society 2024