Nondissociative and dissociative ionization of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mi>CO</mml:mi> </mml:mrow> <mml:mo>+</mml:mo> </mml:msup> </mml:math> in intense ultrashort laser pulses

We have investigated the dissociative and nondissociative ionization of a ${\mathrm{CO}}^{+}$ beam, which lead to ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ and ${\mathrm{CO}}^{2+}$ final products, respectively, in intense ultrashort laser pulses. This study demonstrates an additional capability of our coincidence momentum imaging technique. We show that the relative yield of the metastable dications increases when the laser pulse duration is reduced from 30 to 10 fs, most likely by suppressing the enhanced-ionization mechanism that requires stretching of the parent ${\mathrm{CO}}^{+}$ before the ionization step. This process does not populate the long-lived states of ${\mathrm{CO}}^{2+}$, but direct (vertical) ionization does. In contrast, dissociative ionization, which leads to the ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ final products, is driven by both direct- and enhanced-ionization mechanisms and therefore it dominates, especially for longer pulses. Curiously, the ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ kinetic energy release spectrum exhibits peaks separated by approximately the photon energy, suggesting above-threshold dissociation as a possible driving mechanism. We rule out this possibility by showing that the peak spacing does not change as expected when doubling the photon energy, specifically using 786- and 393-nm pulses at high intensity.