Vibrationally cold ${\mathrm{CO}}^{2+}$ in intense ultrashort laser pulses

By virtue of the short lifetime of excited states, we have performed three-dimensional (3D) momentum imaging on the fragments from an electronically and vibrationally cold metastable ${\mathrm{CO}}^{2+}$ beam following irradiation by intense ultrashort laser pulses. This unique target can be described as a two-channel system, since most low-lying electronic states are not accessible by dipole transitions due to their spin state. Laser excitation between the ground $X^3\Pi$ $v=0$ state and the excited ${}^3{\Sigma }^{-}$ state leads to bond softening and above-threshold dissociation, with peaks in kinetic energy release spaced by the photon energy and interesting angular distributions that peak perpendicular to the laser field. These results are compared with our solutions of the 3D time-dependent Schrödinger equation.

Figure 1

(a) Born-Oppenheimer potentials for ${\mathrm{CO}}^{2+}$ calculated by Šedivcová et al. [12]. (b) The diabatic Floquet potentials for the two lowest triplet states. Each curve carries a photon number label. The energy scale is relative to the dissociation limit of $|X^3\Pi -0\omega \rangle$.

Figure 2

Coincidence dissociation KER spectra of ${\mathrm{CO}}^{2+}$ $\to$ ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ using a target of (a)–(g) vibrationally cold ${\mathrm{CO}}^{2+}$ beam, and (h) CO gas and ${\mathrm{CO}}^{+}$ beam. The top row of spectra are using 40-fs pulses and the bottom row 7-fs pulses. Intensities are as indicated in units of W/cm${}^2$. In (a)–(g), the region KER larger than 7.6 eV has been scaled times 10 as indicated to emphasize the ATD $2\omega$ peak. The solid curve in (d) is a calculation (see text) for the same laser conditions as the experiment, but without focal-volume intensity averaging. Error bars denote statistical error.

Figure 3

Dissociation KER spectrum using (a) 790-nm, 40-fs pulses at $2\times {10}^{15}$ W/cm${}^2$ [same as Fig. 2c] and (b) 395-nm, 40-fs pulses at $1\times {10}^{13}$ W/cm${}^2$. Error bars denote statistical error.

Figure 4

(a) and (b) Angular distribution for $1\omega$ dissociation of (a) ${\mathrm{H}}_2^{+}$ (for KER between 0.74 eV and 0.88 eV corresponding to the $1\omega$ crossing), and (b) ${\mathrm{CO}}^{2+}$, using 40-fs, $1\times {10}^{14}$ W/cm${}^2$ pulses. The data points are the experiment while the solid lines in (b) are from first-order perturbation theory; see text. (c) Momentum plot for ${\mathrm{CO}}^{2+}$ at 40 fs, $2\times {10}^{15}$ W/cm${}^2$.