Resonant excitation during strong-field dissociative ionization

We studied dissociative ionization of oxygen by intense femtosecond laser pulses with central wavelengths between 550 and 1800 nm. We measured kinetic energy release spectra and angular distributions of fragments resulting from symmetric dissociation of doubly charged molecular ions (${\mathrm{O}}_2^{2+}\to {\mathrm{O}}^{+}+{\mathrm{O}}^{+}$ channel). In the kinetic energy release spectra we identified a number of distinct excited states of the molecular ion. Angular distributions for all but one of those states were consistent with recollision excitation following single ionization of ${\mathrm{O}}_2$. For the remaining $\left(B{}^3\Pi _{\mathbf{g}}\right)$ excited state we observed a characteristic resonant dependence of its relative yield on the central optical frequency of the pulse, with the yield peaking at around 890 nm. This presents unambiguous evidence in support of importance of resonant excitation channels in strong field ionization of molecules.

Figure 1

(Color online) Upper panel: A typical KER spectrum of ${\mathrm{O}}^{+}$ fragments at 800 nm and laser intensity $\sim 2\times {10}^{14}\mathrm{W}∕{\mathrm{cm}}^2$. Lower panel right: the angular distribution of the fragments resulting from the dissociation of the $B{}^3\Pi _{\mathbf{g}}$ state in the dication. Lower panel left: the angular distribution of the fragments resulting from other states identified earlier in Ref. 15.

Figure 2

(Color online) KER spectra of ${\mathrm{O}}^{+}$ fragments at different laser intensities ranging from 550 to 1500 nm at the indicated laser intensities in units of $I_o(I_o={10}^{14}\mathrm{W}∕{\mathrm{cm}}^2)$. The dependence of the $B{}^3\Pi _{\mathbf{g}}$ state production on the laser wavelength is shown. In the lower panel, the yield’s ratio of the $B{}^3\Pi _{\mathbf{g}}$ state to all other states is plotted as a function of the laser photon energy. A resonancelike behavior is observed around 1.4 eV.