We investigate early-stage ultrafast nuclear dynamics of ${\mathrm{O}}_2^{+}$ in the electronically highly excited states by ionizing ${\mathrm{O}}_2$ with high-order harmonics of a few-cycle near-infrared (NIR) laser pulse $(\sim 5.8\mathrm{fs})$ in the extreme ultraviolet (XUV) region and by probing the nuclear dynamics of ${\mathrm{O}}_2^{+}$ with the few-cycle NIR laser pulse. The kinetic energy distribution of the yield of ${\mathrm{O}}^{+}$ fragments recorded as a function of time delay of the NIR pulse with respect to the XUV pulse until 100 fs exhibit a periodic parabola-like structure in the low-kinetic-energy region below 0.3 eV and a curvilinear structure in the higher-kinetic-energy region (0.2–1.5 eV). By numerical simulations of a vibrational wave packet in the optically coupled electronic states of ${\mathrm{O}}_2^{+}$, we reveal that the periodic parabola-like structure represents a vibrational motion of a wave packet in the $\mathrm{a}{}^4\mathrm{\Pi }_u$ state of ${\mathrm{O}}_2^{+}$, which appears only when the probe pulse is sufficiently short, and that the curvilinear structure represents dissociating and oscillating components of a vibrational wave packet in the $3{}^2\mathrm{\Pi }_u$ state of ${\mathrm{O}}_2^{+}$, which is predominantly excited to the $8{}^2\mathrm{\Pi }_g$ and $9{}^2\mathrm{\Pi }_g$ states by the probe pulse.

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• Electronic excitation & ionization
• Molecular dissociation
• Photodissociation
• Potential energy surfaces
• Single- and few-photon ionization & excitation
• Ultrafast phenomena
• Vibrational states