Ultrafast dynamics of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi mathvariant="normal">O</mml:mi> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> in the <mml:math xmlns:mml="http://www.w3.org/1998/

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 $(\ensuremath{\sim}5.8\phantom{\rule{0.28em}{0ex}}\mathrm{fs})$ 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 $\mathrm{a}^{4}\mathrm{\ensuremath{\Pi}}_{u}$ 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 $3^{2}\mathrm{\ensuremath{\Pi}}_{u}$ state of ${\mathrm{O}}_{2}^{+}$, which is predominantly excited to the $8^{2}\mathrm{\ensuremath{\Pi}}_{g}$ and $9^{2}\mathrm{\ensuremath{\Pi}}_{g}$ states by the probe pulse.