Using $\mathrm{H}_2{}^{+}$ and $\mathrm{D}_2{}^{+}$, we observe two-surface population dynamics by measuring the kinetic energy of the correlated ions that are created when $\mathrm{H}_2{}^{+}$ ($\mathrm{D}_2{}^{+}$) ionize in short (40–140 fs) and intense (${10}^{14}\mathrm{W}/{\mathrm{cm}}^2$) infrared laser pulses. Experimentally, we find a modulation of the kinetic energy spectrum of the correlated fragments. The spectral progression arises from a hitherto unexpected spatial modulation on the excited state population, revealed by Coulomb explosion. By solving the two-level time-dependent Schrödinger equation, we show that an interference between the net-two-photon and the one-photon transition creates localized electrons which subsequently ionize.

• Received 7 June 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.073003