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Probing the structure of stretched molecular ions with high-harmonic spectroscopy

High-harmonic spectroscopy (HHS) is an emerging tool for probing the structure of molecules in ultrafast timescale. Here, through numerical solution of the time-dependent Schrödinger equation, we show that the present procedures in HHS cannot be directly applied to molecular ions such as H 2 + with large internuclear distances R. For the latter, besides the ground state, the excited state of the system also contributes importantly to the harmonic emission. A simple model based on developed strong-field approximations and quantum-orbit theory is proposed, which allows us to extract the structural and dynamical information of molecular ions with large R from the harmonic spectra obtained with considering only the contributions of the short trajectory. Our work gives suggestions on tracing the bond length of a molecular orbital in a chemical reaction as the molecule is stretched to large R.