We report the observation of a kinetic-energy-release feature at 3.8 eV in the dissociative double ionization of NO dimers driven by intense femtosecond laser fields. This channel is separated from the 5.6 eV signature of direct Coulomb explosion and is absent under electron-impact ionization. Ab initio electronic-structure calculations identify the low-energy feature as arising from intermolecular Coulombic decay initiated by strong-field-induced coupling between the ${\mathrm{NO}}^{+}(1{{}^1\mathrm{\Sigma }}^{+})-\mathrm{NO}(\mathrm{X}{}^2\mathrm{\Pi })$ ground configuration and an excited ${\mathrm{NO}}^{+}(2{}^1\mathrm{\Pi })-\mathrm{NO}(\mathrm{X}{}^2\mathrm{\Pi })$ state, followed by nonlocal relaxation to the ${\mathrm{NO}}^{+}(1{{}^1\mathrm{\Sigma }}^{+})-{\mathrm{NO}}^{+}(1{{}^1\mathrm{\Sigma }}^{+})$ state at enlarged intermolecular separations. This coupling is resonantly enhanced in a two-color field formed by the spatiotemporal overlap of 800 and 400 nm femtosecond pulses, resulting in a pronounced increase of the intermolecular Coulombic decay yield. These findings demonstrate that femtosecond laser excitation can initiate intermolecular Coulombic decay in NO dimers through strong-field population of an electronically excited doorway state.

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• Atomic & molecular processes in external fields
• Interatomic & molecular potentials
• Laser-cluster interaction
• Molecular dissociation
• Strong field ionization & excitation
• Clusters