We present effects of electron energy transfer by electron collisions on high-order-harmonic generation (HHG) in molecular sequential double ionization by intense circularly polarized laser pulses. Results from numerical solutions of time-dependent Schrödinger equations for extended (large internuclear distance) ${\mathrm{H}}_2$ where electrons are entangled and hence delocalized by exchange show that HHG with cutoff energy up to $I_p+24U_p$ can be obtained, where $I_p$ is the molecule ionization potential and $U_p=I_0/4{\omega }_0^2$ (in atomic units) is the ponderomotive energy for pulse intensity $I_0$ and frequency ${\omega }_0$. A time-frequency analysis is employed to identify electron collisions for the generation of harmonics. Extended HHG arises from electron energy exchange, which agrees well with the prediction of a classical two electron collision model. Results for nonsymmetric ${\mathrm{HHe}}^{+}$ where initially electrons are localized on He are also compared and confirm the role of initial electron delocalization via entanglement for obtaining extended HHG plateaus.

• Received 12 April 2015

DOI:https://doi.org/10.1103/PhysRevA.92.023415