High-order-harmonic generation in molecular sequential double ionization by intense circularly polarized laser pulses

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 Schrodinger equations for extended (large internuclear distance) 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 Ip is the molecule ionization potential and U-p = I-0/4 omega(2)(0) (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 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.