Molecules in bichromatic circularly polarized laser pulses: Electron recollision and harmonic generation

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High-order harmonic generation (HHG), the highly nonlinear nonperturbative response of atoms and/or molecules to ultrafast intense laser pulses of femtosecond (1fs=10-15 s) duration, can be modeled as a recollision process after tunneling ionization of an electron in linear polarization. In general, recollision is suppressed at high intensities with monochromatic circularly polarized pulses. Combinations of bicircular pulses with frequencies ω1/ω2= n1/n2 where n1 and n2 are integers, induce recollisions and HHG, seen in a rotating frame Hamiltonian. Molecules are preferred systems for circularly polarized HHG due to lower rotational symmetry as compared to infinite symmetry in atoms where emission selection rules dominate. We simulated HHG spectra from numerical solutions of time-dependent Schrödinger equations for linear and cyclic one and two electron molecular models by intense circularly polarized pulses with both co-rotating and counter-rotating components. Simulations show that the bicircular HHG spectrum is universal, with a cut-off at Nm = (Ip+ 3.17 Up)/ħω, where Ip is the molecular ionization potential and the ponderomotive energy Up= e2E2/4 meω2 for maximum electric field amplitude E. The rotating frame Hamiltonian model predicts the recollision (ponderomotive) frequency ω = (ω1+ ω2)/2. The simulated HHG spectra confirm the recollision model in a rotating frame at frequency ω¯ = (ω2-ω1)/2. The results show that the circularly polarized HHG with definite helicity occurs efficiently when the total electric field E(t) rotational symmetry Cn of bicircular pulses is the same as the symmetry of the molecule. A mismatch of symmetry produces a random HHG spectrum, thus confirming the role of field-molecular symmetry in molecular HHG with circularly polarized pulses.

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Springer Series in Chemical Physics

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