Laser pump-probe schemes are explored numerically from a molecular time-dependent Schrödinger equation for monitoring attosecond electron motion by high-order-harmonic generation (HHG) from a coherent superposition of electronic states. Varying the time delay between the probe and pump pulse on an attosecond time scale alters the HHG signal, leading to universal interference patterns. By using an extended three-step recollision model, we show that various regular interference patterns in the HHG spectra, including continuous harmonic frequency redshifts as a function of pump-probe delay, are related to interference between specific pairs of short or long quantum orbits. For small excited-state population (regime A), interferences are controlled by electron-tunneling times, whereas for equal populations (regime B), recollision time control dominates thus allowing for control of the HHG process by the pump-probe delay time and by electron-state populations. We show that each specific pattern is closely related to the interference between pairs of specific (short or long) quantum orbits originating from a specific electron bound state. One can generate a specific pattern by varying the populations in the coherent superposition, the laser intensity, wavelength, and ionization potential of the electronic states.

• Received 10 May 2013

DOI:https://doi.org/10.1103/PhysRevA.88.033423