High-order-harmonic generation from coherent electron wave packets in atoms and molecules as a tool for monitoring attosecond electrons

We study numerically pump-probe schemes for monitoring electron-nuclear motion in a dissociating molecule using a midinfrared, intense few-femtosecond probe laser pulse which generates molecular high-order harmonics (MHOHG) from a coherent superposition of electron-nuclear wave packets prepared by a weak femtosecond UV pump pulse from an initial bound state. We show that by varying the time delay between the intense probe pulse and the UV pump pulse by a few hundred attoseconds one alters the MHOHG signal intensity by many orders of magnitude. The periodicity of the MHOHG intensity variations as function of the time delay is equal to the period of the electron oscillation in the coherent superposition which varies with internuclear distance. We use the strong field approximation (SFA) and three-step model to explain this high sensitivity of the harmonic intensity to pulse delay time and to the overlap of nuclear wave packets. We also solve numerically the three-dimensional (3D) time-dependent Schrodinger equation describing harmonic generation for a hydrogen atom prepared in a superposition of its two lowest atomic states, in order to investigate the dependence of the same effect (in a simpler system but in 3D) on the probe-pulse carrier-envelope phase (CEP) and on the probe duration. We also relate these strong effects in the intensity of harmonics to the correlation between the velocity of the recolliding electron wave packet and the electron velocity in the coherent superposition of the electron bound states.