Monitoring long-term evolution of molecular vibrational wave packet using high-order harmonic generation

Abstract

We present a method for probing ultrafast nuclear dynamics in molecules using transient enhancement of high-order harmonic generation (HHG). The method exploits strong dependence of the overall harmonic yield in a fixed spectral range on the nuclear separation in a molecule, which is shown to take place for both aligned and randomly oriented molecules. Our numerical simulations show that this method is capable of monitoring long-term evolution of the nuclear vibrational wave packets in molecules, even in light-weight ones, with very high time resolution. By the example of D2+ vibrational wave packet launched via tunnelling ionization of D2 and probed by a time-delayed 8 fs laser pulse with λ = 800 nm and the peak intensity of 1014 W cm-2, we show that the time-delay dependence of the high harmonic signal exhibits pronounced features, which are due to the collapses and revivals of the nuclear wave packet. The time-frequency analysis of the pump–probe signal reveals structures with a periodicity down to 6 fs, which correspond to the fractional revivals of orders 1/5 and 1/10. With the laser parameters used, the deuteron motion during the probe pulse is shown to have almost negligible effect on the resulting signal.