Measurements of electronic coherence and population decay with pulse-shape spectroscopy

Measuring the motion of electrons is at the core of attosecond science, but it is challenging to create laser pulses that are short enough to resolve this motion. Here, we use a pulse shaper to generate few-cycle, pump-probe pulse pairs in the deep ultraviolet that have sub-6 attosecond timing stability. The timing stability is crucial and allows us to measure coupled electronic and nuclear dynamics by interfering multiple ionization pathways, which gives access to the relative phase between different electronic states. This pulse-shape spectroscopy method enables separate tracking of the diagonal and off-diagonal elements of the reduced electronic density matrix. In other words, the probability of being in an electronic state and the ground and excited electronic wavepacket overlap can be measured simultaneously, yet independently. We contrast measurements for molecules where the diagonal matrix elements are more interesting (shedding light on internal conversion) with measurements where the off-diagonal elements show more interesting dynamics (long-lived electronic coherences with revivals). Moreover, we present a method to control electronic coherences in molecules to further understand and influence their dynamics.