Tracking electron motion in real time

Quantum mechanics suggests that the fastest degree of freedom— outside the atomic nucleus— is of electronic nature and that the dynamics of electrons occur on the time scale of attoseconds to femtoseconds.  These—astonishingly short—time intervals on which electron motion influences the transient response of matter to light fields, a has led scientists to name electronic dynamics as instantaneous or immeasurably fast for several decades. With the duration of the optical attosecond pulses to be commensurably short in comparison to the characteristic scale of motion of electrons, one can anticipate that the nonlinear response of quantum systems to such light fields must be –at last—sensitive to the inherent time constants of electrons. Here we give two examples of our research towards probing electronic response in real time:

Tracking the response of bound electrons: In one representative example of relevant studies we have shone optical attosecond pulses to atoms of Krypton—a pure electronic system free from other degrees of freedom— and measured the generated spectrum in the vacuum ultraviolet range. Because the field (here via the carrier envelope phase or more precisely the global phase) of the optical attosecond pulses could be manipulated with sub-fs precision, we performed several measurements where the emission of VUV radiation from the driven electrons in Kr is measured as a function of the field phase. This procedure allowed the composition of spectrograms where the key variable is not a time delay but the absolute phase of the light field. The reconstruction of these spectrograms allowed access into the response of the system and revealed that it takes the electron cloud about ~100 attosecond to adjust in response to the applied optical field (Nature 2016).