  Quantum coherence between electronic states of a photoionized molecule and
the resulting process of ultrafast electron-hole migration have been put
forward as a possible quantum mechanism of charge-directed reactivity governing
the photoionization-induced molecular decomposition. Attosecond experiments
based on the indirect (fragment ion-based) characterization of the proposed
electronic phenomena suggest that the photoionization-induced electronic
coherence can survive for tens of femtoseconds, while some theoretical studies
predict much faster decay of the coherence due to the quantum uncertainty in
the nuclear positions and the nuclear-motion effects. The open questions are:
do long-lived electronic quantum coherences exist in complex molecules and can
they be probed directly, i.e. via electronic observables? Here, we use x-rays
both to create and to directly probe quantum coherence in the photoionized
amino acid glycine. The outgoing photoelectron wave leaves behind a positively
charged ion that is in a coherent superposition of quantum mechanical
eigenstates lying within the ionizing pulse spectral bandwidth. Delayed x-ray
pulses track the induced coherence through resonant x-ray absorption that
induces Auger decay and by the photoelectron emission from sequential double
photoionization. Sinusoidal temporal modulation of the detected signal at early
times (0 - 25 fs) is observed in both measurements. Advanced ab initio
many-electron simulations, taking into account the quantum uncertainty in the
nuclear positions, allow us to explain the first 25 fs of the detected coherent
quantum evolution in terms of the electronic coherence.
