Attosecond Phenomena
Electrons provide the screening to stabilize and bond molecules, and thus electronic redistribution initiates all chemical changes. In order to understand the earliest processes involved in chemical change, we attempt to track the evolution of electrons on their intrinsic timescales. Electrons move across a molecular bond in 0.1 to 1 femtosecond. Ultrafast optical techniques are routinely used to interrogate chemical reactions on the timescale intrinsic to chemical change (i.e. the time scale for nuclear dynamics). These observations have given birth to the field of “femtochemistry.” But nuclear motion is rather slow compared to the rapid motion of electrons, which occurs on the attosecond timescale. The attosecond science group focuses on understanding electronic motion on its natural timescale.
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Recent Research Highlights in Attosecond Science
Probing Electronic Coherence between Core-Level Vacancies at Different Atomic Sites
We observe a new modality of electronic evolution driven by electronic coherence, in the near absence of any change in the charge density. Probing coherent electron motion is important for understanding charge transfer phenomena and may help to steer photochemical reactions. Additionally, our measurement scheme allows us to probe the interplay between coherence and entanglement at x-ray wavelengths.
Attosecond pulses from LCLS create a superposition of core-ionized states. This superposition state decays via the Auger-Meitner process and the temporal evolution of the decay is probed via angular streaking. The decay reveals a quantum beat between the excited states, indicating partial coherence created by the pump. The localized nature of core-level excitations produced an excitation between states with no spatial overlap. The resultant superposition produced no charge density oscillations, unlike all previous measurements.
Attosecond Delays in X-ray Molecular Ionization
Laser-dressed photoionization measurements reveal the correlation-induced delay in the emission of electrons from different K-shells. The measured delay in the emission of electrons was larger than expected and provides a sensitive probe of the correlated nature of electrons.
Attosecond pulses from LCLS drive ionization in the presence of a strong, circularly polarized IR field which measures the relative delay in photoionization channels. Moreover, the tunability of the FEL allows for the full reconstruction of the electronic wavepacket.