The LBNL attosecond dynamics project strives to use its unique ability to generate isolated attosecond pulses to directly study electronic processes in atoms and small molecules in the time domain. In this effort, high harmonic generation and Double Optical Gating are used to produce isolated attosecond pulses in the energy range of 11 to 24 eV. The attosecond pulses are used in combination with a time-delayed perturbative few femtosecond near-infrared (NIR) pulse to measure laser-induced changes in the absorption of a target gas. Recent experimental work has used this attosecond transient absorption technique to measure the autoionization lifetimes of inner-valence excited xenon atoms and to observe quantum beating in a coherent superposition of valence Rydberg states in neon. Future work will focus on expanding the complexity of target systems to small molecules such as N2 and H2O where additional processes such as nuclear-electronic coupling can be investigated.
A. R. Beck, D. M. Neumark, and S. R. Leone, "
Probing ultrafast dynamics with attosecond transient absorption," Chem. Phys. Lett. 624, 119 (2015).
2 B. Bernhardt, A. R. Beck, X. Li, E. R. Warrick, M. J. Bell, D. J. Haxton, C. W. McCurdy, D. M. Neumark, and S. R. Leone, “High-spectral-resolution attosecond absorption spectroscopy of autoionization in xenon,” Phys. Rev. A 89, 023408 (2014).
3 A. R. Beck, B. C. Bernhard, E. R. Warrick, M. Wu, S. Chen, M. B. Gaarde, K. J. Schafer, D. M. Neumark, and S. R. Leone, "Attosecond transient absorption probing of electronic superpositions of bound states in neon: detection of quantum beats," New J. Phys. 16, 113016 (2014).
We employ the use of isolated attosecond pulses to study electron dynamics as they occur on their most fundamental timescales in atoms and molecules. Since isolating a single attosecond pulse is no trivial feat, we have developed our own technique to achieve high-contrast, single attosecond pulses. We refer to this technique as a Polarization ASSisted Amplitude GatE (PASSAGE). The technique employs the use of polarization-tailored, few-cycle pulses to limit the attosecond harmonic emission to less than half an optical cycle of the driving near-infrared field. We are currently using these pulses to study the strong field ionization of atomic Xenon. We find that we are not only able to observe the real-time ionization of the atom, but we can also observe a small time delay in the ionization of different spin-orbit states. This time delay could be a manifestation of many-body correlations that arise during strong field ionization. We hope to extend this technique to study electron rearrangements during the fastest charge transfer reactions that occur in nature.