Department of Physics, NCU
Title
Exploring Ultrafast Quantum Dynamics of Electrons by Attosecond Transient Absorption
Speaker
廖振廷博士
University of Arizona
Date 2017.06.05 (Mon)
Time 14:00
Place 健雄館S4-625
Abstract:
Quantum mechanical motion of electrons in atoms and molecules and their interaction with light is at the heart of many photophysical and photochemical processes. As the natural timescale of electron dynamics is in the range of femtoseconds or shorter, ultrashort pulses are needed for time-resolved measurement of electronic processes. In this research, extreme ultraviolet (XUV) attosecond pulses are used to coherently prepare superposition of excited states in various atomic and molecular systems. This light-matter interaction represents the creation of fast-evolving atomic or molecular polarization. A subsequent phase-locked infrared femtosecond pulse is applied to perturb the polarization, and the transient changes in the transmitted XUV spectrum are measured—and this technique is termed XUV attosecond transient absorption spectroscopy. We applied this technique to study Rydberg states in dilute helium gas and observed several transient changes to the atomic structure. When the experiments were extended to the study of a dense helium gas sample, new spectral features emerged in the transient absorption spectra, which cannot be explained by linear optical response models. We found that these absorption features arise from the interplay between the XUV resonant pulse propagation and the infrared-induced quantum phase shift. Extending our work to argon atoms, we studied how an external infrared field can be used to impulsively control different photo-excitation pathways and the transient absorption lineshape of an otherwise isolated autoionizing state. Unlike atoms, in our study of autoionizing states of the oxygen molecule, we observed both positive and negative optical density changes for states with different electronic symmetries. We relate this symmetry-dependent sign change to the Fano parameters of static photoabsorption. In summary, we experimentally explored ephemeral light-induced phenomena associated with excited states of atoms and molecules. These studies provide real-time information on ultrafast electronic processes and provide strategies for direct time-domain control of the light-matter.