Department of Chemistry Seminar Announcement

1. Department of Chemistry Seminar Announcement About the Speaker Dr Loh Zhi Heng was born and raised in Singapore. He received his S.B. in chemistry from the Massachusetts Institute of Technology in 2003 and his Ph.D. in physical chemistry from the University of California, Berkeley, in 2008. He remained in Berkeley as a postdoctoral researcher before joining NTU as an Assistant Professor in December 2010. During his Ph.D. and postdoctoral studies, both performed under the tutelage of Professor Stephen Leone, he extended transient absorption spectroscopy into the soft x-ray and attosecond time domain; the latter was achieved in collaboration with Professor Ferenc Krausz at the Max-Planck-Institut für Quantenoptik, Germany. His research interests are in employing attosecond to femtosecond time-resolved core-level spectroscopies to study coherent quantum dynamics and strong-field phenomena in atoms, molecules, and nanomaterials. Abstract The wave-like nature of matter is one of the basic tenets of the intriguing world of quantum mechanics. The synchrony of these wave-like states with a definite phase relationship yields a quantum coherence that encodes the motion of quantum mechanical entities. In the case of electrons, the time scale for their ultrarapid motion resides in the attosecond domain, where one attosecond is one billion-billionth of a second. The direct observation of electron dynamics in the time domain has remained only a dream until very recently. In this talk, I shall describe my previous work in applying soft x-ray pulses of femtosecond to attosecond time durations to study ultrafast quantum dynamics. In the femtosecond regime, time-resolved soft x-ray transient absorption spectroscopy reveals the electronic quantum state distribution of ions [1] and time scales for dissociative photoionization [2] following the interaction of intense laser pulses with atoms and molecules. A recent experiment with attosecond soft x-ray pulses allowed the first real-time observation of valence electron motion [3]. In the experiment, valence electron motion that is driven by the spin-orbit superposition of the Kr+2P3/2 and 2P1/2 states is observed after ionizing Kr atoms with an intense, few-cycle near-infrared laser pulse. Retrieval of the entire ion density matrix from the experimental data enables visualization of the Kr 4p valence shell electron motion in real time. If time permits, I will discuss some of the experiments that will be pursued at NTU. [1] Z.-H. Loh, M. Khalil, R.E. Correa, R. Santra, C. Buth, and S.R. Leone, “Quantum state-resolved probing of strong-field-ionized xenon atoms using femtosecond high-order harmonic transient absorption spectroscopy,” Phys. Rev. Lett. 98,143601 (2007). [2] Z.-H. Loh and S.R. Leone, “Ultrafast strong-field dissociative ionization dynamics of CH2Br2 probed by femtosecond soft x-ray transient absorption spectroscopy,” J. Chem. Phys. 128, 204302 (2008). [3] E. Goulielmakis,* Z.-H. Loh,* A. Wirth, R. Santra, N. Rohringer, V.S. Yakovlev, S. Zherebtsov, T. Pfeifer, A.M. Azzeer, M.F. Kling, S.R. Leone, and F. Krausz, "Real-time observation of valence electron motion" Nature 466, 739 (2010). All are Welcome