CeNS Colloquium

Prof. Dr. Shaul Mukamel
Department of Chemistry, University of California

The response of molecules to sequences of ultrafast optical pulses ranging from the infrared to the soft and hard x-ray regimes provides novel probes for elementary molecular events and electronic and nuclear motions. Recent developments in these multidimensional techniques will be discussed.
New free-electron laser and high-harmonic generation x-ray light sources are capable of triggering valence electron motions impulsively by a stimulated Raman process involving core excitations and monitoring them with high temporal and spatial resolution. Attosecond x-ray pulses can probe quantum coherence and correlations of valence electron and hole pairs, in an analogous manner to the way vibrational motions are investigated using femtosecond Raman techniques in the visible regime. By creating multiple core holes at controlled times and locations it becomes possible to study the dynamics and correlations of valence electrons as they respond to core hole perturbations. Such signals may be interpreted in terms of the dynamics of valence electronic wave packets prepared and detected in the vicinity of a selected atom. Applications will be made for energy transfer in porphorin aggregates and long-range biological electron transfer in Azurin.
Excited-state vibrational dynamics in molecules can be studied by Raman processes induced by a probe pulse with variable delay with respect to an actinic pulse. Several possible configurations of the femtosecond stimulated Raman spectroscopy (FSRS) techniques will be presented that involve both spontaneous or stimulated Raman detection and different pulse sequences. Application is made to conical intersections in Uracil and to photoactivated intradimer bond splitting dynamics of cyclobutane thymine dimer, one of the major lesions in DNA under the exposure to UV sunlight.
Entangled photons provide novel nonlinear spectroscopic probes of excitation energy transfer and charge separation processes in chromophore aggregates. Quantum spectroscopy signals are recorded by varying parameters of the photon wavefunction. Rather than classical field envelopes the unusual spectral and temporal characteristics of entangled photon pairs make it possible to monitor and control multiple exciton states in photosynthetic reaction centers.

1. "Watching Energy Transfer in Metalloporphyrin Heterodimers Using X-ray Raman Spectroscopy ", J.D. Biggs, Y. Zhang, D. Healion, and S. Mukamel. PNAS, 110, 15597-15601 (2013).
2. "Probing the Conical Intersection Dynamics of the RNA Base Uracil by UV-Pump Stimulated-Raman-Probe Signals; Ab-Initio Simulations", B.P. Fingerhut, K.E. Dorfman and S. Mukamel. JCTC, DOI: 10.1021/ct401012u (2014).
3. "Suppression of Population transport and Control of Exciton Distributions by Entangled Photons ", F. Schlawin, K.E. Dorfman, B.P. Fingerhut, and S. Mukamel. Nature Communications, 4:1782:DOI:10.1038/ncomms2802 (2013).