Shaul Mukamel, University of California, Irvine, Award No. 1361516Spectroscopic Observation of Ultrafast Light Induced Processes in Molecules – Chasing Schrödinger’s CatS. Mukamel, NSF Grant “Molecular Radiative and Relaxation Processes”, CHE-1361516.M. Kowalewski, K. Bennett, K. E. Dorfman, and S. Mukamel, “Catching Conical Intersection in the Act: Monitoring Transient Electronic Coherences by Attosecond Stimulated X-Ray Raman Signals”, Phys. Rev. Let., 115, 19003 (2015).The conical intersection deactivation pathways are fast and have so far eluded their direct observation in experiment. Understanding and detecting these processes has long been a challenge for scientists. Professor Mukamel and colleagues have designed a spectroscopic technique that makes use of a quaint consequence of quantum mechanics. During the passage through the conical intersection, the molecule can simultaneously exist in both ground and excited state for a short time. This is known as a coherent superposition or a "Schrödinger cat" state. The proposed technique employs novel state-of-the-art X-ray light-free electron lasers (XFEL) sources that provide ultra-short pulses with high-photon energies. A stimulated Raman process creates a signal that is solely generated by the coherent superposition and thus catches these conical intersections in the act. The technique provides an unambiguously clear experimental signature of the underlying dynamics of the nuclei and electrons in great detail and specicity. The method further reveals new ways for controlling and manipulating these important events. This work is likely to inspire novel experiments made possible by newly created X-ray laser light sources.The interaction of light with molecules is of fundamental interest in everyday life and is essential for many chemical and biological processes. When a molecule is excited by light, an electron is lifted into an excited orbit and its way back into the ground state can happen in dierent paths. It can simply fall back into the ground state and emit light of the same or a slightly dierent color. This is used in energy saving light bulbs or neon color dyes, for example. Other pathways are more complex. The excited electron causes the molecule to strongly vibrate, and it gets back to the ground state or to other states without the emission of a photon. Instead, it passes through a funnel known as conical intersection. This passage plays a crucial role in determining the outcomes and timescales of virtually all elementary photochemical and photophysical events. One example is the interaction of our genome under sunlight which can lead to skin cancer and other mutations. The DNA damage and repair mechanisms involve conical intersections.Illustration of the ultrafast spectroscopy method. As the molecule (upper panel) is excited by an ultraviolet laser pulse, its nuclei start to move. Once it is funneled through the conical intersection (lower panel), it can exist simultaneously in both the ground and excited states. This Schrödinger cat state is probed with another laser pulse, giving o a distinct signal which can reveal the energy distance between both cats and their relative phase. This is the rst method that can directly probe the passage through conical intersections.