In this project supported by the Chemical Structure, Dynamics, and Mechanisms Program of the Division of Chemistry, Professor Vincent McKoy and his research group at the California Institute of Technology will develop quantum mechanical methods for simulating femtosecond time-resolved photoelectron spectroscopy (TR-PES) data. TRPES is a technique that can reveal the internal dynamics of a molecule, including the motion of atoms and changes in electronic structure when the molecule is excited by light. The focus of this research will be on non-adiabatic dynamics, especially conical intersections responsible for internal conversion and photodissociation. Because photoionization amplitudes vary rapidly with the nuclear coordinates near conical intersections, calculations that track this geometry dependence are essential to extract the wave-packet dynamics from measured energy- and angle-resolved photoelectron distributions.
The research to be undertaken bears directly on understanding the dynamics in polyatomic molecules, including such important issues as the photostability of the DNA nucleobases and the mechanisms by which light-sensitive proteins function. Besides aiding in the design and interpretation of related experiments, therefore, this research also has wide biophysical and biomedical implications. Detailed knowledge of photon interactions with biomolecules may in the long run assist in developing, for example, biomimetic photosensitive systems or genotoxic phototherapies for cancer. By providing a context for training younger researchers in theoretical molecular photoelectron spectroscopy, moreover, this research project addresses a critical need, since there are very few theorists worldwide to support the many experimental groups active in this field. The research will be carried out within longstanding, productive, and mutually beneficial collaborations with research groups in the U.S., Japan, Canada, and Brazil.
This project uses quantitative numerical simulations of time-resolved photoelectron spectroscopy to explore the ultrafast excited-state dynamics of polyatomic molecules. Its focus is on non-adiabatic dynamics, especially conical intersections responsible for internal conversion and photodissociation. Because photoionization amplitudes vary rapidly with the nuclear coordinates near conical intersections, calculations that track this geometry dependence are essential to extract the wave packet dynamics from measured energy- and angle-resolved photoelectron distributions. Intellectual Merit: The project involved the development and extension of computational procedures and their successful application to molecules such as CS2. The approach followed encompasses a complete ab initio simulation of a time-resolved photoelectron experiment, in which wave packet densities obtained using the ab initio multiple spawning method are combined with energy- and geometry-dependent photoionization amplitudes to compute the photoelectron energy and angular distributions. By making feasible the interpretation of measured photoelectron energy and angular distributions, the project promotes further experimental investigations. Over the course of this project, the following goals were achieved: 1. Development of a practical framework for carrying out ab initio computational simulations of time-resolved photoelectron spectra in polyatomic molecules, including both molecular and photoionization dynamics. 2. Application of that framework to explore and interpret the non-adiabatic dynamics of wave packets near conical intersections in a molecule of experimental interest, CS2. 3. Simulations of time-resolved photoelectron spectra in two other molecules, NO2 and pyrrole. Broader Impacts: The research bears directly on understanding the non-adiabatic dynamics in polyatomic molecules, including such important photobiological processes as vision and photosynthesis and the photostability of the DNA nucleobases and the mechanisms by which light-sensitive proteins function. Besides aiding in the design and interpretation of related experiments, therefore, this research also has far-reaching implications in photochemistry, photophysics, and photobiology. Detailed knowledge of photon interactions with biomolecules may in the long run assist in developing, for example, biomimetic photosensitive systems or genotoxic phototherapies for cancer. By providing a context for training younger researchers in theoretical molecular photoelectron spectroscopy, moreover, this research project addresses a critical need, since there are very few theorists worldwide to support the many experimental groups active in the field. The research has been carried out within longstanding, productive, and mutually beneficial collaborations with research groups in the U.S., Japan, Canada, and Brazil.
