Ab initio-Quantendynamik der Photoabsorption und Dissoziation von Kohlendioxid in den unteren Singulett- und Triplett-Zuständen
Physikalische Chemie von Molekülen, Flüssigkeiten und Grenzflächen, Biophysikalische Chemie
Zusammenfassung der Projektergebnisse
The primary goal of this project was a quantum mechanical investigation of the absorption spectrum and the non-adiabatic mechanisms of the photodissociation of CO2 in the ultraviolet (UV) and vacuum UV wavelength ranges. UV light destroys CO2 with unit quantum yield, and the chosen spectral region is of considerable importance for atmospheric chemistry and CO2 utilization. The project also investigated non-adiabatic interactions between Rydberg and valence states and their impact on the photoreactivity. Research in the second funding period focused on three topics: (1) Absorption spectra; (2) Emission spectra and photofragment distributions and (3) Pump-probe photoelectron spectra. Extensive ab initio calculations of the coupled global potential energy and transition dipole moment surfaces have been performed, followed by multistate quantum dynamical calculations. The project gave an accurate description of the absorption properties of singlet and triplet states and elucidated the non-adiabatic photodissociation and photoionization mechanisms with spatial, temporal, and energy resolution. A stable computational framework has been constructed within which the variation of the UV absorption cross sections in a broad wavelength and temperature range could be efficiently calculated from first principles. It is now possible to reliably calculate CO2 absorption for long wavelengths lambda <= 250 nm and for gas temperatures up to 2500K. This allows simulation driven efficiency optimization of the solar based one-step photolytic reduction of hot CO2 to CO. The studies characterized bent (‘activated’) isomers of neutral CO2 in terms of their infrared spectra and isotope shifts. Five isomers were analyzed: Dioxiranylidene (the cyclic form of CO2 with the equilibrium valence angle of 72°) and four open singlet and triplet isomers with the valence angles of ca. 118° and ca. 127°. All isomers are polar molecules with electric dipoles ranging from 0.25D to 1D. Near a metal surface, the electrostatic interaction with the image dipole lowers their energy relative to the nonpolar ground electronic state by up to 1 eV, making CO2 activation via long wavelength photolysis possible. This result can inspire new heterogeneous schemes of CO2 activation. Potentials of the singlet electronic states were calculated up to the first ionization energy. This allowed theoretical analysis of the unexpected results of the time resolved experiments performed in the research group of Prof. T. Suzuki at the University of Kyoto. They pumped CO2 into the vicinity of ionization threshold and probed its dynamics via photoionization: The resulting pump probe photoelectron spectra were over 3 eV broad. Quantum mechanical calculations performed on a dense network of electronic states explained this unusual broadening by the ultrafast relaxation of high lying Rydberg states on the time scale of 15 fs-65 fs, consistent with the experimental observations. Two novel methodologies have been developed. One allows reconstruction of continuous emission spectra from the internal energy distributions of the photofragments. The other connects these emission spectra to the topographical parameters of conical intersections encountered by interacting photofragments on the way to dissociation. This result is potentially relevant for the design of non-optical detection schemes of weak emissions during molecular fragmentation and for the reconstruction of reactive conical intersections from the experimental resonance Raman spectra of photodissociating molecules.
Projektbezogene Publikationen (Auswahl)
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Intermediate photofragment distributions as probes of non-adiabatic dynamics at conical intersections: Application to the Hartley band of ozone, PCCP 17, 074311 (2015)
D. Picconi and S. Yu. Grebenshchikov
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Partial dissociative emission cross sections and product state distributions of the resulting photofragments, Chem. Phys. 481, 231 (2016)
D. Picconi and S. Yu. Grebenshchikov
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Photochemistry of carbon dioxide from first principles: Application to photoabsorption of hot CO2, J. of CO2 Utilization 15, 32 (2016)
S. Yu. Grebenshchikov
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Infrared spectra of neutral bent carbon dioxide, J. Phys. Chem. A, 2017
S. Yu. Grebenshchikov
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Unexpectedly broad photoelectron spectrum as a signature of ultrafast electronic relaxation of Rydberg states of carbon dioxide, Phys. Rev. A 95, 0033422 (2017)
A. Adachi, M. Sato, T. Suzuki, and S. Yu. Grebenshchikov
