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Photo-induced molecular processes and charge recombination reactions driven by non-adiabatic couplings
Stockholm University, Faculty of Science, Department of Physics. (Chemical Physics)
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is based on a number of theoretical studies of molecular processes in which non-adiabatic effects play a crucial role. The main part is devoted to the photophysics and photochemistry of five-membered aromatic heterocyclic compounds, with particular focus on furan and thiophene. These species absorb light in the UV region of the spectrum, but exhibits neither fluorescence nor phosphorescence. A plausible explanation for this is the presence of very fast radiationless deactivation mechanisms that depopulate the excited states before emission can occur. In the present thesis, several possible such mechanisms are considered based on the results of static electronic structure calculations as well as mixed quantum-classical dynamics simulations. A common feature of the investigated pathways is the presence of easily accessible conical intersections (surface crossings) through which non-adiabatic population transfer to the electronic ground state may take place. Also considered in the present work are the bond-breaking/bond-forming processes that are expected to follow once the ground state has been reached.

Another prominent example of non-adiabatic processes is charge recombination reactions involving either two oppositely charged ions or a cation and an electron. Here, the reactants and products belong to different electronic states of the system as a whole and therefore a non-adiabatic transition is necessary for the recombination to occur. Two such processes are considered in the present thesis; mutual neutralization of oppositely charged hydrogen ions and dissociative recombination of the formyl and iso-formyl cations. The cross sections (rate coefficients) and underlying mechanisms of these reactions are investigated at a fully quantum mechanical level.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2012. , 67 p.
Keyword [en]
Non-adiabatic processes, Photophysics, Photochemistry, Radiationless deactivation, Conical intersections, Five-membered aromatic heterocyclic compounds, Mutual neutralization, Dissociative recombination
National Category
Theoretical Chemistry Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-72456ISBN: 978-91-7447-458-9 (print)OAI: oai:DiVA.org:su-72456DiVA: diva2:498793
Public defence
2012-03-09, lecture room FA32, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: In press. Paper 3: Manuscript. Paper 5. Submitted. Available from: 2012-02-16 Created: 2012-02-12 Last updated: 2012-02-13Bibliographically approved
List of papers
1. A computational study of radiationless deactivation mechanisms of furan
Open this publication in new window or tab >>A computational study of radiationless deactivation mechanisms of furan
2011 (English)In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 379, no 1-3, 6-12 p.Article in journal (Refereed) Published
Abstract [en]

Possible mechanisms for the radiationless deactivation of photo-excited furan have been investigated using high-level electronic structure methods. Two different conical intersections between the S0 and S1 electronic states have been characterized, both involving various degrees of CO bond cleavage. One of these corresponds to a planar ring-opened structure and the other to an asymmetric ring-puckered structure. Calculations have been performed in order to establish the vertical electronic spectrum and to investigate the behaviour of the potential energy surfaces as the intersections are approached. The present results indicate that both crossings can be accessed through exothermic and barrierless processes after vertical excitation into the optically bright S2(ππ∗) state. These features make them good candidates to account for efficient radiationless deactivation in furan. The deactivation pathways considered in the present work are close analogues of those previously described for other five-membered heterocycles.

National Category
Theoretical Chemistry Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-67333 (URN)10.1016/j.chemphys.2010.10.002 (DOI)000286658800002 ()
Available from: 2011-12-28 Created: 2011-12-28 Last updated: 2017-12-08Bibliographically approved
2. Theoretical study of the radiationless deactivation mechanisms of photo-excited thiophene
Open this publication in new window or tab >>Theoretical study of the radiationless deactivation mechanisms of photo-excited thiophene
2012 (English)In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 397, 18-25 p.Article in journal (Refereed) Published
Abstract [en]

The radiationless deactivation mechanisms of photo-excited thiophene have been studied using the multi-reference second-order perturbation theory and linear response coupled cluster methods. The electronic spectrum has been established and various minimum energy structures and conical intersections involving the ground and lowest singlet excited states have been characterized. Simplified reaction paths connecting the optimized geometries have been calculated as well. Based on these investigations, several deactivation mechanisms have been identified leading from the lowest bright 1ππ states back to the electronic ground state. The excited state depletion in each case is possible due to the existence of low-lying conical intersections formed by either cleavage of one of the CS bonds or out-of-plane deformations of the aromatic ring. The deactivation mechanisms suggested in this work should provide some very efficient decay channels after excitation into the first UV absorption band of thiophene, and are good candidates to explain why this compound is non-fluorescent.

Keyword
Radiationless deactivation, Conical intersections, Heterocyclic compounds, Photophysics
National Category
Theoretical Chemistry Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-72436 (URN)10.1016/j.chemphys.2011.12.004 (DOI)000301125700003 ()
Note
1Available from: 2012-02-11 Created: 2012-02-11 Last updated: 2017-12-07Bibliographically approved
3. Radiationless deactivation and intermediate formation in photo-excited thiophene
Open this publication in new window or tab >>Radiationless deactivation and intermediate formation in photo-excited thiophene
(English)Manuscript (preprint) (Other academic)
National Category
Theoretical Chemistry Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-72437 (URN)
Available from: 2012-02-11 Created: 2012-02-11 Last updated: 2012-02-13Bibliographically approved
4. Mutual neutralization in low-energy H+ + H- collisions: A quantum ab initio study
Open this publication in new window or tab >>Mutual neutralization in low-energy H+ + H- collisions: A quantum ab initio study
2009 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 79, no 1, 012713- p.Article in journal (Refereed) Published
Abstract [en]

The mutual neutralization of H+ and H at low collision energies is studied by means of a molecular close-coupling approach. All degrees of freedom are treated at the full quantum level also taking into account the identity of the nuclei. The relevant 1Σg+ and 1Σu+ electronic states as well as the associated nonadiabatic radial couplings are calculated for internuclear distances between 0.5 and 50a0. Following a transformation into a strictly diabatic basis, these quantities enter into a set of coupled equations for the motion of the nuclei. Numerical solution of these equations allows the cross sections for neutralization into the H(1)+H(n), n=1,2,3 final states to be calculated. In the present paper, results are reported for the collision energy region 0.001–100 eV, with special emphasis on the important energy region below 10 eV. The low temperature rate coefficient is obtained from a parametrization of the calculated cross section and is estimated to be valid over the range 10–10 000 K.

National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-60378 (URN)10.1103/PhysRevA.79.012713 (DOI)000262979000094 ()
Available from: 2011-08-18 Created: 2011-08-16 Last updated: 2017-12-08Bibliographically approved
5. The role of the direct mechanism in dissociative recombination of HCO+ and HOC+
Open this publication in new window or tab >>The role of the direct mechanism in dissociative recombination of HCO+ and HOC+
(English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622Article in journal (Refereed) Submitted
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-72439 (URN)
Available from: 2012-02-11 Created: 2012-02-11 Last updated: 2017-12-07Bibliographically approved

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