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Ultrafast photoinduced processes in polyatomic molecules: Electronic structure, dynamics and spectroscopy (in honour of Wolfgang Domcke)
Chemical Physics 347 (2008) 1–2
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Preface
Ultrafast photoinduced processes in polyatomic molecules: Electronic structure, dynamics and spectroscopy (in honour of Wolfgang Domcke)
The excitation of a molecular system by a laser pulse may induce a variety of different dynamical phenomena, including photophysical processes such as radiative or non-radiative electronic transitions, electron and energy transfer, as well as photochemical reactions such as dissociation, proton transfer, or isomerization. The understanding of the fundamental mechanisms underlying photoinduced dynamical processes in molecular systems has been a major goal of experimental and theoretical research in chemical physics and physical chemistry in recent decades. With traditional frequency-resolved spectroscopic methods the dynamics of molecular processes could only be inferred indirectly. The advent of time-resolved spectroscopy techniques that provide a ‘real-time’ picture has revolutionized our knowledge about dynamical processes in molecules. In particular, laser spectroscopy with ever shorter pulse lengths, currently reaching attosecond time scales, has revealed that many elementary processes in photoreactions in molecular systems occur on an ultrafast subpicosecond timescale. Examples of ultrafast photoinduced processes include the photodissociation of diatomic molecules, internal conversion processes in polyatomic molecules, often induced by conical intersections of potential-energy surfaces, as well as charge and energy transfer processes of molecular systems in the condensed phase. The application of non-linear femtosecond spectroscopy techniques has also shown that many of these reactions occur in a partly coherent fashion even in complex environments such as proteins. The combination of novel spectroscopy techniques 0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2008.03.029
employing ultrafast laser pulses with new theoretical methods, from both electronic structure theory and dynamics, has made it possible to unravel the mechanisms of many ultrafast processes at a fundamental level. In some cases even control over the reaction was achieved with carefully designed laser pulse schemes. The aim of this special issue is to present a timely overview of recent theoretical and experimental developments in the field of femtochemistry. Due to the enormous breadth of this field, it is not possible to cover all aspects. Motivated by the occasion of this special issue, the focus is somewhat biased to theoretical work. In an attempt to classify the different topics, the articles have been grouped into two sets. The first part is devoted to theoretical concepts and methodology. The topics discussed in this part include excited-state electronic structure methods and ‘direct’ approaches that combine electronic structure calculations ‘on the fly’ with a description of the dynamics of the nuclei. In recent years, it has been recognized that conical intersections of potential-energy surfaces are fundamental in many photophysical and photochemical processes. A number of articles discuss the concept and the theoretical treatment of conical intersections. Furthermore, several authors present new theoretical methods to simulate frequencyand time-resolved spectra in molecular systems. Contributions on strong field effects and quantum control scenarios, dissipative master equations, as well as electron-molecule resonance complexes conclude the first part of this special issue. The second part comprises articles that present theoretical and experimental state of the art investigations of the photophysics or photochemistry of different dynamical processes in a great variety of molecular systems. The systems investigated include a manifold of small molecules, nucleobases and base pairs, peptides, mixed-valence compounds, as well as large chromophores. Some contributions focus on gas phase processes, while others study systems in solution, proteins, or matrix environments. This special issue is dedicated to Professor Wolfgang Domcke on the occasion of his 60th birthday. His many important contributions to the field of chemical dynamics, and in particular femtochemistry, have had a lasting impact on the theoretical and experimental chemistry communities. Wolfgang’s scientific work stands out for its originality and high quality. His insightful view of the essential physics of a given problem and his precise theoretical treatments of complex chemical systems have influenced many of us. The topics covered in this special issue may serve as a partial list of fields where Wolfgang’s contributions have had a profound impact. The diversity of the articles in this volume
2
Preface / Chemical Physics 347 (2008) 1–2
directly reflects the impressive diversity of Wolfgang’s research in the area of chemical dynamics. His contributions range from fundamental theoretical work on dynamics and spectroscopy, in particular for non-adiabatic excited-state processes at conical intersections, over methodological aspects, such as the development of new theoretical approaches to describe chemical dynamics or the design of efficient ab inito diabatization schemes, to applications as diverse as the influence of relativistic Jahn-Teller effects on excited-state dynamics in small molecules, photochemical reactions in DNA bases and photoacids and the spectroscopy of the hydrated electron. Here, we only want to highlight a few of Wolfgang’s outstanding contributions to two fields that are closely related to the subject of this special issue: The development of new theoretical methods to describe chemical dynamics and spectroscopy and the concept of conical intersections and their importance for photochemical and photophysical processes in chemistry. His work in the former area includes the development of wave-packet, densitymatrix and path-integral methods for quantum dynamics of molecular systems in both gas and condensed phases. Based on a novel non-perturbative approach, he established the theory of time-resolved non-linear spectroscopy of multidimensional molecular systems with multiple vibronically coupled electronic states. In addition to photoinduced dynamical processes, he worked on numerous aspects of electron-molecule scattering, in particular the role of nuclear dynamics in low-energy resonant electron-molecule scattering and single molecule conductance. The timedependent version of the projection-operator approach which he developed in this context has provided a new dynamical perspective of electron-molecule resonance states. In his comprehensive and pioneering work on conical intersections, Wolfgang has covered many aspects of this topic, from the quantum-chemical characterization of photochemical reaction mechanisms, e.g., in nucleic acids, to the quantum-mechanical modeling of time-dependent dissipative dynamics at conical intersections and their manifestation in frequency- and time-resolved spectroscopy. Using well-chosen examples, he demonstrated the importance of conical intersections for the dynamics of ultrafast photophysical or photochemical processes. Most importantly, his work in this area, together with others, has established a paradigm shift in our understanding of excited-state dynamics. The traditional description of radiationless decay in terms of Fermi’s Golden
Rule has been largely supplanted by a picture in terms of wavepacket dynamics on coupled potential-energy surfaces, which can be determined by ab initio electronic structure calculations. The modern concept based on conical intersections has proven to be remarkably rich and predictive, as shown by Wolfgang’s extensive studies of nucleobases and photoacids. These studies revealed that photostability can be related directly to the energetic accessibility of conical intersections. Furthermore, one can control photostability by raising the energy of the relevant conical intersections, for example by appropriate chemical substitution. These ideas may have considerable relevance in understanding the evolution of DNA as Nature’s molecular carrier of the genetic code. As guest editors, we are grateful to the authors who contributed to this special issue. We thank the Editor, Professor G.-L. Hofacker, for his guidance and encouragement, as well as the staff of Elsevier for their support. Finally, but most importantly, we are deeply indebted to Wolfgang Domcke and hope that he will enjoy his special issue. M. Thoss Department Chemie, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany E-mail address: [email protected] G. Stock Institut für Physikalische und Theoretische Chemie, J.W. Goethe Universität, Max-von-Laue-Str. 7, 60438 Frankfurt, Germany E-mail address: [email protected] T.J. Martinez Department of Chemistry, University of Illinois, Urbana, Illinois 61801, USA E-mail address: [email protected]
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Preface
Ultrafast photoinduced processes in polyatomic molecules: Electronic structure, dynamics and spectroscopy (in honour of Wolfgang Domcke)
The excitation of a molecular system by a laser pulse may induce a variety of different dynamical phenomena, including photophysical processes such as radiative or non-radiative electronic transitions, electron and energy transfer, as well as photochemical reactions such as dissociation, proton transfer, or isomerization. The understanding of the fundamental mechanisms underlying photoinduced dynamical processes in molecular systems has been a major goal of experimental and theoretical research in chemical physics and physical chemistry in recent decades. With traditional frequency-resolved spectroscopic methods the dynamics of molecular processes could only be inferred indirectly. The advent of time-resolved spectroscopy techniques that provide a ‘real-time’ picture has revolutionized our knowledge about dynamical processes in molecules. In particular, laser spectroscopy with ever shorter pulse lengths, currently reaching attosecond time scales, has revealed that many elementary processes in photoreactions in molecular systems occur on an ultrafast subpicosecond timescale. Examples of ultrafast photoinduced processes include the photodissociation of diatomic molecules, internal conversion processes in polyatomic molecules, often induced by conical intersections of potential-energy surfaces, as well as charge and energy transfer processes of molecular systems in the condensed phase. The application of non-linear femtosecond spectroscopy techniques has also shown that many of these reactions occur in a partly coherent fashion even in complex environments such as proteins. The combination of novel spectroscopy techniques 0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2008.03.029
employing ultrafast laser pulses with new theoretical methods, from both electronic structure theory and dynamics, has made it possible to unravel the mechanisms of many ultrafast processes at a fundamental level. In some cases even control over the reaction was achieved with carefully designed laser pulse schemes. The aim of this special issue is to present a timely overview of recent theoretical and experimental developments in the field of femtochemistry. Due to the enormous breadth of this field, it is not possible to cover all aspects. Motivated by the occasion of this special issue, the focus is somewhat biased to theoretical work. In an attempt to classify the different topics, the articles have been grouped into two sets. The first part is devoted to theoretical concepts and methodology. The topics discussed in this part include excited-state electronic structure methods and ‘direct’ approaches that combine electronic structure calculations ‘on the fly’ with a description of the dynamics of the nuclei. In recent years, it has been recognized that conical intersections of potential-energy surfaces are fundamental in many photophysical and photochemical processes. A number of articles discuss the concept and the theoretical treatment of conical intersections. Furthermore, several authors present new theoretical methods to simulate frequencyand time-resolved spectra in molecular systems. Contributions on strong field effects and quantum control scenarios, dissipative master equations, as well as electron-molecule resonance complexes conclude the first part of this special issue. The second part comprises articles that present theoretical and experimental state of the art investigations of the photophysics or photochemistry of different dynamical processes in a great variety of molecular systems. The systems investigated include a manifold of small molecules, nucleobases and base pairs, peptides, mixed-valence compounds, as well as large chromophores. Some contributions focus on gas phase processes, while others study systems in solution, proteins, or matrix environments. This special issue is dedicated to Professor Wolfgang Domcke on the occasion of his 60th birthday. His many important contributions to the field of chemical dynamics, and in particular femtochemistry, have had a lasting impact on the theoretical and experimental chemistry communities. Wolfgang’s scientific work stands out for its originality and high quality. His insightful view of the essential physics of a given problem and his precise theoretical treatments of complex chemical systems have influenced many of us. The topics covered in this special issue may serve as a partial list of fields where Wolfgang’s contributions have had a profound impact. The diversity of the articles in this volume
2
Preface / Chemical Physics 347 (2008) 1–2
directly reflects the impressive diversity of Wolfgang’s research in the area of chemical dynamics. His contributions range from fundamental theoretical work on dynamics and spectroscopy, in particular for non-adiabatic excited-state processes at conical intersections, over methodological aspects, such as the development of new theoretical approaches to describe chemical dynamics or the design of efficient ab inito diabatization schemes, to applications as diverse as the influence of relativistic Jahn-Teller effects on excited-state dynamics in small molecules, photochemical reactions in DNA bases and photoacids and the spectroscopy of the hydrated electron. Here, we only want to highlight a few of Wolfgang’s outstanding contributions to two fields that are closely related to the subject of this special issue: The development of new theoretical methods to describe chemical dynamics and spectroscopy and the concept of conical intersections and their importance for photochemical and photophysical processes in chemistry. His work in the former area includes the development of wave-packet, densitymatrix and path-integral methods for quantum dynamics of molecular systems in both gas and condensed phases. Based on a novel non-perturbative approach, he established the theory of time-resolved non-linear spectroscopy of multidimensional molecular systems with multiple vibronically coupled electronic states. In addition to photoinduced dynamical processes, he worked on numerous aspects of electron-molecule scattering, in particular the role of nuclear dynamics in low-energy resonant electron-molecule scattering and single molecule conductance. The timedependent version of the projection-operator approach which he developed in this context has provided a new dynamical perspective of electron-molecule resonance states. In his comprehensive and pioneering work on conical intersections, Wolfgang has covered many aspects of this topic, from the quantum-chemical characterization of photochemical reaction mechanisms, e.g., in nucleic acids, to the quantum-mechanical modeling of time-dependent dissipative dynamics at conical intersections and their manifestation in frequency- and time-resolved spectroscopy. Using well-chosen examples, he demonstrated the importance of conical intersections for the dynamics of ultrafast photophysical or photochemical processes. Most importantly, his work in this area, together with others, has established a paradigm shift in our understanding of excited-state dynamics. The traditional description of radiationless decay in terms of Fermi’s Golden
Rule has been largely supplanted by a picture in terms of wavepacket dynamics on coupled potential-energy surfaces, which can be determined by ab initio electronic structure calculations. The modern concept based on conical intersections has proven to be remarkably rich and predictive, as shown by Wolfgang’s extensive studies of nucleobases and photoacids. These studies revealed that photostability can be related directly to the energetic accessibility of conical intersections. Furthermore, one can control photostability by raising the energy of the relevant conical intersections, for example by appropriate chemical substitution. These ideas may have considerable relevance in understanding the evolution of DNA as Nature’s molecular carrier of the genetic code. As guest editors, we are grateful to the authors who contributed to this special issue. We thank the Editor, Professor G.-L. Hofacker, for his guidance and encouragement, as well as the staff of Elsevier for their support. Finally, but most importantly, we are deeply indebted to Wolfgang Domcke and hope that he will enjoy his special issue. M. Thoss Department Chemie, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany E-mail address: [email protected] G. Stock Institut für Physikalische und Theoretische Chemie, J.W. Goethe Universität, Max-von-Laue-Str. 7, 60438 Frankfurt, Germany E-mail address: [email protected] T.J. Martinez Department of Chemistry, University of Illinois, Urbana, Illinois 61801, USA E-mail address: [email protected]