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PREFACE

Since the late 1920s, most of the many thousands of publications contributingto quantum chemistry have dealt with issues and problems that essen-tially concern, or are applicable to, the ground or the low-lying discretestates of atoms and molecules and of electronic matter in general. In thiscontext, samples of topics that have been examined are many-faceted for-malisms, analysis and computation of various features of the many-electronproblem, computational methodologies and techniques, results of compu-tation of properties and of low-energy chemical reactions, computation ofspectroscopic data involving mainly discrete states, etc.

On the other hand, significant advances have also been made in thebroader domain of quantum chemistry, a prime example being areas ofresearch that involve the continuous spectrum and, as such, are more com-plex, conceptually, formally, and computationally. When the continuousspectrum of a quantum system acquires physical significance, a plethora ofspecial and challenging physical and mathematical features and questionsemerge that are absent in problems involving just the discrete spectrum.

In the variety of excitation or de-excitation processes that allow thepreparation and/or observation of the system via the participation of thecontinuous spectrum, the dominant and most interesting characteristics aregenerated by the transient formation of nonstationary or unstable states. Forexample, the excitation may be caused by the absorption of one or of manyphotons during the interaction of an initial atomic or molecular state withpulses of long or of short duration. Or, the transient formation and influenceon the observable quantity may occur during the course of electron–atomscattering or of chemical reactions.

In principle, the physics involving unstable states ought to engage descrip-tions that are time dependent. Yet, in the formulation and practical solutionof related problems, both time-dependent and time-independent treatmentsare pertinent and necessary. Furthermore, in certain theoretical approaches,the phenomenologies as well as the computational methodology are basedon constructions that are non-Hermitian. We add that the Hamiltonians may

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To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal businessuse only by the author(s), editor(s), reviewer(s), Elsevier and typesetter diacriTech. It is not allowed to publish this proof onlineor in print. This proof copy is the copyright property of the publisher and is confidential until formal publication.

AQC Preface-9780123970091 2011/12/13 12:37 Page viii #2

viii Preface

or may not include the coupling of atomic or molecular states to externalelectromagnetic fields.

The two volumes of Unstable States in the Continuous Spectra, which wehave edited (Part I is AQC volume 60 and Part II is the present volume,63), contain a total of 15 review articles on topics covered by the generaltheme. The invitation of the contributing experts had as one of its purposesto create a book on the above theme where the spectrum of the informationcontained in it is wide, authoritative, and relevant to quantum chemistry. Theinvited authors were free to choose their topic(s) and style of presentation.Before final acceptance, their manuscripts were subjected to “friendly yetcritical” review by referees suggested by the authors, aiming at improvingthe contents as much as possible.

The first volume contained nine state-of-the-art chapters on fundamen-tal aspects, on formalism, and on a variety of applications. The variousdiscussions employ both stationary and time-dependent frameworks, withHermitian and non-Hermitian Hamiltonian constructions. A variety of for-mal and computational results address themes from quantum and statisticalmechanics to the detailed analysis of time evolution of material or photonwave packets, from the difficult problem of combining advanced many-electron methods with properties of field-free and field-induced resonancesto the dynamics of molecular processes and coherence effects in strongelectromagnetic fields and strong laser pulses, from portrayals of novelphase space approaches of quantum reactive scattering to aspects of recentdevelopments related to quantum information processing.

The present volume of the Advances in Quantum Chemistry is the sequelof the first volume, mentioned above, i.e., Unstable States in the ContinuousSpectra, Part II: Interpretation, Theory and Applications. It contains six chap-ters with contents varying from a pedagogical introduction to the notion ofunstable states to the presence and role of resonances in chemical reactions,from discussions on the foundations of the theory to its relevance and pre-cise limitations in various fields, from electronic and positronic quasi-boundstates and their role in certain types of reactions to applications in the fieldof electronic decay in multiply charged molecules and clusters, as well.

Given the plurality of the aforementioned discussions in both volumes, wehope that both senior and young quantum chemists and physicists with aninterest in the specific theme of “unstable states in the continuous spectra”and in quantum theory, in general, will find the present set of two volumesresourceful, innovative, and helpful.

Cleanthes A. NicolaidesAthens, Greece

Erkki J. BrandasUppsala, Sweden