PrefaceShaul Mukamel, Robin M. Hochstrasser
Pulsed laser techniques have made giant strides in recent years with the result that the complete control of optical and infrared electric ®elds is now possible: the shapes, frequencies, timing and pha- ses of laser pulses now can be manipulated in ways that were recently thought to be possible only for radio waves. The feasibility of impulsive excita- tions, which require laser pulses shorter than the corresponding nuclear motion periods has opened up new possibilities of conducting multiple-pulse coherent measurements for disentangling complex electronic and nuclear motions. This Special Issue covers recent experimental and theoretical ad- vances in this rapidly developing field.
One of the major accomplishments of nuclear magnetic resonance (NMR) has been its capacity to disentangle hopelessly complicated spectra by spreading them into more dimensions onto several frequency or time axes. Carefully designed se- quences of radio frequency (RF) pulses induce coherence transfers that make it possible to expose coupling between the spins on different nuclei or to eliminate dipolar interactions which dominate the line widths in solid-state NMR. As a result, NMR has evolved into an immensely powerful tool with one of its major accomplishments being the atomic scale resolution of the structures and dynamics of medium sized proteins in solutions under am- bient conditions. Magnetic resonance is the elder brother of coherent laser spectroscopy, and the two share many basic concepts. The ability to shape and control RF pulses predated similar ad- vances in short pulse laser technology by many decades, thus many of the optical and infrared nonlinear techniques have their analogues in these early NMR developments. Extension of multidi- mensional spectroscopy to the visible, infrared and far infrared regimes leads to novel classes of spectroscopies which can also probe complex molecular structures and their motions. Linear spectroscopies such as optical or infrared absorp- tion, and spontaneous Raman scattering give a one-dimensional (1D) projection of electronic and nuclear interactions onto a single frequency (or time) axis. For simple molecules with well-sepa- rated eigenstates these spectroscopic experiments give direct information on energy levels and absorption cross-sections. The situation is very different in complex molecules with strongly con- gested levels. Here the microscopic information is highly averaged, and is often totally buried under broad, featureless line shapes, whose precise in- terpretation often remains a mystery. Multiple- pulse techniques have the capacity to prepare electronic and vibrational degrees of freedom in nonequilibrium states and monitor their subse- quent evolution, yielding ultrafast snapshots of dynamical events such as energy transfer path- ways, charge transfer, photoisomerization, and structural Żuctuations.
The time-window and information content of electronic and vibrational multidimensional techniques are very different than in NMR. For example a sequence of IR pulses will transfer co- herences amongst the components of a network of vibrators. This network has much faster and dif- ferent dynamical properties from the network of spins that is interrogated by a sequence of RF pulses in NMR. The assumptions and theoretical techniques necessary to convert mid IR or optical data into the relevant structural and dynamical information are fundamentally different from those invoked in NMR. However, such multidi- mensional techniques provide a wealth of new information beyond that obtained from linear spectroscopy making them essential for structural and dynamical diagnostics. The new peaks repre- senting the couplings between excitations, the distributions of structures in the equilibrium en- semble, the intensities of the signals and their line shapes present direct signatures of molecular structure through distances and angular relations between chromophores or vibrators. They expose also the dynamics of electronic or nuclear excita- tions through the spectral density of their local environments. Amongst systems already studied are aggregates of chromophores, hydrogen bonded complexes and liquids, the secondary and tertiary structure of polypeptides, and protein conforma- tional dynamics. Doubtless this list will soon ex- pand signiifcantly.
The multiple-pulse infrared and optical laser experiments have generally involved the controlled transfer of coherences each representing the vari- ous vibrational or electronic Bohr frequencies that characterize the structure of the system. The abil- ity to manipulate coherences makes multidimen- sional techniques particularly suitable for coherent control of chemical systems as well as to quantum computing applications which involve the entan- glement of various degrees of freedom.
PrefaceShaul Mukamel, Daniel S. Chemla
The dynamics of confined electronic excitations is an important fundamental problem which cuts across all types of materials: semiconductors, molecular, metals.
Recent progress in the fabrication of semiconductor and molecular nanotrsuctures has made it possible to control the degree of confinement, thus creating two-dimensional (quantum wells), one-dimensional (quantum wires) and zero-dimensional (quantum dots) structures. In addition, significant advances in laser spectroscopy and particularly the development of ultrafast techniques have made it possible to probe electronic excitonic motions with great accuracy.
The nature of the elementary excitations is strongly affected by confinement. This results in spectral shifts, the redistribution of oscillation strength, and cooperative response which is reflected in the radiation lifetime as well as the magnitude of optical nonlinearities. Optical excitations result in the creation of electron-hole pairs. In molecular crystals they are tightly bound and form Frenkel excitons. In semiconductors they are loosely bound and form hydrogen-atom-like quasi-particles, the Wannier excitons. Charge transfer excitons are intermediate: the electron and hole are separated but still tightly bound. Conjugated polymers bear close resemblance to charge transfer excitons. In describing Wannier excitons we need to consider both the electron-hole center of mass and their relative motion. In Frenkel excitons the relative motion is frozen and we only need the center of mass coordinate.
Apart from the basic interest in finite size effects and the interpolation between microscopic and macroscopic phenomena, the studies of confined systems have important technological implications. Energy and charge transfer processes in molecular aggregates control key biological events such as photosynthesis, as well as color photography. Nonlinear optical materials with large optical susceptibilities with fast switching times are crucial for many devices. Controlling the fluorescence yield and spectral lineshape is key to making electroluminescent displays.
This issue covers recent developments in the studies of a broad range of systems. These include semiconductor quantum wells, magnetically confined excitons, semiconductor nanocrystals, molecular J aggregates, biological complexes (the photosynthetic antennae and the reaction center), molecular superlattices, conjugated polymers and metal clusters. In addition to experimental works, the present issue contains recent theoretical developments which address the many-body aspects of optical properties of confined excitons.
While these diverse systems share many common basic problems, interaction between researchers in these various fields is limited by a serious communication barrier. Molecular systems are usually described using the global many-electron eigenstates, whereas semiconductors are treated within a band theory using the electron-hole picture. These different viewpoints are to a large extent historical. If we look at many observables such as pump-probe and hole-burning spectra in semiconductors and molecular aggregates, they look remarkably similar. Yet the terminology used in their interpretation is completely different. Conjugated polyenes are an important class of systems that have been treated by different researchers using both viewpoints with little attempt to bridge the gap. This special issue will hopefully make a contribution towards resolving these difficulties and provide a common language for these fields.
Rochester Symposium on Charge Transfer in Restricted Geometries
Center for Photoinduced Charge Transfer
Rochester, New York
July 10-13, 1991
Department of Chemistry, University of Rochester, Rochester, New York 14627
This issue of The Journal of Physical Chemistry contains a selection of papers presented at and related to the Conference on Charge Transfer in Restricted Geometries held at the Center for Photoinduced Charge Transfer at the University of Rochester, July 10-13, 1991.
The Center for Photoinduced Charge Transfer is part of the Science and Technology Centers program sponsored by the National Science Foundation and has been in operation since February 1989. The Centers mission is to carry out interdisciplinary research on fundamental and technologically significant problems. The Center supports research programs aimed toward processes which are of fundamental importance in the areas as diverse as imaging science (e.g., conventional photography and electrophotography), surface science, and biological systems (e.g., photosynthesis). Research scientists study the factors controlling the dynamics and energetics of electron and hole transfer, including theoretical work on solvation, aggregation, and other phenomena related to the movement of positive and negative charges, as well as the study of chemical transformations which are induced by electron-transfer processes. The research projects at the Center are carried out in close collaboration between scientists at the University of Rochester, the Eastman Kodak Company, and Xerox Corp.
An annual conference is part of the activities at the Center. The first symposium on Photoinduced Charge Transfer was held in June 1990, and its proceeding were published in a special issue of Molecular Crystals and Liquid Crystals (1991, 194). The present, second, symposium had 172-registered participants including scientists from U.S.A., Japan, Germany, USSR, France, The Netherlands, Canada, and the People's Republic of China. The conference was chaired by Shaul Mukamel, University of Rochester. The organizing committee included R. J. Dwayne Miller of the University of Rochester, Esther Conwell and Thomas Orlowski of Xerox Webster Research Center, and Alfred Marchetti and Annabel Muenter of Eastman Kodak Co.
Geometrical confinement has a profound effect on the nature of electronic states and consequently on electron transfer and transport processes. Recent rapid progress in the fabrication of nanostructures has made it possible to study these phenomena with a remarkable spatial and temporal resolution. A unified picture of organic and inorganic (semiconductor) microstructures is emerging out of these developments. The symposium covered in depth the experimental and theoretical developments in this field. The main topics included molecular monolayers and multilayers, molecular clusters, semiconductor quantum dots, quantum wires and quantum wells, conjugated donor acceptor structures, the effects of electron delocalization on optical nonlinearities, charge transfer at surfaces, and photoconductivity.
The conference was interdisciplinary bringing together chemists, physicists, biologists, chemical and electrical engineers, and material scientists from academic institutions and industrial labs. It was divided into six sessions of oral presentations. Its format was designed to encourage exchange of ideas and cross-fertilization among the various disciplines: An extended discussion period at the end of each session provided an overview of the current status and future directions of these research areas. The provocative comments of the discussion leaders were instrumental in these lively discussions. In addition, the conference had a poster session consisting of 34 posters. The following is a list of session titles, chairpersons, speakers, and discussion leaders.
I. Surfaces: Chairman, B. Kohler, University of California at Berkeley, K. G. Hancock, Natioanl Science Foundation, "The Role of the Science and Technology Centers in the Social Fabric of Chemistry"; J. C. Polanyi, University of Toronto, "The photochemistry of Adsorbates with Special Reference to the Harpooning of Fixed Targets"; J. C. Tully, AT&T Bell Labs, "Chemistry at Metal Surfaces: Molecular Dynamics with Electronic Transitions"; K. B. Eisenthal, Columbia University, "Laser Chemistry at Liquid Interfaces"; K. Yoshihara, Institute for Molecular Science, Okazaki, "Femtosecond Electron Transfer in Diffusionless Weakly Polar Systems"; R. Hochstrasser, University of Pennsylvania, "Spectroscopic and Dynamical Studies of Excitations on Polysilane Chains".
The generous support of The National Science Foundation, The Office of Naval Research, The University of Rochester, Eastman Kodak Co., and Xerox Corp. is greatly appreciated. I would like to extend special thanks to Peter Schmidt, Peter Reynolds, and Michael Shlesinger of the ONR for their efforts which made this participation of many graduate students and young scientists possible. The enthusiasm and efforts of Mostafa El Sayed and Welford Castleman, Jr., who arranged for this special issue and its timely publication, are most appreciated.