Modern aspects of the Jahn-Teller effect theory and applications to molecular problems.

Vibrational and vibronic relaxation and related energy redistribution play an important role in many photoinduced processes of molecules. In the condensed phase, in which collision-induced relaxation is of particular relevance, these phenomena occur on a very rapid time scale between several tens of femtoseconds and 100 ps (1, 2). Spectroscopy with ultrashort laser pulses has been widely applied to elucidate the dynamics and physical mechanisms of intraand intermolecular energy redistribution (3, 4). Indeed, time resolved experiments have provided extensive information not accessible by other experimental techniques. In this article, we present a review on vibrational and vibronic relaxation of large molecules in liquids. Photoinduced redistribution processes of molecules, which consist typically of 20 or more atoms, are considered. We discuss. femtoand picosecond spectroscopy of the intramolecular redistribution and the intermolecular energy transfer from vibrationally hot molecules to their (cold) surrounding. We focus on measurements of vibrational and vibronic lifetimes, as well as on studies of energy redis­ tribution and transport. However, we will not consider dephasing of vibranic or vibrational states, relaxation processes connected to the

VIBRATIONAL AND VIBRONIC RELAlXATION OF LARGE POL-yrATOMIC MOLECULES IN LIQUIDS

Scientists have sought for over two decades to incorporate the necessary attributes of transparency, stability, and high nonlinear susceptibilities into optimized organic or organometallic chromophores for third-order nonlinear optical (NLO) applications. These investigations have provided an ever-expanding understanding of structure−function relationships for the second hyperpolarizability γ and the bulk third-order nonlinear optical susceptibility χ(3) in organic materials, which are reviewed herein. Contributing to this understanding are the studies of the third-order NLO properties displayed by an array of structurally related organic chromophores based on the conjugated carbon skeletons of hex-3-ene-1,5-diynes (1,2-diethynylethenes, DEEs) and 3,4-diethynylhex-3-ene-1,5-diynes (tetraethynylethenes, TEEs). A comprehensive series of donor (D) and/or acceptor (A) substituted derivatives of DEEs and TEEs has been measured by third-harmonic generation (THG) experiments, and the investigations on these one-...

Structure−Property Relationships in Third-Order Nonlinear Optical Chromophores

Two-photon absorption, the process by which an excited molecule or material is produced by the simultaneousabsorption of two photons, has been studied extensively in recent years, from both fundamental and applicationpoints of view. On one side, the field has been expanded with the measurement of two-photon absorptionspectra and cross sections for a wide range of conjugated molecules. In this contribution, we willreview the two-photon properties of some of these classes of materials, and we will discuss structure/propertyrelationships that have been developed from these investigations. On the other side, two-photon absorptionhas been exploited as a means to activate a variety of chemical and physical processes with sub-diffraction–limitedresolution, because three-dimensional spatial confinement of the excitation volume in a material irradiatedby a tightly focused laser beam can be achieved via two-photon or, more generally, multi-photon absorptionprocesses. This characteristic has led to applications in a number of technological fields, such asmicrofabrication and laser scanning fluorescence microscopy. Here, we will survey material systems thathave been developed to activate radical or cationic polymerization reactions, deprotection of functionalgroups, and singlet oxygen generation via two-photon excitation of one of the components in the system.

Two-Photon Absorbing Materials and Two-Photon-Induced Chemistry

Organic materials with large third-order nonlinearities in the near-infrared spectral regime are critical in the development of photonic devices to be utilized in all-optical signal processing. We have developed polymethine materials, specifically bisdioxaborine-terminated polymethine dyes, which possess large ultrafast third-order nonlinearities and low nonlinear loss all in the near-infrared spectral regime. An extended bisdioxaborine polymethine anion exhibited the largest value of γ (third-order microscopic nonlinearity) at 1.3 μm (|γ| = 5.7 × 10-32 esu) and showed no characteristics of symmetry breaking, unlike other polymethines of similar lengths. A neat film of this molecule maintained relatively low linear loss in the near-infrared and showed a large third-order macroscopic nonlinearity at 1.3 μm (|χ(3)| = 3.6 × 10-10 esu), with a temporal response of less than 8 ps. Furthermore, the real part of χ(3) was nearly an order-of-magnitude larger than the imaginary component. Consequently, this materia...

A. Lau

Papers

Third-order nonlinear polarizabilities of a homologous series of symmetric cyanines

Rapid Raman spectroscopy of the excited electronic state of chrysene by coherent anti-stokes Raman scattering

Picosecond time-resolved CARS spectroscopy of a mixed excited singlet state of diphenylhexatriene

Increase and saturation of the third order hyperpolarizabilities in homologous series of symmetric cyanines

Transient resonance cars study on the photoisomerization process of bis-dimethylaminoheptamethine perchlorate (BMC)