CdSe is used as a prototype to show the implications of valence-band degeneracy for the optical properties of strongly quantum-confined nanocrystals. Absorption spectra and photoluminescence spectra obtained under intermediate and strong pulsed excitation show the presence of new structures. The energy levels for the electron and the hole are calculated with the spherical confinement, the nonparabolicity of the conduction band, and the valence band degeneracy taken into account. The oscillator strengths of the dipole-allowed transitions are also calculated. This model is found to be in good agreement with the experimental observations, which originate mainly from the quantization of the energy spectrum of holes with due account given to valence-band degeneracy.

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A pump-probe experiment is described to study femtosecond dynamics of hydrogen bonds in liquid water. The key element of the experimental setup is a laser source emitting 150 fs pulses in the 2.5 -4.4 mm spectral region, at a 10 mJ power level. The OH-stretching band is recorded for different excitation frequencies and different pump-probe delay times. Time-dependent solvatochromic shifts are observed and are interpreted with the help of statistical mechanics of nonlinear optical processes. Using these spectral data, the OH · · · O motions are "photographed" in real time. [S0031-9007(98)08298-2] PACS numbers: 82.20.Wt, 78.47. + pElementary chemical processes in gases and liquids take place on time scales ranging from 10 fs to 10 ps. It took ultrafast laser technology over 20 years to reach these scales. An important breakthrough was accomplished when Zewail and his colleagues, applying laser spectroscopic methods, succeeded in following the photochemical dissociation of gaseous iodine cyanide (ICN) in real time [1], producing in this way a "photographic" picture of the process. A number of related studies were published later; see, e.g., . Although essential, this was only the first step; thermally activated chemical reactions in liquids necessitate further studies. They require the availability of femtosecond infrared sources, the knowledge of statistical mechanics of nonlinear optical processes, and that of large scale quantum molecular dynamics simulations.Published work on femtosecond dynamics of thermally activated processes mainly refers to an isotopic variety of liquid water, the diluted solution HDO͞D 2 O. The following directions of research were explored: (i) Pumpprobe experiments were realized by Graener, Seifert, and Laubereau at 10 ps [5] and by Laenen, Rauscher, and Laubereau at 1 ps [6] time scales. Hole burning of the OH-stretching band was observed for the first time and a population relaxation time of 1.5 ps was inferred. (ii) Another midinfrared pump-probe study of HDO dissolved in D 2 O, realized with polarization resolution at 250 fs time scales, was published by Woutersen, Emmerichs, and Bakker [7]. The orientational relaxation of HDO was found to be biexponential, with time constants of 0.7 and 13 ps. (iii) Theoretical work on this subject, mainly due to Bratos and Leicknam [8,9], employed the correlation function approach of statistical mechanics of nonlinear optical processes. A method was proposed to study OH · · · O motions of water in real time.The purpose of this paper is to realize the experiment suggested in Ref.[9], "photographing" H-bond motions in this way. The principle of the experiment is as follows.Its key ingredient is the well-known relation between the OH-stretching frequency and the length of an H bond; it is described in Refs. [10,11] and is illustrated in Fig. 1. Pumping the HDO͞D 2 O solution at a given frequency V 1 thus results in selecting the OH · · · O bonds of a specified length; an alternative statement is that the pump pulse generates a coherently excited wave ...

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We review nonlinear optical processes that are specific to chiral molecules in solution and on surfaces. In contrast to conventional natural optical activity phenomena, which depend linearly on the electric field strength of the optical field, we discuss how optical processes that are nonlinear (quadratic, cubic, and quartic) functions of the electromagnetic field strength may probe optically active centers and chiral vibrations. We show that nonlinear techniques open entirely new ways of exploring chirality in chemical and biological systems: The cubic processes give rise to nonlinear circular dichroism and nonlinear optical rotation and make it possible to observe dynamic chiral processes at ultrafast time scales. The quadratic second-harmonic and sum-frequency-generation phenomena and the quartic processes may arise entirely in the electric-dipole approximation and do not require the use of circularly polarized light to detect chirality. They provide surface selectivity and their observables can be relatively much larger than in linear optical activity. These processes also give rise to the generation of light at a new color, and in liquids this frequency conversion only occurs if the solution is optically active. We survey recent chiral nonlinear optical experiments and give examples of their application to problems of biophysical interest.

We performed Hyper-Rayleigh Scattering (HRS) experiments to measure the second-order nonlinear optical response of the collagen triple helix and determine the physical origin of second harmonic signals observed in collagenous tissues. HRS experiments yielded a second-order hyperpolarizability of 1.25 x 10(-27) esu for rat-tail type I collagen, a surprisingly large value considering that collagen presents no strong harmonophore in its amino acid sequence. Polarization-resolved experiments showed intramolecular coherent contributions to the HRS signal along with incoherent contributions that are the only contributions for molecules with dimensions much smaller than the excitation wavelength. We therefore modeled the effective second-order hyperpolarizability of the 290 nm long collagen triple helix by summing coherently the nonlinear response of well-aligned moieties along the triple helix axis. This model was confirmed by HRS measurements after denaturation of the collagen triple helix and for a collagen-like short model peptide [(Pro-Pro-Gly)(10)](3). We concluded that the large collagen nonlinear response originates in the tight alignment of a large number of small and weakly efficient harmonophores, presumably the peptide bonds, resulting in a coherent amplification of the nonlinear signal.