We have studied the pump-beam-wavelength (Xr) dependence of the 77-K photoreflectance (PR) of a GaAs/Gap 67Alp33As multiple-quantum-well structure. The PR signal as a function of kr is a new spectroscopy denoted as photoreflectance-excitation spectroscopy (PRE). Both PR and PRE exhibit features from the 1S exciton states as we11 as continuum transitions of the fundamental heavyand light-hole to conduction-band states, thus yielding exciton binding energies. The advantages of PRE are discussed. Optical techniques are extremely powerful tools for studying the fundamental nature of the quantum transitions in semiconductor quantum-well structures. These methods include absorption, ' photoconductivity, ' resonance Raman scattering, photoluminescence (PL), ' photoluminescence-excitation (PLE), ' ' and modulation spectroscopy. " Until recently, the predominant optical techniques were PL and PLE. Modulation spectroscopy, particularly the contactless method of photoreflectance (PR), ' ' is proving to be very useful since it produces sharp, derivativelike spectral features. Even at room temperature it is possible to observe all the allowed, confined transitions as well as symmetryforbidden features and unconfined states. ' ' ' In general, modulation spectroscopy can be performed over a wide range of temperatures (except for electrolyte electroreflectance"). Although features due to groundstate (1S) excitonic transitions have been reported in modulation spectroscopy, there has been, as yet, no direct evidence for excited exciton (or continuum) transitions in order to obtain exciton binding energies. Recently, low-temperature PLE studies have produced structures related to the 2S exciton (or continuum) states of the fundamental heavyand light-hole to conduction-band transitions, ' thus making it possible to deduce exciton binding energies. From a study of PR and photoreflectance-excitation spectroscopy (PRE) at 77 K from a GaAs/Gao 67Alo 33As multiple-quantum-well (MQW) structure we have found evidence for the continuum transitions. This has allowed us to determine, for the first time using modulation spectroscopy, the binding energies of the fundamental heavyand light-hole excitons. Also, in the process we have developed the new spectroscopy of PRE. In this article, we report a new spectroscopy based on an investigation of the pump-wavelength (X~) dependence of the 77-K PR from a GaAs/Ga067Alo 33As MQW. We denote the intensity of the PR spectrum as a function of k~( or the pump photon energy, hco~) as PRE. The PRE spectrum exhibits very strong resonances when Aco~c or-responds to the 1S exciton of 11H and 11L, where mnH(L) denote the rrtth conduction and nth valence subband of heavy-(H) or light-(L) hole character. The energies of these PRE peaks are found to be in excellent agreement with corresponding PR features. In addition, there are small high-energy features in PRE on each side of the fundamental exciton-creation peaks somewhat similar to recent PLE observations, ' which have been identified as the 2S (or continuum a...

When C 60 is photoexcited, a self-trapping exciton ͑STE͒ is formed: The bond structure is distorted from symmetry I h to D 5d while the states A 1u and A 2u are pulled into the energy gap from highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. A dynamical scheme is employed to simulate the relaxation process of STE. The evolution of both bond structure and electronic states shows that the relaxation time to form STE is about 100 fs. It should be noted that this relaxation time is much shorter than that of charge transfer in C 60 . The origin for this large difference is discussed.

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Samples with low-energy C60 ions implanted into porous silicon were fabricated with the ionized cluster beam deposition approach for improving the light emission of C60. Depth analysis by secondary-ion mass spectroscopy showed that C60 had been incorporated into porous silicon. The photoluminescence spectrum measured under excitation by an Ar+ laser (514.4 nm) at room temperature showed a large number of intense and well resolved fine-structure peaks. These features indicated the strong coupling of vibrational progressions with the electron states of C60, induced by the interaction between the C60 molecule and the nanometre-sized silicon particles.