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High-resolution Fourier-transform photoluminescence spectroscopy combined with the resonant photoexcitation technique was used to study in detail the Zeeman eff'ect on excitons bound to the neutral donors in high purity epitaxial GaAs. The neutral donor g-factor depends on the magnetic-field intensity and orientation in agreement with the predictions of the five-level k p theory. The neutral-donor excited-state energies measured in a range of magnetic field from 0 to l2 T are in an excellent agreement with appropriately scaled calculations for the hydrogen atom. The correspondence between transitions having the same initial state but having as a final state either the nuetral-donor ground state (principal transitions) or a neutral donor excited state (two electron satellites) was established using resonant excitation and was verified by the angular dependences of the peak energies. Linear and circular polarizations of the 2p, 2po, and 2p+ two-electron satellites are consistent with the assignment of zero angular momentum to the ground state of the donor-bound exciton and we show that the transition energies for these components can be calculated with a 0.03-meV accuracy over the range from 0 to 12 T.

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The photoluminescence spectra from a number of high-quality GaAs single-quantum-well samples grown by molecular-beam epitaxy reveal a doublet emission having an energy separation of -1.25 meV. A similar doublet was observed in a sample for which the interrupted growth technique was used. Using excitation-intensity-dependent luminescence and time-resolved spectroscopy,~e will show that the lower-energy components of these doublets have diN'erent origins in di6'erent samples and can be attributed either to biexcitons or to impurity-bound excitons.Using low-temperature photoluminescence (PL) from a number of high-quality GaAs multiple-quantum-well samples grown by molecular-beam epitaxy (MBE), Miller and co-workers'2 first reported a double peak whose splitting was -1 meV. The high-energy peak was attributed to the n 1 heavy-hole-free-exciton transition. Based on the excitation intensity, temperature, and polarization dependencies of the lower-energy peak, they concluded that this transition was due to biexcitons with a binding energy of about Bb;," I meV. These results were also supported by Kleinman3 who, using the six-parameter wave function of Brinkman, Rice, and Bell, 4 calculated variationally the binding energy of the biexcitons and the bound excitons in GaAs quantum wells (QW) for various well thicknesses (L, ). This calculation contained no adjustable parameters other than the variational parameters and yields a value of Bb;,"0. 13 meV for bulk GaAs. For the GaAs QW it was found that the calculated biexciton binding energy obeyed Haynes's rule 5 6 and that in the two-dimensional (2D) limit the ratio of the biexciton binding energy over the exciton binding energy was 3-4 times larger than the corresponding quantity in the threedimensional (3D) case. The calculated value of Bb;,"was found to be in good agreement with the 1 meV splitting mentioned above for wells narrower than 25 nm (L, (25.0 nm). The fit was found not to be as good for the wider wells (L, )25. 0 nm).It has recently been proposed that interrupting the molecular-beam epitaxial growth momentarily when changing from one type of semiconductor layer to another leads to smoother interfaces.The low-temperature PL spectra of QW structures prepared with interrupted growth showed multiple peaks which have been interpreted as originating from within different smooth regions in the QW layer which differ in width by exactly one monolayer. However, it must be remembered that fine structure in low-temperature PL spectra may have other causes such as impurity transitions.Using this interrupted growth technique, one would expect the concentrations of residual impurities to increase at the interface which could give rise to additional structure in the lowtemperature PL. Kleinman3 has also estimated the binding energy of excitons bound to neutral donors for the GaAs QW. These values are slightly larger than Bh;,"and are also found to obey Haynes's rule. Nomura, Shimozaki, and Ishikis used PL measurements to study single quantum wells inhomogeneously doped with S...

Excitation spectra of chlorophyll-u (Chl-a) fluorescence in intact cells of CryptonzonaA ovatu, Chroonionas pauciplustidu and Chroornonus sulina were determined at 77 K. For all species thc excitation spectra for emission from Chl-a associated with photosystem I1 (PSII) showed increased Contributions by a carotenoid (493 nm) and phycobiliproteins, and decreased contributions by carotenoid (417 nm, SO5 nni) and Chi-a (445 nm) as compared to excitation spectra for emission from Chl-a associated with photosystem I (PSI). Excitation spectra of C . sulina and C . ovata showed an increased contribution by Chl-c2 to PSII Chl-a fluorescence emission. In all three species the absorbance band positions of Chi-u, as determined from the excitation spectra. were similar to those previously described in green plants. green algae and phycobilisome-containing organisms. Time-rcsolved 77 K fluorescence emission spectra of C. ovum and C. salinu showed successive emission from both phycoerythrin and Chl-cz, PSII Chl-a, and PSI Chl-a. C. puuciplustidu showed successive emission from phycocyanin, PSII Chl-a, and PSI Chl-(1. Spectral red-shifts with time were observed for the phycobiliprotein peaks in all three specics. The fluorescence decay of phycoerythrin in C . ovafa and C. salinu was faster than that of phycocyanin in C. pauciplasfida. The results are discussed in relation to the organization of the antenna pigments of PSI1 and PSI in the cryptophyte algae.