Silicon nitride films were deposited at low temperatures (245370 "Cj and high deposition rates @OO-1700 A/mm) by hot filament assisted chemical vapor deposition (HFCVD). Optical properties of these amorphous silicon nitride thin films have been extensively characterized by absorption, photoluminescence (PL), photoluminescence excitation, and electroluminescence measurements. The optical band gap of the films was varied between 2.43 and 4.74 eV by adjusting the flow rate of the disilane source gas. Three broad peaks at l&2.4, and 3.0 eV were observed in the PL spectra from these films. A simple qualitative model based on nitrogen and silicon dangling bonds adequately explains the observed PL features. The photoluminescence intensity observed in these films was S-10 times stronger than films de.posited by plasma enhanced chemical vapor deposition, under similar conditions. The high deposition rates obtained by HFCVD is believed to introduce a large number of these optically active defects. 0 199.5 Atnerican Zmstitme qf Physics.

Ab initio calculations have been made of possible excited electronic structure of the N-V center in diamond. Molecular-orbital basis states for a center of C 3v symmetry with nϭ2, 4, or 6 active electrons, which account fully for spin symmetries of the wave functions, were constructed to permit predictions of level structures, degeneracies, and splitting patterns under the action of several magnetic and nonmagnetic interactions. Detailed predictions of the resulting three models taking spin-orbit, spin-spin, strain, and Jahn-Teller interactions into account are given in the form of term diagrams.

A set of stilbene-substituted octasilicates [p-RStil(x)Ph(8-x)SiO(1.5)](8) (R = H, Me, MeO, Cl, NMe(2) and x = 5.3-8) and [o-MeStilSiO(1.5)](8) were prepared. Model compounds were also prepared including the corner and half cages: [p-MeStilSi(OEt)(3)], [p-Me(2)NStilSi(OSiMe(3))(3)], and [p-Me(2)NStilSi(O)(OSiMe)](4). These compounds were characterized by MALDI-TOF, TGA, FTIR, and (1)H NMR techniques. Their photophysical properties were characterized by UV-vis, two-photon absorption, and cathodoluminescence spectroscopy (on solid powders), including studies on the effects of solvent polarity and changes in concentration. These molecules are typically soluble, easily purified, and robust, showing T(d(5%)) > 400 degrees C in air. The full and partial cages all show UV-vis absorption spectra (in THF) identical to the spectrum of trans-stilbene, except for [o-MeStilSiO(1.5)](8), which exhibits an absorption spectrum blue-shifted from trans-stilbene. However, the partial cages show emissions that are red-shifted by approximately 20 nm, as found for stilbene-siloxane macrocycles, suggesting some interaction of the silicon center(s) with the stilbene pi* orbital in both the corner and half cages. In contrast, the emission spectra of the full cages show red-shifts of 60-100 nm. These large red-shifts are supported by density functional theoretical calculations and proposed to result from interactions of the stilbene pi* orbitals with a LUMO centered within the cage that has 4A(1) symmetry and involves contributions from all Si and oxygen atoms and the organic substituents. Given that this LUMO has 3-D symmetry, it appears that all of the stilbene units interact in the excited state, consistent with theoretical results, which show an increased red-shift with an increase in the functionalization of a single corner to functionalization of all eight corners with stilbene. In the case of the Me(2)N- derivatives, this interaction is primarily a charge-transfer interaction, as witnessed by the influence of solvent polarity on the emission behavior. More importantly, the two-photon absorption behavior is 2-3 times greater on a per p-Me(2)Nstilbene basis for the full cage than for the corner or half cages. Similar observations were made for p-NH(2)stilbenevinyl(8)OS cages, where the greater conjugation lengths led to even greater red-shifts (120 nm) and two-photon absorption cross sections. Cathodoluminescence studies done on [p-MeStilSiO(1.5)](8) or [p-MeStilOS](8) powders exhibit essentially the same emissions as seen in solution at high dilution. Given that only the emissions are greatly red-shifted in these molecules, whereas the ground-state UV-vis absorptions are not changed from trans-stilbene, except for the ortho derivative, which is blue-shifted 10 nm. It appears that the interactions are only in the excited state. Theoretical results show that the HOMO and LUMO states are always the pi and pi* states on the stilbene, which show very weak shifts with increasing degrees of functionalization, consistent with the small chan...

We applied a new phase-modulation technique for nonlinear laser spectroscopy with sub-Hz relative resolving power to measure fundamental relaxation processes of the N-V center in diamond. Complementary EPR experiments versus temperature establish the ground-state spin character in the absence of optical illumination and show that spin-lattice decay occurs via two-phonon processes involving the dominant vibrational mode. The combined results permit deduction of reliable fine-structure assignments for three states of the center and accurate values for zero-field intersystem crossing and spin-lattice relaxation rates from linewidths of individual resonances in the four-wave-mixing spectrum. PACS numbers: 71.55.Ht, 42.65.Ma, 76.30.Mi, 78.50.Ec Nearly degenerate four-wave-mixing (NDFWM) spectroscopy with 1-Hz resolution based on acousto-optic frequency-modulation techniques has previously been applied to frequency-domain measurements of slow relaxation processes in impurity-doped solids [1] and pointdefect systems [2]. This coherent spectroscopy has proven very useful for precise measurements of decay processes too slow for eA'ective signal averaging in real time.Here we introduce a new optical technique for the performance of NDFWM with much higher resolution (10 mHz), and apply it in concert with double-cavity EPR experiments to resolve the controversy over the energylevel scheme of the N-V center in diamond and to study system dynamics.We provide evidence for a hypothesized metastable state and demonstrate that precise measurements of intersystem crossing and spin-lattice relaxation rates can be obtained from linewidth measurements of individual, ultranarrow resonances in the fourwave-mixing spectrum recorded with sub-Hz resolution.The N-V center is a product of irradiation and annealing processes in diamond crystals containing nitrogen [3] and exhibits a zero-phonon line at 637 nm assigned by uniaxial stress measurements[4] to an 2 E electricdipole transition at a site of trigonal symmetry. It consists of substitutional-nitrogen-vacancy pairs oriented along equivalent (111) directions, and exhibits triplet spin resonance which was first attributed to a metastable excited state [5] because it required optical illumination in the N-V absorption band. This conclusion and even the existence of a metastable state was challenged recently in a series of papers reporting hole burning [6], optically detected magnetic resonance [7], and Raman heterodyne experiments [g], which indirectly suggested that the triplet state occurs in the ground-state manifold rather than the metastable manifold.However, these experiments were performed with optical illumination of the centers, and are consistent with alternative explanations based either on absorption at 637 nm by a metastable excited triplet population or on a third possible energy-level structure of the center. To examine these possibilities, the current work was undertaken to determine fine-3A 1E 'A -'E= I» nm g (1a& b) I 'A (c) FIG. I. Possible energy level schemes of the N-...

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