We have calculated the electronic structures of five different manganese-oxo dimer complexes using density functional methods combined with the broken symmetry and spin projection concepts. The number of carboxylate, oxo, and peroxo bridging ligands was varied, and the terminal ligands were triazacyclononane (TACN). The formal Mn oxidation states varied from Mn(III)(2) and Mn(III)Mn(IV) to Mn(IV)(2). These complexes have been synthesized and their X-ray structures and magnetic properties measured previously. We have calculated the Heisenberg spin coupling parameters J and resonance delocalization parameters B for all of these systems. Despite the very small energy differences involved, there is a good correspondence between calculated and experimental Heisenberg J parameters. We have analyzed potential changes in the calculated effective Heisenberg coupling J(eff) for the mixed-valence Mn(III)Mn(IV) complexes when partial or complete delocalization due to the B parameter is taken into account. These changes depend also on the energy of the relevant intervalence band. Surprisingly, in the two mixed-valence systems studied, the high spin S = (5)/(2) state lies below S = (7)/(2). This is consistent with spin coupling between Mn with site spins S(1) = 1, S(2) = (3)/(2), corresponding to intermediate spin Mn(I) and Mn(II) respectively, instead of the coupling expected from the formal oxidation states, S(1) = 2, S(2) = (3)/(2) from high spin Mn(III) and Mn(IV). The spin and charge distributions in the broken symmetry ground states are also consistent with intermediate spin S(1) = 1, S(2) = (3)/(2). The calculated charge distributions show strong metal-ligand covalency. In fact, as the formal oxidation states of the Mn sites increase, the net Mn charges generally show a slow decrease, consistent with a very strong ligand --> metal charge transfer, particularly from &mgr;-oxo or &mgr;-peroxo ligands. TACN is a better donor ligand than carboxylate, even when calculated on a per donor atom basis. The ligand atom charge transfer order is peroxo >/= oxo >> TACN > acetate. The TACN > acetate ordering is expected from the spectrochemical series, but the strong charge transfer and strong metal-ligand covalency of peroxo and oxo ligands with the Mn sites cannot be simply related to their positions in the spectrochemical series. In the Mn(IV)(2)(&mgr;-O)(2)(&mgr;-O(2))(TACN)(2), each peroxo oxygen has a small charge (-0.3), much less than found for each &mgr;-O atom (-0.7). The high-spin S = 3 state lies quite low in energy, 8 kcal/mol from our calculations and about 4 kcal/mol based on the experimental Heisenberg spin coupling parameters. Potential molecular oxygen dissociation pathways involving a spin S = 1 state are discussed. Effective ligand field diagrams are constructed from the calculated energy levels which display the competition between spin polarization splitting and the ligand field t(2g)-e(g) splitting and allow comparisons of electronic structure among different complexes. The electronic structure and spin couplin...

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Amplitude noise on the light from a semiconductor laser produced a photocurrent fluctuation spectrum that was a maximum of 85/o ( -8.3 dB) below the shot-noise limit. Squeezing in semiconductor lasers is not limited by the overall quantum, or current transfer, efticiency from the laser injection current to the detector photocurrent. Current leakage away from the lasing junction does not introduce Poissonian partition noise. PACS numbers: 42.50.Dv Photon-number eigenstates or Fock states of the electromagnetic field appear to be ideal for optical communication' and for a variety of high-precision measurements.It has been shown theoretically" that the amplitude fluctuation on the field from a pump-noisesuppressed laser can be reduced to below the standard quantum limit (SQL). In the case of a semiconductor laser, the pump noise is suppressed by a high-impedance constant-current source. Light from constant-currentdriven semiconductor lasers featuring squeezed amplitude fluctuation has been demonstrated. ' However, the modest degree of squeezing observed was ascribed to various deficiencies associated with the measurement system. In this experiment, we were able to reduce the eff'ects of optical feedback from the measurement system, and increase the light collection efficiency by direct "face-to-face" coupling of the laser and detector. Consequently we improved the noise reduction and further elucidated the basic physical processes that acct the noise properties of the device. The spectrum of photocurrent fluctuation was measured to be 85 -+7% below the shotnoise limit (SNL). Given that the detection quantum efficiency was approximately 89%, this corresponds to a squeezed amplitude fluctuation (at the front facet) of 96% ( -14 dB) below the SQL. This is by far the largest quantum noise reduction observed in any experiment; and it represents the closest approach so far to a number state. Apparently, amplitude squeezing in semiconductor lasers is not limited by the overall quantum efficiency but only by the optical output coupling efficiency. An explanation of this unexpected result is that current division does not introduce Poissonian partition noise, in contrast to the Poissonian partition noise associated with optical field division.Quadrature fluctuations of a number state, =~z =(2n+1)/4, are larger than the vacuum noise floor. This is in contrast to a quadrature squeezed state, which is a minimum-uncertainty state, where the fluctuation in the quadrature with reduced noise (A'i for example) is below that of the vacuum (~i~-, ' ). The field from a semiconductor laser is not in a minimum-V+ LED T LASER +A1 ZS D2 o RF oDC we, Rs V 50( 1K FIG. 1. The principal elements of the experimental ap-paratus that were inside the cryostat. The power spectrum of the amplified photocurrent is displayed on a spectrum analyzer (not shown), and the average detector current is monitored at the output labeled dc. uncertainty state; more importantly, the photon-number fluctuation may approach that of a number state. Removal of the re...

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The centroid of a single-transverse-mode laser beam fluctuates in position because of spontaneous emission of the laser medium into higher transverse cavity modes.

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