Nonlinearly limited saturable-absorber mode locking of an erbium fiber laser

We describe an erbium fiber laser that is passively mode locked by a novel, precision antireflection-coated semiconductor saturable-absorber mirror that incorporates an additional two-photon absorber. It is shown that passive mode locking evolves from a Q-switching instability. The results are achieved by use of saturable absorbers that provide a large (15%) nonlinear (saturable) loss. Exploiting two-photon absorption can substantially reduce the peak power of the Q-switched pulses, which results in improved reliability of the laser. Moreover, two-photon absorption can be used to produce an optimal stability range for saturable-absorber mode locking.     

The radial distribution of the 1f72 neutron in 41Ca from the 40Ca(13C, 12C)41Ca reaction

The reaction ⁴⁰Ca(¹³C, ¹²C)⁴¹Ca leading to the ground and low-lying $\text{3}\text{2}$^? levels has been studied at bombarding energies of 18.5 and 19.0 MeV, close to the Coulomb barrier. The cross sections have been analysed using the DWBA with previously measured calibrations to obtain the rms radii of 1f_{$\text{7}\text{2}$} and 2p_{$\text{3}\text{2}$} orbits in the ⁴¹Ca levels. The rms radius of the 1f_{$\text{7}\text{2}$} point neutron orbit in the ground state relative to the ⁴⁰Ca core was determined to be 3.89 ± 0.12 fm. This is compared with various theoretical predictions based on Hartree-Fock theory. The 1f_{$\text{7}\text{2}$} orbit radius gives the point neutron excess size to a good approximation for comparison with the value of 3.45 fm deduced from Coulomb energy differences. The consistency of the results with ⁴⁰Ca(t, d)⁴¹Ca experiments using a (t/d), zero-range normalization of 3.26 × 10⁴ MeV² · fm³ is demonstrated, and trends in neutron size parameters through the Ca isotopes are discussed.     

Line frequency expressions for triply degenerate fundamentals of spherical top molecules appropriate for large angular momentum

For states of high angular momentum we obtain expressions for the transition frequencies in the triply degenerate fundamental of a “heavy” spherical top, that are accurate to spectroscopic resolution, without tedious calculation of octahedral (or tetrahedral) vector coupling coefficients or diagonalization of matrices. Our calculations are based upon the physical assumption that as the molecule rotates with larger angular momentum it behaves like a symmetric top with the coupled pure rotational angular momentum quantized along either the four- or threefold body-fixed axes. We find that the approximate calculation which involves only 3-J symbols quite accurately reproduces the result of a complete diagonalization of the Hamiltonian, so that for all but the most accurate saturated absorption spectra theoretical analysis may be carried out without reference to the octahedral (or tetrahedral) subgroup of the rotation group.     

Single-crystal 55Mn NMR spectra of two Mn12 single-molecule magnets

The initial application is reported of single-crystal 55Mn NMR spectroscopy, and associated orientation dependence studies, to single-molecule magnets (SMMs). The studies were performed on two members of the Mn12 family of SMMs, [Mn12O12(O2CMe)16(H2O)4].2MeCO2H.4H2O (Mn12-Ac) and [Mn12O12(O2CCH2Br)16(H2O)4].4CH)Cl) (Mn12-BrAc). Single-crystal spectra give a dramatic improvement in the spectral resolution over oriented powder spectra, allowing the clear observation of quadrupolar splittings, the determination of quadrupole coupling parameters (e2qQ), and an assessment of the symmetry-lowering perturbation of the core of Mn12-Ac by hydrogen-bonding interactions with lattice solvate molecules of crystallization. The results emphasize the utility of single-crystal NMR studies to probe the cores of these magnetic molecules.