ALBERT

Notes: The magnetic behavior of Ni(SCN)2 has been studied at low temperatures for the first time. A Curie–Weiss fit, χM=C/(T−θ), to the susceptibility between 100 and 300 K yields g=2.13±0.01 (S=1) and θ=39.8±1.4 K. Systematic curvature in χ−1 vs T is evident, however. Despite the large positive θ Ni(SCN)2 appears to order antiferromagnetically at Tc=52±1 K, slightly below a maximum in χ(T) at T(χmax)=57.2±0.5 K, with χmax=0.0331±0.0003 emu/mol. The ratio Tc/T(χmax)=0.91±0.02 does not suggest lower magnetic dimensionality. Magnetization isotherms are linear to 16 kG; some features suggesting lower temperature transitions occur. Well above Tc the susceptibility is analyzed assuming axial and rhombic crystal field distortions, i.e., D[Sˆz2−S(S+1)/3] and E[Sˆx2−Sˆy2] spin Hamiltonian terms, with exchange incorporated in a mean field approximation. An extraordinarily large ||D/k||≈119 K seems to emerge, a result which is very provisional lacking single crystal data. A mean field analysis of Tc and θ yields ferromagnetic intrachain exchange J1/k=8.6±0.5 K and antiferromagnetic interchain exchange J2/k=−0.76±0.4 K. It seems more likely that D is negative, but even if it is positive the exchange interaction is large enough to induce magnetic order at finite Tc in light of theories relating Tc, J, and D.

Notes: The magnetic properties of Ni(SCN)2(C2H5OH)2 have been studied, the first Ni(II) system in the general family of compounds M(SCN)2(ROH)2 (where M=divalent Mn, Fe, Co, or Ni and R=CH3, C2H5, i- or n-C3H7) to be examined at low temperatures. In contrast to previously studied Mn(II) and Co(II) members of this family, which exhibit predominant antiferromagnetism, the present compound is ferromagnetic. The susceptibility of a polycrystalline sample is of Curie–Weiss form only above 75 K, with g¯ = 2.175 ± 0.01 and S=1 and with θ=24.1±1.0 K. The initial susceptibility is well accounted for by an asymptotic critical law, χ0 = Γ[T/Tc − 1]−γ, in the reduced temperature range 0.147–0.013, with Tc = 13.081 ± 0.01 K, γ=1.354±0.02, and Γ=0.0925±0.003 emu/mol. The γ value is between 3D-XY and 3D-Heisenberg model predictions. The susceptibility in the paramagnetic regime well above Tc is analyzed including the effects of axial and rhombic crystal field distortions, represented by D[Sˆ2z − S(S + 1)/3] and E[Sˆ2x − Sˆ2y] terms in the spin Hamiltonian, and incorporating exchange interactions in a mean-field approximation. An excellent fit is obtained with g=2.175, D/k=−64.3±5 K, E/k=23.1±4 K, and zJ/k=19.4±1 K. The magnitude of D is the largest we know of in a Ni(II) compound. The three spin states of the 3A crystal field ground term are strongly split, probably contributing to the relatively low value of Tc compared to zJ/k. Some two-dimensional character in the exchange, as has been found for other members of this family of compounds, may also contribute.

Notes: In situ growth of highly oriented YBa2Cu3O7−x thin films (200–500 nm in thickness) has been obtained by pulsed KrF (248 nm) laser ablation on both rigid and flexible randomly oriented polycrystalline yttria-stabilized zirconia substrates. It is shown that c-axis-perpendicular YBa2Cu3O7−x films with a mosaic spread of only 1.0° can be grown on these randomly oriented polycrystalline substrates. Superconducting thin films were obtained with Tc(R=0)∼89 K on well-polished substrates. For the films deposited on the flexible substrates, the superconducting Tc is not degraded by repeated bending of the flexible substrate/film composite over a 2.25-cm-radius arc although the normal-state resistivity increases slightly, suggesting the creation of microcracks. The YBa2Cu3O7−x films grown on rigid polycrystalline yttria-stabilized zirconia substrates have a critical current density Jc(H=0)∼1400 A/cm2 at 77 K.

Notes: Previous magnetization and susceptibility measurements on the mixed magnet Co1−xMnxCl2⋅2H2O disclosed a spin glass transition near 2.45 K over a wide composition range.1 Recent heat capacity and NMR measurements have confirmed and extended this finding.2 The time dependence of the thermoremanent magnetization (TRM) below Tg for an x=0.452 mixture was studied in some detail1 and was found to conform approximately to a decay of stretched exponential type, or perhaps slightly more accurately to the product of a stretched exponential and a power law. Small systematic deviations of data from fitted curves were apparent however. Recently a percolation model for relaxation in random systems was proposed,3 and yielded significantly improved fits to the TRM decay in an Au:Fe spin glass. The model assumes dispersive excitations within fixed finite domains, and includes as parameters the fastest and slowest relaxation rates characterizing the spectrum of domains. We find that this model also permits much better fits to be obtained for the TRM decay in Co1−xMnxCl2⋅2H2O, x=0.452. Systematic deviations that were present when using more traditional decay functions are virtually eliminated. The variation of fitted parameters with cooling field and temperature is also explored.

Notes: Time-dependent quantum-mechanical theories and simulations provide a clear and intuitive description of molecular processes. Due to ensuing simplification of the theory and the generally employed numerical algorithms, the vast majority of these treatments are based upon perturbation theory. Especially in light of the current level of experimental sophistication, with experiments being realized which are influenced by the spectral, temporal, and spatial shape of the laser pulse, it is important to move beyond treatments limited to weak fields or idealized δ-function wave forms. Various methods to examine the results of high-field simulations are presented. All of the techniques are shown to have the familiar linear response form in the weak-field limit. In a time-dependent framework the difference between the linear and nonlinear response expressions can be seen from expectation values over stationary versus nonstationary states. The high-field photodissociation of methyl iodide illustrates this approach. Methyl iodide represents a physical system well suited for examining the effects of such exciting laser-field characteristics as strength, linewidth, and frequency upon the photodissociation dynamics. Its dissociation occurs upon coupled repulsive excited electronic potential-energy surfaces which have recently been revised to fit the most current experimental data. The effect of the surface intersection has previously been typically studied by examining the branching and the internal state distributions of the products in the two channels as a function of excitation frequency only.The collinear photodissociation dynamics is examined using a numerically exact time-dependent quantum-mechanical method. The equations of motion for the amplitudes upon the ground and two coupled excited electronic surfaces, explicitly incorporating the laser field, are integrated by a scheme which employs a low-order polynomial approximation to the evolution operator. The effects of the three field characteristics upon the branching ratio and internal state distributions of the products and the spectroscopy of the process are delineated. The course of the photodissociation dynamics is shown to be affected by these characteristics. The results demonstrate the causal connections between the pulse shape and the resulting photoprocesses. Practical manifestations of strong fields (power broadening, sub-threshold absorption, higher harmonic generation, emission shaping of the ground state, temporal development) are stressed.

Notes: Photodissociation of vibrationally excited CH3I is studied using a time-dependent quantum mechanical formalism based on the fast Fourier transform (FFT) method. The dissociation dynamics is modeled with two active degrees of freedom, i.e., the dissociation coordinate and the C–H3 umbrella coordinate. The ground state vibrational wave functions are calculated using a time-dependent relaxation method proposed by Kosloff and Tal-Ezer. Two coupled excited states are explicitly considered in this model and the potential energy functions are taken from a previous study that was able to reproduce experiments for photodissociation of the CH3I ground state. We investigate the dissociation dynamics of the system after initial vibrational excitation, with particular attention paid to nonadiabatic transitions during the dissociation process. Our calculations show that vibrational excitation can significantly change the product I*/I branching ratio. In particular, it is found that there are significant dips in the I* yield at energies associated with minima in the absorption spectrum. These dips can be attributed to differences in Franck–Condon factors associated with the two excited state potential surfaces. Other observables of the dissociation process, such as the absorption spectrum and fragment vibrational state distributions, have also been investigated.

Notes: Advances in the time propagation of multidimensional wave packets are exploited to present the A-band photodissociation dynamics of methyl iodide for five active vibrational modes on the three relevant excited ab initio potential surfaces. The five modes considered represent all of the experimentally observed dynamical activity. The only modes neglected are the asymmetric C–H stretch and the asymmetric deformation of the methyl group. The kinetic energy operator corresponding to these five degrees of freedom is derived. The fully quantum mechanical calculation was implemented upon grids using 2880 distinct time-dependent configurations, determined by the multiconfigurational time-dependent Hartree algorithm, for each electronic state. All of the currently known experimental results regarding the umbrella vibration, symmetric C–H stretching vibration, perpendicular rotation, and parallel rotation of the photodissociated methyl radical fragment are well reproduced. The full wavelength dependence of all of these quantities is determined. The wavelength dependence of the energy deposited into translational, vibrational, and rotational motion is also given. The time evolution of the modes is presented in the context of correlated motion and its effect upon the dissociative process. Many of the details of the dynamics inherent to the conically intersecting nature of the excited surfaces is delineated. In particular it is shown that the Jahn–Teller distortion of the 1Q1 state is irrelevant in contributing to the perpendicular character of resonance Raman depolarization ratios. Results are compared and contrasted to previous calculations employing the collinear pseudotriatomic model with optimized empirical surfaces or the bent pseudotriatomic model with the same ab initio surfaces.