ALBERT

Notes: The electron barrier transmission probability, T, is calculated for planar-doped potential barrier devices. The calculations are based on the analytic expression derived by Christodoulides et al. obtained from the exact solution to the Schrödinger time-independent equation. The original Airy function solution was recast in Bessel function form for ease of computer evaluations. The barrier is assumed to be triangular with the forward slope S1, much larger than the reverse slope S2. The quantity T is calculated for values of electron energy, E, above and below the barrier peak, Um. It is found that T is a sensitive function of S1 for all values of E. The reverse slope, S2, is observed to have very little effect on T for values of (E−Um)≥10 meV, but have a large effect on T for values of (E−Um)≤0. The quantity T is observed to increase monotonically with increasing E, for practical values of device parameters. These results are in qualitative agreement with those found from the numerical solution of Chandra and Eastman. However, their results give values of T that deviate significantly from those obtained in the present work.

Notes: The equilibrium structures, dipole moments, harmonic vibrational frequencies, and infrared intensities of NH3, FON, Be3, BeC2, and BeO2 have been determined using the coupled-cluster and Brueckner electron correlation methods. The singles and doubles coupled-cluster (CCSD) and the Brueckner doubles (BD) methods have been employed and the corresponding methods that include a perturbational estimate of connected triple excitations [i.e., CCSD(T) and BD(T)] have also been investigated. The T1 diagnostic [Int. J. Quantum Chem. Symp. 23, 199 (1989)] is found to provide a good indication of the magnitude of the difference between the results obtained with the coupled-cluster and Brueckner methods. For NH3, the T1 diagnostic is small and so the differences between results obtained from coupled-cluster and Brueckner theories are quite small. For the other four molecules the T1 diagnostic is larger, and so the differences between the coupled-cluster and Brueckner methods become larger. However, it is found for all of the molecules considered in this study that inclusion of the contribution from connected triple excitations is more important than the differences between the Brueckner and coupled-cluster correlation methods.

Notes: Existing methods for the calculation of the correlation energy of open shell atoms and molecules using Møller–Plesset perturbation theory are discussed with special emphasis on recent advances in methods based on the restricted Hartree–Fock wave function. The convergence of the Møller–Plesset series is examined to high order for a number of small systems and it is shown that some of the methods offer significant advantage over traditional unrestricted Møller–Plesset theory. The optimized bond length and vibrational frequencies of CN, NO, and O2 are also compared for second and fourth order Møller–Plesset theory, and again it is found that the same methods give much better results than the corresponding unrestricted results. Further supporting calculations are presented on the electron affinity of CN and the dissociation energy of HCN. The new methods involve relatively minor adaptation of existing unrestricted Møller–Plesset (UMP) codes, and in the light of their superior performance are recommended instead of UMP theory for open shell molecules.

Notes: The equilibrium structure of HCN has been determined from the previously published ground state rotational constants of eight isotopomers by using (B0-Be) values obtained from a variational calculation of the vibration–rotation spectrum. The results are re(CH)=1.065 01(8) A(ring), and re(CN)=1.153 24(2) A(ring).

Notes: The distributions of metastable excited state (4F7/2) and ground state (4F9/2) Rh atoms ejected from Ar+-bombarded Rh{100} are experimentally determined as a function of ejection velocity and angle. Corresponding theoretical predictions are made by incorporating a nonradiative deexcitation model into molecular dynamics simulations of the bombardment process. There is good agreement between the experimental and theoretical distributions. The simulations show that a fraction of the ejected atoms are excited via collisions 1–20 A(ring) above the surface, and that these atoms make a significant contribution to the excited atom yield at low ejection velocities.

Notes: Infrared absorption spectra of jet-cooled CH3Mn(CO)5 and CD3Mn(CO)5 in the region of the two strongly allowed CO vibrations have been measured using diode lasers. Analysis of the spectra yielded values for the band centers and rotational constants for parallel bands of both isotopic species, and for the perpendicular band of CH3Mn(CO)5; the latter also provided an estimate of the Coriolis constant for this vibration. The spectra were consistent with essentially free rotation of the methyl group. The rotational constants were in close agreement with those derived from electron diffraction data: however, the B-rotational constants provide a revised estimate for the axial Mn–CO bond length.

Notes: This is the fourth in a series of papers on the ab initio calculation of the third and fourth derivatives of the energy of a molecule. In this paper we examine anharmonic effects in the infrared and Raman spectra of benzene. The following spectroscopic properties have been calculated; ab initio anharmonic corrections (ω−ν) and estimates of the harmonic frequencies ω for all 30 vibrational modes of C6H6 and C6D6, a complete set of anharmonic constants x and g for C6H6, intensities for the infrared spectrum of C6H6 up to 6148 cm−1, and anharmonic corrections to the Raman scattering factors for the fundamental modes of C6H6. In addition, we have improved on previous calculations of the equilibrium geometry of benzene, using Møller–Plesset perturbation theory and a triple zeta plus double polarization (TZ2P) basis. We have also calculated a zero-point vibrationally averaged geometry which is in good agreement with the experimental R0 value. All these calculations are based on a Hartree–Fock quartic potential, cubic dipole surface, and quadratic polarizability surface, using a double-zeta plus polarization (DZP) basis. This is the first time a complete anharmonic potential has been obtained for a molecule of this size; the computer time required was minimized by the use of analytic derivative programs in favor of finite-difference programs. The quartic potential is presented in three coordinate systems.We discuss efficient algorithms for the nonlinear transformation of the potential from normal coordinates to valence coordinates and for symmetry checking the potential. The approximations used in our calculations have been examined and we find that the use of a Hartree–Fock DZP potential together with a perturbative treatment of the vibrational Hamiltonian is just as accurate for D6h benzene as for smaller molecules. In order to examine correlation effects in the B2u modes 14 and 15, basis-set limit second-order Møller–Plesset TZ2P+f harmonic frequencies have been calculated for these modes. It is suggested that, while these modes are very sensitive to correlation, anharmonicity has only a small effect, so a Hartree–Fock DZP anharmonic potential is adequate. Furthermore, experimental determination of anharmonic corrections to frequencies is very difficult for a molecule of this size so we hope our calculations will fill this gap.

Notes: Three models for the relaxation kinetics of a reversible unimolecular isomerization reaction are formulated and analyzed: a generalization of the simple Lindemann–Hinshelwood scheme, a detailed model with the strong collision approximation, and a master equation solution. For such systems the use of a classical relaxation analysis has been questioned. In each case it is found that the relaxation analysis does not give forward and reverse rate constants appropriate to the pure irreversible reactions, but that the rate constants so obtained can be interpreted in terms of irreversible schemes which allow for back reaction before collisional stabilization. The accuracy of this decomposition is linked with the applicability of the steady-state approximation for the populations of the reactive states, as is demonstrated analytically under the strong collision approximation, and numerically with the full master equation. An alternative approach using perturbation theory is shown to be unacceptably inaccurate.

Notes: Low energy helium diffraction has been used to determine the unit mesh parameters of overlayers of CH3Br physisorbed on C(0001), NaCl(001), and LiF(001) at ≈35 K. CH3Br forms a uniaxially commensurate overlayer on C(0001) with unit mesh parameters 4.26 A(ring)×6.75 A(ring). On NaCl(001), CH3Br forms a high coverage and a low coverage phase. The high coverage phase is incommensurate and has unit mesh parameters 4.54 A(ring)×6.73 A(ring), whereas the low coverage phase is commensurate with a ((2)1/2×3(2)1/2)R45° unit mesh. The structure of CH3Br/LiF(001) is essentially the same as that of the high coverage phase of CH3Br/NaCl(001) with unit mesh parameters 4.52 A(ring)×6.71 A(ring). The unit mesh parameters (with the exception of low coverage CH3Br/NaCl ) are very similar to the lattice parameters of the a-b [or (001)] plane of bulk crystalline CH3Br at ≈153 K. By analogy with the bulk crystal, it is likely that there are two molecules per unit mesh and that the CH3Br dipoles are nearly perpendicular to the surface and antiferroelectrically ordered. Our results suggest that the unit mesh for the low coverage phase of CH3Br/NaCl contains four molecules and that the molecular axes are parallel to the substrate surface. The implications of these results for photodissociation studies of physisorbed CH3Br are briefly discussed.

Notes: The addition of a piezoelectric field in AlGaAs/InGaAs/GaAs HEMT structures is shown to lead to enhanced electron densities and hence improved device performance. Growth of a strained InxGa1−xAs layer is in [111]A direction causes a piezoelectric field to be built into the quantum well of a pseudomorphic HEMT, which opposes the electric field due to charge transfer and hence lowers the confinement energy. This leads to carrier densities 50% larger than in equivalent [100] structures, with the wave function also spaced further away from the dopant impurities and the well interfaces. We expect these factors to give improved device performance.