ALBERT

Notes: We complete the study of the HF stretches (v1 and v2) of (HF)2 at N=v1+v2=3. A previous publication [J. Chem. Phys. 98, 9266 (1993)] reported the observations of the free-HF and hydrogen-bonded-HF stretches at (v1,v2)=(3,0) and (0,3). In this paper, second overtone (ΔN=3←0) spectra of the vibrations mixed between the two HF subunits are presented. Spectroscopic constants of the K subbands and tunneling states (A+ and B+) of the two mixed modes (2,1) and (1,2) are determined from their lifetime-broadened but rotationally resolved manifolds. For the (2,1) mode, we observe only a parallel band, K=0←0, and obtain band origins ν0=11 552.897 cm−1 (A+), 11 552.509 cm−1 (B+), rotational constants B¯=0.220 86 cm−1 (A+), 0.220 94 cm−1 (B+). For the (1,2) mode, a perpendicular band, K=1←0, is observed at ν0=11 536.95 cm−1 (A+), 11 536.93 cm−1 (B+) with B¯=0.222 cm−1 for both A+ and B+ states. The hydrogen interconversion tunneling splittings are determined to be 0.387 and 0.02 cm−1 for the K=0 levels of (2,1) and the K=1 levels of (1,2), respectively, demonstrating a strong dependence on K rotation and the importance of transition-dipole coupling in the tunneling process. Based on our present and previous results, we provide an overview of all the four components of the quartet by comparing five unique characteristics: vibrational symmetry, band origin, relative transition strength, hydrogen interconversion tunneling, and vibrational predissociation. Systematic comparison is also made against ab initio calculations of Jensen, Bunker, Karpfen, Kofranek, and Lischka [J. Chem. Phys. 93, 6266 (1990)]. A brief analysis suggests that the pure overtone modes can be described sufficiently by a local mode picture, whereas the mixed modes have strong normal mode characters. It is also concluded that the ab initio calculations do not reproduce the observations correctly and more adequate representation of the high vibrationally excited states of the HF dimer is required.

Notes: Three new combination bands, at the second overtone of the HF intramolecular stretch, 3ν1, with each of the three low frequency intermolecular modes, have been spectroscopically characterized by intracavity laser-induced fluorescence for the N2HF complex. The van der Waals stretching, HF and N2 bending frequencies at vHF=3 are determined to be 98.6, 328.6, and 68.5 cm−1, respectively. State dependent vibrational predissociation is observed in these three bands. The variation in the vibrational predissociation rate in these three bands suggests a strong angular dependence of the intermolecular potential with the HF internuclear distance in the complex. © 1996 American Institute of Physics.

Notes: Argon incorporation and the formation of silicon carbide in Si(100) by low energy Ar+ ion bombardment have been studied by angle-resolved x-ray photoelectron spectroscopy (XPS). The bombardment was performed at ion energies of 1, 1.5, and 2 keV and various ion fluences in an ultrahigh vacuum chamber equipped with XPS. The XPS measurements showed that the incorporated Ar concentrations achieved saturation in the near-surface region at ion bombardment fluences (approximately-greater-than)1016 cm−2. The surface Ar concentrations decreased with increasing bombardment energy. No Ar bubbles on the surface of Ar+-bombarded samples were observed by atomic force microscopy under these experimental conditions suggesting that Ar bubble formation was not the main Ar trapping mechanism in our study. The SiC formation was confirmed by characteristic XPS peaks of Si 2p and C 1s for SiC. The carbide formed at lower ion fluence was of a metastable structure as inferred by XPS. Bombardment at higher ion fluence yielded a stable carbide phase through continuous ion beam mixing. No strong dependence of carbide depth distribution on bombardment energy was observed suggesting that the carbide phase is probably dispersed inside the bombarded layer and that carbon is bonded to silicon at localized defect sites. © 1996 American Institute of Physics.

Notes: The vibrational dependence of the intermolecular potential of Ar–HF is investigated through the spectra of levels correlating with HF(v=3). We have previously reported measurements of the (vbKn)=(3000), (3100), and (3110) levels of Ar–HF using intracavity laser-induced fluorescence in a slit supersonic jet [J. Chem. Phys. 98, 2497 (1993)]. These levels are found to be well reproduced (within 0.1 cm−1) by the Ar–HF H6(4,3,2) potential [J. Chem. Phys. 96, 6752 (1992)]. The second overtone experiments are extended to include the (3002) state which is coupled to (3110) through Coriolis interaction, and the (3210) state which is more sensitive to higher-order anisotropic terms in the potential. The observations establish that the level (3002) lies 0.229 cm−1 below (3110), with upper state rotational constant B=0.085 89 cm−1. This is in good accord with the predictions of the H6(4,3,2) potential. The (3210) state lies at 11 484.745 cm−1 with B=0.099 79 cm−1. The band origin is 1.7 cm−1 higher than predicted, and thus contains important new information on the vibrational dependence of the potential. Several detailed features of the spectra can be explained using the H6(4,3,2) potential. The Q-branch lines of the (3210)←(0000) band show evidence of a weak perturbation, which can be explained in terms of mixing with the (3112) state. The (3210) spectrum exhibits parity-dependent rotational predissociation and the widths of the P- and R-branch lines and the magnitude of the l-type doubling can be explained in terms of coupling to the (3200) state, which is estimated to lie 4 cm−1 below the (3210) state.The Q-branch lines show a predissociation cutoff above Q(16); this is in reasonable agreement with the predictions of the H6(4,3,2) potential, but suggests that the binding energy calculated for the potential may be about 1 cm−1 too large. To examine the potential further, high-level ab initio calculations are performed, with an efficient basis set incorporating bond functions. The calculations give a well depth of 92%–95% of that of the H6(4,3,2) potential at θ=0° for v=0 and v=3, respectively; this is in line with earlier results on rare gas pairs. The calculations also reproduce the anisotropy of the H6(4,3,2) potential and its vibrational dependence. The dependence of the intermolecular potential on HF bond length is found explicitly.

Notes: We report the second overtone (Δv=3←0) spectra of the free-HF (ν1) and bound-HF (ν2) stretches of (HF)2 using laser induced fluorescence. Subbands of K=0←0 and K=1←0 are detected near 900 nm with linewidths spanning almost two orders of magnitude. The line broadening (Δνpd) due to vibrational predissociation is not only mode specific but also is state specific. A fit of the spectral lines to a Voigt profile reveals Δνpd=10 GHz for the parallel band of 3ν2, and 0.10 and 1.9 GHz for the parallel and the perpendicular bands of 3ν1, respectively. The linewidths of these subbands are J and tunneling state independent. The K-dependent vibrational predissociation is attributed to near-resonant centrifugal interaction of the K=1 state with the K=1 combination mode of the bound HF stretch (3ν2) and the antisymmetric bend (ν5). The exceedingly state-specific behavior is at variance with elementary density of states arguments. Spectroscopic constants of these two K subbands and two tunneling states (A+ and B+) of 3ν1 are determined from their rotationally resolved manifolds. For the parallel band, we obtain band origins ν0=11 273.501 cm−1 (A+), 112 73.499 cm−1 (B+), rotational constants B¯=0.221 177 cm−1 (A+), 0.221 179 cm−1 (B+), and centrifugal distortion constants D=2.02×10−6 cm−1 (A+), 2.05×10−6 cm−1 (B+).For the perpendicular band, ν0=11 299.850 cm−1 (A+), 11 299.847 cm−1 (B+), and B¯=0.222 02 cm−1 (A+), 0.222 04 cm−1 (B+). The interconversion tunneling splitting is found to be 0.0024 cm−1, showing that the tunneling motion of the dimer could be quenched entirely. For the 3ν2 where only the R branch is resolved, the breadth of the lines prevents accurate determination of its spectroscopic constants. The band is estimated to center at 11 043.09 cm−1 with a rotational B¯ constant of 0.2240 cm−1. All the constants indicate that a stronger hydrogen bond is formed at higher valence vibrational states. The shifts of the free- and the bound-HF stretching frequencies from that of the monomer are −99.306 and −329.72 cm−1, respectively. Finally, we present an analysis for the rotational dependence of the tunneling in states of v1, which suggests that the transition state, under the assumption of C2h geometry, has the HF units oriented at 33° with respect to the F–F axis.

Notes: Laser-induced fluorescence is used to obtain the second overtone spectrum of ArHF. The method exploits intracavity circulating power of a Ti–sapphire ring laser to pump the weakly bound complex generated in a supersonic slit jet from v=0 to v=3. Fundamental (Δv=−1) emission is monitored using an infrared PbS detector. Intense fluorescence allows recording of the rotationally resolved sub-Doppler spectra of (3000)←(0000), (3100)←(0000), and (3110)←(0000) transitions. We determine vibrational band origins of ν0=11 339.034 cm−1, 11 412.438 cm−1, 11 422.378 cm−1 and rotational constants of B=0.103 30 cm−1, 0.102 76 cm−1, 0.101 18 cm−1 for the (3000), (3100), and (3110) bands, respectively. Both the band origins and the rotational constants indicate that the weak Ar–HF van der Waals bond is strengthened as the HF stretch is vibrationally excited to higher states. All the observations are in near perfect accord with extrapolations of related constants in the HF stretching states of v=0–2.

Notes: Ferroelectric SrBi2Ta2O9 (SBT) thin films were deposited on Pt/TiOx/SiO2/Si substrates using the off-axis radio frequency magnetron sputtering technique. X-ray diffraction and atomic force microscopy experiments showed that the crystallization of SBT thin films at 〉700 °C correlated with the formation of rod-like grains. Cross-sectional field emission scanning electron microscopy images revealed that the apparent thickness of SBT decreased while the thickness of Pt increased as the annealing temperature was increased. The apparent decrease in the thickness of SBT was attributed to crystallization and densification in the film whereas the apparent increase in Pt thickness was due to diffusion of Ti and Bi into the Pt layer. This diffusion at high annealing temperatures (800 °C and above) alters the Pt purity and degrades the Pt as the bottom electrode for the ferroelectric capacitor. Good insulating properties were obtained when the SBT film was annealed at 700 and 750 °C whereas higher leakage currents were observed at annealing temperatures 〉800 °C. A remnant polarization (Pr) of 4.35 μC/cm2 and coercive field (Ec) of 31.5 kV/cm were obtained for the SBT thin film annealed at 750 °C with a leakage current density of 〈10−7 A/cm2. © 2000 American Institute of Physics.

Notes: The effect of elastic strains on antiferromagnetic phase transitions is considered. For cases in which the magnetic and chemical unit cells coincide, the combination of a strain and an applied field is found to lead to the possibility of a linear magnetoelastic (LME) coupling which may induce antiferromagnetic order, even in the normally paramagnetic phase. Such an effect can, in principle, destroy any second-order phase transition. An order of magnitude estimate shows that the effect is small but not negligible, and that it may explain a number of unusual effects observed in dysprosium aluminum garnet, including anomalous neutron scattering, magnetic hysteresis and magnetostriction. Similar strain-induced effects may be important in many other antiferromagnets, including CoF2, FeF2, MnF2, and αFe2O3, as well as in mixed crystals with the same structures. Strain gradients may produce similar effects in other antiferromagnets which are magnetoelectric, including DyPO4, DyAlO3, and Cr2O3.