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15 Dec 1985

Volume 83, Issue 12, pp. 6071-6546

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Bonding in d9 complexes derived from EPR: Application to CuCl2−4, CuBr2−4, and CdCl2:Cu2+

J. A. Aramburu and M. Moreno

J. Chem. Phys. 83, 6071 (1985); http://dx.doi.org/10.1063/1.449597 (13 pages) | Cited 47 times

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In this work are reported the theoretical expressions for the [g], hyperfine, and superhyperfine (shf) tensors of a d9 square‐planar complex within a molecular orbital (MO) scheme. These expressions include contributions arising from crystal field and charge transfer excitations calculated up to third and second order perturbations, respectively. This makes the present framework more general than those previously used. Through those expressions we have derived from the experimental EPR and optical data the MO coefficients corresponding to the valence b1g(x2−y2), b2g(xy), and eg(xz,yz) levels and also the core polarization contribution K to the hyperfine tensor for the systems CuCl2−4, CuBr2−4, and CdCl2:Cu2+. The 3d charge obtained for CuCl2−4 is equal to 0.61, 0.83, and 0.85 for the antibonding 3b1g, 2b2g, and 2eg levels, respectively. These figures are much closer to the Xα results by Bencini and Gatteschi [J. Am. Chem. Soc. 105, 5535 (1983)] than to those by Desjardins et al. [J. Am. Chem. Soc. 105, 4590 (1983)]. The σ and π covalency for CuBr2−4 are both higher
than for CuCl2−4 in accord to the lower electronegativity for bromine. However, only for the antibonding 3b1g level of CuBr2−4 have we obtained an electronic charge lying mainly on ligands. The covalency of CdCl2:Cu2+ is smaller than that found for CuCl2−4, a fact associated to a higher metal–ligand distance for the former. Evidence of this statement are also given from the analysis of crystal‐field spectra and isotropic shf constant. The values of K derived for CuCl2−4 (128.1×10−4 cm−1), CuBr2−4 (103.6×10−4 cm−1), and CdCl2:Cu2+ (123.9×10−4 cm−1) point out the dependence of K on the equatorial covalency but also on the existence of axial ligands. The [g] tensor of CuBr2−4 is dominated by the charge transfer contribution while the crystal field one is negative. Finally an analysis of the importance of each one of the involved contributions to the spin‐Hamiltonian parameters is reported for the three systems, together with the results obtained through a full diagonalization within crystal field and charge transfer states.
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76.30.Fc Iron group (3d) ions and impurities (Ti-Cu)
71.70.Ch Crystal and ligand fields

High resolution optogalvanic spectroscopy of He 3 1,3D–2 3P transitions

Kazuhide Fukuda, Shigehiko Fujimaki, Yukio Adachi, and Chiaki Hirose

J. Chem. Phys. 83, 6084 (1985); http://dx.doi.org/10.1063/1.449598 (4 pages) | Cited 1 time

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He 3 1,3D–2 3P lines have been investigated by optogalvanic spectroscopy in the hollow cathode discharge of He to reveal the presence of the 3 1D2–2 3P2 component which is predicted at 0.0764 cm−1 above the 3 1D2–2 3P1 line appearing at 17 018.137±0.004 cm−1. The assignment of the latter has also been experimentally established by the present study through the use of optogalvanic double resonance methods. The spin‐orbit interaction of the 3 1D2 level with the 3 3D2 level is responsible for the appearance of the 3 1D2–2 3P2 line. The relaxation processes among the 3 1D, 3 3D, and 2 3P levels have also been briefly discussed on the basis of the observations of mysterious dips on the double resonance signals and of a broad background signal on Doppler‐free intermodulation spectrum.
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32.30.Jc Visible and ultraviolet spectra
32.70.Jz Line shapes, widths, and shifts

EPR of Gd3+ in Na2Cd(SO4)22H2O, comparison with previous results obtained for Fe3+

J. M. Gaite, G. R. Bulka, N. M. Hasanova, N. M. Nizamutdinov, and V. M. Vinokurov

J. Chem. Phys. 83, 6088 (1985); http://dx.doi.org/10.1063/1.449599 (3 pages) | Cited 3 times

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An EPR experiment on Gd3+ in a single crystal of Na2Cd(SO4)22H2O (CdK) was carried out at Q band frequency and nitrogen temperature. Two spectra related by symmetry were observed. All spin Hamiltonian constants have been calculated. The pseudosymmetries of the fourth‐order term of the spin Hamiltonian are compared to those of Fe3+. It is observed that the substitution of Gd3+ for Cd2+ induced a very important local distortion of the host lattice, which is discussed.
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76.30.Kg Rare-earth ions and impurities

Triplet state spectroscopy and photophysics of As2 in solid neon

L. A. Heimbrook, N. Chestnoy, M. Rasanen, G. P. Schwartz, and V. E. Bondybey

J. Chem. Phys. 83, 6091 (1985); http://dx.doi.org/10.1063/1.449600 (8 pages) | Cited 11 times

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As2 dimer molecules generated by evaporating gallium arsenide were trapped and studied in solid neon. Components of at least three different excited electronic states are populated by direct tunable laser excitation and observed in phosphorescence. Nonradiative vibrational and electronic relaxation processes occurring in the matrix have been investigated. The electronic structure and spectroscopy of As2 is reviewed and summarized.
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33.50.Dq Fluorescence and phosphorescence spectra
36.40.-c Atomic and molecular clusters

Resonances in the K shell excitation spectra of benzene and pyridine: Gas phase, solid, and chemisorbed states

J. A. Horsley, J. Stöhr, A. P. Hitchcock, D. C. Newbury, A. L. Johnson, and F. Sette

J. Chem. Phys. 83, 6099 (1985); http://dx.doi.org/10.1063/1.449601 (9 pages) | Cited 154 times

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K shell excitation spectra of the aromatic molecules benzene and pyridine in the gas phase are compared to those for the solids (ices) and for monolayers chemisorbed on Pt(111). The gas phase and solid spectra are essentially identical and even the spectra for the chemisorbed molecules exhibit the same resonances. Because of the orientation of the molecules upon chemisorption the latter spectra show a strong polarization dependence as a function of x‐ray incidence. This polarization dependence in conjunction with a multiple scattering Xα calculation for the benzene molecule allows us to assign the origin of all K shell resonances. The resonances are found to arise from transitions to π∗ antibonding orbitals and to σ∗ shape resonances in the continuum. The shape resonances are characterized by potential barriers in high (l=5 and 6) angular momentum states of the excited photoelectron. The polarization dependence and energy position of the resonances allow the molecular orientation on the surface to be determined and show that the change in the carbon–carbon bond length is less than 0.02 Å.
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33.20.Rm X-ray spectra
78.70.Dm X-ray absorption spectra

The effect of high pressure on the electronic and infrared spectra of TCNE and its charge transfer complexes with hexamethylbenzene

C. W. Jurgensen, M. J. Peanasky, and H. G. Drickamer

J. Chem. Phys. 83, 6108 (1985); http://dx.doi.org/10.1063/1.449602 (5 pages) | Cited 7 times

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The visible and infrared spectra of the 1 : 1 and 2 : 1 solid‐state complexes of hexamethylbenzene with tetracyanoethylene were investigated as a function of pressure up to 100 kbar. The degree of charge transfer is estimated as a function of pressure from the vibrational frequencies. A comparison with the visible spectra is used to check the internal consistency of the results. The observed pressure shift of the charge transfer peak is discussed in terms of the environmental and resonance contributions.
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78.40.Ha Other nonmetallic inorganics
78.30.Jw Organic compounds, polymers

EPR and thermoluminescence investigations of Mn2+ in LuPO4, YPO4, and ScPO4

J. L. Boldú O., E. Muñoz P., M. M. Abraham, and L. A. Boatner

J. Chem. Phys. 83, 6113 (1985); http://dx.doi.org/10.1063/1.449603 (8 pages) | Cited 7 times

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Divalent manganese has been incorporated in both single crystals and powders of the zircon‐structure hosts LuPO4, YPO4, and ScPO4. The single‐crystal specimens were grown by slowly cooling a Pb2P2O7 flux from 1360 °C, and finely divided powders of these materials were also prepared using a molten urea precipitation technique. Electron paramagnetic resonance studies have shown that tetragonal symmetry Mn2+ sites are present in both single crystals and powders of ScPO4 and LuPO4 but are not present in either YPO4 single crystals or powders. An orthorhombic symmetry spectrum was observed for Mn2+ in single crystals of all three hosts and in YPO4 powders. Orthorhombic spectra were not found in Mn2+ doped powders of ScPO4 and LuPO4. In the case of the orthorhombic‐symmetry spectra, four sites were present in these tetragonal zircon‐structure hosts. The sites were magnetically inequivalent but were equivalent in terms of the strength and symmetry of the crystalline electric field. The orientations of the orthorhombic‐site principal axes relative to the crystallographic axes were determined in each case. The axial spin‐Hamiltonian parameter b02 was found to be positive in the case of the tetragonal‐symmetry sites and negative for the orthorhombic‐symmetry spectra. Thermoluminescent properties of these materials were also investigated.
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28.41.Kw Radioactive wastes, waste disposal
76.30.Fc Iron group (3d) ions and impurities (Ti-Cu)
78.60.Kn Thermoluminescence

Q‐band ESR studies on the origin of magnetic anisotropy in tetrahalogenated semiquinones

B. S. Prabhananda, Christopher C. Felix, James S. Hyde, and Anand Walvekar

J. Chem. Phys. 83, 6121 (1985); http://dx.doi.org/10.1063/1.449604 (7 pages) | Cited 13 times

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The g‐tensor components associated with tetrachloro‐, tetrabromo‐, and tetraiodo‐benzosemiquinones in frozen solutions of dimethylsulfoxide and ethanol‐D at 135 K have been measured by ESR spectroscopy at Q band (35 GHz). Spin‐orbit interactions on the halogens make a significant contribution to the g tensor; components in the plane of the aromatic ring, (gx and gy) are enhanced, whereas the component perpendicular to the ring (gz) is decreased. Related solvent effects have been observed in the spectrum of tetraiodobenzosemiquinone; in going from dimethylsulfoxide to ethanol gx and gy were found to increase, whereas gz decreased. It is argued that the positive contribution to gx and gy arises from the participation of halogen pz orbitals in the molecular π orbitals and that the negative contribution to gz in part reflects spin density on the halogens due to σ–π interaction induced configurational admixture in the wave function. Possible participation of halogen d orbitals is discussed. The dominant part of the solvent dependence of the g tensor components is suggested to result from changes in the energy separation between the orbital of the unpaired electron and lone‐pair orbitals on the halogens.
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33.35.+r Electron resonance and relaxation
31.70.Dk Environmental and solvent effects

Laser‐induced emission in the 1.9 μm region from excited state–excited state transitions in PuF6

W. W. Rice and J. E. Barefield

J. Chem. Phys. 83, 6128 (1985); http://dx.doi.org/10.1063/1.449605 (2 pages) | Cited 2 times

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Laser‐induced fluorescence (LIF) in the 1.9 μm region has been observed from room temperature PuF6 excited at 795 nm. The LIF has peaks at 1864±2, 1918±8, and 1983±2 nm and the emission waveforms have a temporal behavior similar to the PuF6 1 μm LIF that we have previously reported [J. Chem. Phys. 79, 2621 (1983)]. These spectral and temporal data suggest that the 1.9 μm LIF arises from vibronic transitions between electronically excited states of PuF6, i.e., 3Γ4(2)→3Γ4(1).
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33.50.Dq Fluorescence and phosphorescence spectra
31.50.Df Potential energy surfaces for excited electronic states

Structure and motion of hydrogen in molybdenum bronzes HxMoO3 as studied by nuclear magnetic resonance

Cl. Ritter, W. Müller‐Warmuth, and R. Schöllhorn

J. Chem. Phys. 83, 6130 (1985); http://dx.doi.org/10.1063/1.449606 (9 pages) | Cited 31 times

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1H NMR solid state techniques have been used to study bonding properties, location, and mobility of hydrogen in various phases of the hydrogen bronze HxMoO3. Temperature‐dependent spectra characteristic of different degrees of intercalation have been observed, and furthermore, from measurements of the relaxation rate, dynamic properties have been derived. There is strong evidence of intralayer hydrogen positions on a quasi‐one‐dimensional zig–zag line connecting the vertex‐sharing oxygen atoms of the MoO6 octahedra and these are first occupied if the degree of intercalation x is low. For x>0.85 the hydrogen in excess starts to fill up interlayer positions coordinated with terminal oxygen atoms at the van der Waals gap. Both isolated and paired protons have been detected in the interlayers, whereas clusters or pairs appear along the zig–zag lines. Hydrogen separations within the clusters, bonding with oxygen and charge transfer to the conduction band of the host lattice, are discussed. Hydrogen diffusion changes from being predominantly one‐dimensional to three‐dimensional in character as x increases. The activation energies of the motion are of the order of magnitude of 15 to 30 kJ/mol.
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76.60.Es Relaxation effects
61.50.Lt Crystal binding; cohesive energy

Pressure broadening of rotational bands. I. A statistical theory

P. W. Rosenkranz

J. Chem. Phys. 83, 6139 (1985); http://dx.doi.org/10.1063/1.449607 (6 pages) | Cited 56 times

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Absorption of electromagnetic waves by rotational transitions of molecules is formulated for the case in which the wave frequency is displaced from resonance by an amount large compared to the reciprocal duration of a typical binary collision, and also large compared to the differences between frequencies of the strong resonances of the gas. In this far‐wing limit, Fano’s relaxation operator is reduced to a scalar parameter which depends on the frequency displacement. This relaxation parameter is not symmetric with respect to reflection about resonance, but becomes symmetric when multiplied by the factor exp(ℏωd/2kT) where ωd is the frequency displacement. The theory applies to dipolar molecules of any shape, in collisions with either dipolar or quadrupolar molecules.
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33.70.Jg Line and band widths, shapes, and shifts
34.50.Ez Rotational and vibrational energy transfer

Optical spectra of matrix‐isolated palladium atoms

W. Schrittenlacher, H. H. Rotermund, and D. M. Kolb

J. Chem. Phys. 83, 6145 (1985); http://dx.doi.org/10.1063/1.449608 (5 pages) | Cited 3 times

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The optical properties of Pd atoms isolated in Ne, Ar, Kr, and Xe have been studied in the photon energy range of 2–10 eV by absorption, emission, and emission‐yield spectroscopy. Strong matrix effects on the optical transitions are observed which lead to a breakdown of the selection rules and, hence, make a band assignment by AMCOR (atom to matrix correlation) impossible. The unusually large blue shift of the Pd 4d→5p transitions is rationalized by an interstitial site for the Pd atom in all four noble gas matrices. Such a site assignment is supported by the high mobility of the Pd atoms which becomes apparent in annealing experiments. For Pd in Ne, absorption bands due to 4d→4f transitions were found around 8 eV. Finally, a transient‐absorption spectrum from a metastable state has been observed, which is in agreement with previously reported MCD measurements.
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32.30.Jc Visible and ultraviolet spectra

Fast orientational motional correlation time measurements from ESR spectra of perdeuterated, normal nitroxide radicals with 14N(I=1)

Anil Shetty and Sook Lee

J. Chem. Phys. 83, 6150 (1985); http://dx.doi.org/10.1063/1.449609 (4 pages) | Cited 3 times

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The new method recently formulated for analyzing the fast tumbling motional ESR hyperfine spectra of perdeuterated nitroxide radicals enriched with 15N(I=1/2) TANOLD‐15 (4‐hydroxy, 2,2,6,6‐tetranethylpiperidine‐d17‐1‐oxyl) are applied to perdeuterated, normal nitroxide radicals with 14N (I=1), TANOLD‐14, which are widely utilized as spin probes and labels. The fast motional correlation times τ obtained from different pairs of ESR hyperfine lines from TANOLD‐14 spin probes in poly(ethylene oxide) (PEO) polymer matrix are not only mutually consistent, but also agree with those obtained from TANOLD‐15 spin probes on the same host matrix. The τ values determined with the present method for normal nitroxide radicals containing 14N should be more accurate and reliable than those obtained with other conventional methods.
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33.35.+r Electron resonance and relaxation
33.15.Vb Correlation times in molecular dynamics
76.30.Rn Free radicals
33.15.Pw Fine and hyperfine structure

Hyperfine coupling constants of the CCN radical in the math 2Δ(000) state by microwave–optical double resonance spectroscopy

Tetsuo Suzuki, Shuji Saito, and Eizi Hirota

J. Chem. Phys. 83, 6154 (1985); http://dx.doi.org/10.1063/1.449610 (4 pages) | Cited 10 times

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Electric‐dipole allowed transitions between hyperfine components and between rotational levels have been observed for CCN in the math 2Δ(000) state by a technique of microwave–optical double resonance. The observed transition frequencies have been analyzed to yield 14N hyperfine coupling constants: a=50.22(19), b=−5.9(12), c=7.8(20), and eQq=0.75(54), all in MHz with one standard error in parentheses. An attempt was made to observe Λ‐doubling transitions and rotational transitions in the math 2Π(000) state without success.
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33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
33.15.Pw Fine and hyperfine structure
33.20.Bx Radio-frequency and microwave spectra

Picosecond CARS as a probe of ground electronic state intramolecular vibrational redistribution

David J. Tannor, Stuart A. Rice, and Peter M. Weber

J. Chem. Phys. 83, 6158 (1985); http://dx.doi.org/10.1063/1.449611 (7 pages) | Cited 22 times

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We describe a representation of coherent anti‐Stokes Raman spectroscopy (CARS) suitable to the description of time‐dependent measurements. This representation includes the standard energy frame formulation as a special case when cw fields are involved. The traditional CARS cw field frequency matching condition ω0=ω1−ω2+ω3 must be generalized for non‐cw fields; in that case it refers to the Fourier component at ω0 of the convolution of the radiation field with the wave packet recurrences. The influence of resonance, both in the ground and electronically excited states, on the decay of time delayed CARS signals is discussed. As expected, intramolecular vibrational redistribution on the ground state potential surface of a molecule causes the CARS signal to decay in time. Model calculations show that quantum beats in the CARS signal may be observed, reflecting either a small number of coupled states (strong and regular recurrences) or sequential coupling of states (weak and early recurrences).
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
42.65.Dr Stimulated Raman scattering; CARS
42.65.Es Stimulated Brillouin and Rayleigh scattering
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Raman spectra of solid CH4 under high pressure. III. New high pressure phases in solid CH4 and CD4

M.‐M. Thiéry, D. Fabre, and K. Kobashi

J. Chem. Phys. 83, 6165 (1985); http://dx.doi.org/10.1063/1.449612 (8 pages) | Cited 18 times

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We have observed the Raman spectra of the intramolecular vibrations in solid CH4 and CD4 at T=4.2 K under high pressure. For solid CH4, the spectra changed markedly at P≂9 kbar, indicative of a new phase VI above this pressure. For solid CD4, an analogous change was observed at P≂5.4 kbar, indicating a phase transition from phase III to a new phase IV. The spectra of these new phases, CH4 VI and CD4 IV, are very similar so that it is concluded that the crystal structures of the phases are isomorphous. The observed spectra seem to indicate that the molecular orientations of methane molecules in the new high pressure phases are fully ordered.
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78.30.Jw Organic compounds, polymers
64.70.K- Solid-solid transitions
62.50.-p High-pressure effects in solids and liquids

HeI α photoelectron modulation spectroscopy of H2 and D2

W. J. van der Meer, H. van Lonkhuyzen, R. J. Butselaar, and C. A. de Lange

J. Chem. Phys. 83, 6173 (1985); http://dx.doi.org/10.1063/1.449613 (8 pages) | Cited 4 times

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A new method for the determination of photoionization cross sections is presented. The technique is used to obtain cross sections of H2 and D2 relative to those of H and D atoms from accurate HeI α photoelectron intensity measurements. With the virtually exact atomic cross section calculations available for H and D atoms our experiments provide absolute H2 and D2 photoionization cross sections. Ultraviolet photoelectron (UV PE) spectra of vibrationally excited hydrogen (v″=1,2) and deuterium (v″=1,2,3), free from overlapping bands due to vibrational ground state molecules, are presented for the first time. Positions and intensities of all observed features are excellently reproduced using well‐known molecular and ionic constants and accurate ab initio potential energy curves. Combination of the cross section determinations of H2 and D2 with intensity measurements of vibrationally excited H2 and D2 reveals some qualitative information on H@B|2 and D∗2 cross sections and  β parameters.
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33.60.+q Photoelectron spectra
33.80.Eh Autoionization, photoionization, and photodetachment

Si2, SiH3, and HSiO molecules: ESR at 4 K

R. J. Van Zee, R. F. Ferrante, and W. Weltner

J. Chem. Phys. 83, 6181 (1985); http://dx.doi.org/10.1063/1.449614 (7 pages) | Cited 31 times

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ESR spectra of 29Si2, 29SiH3, and H29SiO molecules in argon and/or neon matrices at 4 K have yielded the following new data: Si2 has a zero‐field splitting in its ground3Σ−g state of ‖D‖=2.53(1) cm−1 in an argon matrix, which extrapolates to a gas‐phase parameter of ‖λ‖=1.35(5) cm−1. The hyperfine interaction constant ‖A⊥(29Si)‖=40(2) MHz. SiH3 has g∥=2.0043(5), g⊥=2.0035(5), ‖Aiso(H)‖=25(3), ‖Aiso(29Si)‖=665(4), and ‖Adip(29Si)‖=166(3) MHz. Probable spin densities are 15% sSi+91% pSi−5% sH with a calculated bond angle of 113° in solid argon. HSiO has been observed for the first time. Its g components and spin distribution are very similar to those of the HCO radical. It is bent with a presumed 2A′ ground state, in accord with unpublished calculations of Bruna and Peyerimhoff and the recently published study of Frenking and Schaefer. The more stable isomer HOSi may have also been detected here.
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33.35.+r Electron resonance and relaxation
33.15.Pw Fine and hyperfine structure

Rovibrational analysis of ν3 HCN‐‐‐HF using Fourier transform infrared spectroscopy

B. A. Wofford, J. W. Bevan, W. B. Olson, and W. J. Lafferty

J. Chem. Phys. 83, 6188 (1985); http://dx.doi.org/10.1063/1.449564 (5 pages) | Cited 31 times

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The gas phase rovibrational spectrum of the ν3 band arising from the C≡N stretching vibration in the hydrogen bonded heterodimer HCN‐‐‐HF has been observed at 0.004 cm−1 instrumental resolution using a Fourier transform infrared spectrophotometer. Analysis of the spectrum gave the following molecular parameters (in cm−1): ν3=2120.935(12), α3=+5.06(19)×10−4, B′=0.119 283(19), D′J=2.30(7)×10−7. Excited state amplitude lifetimes of observed transitions are demonstrated to be 5.6(4)×10−10 s.
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33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Vq Vibration-rotation analysis

The bound and quasibound levels of 6Li2 a 3Σ+u

Xingbin Xie and R. W. Field

J. Chem. Phys. 83, 6193 (1985); http://dx.doi.org/10.1063/1.449565 (4 pages) | Cited 20 times

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Rotationally resolved 6Li2 2 3Πg→a 3Σ+u fluorescence spectra have been obtained by perturbation facilitated optical–optical double resonance spectroscopy. Principal molecular constants for the a 3Σ+u first excited state of 6Li2 are (2σ uncertainties): Te =8180.9±5.4 cm−1, ωe =73.0±1.4 cm−1, ωexe =3.95±0.14 cm−1, Re =4.13±0.08 Å, and De =336.0±5.4 cm−1. Transitions into 0≤v≤6 and N=18, 20, 31, 33 were observed and the total number of bound rotation–vibration levels was computed to be 193 for the a 3Σ+u state of 6Li2.
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33.50.Dq Fluorescence and phosphorescence spectra
33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Dj Interatomic distances and angles

Optical spectroscopy of PbCl2 particles embedded in a NaCl host matrix

C. Zaldo, J. García Solé, E. Diéguez, and F. Agulló‐López

J. Chem. Phys. 83, 6197 (1985); http://dx.doi.org/10.1063/1.449859 (4 pages) | Cited 5 times

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The absorption and luminescence data reported in this paper definitely confirm that the precipitates formed during high‐temperature aging of Pb‐doped NaCl are PbCl2 microcrystals. A detailed comparison of the optical spectra of the precipitates and free PbCl2 is presented. The absorption and luminescence spectra are essentially similar in both cases, although the splitted structure of the prominent exciton band has now been resolved for the first time in the absorption spectrum of the precipitates. The splitting has been interpreted in terms of the noncubic crystal field effect on the 3P1 excited level of Pb+2 which is considered as a cationic exciton. The gap energy Eg and the binding energy ϵb of the exciton have also been determined (Eg=4.86 eV, ϵb=0.18 eV). The same UV and blue luminescence bands, as well as a thermally induced interconversion among them, have now been observed for the precipitates as well as for free PbCl2. This luminescence behavior, not previously understood, has been explained in terms of two coexisting AT and Ax minima in the adiabatic energy surface of the 3P1 level of a Pb+2 localized exciton (Fukuda’s model).
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78.40.Ha Other nonmetallic inorganics
78.55.Hx Other solid inorganic materials
78.55.Fv Solid alkali halides

Ammonia dimer: A surprising structure

D. D. Nelson, G. T. Fraser, and W. Klemperer

J. Chem. Phys. 83, 6201 (1985); http://dx.doi.org/10.1063/1.449566 (8 pages) | Cited 96 times

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High resolution rotational spectra for (14NH3)2, (14ND3)2, 15NH3–14NH3, and 14NH3–15NH3 have been obtained using the molecular beam electric resonance technique. For 14NH3 dimer the spectra are simple and consist only of the J=0–1 and J=1–2 transitions with K=0 for two vibrational states (hereafter designated α and β) of the complex. The spectroscopic constants for (14NH3)2 are: 1/2 (B+C)α=5110.412(2), 1/2 (B+C)β=5110.564(2), DJ=0.0529(5), eqQ1aa =−0.639(13), eqQ2aa =0.904(8) (MHz);  μαa =0.7497(10),  μ βa =0.7376(9) (D). For (ND3)2 the vibrational states are unsplit 1/2 (B+C)=4190.300(50) MHz and  μa=0.567(4) D. These spectroscopic constants are inconsistent with the theoretically predicted structure for NH3 dimer. In the predicted structure one NH3 unit hydrogen bonds to the other with a nearly linear N–H‐ –N arrangement so that the C3 axis of the basic NH3 is collinear with the hydrogen bond. If θi is the angle between the C3 axis of the ith NH3 unit and the a‐inertial axis of the complex, θ1∼0° and θ2∼68° for
the theoretically predicted structure. In contrast, for 14NH3 dimer we measure the average values of θ1 and θ2 as 48.6(1)° and 115.5(1)°, respectively, using the quadrupole hyperfine structure. Furthermore, the component of the dipole moment along the a‐inertial axis,  μa, is found to be much smaller (0.75 D) than expected for the theoretical structure (∼2 D). The distance between the centers of mass of the NH3 subunits, RCM, is also determined and is found to be 3.3374(1) Å. No evidence exists for nonrigidity in the ground vibrational state of this complex, i.e., no high frequency tunneling splittings were observed in either the microwave or the infrared spectrum of the complex. The relationship between these structural results and the concept of hydrogen bonding in NH3 systems is discussed. The apparent inability of measured values of heats of formation for NH3 dimer to correctly predict dissociation energies of the complex is also commented upon. The dissociation energy of NH3 dimer has been previously determined as less than 2.8 kcal/mol.
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33.20.Bx Radio-frequency and microwave spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants

Observation of ns 3Σ+u(1u, 0−u)→mp 3Πg Rydberg transitions of Ar2 (n=4; 5≤m≤15) and Kr2(n=5; 6≤m≤16) in absorption

K. P. Killeen and J. G. Eden

J. Chem. Phys. 83, 6209 (1985); http://dx.doi.org/10.1063/1.449567 (10 pages) | Cited 24 times

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Rydberg series of Ar2 and Kr2 have been observed in the visible and ultraviolet (350≤λ≤560 nm) absorption spectra of the lowest‐lying metastable states of the dimer (ns 3Σ+u(1u, 0−u); Ar: n=4, Kr: n=5). The most prominent series for Ar2 and Kr2 are assigned to ns 3Σ+u → mp 3Πg (Ar: 5≤m≤15; Kr: 6≤m≤16) Rydberg transitions of the diatomic molecule and adiabatic ionization potentials are determined from an extrapolation of the series to their limits (m → ∞) to be 116 356±340 cm−1 (14.43±0.04 eV) for Ar2 and 104 616±150 cm−1 (12.97±0.02 eV) for Kr2. The dissociation energies of the A2Σ+u[1( (1)/(2) )u] dimer ion states of Ar2 and Kr2 are, therefore, 1.33±0.04 and 1.03±0.02 eV, respectively. All of the observed Rydberg states have an A2 Σ+u ion core. Binding energies of the lowest observed 3Πg states, were determined to be (1.331±0.005) eV for Ar2 and (1.030±0.001) eV for Kr2. The quantum defects for the Ar2 and Kr2 series (1.656±0.006 and 2.621±0.004, respectively) and ωe−2ωexe for the ns 3Σ+u metastable state of Ar2 (303±20 cm−1) and Kr2 (172±10 cm−1) were also measured and the vibrational frequencies are consistent with previously reported values. Absorption peaks which are attributed to ns 3Σ+u→m′p 3Σ+g Rydberg transitions of Ar2 (6≤m′≤10) and Kr2 (7≤m′≤10) are also reported.
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33.20.Lg Ultraviolet spectra
32.80.Rm Multiphoton ionization and excitation to highly excited states
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.15.Fm Bond strengths, dissociation energies

Temperature dependence of the isotope effect in dissociative attachment

L. G. Christophorou

J. Chem. Phys. 83, 6219 (1985); http://dx.doi.org/10.1063/1.449568 (3 pages) | Cited 4 times

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The isotopic dependence of the cross section for dissociative attachment is shown to be a function of temperature.
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34.80.Gs Molecular excitation and ionization

State to state study of the dissociation of doubly charged carbon dioxide cations

Gérald Dujardin and Dominique Winkoun

J. Chem. Phys. 83, 6222 (1985); http://dx.doi.org/10.1063/1.449569 (7 pages) | Cited 29 times

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Double photoionization of CO2 has been studied in the photon energy range 37–52 eV by the photoion–photoion coincidence (PIPICO) method. Three dissociative electronic states (α, β, and γ) of CO++2 have been observed with threshold energies at, respectively, 37.9, 40.5, and 45.0 eV. Comparison of these energies with those recently calculated by Flament et al. indicates that the lowest electronic state at 37.9 eV most probably corresponds to the 1∑+g state (1π−2g main configuration). CO++2 in its α, β, and γ states dissociates into CO++O+ with a high exothermicity since the total kinetic energy released was found to be respectively 4.5, 6.5, and 9.4 eV. From these results we concluded that CO++2 in its α(1∑+g ) state dissociates into CO+(X 2∑+)+O+(4Su) via an indirect process (predissociation). Absolute values of the partial double photoionization cross sections σ++n  (E) (n≡α, β, and γ) were also determined as a function of the photon excitation energy E. A ratio of double to total photoionization cross sections as high as 3% at a 52 eV photon energy was observed.
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33.80.Eh Autoionization, photoionization, and photodetachment
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