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Volume 96, Issue 8, 15 April 1992

Site‐selective resonance fluorescence of Eu^{+} ^{3}‐exchanged sodium β‘‐alumina
View Description Hide DescriptionSite‐selective excitation of Eu^{+3} β‘‐alumina into the ^{5} D _{0} level (568–579 nm) shows that the europium ion occupies at least two types of site distributions. The first distribution occurs at the higher energy of the tuning range and has site symmetry C _{3v } or C _{3}. It corresponds to the crystallographic Beevers–Ross position. The other site distribution is observed at lower energy and is associated with the mid‐oxygen crystallographic position (site symmetry: C _{2h } ). We also show that interpretation of the site‐selective spectroscopy of Eu^{+} ^{3} β‘‐alumina must take into account the occurrence of site‐to‐site energy transfer linked to vibronic coupling phenomena.

A laser study of the blue electronic transitions of CaS
View Description Hide DescriptionThree new electronic transitions of CaS have been observed in the blue region of the spectrum. The vibrational bands in these transitions span thousands of cm^{−} ^{1} owing to a large change in geometry. Approximately 1000 cm^{−} ^{1} portion of the spectrum containing these bands has been recorded at high resolution by dye laser excitation spectroscopy and analyzed. The new states all have Ω=1, and are tentatively labeled G ^{1}Π, F ^{1}Π, and f ^{3}Π_{1}. Spectroscopic constants have been generated for all three upper states and for the X ^{1}Σ^{+} state from a fit to 4340 individual lines. These constants have been used to generate potential curves and Franck–Condon factors.

The rotational spectrum, internal rotation, and structure of H_{2}O–NCCN and D_{2}O–NCCN
View Description Hide DescriptionThe radio frequency and microwave spectra of H_{2}O–NCCN and D_{2}O–NCCN have been measured using molecular beam electric resonance spectroscopy. The spectrum is characteristic of a planar T‐shaped asymmetric top in which the H_{2}O subunit exhibits hindered twofold internal rotation. The spectroscopic parameters for H_{2}O–NCCN are A=4692.11(2) MHz, A’=4689.45(25) MHz, B=4258.06(1) MHz, C=2219.34(1) MHz, and μ_{ a } =2.06(3) D and for D_{2}O–NCCN are A=4644.31(27) MHz, B=3822.76(10) MHz, C=2084.86(10) MHz, and μ_{ a } =2.1617(10) D. The H_{2}O subunit is bound to NCCN through the oxygen atom with a twofold barrier of 285(6) cm^{−} ^{1} hindering the internal rotation of H_{2}O about the a axis of the complex.

Vibrational band oscillator strengths and dipole transition moment of the A ^{1}Π–X ^{1}Σ^{+} system of CO
View Description Hide DescriptionRotationally integrated vibrational absorption cross sections of the A ^{1}Π−X ^{1}Σ^{+} (v’−0) bands of CO have been measured in the wavelength range 125–155 nm using monochromatized synchrotron radiation as background light source. Absorption cross sections for a few singlet‐triplet transitions are also obtained. Vibrational band oscillator strengths are determined for 0≤v’≤12 and compared to other experimental and theoretical values. It is found that the new data are well reproduced by extrapolating to shorter internuclear distances the parabolic dipole transition moment function determined by De Leon [J. Chem. Phys. 8 9, 20 (1988); 9 1, 5859 (1989)]. Deperturbed lifetimes for levels 0≤v’≤13 are calculated from the parabolic dipole moment function and, except for v’=13, are found in good agreement with available experimental results.

Pressure dependence of zero‐field splittings in organic triplets. III. Vibronic systems
View Description Hide DescriptionWe report optically detected magnetic resonance(ODMR) experiments at pressures up to 40 kbar for p‐benzoquinone (BQ) in dibromobenzene (DBB) and for a deep x‐trap in a BQ‐d_{4} neat crystal. It is known from previous works that the phosphorescent triplet state (^{3} B _{1g }) of these systems is vibronically coupled to a nearby ^{3} A _{ u } state, such that the former exhibits a double‐minimum potential well. The zero‐field splitting (ZFS) parameter D of both systems changes by several gigahertz over our pressure range. The greatest slope occurs at zero pressure, and the rate of change slows down exponentially at higher pressure. In addition, we observed a phase transition in BQ‐d_{4} crystals at 6 kbar, which causes a huge step‐discontinuity in both D and E. In contrast to the enormous change in D, the E‐value is not sensitive to pressure except for the step‐change during the phase transition. Triplet state kinetics and phosphorescence spectra are used to establish the existence of an in‐plane distortion at least in the high‐pressure phase. A model is proposed to interpret these results based on the pressure‐dependent vibronic interaction, leading to a reduction of the spin–orbit coupling contribution to the ZFS.

Algebraic determination of the vibrational quantum numbers of a diatomic molecule
View Description Hide DescriptionThe quantum numbers of electronic‐vibrational levels of a diatomic molecule are assigned using a new and simple algebraic procedure which is applied to data from a single absorptionspectrum of the molecule. This procedure represents a considerable simplification in the solution to the problem of quantum numbering of a molecule since the method does not need additional spectra of other isotopic species, nor does it use advanced, yet more complicated, molecular potentials different from the usual Morse potential. In this paper the new method, applied to the absorption bands from the lowest X ^{1}Σ^{+} _{ g } state of I_{2} to the vibrational manifold of its B ^{3}Π^{+} _{0 u }excited electronic state, allows the assignation of the quantum numbers of the vibrational levels and, in addition, the evaluation of the main spectroscopic constants with reasonable accuracy even when a low‐resolution spectrum is used as the input data.

Laser vaporization generation of B ^{1} ^{4}NH, B ^{1} ^{5}NH, B ^{1} ^{4}ND, B ^{1} ^{6}O, and B ^{1} ^{7}O: Electron‐spin‐resonance investigation in neon matrices under ultracold trapping conditions
View Description Hide DescriptionReactive laser vaporization conditions have been employed to generate various isotopic combinations of the new radical species, BNH, for neon matrix electron‐spin‐resonance (ESR) study. The BNH radical was found to have a linear geometry with an X ^{2}Σ electronic ground state, in agreement with a b i n i t i o theoretical calculations, which were conducted as part of this experimental investigation. ESR results for the nuclear hyperfineinteractions (Atensor) indicate that the electronic structure of BNH is quite similar to that of the isoelectronic BO radical. Results of a detailed reinvestigation of the ^{1} ^{1},^{1} ^{0}B ^{1} ^{6},^{1} ^{7}O radicals trapped in neon matrices are reported which now show excellent agreement with earlier gas‐phase microwave spectroscopicmeasurements of the dipolar boronhyperfineinteraction. The ^{1} ^{7}O Atensor for BO has been fully resolved for the first time into its isotropic and anisotropic components. A new ultracold neon trapping procedure is described which was used to produce matrix samples of randomly oriented and nonrotating radicals in order to measure the full extent of the dipolar nuclear hyperfineinteraction. The neon matrix magnetic parameters for ^{1} ^{1}B ^{1} ^{4}NH are g _{∥}=2.0020(5) and g _{⊥}=2.0010(5), A _{∥}=1089(1) and A _{⊥}=1013(1) MHz for ^{1} ^{1}B, A _{∥}=49(1) and A _{⊥}=37(1) MHz for ^{1} ^{4}N, and A _{∥}=29(1) and A _{⊥}=23(1) MHz for H.

SU_{ q }(2) quantum group analysis of rotational spectra of diatomic molecules
View Description Hide DescriptionThe spectra of some diatomic molecules are analyzed in terms of a SU_{ q }(2) symmetry. Particular emphasis is put on those molecules with relatively high values of D _{ n } and H _{ n }. The excellent agreement found between the observed and calculated wave numbers can be seen as an indication that an up to now hidden symmetry SU_{ q }(2) is present.

Photon echoes and related four‐wave‐mixing spectroscopies using phase‐locked pulses
View Description Hide DescriptionThe use of phase‐locked pulses in various spectroscopic techniques related to the third‐order polarizationP ^{(3)} is analyzed. Using correlation function expressions for the nonlinear response function, we clarify the interrelationship among several photon echo, pump–probe, and spontaneous light emission techniques, without alluding to any specific model for the material system. By combining phase‐locked pulses and heterodyne detection it becomes possible to probe separately the real and imaginary parts of the nonlinear response function. Combining two phase‐locked pulse excitation with time‐resolved detection of the spontaneous light emission allows direct separation of the Raman and fluorescence contributions.

Photogeneration of ions via delocalized charge transfer states. I. Xe_{2}H^{+} and Xe_{2}D^{+} in solid Xe
View Description Hide DescriptionDelocalized charge‐transfer excitations in solid xenon multiply doped with atomic halogens (I, Br, Cl) and hydrogen are demonstrated to lead to charge separation by trapping of the positive charge. As evidence of such a concept we present the first vibrational spectra of Xe_{2}H^{+} and Xe_{2}D^{+}, which are believed to be vibrationally bound ions.

Scattering delay times and transition rates for continuum resonance Raman scattering: Detailed derivations via the time‐dependent approach and applications to ^{7} ^{9}Br_{2}
View Description Hide DescriptionScattering delay times between photon absorption and emission, and transition rates for continuum resonance Raman scattering are derived in a coherent and detailed fashion, within the time‐dependent approach pioneered by Heller, Imre, and others. The resulting expressions for the Raman scattering delay times are related to similar expressions for particle scattering delay times derived by Eisenbud, Wigner, and Smith in terms of S matrices. These expressions are valid both for the ideal cases of sharp photon frequencies, and for realistic cases of finite frequency distributions or laser profiles. The Raman transition rates for this type of resonance scattering have a golden‐rule‐type expression implying the familiar selection rules and symmetry of two‐photon transitions, similar to the Fermi–Pauli golden rule for single photon transitions. Applications to ^{7} ^{9}Br_{2} yield ultrashort Raman scattering delay times in the 10 fs domain.

Optical spectra and crystal‐field analysis of europium double nitrates
View Description Hide DescriptionLocations and assignments of crystal‐field levels in low‐temperature spectra are reported for Eu^{3+} in the europium double nitrate system [Eu_{2}M_{3}(NO_{3})_{12}⋅24H_{2}O with M=Mg, Zn]. These energy levels are assigned from the polarized luminescence and the polarized absorption measurements on single crystals at 77 K. The arrangement of the oxygen atoms around the europium ion has approximate icosahedral symmetry and we have tried to find out whether the spectroscopicproperties of Eu_{2}M_{3}(NO_{3})_{12}⋅24H_{2}O reflects this pseudosymmetry, even though the site symmetry of the europium ion is C _{3}. Quantitative crystal‐field calculations have first been performed assuming a C _{3v } symmetry which is close to icosahedral. The levels are analyzed in terms of 20 free‐ion and 6 crystal‐field parameters. Afterwards it is examined how far the C _{3v } parameters can be used to approximate the C _{3} symmetry. In this case, three additional imaginary crystal‐field parameters are taken into account.

Vibrational overtone spectroscopy of the 4ν_{OH}+ν_{OH’} combination level of HOOH v i a sequential local mode–local mode excitation
View Description Hide DescriptionSequential pumping of the local OH stretch vibrations in hydrogen peroxide using infrared‐optical double resonance permits spectroscopic access to the 4ν_{OH}+ν_{OH’} combination level. Analysis of the rotationally resolved vibrational overtone spectra generated by this technique determines approximate rotational constants for this level and a value of 17 051.8±3.4 cm^{−} ^{1} for the O–O bond dissociation energy. The linewidths of individual zeroth‐order rotational transitions increase sharply with increasing K and change from smooth Lorentzian profiles to clumps of individual lines. The K dependence of the clump widths suggests that an a‐axis Coriolis interaction is the primary coupling mechanism between the zeroth‐order bright state and dark bath states. As a function of increasing J, each clump coalesces into a smooth Lorentzian profile. We interpret this J dependence in terms of a model that includes rotationally induced vibrational coupling among zeroth‐order dark states.

Enhanced double‐quantum nuclear magnetic resonance in spinning solids at rotational resonance
View Description Hide DescriptionThe excitation efficiency of double‐quantum (2Q) coherences between pairs of spin‐1/2 nuclei in rotating powdered solids is greatly enhanced at rotational resonance, where an integer multiple of the spinning speed matches the difference in isotropic chemical shifts. In powdered samples it is possible to transfer up to 50% of the single‐quantum magnetization through the 2Q transitions, an order of magnitude more than is achievable in magic‐angle‐spinning experiments performed off rotational resonance. The effect is simulated numerically by integration of the homogeneous evolution propagator and described analytically using first‐order average Hamiltonian theory. Rotational resonance enhanced 2Q filtering is demonstrated experimentally for doubly ^{1} ^{3}C‐labeled samples of zinc acetate and diammonium oxalate monohydrate.

Optical–optical double resonance spectroscopy of Cl_{2}: Analysis of the 1_{ g }(^{3} P _{1})–A ^{3}Π(1_{ u }) system
View Description Hide DescriptionThe 1_{ g } (^{3} P _{1})–A ^{3}Π(1_{ u }) transition is analyzed by optical–optical double resonance using the 1_{ g } (^{3} P _{1})–A ^{3}Π(1_{ u })–X ^{1}Σ^{+} _{ g }photoexcitationsequence. The analysis of the 409 transitions in the v’=0–14 and J’=2–51 range of ^{35}Cl_{2}isotope species yields a set of molecular parameters for the 1_{ g }(^{3} P _{1}) state in a Dunham‐type expansion: Y _{00}=59 295.651(5), Y _{10}=256.6412(30), Y _{20}=−1.205 36(48), Y _{30}=3.724(22)×10^{−3}, Y _{01}=0.114 313 8(60),Y _{11}=−7.507(15) ×10^{−4}, Y _{21}=1.96(13)×10^{−6}, and Y _{02}=−8.10(22)×10^{−8} with the Ω‐type doubling constant, q _{ v }=B ^{(f)} _{ v }−B ^{(e)} _{ v } =[3.176(31)−0.0660(50)×(v+1/2)]×10^{−4} (all in cm^{−1} and σ in parentheses). An empirical model is given to interpret the spin–orbit couplings between the nascent six ion‐pair states of g‐type symmetry correlating to Cl^{−}(^{1} S)+Cl^{+}(^{3} P).

Velocity‐gauge formalism in the theory of vibrational circular dichroism and infrared absorption
View Description Hide DescriptionThe first use of velocity‐gauge factors in the theory of vibrational circular dichroism (VCD) and infrared absorption intensities is described. The approach involves the exact incorporation of all or part of the dependence of the electronic wave function on an electron‐velocity perturbation, such as the vector potential or the velocities of the nuclei, into the atomic orbital basis functions as a gauge transformation. Any remaining dependence of the wave function on these perturbations is carried by the basis‐function coefficient derivatives which are determined to first order by coupled Hartree–Fock perturbation theory. The magnetic fieldperturbation formulations of VCD in the common origin and distributed origin gauges are identified within the new formalism, providing a new direct derivation of the distributed origin gauge theory. The formalism also yields new a p r i o r i VCD intensity expressions, derived using nuclear velocity‐gauge factors, in the complete adiabatic approximation. Several distinct a p r i o r i computational approaches to VCD intensities can now be identified—the vibronic coupling theory implemented with a direct sum over states (VC/SOS), the field adiabatic theory implemented with magnetic fieldperturbation (FA/MFP), and the complete adiabatic theory implemented with nuclear velocity perturbation (CA/NVP). In addition, basic expressions are presented for an energy gradient theory of VCD that employs both magnetic‐field and nuclear‐velocity perturbations (EG/MFNVP). It is shown that the CA/NVP theory of VCD possesses a higher Born–Oppenheimer content than the VC/SOS or FA/MFP theories and provides an improved basis for reducing a p r i o r i VCD theory to various models of VCD intensity.

Sideband modeling in molecular crystals N_{2} and CO_{2}
View Description Hide DescriptionVibron‐phonon excitation bands, phonon sidebands to the zero phonon line, mirror the one‐phonon density of statesg(ω) calculated in the harmonic approximation. The origin of bands in the sideband is investigated, e.g., phonon contributions from librational and translational modes or phonons at special points of the Brillouin zone. The temperature‐dependent structure of the sideband (frequency shifts, line broadening) is due to anharmonic processes which modify g(ω): Temperature‐dependent frequency shifts of maxima in the sideband are shown to depend mainly on the volume effect, whereas line broadening is due to phonon–phonon interactions, which are simulated in the lattice dynamics calculations of g(ω) by a special modeling procedure. Compared to the CO_{2}solid, effects are more pronounced in the N_{2} crystal due to the presence of strong mechanical and electrical anharmonicities. The latter give rise to multiphonon contributions in the vibron‐phonon excitation process. Intensity changes with temperature can be explained by the thermal weighting of the one‐phonon density of states.

Rigid bender analysis of van der Waals complexes: The intermolecular bending potential of a hydrogen bond
View Description Hide DescriptionHigh resolution ir data on weakly bound OCOHF complexes formed in a slit supersonic expansion reveal a progression of extremely low frequency vibrational levels associated with the bending of the OCO–HF hydrogen bond. In a previous paper [J. Chem. Phys. 9 3, 7716 (1990)], we presented a spectroscopicanalysis of the fundamental, combination and hot bands observed, corresponding to transitions between v ^{ l } _{bend}=0^{0}, 1^{1}, 2^{0}, 2^{2}, and 3^{1}, where v ^{ l } _{bend} denotes quanta of OCOHF skeletal bend excitation with l units of vibrational angular momentum. In this paper, we analyze the rotationally resolved data in terms of the rigid bender formalism of Hougen, Bunker and Johns to determine an explicit angular potential, V(θ), for the OCOHF complex in both the HF ground (v _{HF}=0) and vibrationally excited (v _{HF}=1) state.
The OCOHF ground state (v _{HF}=0) potential is dominated by quartic and sextic angular terms, and thus is surprisingly shallow with respect to the bending angle. This q u a s i l i n e a r vibrational behavior is characterized by wide amplitude bending wave functions with zero point motion extending from −38° to +38°. In contrast, the OCOHF excited state (v _{HF}=1) exhibits a significantly b e n t equilibrium geometry with a hydrogen bond bend angle of 31°±5°, corresponding to a cylindrically symmetric, noncolinear minimum in the potential. This shift in equilibrium geometry upon v _{HF} excitation is quantitatively responsible for promoting Δv _{bend}=0,2,... combination band vibrational modes, in analogy with Franck–Condon progressions in a bent←linear electronic transition.
The predissociation lifetimes for v _{HF}=1 excited OCOHF vary systematically with v ^{ l } _{bend}, and can be analyzed in terms of a geometry dependent predissociation rate which increases with bending of the hydrogen bond angle. These empirical bending potentials are in qualitative agreement with, but quantitatively much shallower than predicted by previous electrostatic and a b i n i t i o calculations, and differ fundamentally from the traditional notions of a relatively s t i f f, linear hydrogen bond. The present results on the hydrogen bond potential surface for O=C=O‐‐‐HF are consistent with statistical analyses of–C=O‐‐‐H–N hydrogen bond angles obtained from x‐ray crystallographic studies of proteins.

Rotationally resolved photoionization of molecular oxygen
View Description Hide DescriptionWe report the results of theoretical studies of the rotationally resolved photoelectron spectra of ground state O_{2} leading to the X ^{2}Π_{ g } state of O^{+} _{2} via the absorption of a single vacuum ultraviolet photon. These studies elaborate on a recent report [M. Braunstein e t a l., J. Chem. Phys. 9 3, 5345 (1990)] where we showed that a shape resonance near threshold creates a significant dependence of the rotational branching ratios on the ion vibrational level. We also showed that analysis of the rotational branches yields detailed information on the angular momentum composition of the shape resonance. We continue this analysis giving a comprehensive derivation of the rotationally resolved cross sections and photoelectron angular distributions. We discuss the selection rules implied by these expressions and present very high resolution cross sections (J→J ^{+}) obtained using static‐exchange photoelectron orbitals and explicitly taking into account the internuclear distance dependence of the electronic transition moment. These cross sections illustrate the selection rules and show more explicitly the angular momentum composition of the shape resonance. We also present rotationally resolved photoelectron angular distributions which would be expected at low energy.

Useful results for the determination of excited state geometries from resonance Raman spectra: Application to inorganic complexes
View Description Hide DescriptionThe time‐dependent formulation of Raman scattering is used to derive simple expressions for fundamental and overtone intensities that depend on potential energy features in the Franck–Condon region and the homogeneous damping constant due to the bath modes. From the Raman excitation profiles, the dependence of the full width at half‐maximum on the damping constant is calculated. The results are applied to the rich resonanceRaman spectra and Raman excitation profiles of transition metal complexes, in particular, Cs_{3}[Re_{2}OCl_{10}] and Cs_{4}[W_{2}OCl_{10}], to determine the magnitude of the geometric changes occurring upon excitation of the molecule from the ground to the excited electronic state. For each compound, the multidimensional harmonic potential surface and damping constant derived for the excited electronic state are then used to simulate the observed Raman excitation profiles and resonanceRaman spectrum.