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Volume 101, Issue 3, 01 August 1994

Pressure‐induced disappearance of the in‐plane lattice distortion in layered cupric chloride: (C_{2}H_{5}NH_{3})_{2}CuCl_{4}
View Description Hide DescriptionEffects of hydrostatic pressure on Raman scattering and electronic absorption spectra have been investigated in layered cupric chloride (C_{2}H_{5}NH_{3})_{2}CuCl_{4}, consisting of strongly Jahn–Teller (JT) deformed CuCl_{4} layers. We observed selective deactivation of the Cu–Cl stretching modes at a pressure above ∼4 GPa, and interpreted the phenomena in terms of disappearance of the in‐plane lattice distortion of the CuCl_{4} layer. The pressure‐induced structural change reduces the charge‐transfer (CT)‐type gap from the ligand (Cl3p) to metal (Cu3d _{ x 2−y 2 }) sites by ∼0.4 eV, and perhaps produces the two‐dimensional electronic states analogous to that of the parent compounds for the cupric superconductors.

A Raman investigation of lead haloborate glasses
View Description Hide DescriptionGlasses of the (65−x)PbO⋅xPbX_{2}⋅35B_{2}O_{3} system (where X=F, Cl, Br, and I) have been made and investigated by Raman measurements. Three low frequency bands with high Raman intensities have been observed at about 40, 105, and 135 cm^{−1}, respectively. It is found that the intensities of the depolarized low frequency Raman contour peaked at 40 cm^{−1} are exceptionally high and strongly dependent on the halogen type and content. We attribute this nonsymmetric low‐frequency Raman contour to the collective modes of the BO_{3}–PbO_{4} bridged local structures and propose a model to interpret the experimental data. Our results suggest that the liberation of these local structures is related to the lead as well as the halogen. Cl, Br, and I enter the glass as nonbridging anions and weaken the cohesion of the glass network in the increased order of Cl, Br, and I.

Cage‐to‐cage migration rates of Xe atoms in zeolite NaA from magnetization transfer experiments and simulations
View Description Hide DescriptionXenon trapped in the alpha cages of zeolite NaA exhibits distinct NMR signals for clusters Xe_{1}, Xe_{2}, Xe_{3},..., up to Xe_{8}. Using multisite magnetization transfer experiments, we have measured the rate constants k _{ mn } for the elementary processes that are involved in the cage‐to‐cage transfer of Xe atoms in the zeolite NaA, that is, for a single Xe atom leaving a cage containing Xe_{ n } to appear in a neighboring cage containing Xe_{ m−1}, thereby forming Xe_{ m }. In a random walk simulation, these rate constants reproduce over a hundred magnetization decay/recovery curves that we have measured in four samples of Xe in zeolite NaA at room temperature, in selective inversion, and complementary experiments for all the significantly populated clusters. The simulations also lead to the correct experimental equilibrium distributions, that is, the fractions of the alpha cages containing Xe_{1},Xe_{2},...,Xe_{8}.

Triplet‐state photoexcitations of oligothiophene films and solutions
View Description Hide DescriptionWe present studies of steady‐state photoinduced absorption (PIA) spectroscopy on photoexcitations in a series of well‐defined α‐oligothiophene (T _{ n }, n=6, 7, 9, and 11) films and solutions. The PIA spectra and the excited state lifetimes are consistent with the signatures of a photoexcited triplet state. The PIA spectra consist of a strong vibronically resolved subgap absorption, which is readily observed in solid‐state films and in solutions at ambient and cryogenic temperatures. The transition energy is linearly dependent on the reciprocal chain length and shifts to lower energy for longer oligomers. Variation of the modulation frequency and the pump intensity under matrix‐isolated conditions reveals that the photoexcitation is created via an intrachain mechanism and decays nonradiatively with monomolecular kinetics. In solid films we find a significant contribution of a bimolecular decay process to the relaxation rate.

Tunneling frequencies of NH_{2}D^{+} _{2} and CH_{2}D_{2} in crystalline fields of low symmetry
View Description Hide DescriptionProcedures for calculating the rotational energy levels of a hindered asymmetric top in crystalline fields of C _{3}, C _{2}, and C _{1} symmetry are outlined. These procedures are used to compute the splittings between tunneling states of NH_{2}D^{+} _{2} and CH_{2}D_{2} in various crystalline environments. These splittings have been computed for NH_{2}D^{+} _{2} in (NH_{4})_{2}SiF_{6}, (NH_{4})_{2}GeF_{6}, (NH_{4})_{2}TiF_{6}, NH_{4}F, NH_{4}ReO_{4}, NH_{4}IO_{4}, NH_{4}HF_{2}, and NH_{4}ClO_{4}. Large splittings between tunneling states are computed for this asymmetric top in these compounds, all of which have site symmetries for the ammonium ion that are less than tetrahedral. Two groups of tunneling states are found when the site symmetry of NH_{2}D^{+} _{2} is C _{3v } and four groups of states are found when the site symmetry of NH_{2}D^{+} _{2} is S _{4}, C _{2}, or C _{ s }. These large tunneling frequencies are similar to those computed for NH_{3}D^{+} and NHD^{+} _{3} in these solids. Tunneling frequencies have also been calculated for the asymmetric top CH_{2}D_{2} in phase II of solid methane and for CH_{2}D_{2} adsorbed on the surfaces of graphite and MgO. For the latter two environments, the splittings between the groups of tunneling states are larger than was found for CH_{3}D, but the tunneling frequencies within a group of tunneling states is smaller than for CH_{3}D in the same environment.

Double‐quantum homonuclear rotary resonance: Efficient dipolar recovery in magic‐angle spinning nuclear magnetic resonance
View Description Hide DescriptionWe describe an efficient method for the recovery of homonuclear dipole–dipole interactions in magic‐angle spinning NMR. Double‐quantum homonuclear rotary resonance (2Q‐HORROR) is established by fulfilling the condition ω_{ r }=2ω_{1}, where ω_{ r } is the sample rotation frequency and ω_{1} is the nutation frequency around an applied resonant radio frequency (rf) field. This resonance can be used for double‐quantum filtering and measurement of homonuclear dipolar interactions in the presence of magic‐angle spinning. The spin dynamics depend only weakly on crystallite orientation allowing good performance for powder samples. Chemical shift effects are suppressed to zeroth order. The method is demonstrated for singly and doubly ^{13}C labeled L‐alanine.

Anomalous pressure effects on the Raman spectra in hydrogen‐bonded molecular chain systems
View Description Hide DescriptionEffects of hydrostatic pressure on the Raman spectra have been investigated for four kinds of 1,3‐diketone crystals with hydrogen‐bonded molecular chains. In all the crystals we studied, intense CO stretching Raman mode shows pressure‐induced softening reflecting compression of the hydrogen bonds. Furthermore, application of pressure broadens the both C=O and C–O stretching modes and intensifies several specific vibrational bands. We ascribed these spectral changes to formation or growth of the kink‐type defects of the hydrogen‐bonded sequence.

Many‐body potentials of an open shell atom: Spectroscopy of spin–orbit transitions of iodine in crystalline Xe and Kr
View Description Hide DescriptionTemperature dependent emission spectra of spin excited iodine in crystalline Xe and Kr are presented and analyzed in terms of nonadditive anisotropic pair interactions. In the octahedral trap site, the atomic ^{2} P states split into E _{1/2} and G _{3/2} groups of the double valued representation. The fourfold degenerate G _{3/2} state is subject to strong Jahn–Teller instability and further splits by coupling to phonons into E _{1/2} and E _{3/2} Kramers pairs. Accordingly, the observed emission spectra are composed of two bands: 2E _{1/2}→1E _{1/2} and 2E _{1/2}→E _{3/2} transitions. Two pairs of bands are observed each in Xe and Kr. The long‐lived pairs (at 15 K, τ=250 μs and 930 μs in Xe and Kr, respectively) are assigned to the isolated atom, while a short lived pair of bands (at 15 K, τ<1 μs in Xe, and τ=2.2 μs in Kr) are assigned to I atoms trapped as nearest neighbor to a localized charge, identified as (HRg)^{+}. The isolated atom spectra are simulated by Monte Carlo methods which assume classical statistics in the heavy atom coordinates, and adiabatic following of the electronic coordinate. Angle dependent, gas phase pair interactions are used as a starting point. Minor modifications to the pair interactions, and a temperature dependent spin–orbit splitting constant, adequately reproduce the experimental spectra. Many‐body contributions to the effective pair potentials can be estimated to change pair parameters by less than ∼3%.

^{2}H electron spin echo envelope modulation spectroscopy of strong, α‐hydrogen hyperfine coupling in randomly oriented paramagnetic systems
View Description Hide DescriptionExperimental characterization of strong, anisotropic^{2}H hyperfine interactions in randomly oriented organic radicals in the solid state by using electron spin echo envelope modulation (ESEEM) spectroscopic techniques has been examined systematically. The α‐3,5‐^{2}H coupling in the tyrosine neutral radical in a low temperature aqueous glass was used as a model. Envelope modulation was obtained by integration of the stimulated‐echo generated by a three‐pulse, microwave pulse‐swapping sequence. Division of envelope modulation from the ^{2}H‐substituted radical by that from the per‐protonated radical remedies discontinuities introduced in the envelope by the eclipse of the second and third pulses. Envelope modulation data was collected for τ values from 214 to 1295 ns (9.132 GHz, 0.3265 T). The common, spectrometer dead‐time‐limited data collection start‐point (140 ns) for the different τ values attenuates loss of anisotropic information and allows summation of time domain data. The Fourier transform of the envelope summation shows the line shape determined only by the orientation‐dependencies of the modulation depth and the random distribution of spin systems. The systematic influence of τ‐suppression on the line shapes is examined in the individual spectra. Intensity in the region of the Δm _{ I }=±2, double quantum transition is revealed at values of τ sufficiently long to relieve τ‐suppression of this feature. Algebraic expressions for the orientation‐dependence of the modulation frequencies and amplitudes are given for the general case of rhombic hyperfine interaction in S=1/2, I=1 systems where hyperfine interactions dominate nuclear quadrupole coupling, and are used to describe the changes in the experimental line shapes wrought by variation of τ and magnetic field strength. Theoretical simulation of the spectra yields the complete 3,5‐^{2}H hyperfine tensor, (−3.0, −3.9, −1.1)±0.1 MHz. The results show that the sculpting of the ESEEM line shape by the suppression effect allows retrieval of information, obscured by the orientation dependence of the modulation depth, that is necessary to determine the rhombic hyperfine tensor.

Franck–Condon modeling of the structure of the S _{0}→S _{2} transition of trans, trans‐, cis, trans‐, and cis, cis‐octatetraene
View Description Hide DescriptionThe Franck–Condon structure of the lowest lying intense polyenic electronic transition of trans, trans‐, cis, trans‐, and cis, cis‐octatetraene is investigated through model calculations. To avoid a possible bias in the parameters of the model, the starting inputs are obtained ab initio. The molecular orbital (MO) procedure consists in first optimizing the structures of S _{0} and S _{2} and then calculating the vibrational frequencies at the stationary points on the potential energy surface of the three isomers. Together with the minima associated with the three isomers, we find one more saddle point in S _{0} and two more in S _{2}. These three saddle points correspond to planar S _{0} cis, cis‐octatetraene, planar S _{2} cis, cis‐octatetraene, and planar S _{2} cis, trans‐octatetraene. The displacement, between the surfaces, of the harmonic oscillators associated with the normal modes, are obtained and used to simulate the Franck–Condon activity of the S _{0}→S _{2} transitions of the three isomers. Such displacements are calculated in two schemes, the first uses the variation of the equilibrium position of the vibrational oscillators in the two states involved in the transition and requires full geometry optimization of all the geometrical parameters of the two states; the second, approximate, scheme requires only a single point calculation on the excited statesurface and is therefore far less demanding. A simple scaling procedure, proposed before for hexatriene, is used to improve the agreement between theory and experiment. The model calculations of the Franck–Condon structure simulate very well the S _{0}→S _{2}absorption spectra of trans, trans‐ and cis, trans‐octatetraene. It is further proposed that the large homogeneous linewidth in the S _{0}→S _{2} transitions of polyenic systems is a function of the nonplanarity of these molecules in S _{2}.

Three‐dimensional variable‐angle nuclear magnetic resonance exchange spectroscopy without rotor axis hopping
View Description Hide DescriptionSlow, large‐amplitude chain motions play an important role in determining the macroscopic mechanical properties of polymers. Although such motions have been studied quantitatively by two‐dimensional (2D) nuclear magnetic resonance(NMR) exchange experiments, overlapping anisotropic patterns hamper spectral analysis, and limit applications. Variable angle correlation spectroscopy (VACSY) has proven useful in resolving such problems for rapidly spinning samples by separating anisotropic spectral patterns according to isotropic chemical shifts. In a previous study [J. Am. Chem. Soc. 115, 4825 (1993)], we described a three‐dimensional (3D) NMR experiment that incorporates the VACSY method and a hop of the rotor axis to correlate the isotropic chemical shifts to 2D anisotropic exchange patterns. The hop of the rotor axis, however, presents experimental difficulties and limits the range of motional rates that may be studied. We present in this paper a new 3D VACSY exchange experiment that obtains the same correlations without the need for the rotor axis hop. A series of 2D exchange spectra are recorded with the sample spinning at different rotation axis angles. Then using the scaling of the anisotropic frequency at the different angles, we construct the data onto a 3D matrix so that a Fourier transformation directly yields the desired correlations. The technique is applied to ^{13}C exchange NMR to study the slow molecular motion of ordered isotactic polypropylene.

Molecular spectroscopy with light pulses of arbitrary pulse shape and field strength: A nonperturbative approach
View Description Hide DescriptionWe present a novel approach to molecular spectroscopy with light pulses of arbitrary strength and duration. The key quantity is the frequency‐resolved net energy transferdE/dω which reveals at which frequencies energy is transferred from the field to the molecule (absorption) or from the molecule to the field (stimulated emission). It is shown that dE/dω can be expressed as the Fourier transform of the cross‐correlation function of the molecular polarizationP(t) and the time derivative of the applied field, dE/dt. In this sense, it is formally equivalent to the absorption cross section under weak‐field conditions which, as commonly known, can be represented as the Fourier transform of the autocorrelation function S(t). The time‐dependent polarizationP(t) is determined by exact integration of the time‐dependent Schrödinger equation including the light‐matter interaction to all orders. It is shown that under weak‐field conditions the expression for dE/dω reduces to the well‐known cross section formula in the time‐dependent picture of spectroscopy, multiplied by the spectral intensity of the light pulse. Therefore, we consider the expression for the frequency‐resolved energy transfer, which is valid for arbitrary electric fields, as the natural extension of the absorption cross section in the weak‐field limit. Furthermore, dE/dω is shown to be formally equivalent to the change of the spectral intensity, ΔI(ω), of an optical pulse after transmission through a sample, the latter being derived by solving Maxwell’sequations under well‐known approximations. The theory is applied to a simple one‐dimensional model with two electronic states and the frequency‐resolved energy transfer is investigated as a function of the field strength. For sufficiently strong fields, dE/dω exhibits transitions between essentially all vibrational levels in the ground and all states in the excited electronic manifold. The new expression distinguishes between absorption and emission and that is clearly seen in the spectra.

Experimental study of the CO:O_{3} complex in argon matrices: Irradiation at 266 nm
View Description Hide DescriptionCodeposition of argon dilute samples of O_{3} and CO produced new narrow absorption lines which are assigned to the CO:O_{3} complex. One line appears at 2140.44 cm^{−1}, 2 cm^{−1} above the CO frequency which shows that it is a weakly bonded complex. Its spectrum exhibits three new lines near the ν_{3} doublet of O_{3}. The two lines observed at 1042.56 and 1043.37 cm^{−1} appear clearly at 5 K; the lowest frequency one, at 1041.35 cm^{−1}, is overlapped at 5 K by the high frequency component of the ν_{3} doublet of isolated O_{3} and can only be detected when the temperature is raised, as it is almost not temperature dependent up to 20 K. Irradiation of the sample at 5 K with the 266 nm emission of a quadrupled ND‐YAG laser leads to an efficient decomposition of free and complexed ozone and an increase of CO_{2} with two regimes, a fast beginning similar to the O_{3} and CO:O_{3} decays followed by a much slower one. After such an irradiation at 5 K, some recombination of O_{3} and CO:O_{3} is observed during a warming at 10 K. A careful analysis of the relative intensities variation shows that CO_{2} is only produced inside the CO:O_{3} complex, from the reaction of CO with ^{1} D atomic oxygen. As ^{1} D oxygen atoms are very fastly relaxed to ^{3} P ones during their migration through the matrix, isolated CO does not react. We have shown evidence of a very limited migration of atomic oxygen at 5 K in solid argon, and of an efficient one at 10 K.

Stark energy levels of symmetric top dipoles: Analytical expressions for arbitrary field strengths
View Description Hide DescriptionAnalytical expressions are presented which interpolate between weak field and strong field expansions of rotational energy levels of polar symmetric top molecules in homogeneous electrical fields. Besides Stark spectroscopy, the derived formulas can be used as well for calculating adiabatic channel potential curves in capture processes of species whose interaction is characterized by an anisotropy of the cos θ type.

Overtone resonance Raman scattering beyond the Condon approximation: Transform theory and vibronic properties
View Description Hide DescriptionThe time correlator formalism is used to develop the expression for nth order (overtone) resonance Raman scattering (RRS) to include both Raman frequency shifts upon electronic excitation as well as non‐Condon vibronic coupling. In particular the compact operator formalism recently introduced by Hizhnyakov and Tehver [J. Raman Spectrosc. 19, 383 (1988)] to obtain several RRS correlators (including overtone scattering with frequency shift, but in the Condon approximation) is used to extend the theory. At the same time a formal advantage is achieved by the limited introduction of the Born–Oppenheimer approximation. Also transform relationships including non‐Condon effects are given that link the Raman excitation profile of nth order scattering to the absorptionspectrum. Finally, it is emphasized how all three vibronic parameters—potential energy surface displacement, Raman mode frequency changes, and the linear non‐Condon coupling parameter—can be quantitatively determined without the need for absolute Raman cross‐section measurements. The relative scattering intensity of the fundamental and three (or more) overtones suffices to fix the three. By way of application, the vibronic parameters are determined from published single wavelength overtone RRS in six molecules.

Stationary approaches for solving the Schrödinger equation with time‐dependent Hamiltonians
View Description Hide DescriptionAccurate and efficient computational schemes are utilized for solving the Schrödinger equation with time‐dependent Hamiltonians. These schemes, based on an extended Hilbert space in which time is treated like a space coordinate, allow essentially all of the computational strategies for stationary problems to be equally applicable to explicitly time‐dependent problems. In particular, variational principles, the discrete variable representation in time, the fast Fourier transform in time, the recursive residue generation method (RRGM), and the Chebyshev propagator can be employed. Some of these methods are implemented and tested for a rigid rotor interacting with various laser pulses. The fast exponential convergence with respect to the number of time grid points is illustrated, which accounts for the high accuracy of the methods (limited only by the precision of computer), a feature which is hardly achievable by other methods.

High‐accuracy measurement of vibrational Raman bands of ozone at 266 and 270 nm excitations
View Description Hide DescriptionResonance Raman spectra of ozone at two excitation wavelengths (266 and 270 nm) have been measured up to 11 000 cm^{−1}. Band origins have been measured to high accuracy and determined to within a few cm^{−1}. Several bands beyond the dissociation limit have been observed. All prominent bands observed in this work fit to a two‐oscillator Darling–Dennison model. An analytical two‐dimensional potential energy surface has been constructed based on the new experimental data.

Experimental investigation of vibrational radiative lifetimes: H_{2}O^{+} and D_{2}O^{+} ions in their ground electronic state (X ^{2} B _{1})
View Description Hide DescriptionRadiative lifetimes of vibrationally excited H_{2}O^{+} and D_{2}O^{+} ions in their ground electronic state (X ^{2} B _{1}) have been determined using the monitor ion technique in a triple cell ion cyclotron resonancespectrometer with Fourier transform detection. The monitor reactions are proton or deuteron transfer from H(D)_{2}O^{+} to CO_{2} and N_{2}O. The lifetimes are corrected for collisional deactivation and reactions with the background gases occurring during the relaxation time of the ions. N_{2}O probes all the excited vibrational levels of H_{2}O^{+} and D_{2}O^{+}. For H_{2}O^{+} only the bending modes (0,v≥1,0) contribute to the decay curve. The corresponding overall lifetime, 26.8±3 ms, is in very good agreement with the computer simulated overall lifetime including the theoretical lifetimes of Weis et al. [J. Chem. Phys. 91, 2818 (1989)] and estimated populations of the bending vibrational levels. For D_{2}O^{+}, the overall lifetime of the (0,v≥1,0) bending modes, 99.5±15 ms, and the lifetime of the (1,0,0) stretching mode, 27.5±4.5 ms, are observed, also in good agreement with the computer simulated and theoretical values, respectively. For both ions the overall lifetime of the (0,v≥1,0) levels may be considered as a good approximation for the radiative lifetime of the (0,1,0) level. The overall lifetimes determined with CO_{2} as a monitor may be attributed to the (0,v≥4,0) bending modes: 8.1±1 ms for H_{2}O^{+} and 44±12 ms for D_{2}O^{+}. In this case, several levels having similar populations and lifetimes contribute to the decay curve, therefore the lifetimes of the individual levels cannot be determined. The agreement with computer simulated lifetimes is an indication for the validity of the theoretical lifetimes.

Electronic structure of the copper(II) ion doped in cubic KZnF_{3}
View Description Hide DescriptionThe absorption and magnetic circular dichroismspectra of 1%–3% copper(II) doped into the cubic perovskite host KZnF_{3} are measured over the temperature range 1.8–300 K. Sharp magnetic dipole allowed transitions^{2} E _{ g }(Γ_{8})→^{2} T _{2g }(Γ_{7},Γ_{8}) are observed together with accompanying vibrational fine structure. The spectra are interpreted on the basis of a tetragonally elongated CuF^{4−} _{6}ground state geometry which arises from strong Jahn–Teller coupling. This results in a statistical distribution of three equivalent elongations which can undergo reorientation on the electron paramagnetic resonance time scale. The Jahn–Teller coupling within the ^{2} T _{2g } multiplet is partially quenched by spin–orbit coupling and the lowest Kramers doublet Γ_{7} has an octahedral geometry, while the higher lying Γ_{8} state has a small tetragonal distortion. This system presents an excellent opportunity to study the spectroscopy of the copper(II) ion in a cubic environment.

Atom–molecule van der Waals complexes containing open‐shell atoms. I. General theory and bending levels
View Description Hide DescriptionThe theory needed to carry out calculations on atom–molecule van der Waals complexes containing open‐shell atoms is developed. The discussion concentrates on complexes containing atoms in P states. Several possible expansions of the total wave function are described, and the matrix elements needed to construct the Hamiltonian matrix are set out. Several different angular momentum coupling cases may arise, analogous to Hund’s coupling cases in diatomic molecules. The bending energy levels of Ca–HCl, B–H_{2}, F–H_{2}, Cl–Cl_{2}, and F–N_{2} are calculated, using simple models of the interaction potentials.