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Volume 104, Issue 13, 01 April 1996

Multidimensional nuclear magnetic resonance in complex liquids analyzed by a simple model for non‐Markovian molecular reorientation
View Description Hide DescriptionA simple model of non‐Markovian molecular reorientation is applied to multidimensional NMRexperiments in supercooled liquids and polymers close to the glass transition. The process of exchange between components of a correlation time distribution is related with the notions ‘‘exponential/nonexponential, homogeneous/heterogeneous, Markovian/non‐Markovian, and ergodic/nonergodic’’ often used in relation with molecular dynamics in viscous systems. By working out a detailed example of two‐state exchange the difference between 2D and reduced 4D‐spectra with respect to the mechanism and dynamics of molecular reorientation in complex liquids is elucidated.

The S _{1}–S _{0}(^{1} B _{2}–^{1} A _{1}) transition of jet‐cooled toluene: Excitation and dispersed fluorescence spectra, fluorescence lifetimes, and intramolecular vibrational energy redistribution
View Description Hide DescriptionThe fluorescence excitation spectrum of the S _{1}–S _{0}(^{1} B _{2}–^{1} A _{1}) transition in jet‐cooled toluene has been measured up to 2000 cm^{−1} above the origin band. Dispersed fluorescence spectra of the major features have been recorded and used to assign the levels observed in excitation. Collisional energy transfer experiments have been used to confirm assignments for some of the lower lying S _{1} fundamentals that were not accessible via direct optical pumping. The number of known S _{1} fundamentals has been extended to 13. The dispersed fluorescence spectra reveal the onset of intramolecular vibrational energy redistribution (IVR) at low S _{1} vibrational energies.Fluorescence lifetimes of all of the major bands observed in the excitation spectrum have been measured. The lifetimes decrease from 86 ns for 0^{0} to 48 ns at an S _{1} vibrational energy of 1900 cm^{−1}. To alleviate the confusion that exists over the mode numbering in toluene a new scheme is proposed which obviates this problem. This system is similar to that used for other substituted aromatics and should rationalize future work.

Photoelectron spectroscopy of mixed metal cluster anions: NiCu^{−}, NiAg^{−}, NiAg^{−} _{2}, and Ni_{2}Ag^{−}
View Description Hide DescriptionNegative ion photoelectron spectra of NiCu^{−}, NiAg^{−}, NiAg^{−} _{2}, and Ni_{2}Ag^{−} are presented for electron binding energies up to 3.5 eV. The metal cluster anions were prepared in a cold cathode dc discharge flowing afterglow source. The dimer spectra exhibit three low lying electronic states; the ground electronic states are vibrationally resolved. The dimer electron affinities are determined to be EA(NiCu)=0.889±0.010 eV and EA(NiAg)=0.979±0.010 eV. Two excited state electron configurations are assigned as (d ^{8} _{Ni} d ^{10} _{Cu}σ^{2}σ*^{1}) and as ^{4}Δ (d ^{9} _{Ni} d ^{10} _{Cu}σ^{1}σ*^{1}). The NiCu^{−}ground state is assigned as ^{3}Δ (d ^{9} _{Ni} d ^{10} _{Cu}σ^{2}σ*^{1}), and has a vibrational frequency of 235±25 cm^{−1}. The photoelectron spectrum of NiAg^{−} strongly suggests that the electronic configurations of the three observed states are the same as those of NiCu^{−}. The NiAg ground state vibrational frequency is 235±25 cm^{−1} and the NiAg^{−} frequency is 185±25 cm^{−1}. The chemical bonding in both NiCu and NiAg dimers is dominated by a sσ molecular orbital, and the extra electron in the anions has primarily sσ* character. The photoelectron spectra of the trimers, NiAg^{−} _{2} and Ni_{2}Ag^{−}, are remarkably similar to those obtained for the coinage metal trimers, and are consistent with a transition between a linear anion ground state and a linear excited state of the neutral.

Zero kinetic energy‐pulsed field ionization and resonance enhanced multiphoton ionization photoelectron spectroscopy: Ionization dynamics of Rydberg states in HBr
View Description Hide DescriptionThe results of rotationally resolved resonance enhanced multiphoton ionizationphotoelectron spectroscopy and zero kinetic energy‐pulsed field ionization studies on HBr via various rotational levels of the F ^{1}Δ_{2} and f ^{3}Δ_{2}Rydberg states are reported. These studies lead to an accurate determination of the lowest ionization threshold as 94 098.9±1 cm^{−1}. Observed rotational and spin–orbit branching ratios are compared to the results of ab initio calculations. The differences between theory and experiment highlight the dominant role of rotational and spin–orbit interactions for the dynamic properties of the high‐nRydberg states involved in the pulsed field ionization process.

Curvature‐based analysis of nonmonotonic forced Rayleigh scattering signals
View Description Hide DescriptionWe propose a new method for analysis of nonmonotonic forced Rayleigh scattering (FRS) decay profiles. It is shown that the curvature near the local maximum of a decay–grow–decay profile or a grow–decay profile can be used to determine directly the geometric mean of the unperturbed and photoproduct diffusion coefficients, which will in many cases mitigate the need to apply multiparameter nonlinear fitting routines. FRS tracer‐diffusion measurements of methyl red in poly(vinyl acetate)/toluene solutions at two different probe wavelengths, analyzed using the new method, are in good agreement with results of previous studies. Simulations are used to estimate the systematic error due to the approximation employed, and it is concluded that accuracies of a few percent or better should be routinely achievable in most cases. We have, in addition, investigated the probe‐wavelength dependence of the profile shape for this system.

An electron spin resonance investigation of the ^{12}C^{11}B^{12}C, ^{12}C^{11}B^{13}C, and ^{13}C^{11}B^{13}C radicals in neon, argon, and krypton matrices: Comparison with ab initio calculations
View Description Hide DescriptionThe 11 electron CBC radical has been generated by the pulsed laservaporization of elemental carbon–boron mixtures and trapped in neon, argon and krypton matrices for detailed electron spin resonance(ESR) studies. Extensive comparisons of the experimental nuclear hyperfineAtenors for carbon and boron were made with a variety of ab initio computational results that involved different levels of theory and basis sets. These new ESR results agree with recent vibrational studies of CBC that show it to have a nonlinear symmetric geometry with a ^{2} A _{1} electronic ground state. These ESR results provide a description of the singly occupied molecular orbital. The spin density resides primarily on boron in 2s and 2p _{ z } orbitals, however a complete resolution of the ^{13}C hyperfine structure does show that approximately 20% resides on the carbon atoms.

Spin–orbit branching ratios for photoionization of the 3dπ gerade states of O_{2}: Evidence for preferential ionization of the Ω_{ c }=3/2 core states
View Description Hide DescriptionWe have obtained high‐resolution photoelectron spectra for eight of the ten 3dπ electronic states of O_{2}, using a rotationally resolved, two‐color 2+1′ excitation and ionization scheme. A magnetic bottle photoelectron spectrometer, used to acquire the spectra, provided a resolution sufficient to observe the yields of the individual spin–orbit states of the ground electronic state of the ion. Branching ratios were obtained from the spectra for three different photoionization wavelengths. The overall behavior of the branching ratios for different electronic states can be explained by calculating the core state composition of the intermediate states, taking into account the spin–orbit mixing of Hund’s case (a) basis states, and assuming that electronic states having the core state Ω_{ c }=3/2 have a larger photoionization probability than those having an Ω_{ c }=1/2 core.

Ab initio calculations and high resolution infrared investigation on XeF_{4}
View Description Hide DescriptionAb initio calculations employing effective core potentials and polarized triple‐zeta basis sets have been carried out for XeF_{4} at the following levels: self‐consistent‐field (SCF) theory, Mo/ller–Plesset second‐order perturbation theory (MP2), and coupled clustertheory with single and double excitations and a perturbational treatment of triple excitations (CCSD(T)). Pronounced correlation effects are found, especially for the Xe–F bond length and the vibrational frequencies. The theoretical predictions for the harmonic and anharmonic force fields and the associated spectroscopic constants have guided the analysis of the experimental data. Fourier transforminfrared spectra of monoisotopic ^{136}XeF_{4} have been recorded between 100–1170 cm^{−1} with an effective resolution of 0.002–0.004 cm^{−1}, and a rotational analysis has been performed for the ν_{2} band. The high resolution results provide accurate molecular parameters for the ground state and the v _{2}=1 excited state of XeF_{4} and allow a precise determination of the ground state Xe–F bond length, r _{0}=193.487(3) pm. The agreement between the experimental and the correlated theoretical results is generally quite good.

Electronic absorption spectra of linear carbon chains in neon matrices. IV. C_{2n+1} n=2–7
View Description Hide DescriptionThe ^{1}Σ^{+} _{ u }←X ^{1}Σ^{+} _{ g } electronic absorption system of the carbon chains C_{2n+1} (n=3–7) trapped in 5 K neon matrices has been identified. The ^{1}Π_{ u }←X ^{1}Σ^{+} _{ g } transition has also been observed for C_{5} and C_{7}, as well as a dipole forbidden one for all the species. The spectroscopic information was derived from absorption spectra recorded after neutralization of mass‐selected anions and cations that were codeposited with an excess of neon to form a matrix. The assignment is based on the mass‐selection, theoretical predictions, and trends observed for this homologous series. These results are related to published absorption spectra of graphite vapor trapped in rare gas matrices. The suggestion that such species may be potential carriers of diffuse interstellar bands is considered.

Dye laser excitation studies of the Ã ^{2}Π(100)/(020)–X̃ ^{2}Σ^{+}(020)/(000) bands of CaOD: Analysis of the Ã ^{2}Π(100)∼(020) Fermi resonance
View Description Hide DescriptionThe CaOD Ã ^{2}Π(100)/(020)–X̃ ^{2}Σ^{+}(020)/(000) bands have been rotationally analyzed via high resolution laser excitation. All measured line positions have been included in a global matrix deperturbation that takes account of the Renner–Teller, spin–orbit, and Fermi resonance interactions occurring in the Ã(100)(020) ^{2}Π vibronic manifold. The corresponding bands of CaOH were studied previously; in the present work, two new CaOH subbands, Ã(020)κ ^{2}Π–X̃(020), were recorded, and the complete data set for CaOH has been refitted using the improved model reported in this paper. The Fermi resonance parameter for CaOD has been determined as ‖W 1‖=5.2707(22) cm^{−1}; for CaOH, the newly determined value, ‖W 1‖=10.3256(5) cm^{−1} is very close to that determined originally. The (100)∼(020) Fermi interaction in the X̃ ^{2}Σ^{+} state has also been investigated for both isotopomers. The vibrational dependence of the Renner–Teller parameter εω2 has been characterized, yielding values of the anharmonic quartic parameter, ĝ4=−0.1002(3) and −0.0666(5) cm^{−1} for CaOH and CaOD, respectively. The ‘‘harmonic’’ Renner–Teller parameters are thus deduced as εω2=−35.6622(19) and −26.5605(31) cm^{−1} for CaOH and CaOD, respectively. The equilibrium bond lengths, molecular force constants and Coriolis coupling constants for both the Ã and X̃ states have been evaluated.

Electron transfer dynamics in MoS_{2} nanoclusters: Normal and inverted behavior
View Description Hide DescriptionThe photophysics and electron transfer(ET) dynamics of quantum confined MoS_{2}nanoclusters have been studied using static and time resolvedemission spectroscopy. The MoS_{2}nanoclusters consist of a single S–Mo–S trilayer, having diameters of ∼2.5 or 4.5 nm. Two types of electron acceptors are adsorbed on these nanoclusters: 2,2′‐bipyridine (bpy) and 4,4′,5,5′‐tetramethyl‐2,2′‐bipyridine (TMB). The ET reaction exothermicities may be varied by changing the electron acceptor or by varying the size of the MoS_{2}nanocluster. TMB is harder to reduce, and thus has a smaller ET driving force than bpy. The smaller nanoclusters have a higher energy conduction band, and thus have a larger ET driving force. In all cases, the ET driving force may be calculated from bulk MoS_{2} properties and quantum confinement theory. Both ‘‘normal’’ and ‘‘inverted’’ behaviors are observed. A reorganization energy of 0.40 eV is calculated from energy dependent ET rates.

Refractive index change by photothermal effect with a constant density detected as temperature grating in various fluids
View Description Hide DescriptionTwo origins of the refractive index variation after depositing heat energy to a fluid are separately investigated by using the transient grating (TG) method; the variations are caused by a density fluctuation and a temperature fluctuation with a constant density. Although the relative contribution of the temperature rise (constant density) component is small in the total refractive index change, a precise measurement of the TG signal enables us to determine the magnitude and it is found that they are as large as 3%–6% in magnitude of dn/dT except water, in which the relative contribution strongly depends on temperature. The origin of the temperature rise component is interpreted in terms of the interaction induced polarizability change of the fluid molecules. An application of this component to the ultrafast detection of the photothermal techniques is discussed.

Quantum dynamics of ArI_{2} vibrational predissociation including low total angular momenta: The role of intramolecular vibrational energy redistribution
View Description Hide DescriptionFull‐dimensional quantum dynamics calculations on ArI_{2}(B,v _{ i }) vibrational predissociation with total angular momenta J=0, 1, and 2 are presented. Models based on a few interacting states are shown to reproduce important aspects of the dynamics, confirming that vibrational predissociation is mediated by a few‐state intramolecular vibrational energy redistribution effect. As a consequence, vibrational predissociation rate constants exhibit large oscillations with v _{ i }, the initial I_{2} vibrational quantum number in the complex. The qualitative effect persists when alternative choices for the interaction potential parameters are considered. Similarly, despite the importance of Coriolis coupling, the effect persists as J is varied from 0 to 2. We also discuss how the effect may be difficult to observe in typical experiments that involve higher J values.

Radiative cooling of fullerenes
View Description Hide DescriptionWe have measured the metastable fragmentation of fullerene ions in molecular beams. The rates are significantly smaller than the evaporative ensemble prediction, consistent with an alternative cooling mechanism through emission of electromagnetic radiation. Modeling the competition between radiative and evaporative cooling yields information about the evaporative activation energy and emissivity of the carbon clusters.

Accurate high‐pressure and high‐temperature effective pair potentials for the systems N_{2}–N and O_{2}–O
View Description Hide DescriptionStatistical mechanical chemical equilibrium calculations of N_{2} and O_{2} show that these molecules dissociate behind strong shock waves. Our determination of accurate intermolecular potentials has required the consideration of the dissociation products N and O. Our previous theoretical efforts to predict the thermodynamic properties of these molecules relied in part on corresponding states theory and shock wave data of argon, without consideration of the dissociation products. Recent high‐pressure Hugoniot measurements, however, allowed a more accurate determination of the potentials and the explicit inclusion of the dissociation products. The best fit to the data is obtained with the exponential‐6 coefficients, for O_{2}–O_{2}: ε/k=125 K, r*=3.86 Å, α=13.2; for O–O: ε/k=700 K, r*=2.40 Å, α=11.0; for N_{2}–N_{2}: ε/k=293 K, r*=3.91 Å, α=11.5; and for N–N: ε/k=600 K, r*=2.47 Å, α=10.0. The unlike pair interactions are obtained from these like interactions with a modified Lorentz–Berthelot rule. The coefficients in the modified Lorentz–Berthelot equations are k/l/m=1/1/0.93 for O_{2}–O– and k/l/m=1/1/0.90 for N_{2}–N interactions.

Photodissociation spectroscopy and dynamics of the N_{2}O^{−} _{2} anion
View Description Hide DescriptionThe spectroscopy and dissociation dynamics of the N_{2}O^{−} _{2}(C_{2v }) anion have been investigated using the technique of fast ion beam translational spectroscopy. A newly developed pulsed supersonic discharge source is described for the production of internally cold N_{2}O^{−} _{2}. A structured absorption band beginning near 580 nm is observed, and is assigned to the B̃(^{2} A _{2})←X̃(^{2} B _{2}) transition with the aid of ab initio calculations. Two dissociation channels from the upper state are observed: (1) O^{−}+N_{2}O and (2) NO^{−}+NO. Translational energy and angular distributions are measured for both channels at several excitation energies. The translational energy distribution for channel (1) at 570 nm shows resolved structure corresponding to N_{2}O vibrational excitation. The translational energy distributions for channel (1) are reasonably well described by prior distributions, indicating this channel results from dissociation from the N_{2}O^{−} _{2}ground electronic state. In contrast, channel (2) appears to result from dissociation on a repulsive excited electronic state. From the translational energy distributions for channel (1), we obtain the bonddissociation energy and heat of formation (at 0 K) for N_{2}O^{−} _{2}: D _{0}(O–N_{2}O)=1.40±0.03 eV and ΔH ^{0} _{ f,0}(N_{2}O^{−} _{2})=0.58±0.04 eV.

Further direct evidence for stepwise dissociation of acetone and acetone clusters
View Description Hide DescriptionThe (pre)dissociation of acetone and acetone clusters after excitation to states corresponding to upper {S _{1},T _{1}} and 3sRydberg states of the acetone monomer are investigated through femtosecond pump–probe experiments coupled with molecular beam time‐of‐flight mass spectrometry techniques. Upon excitation to either state, [(CH_{3})_{2}CO]_{ n } ^{*} dissociates rapidly. Acetyl fragments, [(CH_{3})_{2}CO]_{ n−1}CH_{3}CO^{+} may arise from ionization of an excited species formed by (pre)dissociation of intact precursors or by dissociation after the intact species has been ionized. The method employed to separate these two channels is discussed herein; the resulting transients are fit to a kinetic model to elucidate intermediate lifetimes and dissociation mechanisms. The present experiments establish that a stepwise dissociation mechanism is operative upon excitation to the 3sRydberg state for the acetone monomer and dimer, with their corresponding acetyl fragments having lifetimes on the order of picoseconds. Larger cluster species, [(CH_{3})_{2}CO]_{ n } (n≳2), do not exhibit the predissociative behavior evident in the monomer and dimer. Conversely, dissociation upon excitation to the {S _{1},T _{1}} state exhibits an initial loss of a methyl unit, with the acetyl intermediate being considerably more stable than those created by dissociation of acetone from higher lying states. A strong dependence on the internal energy available after the dissociation event is noted.

Charge transport in lithium phthalocyanine
View Description Hide DescriptionThe acelectrical properties of monoclinic lithium phthalocyanine (LiPc) and of the iodinated compound LiPcI have been investigated in the frequency and temperature regimes 20 Hz≤ν≤1 GHz and 1.5 K≤T≤300 K, respectively. Both compounds are semiconductors with dielectric constants ε_{∞}≊6 (LiPc) and 20 (LiPcI). Up to room temperature tunneling of charge carriers is the dominant conduction process in both compounds, yielding intrinsic dc conductivities σ_{dc}≊5.3×10^{−4} Ω^{−1} cm^{−1} (LiPc) and 0.2 Ω^{−1} cm^{−1} (LiPcI). The frequency and temperature dependence of the complex ac conductivity suggests polarons as the dominant species of charge carriers. The higher conductivity of the iodinated compound can be attributed to an enhanced mobility of the polaronic charge carriers which is most probably due to a better overlap of the π orbitals along the stacking direction of the molecules.

Configuration interaction and the theory of electronic factors in energy transfer and molecular exciton interactions
View Description Hide DescriptionThe theory established in J. Chem. Phys. 101, 10521 (1994), for electronic factors which promote interchromophore electronic energy transfer,excitoninteractions and which provide the stabilization of excimers, is extended; first so as to include the possible contribution of doubly excited configurations. It is ascertained that there is a resultant effect upon the (interchromophore orbital overlap‐dependent) through‐configuration interaction, and a significant correction to the simple expression obtained previously for the Coulombic interaction. These CI effects are admitted to the general theory of the previous work and the cases of nonidentical, identical, and orthogonal donor and acceptor are discussed. Second, a description of superexchange effects is admitted to the theory. Two possible formalisms are developed and discussed.

Relaxation of the product state coherence generated through the photolysis of HgI_{2} in solution
View Description Hide DescriptionExperimental results are presented for wave packet dynamics of the HgI molecules produced through the impulsive photolysis of HgI_{2} in ethanol solution. The results demonstrate that the HgI is formed in a vibrational superposition state. The phase of the beats fit to a value of 0.9π. This information is used to estimate the frequency of a transition state vibration of the HgI_{2} reactive state in solution. Both the oscillation frequency and decay rate exhibit a probe wavelength dependence. This is described as a result of each of the finite probe spectral bandwidths interrogating a specific set of vibrational eigenstates. The anharmonicity of HgI leads to a distinct oscillatory signal for each of the probes used. From this an approximate assignment of the level pairs being sampled by each probe is obtained. This information is used to construct solvated potential energy surfaces consistent with the data. The level dependent relaxation was not interpretable by a simple Bloch‐type picture, in which each coherence is characterized by a complex frequency. A quantum mechanical relaxation matrix including coherence transfer terms adequately modeled the observed coherence retention. The data also show that an energy dependent pure dephasing rate is required for the anharmonic HgI level pairs. Other aspects of solvated wave packet dynamics are discussed.