Volume 100, Issue 1, 01 January 1994
Index of content:

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

Structure of the propene⋅sulfur dioxide complex
View Description Hide DescriptionThe rotational spectra of eight isotopomers of the propene⋅SO_{2} complex have been observed with a Fourier transformmicrowave spectrometer. The rotational constants of the normal species are A=4269.564 MHz, B=1577.2661 MHz, and C=1469.6335 MHz. The structure of the complex was derived from least‐squares fitting of the 24 moments of inertia. It has a stacked, near‐parallel planes configuration. The distance between the centers of mass of the two monomers is 3.26(5) Å. The sulfur atom is approximately above the propene double bond. The C_{2} axis of SO_{2} nearly eclipses the carbon–carbon single bond with the oxygen atoms towards the methyl group. The dipole moment of the complex was determined by Stark effect measurements to be μ=1.34(3) D. The binding energy is estimated to be 2.9 kcal/mol from the pseudodiatomic model. Both electrostatic and ab initio calculations have been carried out to rationalize the structure and properties of the complex. The effect of methyl group substitution on the structures and properties of the ethylene⋅SO_{2}, propene⋅SO_{2}, and toluene⋅SO_{2} complexes is discussed.

The effect of pressure on the fluorescence lifetime of pentacene in para‐terphenyl at low temperature
View Description Hide DescriptionTime resolved fluorescencemeasurements are presented as a function of pressure for pentacene in para‐terphenyl at low temperature. A photosite specific pressure effect is observed at low temperature for the triclinic (P≤5.5 kbar) phase of para‐terphenyl. In addition, the fluorescence lifetime of pentacene in the monoclinic (high pressure) crystal phase at low temperature is obtained. We also observe unique fluorescence decay dynamics in the pressure region (5.5–6.5 kbar) intermediate to the triclinic and the monoclinic crystal phases. The effect of pressure on the triclinic host crystal is discussed in terms of a lowering of the energy of the lowest singlet excited state (S _{1}) relative to the nearby triplet level (T _{2}) with a corresponding decrease in the intersystem crossing rate. The fluorescence dynamics also provide information on the nature of the pentacene environment in the higher pressure (i.e.≳5.5 kbar) structural phases of the p‐terphenyl host crystal.

Multiphoton ionization of uranium hexafluoride
View Description Hide DescriptionMultiphoton ionization (MPI) time‐of‐flight mass spectroscopy (TOFMS) and photoelectron spectroscopy (PES) studies of UF_{6} are reported using focused light from the Nd:YAG laser fundamental (λ=1064 nm) and its harmonics (λ=532, 355, or 266 nm), as well as other wavelengths provided by a tunable dye laser. The MPI mass spectra are dominated by the singly and multiply charged uranium ions rather than by the UF^{+} _{ x } fragment ions, even at the lowest laser power densities at which signal could be detected. In general, the doubly charged uranium ion (U^{2+}) intensity is much greater than that of the singly charged uranium ion (U^{+}). For the case of the tunable dye laser experiments, the U^{ n+} (n=1–4) wavelength dependence is relatively unstructured and does not show observable resonance enhancement at known atomic uranium excitation wavelengths. The MPI‐PES studies reveal only very slow electrons (≤0.5 eV) for all wavelengths investigated. The dominance of the U^{2+} ion, the absence or very small intensities of UF^{+} _{ x } (x=1–3) fragments, the unstructured wavelength dependence, and the preponderance of slow electrons all indicate that mechanisms may exist other than ionization of bare U atoms following the stepwise photodissociation of F atoms from the parent molecule. The data also argue against stepwise photodissociation of UF^{+} _{ x } (x=5,6) ions. Neither of the traditional MPI mechanisms (‘‘neutral ladder’’ or the ‘‘ionic ladder’’) are believed to adequately describe the ionization phenomena observed. We propose that the multiphoton excitation of UF_{6} under these experimental conditions results in a highly excited molecule, superexcited UF_{6} ^{**}. The excitation of highly excited UF_{6} ^{**} is proposed to be facilitated by the well known ‘‘giant resonance,’’ whose energy level lies in the range of 12–14 eV above that of ground state UF_{6}. The highly excited molecule then primarily dissociates, via multiple channels, into U^{ n+}, UF^{+} _{ x }, fluorine atoms, and ‘‘slow’’ electrons, although dissociation into F^{−} ions is not ruled out.

Electronic absorption spectra of large benzene ⋅Ar_{ n } clusters
View Description Hide DescriptionWe report the mass resolved resonant two‐photon ionizationspectra of C_{6}D_{6}⋅Ar_{ n } clusters up to n=70. Shifts of the benzene S _{1}←S _{0} 6^{1} _{0} vibronic band were studied as a function of both cluster size and expansion conditions. We find that clusters in different size ranges exhibit remarkably different spectra, which also depend on expansion conditions and, hence, cluster internal energy. Below n=16, spectral features trend toward the blue with increasing size. Above n=16, all features exhibit weak size dependence over wide size ranges. In the n=20–40 range, four distinct bands were found, which we suggest could be due to fully enclosed fluxional (−55 cm^{−1} vs free C_{6}D_{6}), partially enclosed rigid (−43 cm^{−1}), and one‐sided rigid (−32 cm^{−1}) or one‐sided fluxional (also −32 cm^{−1}, broader) structural types. Above n=40, only one band was definitely identified (at −26 cm^{−1}), which may be due to a one‐sided rigid structural type. The trends in spectral shift vs size give no indication of approaching bulk‐like solvation of the benzene. Only one subpopulation between n=20 and 40 may show similarity to macroscopic benzene–argon solutions.

Intermolecular vibrations of the 2,3‐dimethylnaphthalene⋅Ar van der Waals complex: Experiment and quantum three‐dimensional calculations
View Description Hide DescriptionA combined experimental and theoretical study of the intermolecular vibrations of 2,3‐dimethylnaphthalene⋅Ar (2,3‐DMN⋅Ar), for the first excited electronic state (S _{1}), is reported. Methyl groups at C2 and C3 positions of naphthalene lower the symmetry of the complex, so that transitions involving excitation of the intermolecular long‐axis in‐plane x mode become allowed in electronic spectra, in addition to the out‐of‐plane z mode. Two‐color resonant two‐photon ionization (2C‐R2PI) spectrum of the van der Waals (vdW)‐mode region (0^{0} _{0}+70 cm^{−1}) of 2,3‐DMN⋅Ar exhibits six bands to the high‐frequency side of the electronic origin 0^{0} _{0}, which arise from excitation of low‐frequency intermolecular vibrations of the complex in the S _{1} state. Accurate quantum three‐dimensional (3D) calculations of vdW vibrational (J=0) levels of S _{1} 2,3‐DMN⋅Ar have been performed, using a recently developed quantum method based on the 3D discrete variable representation. Since no approximation is made in the treatment of coupled, very anharmonic vdW vibrations, the calculated eigenstates are essentially exact for the intermolecular potential energy surface (PES) employed, thus enabling direct comparison between theory and experiment. The intermolecular PES was modeled as a sum of atom–atom Lennard‐Jones (LJ) pair potentials. Some of the initial LJ parameters were modified until very good agreement was achieved between the calculated and measured vdW frequencies of S _{1} 2,3‐DMN⋅Ar. This allowed assignment of the vdW bands to the blue side of 0^{0} _{0}, and resulted in an improved intermolecular PES of the complex. In addition, the quantum 3D calculations provided a quantitative description of the vdW vibrational level structure and floppiness of S _{1} 2,3‐DMN⋅Ar up to ∼60–70 cm^{−1} above the ground vdW state. The wave functions of all vdW states below ∼49 cm^{−1}, relative to the ground state, are sufficiently regular to allow assignment of vibrational quantum numbers. At higher excitation energies, mode coupling becomes stronger, and irregular vdW states whose assignment is uncertain, are common.

Optical–optical double resonance polarization spectroscopy of highly excited states of ^{23}Na^{39}K
View Description Hide DescriptionThe Doppler‐free high resolution spectrum of the transitions to highly excited states of the ^{23}Na^{39}K molecule was measured by using the technique of the optical–optical double resonance polarizationspectroscopy. Several highly excited states^{1}Σ^{+}, ^{1}Π, and ^{1}Δ were found, and the molecular constants for these states were determined. The vibrational numbering of the ^{1}Δ state (G ^{1}Δ state) was estimated, and the potential energy curve was calculated by the Rydberg–Klein–Rees method. Many perturbed lines were observed. The indirect perturbation between ^{1}Σ^{+} and ^{1}Δ levels through the L‐uncoupling interaction with a distant ^{1}Π state was identified. The hyperfine splittings were observed in transition lines to a ^{3}Π state, which is perturbed by a ^{1}Σ^{+} state.

Electron‐energy‐loss spectroscopy of the low‐lying triplet states of styrene
View Description Hide DescriptionLow‐energy electron‐energy‐loss spectra of styrene deposited on a thin film of solid argon are measured at a temperature of 15 K. The spectra show vibrationally resolved bands in the region of the lowest valence transitions thus allowing to locate the 0–0 transition to the lowest triplet state at 2.69 eV. The second triplet state of styrene is detected for the first time with a 0–0 transition at 3.98 eV. Semiempirical calculations are performed to characterize the bands observed in the spectrum considering the nomenclature of Platt. They suggest that the lowest triplet state has the same spacial wave function as the second singlet state and is closely related to ^{3} L _{ a } benzene. The second triplet state which has most likely B _{ a } character cannot directly be related to a specific singlet state because the B _{ a } and B _{ b } states are found to mix strongly in the singlet manifold whereas among the triplets they do not.

Molecular beam stimulated emission pumping spectroscopy of propynal
View Description Hide DescriptionStimulated emission pumping (SEP) experiments were performed on propynal (H–C≡C–CHO). The SEP spectra probed the overtones of the pure C=O stretch (ν_{4}) and a combination of the C=O stretch with the C≡C–C bend (ν_{9}). Term energies for J≤8, K≤2 of the vibrations 4_{2}, 4_{3}, 4_{4}, 4_{5}, 4_{3}9_{1} as well as two unidentified perturbing vibrational states were obtained. The SEP spectra show simple rotational structure that is assignable by means of a near prolate asymmetric top model. Rotational constants were obtained for all the vibrational statesmeasured.Spectroscopic perturbations are observed for ν_{4}=2,4,5 as either anomalous rotational constants, anomalous energy level shifts, or as extra spectral transitions. Complex state mixing is not observed at the current spectroscopic resolution (0.04 cm^{−1}) even though vibrational state densities as high as 400 states per cm^{−1} were probed. The propynal SEP results are compared to other SEP experiments on aldehyde containing molecules and inferences are made.

The nonlinear optical spectrum of polydiacetylene in the near infrared
View Description Hide DescriptionThe spectrum of the modulus and the imaginary part of the third order nonlinear optical susceptibility χ^{(3)} of polydiacetylene was studied over the range 750–1000 nm by a combination of two‐photon absorption and degenerate four wave mixing measurements. A peak in the two‐photon absorptionspectrum near 880 nm was found to be due to induced absorption, probably caused by direct optical excitation of the long‐lived triplet exciton. No clear evidence was observed for resonances due to two‐photon accessible A _{ g } symmetry states in the spectral range studied.

Wentzel–Kramers–Brillouin theory of multidimensional tunneling: General theory for energy splitting
View Description Hide DescriptionA general Wentzel–Kramers–Brillouin (WKB) theory of multidimensional tunneling is formulated and an illuminating physical picture of the effects of multidimensionality is provided. Two basic problems are solved: (i) Maslov’s semiclassical wave function in the classically accessible region is connected to the wave function in the classically inaccessible region and (ii) the latter is propagated into the deep tunneling region. It is found that there exist two distinct types of tunneling: pure tunneling and mixed tunneling. The former is the usual one in which the tunneling path can be defined by a certain classical trajectory on the inverted potential and its associated action is pure imaginary. In the latter case, no tunneling path can be defined and the Huygens‐type wave propagation should be carried out. In this case, tunneling is always accompanied by classical motion in the transversal direction and the associated action is complex. A general procedure is presented for the evaluation of energy splitting ΔE in the double well. Moreover, under the locally separable linear approximation, a simple and convenient formula for ΔE is derived and is confirmed to work well by comparison with the exact numerical calculations.

An interpretation of the vibrational spectra of insulating and electrically conducting poly(3‐methylthiophene) aided by a theoretical dynamical model
View Description Hide DescriptionIt is shown that the relevant spectral features which arise in the infrared spectrum of poly(3‐methylthiophene) upon chemical doping or photoexcitation (i.e., in the electrically conducting form) can be properly explained by means of the effective conjugation coordinate (ECC) formalism. This theoretical dynamical model accounts for the intramolecular hopping of π electrons in the class of polyconjugated aromatic systems. A complete assignment for the infrared and Raman spectra of the polymer in the pristine state (i.e., in the insulating form) is proposed as the result of a theoretical vibrational potential function derived from semiempirical calculations on short oligomers. The dependence of the bandgap energy on the internal rotation about the inter‐ring single bond is analyzed theoretically in dimers as model molecules and the results are compared with experiments.

Suppression of sidebands in magic‐angle‐spinning nuclear magnetic resonance: General principles and analytical solutions
View Description Hide DescriptionSeveral theoretical and experimental aspects of sideband suppression in the nuclear magnetic resonance(NMR) spectra of rotating solids are considered. The principles of sideband suppression are explored using general symmetry arguments and previous treatments are examined critically. Analytical solutions are given for sideband suppression pulse sequences employing four, five, six, and nine π pulses. The analytical solutions for four π pulses are complete. Experimental demonstrations are given.

Vacuum‐ultraviolet photolysis of N_{2}H_{2}: Generation of NH fragments
View Description Hide DescriptionThe photodissociation of the double bond in HN=NH yielding electronically excited NH(A ^{3}Π) and ground state NH(X ^{3}Σ^{−}) radicals has been studied in the vacuum‐ultraviolet above 105 nm. Fragment excitation spectra were taken using tunable synchrotron radiation as the photolysis light source. The excited radicals were detected by their triplet emission to the ground state. A very crude estimate results in 10%, 20%, and 70% of the excess energy to be channeled into fragment vibration, rotation, and translation, respectively, at the Kr resonance line at 123.6 nm. This energy distribution supports a repulsive process with almost equal rotation in the two NH fragments and vibration caused by lengthening all bonds during the N=N bond breaking. An upper limit for the energy necessary to break the double bond is measured to be 510.7±1.2 kJ mol^{−1}. This value yields Δ_{ fH0 0 }(N_{2}H_{2})≥204.1±2.2 kJ mol^{−1}.

Toward a working theory of polarized light scattering from helices
View Description Hide DescriptionThe first Born approximation is used to calculate all 16 Mueller scattering matrix elements for a single, continuous helix at any orientation with respect to the incoming light. The results are compared to those from a helix characterized by point‐polarizable subunits using the Born approximation and the coupled‐dipole approximation. The number of point‐polarizable groups necessary to describe the helix is investigated by comparing the calculations using the continuous helix to those using the helix made of discrete subunits. It is found that for large helices, many subunits are necessary, so in these cases, the continuous model may be more applicable. Conditions for the necessity of including dipolar interactions are established as a function of the dielectric constant of the material.

Experiment versus molecular dynamics simulation: Spectroscopy of Ba–(Ar)_{ n } clusters
View Description Hide DescriptionThis work presents a quantitative comparison between experiment and molecular dynamics simulations for the excitation spectra of large van der Waals clusters. The emission and excitation spectra of mixed Ba(Ar)_{ n } clusters have been obtained for average cluster sizes ranging between 300 and 4000. The simulation is performed by using classical dynamics and pairwise additive potentials for two cases corresponding to the barium atom at the surface or inside the argon cluster. A very good agreement with the experiment is found when the barium atom is at the surface.

A study of the singlet–triplet perturbations in the Ã ^{1} A _{ u } state of acetylene by high resolution ultraviolet spectroscopy
View Description Hide DescriptionLaser‐induced fluorescence spectra of the 3^{3} _{0} K ^{1} _{0} and 3^{4} _{0} K ^{1} _{0} vibronic bands of the Ã ^{1} A _{ u }←X̃ ^{1}Σ^{+} _{ g } transition in acetylene have been recorded with a resolution of 18 MHz. Each rotational transition consists of a group of lines due to coupling of the electronically excited singlet state with isoenergetic triplet states. Using the standard deconvolution procedure the singlet–triplet coupling elements and the density of coupled triplet states are derived for rotational levels up to J=4 in both bands. From the density of coupled triplet states it is concluded that the Ã ^{1} A _{ u } state is perturbed by the T _{1} ^{3} B _{2} state. Magnetic field measurements have shown that the predissociation of acetylene in the 4ν_{3} ^{’} vibrational level of the Ã state is caused by a coupling via the T _{1} ^{3} B _{2} state with predissociating vibrational levels of the electronic ground state.

An ab initio study of the structure and infrared spectrum of Si_{2}C_{3}
View Description Hide DescriptionThe geometry and vibrational spectrum of the previously not studied silicon–carbon cluster Si_{2}C_{3} has been investigated. Geometries and frequencies for a number of isomeric structures are presented at the Hartree–Fock level. In agreement with a concurrent experimental observation the ground state is found to be linear. Harmonic frequencies, isotopic shifts, and infrared intensities calculated using many‐body perturbation theory to second order are found to be in good agreement with the experimental results supporting the identification of a new penta‐atomic silicon–carbon cluster. The general behavior of penta‐atomic silicon–carbon clusters is discussed and preliminary ground state geometries and harmonic frequencies are presented for Si_{3}C_{2} and Si_{4}C.

Vibrational spectra of penta‐atomic silicon–carbon clusters. II. Linear Si_{2}C_{3}
View Description Hide DescriptionFourier transform infrared matrix measurements carried out in conjunction with ab initio calculations reported in a companion paper by Rittby have resulted in the first identification of two fundamental vibrations, the C=C stretching mode ν_{3}(σ_{ u })=1955.2 cm^{−1}, and the Si–C stretching mode ν_{4}(σ_{ u })=898.9 cm^{−1}, of the SiC_{3}Si cluster formed by trapping the products of the vaporization of silicon/carbon mixtures in Ar at 10 K. The observed frequencies, relative intensities, and ^{13}C, ^{29}Si, and ^{30}Si isotopic shifts for the ν_{3} and ν_{4} vibrations are in good agreement with the results of the ab initio calculations at the second‐order many‐body perturbation theory [MBPT(2)] level which predict a linear centrosymmetric geometry for the ground state of SiC_{3}Si. The results of force constant adjustment calculations are consistent with the proposed vibrational assignments and structure.

On the field ionization spectrum of high Rydberg states
View Description Hide DescriptionWe discuss the spectrum of very high Rydberg states as detected via ionization in weak external electric fields. For the conditions of interest, namely, states just below the ionization continuum and weak fields, the classical barrier to dissociation is extremely far out from the core. About the saddle point the potential is very shallow. It is concluded that ionization by tunneling is far too slow. Only electrons whose energy is above the classical barrier can be detected via ionization. However, not all electrons which energetically can ionize will necessarily do so. Electrons may fail to ionize if the fraction of their energy which is in the direction perpendicular to the field is high. The computed fraction of electrons which fails to ionize does depend, in a sensitive way, on the diabatic vs adiabatic switching on of the external field. More experiments and theoretical work is needed on this point. A classical procedure based on the adiabatic invariance of the volume in phase space is developed for the computation of the fraction of electrons that can surmount the classical barrier for a given field. Analytically exact results are obtained for adiabatic switching and for the sudden limit where the rise time of the field is shorter than the period of the orbit. For the case of diabatic switching (which is appropriate for very high n values), the exact classical computations on the yield of ionization show that the onset of ionization is at an energy of 4.25 F ^{1/2} cm^{−1} below the ionization potential and the 50% point it as 3.7 F ^{1/2} cm^{−1} for a field F in V/cm.