Volume 96, Issue 9, 01 May 1992
Index of content:

Optical measurements of methyl group tunneling in molecular crystals: Temperature dependence of the nuclear spin conversion rate
View Description Hide DescriptionThe tunneling methyl groups in dimethyl‐s‐tetrazine (DMST) dopedsingle crystals of durene were investigated by high resolution optical spectroscopy using spectralhole burning. The experiments probe the level structure as well as the relaxation dynamics of the tunneling methyl groups in different electronic states of DMST. The tunneling splitting differs by 1.24 GHz in the ground and the first excited singlet states of DMST. In the ground electronic state, relaxation (spin conversion) between the spin 3/2 (A) and 1/2 (E) tunneling levels was measured between 1.5 and 12 K. The spin conversion time is larger than 100 h at 1.5 K and decreases with Arrhenius‐type behavior above 3.5 K. The activation energy of 20 cm^{−} ^{1} also is observed as a phonon sideband in emission, and is, in agreement with theoretical predictions, tentatively assigned to a librational mode of the methyl group.

Fourier transform emission spectroscopy of ScN
View Description Hide DescriptionThe near‐infrared electronic emission spectrum of ScN has been observed in the 1.6–1.8 μm spectral region using a Fourier transformspectrometer. The three bands with origins at 5820.0944(5), 6051.2858(20), and 6266.1290(13) cm^{−} ^{1} have been assigned as the 0‐0, 1‐1, and 2‐2 bands of the A ^{1}Σ^{+}−X ^{1}Σ^{+} electronic transition. The principal equilibrium molecular constants for the ground state obtained from a fit of the observed lines are B ^{‘} _{ e }=0.554 609(17) cm^{−} ^{1}, r ^{‘} _{ e }=1.687 23(3) Å and the corresponding values for the excited state are B ^{’} _{ e }=0.549 277(17) cm^{−} ^{1}, r _{ e }=1.695 40(3) Å. Although ScN is a well‐known refractory solid, our work is the first observation of the gas‐phase ScN molecule.

Fermi resonance states in the natural isotopically mixed CS_{2} crystal
View Description Hide DescriptionHigh‐resolution Raman spectroscopy (HRRS) is used to analyze the anharmonic interactions between the modes ν_{1} and ν_{2} in the CS_{2} crystal and its natural isotopic aggregates. Statistics of aggregates (mainly formed with molecules containing ^{34}S and ^{33}S isotopes) are calculated. Using the exciton theory formalism, a unified description of the Fermi resonance going from the molecule to the crystal is provided and used to calculate the various eigenmodes of aggregates. HRRS allows an accurate analysis, in the ν_{1} frequency domain, of the complex band structure due to the various configurations of isotopic aggregates (monomers, dimers, trimers,...). In the crystal, the weak Fermi resonance interaction (ν_{1}, 2ν_{2} ) leads to the appearance of bound states ν^{+} and ν^{−}. HRRS allows one to clearly separate the two Raman active Davydov components (A _{ g } and B _{3g } ) in both the ν^{−} and ν^{+} bands. Their thermal evolution is also given and compared to other incoherent as well as coherent Raman data.

The structure of water and methanol in p‐dioxane as determined by microwave dielectric spectroscopy
View Description Hide DescriptionDielectricmeasurements were performed on water–p‐dioxane and methanol–p‐dioxane mixtures using time domain reflectometry over the frequency range 0.1–10 GHz. In the case of water–p‐dioxane mixtures, the relaxation strength normalized by the number of water molecules per unit volume is independent of the molar fraction of water x _{ W } if x _{ W }<0.83. There are no ordered micellelike clusters in the mixture. On the other hand, if x _{ W } is larger than 0.83, the normalized strength increases linearly with x _{ W }. The clusters of pure water which appear in this region are cyclic and consist of six molecules. In the case of methanol–p‐dioxane, the normalized strength is independent of x _{ M } for x _{ M }<0.66 and increases linearly with x _{ M } for x _{ M }>0.66. However, the relaxation time of pure methanol is too large for clusters consisting of three molecules. It is suggested that the chainlike clusters form network structures.

p‐fluorotoluene. I. Methyl (CH_{3} and CD_{3}) internal rotation in the S _{1} and S _{0} states
View Description Hide DescriptionSupersonic jet S _{1}‐S _{0}spectroscopy (resonance‐enhanced multiphoton ionization,fluorescence excitation, and dispersed single vibronic level fluorescence) has been used to determine the S _{1} and S _{0} internal rotation energy level structure of p‐fluorotoluene with a CD_{3} methyl rotor as well as to extend observations of the CH_{3} rotor structure. The observed rotor energy levels 2≤m≤8 for both species in both states are fit by a simple sixfold hindered rotor Hamiltonian for which the rotor inertial constants B and the internal rotation potential energy barriers V _{6} are evaluated. V _{6} may be obtained independently from B by observations of ΔE _{3}, the observed splitting of the 3a‘_{1} and 3a‘_{2} rotor levels. Numerical solution of the wave equation shows that the perturbation theory relationship V _{6}=−2ΔE _{3} holds well for any reasonable B value. Correspondingly, the B constant may be obtained from other level energies without appreciable sensitivity to (reasonably) assumed barrier heights. Earlier microwave and S _{1}‐S _{0}fluorescence results are combined with the present work to produce a set of preferred values for these constants. The values in cm^{−} ^{1} for the S _{0} state are B=5.46 (2.82) and V _{6}=−4.77 (−4.77) for CH_{3} (CD_{3}) rotors. The S _{1} values are B=4.90 (2.54) and V _{6}=−33.0 (−25.2). The 20% barrier height reduction occurring on transformation from a CH_{3} to a CD_{3} rotor is similar to that observed in other systems. Calculation implies that the staggered conformer is the minimum energy configuration for both electronic states. Many of the S _{1}‐S _{0} rotor transitions are forbidden, and a discussion is given of induced intensity mechanisms that involve coupling of internal rotation to overall rotation or coupling of internal rotation to electronic motion. Substantial energy‐level perturbations often occur for states with m≥5. A survey of B values and hindered rotation constants for 30 species with methyl rotors attached to aromatic rings reveals some general correlations.

Simple theory of diffuse structure in continuous ultraviolet spectra of polyatomic molecules. III. Application to the Wulf–Chappuis band system of ozone
View Description Hide DescriptionWe apply a simple model for the photodissociationabsorption spectra of bent symmetric triatomic molecules to the Wulf–Chappuis band system of ozone (10 000–22 000 cm^{−1} ) to assign the electronic states and the diffuse vibrational bands involved. The conical intersection between the two lowest ^{1} A‘ states is treated in an approximate way, and the role of the lowest excited triplet states is explored. The results indicate that the Wulf band is probably due to the ^{3} A _{2} state of ozone which gains intensity through spin–orbit coupling. The 1 ^{1} A‘ (^{1} A _{2}) state gives rise to the featureless red wing of the Chappuis band. Most of the structure in the Chappuis band is reproduced in the model and is due to the 2 ^{1} A‘ (^{1} B _{1}) state as was previously supposed. A more complete treatment of the conical intersection and nonadiabatic effects will be necessary to quantitatively reproduce all features of the experimental spectra.

The C_{1s } core shake‐up spectra of alkene molecules: An experimental and theoretical study
View Description Hide DescriptionThe C_{1s } core photoelectron spectra of a series of alkene molecules, ethene, propene, 1‐butene, c i s and t r a n s 2‐butene, 2‐methyl‐propene, and 1‐pentene are discussed. The experimental spectra are assigned using intermediate neglect of differential overlap‐configuration interaction (INDO‐CI) calculations and comparative discussions. It is shown that hyperconjugation is a useful concept in the assignment of the transitions. INDO‐CI is shown to give a reasonable description of the low energy part of the spectra. The results are used in the discussion of molecular models for the interpretation of the electronic structure of polyacetylene.

Vibrational spectra of aniline–Ar_{ n } van der Waals cations (n=1 and 2) observed by two‐color ‘‘threshold photoelectron’’ [zero kinetic energy (ZEKE)‐photoelectron] spectroscopy
View Description Hide DescriptionMeasurements of mass‐selected ion‐current and threshold photoelectron spectra of jet‐cooled aniline–Ar_{ n } van der Waals complexes (n=1 and 2) have been carried out with a two‐color resonantly enhanced multiphoton ionization (REMPI) technique using a high‐resolution threshold photoelectron analyzer developed in this laboratory. From our (1+1’) REMPI experiments via the respective excited S _{1} states, we have obtained photoelectron spectra with well‐resolved vibrational progressions due to ‘‘low‐frequency van der Waals modes’’ of the cations; ν_{vdW}=16 cm^{−1} (n=1) and ν_{vdW}=11 cm^{−1} (n=2). From Franck–Condon calculations, we have assigned these low‐frequency vibrations to the ‘‘van der Waals bending’’ of the cations. We have also found that the angles of the van der Waals bonds in the cations are changed by 8.2 (n=1) and 8.8 (n=2) degrees with respect to the S _{1} states. The adiabatic ionization potentials (I _{ a }) of aniline and the aniline–Ar_{ n } complexes (n=1 and 2) have been determined as 62 268±4 cm^{−1} (aniline), 62 157±4 cm^{−1} (n=1), and 62 049±4 cm^{−1} (n=2). Their shifts ΔI _{ a } are 111 cm^{−1} (n=1) and 219 cm^{−1} (n=2) with respect to aniline. Spectral shifts due to complex formation have been observed for a total of 13 ring modes of the cations.

Rydberg states of the Ar_{2} molecule
View Description Hide DescriptionExtensive spectra attributable to transitions from the 4sσ a ^{3}Σ^{+} metastable state of Ar_{2} to excited Rydberg states have been observed by intracavity absorption spectroscopy and by laser excitation spectroscopy in the afterglow of a pulsed corona discharge. Of these the most extensive and best resolved were the laser induced fluorescencespectra. Most of the spectra can be assigned to vibronic transitions in the n fλ (^{3}Π_{ g },^{3}Σ^{+} _{ g }) and n pλ (^{3}Π_{ g },^{3}Σ^{+} _{ g })←a ^{3}Σ^{+} _{ u } series. The 5pπ^{3}Π_{ g }←a ^{3}Σ^{+} _{ u } and 7pσ ^{3}Σ^{+} _{ g }←a ^{3}Σ^{+} _{ u } transitions observed near 19 823 and 19 529 cm^{−} ^{1}, respectively, exhibit many bands with v’≠0 in their (v’–v‘) vibrational band system developments. The higher n value members of these and other observed series are dominated by (0–0) transitions. Rotational structure is partially resolved in a few bands of the 7pσ ^{3}Σ^{+} _{ g }–a system, but most of the spectra observed appear to be either rotationally unresolved or made up of blended collections of rotational lines. Above the 7pσ, 5pπ pair, the (n+2)pσ ^{3}Σ^{+} _{ g } and n pπ ^{3}Π_{ g } members of the n pλ series rapidly coalesce, indicating a rapid onset of decoupling of the electronic orbital angular momentum,L̂, from the internuclear axis.
Such decoupling leads to n p‐complex formation at n values much lower than observed in the n pλ series of the lighter dimers: He_{2} and Ne_{2}. From the data for the observed series, the lowest ionization limit of Ar_{2} (relative to a ^{3}Σ^{+} _{ u },v=0 ) was determined to be 29 373±3 cm^{−} ^{1}. Vibrational intervals ΔG(v+1/2) for a ^{3}Σ^{+} _{ u }(v≤4) and 7pσ ^{3}Σ^{+} _{ g }(v≤2) yield the vibrational constants ω_{ e }=296 and 282 cm^{−} ^{1} and xω_{ e }=2.5 and 17 cm^{−} ^{1}, respectively.

Hyperfine structure of the NaK c ^{3}Σ^{+} state and the effects of perturbation
View Description Hide DescriptionScanning the frequency of a single‐mode dye laser crossed with a molecular beam of NaK, we have measured the excitation spectrum by monitoring selectively the fluorescence intensity of transition to the a ^{3}Σ^{+} state. The B ^{1}Π(v=8) and c ^{3}Σ^{+}(v=22) levels are perturbed around J=5 and the c ^{3}Σ^{+}(v=22) and b ^{3}Π(v≊62) levels are perturbed around J=24. The transition lines to the perturbed levels are fully resolved. The hyperfine splitting, which is induced by the coupling with the nuclear spins of the Na and K atoms, is observed for levels of the c ^{3}Σ^{+} state and the perturbed states. The magnitude of the hyperfine splitting, the line intensity, and the energy shift are analyzed, and their relation with the perturbation is studied. The ratio of electron spin densities at the sodium and potassium atoms in the c ^{3}Σ^{+} state is estimated to be 0.71:0.15 from the magnitudes of hyperfine splitting.

Solvent effects on excited‐state torsional motion and electronic relaxation of c i s‐1,3,5‐hexatriene
View Description Hide DescriptionResonanceRaman spectra of c i s‐1,3,5‐hexatriene, including absolute cross sections, have been obtained in both vapor and solution phases and compared with previous results for the t r a n s isomer. Quantitative modeling of the resonance Raman intensities provides information on the excited‐state geometry and dynamics and solvent effects on the excited‐state potential surfaces.C i s‐hexatriene is nonplanar in both ground and excited states as evidenced by the appearance of out‐of‐plane fundamentals in the resonanceRaman spectra, but the Franck–Condon activity of these modes is weak. Population decay from the lowest allowed singlet state on time scales of approximately 20 fs (c i s‐hexatriene) and 40 fs (t r a n s‐hexatriene) is the major source of diffuseness in the gas‐phase absorption spectra of both molecules. Solvation decreases the resonance Raman activity of out‐of‐plane vibrations, although the effects are smaller in c i s‐hexatriene than previously observed in the t r a n s isomer. Simulation of the spectra indicates that the allowed excited‐state potential surfaces of both isomers undergo an increase in the local force constants for double‐bond torsional motion upon solvation in increasingly polarizable environments.

Nonperturbative susceptibility of a three‐level system interacting with a monochromatic field
View Description Hide DescriptionWe derive an exact, intensity‐dependent expression for the susceptibility of a three‐level system interacting with one monochromatic electric field. This expression, once expanded, is equivalent to the usual perturbative series χ=χ^{(1)}+χ^{(3)} E ^{2}+⋅⋅⋅ within the rotating wave approximation. We consider two types of resonant processes: one‐ and two‐photon resonances corresponding to the intensity‐dependent susceptibility χ^{(3)}(−ω;ω,−ω,ω), and one‐, two‐, and three‐photon resonances corresponding to χ^{(3)}(−3ω;ω,ω,ω). As an example of current interest, we use a model three‐level system that mimics the excited electronic states of typical nonlinear optical polymers and show that near resonance, successive terms in the perturbative series approach the same order of magnitude for experimentally realizable fields.

Study of butterfly inversion of perfluoronaphthalene by laser‐induced fluorescence in supersonic jet
View Description Hide DescriptionFluorescence excitation and dispersed emission spectrum of jet‐cooled perfluoronaphthalene is reported. The origin region of the excitation spectrum exhibits a clear progression for a very low‐frequency vibration which indicates that the molecule is highly floppy in the electronic excited state. The low‐frequency progression has been assigned to the butterfly inversion of the fluorine atoms perpendicular to the ring plane. Barrier height of the butterfly inversion mode for the S _{1} electronic state, calculated by a quadratic Gaussian‐type potential function, have been found to be 14 cm^{−} ^{1} only. A splitting if 7 cm^{−} ^{1} between the zero‐point and the first vibronic levels indicates that butterfly inversion is active even in the zero‐point level of the S _{1} electronic state. Simulated intensity distribution pattern over the cold progression fits well with the observed spectrum. Other vibrational frequencies of both the excitation and the dispersed emission spectrum have been tentatively assigned by correlating the observed frequencies with the values obtained from Raman and IR spectrum.Analysis of the dispersed fluorescencespectrum indicates that only totally symmetric modes are active in emission.

Fourier transform electron spin echo envelope modulation of a S=1/2, I=5/2 spin system: An exact analysis and a second order perturbation approach
View Description Hide DescriptionElectron spin echo envelope modulation (ESEEM) induced by ^{27}Al nuclei can be used to characterize the interactions of paramagnetictransition metal cations with the framework of various aluminosilicates. The quantitative analysis of such systems is complex due to the ^{27}Al nuclear quadrupoleinteraction which is often large and unknown. In order to obtain a better understanding of the various spectral features of the ^{27}Al modulation, the ESEEM of a S= (1)/(2) I= (5)/(2) spin system in orientationally disordered systems was investigated in the frequency domain. The relative contributions of the various electron‐nuclear double resonance(ENDOR) frequencies to the Fourier transform (FT) ESEEM spectrum were studied as a function of the size of the nuclear quadrupole coupling constant and of the relative orientation of the nuclear quadrupoletensor with respect to the g‐tensor. The parameters range investigated is that expected from Cu^{2+} interacting with framework Al in zeolites. Two approaches were employed in the calculations, exact diagonalization of the nuclear Hamiltonian and a second order perturbation treatment of the quadrupoleinteraction. It is found that the second order perturbation approach applies up to e ^{2} q Q/h<7 MHz. The conditions under which the FT‐ESEEM spectrum is dominated by the ‖ (1)/(2) 〉−‖− (1)/(2) 〉 ENDOR transitions were explored as well. This occurs when e ^{2} q Q/h is relatively large and it strongly depends on the orientation of the quadrupoletensor. When the FT‐ESEEM can be described only by the ‖ (1)/(2) 〉−‖− (1)/(2) 〉 ENDOR transitions, the simulations are significantly simplified since these can be calculated using the analytical expressions obtained by perturbation theory also for a quadrupole coupling constant as large as 10 MHz.

Optical response of concentrated colloids of coinage metals in the near‐ultraviolet, visible, and infrared regions
View Description Hide DescriptionThe optical response of two‐phase composite materials(cermettopography with coinage metals) at high filling factors under a wide range of particles sizes, frequencies, and experimental conditions is calculated using a model that combines multipolar interactions and particle size effects. The validity of the model is established for a real system of a silvercolloid and the simulations are discussed in terms of the microscopic structural parameters and the interaction between the particles. The limitations of the Maxwell–Garnettmodel are explored and the transition between a dense system which primarily reflects light and a dilute composite which mostly transmits light is followed with the same model.

Two‐color double resonance spectroscopy via Ã ^{1} A _{ u } state of acetylene: 3p Rydberg state and its Renner–Teller effect
View Description Hide DescriptionThe two‐color double resonance multiphoton ionization spectra due to the transitions from various rovibrational levels of the Ã ^{1} A _{ u } state of acetylene to the 3pRydberg state have been measured. The observed spectra showed large dependence on the quantum number K. It was concluded from the spectral analysis that the geometry of the 3pRydberg state is linear. The spectra also showed a long progression of the t r a n s‐bending vibration, supporting the linear structure. The t r a n s‐bending progression belonging to the ^{1}Δ_{ g } electronic state showed the Renner–Teller splittings. The observed splittings were reproduced by the calculation using the formulas for a linear triatomic molecule in a Δ electronic state.

Intra–inter polyad mixing and breaking of symmetric–antisymmetric selection rule in the vibrational spectra of CS_{2} molecule
View Description Hide DescriptionA laser system composed of tunable lasers pumped by a copper vapor laser (Oxford Lasers Cu60) is described in this paper. The high resolution obtained with this system has allowed excitation of selective rotational levels of the 15 V and 10 V vibrational bands of the V ^{1} B _{2}excited electronic state of the CS_{2} molecule in its vapor phase (∼100 mTorr). The rotational assignment of the excitation spectra was accomplished by observing the dispersed fluorescence. We show that it is not necessary to use a supersonic jet in order to assign the emission spectra of CS_{2}. The goal of this work is to study the highly excited vibrational states of the ground electronic state of CS_{2} up to the first dissociation limit. For our purpose, there are two important consequences of the particular geometry of the 15 V excitation, which is just below and close to the bending potential barrier of the V ^{1} B _{2} state. First, a very good Franck–Condon overlap in the excitation and a large Franck–Condon access to high vibrational states, as high as 20 000 cm^{−1}, allow observation of the dispersed fluorescence through a high resolution monochromator with an OMA detector. This avoids the need for more complicated techniques, like SEP spectroscopy. Moreover, we show for the first time, that, in the V ^{1} B _{2}excited state, a strong Coriolis interaction Q ^{3} _{2} Q _{3} J _{ c } couples the bending (0,3,0) and antisymmetric stretching levels (0,0,1). This breaking of the symmetric–antisymmetric selection rule gives access to the antisymetric stretching levels of the ground electronic stateX̃ Σ^{+} _{ g } from the 15 V excitation. We also show that, below 12 000 cm^{−1}, the vibrational couplings of CS_{2} can be described by a model of 2 degrees of freedom, which includes a strong 1:2 Fermi resonance and accidental resonant perturbations between adjacent polyads which is probably a first step in the transition to a chaotic regime in CS_{2}.

Vibrational spectra of tetra‐atomic silicon–carbon clusters. I. Rhomboidal Si_{3}C in Ar at 10 K
View Description Hide DescriptionThe vibrational spectrum of Si_{3}C has been observed for the first time in a Fourier transform infrared study of the products of the vaporization of carbon/silicon mixtures trapped in Ar at 13 K. Five of the six fundamental modes have been assigned: the symmetric breathing vibration, ν_{1}(a _{1})=658.2 cm^{−} ^{1}; the Si_{β}–Si_{α}–Si_{β} symmetric deformation vibration, ν_{2}(a _{1})=511.8 cm^{−} ^{1}; the Si_{β}–C–Si_{β} symmetric deformation vibration, ν_{3}(a _{1})=309.5 cm^{−} ^{1}; the Si_{β} –C antisymmetric stretching vibration, ν_{5}(b _{2})=1101.4 cm^{−} ^{1}; and the Si_{α}–Si_{β} antisymmetric stretching vibration, ν_{6}(b _{2})=357.6 cm^{−} ^{1}. The assignments are supported by ^{1} ^{3}C, ^{2} ^{9}Si, and ^{3} ^{0}Si isotopic data and are in excellent agreement with the predictions of an a b i n i t i o study carried out by Rittby in collaboration with this work. The results of force constant adjustment calculations are consistent with the ground state geometry established by the a b i n i t i o calculation, a rhomboidal structure of C _{2v } symmetry, with carbon–silicon transannular bonding between the two equivalent Si_{β} atoms.

Mean‐field treatment of local binding processes
View Description Hide DescriptionA mean‐field treatment of ligand binding to a biological macromolecule containing multiple binding sites correctly describes the essential thermodynamic features involved in the effects observed locally at the level of individual sites. Equilibrium and steady state phenomena can be analyzed within a simple framework that is particularly useful in practical applications. Analysis of the complex electron binding properties of cytochrome a a _{3} reveals the origin of the negative electron binding capacity observed experimentally for cytochrome a.

Collision energy‐resolved Penning ionization electron spectra of unsaturated hydrocarbons with He*(2 ^{3} S) metastable atoms
View Description Hide DescriptionCollision energy dependencies of the partial ionization cross sections of ethylene, propene, butadiene, and benzene upon collision with He*(2 ^{3} S) metastable atoms were measured. Band intensities of the π bands decrease with the increase of the collision energy. This tendency indicates that interaction potentials are attractive when He*(2 ^{3} S) approaches the π orbitals of unsaturated hydrocarbons. The intensity of a strong extra band at an electron energy of 3.7 eV for benzene shows a similar collision‐energy dependence with those of π bands. This extra band is assigned to the ionization from the π(1e _{1g }) orbitals associated with the excitation from the π(1e _{1g }) orbitals to the π*(1e _{2u }) orbitals. The reason why the extra band of the Penning ionization electron spectrum of benzene is more enhanced than that of the ultraviolet photoelectron spectrum can be ascribed to the existence of the He*(2 ^{3} S) atom at the time the ionization occurs. The shapes of the σ and π bands change with the collision energy, which reflect the nature of the interaction potential.