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Volume 89, Issue 3, 01 August 1988

A study of the D ^{1}Σ^{+} _{u}–X ^{1}Σ^{+} _{g} band system of Cs_{2}
View Description Hide DescriptionThe transition lines to the low v’ levels of the D ^{1}Σ^{+} _{ u } state are overlapped with the lines of the C ^{1}Π_{ u } –X ^{1}Σ^{+} _{ g } transition. By choosing an appropriate wave number through a monochromator to observe a selected series of the D ^{1}Σ^{+} _{ u } (v’,J±1)–X ^{1}Σ^{+} _{ g } (v‘,J) transition of Cs_{2}, we measured the fluorescence intensity as a function of wave number of the laser line. 1237 spectral lines have been assigned to this band system of J’=4–108, v’=0–5. The Dunham coefficients of the D ^{1}Σ^{+} _{ u } state, which are useful to reproduce the measured line positions and also to construct a reasonable RKR potential curve, were determined by an iterative procedure. The molecular constants reproduced the observed 983 nonoverlapping line positions with a standard deviation of 0.0106 cm^{−} ^{1}. The electronic term energy T _{ e }, the dissociation energyD _{ e }, and the equilibrium internuclear distance R _{ e } of the D ^{1}Σ^{+} _{ u } state were determined to be 16 698.997 cm^{−} ^{1}, 1547.6±0.8 cm^{−} ^{1}, and 5.712 Å, respectively. The potential curves of the D ^{1}Σ^{+} _{ u } and C ^{1}Π_{ u } states were estimated to cross at R=5.85 Å and E=16 700 cm^{−} ^{1}.

Optical luminescence excitation spectra of molecular oxygen in the soft x‐ray region
View Description Hide DescriptionThe observation of an anomaly in the optical luminescence excitation spectrum of oxygen in the region of the oxygen K edge is reported. Dispersed luminescencespectra were obtained for x‐ray excitation at the pi and sigma resonances, at the anomaly, and in the continuum. These spectra indicate enhanced production of O^{2+} _{2} ions at both the sigma resonance and at the anomaly. The anomaly thus is attributed to a shake‐up or shake‐off state associated with an antibonding sigma molecular orbital of oxygen. This work also demonstrates that optical luminesence spectra provide state‐specific information about the products of core hole excitation and relaxation.

Two‐dimensional exchange NMR of powder samples. I. Two‐time distribution functions
View Description Hide DescriptionA systematic description of the two‐dimensional (2D) NMR exchange experiment for studying molecular motions in static powder samples is presented in terms of two‐time distribution functions. Various angular distributions and their transformation to the NMR frequency domain yielding 2D absorption spectra are discussed. The concept of jump angle distributions is introduced. General and isotropic reorientation as well as different symmetries of the tensorial interaction between spin and lattice are distinguished in the analysis. Special attention is directed to the question, to what extent can angular information be reextracted from a 2D exchange spectrum without referencing any model of reorientation. For tensorial interactions of axial symmetry, projections of the 2D spectrum are also calculated and their usefulness is compared to that of the complete 2D spectra. The close relationship between the dynamic exchange experiment and several static NMR experiments like 2D separated local field spectroscopy of powders and NMR of oriented samples is pointed out.

Two‐dimensional exchange NMR of powder samples. II. The dynamic evolution of two‐time distribution functions
View Description Hide DescriptionTheoretical as well as experimental examples concerning the evolution of the two‐time distribution S _{2‖0} (ω_{1},ω_{2} ;t _{ m }) as a function of the mixing time t _{ m } are presented, where S _{2‖0} is identical with the two‐dimensional (2D) absorptionspectrum rendered by 2D exchange NMR spectroscopy of static powder samples. The model calculations comprise standard models like isotropic rotational diffusion or overall isotropic reorientation combined with discrete internal rotational jumps to simulate the chain dynamics of polymers. In any case, the 2D spectrum directly reflects the main aspects of the underlying motional mechanism. An axially symmetric coupling (η=0) between spin and lattice is assumed throughout. Thus, the angular information contained in a 2D spectrum is completely specified by a one‐dimensional jump angle distribution supplied with each spectrum. In connection with the simulations the numerical mapping of a discrete distribution function into a space of new variables is discussed. In the experimental section ^{2}H NMR spectra of chain deuterated linear polystyrene above its glass transition temperature are shown, which are compared with the model calculations.

Vibration–rotation spectrum of the carbon dioxide–acetylene van der Waals complex in the 3 μ region
View Description Hide DescriptionThe infrared absorptionspectrum of the carbon dioxide–acetylene van der Waals complex has been observed in a pulsed molecular beam. A color center laser was used to excite the vibration associated with the antisymmetric hydrogen stretching mode of the acetylene monomer. The vibrational origin is 3281.740 cm^{−} ^{1} and the rotational constants are A‘=8876, B‘=2859, C‘=2155, A’=8864, B’=2855, and C’=2154 MHz. The axes of the two monomer are parallel to one another and the complex has C _{2v } symmetry. The separation of the monomer units is 3.285 Å. The angular part of the intermolecular potential function is discussed in terms of electrostatic interactions between distributed multipole moments.

The fluorescence lifetime of excited cyclohexane in an argon matrix and in solid state cyclohexane at low temperature
View Description Hide DescriptionElectronic fluorescence has been observed from cyclohexane isolated in an Ar matrix in the temperature range 16–30 K. Excitation spectra have been obtained, and the behavior of the fluorescence lifetime has been determined as a function of excitation wavelength. The variation of the fluorescence lifetime as a function of excitation wavelength in the matrix resembles that seen in the gas phase. The fluorescence of neat, solid cyclohexane has also been observed at low temperature, and its decay rate can be used to refine the parameters obtained near room temperature which describe the decay rate. The fluorescence lifetime (τ) of excited cyclohexane can be described by an expression of the form 1/τ=[4.1×10^{8}+8.8×10^{1} ^{1} exp(−4100/1.987 T)] s^{−} ^{1}.

Reinterpretation of the main absorption band of 1,3‐butadiene
View Description Hide DescriptionWe have measured the near ultraviolet absorptionspectrum of 2,3‐dideuterobutadiene to provide a complete set of experimental B _{ u }←X vibrational intervals and bandwidths for all symmetrically deuterated butadienes. These vibrational intervals and bandwidth ratios are compared with the ground state vibrational frequencies and frequency ratios of the molecules. The prominent vibrational frequency interval observed in transitions to the B _{ u } state of butadiene is demonstrated to arise predominantly from a kinetic coupling of the C=C stretching and CH wagging vibrations. The experimental bandwidth ratios are shown to correlate with single quanta of the ground statea _{ u } CH_{2} twist frequency interval ratios. From the latter, a plausable decay path for the B _{ u }excited state of butadiene is deduced. The implications of these conclusions on prior and present attempts to determine the butadiene B _{ u } equilibrium geometry and to understand polyene spectroscopy, photochemistry, and photophysics are discussed.

Sub‐Doppler resolution infrared spectra of the isoelectronic pair: N_{2}–HCN and OC–HCN
View Description Hide DescriptionSub‐Doppler resolution infrared spectra have been obtained for the ν_{1} bands of N_{2} –HCN and OC–HCN using the opto‐thermal detection method, from which accurate ground and excited state molecular constants are determined. Vibrational predissociation lifetimes are estimated from the homogeneous broadening of the observed transitions, giving lifetimes of 80 and 2.6 ns for N_{2} –HCN and OC–HCN, respectively. These lifetimes are considerably longer than those obtained previously for N_{2} –HF and OC–HF. This difference can be understood in terms of the strength of the coupling between the intra‐ and intermolecular motions, which is also reflected in the vibrational frequency shift associated with complex formation.

Tunable far infrared laser spectroscopy of van der Waals bonds: Extended measurements on the lowest Σ bend of ArHCl
View Description Hide DescriptionA tunable far infrared laser system has been used to measure the vibration–rotation spectrum of the lowest Σ bending state of ArHCl near 24 cm^{−} ^{1} in a cw planar jet operating with a terminal jet temperature near 3 K. Over 60 transitions have been observed for both ^{3} ^{5}Cl and ^{3} ^{7}Cl isotopes with resolution of the quadrupolehyperfine structure. An improved set of molecular parameters was determined, including B, D, H, and e q Q for both upper and lower states. Very narrow linewidths (approximately 300 kHz) resulting in high resolution and sensitivity make this technique a powerful new method for the detailed investigation of intermolecular forces.

Rydberg fluorescence of NO trapped in rare gas matrices
View Description Hide DescriptionFluorescence spectra from the RydbergA ^{2}Σ^{+} (v=0) level of NO trapped in Ne, Ar, Kr, and Xe matrices have been obtained for two different sites. The main site is characterized by broad A(0,v‘) bands (FWHM≂80 meV), blue gas‐to‐matrix shifts of 70 to 300 meV, and by absorption–emission Stokes shifts of 300 meV in Xe to 800 meV in Ne matrices. The line shapes and Stokes shifts are treated within a configuration coordinate model by a moment analysis. A linear and quadratic coupling is invoked with relaxation energies ranging for the excited state from 160 meV in Xe to 540 meV in Ne and for the ground state from 150 meV in Xe to 280 meV in Ne and an increase of the cage radius of 3% in Xe to 15% in Ne. The ‘‘red’’ site fluorescence shows emission bands with matrix shifts of −140 meV in Xe to 80 meV in Ne and absorption–emission Stokes shifts of 210 meV in Xe to 830 meV in Ne. Red site ‘‘hot’’ A (v=1) fluorescence is also observed. Red sites are attributed to local disorder around the molecule in Ar, Kr, and Xe matrices and to hexagonal close packed (hcp) pockets within the face centered cubic (fcc) lattice in the case of Ne matrices.

Conformational stability, barriers to internal rotation, vibrational assignment, and a b i n i t i o calculations of chloroacetyl fluoride
View Description Hide DescriptionThe far infrared spectrum of gaseous chloroacetyl fluoride, CH_{2}ClC(O)F, has been recorded at a resolution of 0.10 cm^{−} ^{1} in the 350 to 35 cm^{−} ^{1} region. The fundamental asymmetric torsional frequencies of the more stable t r a n s (two halogen atoms oriented t r a n s to one another) and high energy g a u c h e (Cl–C–C=O torsional dihedral angle of 122°) have been observed at 86.5 and 48.8 cm^{−} ^{1}, respectively, each with excited states falling to lower frequency. From these data the asymmetric torsional potential function governing internal rotation about the C–C bond has been determined. This potential function is consistent with torsional potential coefficients of: V _{1}=350±12, V _{2}=306±6, V _{3}=420±1, V _{4}=44±1, and V _{6}=2±1 cm^{−} ^{1}. The t r a n s to g a u c h e, g a u c h e to g a u c h e, and g a u c h e to t r a n s barriers have been determined to be 796, 245, and 271 cm^{−} ^{1}, respectively, with an energy difference between the conformations of 525±24 cm^{−} ^{1} (1.50±0.07 kcal/mol). From studies of the Raman spectrum at variable temperatures the conformational energy difference has been determined to be 445±80 (1.27±0.2 kcal/mol) and 534±68 cm^{−} ^{1} (1.53±0.2 kcal/mol) for the gaseous and liquid phases, respectively. A complete assignment of the vibrational fundamentals observed from the infrared (3500 to 50 cm^{−} ^{1}) spectra of the gaseous and solid states and Raman (3200 to 10 cm^{−} ^{1}) spectra of the gaseous, liquid, and solid states is proposed. All of these data are compared to the corresponding quantities obtained from a b i n i t i o Hartree–Fock gradient calculations employing both the 3‐21G* and 6‐31G* basis sets. Additionally, complete equilibrium geometries have been determined for both rotamers. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.

Nonadiabatic theory of triatomics: General formalism and application to Renner–Teller and conical‐intersection effects
View Description Hide DescriptionA general theory of nonadiabatic effects in molecular spectra of bound triatomic molecules is presented. Within the nonrelativistic approximation and the expansion method, the formalism is exact and allows a unified study of the various nonadiabatic interactions between two or more electronic states. A full, spinless rovibronic Hamiltonian is used and the rotation, inversion, and nuclear permutation symmetries are considered for defining a rovibronic expansion basis which is symmetry adapted for ABC and AB_{2} molecules. The Hamiltonian matrix elements on the rotoelectronic‐angular basis are obtained for singlet electronic states, by fully considering both the nuclear and the electronic structure. The equations are then specialized to three‐state Renner–Teller and conical intersection effects in the diabatic representation, and are approximated and compared with previous works. Finally, the expressions of the dipole line strengths are derived and the singularities of the adiabatic representation of conical intersections are discussed.

Benzene clustered with N_{2}, CO_{2}, and CO: Energy levels, vibrational structure, and nucleation
View Description Hide DescriptionTwo‐color time‐of‐flight mass spectroscopy is employed to study the van der Waals (vdW) clusters of benzene(N_{2})_{ n } (n≤8), benzene(CO_{2})_{ n } (n≤7), and benzene(CO)_{ n } (n=1,2) created in a supersonic molecular jet. Potential energy calculations of cluster geometries, normal coordinate analysis of vdW vibrational modes, and calculations of the internal rotational transitions are employed for the assignment of the benzene(solvent)_{1}cluster spectra in the 0^{0} _{0} and 6^{1} _{0} regions of the benzene ^{1} B _{2u }←^{1} A _{1g } transition. The respective vibronic and rotational selection rules for these clusters are determined based on the appropriate point groups and molecular symmetry groups of the clusters. Good agreement between the calculated and experimental spectra is obtained with regard to the vdW vibrational and internal rotational modes. The solvent molecules rotate nearly freely with respect to benzene about the benzene–solvent bond axis in the benzene(solvent)_{1} clusters. In the excited state a small ∼20 cm^{−} ^{1} barrier to rotation is encountered. Studies of larger clusters (n>2) reveal a broad red shifted single origin in the 6^{1} _{0}spectra. A linearly increasing cluster energy shift is observed as a function of cluster size. The cluster energy shifts are not saturated by one solvent molecule on each side of the aromatic ring; several solvent molecules effectively interact with the solute π electronic cloud. Both homogeneous and inhomogeneous nucleation take place for the clusters studied depending on the ratio of the solvent–solvent binding energy to the cluster binding energy.

Laser induced vibration–rotation excitation of hydroxyl ions: OH^{−} and OH^{+}
View Description Hide DescriptionThe quantum dynamics of vibration–rotation excitation of hydroxyl ions (e.g., OH^{−} and OH^{+} in the gaseous phase) in the presence of a laser beam is investigated. The nonperturbative Floquet method is used to solve the equation of motion with an explicitly time‐dependent Hamiltonian. Convergent results are obtained for the long‐time averaged transition probabilities for various transitions by including a sufficiently large number of rovibrational and photon states. In order to understand the roles played by power broadening, dynamic stark shifts, and non‐Lorentzian line shapes, higher laser intensities (in the GW/cm^{2} range) are considered in the molecular excitation process.

The potential surface and stretching frequencies of X̃ ^{3} B _{1} methylene (CH_{2}) determined from experiment using the Morse oscillator‐rigid bender internal dynamics Hamiltonian
View Description Hide DescriptionThe Morse oscillator‐rigid bender internal dynamics (MORBID) Hamiltonian [P. Jensen, J. Mol. Spectrosc. 1 2 8, 478 (1988)] has been used in a fitting to all extant rotation–vibration data for X̃ ^{3} B _{1} methylene CH_{2}. This fitting leads to an improved determination of the potential energy surface, and in particular to reliable predictions for the stretching frequencies. We predict ν_{1}=2992 cm^{−} ^{1} and ν_{3}=3213 cm^{−} ^{1} for ^{1} ^{2}CH_{2}, and we hope that the new predictions will encourage the experimental search for these weak fundamentals. In the MORBID approach the rotation–vibration energies are obtained from the potential energy surface in a purely variational calculation, and consequently the present work is an improvement over previous determinations of the CH_{2}potential energy surface from experiment that used the nonrigid bender formalism [see P. R. Bunker e t a l., J. Chem. Phys. 8 5, 3724 (1986), and references therein]; this latter approach treats the stretching vibrations by second order perturbation theory. A fitting to the J=0 vibrational energy data for ã ^{1} A _{1} methylene has also been made here using the MORBID Hamiltonian. Combining the results of these MORBID fittings to experimental data for the (X̃) and (ã) states of CH_{2} we obtain the singlet–triplet splittings T _{0}(ã ^{1} A _{1})=3147 cm^{−} ^{1} (8.998 kcal/mol) and T _{ e }(ã ^{1} A _{1})=3223 cm^{−} ^{1} (9.215 kcal/mol).

Multiple‐pulse NMR in inhomogeneously broadened rotating solids: Theory of sideband suppression experiments
View Description Hide DescriptionA detailed analysis of a wide range of magic angle sample spinning (MASS)/multiple‐pulse NMRexperiments is presented. The analysis depends upon the calculation of the magnetization trajectories of individual crystallites, and thus is valid for inhomogeneously broadened systems. Using this model we examine a variety of experiments used to control and manipulate rotational sidebands in MASS spectra. This model provides a straightforward analysis of both the two and four π pulse experiments used for total suppression of sidebands (TOSS), and allows us to derive new timings for both sequences. We are able to provide the first proof that the TOSS sequence exactly eliminates sidebands, and that the two‐pulse sequence eliminates first‐order sidebands. We also demonstrate that the TOSS cycle can actually lead to inverted spectra, and we have accurately quantified intensity losses which occur in both the two‐ and four‐pulse versions. The effect of pulse errors on sideband suppression experiments is also briefly discussed, as well as one possible means to reintroduce information about shift anisotropies into TOSS‐type experiments.

Molecular spectra in chaotic regime expressed by the Wigner function
View Description Hide DescriptionWe have shown in this paper that the rovibrational spectrum of a molecule corresponding to the classical chaotic regime can be expressed by the dipole correlation averaged over the Wigner function, and that the commonly used dipole correlation averaged over the classical microcanonical ensemble is not generally valid in the chaotic regime. The spectacular characteristics of chaotic trajectories, e.g., the exponential growth of separation of initially nearby trajectories, the decaying correlation, etc., make the molecular spectra diffuse and obscure its line structure. It is suggested that the correlation expression obtained in this paper may be used to construct the approximate envelope of the actual spectra.

Deexcitation of metastable Ba^{+}
View Description Hide DescriptionThis is a study of the deexcitation mechanisms of the metastable 5 ^{2} D _{3} _{/} _{2} state of Ba^{+} in about 1 atm He buffer gas in the temperature range of 500–650 °C. The ions were produced by three‐photon ionization with light tuned to a resonance between two excited states of Ba, enabling the electron density to be varied independently of the temperature and pressure. The deexcitation is dominated by superelastic scattering of electrons with a cross section of σ_{se} =45±20 Å^{2} (corresponding to an electron‐impact excitation cross section of 11 Å^{2}) and quenching by Ba atoms with a cross section of σ_{Ba} =130±25 Å^{2}. No dependence of the deexcitation rate on He density was detected, and we infer an upper limit on the cross section of σ_{He} ≤9×10^{−} ^{2} ^{2} cm^{2}.

Excimer laser multiphoton dissociation of Cr(CO)_{6}: Evidence for two distinct dissociation processes
View Description Hide DescriptionThe excimer lasermultiphotondissociation of Cr(CO)_{6} has been investigated in the gas phase using emission spectroscopy to detect excited state photoproducts. Following laser irradiation at 193 nm (ArF*), 248 nm (KrF*), and 351 nm (XeF*) well‐resolved Cr(I) emission was detected. The photodissociation mechanism was studied by determining the laser fluence dependence, buffer gas pressure dependence, and temporal profiles of the emission intensity for the various Cr(I) excited states. The data suggest that dissociation occurs via two distinct processes, sequential and direct. The sequential process is found to be extremely sensitive to buffer gas pressure, while the direct mechanism is pressure invariant. The Cr(I) excited state distributions formed in the direct process, following irradiation at the three laser wavelengths used, appear to be statistical.

Energetics and spin‐ and Λ‐doublet selectivity in the infrared multiphoton dissociation DN_{3}→DN(X ^{3}Σ^{−}, a ^{1}Δ)+N_{2}(X ^{1}Σ^{+} _{ g }): Experiment
View Description Hide DescriptionMultiphoton vibrational excitation of deuterated hydrazoic acid, DN_{3}, by a CO_{2} laser (I=10 GW/cm^{2}) leads to dissociation forming DN in both X ^{3}Σ^{−} (spin forbidden) and a ^{1}Δ (spin allowed) electronic states. Under collisionless conditions, the nascent DN fragments were probed via laser induced fluorescence, to determine initial product state distributions. The DN(X ^{3}Σ^{−}) molecules are formed predominantly in the symmetric F _{1} and F _{3} spin–rotation states with little population (≤6%) in the antisymmetric F _{2} levels. There is no significant population (<3%) in excited DN(^{3}Σ^{−}) vibrational levels. The distribution of rotational states is Boltzmann‐like, characterized by a rotational ‘‘temperature’’ of about 920 K for the F _{1}, F _{3} states and 500 K for F _{2} levels. Doppler profiles showed a large kinetic energy release of about 10 100 cm^{−} ^{1} total in the triplet channel. The DN(^{1}Δ) products are formed preferentially in the symmetric Δ(A’), e‐labeled lambda doublet levels: Δ(A’)/Δ(A‘)=1.44. The DN(^{1}Δ) is formed with no vibrational excitation (<2%); the rotational states are populated Boltzmann‐like with a rotational ‘‘temperature’’ of 425 K. Doppler profiles give a total kinetic energy of about 1500 cm^{−} ^{1} in this channel. These observations give information about the distribution of energy in the reactant, the location of the barriers to dissociation, and the geometry of the transition states. Alexander, Werner, and Dagdigian (accompanying article) show that the observed DN(^{3}Σ^{−}) spin‐ and DN(^{1}Δ) Λ‐doublet selectivities reflect the symmetry properties of a planar transition state and that the low degree of DN(^{3}Σ^{−}) rotational and vibrational excitation is also expected from the transition state geometry.