Volume 93, Issue 9, 01 November 1990
Index of content:

Water vapor continuum in the millimeter spectral region
View Description Hide DescriptionA theory is presented for the calculation of the continuous absorption of water molecules in the millimeter spectral region. The theory is based on a generalization of Fano’s theory in which the spectral density, the Fourier transform of the dipole‐moment correlation function, is calculated for a system consisting of a pair of molecules. The internal states are written in terms of the line space of the system, and the resolvent operator is obtained using the well‐known Lanczos algorithm. For the interaction between two water molecules, we include only the leading dipole–dipole term of the long‐range anisotropic potential, and model the isotropic interaction, used to calculate the statistical weight within the quasi‐static approximation, by a Lennard–Jones potential. Using reasonable values for the two Lennard–Jones potential parameters, and the known rotational constants and permanent dipole moment of a water molecule, we calculate the absorption coefficient for frequencies up to 450 GHz for temperatures between 282 and 315 K. The present results are in good agreement with an empirical model for the water continuum based on combined laboratory and atmospheric measurements. We conclude from our results that, contrary to some previous assertions, the strong negative temperature dependence as well as the magnitude of the continuum absorption, at least for the millimeter spectral region, can be explained in terms of the far‐wings of allowed rotational transitions.

Where does overtone intensity come from?
View Description Hide DescriptionThe factors that influence overtone intensity are examined using realistic potential and dipole moment functions that model the acetylenic stretching vibration. It is found that mechanical anharmonicity is more important than electrical anharmonicity, especially at higher overtone levels but a quantitative treatment requires both contributions which are not simply additive. It is found numerically that the inner wall of the potential dominates in determining overtone intensity. This can be justified by heuristic arguments and by the properties of the sensitivity functional of the transition dipole matrix elements to changes in V(r). It is suggested that absolute overtone intensities could be useful in determining the inner walls of hydrogen stretching vibrations.

Investigation of very fast motions in electrolyte solutions by far infrared spectroscopy
View Description Hide DescriptionThe far infrared spectra of solutions of a number of alkali halide salts in protic (methanol) and dipolar aprotic solvents (acetonitrile, acetone, dimethyl sulfoxide) have been investigated in the frequency range 25–650 cm^{−1}. Special attention has been paid to detect weak bands in making the difference between the absorptionspectrum of the solution and that of the pure solvent. These spectra are characterized by several absorption peaks spread all over the FIR spectral range. Introducing a chemical model of the electrolytesolution, it is shown that the time correlation function at the origin of the absorption phenomenon is governed essentially by three processes, namely, (i) the ion‐induced dipole mechanism, (ii) the ionic association, and (iii) the relaxation of the solvent molecules within the ionic solvation shell. A band shape analysis, based upon the Mori theory, permits us to identify the molecular motions which give rise to specific frequency modes on the absorptionspectrum. In particular, the role played by the time scale separation between the fast dynamics occuring within the first solvation shell of ionic species and the time spent to interconvert inner and outer solvation shells, is emphasized.

A quantum electrodynamical study of intermolecular line broadening and line shift
View Description Hide DescriptionIn this paper quantum electrodynamics is used to investigate the effects of radiation field‐induced energy transfer as a mechanism for the manifestation of frequency shift and both homogeneous and inhomogeneous line broadening. Using resolvent operator methods and the Power–Zienau–Woolley multipolar Hamiltonian to derive an effective Hamiltonian, it is illustrated how all coherent and incoherent field‐induced line shift and line broadening including Förster energy transfer, dispersion, and cooperative Rayleigh scattering can be described solely in terms of a single multipolar interaction. The detailed analysis reveals that a description of intermolecular interactions in terms of virtual photon coupling leads a detailed range dependence that is not only different to nonretarded semiclassical theory, but also complex. This paper therefore illustrates for the first time that all intermolecular processes contribute to b o t h line broadening and energy shift, and that the imaginary contributions to virtual photon coupling lead to significant retardation effects in condensed media. Finally by applying the results within a density matrix framework important aspects regarding the statistical averaging of these additional terms in the Hamiltonian are discussed, and it is illustrated how intermolecular coupling leads to non‐Lorentzian, redshifted, asymmetric line profiles.

Laser multiphoton ionization and photoelectron spectroscopy of Co(CO)_{3}NO and Fe(CO)_{5}
View Description Hide DescriptionLaser multiphoton dissociation‐resonance‐enhanced multiphoton ionization (MPD‐REMPI) and time‐of‐flight photoelectron spectra (TOF‐PES) of Co(CO)_{3}NO and Fe(CO)_{5} have been obtained in the range 445–455 nm. The only ions produced by the pulsed dye laser are Co^{+} and Fe^{+}. Transitions observed in the MPD‐REMPI spectra are assigned to resonant states of the neutral atoms. Final states of the atomic ions are determined from the TOF‐PES spectra. The multiphoton dissociation process produces metal atoms in a broad distribution of states, ranging in energies up to 33 000 cm^{−1} for Co, and 32 000 cm^{−1} for Fe. The most intense REMPI lines are associated with low‐lying electronic states (<8500 cm^{−1} for Fe and Co). By tuning the laser to appropriate wavelengths, neutral metal atoms in selected electronic states may be ionized. At most laser wavelengths, the atomic metal ions are formed in a distribution of states, only some of which are consistent with preservation of the core configuration of the Rydberg intermediate in the ionization step. An exception appears for ionization of Co atoms via 4dRydberg intermediate states.

Laboratory observation of hot bands of H^{+} _{3}
View Description Hide DescriptionThe (2ν_{2},l=2←ν_{2}), (2ν_{2},l=0←ν_{2}), and (ν_{1}+ν_{2}←ν_{1}) hot bands of H^{+} _{3} were observed. The vibrationally hot ions were produced in a liquid nitrogen cooled 6 kHz ac discharge using gas mixtures of H_{2} and He. The spectra were detected in direct absorption using a newly extended tunable difference frequency spectrometer using both LiNbO_{3} and LiIO_{3} crystals as nonlinear optical elements. The range of this spectrometer is now ∼5300–∼1900 cm^{−1}. The positions of the rovibrational transitions compare extremely well with the theoretical predictions of Miller and Tennyson. A vibrational temperature study of the discharge indicates a significant population inversion between the ν_{1} and ν_{2} levels.

Observation of the 2ν_{2}(l=2)←0 overtone band of H^{+} _{3}
View Description Hide DescriptionThe overtone band of 2ν_{2}(l=2)←0 of H^{+} _{3} in the 2 μm infrared region has been observed in absorption by using a difference frequency laser spectrometer. The LiIO_{3} nonlinear optical system assembled by Bawendi and the bidirectional multiple path optical arrangement with velocity modulation have enabled us to observe 34 vibrational–rotation transitions from 5094 to 4557 cm^{−1}. Liquid‐N_{2} cooled plasmas were used for transitions involving low rotational levels, while gaseous‐N_{2} cooled plasmas were used for high rotational levels, up to J,K=(9,9). From the relative intensities of the transitions, rotational temperature has been estimated to be ∼270 K for the former and ∼1300 K for the latter. The Δ‖k−l‖=±3 selection rule of the overtone band allows us to determine the absolute energy level values by combining the frequencies of the overtone band with those of the fundamental and hot bands.

Rotational spectrum and potential surface for Ar_{2}–HCN: A T‐shaped cluster with internal rotation
View Description Hide DescriptionThe rotational spectrum of Ar_{2}–HCN has been observed between 2.5 and 11.5 GHz with the pulsed nozzle, Fourier transform, Balle/Flygare Mark II microwave spectrometer. Eighteen transitions were found and their ^{14}N quadrupolehyperfine structure analyzed. The line centers were fitted with the Watson Hamiltonian giving ground state rotational constants of 1769.366, 1743.854, and 857.600 MHz. The effective geometry of the cluster is found to be T‐shaped with C_{2v } symmetry and the H end of the HCN closest to the Ar_{2}. However, the rms deviation of the fit is poor (300 kHz), the centrifugal distortion and inertial defect are huge, the Ar to HCN c.m. distance is nearly 0.2 Å shorter than in the Ar–HCN dimer, and the average angular displacement of the HCN from the C_{2} axis is both large (39°) and highly anisotropic (10°). In contrast, the Ar_{2} subunit exhibits an in‐plane, average angular displacement of only 6°. These anomalies led us to calculate potential surfaces for Ar_{2}–HCN and Ar_{2}–HF using the molecular mechanics for clusters scheme. A comparison of the surfaces and the rotational properties of the two species prompts us to propose that in Ar_{2}–HCN the HCN axis rotates about the C_{2} axis maintaining an angle of ∼40° between them for the m=0 internal rotation state. Such internal rotation accounts at least qualitatively for the otherwise anomalous rotational behavior of the Ar_{2}–HCN cluster.

Theoretical studies of the spectroscopy of excess electrons in water clusters
View Description Hide DescriptionVariational calculation based on a continuum dielectric model, and numerical simulations based on the RWK2‐M water potential and on a pseudopotential for the electron–water interaction, are used to evaluate excitation energies and optical spectra for bound interior states of an excess electron in water clusters and in bulk water. Additionally, optical data for surface states are obtained from numerical simulations. The simulation approach uses adiabatic dynamics based on the quantum‐classical time‐dependent self‐consistent field (TDSCF) approximation and the fast‐Fourier transform (FFT) algorithm for solving the Schrödinger equation. Both approaches predict very weak or no cluster size dependence of the excitation spectrum for clusters that support interior solvated electron states. For an electron attached to the cluster in a surface localization mode, bound excited states exist for most nuclear configurations of clusters down to (H_{2}O)^{−} _{18}, and the corresponding excitation energy is strongly shifted to the red relative to that associated with stable internal states in larger clusters. Binding and excitation energies associated with surface states are about half the value of these quantities for interior states. The present variational continuum dielectric theory is in relatively good agreement with the simulation results on the size dependence of the relative stability of interior states. However, it strongly underestimates the vertical excitation energy of the solvated electron. It is suggested that optical spectroscopy of excess electrons in water clusters could serve as a sensitive probe of the transition from surface to interior localization modes as the number of water molecules in the cluster is increased.

Intensities in local mode overtone spectra: Propane
View Description Hide DescriptionThe gas phase vibrational overtone spectrum of propane is measured using conventional near infrared (NIR) spectroscopy for the Δv _{CH}=2–5 regions and intracavity dye laserphotoacoustic spectroscopy (IDL‐PAS) for the Δv _{CH}=5 and 6 regions. The peaks are assigned in terms of the local mode model. Experimental oscillator strengths are compared to values calculated for the CH‐stretching components of the spectrum. The calculations use a harmonically coupled, anharmonic oscillator local mode model to obtain the vibrational wave functions, and a b i n i t i o MO calculations at the SCF level with a 6‐31G* basis set to obtain the dipole moment function. The importance of intermanifold coupling is explored. The calculations can account for the fall‐off in intensity with increasing v, and can give a reasonably quantitative account of the relative intensities of the individual peaks within a given vibrational manifold. The questions of the relative intensities of primary and secondary CH bonds, and of the relative intensities of different methyl CH bonds are also explored.

An investigation of hydrogen bonding between HCl and vinylacetylene: A molecule with two different π‐acceptor sites
View Description Hide DescriptionThe ground state rotational spectrum of a hydrogen‐bonded dimer formed by vinylacetylene and hydrogen chloride has been detected by the pulsed‐nozzle, Fourier‐transform microwave technique. Vinylacetylene has been chosen as a prototype acceptor molecule containing two different π‐acceptor sites. Rotational constantsA_{0}, B_{0}, C_{0}, centrifugal distortion constants Δ_{ J }, Δ_{ J K }, δ_{ J }, δ_{ K }, and three components χ_{ a a }, χ_{ b b }−χ_{ c c }, and χ_{ a b } of the Cl nuclear quadrupole coupling tensor have been determined for each of the three isotopomers CH_{2}CHCCH⋅⋅⋅ H^{35}Cl, CH_{2}CHCCH⋅⋅⋅H^{37}Cl, and CH_{2}CHCCH⋅⋅⋅D^{35}Cl. These spectroscopic constants have been interpreted in terms of a dimer in which the HCl subunit forms a hydrogen bond to the C 3/4 C triple bond in a T‐shape configuration, but is displaced from the center of the triple bond by d=0.04 Å towards the inner C atom, and makes an angle φ=34° with the vinylacetylene plane. The experimental angular geometry is in excellent agreement with that predicted by the Buckingham–Fowler electrostatic model which gives φ=27°.

Rotational spectrum and internal rotation of a methane–HCl complex
View Description Hide DescriptionRotational spectra of CH_{4}⋅HCl and CD_{4}⋅HCl have been studied using a pulsed‐nozzle Fourier‐transform microwave spectrometer. The K=0 and K=1 components of the J=1–0,2–1, and 3–2 transitions have been detected in 4–18 GHz and assigned through the Cl‐nuclear quadrupolehyperfine structure characteristic for a symmetric top. The Cl‐isotopic dependence of the determined rotational constants is consistent with a methane...HCl geometry, i.e., methane acts as a proton acceptor and hydrogen chloride as a donor, respectively. The K=0 transitions were observed to be split into a doublet while only one component was detected for K=1. This anomaly has been explained as it is due to two‐dimensional internal rotation of methane. An analysis considering the angular momentum coupling between the internal rotation and overall rotation of the complex gives a correlation between a free‐internal rotor and a rigid symmetric top. The two observed K=0 and one observed K=1 components correspond to the three lowest states with different symmetries, A, F, and E, which correlate respectively to the j=0, 1, and 2 states of the freely rotating methane.

An a b i n i t i o calculation of the intramolecular stretching spectra for the HF dimer and its D‐substituted isotopic species
View Description Hide DescriptionWe have carried out an a b i n i t i o calculation of the intramolecular stretching spectra (wave numbers and transition moments) of (HF)_{2}, (DF)_{2}, and HFDF involving v _{1}+v _{2}≤3, where v _{1} and v _{2} are the local mode quantum numbers for the two intramolecular (HF or DF) stretches. The a b i n i t i o surface used as a basis for these calculations has already been published [M. Kofranek, H. Lischka, and A. Karpfen, Chem. Phys. 1 2 1, 137 (1988); P. R. Bunker, P. Jensen, A. Karpfen, M. Kofranek, and H. Lischka, J. Chem. Phys. 9 2, 7432 (1990)], but in the present work we have extended the 1520 nuclear geometry points previously available with 198 points in order to explore further the variation of the intramolecular stretching energies and the dipole moment along the minimum energy (t r a n stunneling) path. We compute the intramolecular stretching energies and transition moments by making an adiabatic separation of the intramolecular stretching motion and the other vibrational motions of the molecules, and we use the semirigid bender Hamiltonian to average over the t r a n s‐tunneling motion. For HFHF, we obtain the fundamental level corresponding to the ‘‘free‐H’’ stretch ν_{1} at 3925 cm^{−1} and that corresponding to the ‘‘bound‐H’’ stretch ν_{2} at 3874 cm^{−1}, in very good agreement with the experimental results of 3930.9 and 3868.1 cm^{−1}, respectively [A. S. Pine, W. J. Lafferty, and B. J. Howard, J. Chem. Phys. 8 1, 2939 (1984)]. For the higher excited states, we obtain the 2ν_{1} energy level at 7674 cm^{−1} (7700±20 cm^{−1}), 2ν_{2} at 7570 cm^{−1} (7555±15 cm^{−1}), 3ν_{1} at 11 259 cm^{−1} (11 260 cm^{−1}), and 3ν_{2} at 11 085 cm^{−1} (11 060 cm^{−1}), where the experimental values [K. von Puttkamer and M. Quack, Chem. Phys. 1 3 9, 31 (1989)] are given in parentheses.

The Na ^{7}Li 3 ^{1}Σ^{+}(C) and 1 ^{1}Π(B) electronic states through collision energy transfer
View Description Hide DescriptionThe NaLi 3 ^{1}Σ^{+}(C)→2 ^{1}Σ^{+}(A) electronic transition has been observed in the infrared region after laser excitation of the 1 ^{1}Π(B) electronic state and subsequent collisional energy transfer between the 1 ^{1}Π(B) and 3 ^{1}Σ^{+}(C) electronic states. The spectra were recorded at high resolution by Fourier‐transform spectroscopy. Thirteen vibrational bands were analyzed, providing detailed information for the 2 ^{1}Σ^{+}(A) (v=0,...,4) and 3 ^{1}Σ^{+}(C) (v=5,...,13) vibrational levels. Rotational perturbations have been observed in the spectra. The nearly 1200 observed lines belonging to 1 ^{1}Π(B)→2 ^{1}Σ^{+}(A) and 3 ^{1}Σ^{+}(C)→2 ^{1}Σ^{+}(A) transitions have been assigned and reduced to molecular constants in a linear least‐squares fit. Perturbations observed in the upper electronic states have been reduced using a nonlinear least‐squares fit to a ^{1}Σ∼^{1}Π effective Hamiltonian matrix model. Deperturbed molecular constants and perturbation parameters are obtained for the 1 ^{1}Π(B) electronic state (v=0,...,6) and the 3 ^{1}Σ^{+}(C) electronic state (v=5,...,13) levels. Propensity rules concerning the energy gaps and the conservation of angular momentum, during the energy transfer, are inferred from the intensity distributions of anomalous lines.

Far‐infrared spectra and two‐dimensional potential energy surface of silacyclopentane and its deuterated isotopomers
View Description Hide DescriptionThe far‐infrared spectra of silacyclopentane and its 1‐d _{1} and 1,1‐d _{2} isotopomers have been recorded and analyzed. Spectral series corresponding to the bending and twisting transitions were observed. Bend‐twist combination bands and bending overtone spectra were also detected. Kinetic energy (reciprocal reduced mass) expansions were calculated for the bending and twisting motions. These were used along with a five‐term two‐dimensional potential energy surface in the vibrational Hamiltonian in order to calculate the energy states. The basis sets were carefully generated to ensure that these levels were accurately calculated. The potential energy surface calculation does an excellent job of reproducing the 103 observed transition frequencies. The potential surface has energy minima at twist angles of 30°. The barrier to planarity is 2110±200 cm^{−1}. The bent structure, which corresponds to a saddle point on the surface, has an energy about 1500 cm^{−1} above the twisted conformation.

Measurement of the lifetime of metastable triatomic hydrogen
View Description Hide DescriptionWe measured the survival probability of the metastable (N=0, K=0) level of the 2p ^{2} A ^{‘} _{2} of H_{3} as a function of time elapsed from formation of the molecule by probing its population with photoionization. We find lifetimes τ=640^{+300} _{−100} ns for the ground vibrational state and τ=740^{+300} _{−100} ns for the symmetric stretch‐excited level. Equally short lifetimes are obtained from an analysis of the photoinduced bleaching of the spontaneous dissociation signal of metastable H_{3}. These lifetimes are about 2 orders of magnitude shorter than those expected on the basis of the allowed radiative transition 2p ^{2} A‘_{2}→2s ^{2} A ^{’} _{2}. We attribute the faster decay channel to weakly allowed radiative transitions between the metastable state and the degenerate mode‐excited repulsive ground state of H_{3}, as well as to predissociation of the metastable levels by the repulsive ground state of H_{3} induced by spin–orbit coupling.

Photoelectron spectroscopy and electronic structure of clusters of the group V elements. I. Dimers
View Description Hide DescriptionThe HeI (584 Å) high resolution photoelectron spectra of As^{+} _{2}, Sb^{+} _{2}, and Bi^{+} _{2} have been obtained with a high temperature molecular beamsource. A pure As_{2}beam was produced by evaporating Cu_{3}As. Sb_{2} was generated as a mixture with the atoms and tetramers by evaporating the pure element, while Bi_{2} was generated as a mixture with only the atoms from the pure element. Vibrational structure was well resolved for the As^{+} _{2} spectrum. Spectroscopic constants were derived and reported for the related ionic states. In addition, we have carried out relativistic complete active space self‐consistent field followed by multireference single + double configuration interaction calculations on these dimers both for the neutral ground states and the related ionic states. The agreements between the calculated and experimentally derived spectroscopic constants were fairly good, although the calculations tended to underestimate consistently the strength of the bonding in these heavy homonuclear diatomics.

Photoelectron spectroscopy and electronic structure of clusters of the group V elements. II. Tetramers: Strong Jahn–Teller coupling in the tetrahedral ^{2} E ground states of P^{+} _{4}, As^{+} _{4}, and Sb^{+} _{4}
View Description Hide DescriptionHigh resolution HeI (584 Å) photoelectron spectra have been obtained for the tetrameric clusters of the group V elements: P_{4}, As_{4}, and Sb_{4}. The spectra establish that the ground ^{2} E states of tetrahedral P^{+} _{4}, As^{+} _{4}, and Sb^{+} _{4} are unstable with respect to distortion in the ν_{2}(e) vibrational coordinate. The E⊗e Jahn–Teller problem has been treated in detail, yielding simulated spectra to compare with experimental ones. Vibronic calculations, extended to second order (quadratic coupling) for P^{+} _{4}, account for vibrational structure which is partially resolved in its photoelectron spectrum. A Jahn–Teller stabilization energy of 0.65 eV is derived for P^{+} _{4}, which can be characterized in its ground vibronic state as being highly distorted, and highly fluxional. Linear‐only Jahn–Teller coupling calculations performed for As^{+} _{4} and Sb^{+} _{4}, show good qualitative agreement with experimental spectra, yielding stabilization energies of 0.84 and 1.4 eV, respectively.

Photoelectron spectroscopy and electronic structure of clusters of the group V elements. III. Tetramers: The ^{2} T _{2} and ^{2} A _{1} excited states of P^{+} _{4}, As^{+} _{4}, and Sb^{+} _{4}
View Description Hide DescriptionMethods employing high resolution HeI (584 Å) photoelectron spectroscopy have been applied to the tetrameric clusters of the group V elements, to resolve details of vibronic and spin–orbit structure in the first three electronic states of P^{+} _{4}, As^{+} _{4}, and Sb^{+} _{4}. Measured spacings of distinct vibrational progressions in the ν_{1} mode for the ^{2} A _{1} states of P^{+} _{4} and As^{+} _{4}, yield vibrational frequencies of 577 (5) cm^{−1} for P^{+} _{4} and 350 (6) cm^{−1} for As^{+} _{4}. Franck–Condon factor calculations suggest bond length changes for the ions in the ^{2} A _{1} states of 0.054 (3) Å for P^{+} _{4} and 0.060 (3) Å for As^{+} _{4}. Strong Jahn–Teller distortions in the ν_{2}(e) vibrational mode dominate the structure of the ^{2} E ground states of the tetrameric ions. Both Jahn–Teller and spin–orbit effects appear in the spectra of the ^{2} T _{2} states of the tetrameric ions, with the spin–orbit effect being dominant in Sb^{+} _{4} and the Jahn–Teller effect dominant in P^{+} _{4}. Vibrational structure is resolved in the P^{+} _{4} spectrum, and the ν_{3}(t _{2}) mode is found to be the one principally active in the Jahn–Teller coupling. A classical metal‐droplet model is found to fit well with trends in the IPs of the clusters as a function of size.

Calculation of van der Waals spectra for H_{2}HF, D_{2}HF, and H_{2}DF
View Description Hide DescriptionCalculations of spectra for the excitation of the van der Waals modes in the weakly bound complexes H_{2}HF, D_{2}HF, and H_{2}DF are reported. An a b i n i t i opotential energy surface has been computed using the coupled electron pair approximation with a large basis set. The rovibrational bound states for the complexes are calculated using a variational method. A self‐consistent field approach is used to optimize the basis sets for bending and stretching motion in the van der Waals complex. The calculated spectra compare very well with those measured in near‐infrared experiments. The computations provide a systematic analysis of the relative stability and rigidity of the different complexes, the very large zero‐point energy effects, and the unusual nature of the rovibrational wave functions in these van der Waals molecules. The predicted spectra contain some new bands with surprisingly large intensities that might be detectable in experiments.