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Volume 104, Issue 24, 22 June 1996

Analysis of the mass‐resolved two‐photon spectra of jet‐cooled ArKr near Kr^{*}(5p) and Ar^{*}(4s)
View Description Hide DescriptionNew (2+1) resonantly enhanced multiphoton ionization (REMPI) spectra of ArKr in the region of Kr*(5p) and Ar*(4s) between ≊92378.8 and 94250.7 cm^{−1} are presented. A time‐of‐flight (TOF)mass spectrometer was used to obtain single isotopomer data. Four band systems, two previously observed by Dehmer and Pratt [J. Chem. Phys. 88, 4139 (1988)], and two new ones, have been vibrationally analyzed.Excited state bond lengths have been found from Franck–Condon factor calculations while electronic symmetries were assigned from REMPI spectra recorded with circularly polarized light. Our excited state symmetry assignments differ from those recently proposed by Heck et al. [J. Phys. Chem. 99, 17700 (1995)]. The unusual vibrational band intensity distributions observed for some of the electronic systems are rationalized qualitatively in terms of interstate avoided crossings.

The frequency detuning and band‐average approximations in a far‐wing line shape theory satisfying detailed balance
View Description Hide DescriptionWe develop the basic formalism of a far‐wing line shape theory that satisfies the detailed balance principle. For molecular systems of interest, e.g., CO_{2}–Ar at room temperature or higher, there are many individual vibration–rotational lines in a given band and many bands in the spectrum. In such cases, one must make additional approximations in order to carry out accurate calculations of the absorption coefficient using a reasonable amount of computer time. In the present paper, we discuss two such simplifications: the frequency detuning approximation of the line‐coupling functions and the band‐average approximation. We then apply the theory to a calculation of the far‐wing absorption of the ν_{3} band of CO_{2} perturbed by Ar, successively including the effects of more lines in the calculations by increasing J _{max} from 40 to 108. From the results of this work, we find that the frequency detuning approximation is good only for frequencies of interest far from the band center. In addition, we find that contrary to previous assertions of the adequacy of the first‐order band‐average approximation, the higher‐order terms are significant. To a good approximation these can be incorporated by introducing a frequency shift in the first‐order results so that extensive additional calculations are not required.

Laser cooling of molecules: A sequential scheme for rotation, translation, and vibration
View Description Hide DescriptionA novel scheme is proposed for sequential cooling of rotation, translation, and vibration of molecules. More generally, this scheme manipulates and controls the states and energies of molecules. The scheme, while somewhat complex, is simpler and more feasible than simply providing a large number of synchronously but independently tunable lasers. The key component is a multiple single frequency laser (MSFL) in which a single narrow band pump laser generates an ensemble of resonant ‘‘stimulated Raman’’ (RSR) sidebands (subsequently amplified and selected) in a sample of the molecules to be cooled. Starting with a relatively cold molecular sample (e.g., a supersonic beam of Cs_{2}), the rotation of molecules is cooled by sequential application of P branch electronic transition frequencies transverse to the molecular beam beginning at higher rotational angular momentumJ. Then translation of molecules is cooled by application of multiple low J, P, and R branch transition frequencies which counterpropagate with the molecular beam and are synchronously chirped over their Doppler profiles. Finally, vibration of molecules is cooled by blocking the R(0) line of the 0–0 band. Only this specific order of rotation–translation–vibration appears feasible (using molecules produced by photoassociation of ultracold atoms avoids the requirement for translational cooling). Each step employs true dissipative cooling (i.e., reduction of system entropy in three degrees of freedom) by spontaneous emission and should yield a large translationally cold sample of molecules in the lowest (v=0, J=0) level of the ground electronic state, suitable for studies such as molecule trapping, ‘‘molecule optics,’’ or long range intermolecular states.

Temperature determination of transient species by degenerate four wave mixing: Application of the independently determined power law of the transition dipole moment and geometric factors
View Description Hide DescriptionWe investigated the dependence of the degenerate four wave mixing(DFWM) signal intensities on the electronic transition dipole moment of isolated lines in the A ^{1}Σ^{+}←X ^{1}Σ^{+} transition band of NaH. We applied a new method to determine this dependence in transient species without previous knowledge of the sample temperature. By using different appropriate pairs of DFWM lines sharing common lower level, the relative population difference is eliminated. We found that this ratio is well described by a power law (μ_{1}/μ_{2})^{ x } where μ_{ i } is the electronic one‐photon transition dipole moment. As a result of saturation the exponent x depends on the total laser energy deposited in the medium. The observations are in good agreement with a modified two‐level model, which includes the effects of polarization of the laser beams used in the experimental setup by introducing geometric factors. The exponent of the integrated signal intensities varies between 3 and 8 for high and low laser intensities with a rapid decrease to high intensities. This makes it possible to compare signal intensities at different DFWM laser intensities in order to obtain relative level populations, i.e., temperature, with high sensitivity.

The intramolecular vibrations of prototypical polythiophenes
View Description Hide DescriptionInelastic neutron scattering experiments are combined with infrared and Raman data to obtain a uniquely defined description of the intramolecular vibrations of three oligomers of polythiophene. Through refinement of ab initio force fields, the three vibrational spectra of each oligomer are simulated with remarkable accuracy. Two different basis sets of atomic orbitals are used: the first, is 6‐31G* and is used to optimize the geometries and calculate the relevant force fields of α‐2T and α‐4T, the second is 3‐21G* and is used for the same purpose for α‐4T and α‐6T. To improve agreement with the experiment, the force fields are scaled. In this way, one set of scaling parameters is generated for the 6‐31G* basis and another for the 3‐21G* basis. The parameters are common to both molecules calculated with either basis sets and are believed to be transferable to higher isomers. The fitting procedure is applied in steps: first, the calculated vibrational frequencies are assigned on the basis of the experimental infrared and Raman activity, then a fitting of the Inelastic Neutron Scattering profile is performed, finally, the infrared and Raman spectra are calculated with the new normal modes and the ab initio derivatives of the dipole moment and the polarizability. The procedure is iterated until the three spectra of each oligomer are satisfactorily reproduced. For α‐4T, two scaled force fields are obtained—one for each basis set—and are shown to yield very similar normal modes. It is important to emphasize that not only the vibrational frequencies but also the spectral intensities are well reproduced by the simulations. Implicitly, this means that the dipole moment and the polarization tensor surfaces calculated ab initio at the potential energy surface minimum are of good quality. The procedure is absolutely general and can be applied to any molecular system. In the present case, it leads to well defined force fields that give us a stringent picture of the vibrations of these molecules.

Photoelectron investigations and density functional calculations of anionic Sb_{ n } ^{−} and Bi_{ n } ^{−} clusters
View Description Hide DescriptionWe recorded photoelectron spectra of antimonySb^{−} _{ N=2–9} and of bismuth clusters Bi^{−} _{ N=2–9} with a photon energy of 4.03 eV, as well as of Bi^{−} _{ N=2–21} with a photon energy of 5.0 eV. The experimentally determined photoelectron thresholds and peak positions of Sb^{−} _{ N=2–5} and Bi^{−} _{ N=2–5} are compared with the results of ab initio density‐functional (LCAO) calculations. The agreement between the experimental thresholds and the calculated adiabatic electron affinities, as well as between the first maxima in the spectra and the calculated vertical detachment energies is fair to good for the antimony clusters and qualitative for the bismuth systems. For the calculation of the ionization (detachment) energies we determined for the neutral and anionic clusters the most stable structures by LCAO calculations. In particular, the tetramer cluster anions have a ‘‘roof’’ structure, while the negatively charged pentamers are planar rings [with similarities to the (C_{5}H_{5})^{−} anion]; positive and negative trimers are nonlinear. Furthermore, the ionization energies and affinities of larger antimony and bismuth clusters are discussed qualitatively and compared to jellium calculations of Seidl and Brack.

Rotational spectrum, internal rotation barrier and ab initio calculations on 1‐chloro‐1‐fluoroethane
View Description Hide DescriptionThe microwave spectrum of 1‐chloro‐1‐fluoroethane (CFC 151‐a) has been studied in the frequency region 8–250 GHz using waveguide Fourier transform, Stark, and source modulation spectrometers. Accurate rotational, quartic, and sextic centrifugal distortion and quadrupole coupling constants have been obtained from a global fit for the ground, v _{17}=1 (Cl–F skeletal bending mode), and v _{18}=1 (CH_{3} torsional) vibrational states of the ^{35}Cl isotopomer and for the ground state of the ^{37}Cl isotopomer. The larger off‐diagonal element of the χ tensor was also determined for the ^{35}Cl isotopomer. Assignment of the v _{17}=1 and v _{18}=1 states was confirmed by the presence of small A–E internal rotation splittings in the v _{18}=1 state, in agreement with ab initio calculations, but in contradiction with previous assignment of the microwave spectrum by Thomas et al. [J. Chem. Phys. 61, 5072 (1974)]. The barrier height for the internal rotation of the methyl group was determined to be 3814(11) cal/mol, and compared with the result of ab initio calculations made for 1‐chloro‐1‐fluoroethane and other related chlorine or fluorine substituted ethanes.

Effects of various halide ions and probe molecules on inelastic Mie scattering from surface enhanced Raman scattering active surfaces: Determination of particle size distributions from band shapes simulation
View Description Hide DescriptionExtensive Raman experiments at very low frequencies arising from the scattering of mechanical vibrations of surface roughness features and colloidal particles (inelastic Mie scattering of localized acoustic vibrations) have been carried out at various laser excitation wavelengths. These low frequency bands have been studied either on electrochemically roughened silver electrodes or in colloidalsilver sols, in the presence of polarizable molecules (pyridine, benzoic acid, acridine) or only with various salts. A simple model has been built and allows to account satisfactorily for the experimental band shapes. From the fit between experimental and calculated curves, we can approach the size distribution for resonant particles. These distributions display a shift of their maximum toward large size particles and a broadening of their widths when the laser excitation wavelength is turned from violet to red. An unexpected intensity enhancement of these acoustic modes, detected when excitation takes place in the red, cannot be explained as originating solely from electric dipolar plasmon resonance. Likewise, surface enhanced Raman scattering(SERS) spectra, molecular mechanisms via charge transfer complexes and/or dipolar magnetic scattering are invoked in an attempt of explanation.

Direct absorption observation of the van der Waals bending band of ArHCN by millimeterwave spectroscopy combined with pulsed‐jet expansion technique
View Description Hide DescriptionMillimeterwave absorption spectroscopy combined with the pulsed‐jet expansion technique was applied to the measurement of rovibrational transitions in the van der Waals band of ArHCN near 200 GHz. Observations were extended to the higher millimeterwave frequency region up to 260 GHz, and 17 rovibrational transitions split into hyperfine components due to the nitrogen nucleus were newly observed for both the Σ_{1}–Σ_{0} and Π_{1}–Σ_{0} bands. An improved set of molecular constants, including the band origins, rotational constants,quadrupole coupling constants, and the Coriolis coupling constant between the Σ_{1} and Π_{1} bending substrates, was determined.

The Auger spectra of CF_{4} in the light of foreign imaging
View Description Hide DescriptionThe fluorine and carbon Auger spectra of CF_{4} are investigated by computing very many dicationic states in the valence region up to 120 eV with the Green’s function method. An analysis of the double hole density in the correlated states of CF_{4} ^{++} proves that pronounced hole localization phenomena at the fluorine atoms take place in almost all the final states of the Auger decay. We discuss how these phenomena are at the origin of the observed fluorine and carbon Auger spectral profiles and, in particular, how they provide a complete and conclusive interpretation of the spectra. The intra‐atomic nature of the Auger process allows us, by a simple convolution of appropriate (localized) one‐site components of the computed two‐hole density distribution, to obtain line shapes which are in close agreement with experiment. To show the general validity of the presented arguments we also compare the results for CF_{4} to the Auger spectra of BF_{3}. The central atom spectrum of these molecules can be understood in the light of the recently introduced foreign imaging picture of Auger spectroscopy.

Photodissociation study on Ca^{+}(H_{2}O)_{ n }, n=1–6: Electron structure and photoinduced dehydrogenation reaction
View Description Hide DescriptionThe photodissociation spectra of size‐selected Ca^{+}(H_{2}O)_{ n } (n=1–6) ions are examined in the wavelength region from 330 to 1440 nm by monitoring the total yield of the fragment ions. The absorption bands exhibit redshifts as large as ∼16 000 cm^{−1} with respect to the ^{2} P–^{2} S resonance line of the free Ca^{+} ion and are explained by the shift of this transition as a result of hydration. The converging trend in the spectral shifts at n∼6 is discussed in relation to the filling of the first solvation shell for Ca^{+} undergoing the sdσ hybridization. We also discuss the possible contribution of charge‐transfer character in the observed transitions in conjunction with our recent results on the photoelectron spectra of Na^{−}(NH_{3})_{ n }. The mass spectra of the fragment ions show the existence of two dissociation channels: The evaporation of water molecules and the dehydrogenation reaction to produce the hydrated CaOH^{+} ions. The evaporation process is the dominant decay channel for n=1, while the reaction is the only one for n≥3. The former process is found to take place from the higher vibrational levels in the ground state being populated through the fast internal conversion induced by the presence of the low‐lying ^{2} D‐type states. As for n=1, the reaction takes place only in an excited state which has the Ca^{+} p orbital aligned on the intermolecular axis. The state‐specific reaction for n=1 is explained in terms of charge‐transfer interaction between the Ca^{+} ion and the water molecule. On the other hand, the reactivity for the larger clusters drastically increases with increasing cluster size. These reaction features are discussed in comparison with those for Mg^{+}(H_{2}O)_{ n } reported previously.

Correlated static‐exchange interaction for electron–molecule scattering: Case study for LiH and H_{2}
View Description Hide DescriptionThe correlated static‐exchange potential for LiH and H_{2} molecules has been studied using many‐body coupled cluster technique. A general trend has been observed. Its importance to the low energy scattering of electrons from these diatomic targets has been pointed out.

Full nine‐dimensional ab initio potential energy surfaces and trajectory studies of A‐band photodissociation dynamics: CH_{3}I^{*}→CH_{3}+I, CH_{3}+I^{*}, and CD_{3}I^{*}→CD_{3}+I, CD_{3}+I^{*}
View Description Hide DescriptionThe full nine‐dimensional potential energy surfaces (PESs) of the ^{3} Q _{0} and ^{1} Q _{1} states of CH_{3}I have been calculated with the ab initio contracted spin–orbit configuration interaction method. The results are fitted to three diabatic potential terms and their couplings as functions of all the internal degrees of freedom. The transition dipole at the Franck–Condon region has also been calculated. Surface hopping quasiclassical trajectory calculations on these potential energy surfaces have been performed to examine the photodissociationdynamics of both CH_{3}I and CD_{3}I in the A‐continuum. The results are in general good agreement with the recent experimental findings. The reasonable I*/(I*+I) branching ratio can be obtained with these PESs when the contribution of direct transition to the ^{1} Q _{1} state is considered. The rotational distribution of the CH_{3} and CD_{3} fragments and its I*/(I*+I)‐channel selectivity are determined by the shape of the PESs with respect to the bending angle outside the conical intersection region. The vibrational distribution of umbrella mode is closely related to the shape of PESs for the umbrella angle; the sudden switch of reaction coordinate from ^{3} Q _{0} to ^{1} Q _{1} at the conical intersection is the origin of vibrational excitation in the I* channel. The larger umbrella excitation of the CD_{3} fragment in both I and I* channels, in comparison with the CH_{3} fragment, is related to the larger separation of the reaction coordinate from the Franck–Condon geometry. The symmetric stretching energy increases during the dissociation, which is related to the shape of PESs with respect to this coordinate, and the excitation of symmetric stretching mode seems to be possible.

Curve crossing problem with dissipation: Uniform rate expression in diabatic representation
View Description Hide DescriptionThe approach based on identification of the quasi‐ballistic collective mode using the variational transition state theory (VTST) is applied to evaluation of the radiationless transition rate in the diabatic representation. The results for the rate match smoothly with the corresponding results derived in the adiabatic representation. This implies that the uniform expression for the rate constitutes a good approximation to the exact result. Analytic expression for the renormalized barrier frequency for Ohmic dissipation in the high barrier limit is derived. The result for the adiabaticity parameter in the strong damping regime reduces to Zusman’s result. The effect of non‐Ohmic dissipation on the electron transfer kinetics in polar solvents is explored. A new procedure for the determination of the quasi‐ballistic mode, based on variation of the total flux, is suggested. The procedure reduces to the VTST procedure in the adiabatic limit. In the nonadiabatic limit the quasi‐ballistic mode coincides with the original reaction coordinate independently of the dissipation strength.

The reaction between N(^{4} S) and C_{2}H_{3}: Rate constant and primary reaction channels
View Description Hide DescriptionThe rate constant and the product branching ratios have been determined at T=298 K for the reaction between ground state atomic nitrogen (^{4} S) and the vinyl radical (C_{2}H_{3}) at a nominal pressure of 1 Torr He. The kinetic technique employed was discharge‐flow coupled to a collision‐free sampling mass spectrometer. The rate constant was determined by monitoring the decay of the vinyl radical in the presence of excess [N], yielding a value for k(N+C_{2}H_{3}) of (7.7±2.9)×10^{−11} cm^{3} molecule^{−1} s^{−1}. Three primary reaction channels have been experimentally observed: N+C_{2}H_{3}→C_{2}H_{2}+NH (1a), C_{2}H_{2}N+H (1b), and C_{2}H_{3}N (1c). The lowest energy isomers of the C_{2}H_{2}N radical and the C_{2}H_{3}N adduct molecule are CH_{2}CN and CH_{3}CN, respectively and their identification as products of the reaction is consistent with experimental results. Contributions from the higher energy isomers CH_{2}NC or cyc‐C_{2}H_{2}N in channel (1b) and CH_{3}NC or H_{2}C=C=NH in channel (1c) are not consistent with the experimental results and can be ruled out. Contribution from other higher energy isomers such as the radicals HC–CH=N, HC=C=NH and H_{2}C=C=N in channel (1b) and the adduct species vinyl nitrene and 2H‐azirine in channel (1c) cannot be ruled out in the absence of knowledge of heats of formation of the radical species and ionizationenergies for both the radical and adduct species. The following branching ratios were determined at T=298 K: Γ_{1a}=0.16, Γ_{1c}=0.04. No other potential products were detected. It can therefore be inferred that channel (1b) accounts for most if not all of the remaining products, i.e., Γ_{1b}=0.80. The magnitude of the rate constant and the nature of the observed products for N+C_{2}H_{3} are compared with those for the reactions N+CH_{3} and N+C_{2}H_{5}. The formation of the adduct molecule is considered in terms of initial formation of vinyl nitrene or 2H‐azirine followed by a series of ring openings, ring closings, and an H atom transfer to yield the lowest energy isomer acetonitrile, CH_{3}CN. The possible role of the N+C_{2}H_{3}reaction in the atmospheric chemistry of Titan, Neptune, and Triton is briefly considered.

Early‐time photodissociation dynamics of chloroiodomethane in the A‐band absorption from resonance Raman intensity analysis
View Description Hide DescriptionWe have obtained resonance Raman spectra and absolute Raman cross sections for h _{2}‐chloroiodomethane (fourteen excitation wavelengths between 200 nm and 355 nm) and d _{2}‐chloroiodomethane (for 282.4 nm excitation) in cyclohexane solution. Most of the intensity in the A‐band resonance Raman spectra appears in the nominal C–I stretch overtones progression and combination bands of the nominal C–I stretch overtones with the fundamentals of the CH_{2} wag, CH_{2} scissor, and the Cl–C–I bend or C–Cl stretch fundamentals. The A‐band absorption and absolute resonance Raman intensities were simulated using a simple model which included preresonant contributions to the fundamental Raman peaks and time‐dependent wave packet calculations. The motion of the wave packet on the excited statesurface was converted from dimensionless normal coordinates into internal coordinates using the results of normal coordinate calculations. The A‐band short‐time photodissociation dynamics of chloroiodomethane shows that the C–I bond lengthens, the I–C–Cl and H–C–I angles become smaller, and the H–C–Cl angles become larger. These internal coordinate motions which are associated with relatively low frequency modes are consistent with a simple impulsive ‘‘soft’’ radical model of the photodissociation and the CH_{2}Cl group changing to a more planar structure. However, the C–H bond length does not change much and the H–C–H angle (associated with higher frequency modes) becomes slightly smaller which is inconsistent with the ‘‘soft’’ radical model and the CH_{2}Cl group changing to a more planar structure. This suggests that an impulsive ‘‘semirigid’’ radical model may be more appropriate than the ‘‘soft’’ radical model to qualitatively describe the chloroiodomethane photodissociation. An ambiguity in the assignment of the 724 cm^{−1} Raman peak and its associated combination bands to combination bands of the nominal C–I stretch overtones with the fundamentals of the Cl–C–I bend or C–Cl stretch fundamentals limits what we are able to determine about the C–Cl bond length changes during the initial stages of the photodissociation.

Combined tight‐binding and density functional molecular dynamics investigation of Si_{12} cluster structure
View Description Hide DescriptionAn extensive search for the lowest energy structure of Si_{12} has been carried out using a combination of simulated annealing studies based on tight‐binding molecular dynamics and density functional based Car–Parrinello calculations. This investigation revealed three families of cluster structures that are low in energy. The potential energy surface in the vicinity of these structures has corrugated landscape, similar to that associated with the conformations of long chain polymers and proteins. The lowest energy structure is a hexacapped trigonal prism, which is a continuation of the growth pattern started at Si_{6}, whereby the faces of a trigonal prism or anti prism seed are terminated by adatoms. This finding reveals emergence of a nucleation pattern in the growth of silicon clusters in the 6–13 atom size range.

The GeOH–HGeO system: Are the 3d electrons core or valence?
View Description Hide DescriptionThe ^{2} A′ ground state of GeOH–HGeO system has been investigated by ab initio electronic structure theory. The equilibrium geometries and physical properties including dipole moments, harmonic vibrational frequencies, and associated infrared (ir) intensities for GeOH, HGeO, and the isomerization (1,2 hydrogen shift) transition state are determined at the self‐consistent‐field (SCF) and configuration interaction with single and double excitations (CISD) levels of theory with four basis sets. There appear to be two minima for the bent HGeO (isomers A and B) on its SCF and CISD potential energy hypersurfaces. At the Hartree–Fock level the structure with HGeO angle near 90° (isomer B) lies lower, but correlated methods show that the structure with HGeO angle near 120° (isomer A) actually lies lower. At the optimized CISD geometries, the single point energies of coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] methods are also determined. In the correlated procedures three different types of frozen core orbital approximation (15 frozen core, 10 frozen core, and 6 frozen core orbitals) have been examined. The energetics based on the first (15 frozen core orbitals) approximation present errors of about 1 kcal/mol compared to more accurate second (10 frozen core orbitals) and third (6 frozen core orbitals) approximations. At the highest level of theory employed in this research, CCSD(T) with triple zeta plus double polarization with diffuse and higher angular momentum functions [TZ2P(f,d)+diff] basis set, the bent GeOH molecule is predicted to be lower in energy than the bent HGeO molecule by 28.5 kcal/mol. This energy separation becomes 25.7 kcal/mol with the zero‐point vibrational energy (ZPVE) correction. The classical energy barrier for the exothermic isomerization reaction [HGeO(B)→GeOH] is determined to be 11.8 kcal/mol and the activation energy (with the ZPVE correction) 10.7 kcal/mol. The theoretically predicted isotope shifts for the GeO stretching vibrational frequency of GeOH agree very well with experimental assignments by Withnall and Andrews [J. Phys. Chem. 94, 2351 (1990)].

Improved radial grids for quadrature in molecular density‐functional calculations
View Description Hide DescriptionA new radial coordinate transformation and associated integration grid scheme is presented for the problem of integrating functions of atomic electron density. Remarkable accuracy and stability is attained. Together with standard atomic partitioning and angular quadrature schemes, the new radial grid is applied to molecular density‐functional theory, and it is shown that acceptable accuracy is attained with significantly fewer grid points than in previously presented schemes.

The first excited singlet state of s‐tetrazine: A theoretical analysis of some outstanding questions
View Description Hide DescriptionThe equation‐of‐motion coupled cluster method for excited electronic states (EOMEE‐CC) is applied to study the structure and selected properties of the first excited singlet state of s‐tetrazine. Adiabatic S _{1}←S _{0}excitation energies obtained with large basis sets containing up to 270 functions are uniformly somewhat above the experimental 0–0 value of 2.238 eV, but nevertheless are the most accurate calculations reported to date for this quantity. The equilibrium geometry of S _{1} predicted in this study is in excellent agreement with another high‐level calculation, and moreover is quantitatively consistent with both the intensity of vibrational progressions observed in absorption and measuredrotational constants for S _{1}. The EOMEE‐CC harmonic force field of S _{1} is the first to satisfactorily describe the low frequency in‐plane b _{3g } bending mode, notably its marked reduction in frequency upon excitation and characteristic positive anharmonicity. Both of these effects are due to a strong second‐order Jahn–Teller interaction between S _{1} and the nominal S _{2}(^{1} A _{ u }) state, which is also investigated superficially in this work. Finally, results presented for the static dipole polarizabilies of the S _{1} state join others in calling for a reinvestigation of the experimentally determined parameters.