Volume 93, Issue 6, 15 September 1990
Index of content:

Conformational instability of the lowest triplet state of the benzene nucleus. I. The unsubstituted molecule
View Description Hide DescriptionExperiments on benzene have established that its lowest triplet state (^{3} B _{1u }) is conformationally unstable owing to vibronic coupling with the next higher state (^{3} E _{1u }). This instability was found to be critically dependent on the influence of a crystal field. An analogous vibronic coupling is to be expected in the singlet manifold, but here no direct evidence is available for a conformational instability. The distortion behavior of benzene is of importance for the interpretation of its photophysical and photochemical properties. We have therefore determined the potential‐energy surfaces of the ^{1,3} B _{1u } and ^{1,3} E _{1u } states along the two‐dimensional distortion coordinate S _{8}(ρ,φ) using a b i n i t i o multireference single and double excitation‐configuration‐interaction calculations. The results show that for both B _{1u } states the hexagonal conformation is unstable and lies 800 cm^{−1} above a wide, virtually cylindrical trough. A calculation of the vibrational spacing in the ^{3} B _{1u } state yields good agreement with the experimentally observed frequency. The calculation of intensities in the absorption and emission spectrum for this state qualitatively agrees with the experiment. An estimate is made of the interaction of the excited molecule with neighboring molecules in a crystal, which indicates that the crystal‐field induced energy variations in the trough should be of the order of 10 cm^{−1}. Combination of our calculations with experimental data shows that the vibronic coupling in the B _{1u } states of benzene should not be looked upon as a static coupling in which the molecule is permanently distorted to one conformation but as a dynamic one in which the molecule makes excursions over the entire potential‐energy surface.

Conformational instability of the lowest triplet state of the benzene nucleus. II. p‐Xylene, the influence of substituents
View Description Hide DescriptionA calculation of the potential‐energy surface of the lowest triplet state of p‐xylene as a function of the S _{8}(ρ,φ) distortion coordinate of the benzene skeleton has been made to learn more about the influence of substituents on the vibronically induced distortion of benzene in its metastable triplet state.The results show the hexagonal conformation of the benzene nucleus in p‐xylene to be unstable with respect to distortions along S _{8}(ρ,φ). In contrast with benzene, for which a flat, virtually cylindrical trough was calculated, the lowest triplet state of p‐xylene shows a preference for quinoidal conformations. The variation in energy with the position of the methyl groups in the quinoidal structure is insignificant within the accuracy of the calculation.

The emission spectrum of helium hydride. II. Bands near 5500 and 6400 Å
View Description Hide DescriptionThe emission bands of helium hydride near 5500 and 6400 Å were analyzed for ^{4}HeH, ^{3}HeD, and ^{4}HeD. They are assigned to the emission of the coupled states D ^{2}Σ^{+}, (3d, L=2) and (for the deuterides) C ^{2}Σ^{+}, v=3 to the A ^{2}Σ^{+} (5500 Å) and B ^{2}Π (6400 Å) states. The 3d, L=2 state is treated as pure Hund’s case (d). The coupling of the electronic states is homogeneous and described by constant matrix elements. Only in the case of ^{4}HeH, strong predissociation of the D state was observed for N’≥3. The emission spectra were observed after neutralization of a fast (15 keV) mass‐selected HeH^{+} beam in potassium vapor.

The emission spectrum of helium hydride. III. Bands near 5200, 5300, and 6000 Å
View Description Hide DescriptionThe emission bands of helium hydride near 5200, 5300, and 6000 Å were analyzed for ^{4}HeH and ^{3}HeD. They are assigned to the emission of nine (HeD) or eight (HeH) coupled electronic states to the A ^{2}Σ^{+} (5200 and 5300 Å) and B ^{2}Π (6000 Å) states. Because of the high rotational temperature of 2500 – 3500 K and several perturbations, a very complex rotational structure was observed. The strongest band near 6000 Å is emitted by the five components of the 3d states, which show strong uncoupling of the 3d electron from the symmetry axis. The D ^{2}Σ^{+} and 3p E ^{2}Π states interact by Λ‐type doubling interaction and with the 3d states since interactions with Δl=±1 are allowed in a strongly polar molecule as HeH. The line intensities are affected by interference effects due to the interactions of the electronic states, by different lifetimes of the upper states and by predissociation, which is only strong for the A’parity component of ^{4}HeH.

Vibrational dynamics of the bifluoride ion. III. F–F (ν_{1}) eigenstates and vibrational intensity calculations
View Description Hide DescriptionThis paper concludes a theoretical study of vibrational dynamics in the bifluoride ion FHF^{−}, which exhibits strongly anharmonic and coupled motions. Two previous papers have described an extended model potential surface for the system, developed a scheme for analysis based on a zero‐order a d i a b a t i c separation of the proton bending and stretching motions (ν_{2},ν_{3}) from the slower F–F symmetric‐stretch motion (ν_{1}), and presented results of accurate calculations of the adiabatic protonic eigenstates. Here the ν_{1} motion has been treated, in adiabatic approximation and also including nonadiabatic couplings in close‐coupled calculations with up to three protonic states (channels). States of the system involving more than one quantum of protonic excitation (e.g., 2ν_{2}, 2ν_{3} σ_{ g } states; 3ν_{2}, ν_{2}+2ν_{3} π_{ u } states; ν_{3}+2ν_{2}, 3ν_{3} σ_{ u } states) exhibit strong mixing at avoided crossings of protonic levels, and these effects are discussed in detail.
Dipole matrix elements and relative intensities for vibrational transitions have been computed with an electronic dipole moment function based on a b i n i t i o calculations for an extended range of geometries. Frequencies, relative IR intensities and other properties of interest are compared with high resolution spectroscopic data for the gas‐phase free ion and with the IR absorption spectra of KHF_{2}(s) and NaHF_{2}(s). Errors in the a b i n i t i opotential surface yield fundamental frequencies ν_{2} and ν_{3} 100–250 cm^{−1} higher than those observed in either the free ion or the crystalline solids, but these differences are consistent and an unambiguous assignment of essentially all transitions in the IR spectrum of KHF_{2} is made. Calculated relative intensities for stretching mode (ν_{3}, σ_{ u } symmetry) transitions agree well with those observed in both KHF_{2} [e.g., bands (ν_{3}+nν_{1}), (ν_{3}+2ν_{2}), (3ν_{3}), etc.] and the free ion (ν_{3},ν_{3}+ν_{1}). Calculated intensities for bending mode (ν_{2}, π_{ u } symmetry) transitions agree well with experiment for the ν_{2} fundamental in the free ion and KHF_{2}(s), and for a π_{ u } transition in KHF_{2} which we assign to ν_{2}+2ν_{3}, but are far too small to explain the prominence of progression bands (ν_{2}+nν_{1}) and especially the strong overtone 3ν_{2} in the spectrum of KHF_{2}(s). Intensity of the progression bands (ν_{2}+nν_{1}) in KHF_{2} can be explained by hydrogen bonding between adjacent FHF^{−} ions; in NaHF_{2}(s) where such interaction is absent, the band (ν_{2}+ν_{1}) is 50–100 times weaker, in agreement with calculations.
The relatively high intensity of the 3ν_{2} band, which also appears strongly in NaHF_{2}(s), remains the major unexplained feature of the bifluoride spectrum in these solids. Suggestions are made for further experiments on the FHF^{−} and FDF^{−} systems which could test predictions of this dynamical analysis.

Optical absorption spectroscopy of sodium clusters as measured by collinear molecular beam photodepletion
View Description Hide DescriptionCollinear molecular beam photodepletion was used to obtain particle specific electronic absorption information for Na_{3}, Na_{4}, and Na_{8} in a wavelength range from 370–835 nm. We critically discuss the experimental method used and the deconvolution procedure applied to the resulting data to yield absolute absorption cross sections. The spectra contain much information on the cluster‐size–dependent transition from molecular to bulk‐like optical response and are interpreted in terms of various computational approaches ranging from classical electrostatic to a b i n i t i o large scale configuration interaction.

Theoretical interpretation of the photoelectron detachment spectra of Na^{−} _{2–5} and of the absorption spectra of Na_{3}, Na_{4}, and Na_{8} clusters
View Description Hide DescriptionThe configuration‐interaction (CI) study of excited states of alkali metal clusters accounts for spectroscopical patterns obtained from (i) the photoelectron detachment spectra of their anions and from (ii) the photodepletion spectra of the neutral species, reproduces observed excitation energies, intensities for allowed transitions, and permits an assignment of cluster structures. For Na^{−} _{2–4} the linear anionic geometries are responsible for the photoelectron detachment spectra. In the case of Na^{−} _{5}, both planar and linear anionic isomers seem to contribute to the recorded spectrum. The calculation of optically allowed states for Na_{3}(C _{2v }) and Na_{4}(D _{2h }) structures and oscillator strengths yield rich spectra which have been fully assigned to the observed ones. In the case of Na_{8}, the T _{ d } and the related D _{2d } forms give rise to an intense transition located at ∼495 nm and the weak fine structure shifted to the red in full agreement with the measured spectrum. A molecular versus collective excitation interpretation of absorption spectra is discussed.

Near‐infrared absorption spectra of ^{16}O_{3} and ^{18}O_{3}: Adiabatic energy of the ^{1} A _{2} state?
View Description Hide DescriptionNew absorption spectra of the ^{1} A _{2}(0,v _{2},1)←^{1} A _{1}(0,0,0) bands of ^{16}O_{3} and ^{18}O_{3} near 1 μ are reported. The behavior of vibronic band isotope shifts for low v _{2} suggests that the lowest point on the ^{1} A _{2}surface lies 9990±70 cm^{−1} above the ^{1} A _{1} minimum. This result is relatively insensitive to the vibrational assignment. Accounting for zero‐point and binding energies of the ground state places the ^{1} A _{2} minimum very close to the O+O_{2}(v=0) dissociation limit, not low enough to support the zero‐point energy of a bound state. Implications regarding recent speculation on the role of this and other electronically excited states in ozonephotochemistry are discussed.

Raman scattering study of LuFeO_{3}
View Description Hide DescriptionThe zone‐center magnon modes of LuFeO_{3} have been studied by first‐order Raman scattering. At room temperature, the two magnon modes (M _{1} and M _{2}) associated with the canted, antiferromagnetically ordered Fe^{3} ^{+} moments are seen at 18.5 and 22 cm^{−} ^{1}, respectively. Their corresponding symmetries, determined through polarization measurements on single crystals, are consistent with the magnetic point group (m’m’m) of the crystal. The distinct scattering observed from phonons is also discussed in the context of the dependence of the phononspectra on the mass of the rare‐earth atom.

Microwave and far infrared spectra, r _{0} structure, barriers to internal rotation, and a b i n i t i o calculations for 2‐fluoropropane
View Description Hide DescriptionThe microwave spectra of five isotopic species of 2‐fluoropropane, (CH_{3})CH_{2}DCFH, (CH_{3})_{2}CFD, (CH_{3})CD_{3}CFH, (CD_{3})_{2}CFD, and (CH_{3})_{2} ^{1} ^{3}CFH, have been recorded from 12.4 to 39.7 GHz. The b‐ and c‐type R‐branch transitions have been observed and assigned for the ground state. Utilizing the rotational constants for these five isotopic species along with those reported earlier for the normal species the following r _{0} structural parameters have been determined: r(C–C) =1.522±0.007 Å, r(C–F)=1.398±0.013 Å, CCC =113.37±0.79°, and CCF=108.19±0.41°. All of the carbon–hydrogen parameters have also been determined from the rotational constants except for the r(C–H_{sec}) which was obtained from its frequency in the infrared spectrum. The far infrared spectra of 2‐fluoropropane‐d _{0}, ‐d _{3} and ‐d _{7} in the gas phase were recorded with a resolution of 0.10 cm^{−1}. Both torsional fundamentals along with several hot transitions were assigned for the three isotopic species. The barrier to internal rotation of the methyl rotors has been determined with the two‐coupled rotor model to be 1149±69 cm^{−1} (3.29±0.12 kcal/mole). Both potential coupling terms, V _{3} _{3} and V ^{’} _{33}, have been determined for the ‐d _{0}, ‐d _{3} and ‐d _{7} isotopic species. The complete equilibrium geometry has been determined from a b i n i t i o Hartree–Fock gradient calculations employing both the 3‐21G and 6‐31G* basis sets. These results are compared to the corresponding quantities for some similar molecules.

Optical impulsive excitation of molecular pseudorotation in Jahn–Teller systems
View Description Hide DescriptionWe analyze the use of vibrationally abrupt nonresonant laser pulses to prepare coherently pseudorotating states in a model Jahn–Teller molecule. Our derivation of impulsive excitation invokes the dynamical adiabatic phase for the perturbed electronic ground state.Polarization selection between two Raman active distortion coordinates allows creation of an orbit of arbitrary eccentricity. Repetition of the pulse pair at the pseudorotational frequency amplifies the nuclear motion. Timing of a resonant pulse of given polarization, or choice of polarization for a given delay, transfers the moving wave packet to either or both Jahn–Teller branches of an electronic excited state.

Analysis of the absorption spectrum of the N V _{1} transition of cyclopentadiene
View Description Hide DescriptionThe ultraviolet absorption spectra of cyclopentadiene and cyclopentadiene‐d _{6} were measured in the N V _{1} transition region. The spectra were analyzed to determine the excited stateproperties. The prominent vibrational progression was identified as a progression in the (predominantly) a _{1} C=C stretching vibration. A second, unidentified, vibrational interval was observed in the cyclopentadiene spectrum and inferred to exist in the spectrum of cyclopentadiene‐d _{6}. The excited statepotential surface was deduced to be displaced 0.2 Å from the lower, predominantly along the C=C normal coordinate. The excited state lifetime was determined to be 37 fs at the origin and decrease at a rate inversely related to the excited state vibrational quantum number.

The vibrational spectra of molecular ions isolated in solid neon. III. N^{+} _{4}
View Description Hide DescriptionWhen a Ne:N_{2}=100 or 200 mixture is codeposited at 5 K with a beam of neon atoms excited by a microwavedischarge, a weak to moderately intense infrared absorption appears at 2237.6 cm^{−1} which is assigned to the N^{+} _{4} molecular ion. The analysis of the infrared spectra of the nitrogen‐15 substituted species of N^{+} _{4} supports the conclusion from earlier a b i n i t i o calculations and electron spin resonance observations that N^{+} _{4} has a linear, centrosymmetric ground‐state structure. For the N^{+} _{4} species with noncentrosymmetric isotopic substitution, the in‐phase end‐atom stretching fundamental becomes infrared active and has also been observed. Although the anion responsible for overall charge neutrality of the deposit has not been definitively identified, secondary photolysis studies provide some information regarding its properties.

Intramolecular and solvent dynamics in femtosecond pump–probe spectroscopy
View Description Hide DescriptionA density‐matrix theory is applied to the calculation of femtosecond pump–probe experiments on solvated polyatomic molecules. Specific calculations are performed for a two‐mode system, in which an optically active solute mode is represented by a pair of harmonic oscillators in the state representation, and the solute is modeled as an overdamped harmonic mode, using the Wigner phase space representation. No restrictions are placed on the relative time scales of the solute vibrational period, the solventrelaxation time scale and the pump pulse duration. The calculations demonstrate the physical phenomena observed in recent experiments on solvated dyes, both in the hole‐burning limit, where the pump pulse is long compared to the relevant solute and solvent time scales, and the impulsive limit, for which the pump is short compared to the solute vibrational period.

Double‐Rydberg anions: Ground‐state electronic and geometric stabilities
View Description Hide DescriptionEach member of the class of Double‐Rydberg (DR) molecular anions consists of an underlying closed‐shell cation core around which a pair of highly correlated electrons move in diffuse orbitals. We have examined the geometric and electronic stabilities of the ground states of candidate DR anions resulting from the following cation cores: H^{+} _{3} , NeH^{+}, FH^{+} _{2} , H_{3}O^{+}, NH^{+} _{4} , and CH^{+} _{5} . Near the equilibrium geometry of the cation, all of the DR anions, except H^{−} _{3} , are electronically stable with respect to the corresponding Rydberg radicals. Results of our geometry optimizations indicate, however, that only NH^{−} _{4} and H_{3}O^{−} are locally geometrically stable; the other DR anions undergo fragmentation. Vertical ionization potentials for the T _{ d } isomer of NH^{−} _{4} and the C _{3v } isomer of H_{3}O^{−} are found to be 0.45 and 0.46 eV, respectively.

Microwave spectra and structures of the NNO–HCN, ^{1} ^{5}NNO–HCN, and NNO–DCN complexes
View Description Hide DescriptionA total of 60 a‐ and b‐dipole rotational transitions were measured in the 4–18 GHz range for the NNO–HCN, ^{1} ^{5}NNO–HCN, and NNO–DCN bimolecular complexes using a pulsed‐beam, Fourier transformmicrowave spectrometer. Spectroscopic constants (A−D _{ K }), B, C, D _{ J }, D _{ J K }, e Q q _{ a a } (N of HCN), and e Q q _{ b b } (N of HCN) were obtained by fitting the observed transition frequencies with a first‐order quadrupole coupling interaction Hamiltonian. The structure of the complex appears to be planar with NNO and NCH nearly parallel. It can be described with the distance R _{cm} between the center‐of‐masses of the monomer subunits, the angle θ between HCN and R _{cm}, and the angle φ between N_{2}O and R _{cm}. A least‐squares fit to the nine rotational constants to obtain the structure parameters R _{cm}, θ, and φ, produced three local minimia for bent structures with standard deviations of <25 MHz. A Kraitchman analysis was used to determine magnitudes of principal axes coordinates for the N of HCN, and the terminal N of NNO. The best nonlinear least‐squares fit result (structure I, lowest standard deviation of the fit =7.2 MHz) produced the best match to the coordinates from the Kraitchman analysis. The spectroscopic constants B, C, and e Q q _{ a a } were used in a second structural analysis to determine values for R _{cm}, θ, and φ. These results were compared with the above coordinates. The best least‐squares fit structure parameters for the vibrationally averaged structure are R _{cm} =3.253(4) Å, θ=89.1(5.4)°, and φ =76.4(0.4)°. Comparisons are made with other similar weakly bound complexes.

Determination of the transition dipole moment μ_{ i→b }(R) in H_{2} from the measurement of vibrational wave functions
View Description Hide DescriptionIn this work we present a theoretical and experimental study of the i ^{3}Π^{−} _{ g }→b ^{3}∑^{+} _{ u }transitiondipole moment in molecular hydrogen. By means of translational spectroscopy the functional dependence on internuclear distance of the transitiondipole moment was directly probed. Selective excitation of the i ^{3}Π^{−} _{ g }, N=1 level, being unaffected by rotational couplings with other 3d singly excited states, allows for straightforward comparison with a b i n i t i o calculations. Excellent agreement is found.

Continuous wave multi‐quantum electron paramagnetic resonance spectroscopy
View Description Hide DescriptionThe equation of Anderson [Phys. Rev. 1 0 2, 151 (1956)] (ω_{2}−ω_{1})^{2}=(γH _{0}−ω_{1})^{2}+γ^{2} H ^{2} _{1}, which describes resonance conditions if relaxation times are long and irradiation at two frequencies is applied to a spin system, has been studied experimentally in the context of continuous waveelectron paramagnetic resonance(EPR)spectroscopy. Here ω_{2} and ω_{1} are the frequencies of two incident microwave levels, one of which is much stronger than the other and is of amplitude H _{1}. γH _{0} is the resonant condition if just one frequency is applied. Magnetization at either ω_{1} or ω_{2} has been observed as a function of sweep of the static magnetic field, sweep of ω_{2} and also sweep of the amplitude H _{1}. Observation of magnetization at frequency ω_{1} corresponding to the strong microwave field H _{1} replicates the rotary saturation experiment of Redfield [Phys. Rev. 9 8, 1787 (1955)]. Multi‐quantum effects are studied with the two frequencies well separated and also when they lie within the width of a single homogeneous line. In addition, data are shown when both microwave amplitudes are similar and the Anderson equation is no longer correct. The thrust of the work is not only to study the spin physics, but also to develop a basis for our development of rotary resonance as an alternative to field modulation in EPRspectroscopy [J. Chem. Soc. Faraday Trans. 1 8 5, 3901 (1989)].

Role of photoacoustics in optogalvanics
View Description Hide DescriptionTime‐resolved laser optogalvanic (LOG) signals have been induced by pulsed laser excitation (ls _{ j }→2p _{ k }, Paschen notation) of a ∼30 MHz radio‐frequency (rf)discharge in neon at ∼5 torr. Dramatic changes of the shape/polarity of certain parts of the LOG signals occur when the rf excitation frequency is scanned over the electrical resonance peak of the plasma and the associated driving/detecting circuits. These effects are attributed to ionization rate changes (i.e., laser‐induced alterations of the plasmaconductivity), with concomitant variations in the plasma resonance characteristics. In addition to ionization rate changes, it is shown that photoacoustic (PA) effects also play a significant role in the generation of the LOG signal. Those parts of the LOG signal that are invariant with respect to the rf frequency are attributed to a PA effect. The similarity of LOG signal shapes from both rf and dc discharges suggests that photoacoustics play a similar role in the LOG effect in dc discharges. Contrary to common belief, most reported LOG signal profiles, ones produced by excitation to levels that do not lie close to the ionization threshold, appear to be totally mediated by the PA effect.

Photodissociation dynamics of formaldehyde: H_{2} (v,J) vector correlations
View Description Hide DescriptionSeveral bipolar moments of the H_{2} (v,J) correlated angular momentum and velocity distribution produced from the photodissociation of formaldehyde near the threshold for dissociation have been measured by analysis of Doppler‐resolved LIF line shapes. It is determined that the fragment H_{2} 〈v⋅J〉 correlation is not at the limit of v⊥J, but is closer to the limit of v⊥J than to v∥J. The rotation of the excited H_{2}CO during the 10^{−7}–10^{−8} s before dissociation does not completely wash out the lab‐frame vector correlations.Anisotropy parameters as large as 0.85 and as small as −0.41 have been measured; these are outside the limits imposed by classical models of parent rotation. A quantum mechanical model for parent rotation is introduced that accounts for the large magnitude of the measuredanisotropy parameters. Photolysis on the ^{ r } R _{0}(0) line of the 4^{3} band produces fragments with β<0 while photolysis on the same rotational transition of the 2^{1}4^{1} band produces fragments with β>0. It is not known if the different anisotropies are caused by differences in the parent transition dipole moment or by differences in the dissociation dynamics. The simple impulsive model that reproduces the fragment rotational distributions and product quantum‐state correlations does not adequately describe the measured H_{2} (v,J) vector correlations.