Volume 86, Issue 10, 15 May 1987
Index of content:

A laser‐induced fluorescence study of bands of the red system of gaseous CoO: Evidence for a ^{4}Δ_{ i } ground state
View Description Hide DescriptionThree bands of the ‘‘red’’ system of gaseous CoO have been rotationally analyzed using laser‐induced fluorescence techniques. The available evidence indicates that the lower levels are the Ω=7/2 and 5/2 spin–orbit components of a ^{4}Δ_{ i } electronic state, which is assigned as the ground state of the molecule. The cobalt nuclear hyperfine splittings are small in the ground state, which suggests that no unpaired electron in a σ molecular orbital derived from the Co 4s atomic orbital is present, so that the electron configuration is presumably σ^{2}δ^{3}π^{2}; the ground statebond length (r _{0}) is 1.631 Å. The upper electronic levels are heavily perturbed, both rotationally and vibrationally, and their hyperfine structures, though following case (a) behavior, are large and irregular. This work on CoO completes the determination of the ground state symmetries and bond lengths for the whole series of the 3dtransition metal monoxides; some comparisons for the members of the series are given.

Extended interactions in the ε phase of oxygen
View Description Hide DescriptionThe high pressure infrared and Raman spectra for the ε phase of a series of oxygen samples, ^{1} ^{6}O_{2}, ^{1} ^{6}O_{2} with 10% ^{1} ^{8}O_{2}, and ^{1} ^{8}O_{2} with 6% ^{1} ^{6}O ^{1} ^{8}O, are reported. Assignments are made for the fundamental, combination, and isotope‐induced features of this phase of solid oxygen based on a simple model that incorporates a pairwise interaction betweeen nearest‐neighbor oxygen molecules within the molecular oxygen plane, with pairs forming extended chains. Vibrational analysis on this chain with two interaction force constants reproduces all of the features that we have observed.

Quantitative analysis of third harmonic generation of XUV radiation in a cell and a cw free jet of carbon monoxide
View Description Hide DescriptionIn the theory, the two‐photon resonant third‐order nonlinear susceptibility of a molecular gas is calculated. The variations of this susceptibility, with the rotational quantum numbers of the resonant states, are given in the case of two singlet states of different symmetries, Σ and Π. In the experiment, third harmonic generation of XUV light (94.5–97.5 nm) is obtained with and without two‐photon resonance. The gaseous nonlinear medium is formed by CO as used either in a cell, or in a continuous supersonic free jet zone of silence. The intensity of the third harmonic wave generated is well related to the laser power, phase‐matching conditions, two‐photon resonances, and rotational temperature, through the nonlinear susceptibility, as described theoretically.

Nonadiabatic effects in multiphoton transitions: A coupled equations study
View Description Hide DescriptionA coupled equations method is used to examine nonadiabatic effects on two‐ and three‐photon transitions in diatomics. Using C1_{2} as a test case, it is shown that nonadiabatic corrections are less important for virtual (nonresonant) transitions than in the case of resonant transitions. The contributions from virtual transitions diminish with the order of the multiphoton transition. Finally, the present method enables one to calculate transition amplitudes. The phase information in such amplitudes are shown to be useful to identify photon pathways in some multiphoton transitions especially at high intensities where the coupled equations approach enable us to go beyond perturbative methods.

EPR characterization of Er^{3} ^{+} and Yb^{3} ^{+} in single crystals of synthetic perovskite (CaTiO_{3})
View Description Hide DescriptionSynthetic perovskite(calciumtitanate, CaTiO_{3}) crystallizes in the orthorhombic structure with space group P c m n (Z=4) and with lattice constantsa=5.367, b=7.622, and c=5.425 Å. Single crystals of CaTiO_{3}doped with either Er^{3} ^{+} (4f ^{1} ^{1} electronic configuration) or Yb^{3} ^{+} (4f ^{1} ^{3} electronic configuration) were grown by means of a flux technique. A mixed KF–LiF flux and Pt crucibles were employed, and the CaTiO_{3} crystals were formed by spontaneous nucleation on slow cooling (1.0 °C/h) between 1250 and 875 °C. EPR spectra were observed for both Er^{3} ^{+} and Yb^{3} ^{+} at a frequency of ∼9 GHz and a temperature of ∼15 K. The EPR results establish that both Er^{3} ^{+} and Yb^{3} ^{+} occupy four magnetically inequivalent sites in the CaTiO_{3} lattice and that these sites become equivalent when the applied magnetic field is oriented parallel to the ‘‘pseudocubic’’ b axis of the host synthetic perovskite crystal. The magnetic axes lying in the plane perpendicular to the b axis were found to make an angle with the a and c faces of the crystal that is equal to 21.0(5)° for Yb^{3} ^{+} and 11.5(5)° for Er^{3} ^{+}. Isotopically enriched dopants were employed as an aid in analyzing the spectra, and the spin‐Hamiltonian parameters have been determined for both rare‐earth impurity ions.

Resonantly enhanced two‐photon spectroscopy of HCl and DCl in the 77 000–87 000 cm^{−} ^{1} region
View Description Hide DescriptionThe two‐photon spectra of HCl and DCl were studied using the technique of resonantly enhanced multiphoton ionization. Rovibrational bands were observed for the X ^{1}Σ^{+}→V ^{1}Σ^{+}, E ^{1}Σ^{+}, F ^{1}Δ_{2}, d ^{3}Π_{1}, f ^{3}Δ_{2}, and g ^{3}Σ^{−} transitions. The results are compared with the single photonspectra of Ginter, Douglas, and co‐workers and with the molecular orbital calculations of Bettendorf e t a l.

Evidence for vibronic coupling contributions to overtone intensities in alkyl phenyl ketones
View Description Hide DescriptionLiquid phase overtone spectra are measured in the spectral regions of CH‐stretching local modes corresponding to Δv _{CH}=2–7 for acetophenone, Δv _{CH}=2–8 for isobutyrophenone, and to Δv _{CH}=2–6 for biacetyl and isobutyrophenone‐d _{5}. The spectra are assigned in terms of the local mode model. The intensities of the aryl CH overtones are quantitatively determined as a function of Δv _{CH}. The intensities of the alkyl CH‐stretching overtones of all four molecules and the intensities of the aryl CD‐stretching overtones of isobutyrophenone‐d _{5} appear to decrease smoothly and exponentially with increasing Δv _{CH} (Δv _{CD}). However, the intensities of the aryl CH–stretching overtones of isobutyrophenone actually increase from Δv _{CH}=6 to Δv _{CH}=7 to Δv _{CH}=8. A similar but less pronounced anomaly is observed for the intensities of the aryl overtones of acetophenone. The results are interpreted in terms of a vibronic coupling contribution to overtone intensity which involves excited electronic states.

Susceptibility and Mössbauer study of an antiferromagnetically coupled admixed‐intermediate spin state in Fe(TPP) (FSbF_{5})⋅C_{6}H_{5}F
View Description Hide DescriptionMagnetic properties of the synthetic hexafluoroantimonate complex of iron (III) tetraphenylporphyrin, Fe(TPP) (FSbF_{5})⋅C_{6}H_{5}F, were studied by susceptibility and Mössbauer techniques. The measured effective magnetic moment varies from 1.7 β at 1.5 K to 4.14 β at 300 K. The data were analyzed with the ‘‘Maltempo model’’ combined with a mean fieldantiferromagnetic treatment. This analysis shows that the system is predominantly in the S=3/2 spin state (98%) with an unusually small spin–orbit coupling constant ζ=79 cm^{−} ^{1}. The net interaction of a given spin with the thermal average of all the spins is (1.39 cm^{−} ^{1})S⋅〈S〉_{ T }. Mössbauer spectra were recorded from 4.2 to 64 K in fields 0 to 6 T. The chemical shift δ=0.39 (Fe) mm/s and quadrupole splitting ΔE=4.29 mm/s are temperature independent and typical of ferric ions in the admixed–intermediate spin state. Mössbauer analysis confirms that the exchange interaction must couple many spins rather than forming spin pairs as seen in earlier work. The analysis also provides the rather low hyperfine coupling constant P/g _{ N }β_{ N }=28 T/unit spin, and contact constant κ=0.25. The spectra imply slow relaxation in fields greater than 4 T at 4.2 K, the rate increasing with increasing T and decreasing H. All Mössbauer and susceptibility data were fitted with a common parameter set; only the relaxation parameter was varied from one spectrum to another.

A correlation between NMR chemical shifts and Mössbauer quadrupole splittings for ^{1} ^{1} ^{9}Sn and ^{1} ^{2} ^{5}Te in organotin and organotellurium compounds
View Description Hide DescriptionA correlation has been observed between solutionNMRchemical shifts δ_{NMR} and Mössbauer quadrupole splittings Δ for ^{1} ^{1} ^{9}Sn and ^{1} ^{2} ^{5}Te in a number of organotin and organotellurium compounds. These correlations are discussed in terms of the effects on δ_{NMR} and Δ of the valence p‐orbital populations and the inverse cube of the radii of the 5pelectrons.

Stark effect of polar and unpolar dye molecules in amorphous hosts, studied via persistent spectral hole burning
View Description Hide DescriptionA simple analytical model is presented describing the shape of a persistent spectral hole in a homogeneous electric field for random orientation of the dye molecules in the host matrix. We consider the two cases that the absorbers have either one fixed value or a distribution of values of the electric dipole moment difference Δμ between the ground and excited electronic states. Comparison with experiment yields the Δμ values for chlorin (Δμ=0.214 D) and a substituted Zn‐tetrabenzoporphin (∼(Δμ) =0.174 D) in poly(vinylbutyral). In the latter case the isolated dye molecule has inversion symmetry, yet there is nevertheless a dipole moment through the interaction with the polar matrix of low local symmetry.

An extended theoretical formulation of the circular dichroism band shape of chromophore aggregates by use of the ensemble‐averaged resolvent matrix method
View Description Hide DescriptionA new theoretical formulation is given to the circular dichroism band shape of chromophore aggregates by means of the ensemble‐averaged resolvent matrix method. This formulation can be used to calculate the circular dichroism band shape of chromophore aggregates having arbitrary strengths of the excited‐state interactions between chromophores. Using the resolvent expansion rule, thermal and static effects upon the circular dichroism band shape are incorporated as the mean‐frequency shifts of the normalized band shape functions used for ensemble averaging the resolvent matrix of chromophore aggregates. This formulation is applied to derive a generalized band shape expansion formula which gives a theoretical justification to the practical first‐order Taylor expansion equation used to calculate the circular dichroism band shape of chromophore aggregates. Basic properties of the generalized band shape expansion formula are investigated by comparing it with the usual perturbation expansion equation for the circular dichroism band shape of chromophore aggregates.

Rotational spectra and structures of the Ar_{2}–H^{3} ^{5}Cl/^{3} ^{7}Cl trimers
View Description Hide DescriptionMicrowave rotational spectra have been observed for both Cl isotopes of the Ar_{2}–HCl trimer with the pulsed nozzle Fourier transform method using the Flygare Mark II spectrometer. The Cl nuclear quadrupolehyperfine structure was analyzed for each of the transitions and the coupling constants and line centers determined. Sixteen transitions were observed in the 2 to 15 GHz region for the ^{3} ^{5}Cl species and 11 for ^{3} ^{7}Cl. The line centers were fitted to obtain ground state rotational and quartic centrifugal distortion constants A‘, B‘, C‘, τ_{1}, τ_{2}, τ_{ a a a a }, τ_{ b b b b }, and τ_{ c c c c }. For Ar_{2}–H^{3} ^{5}Cl, the values are 1733.857, 1667.932, 844.491, −0.1170, −0.0292, −0.1199, −0.0802, and −0.0079 MHz, respectively, and for Ar_{2}–H^{3} ^{7}Cl: 1733.824, 1606.877, 828.497, −0.1121, −0.0279, −0.1205, −0.0737, and −0.0075 MHz. The equilibrium geometry is determined to be T shaped with C _{2v } symmetry and the H end of the HCl closest to the Ar_{2}. Large amplitude slightly anisotropic torsional motion of the HCl is evident from the hyperfine constants. The chlorine isotopic substitution enables the torsional displacement of the Ar_{2} dimer to be estimated. It is found to be an average of 7.3° with respect to the b axis. With allowance for the torsional effects, a value of 3.861 Å is obtained for the Ar–Ar distance and 4.005 Å for the Ar to Cl distance. An approximate, harmonic force field analysis is based on the centrifugal distortion constants. A comparison is given of the force constants and Ar–Ar distances for the Ar_{2}–HCl and Ar_{2}–H/DF trimers.

High‐density amorphous ice. IV. Raman spectrum of the uncoupled O–H and O–D oscillators
View Description Hide DescriptionThe Raman spectrum of the uncoupled O–H and O–D stretching vibrations of HDO dissolved in H_{2}O high‐density amorphous ice, that was made from ice Ih, has been measured at 12 K for the sample as recovered at 77 K and after heating it at 5 K min^{−} ^{1} from 12 K to, successively, 80, 90, 100, ..., 170 K and annealing there for 5 min. The distribution of hydrogen‐bonded O–H‐‐O and O–D‐‐O distances was estimated by using an experimental curve of the O–H stretching frequency as a function of the O‐‐O distance in ice VII, as suitably adjusted using the frequency in ice I. The most probable bond length, as estimated from the peak frequency, is 2.83 Å, which is 0.08 Å longer than the length in ice I, and the distribution of bond lengths has a width at half‐height of 0.19 Å. When the sample was heated to 80, 90, 100, 110, and 120 K in succession, the peak frequency and the width at half‐height, as measured at 12 K, decreased by amounts that corresponded to a decrease of the most probable distance from 2.83 to 2.76 Å and a decrease of the width at half‐height from 0.19 to 0.09 Å. The phase produced at 120 K is a low‐density amorph, and it changes little after heating to 130 and 140 K. After heating to 150 K it changes to ice Ic, whose peak frequency implies an O‐‐O distance of 2.75 Å.

A b i n i t i o synthesis of the ozone ultraviolet continuum
View Description Hide DescriptionPotential energy surfaces for the ground and excited electronic states responsible for the Hartley continuum of ozone are used to obtain quadratic, cubic, and quartic force constants. Vibrational dependence of rotational constants to sixth order is calculated by perturbation theory. The spectroscopic constants enable computation of rovibronic energy levels. Overlap of ground state and excited state perturbed vibrational wave functions yield Franck–Condon factors. Electric dipole allowed rovibronic transitions are generated under the I ^{ r } representation. The entire set of results generate the ultraviolet absorptionspectrum. It is shown that inclusion of anharmonic terms in the vibrational Hamiltonian has a small effect upon the final spectrum, whereas rotational broadening plays a greater role in achieving agreement with experiment.

Direct IR laser absorption spectroscopy of jet‐cooled CO_{2}HF complexes: Analysis of the ν_{1} HF stretch and a surprisingly low frequency ν_{6} intermolecular CO_{2} bend
View Description Hide DescriptionHigh sensitivity, tunable laser direct absorption methods are exploited to obtain high resolution IR spectra (Δν≲0.001 cm^{−} ^{1}) of weakly bound CO_{2}HF complexes in a pulsed supersonic slit jet expansion. Transitions from the ground vibrational state corresponding to a single quantum excitation of the ν_{1} HF stretch are observed and analyzed with a semirigid linear molecule Hamiltonian. The observed increase in both B (+1.75%) and D (+55%) upon ν_{1} excitation is inconsistent with the commonly used diatomic approximation, and is not possible to rationalize for a nearly linear upper state geometry with small amplitude zero point motion of the intermolecular CO_{2} bend coordinate. We consider an alternative centrifugal straightening mechanism which predicts large centrifugal distortion effects due to end over end rotation of a complex with a nonlinear vibrationally averaged geometry in a weak bending potential. In support of this interpretation, hot band spectra are observed arising from bend excited complexes significantly populated in the 16 K expansion; intensity based estimates of the internal excitation indicate a surprisingly low bend frequency of 10±5 cm^{−} ^{1}. A preliminary analysis of the spectra as l doubling in a Π←Π vibrational hot band for a linear equilibrium geometry is presented. An alternative interpretation of the spectrum as asymmetry doubling of a K=1←1 rotational hot band for a bent geometry is also considered. This latter interpretation is more consistent with the data and predicts a CO_{2} bend angle in the complex for K=1 between 25° and 30°. Linewidths for the upper vibrational states in CO_{2}HF exhibit homogeneous broadening 3–4 times in excess of the apparatus resolution. Voigt analysis of the absorption line shapes indicates linewidths (FWHM) of 136±16 MHz and independent of J state; this corresponds to a relaxation lifetime of 1.1±0.2 ns for HF in the complex.

Spectroscopy of reactive intermediates: Electron photodetachment from solvated anions
View Description Hide DescriptionElectron photodetachment spectra of two complex anions, C_{6}H_{5}CH_{2}OHF^{−} and CH_{3}OHOCH^{−} _{3}, have been recorded, using an ion cyclotron resonancespectrometer to generate, store, and detect the ions. The threshold energies determine the potential well depths of the ion–molecule reactions for which the anions are presumable intermediates. The detachment energies for C_{6}H_{5}CH_{2}OHF^{−} and CH_{3}OHOCH^{−} _{3} are ≤3.05±0.06 and 2.26±0.08 eV, respectively. The ability to measure absolute photodetachment cross sections is demonstrated in the latter case. The electron affinity of C_{6}H_{5}CH_{2}O is 2.14±0.02 eV (49.3±0.5 kcal/mol). One would predict that complex ions of the form ROHF^{−} where ROH is less acidic than HF would have poor Franck–Condon factors for adiabatic threshold photodetachment. This prediction is confirmed; five such ions display cross sections smaller than about 10^{−} ^{2} ^{0} cm^{2} at all wavelengths above 365 nm. A useful correlation between ion structure and observation or nonobservation of visible/near‐UV photodetachment is therefore demonstrated.

Dynamic quadrupole echo NMR of deuterium in liquid crystalline solutions: The effect of the chemical shift
View Description Hide DescriptionThe problem of calculating dynamic deuterium quadrupole echo spectra in ordered systems in which exchange takes place between noninteracting nuclei with different quadrupole interactions and chemical shifts is solved. For the solution a specific set of basic spin I=1 matrices is chosen which allows casting the solution in a form having the same dimensionality as the Bloch–McConnell equations of spin I=1/2 nuclei. The theory is applied to two dynamic systems, viz., the ring inversion of cyclohexane and the Cope rearrangement in bullvalene, both in liquid crystalline solution. It is shown that even in ordered systems the time interval between the π/2 pulses in the quadrupole echo experiment may strongly affect the shape and intensity of the spectrum, thus providing an additional experimental variable for analyzing dynamic spectra. When large chemical shifts or off‐resonance pulses are involved, distorted spectra are obtained. This distortion can be removed by including π pulses in the quadrupole echo sequence which eliminate the dephasing effect of the chemical shift.

Rotational spectra of s‐t r a n s and s‐c i s glyoxal‐d _{1} (CHO–CDO) observed by microwave Fourier transform spectroscopy
View Description Hide DescriptionRotational spectra of both conformers of glyoxal‐d _{1} have been investigated in the 7.8–18 GHz region. The asymmetric isotopic labeling induces in the s‐t r a n s conformer a small permanent electric dipole moment. Exploiting the unique properties of a pulsed Fourier transform spectrometer 20 of the extremely weak rotational transitions of s‐t r a n s glyoxal‐d _{1} have been observed and assigned. Three rotational constants and four quartic centrifugal distortion constants have been determined from the frequencies of transitions with J≤22. Nuclear quadrupolehyperfine splittings have been resolved for two low J transitions. Approximate values for the quadrupole coupling constants have been obtained therefrom. In addition, 16 transitions of s‐c i s glyoxal‐d _{1} have been observed and assigned. From the transition frequencies with J≤22 improved rotational constants have been derived along with all quartic centrifugal distortion constants.

Gas phase dynamics and spectroscopy probed with picosecond transient grating experiments
View Description Hide DescriptionPicosecond transient grating experiments in sodium and iodine vapors, involving the 3S→3P and X→B transitions, respectively, are discussed in detail. Population gratings in sodium demonstrate that the technique can be used to measurevelocity distributions in the gas phase. It is shown that the time dependent transient grating signal is related to the Fourier transform of the velocity distribution. Similar experiments on iodine illustrate the effect of state changing collisions on the grating signal. Theoretical calculations for a model in which the change of state is caused by a single collision are given. Close agreement with the data is observed for the situation in which the collision takes the initial velocity into a random velocity distribution. From this model a collision cross section is determined. The results demonstrate that information on collision dynamics can be obtained from grating experiments. In addition, the sodium experiments are used to illustrate a new type of time domain high resolution spectroscopy. When the grating excitation pulses have perpendicular polarizations, a polarizationgrating, rather than the usual population grating, is formed. Diffraction from the sodiumpolarizationgrating shows larger time dependent oscillations in the diffraction efficiency. These oscillations yield the ground state and excited state hyperfine frequencies (1.77 GHz and 189 MHz, respectively). A detailed theoretical description of the origin of the oscillations is presented. The results suggest that polarizationgratingspectroscopy can have applications in other areas, such as molecular rotational dynamics.

A general treatment of environmental effects on molecular vibrations
View Description Hide DescriptionVibrational spectroscopy is increasingly used as a selective and sensitive probe of the interaction between a given molecule and its environment. For electrochemistry, in particular, vibrational spectroscopy has been useful in the process of sorting reactive intermediates and mechanisms out of the many possibilities which present themselves. In this paper we discuss a general approach to the analysis of environmental effects on molecular vibrations. The objective is to use vibrational spectroscopy to determine values of specific parameters in a set of intra‐ and intermolecular pair potentials. In the body of this paper, we discuss in detail how one can manipulate a variety of model potential energy functions in order to account for environmental effects. We then apply the method to the adsorption of tetrahedral molecules on a metal surface. We report infrared spectra for sulfate ion adsorbed on a platinumelectrode, and we make use of our analysis of the spectra to suggest that the sulfate anion associates with the metal through only one oxygen atom.