Index of content:
Volume 88, Issue 2, 15 January 1988

Semiclassical treatment of the vibrational spectroscopy of OCS
View Description Hide DescriptionThe utility of a primitive semiclassical method for the quantitative prediction of vibrational eigenvalues and electric dipole transition intensities in triatomic molecules is assessed for the particular case of rotationless OCS in its ground electronic state by comparison to exact quantum calculations. The semiclassical method is based on numerical integration of appropriately selected classical trajectories. The potential energy function determined by Foord, Smith, and Whiffen [Mol. Phys. 2 9, 1685 (1975)] and the electric dipole moment function determined by Tanaka, Tanaka, and Suzuki [J. Chem. Phys. 8 2, 2835 (1985)] provide the model of the OCS system. Eigenvalues are obtained by the method of adiabatic switching, the number of trajectories required for this purpose being minimized to four using an extension of Johnson’s Fourier series method [J. Chem. Phys. 8 3, 1204 (1985)]. The resulting semiclassical vibrational transition frequencies (with respect to the ground state) agree with the corresponding quantum frequencies to within 1 cm^{−} ^{1} (2 cm^{−} ^{1}) for 112 (128) of the 145 converged quantum levels, with the largest discrepancy being 5.2 cm^{−} ^{1}. The semiclassical frequencies are compared to the experimental results of Fayt [Ann. Soc. Sci. Brux. 8 6, 61 (1972)] and to the semiclassical results of Colwell [Chem. Phys. 4 6, 165 (1980)]. The sets of 176 semiclassical and 145 converged quantum transition frequencies reported here are the most extensive and complete to date, the highest energy level being ∼7500 cm^{−} ^{1} above the ground state. The methodology of Wardlaw, Noid, and Marcus [J. Phys. Chem. 8 8, 536 (1984)] for the determination of semiclassical transition intensities in 2Doscillator systems is herein extended to the vibrational degrees of freedom in triatomic molecules. For numerous transitions from the ground state and from several low‐lying excited states, the semiclassical intensities agree with the quantum intensities to within 6% in the absence of resonances in the associated approximate eigentrajectories. When resonances are involved, the primitive semiclassical treatment is found to be far less accurate, as is expected. A
numerical determination of the classical actions, Fourier spectra of the coordinates, surfaces of section, and 2D slices through configuration space are presented for representative resonant and nonresonant approximate eigentrajectories. The inherent uncertainties in the semiclassical energy levels and in the transition intensities (if no resonant trajectories are involved) are found to provide a very reliable upper bound on the difference between the semiclassical and quantum results.

Rotary echo nutation NMR
View Description Hide DescriptionA two‐dimensional solid state NMR experiment which combines rotary echoes and nutation NMR is investigated and used to study different sodium sites in zeolite NaA. It is shown that with this technique sodium ions with different relaxation rates in the rotating frame can be distinguished.

Rare gas matrix ESR investigations of ^{1} ^{2}CH_{3} ^{6} ^{3} ^{,} ^{6} ^{5}CuF, ^{1} ^{3}CH_{3} ^{6} ^{3} ^{,} ^{6} ^{5}CuF, H^{6} ^{3} ^{,} ^{6} ^{5}CuF, and D^{6} ^{3} ^{,} ^{6} ^{5}CuF generated by reactive laser vaporization
View Description Hide DescriptionA new experimental procedure for conducting reactive laser vaporizations is described and applied to the generation of the previously unreported copper radicals, ^{1} ^{3}CH_{3}CuF, ^{1} ^{2}CH_{3}CuF, HCuF, and DCuF, for rare gas matrix ESR investigations. Laser vaporization was conducted on a thin film of reactant condensed on the coppermetal surface. The g and Atensors for these are compared with other small copper radicals in order to determine electronic structure and bonding trends. The neon magnetic parameters (MHz) for ^{1} ^{3}CH_{3} ^{6} ^{3}CuF are: g _{∥} =1.965(1), g _{⊥} =2.3626(3); A _{∥}(^{6} ^{3}Cu) =3076(2), A _{⊥}(^{6} ^{3}Cu) =2993(1); A _{∥}(^{1} ^{9}F) =94(1), A _{⊥}(^{1} ^{9}F) =68(1); A _{∥}(^{1} ^{3}C) =19(1), A _{⊥}(^{1} ^{3}C) <3. The H quartet hfs for CH_{3}CuF was resolved on the perpendicular transitions, having an A value of 9.3(3) MHz. For H^{6} ^{3}CuF, the neon results (MHz) are: g _{∥} =1.975(1); g _{⊥} =2.4075(4); A _{∥}(^{6} ^{3}Cu) =2826(3), A _{⊥}(^{6} ^{3}Cu) =2717(1); A _{∥}(^{1} ^{9}F) =127(2), A _{⊥}(^{1} ^{9}F) =94(1); A _{∥}(H) =61(3), and A _{⊥}(H) =29(1) MHz.

Inelastic electron tunneling spectra of 1‐ and 2‐naphthoic acids
View Description Hide DescriptionInelastic electron tunneling spectra of 1‐ and 2‐naphthoic acids were measured at 4.2 K with the aluminumelectrode biased negatively and positively with respect to the lead electrode. The naphthoic acid molecules were transferred onto the alumina surface by a vacuum deposition technique. The complete inelastic electron tunneling vibrational spectra are presented and the line positions are listed in the table. We have observed an enhanced noise level in the energy range above 1800 cm^{−} ^{1} in the tunneling spectra of some multiring aromatic dopants. The noise appears in a particularly strong form in the spectra of 1‐ and 2‐naphthoic acids. One of the aims of the present experiments was also to study the character of this noise.

Infrared (2 to 8 μm) fluorescence of the W ^{3}Δ_{ u }→B ^{3}Π_{ g } and w ^{1}Δ_{ u }→a ^{1}Π_{ g } systems of nitrogen
View Description Hide DescriptionEleven transitions in the W ^{3}Δ_{ u }→B ^{3}Π_{ g } (W–B) and w ^{1}Δ_{ u }→a ^{1}Π_{ g } (w–a) systems of nitrogen have been observed in the infrared including the previously unobserved (1,0) and (2,1) W–B features at 6.5 and 7.65 μm, respectively. The fluorescence spectra were observed in a cryogenic reaction chamber at pressures of ∼3 mTorr (0.4 Pa), following expansion of flowing N_{2}/Ar mixtures excited by microwavedischarges at ∼1 Torr. Einstein coefficients for the w–a system, calculated using a published transition moment function, predict the radiative lifetimes of the lower vibrational levels of the w ^{1}Δ_{ u } state to be a factor of 3 longer than earlier estimates. Using a spectral simulation and linear least‐squares fitting technique, the published W–B and calculated w–a branching ratios are verified for the transitions observed across the 2 to 4 μm region. The observed vibrational/electronic state distributions are not characteristic of those expected for direct excitation, but appear to result from extensive collisional coupling among excited states of nitrogen which occurs in the high pressure region prior to expansion.

Microwave spectra, structure, and dipole moment of trifluoromethyl silane
View Description Hide DescriptionThe microwave spectra in the region from 18.5 to 39.0 GHz have been measured and assigned for the following isotopic species of trifluoromethyl silane: CF_{3} ^{2} ^{8}SiH_{3}, CF_{3} ^{2} ^{9}SiH_{3}, CF_{3} ^{3} ^{0}SiH_{3}, CF_{3} ^{2} ^{8}SiD_{3}, CF_{3} ^{2} ^{9}SiD_{3}, CF_{3} ^{3} ^{0}SiD_{3}, CF_{3} ^{2} ^{8}SiDH_{2}, CF_{3} ^{2} ^{9}SiDH_{2}, CF_{3} ^{2} ^{8}SiD_{2}H, and CF_{3} ^{2} ^{9}SiD_{2}H. In addition to the vibrational ground state, two torsionally excited states were assigned for some of the asymmetrically substituted molecules whereas as many as thirteen states involving multiple excitation of the torsional and/or CF_{3} rocking modes were assigned for some of the symmetric rotor species. From a least squares adjustment to fit 18 rotational constants for the vibrational ground state, the following r _{0} structural parameters were obtained: r(C–F) =1.3600±0.0029 Å, r(C–Si)=1.8997±0.0071 Å, r(Si–H)=1.4600±0.0007 Å, r(Si–H) =1.4600±0.0007 Å, CSiH=107.15±0.03°, and FCSi=113.28±0.30°. The r _{ s } structural parameters were obtained for the SiH_{3} moiety and were found to be r(Si–H) =1.4598±0.0033 Å and CSiH=107.17±0.09°. The dipole moment was obtained from the ground stateStark effectmeasurements on CF_{3} ^{2} ^{8}SiDH_{2} and was found to be ‖μ_{ a }‖=2.32±0.02 D. The barrier to internal rotation is estimated from relative intensity measurements to be 403±20 cm^{−} ^{1} (1.15±0.06 kcal/mol). These results are compared to the corresponding parameters of some similar molecules.

Temperature dependence of the CIDEP spectrum of acetone
View Description Hide DescriptionThe CIDEP spectrum of acetone has been studied by means of time‐resolved EPR over a wide range of temperature. It was found that the spectrum varied remarkably depending on temperature indicating that different CIDEP mechanisms are involved at different temperatures. The CIDEP mechanisms operating in each temperature region are determined quantitatively mainly from the simulations of the spectra and their dependence on the diffusion constant of the solvent; from room temperature to −90 °C, the radical pair mechanism involving S–T _{0} mixing (S T _{0} M)is always dominant, but the triplet mechanism and the radical pair mechanism involving S–T _{−} _{1} mixing (S T _{−} _{1} M)become important above and below −45 °C, respectively. The relative importance of S T _{−} _{1} M as compared with S T _{0} M determined below −60 °C is in good agreement with the prediction of the theory. We have also found two interesting phenomena that relatively strong E/A peaks appeared in the center portion of the complete E/A spectrum around −45 °C, and that the line shapes of the peaks were seriously distorted below −70 °C. We examined in detail these observations in terms of the contribution of the radical pair spectrum. The temperature dependence of a hyperfine coupling constant for the hydroxyl proton in the acetone ketyl radical is discussed in terms of spin polarization and hyperconjugation. CIDEP spectra of similar aliphatic carbonyls were investigated and the results support the generality of the conclusion obtained in acetone.

The torsional spectrum of CH_{3}CH_{3}
View Description Hide DescriptionThe perturbation‐allowed torsional spectrum of gaseous CH_{3}CH_{3} has been measured between 225 and 340 cm^{−} ^{1} at a resolution of 0.015 cm^{−} ^{1} using a Fourier transform spectrometer. The absorption path length was 124 m; the gas temperature and pressure were 295 K and 107 Torr, respectively. The P, Q, and R branches have been observed for all four torsional sublevels in the bands v _{4}=1←0 and 2←1. A torsion–rotation Hamiltonian containing seven parameters was used to analyze 204 frequencies assigned in the P and R branches of the two bands. Effective values were obtained for the reduced rotational constantA _{ R }, the two leading coefficients V _{3} and V _{6} in the Fourier expansion of the hindering potential, the Brotational constant, and three distortion parameters. Estimates for the three zeroth order torsional parameters free of higher order contributions are: A _{ R } =2.6711(50) cm^{−} ^{1}, V _{3}=1012.0(1.0) cm^{−} ^{1}, and V _{6}=10.71(0.60) cm^{−} ^{1}. The torsional energies calculated from the current model are compared to those obtained by earlier experiments.

Bound and rotational resonance states and the infrared spectrum of N_{2}Ar
View Description Hide DescriptionRotational and van der Waals vibrational states of N_{2}Ar have been computed in the atom–diatom formalism by an expansion in L ^{2} basis functions. The bound states are characterized by a strong coupling between the modes, which results in an irregular spacing of the bound levels. The lower frequency part of the spectrum (<25 cm^{−} ^{1}) is therefore very complicated and sensitive to the detailed intermolecular potential. It is shown that the regular, free internal rotor pattern, which is observed in the higher frequency part of the gas phase infrared spectrum of N_{2}Ar, is due to rotational resonances lying in the collision continuum. The resonances have a width in the range of 0.2–3.5 cm^{−} ^{1} and especially the narrow ones contribute to the free internal rotor structure in the spectrum.

Radiationless transitions and the quantum yield for nonresonant light scattering
View Description Hide DescriptionThe quantum yield for fluorescence can be defined as the ratio of the amount of light scattered as fluorescence or as Rayleigh or Raman scattering to the amount of light removed from an incident beam. Clearly then the quantum yield should go to one for a nonresonant excitation. The conventional theory of radiationless transitons (the Bixon–Jortner model) does not yield this result. We therefore postulate an intermediate state in the radiationless pathway. Then the model does yield the desired frequency dependence of the quantum yield. It may be that the arbitrary variation of the quantum yields of organic molecules may be due to the absence or presence of such an intermediate state.

Fine and magnetic hyperfine structure in the A ^{2}Π and X ^{2}∑^{+} states of yttrium monoxide
View Description Hide DescriptionA molecular beam optical and rf–optical double resonancespectroscopic study of numerous vibrational components in the A ^{2}Π_{3} _{/} _{2}–X ^{2}∑^{+} subband of gas phase YO has been performed. The fine and magnetic hyperfine parameters for v=0–4 of the X ^{2}∑^{+} state have been determined. The observed strong vibrational dependence of the spin–rotation parameter γ(X ^{2}∑^{+}) is proposed to arise from a perturbation from two as yet unobserved low‐lying ^{2}Π states coming from a three open‐shell configuration. The magnetic hyperfine parameters for the X ^{2}∑^{+} state can be interpreted in terms of a single unpaired electron model. An estimate for the A ^{2}Π excited state dipolar parameter has been established.

Nonlinear electron spin resonance techniques for the study of inhomogeneously broadened spectra
View Description Hide DescriptionThe application of nonlinear multiple irradiationelectron spin resonance techniques to the case of inhomogeneously broadened spectra is studied. A detailed theoretical analysis within a unified method shows that the longitudinally detected electron spin resonance (LODESR) and the double modulation electron spin resonance (DOMESR) techniques represent two different aspects of the same physical effect and, under the same conditions, both give information on the longitudinal relaxation timeT _{1} of the single spin packet. Previous papers, giving different interpretation for the double modulation spectrum, are critically reviewed. The usefulness of the two techniques in the case of inhomogeneously broadened lines is put into evidence by experiments with dextrose chars pyrolyzed at different temperatures. The results are in excellent agreement with theoretical results. The optimum application ranges of these nonlinear techniques are discussed and compared.

Spectroscopy of the 3p ^{2}Π Rydberg state of HCO by resonance‐enhanced multiphoton ionization
View Description Hide DescriptionHigh resolution two‐photon resonant, three‐photon ionization spectra of HCO and DCO are presented for wavelengths from 373 to 460 nm. Rotational analysis of the (K’,K‘)=(0,2) and (0,1) subbands and measurements of spin–rotation splitting on the (0,2) subband provide a definitive assignment of the (070)←(000) band of the bent‐to‐linear 3p ^{2}Π(A‘)←X̃ ^{2}Π(A’) Rydberg absorption in HCO. Bands from nine vibrational states in HCO and ten in DCO, attributable to a (0n0)←(000) bending mode progression in the 3pRydberg state, are observed and assigned. Additional bands in HCO and DCO are assignable to (0n1)←(000) 3p progressions. Values for the 3p ^{2}Π fundamentals ν_{2}=822.1(0.7) cm^{−} ^{1}, ν_{3}=2177(3) cm^{−} ^{1} for HCO and ν_{2}=657(2) cm^{−} ^{1}, ν_{3}=1900(5) cm^{−} ^{1} for DCO were measured. Additional unassigned partial progressions are provisionally attributed to 3s ^{2}∑^{+}←X̃ ^{2}Π(A’) transitions.

Laser fluorescence excitation spectrum of jet‐cooled tropolone: The Ã ^{1} B _{2}–X̃ ^{1} A _{1} system
View Description Hide DescriptionFive of the lowest frequency species a _{1}, b _{1}, and b _{2} vibrational modes of tropolone in the Ã ^{1} B _{2} electronic state are studied using high resolution laser fluorescence excitation spectroscopy of the jet‐cooled sample. The ν^{’} _{26}(b _{1}) mode at 38 cm^{−} ^{1}, most probably a ‘‘folding’’ vibration of the seven‐membered and O⋅⋅⋅HO chelated ring systems of tropolone, is observed in the progression 26^{ v } _{0} with primarily even valued quantum numbers to v _{26}=14. Tunneling doublets in the progression are observed to decrease from 18.90 cm^{−} ^{1} in the vibrationless state to unresolvable values when v ^{’} _{26} is larger than 8, thereby demonstrating highly mode‐specific quenching of the ‘‘H atom’’ tunneling process by these low frequency, out‐of‐plane, ring deformation vibrations. The progression 25^{1} _{0}26^{ v } _{0} is observed to v _{26}=9 and no tunneling doubling is resolved for any of the transitions. These progressions show that the tunnelingenergy level splitting of tropolone in the vibrationless level of the ground electronic state can be at most about 0.3 cm^{−} ^{1}. Additional highly mode‐specific interactions with the tunneling process are observed for the in‐plane ring deformation modes ν^{’} _{13}(a _{1}) and ν_{39}(b _{2}), but the ν^{’} _{14}(a _{1}) mode at 378.24 cm^{−} ^{1}, the lowest frequency a _{1} mode, is found to interact very little with the tunneling coordinate.

Measurement of Debye temperature from the zero‐field splitting of Ni^{2} ^{+} in ZnSiF_{6}⋅6H_{2}O
View Description Hide DescriptionThe variation of the zero‐field splitting parameter D for Ni^{2} ^{+} in ZnSiF_{6}⋅6H_{2}O between 4 and 250 K has been used to calculate the temperature dependence of the Debye temperature θ_{ D }(T) in ZnSiF_{6}⋅6H_{2}O. θ_{ D }(T) was determined to be 143 K at a temperature of 30 K and 127 K at 250 K. There was a pronounced minimum of 105 K in θ_{ D }(T) near 60 K. The behavior of θ_{ D }(T) is qualitatively similar to that found for alkali halides from specific heat data.

Ionic rotational branching ratios in resonant enhanced multiphoton ionization of NO via the A ^{2}Σ^{+}(3sσ) and D ^{2}Σ^{+}(3pσ) states
View Description Hide DescriptionWe present the results of a b i n i t i o calculations of the ionic rotational branching ratios in NO for a (1+1) REMPI (resonant enhanced multiphoton ionization) via the A ^{2}Σ^{+}(3sσ) state and a (2+1) REMPI via the D ^{2}Σ^{+}(3pσ) state. Despite the atomic‐like character of the bound 3sσ and 3pσ orbitals in these resonant states, the photoelectron continuum exhibits strong l mixing. The selection rule ΔN+l=odd (ΔN≡N _{+}−N _{ i }) implies that the peaks in the photoelectron spectrum corresponding to ΔN=odd (±1,±3) are sensitive to even partial waves while those corresponding to even ΔN probe the odd partial waves in the photoelectron continuum. Recent experimental high resolution photoelectron studies have shown a strong ΔN=0 peak for ionization via the A ^{2}Σ^{+} and the D ^{2}Σ^{+} states, indicating a dominance of odd‐l partial waves. While this seems natural for ionization out of the 3sσ orbital, it is quite anomalous for 3pσ ionization. Based on extensive bound calculations, Viswanathan e t a l. [J. Phys. Chem. 9 0, 5078 (1986)] attribute this anomaly to a strong l mixing in the electronic continuum caused by the nonspherical molecular potential. We have performed a b i n i t i o calculations of the rotational branching ratios and compared them with the experimental results. The electronic continuum shows a significant p‐wave component which leads to the large ΔN=0 peak in both cases. Calculations are performed for both rotationally ‘‘clean’’ and ‘‘mixed’’ branches. The relative heights of the peaks are very sensitive to the photoelectron kinetic energy for the D ^{2}Σ^{+} state and less so for the A ^{2}Σ^{+} state. This is a direct consequence of the l mixing in the continuum.

Photoelectron spectroscopy of the nitrogen dimer (N_{2})_{2} and clusters (N_{2})_{ n }: N_{2} dimer revealed as the chromophore in photoionization of condensed nitrogen
View Description Hide DescriptionThe He iphotoelectron spectra of gas‐phase nitrogen dimer and nitrogen clusters have been measured in a pulsed cluster beam. The dimer (N_{2})_{2} is characterized by broad bands with vertical ionization energies which are 0.3±0.1 eV lower than for N_{2}monomer. The bands observed for a mixture of small clusters, estimated to be of average size N̄=10, are identical to the dimer bands except for further shifts of 0.3 eV to lower ionization energies. The clusters bandwidths and band shapes are virtually the same as measured for thin films of condensed N_{2}, indicating that the nitrogen dimer (N_{2})_{2} is the ionization chromophore in each case. This offers support for Haberland’s hypothesis that ionization of any M_{ n } cluster produces the ion M^{+} _{2}M_{ n−2} provided M is a closed‐shell atom or molecule. The theory of electronic relaxationpolarization of the dielectric medium, which explains the gas‐to‐solid ionization energy shifts, is modified for the case of finite clusters and to account for dimer ion formation.

Analysis of fluorescence line shapes and Raman excitation profiles of tetradesmethyl‐β‐carotene
View Description Hide DescriptionThe fluorescence lineshapes and Raman excitation profiles of tetradesmethyl‐β‐carotene in isopentane at 190 and 230 K are analyzed and related to the solvation dynamics and vibrational relaxation. We find a solvation time scale of 125 fs at 190 K, 111 fs at 230 K, and a vibrational relaxation time scale of 253 fs at both temperatures. The magnitude of the fluctuations of the electronic energy gap due to interactions with the solvent and with molecular low frequency modes is 423 cm^{−} ^{1} at 190 K and 480 cm^{−} ^{1} at 230 K.

Infrared spectroscopy of carbo‐ions. III. ν_{3} band of methyl cation CH^{+} _{3}
View Description Hide DescriptionThe infrared spectrum of the degenerate ν_{3} vibration–rotation band of methyl cation CH^{+} _{3}, one of the most fundamental carbo‐ions, has been observed and analyzed. The spectrum was observed in the frequency range of 3240–2960 cm^{−} ^{1} using the difference frequency laser system as the tunable coherent infrared source. A gas mixture of He:H_{2}:CH_{4}∼700:20:1 with a total pressure of ∼7 Torr was used for the ac discharge in an air‐cooled and a water‐cooled multiple inlet–outlet discharge tube. The velocity modulation method together with noise subtraction and the unidirectional multiple passing of the beam was used for high sensitivity. The spectral lines of CH^{+} _{3} appeared amid many other lines of C_{2}H^{+} _{2} and C_{2}H^{+} _{3} but their widths and their response to chemical conditions gave a good initial clue for the identification. Over 200 absorption lines have been assigned to CH^{+} _{3} and the isotopic species ^{1} ^{3}CH^{+} _{3}. The spectral pattern clearly shows that the equilibrium molecular structure is a planar, equilateral triangle as expected from Herzberg’s Rydbergspectrum of CH_{3} and from a b i n i t i o theory. The spectrum has been analyzed by using the symmetric rotor Hamiltonian in a degenerate vibrational state with various A _{1}–A _{2} splittings and l resonance. The spectral lines have been least‐squares fit with a standard deviation of 0.0058 cm^{−} ^{1}, and led to 21 vibration–rotation constants of CH^{+} _{3}. Several analytical expressions have been derived among these constants to determine the structure and potential constants. Some perturbation has been noticed and discussed. Also quite a few lines which are definitely due to carbocations with one carbon atom have been left unassigned. The prominence of the CH^{+} _{3}spectrum in hydrocarbon discharges confirms the importance of this species in the ion–molecule reaction scheme in the laboratory and in space which has already been noted by many people in the field. A simple discussion is given about the formation and destruction of CH^{+} _{3} as well as other carbocations.

Photochemical hole burning of phthalocyanine in polymer glasses: Thermal cycling and spectral diffusion
View Description Hide DescriptionIn this paper we report an analysis of hole burningline shapes yielding both reversible and irreversible contributions to the linewidth which are due to spectral diffusion. These terms show a linear T dependence and can, through cycling experiments, be investigated in the temperature range of 0.5<T<25 K. The range 1<T<25 K is particularly interesting because, under normal conditions, Debye contributions and local mode contributions overwhelm the small spectral diffusion terms at these temperatures. Comparison between optical data and specific heat data supports our model of spectral diffusion.