Volume 91, Issue 10, 15 November 1989
Index of content:

Comparison between the density effects on the levels of the Raman spectra of the Fermi resonance doublet of the ^{1} ^{2}C^{1} ^{6}O_{2} and ^{1} ^{3}C^{1} ^{6}O_{2} molecules
View Description Hide DescriptionIsotropic Raman studies of density effects on the lower Fermi doublet ν_{1}, 2ν_{2} of the ^{1} ^{2}C^{1} ^{6}O_{2} and ^{1} ^{3}C^{1} ^{6}O_{2} compressed molecules have been made in the gaseous and liquid phases along the saturation line up to the triple point temperature. After a brief bibliography review of the unperturbed frequency ordering for the ^{1} ^{2}C^{1} ^{6}O_{2} case, the problem of the determination of unperturbed levels from our experimental data is treated for both isotopic species. Comparison between ^{1} ^{2}C and the ^{1} ^{3}C frequency shifts due to the density effect leads to the conclusion that the order of the ν_{1}, 2ν_{2} levels is reversed in the liquid and gaseous phase of ^{1} ^{2}C^{1} ^{6}O_{2} with an exact Fermi resonance ν_{1}=2ν_{2} in the high‐density fluid phase. For the first time, bandwidths of the two components of the Fermi dyad are measured in the liquid.

Infrared spectra of ^{4}HeH^{+}, ^{4}HeD^{+}, ^{3}HeH^{+}, and ^{3}HeD^{+}
View Description Hide DescriptionIsotopic species of the HeH^{+} molecular ion provide an excellent testing ground for studying isotopic dependence of vibration–rotation constants because of the small masses of He and H isotopes. We have observed infrared spectra of the hot band v=2←1 of HeH^{+} and fundamental bands of isotopic species HeD^{+}, ^{3}HeH^{+}, and ^{3}HeD^{+}, and obtained the Dunham coefficients Y _{ k l }, and the isotopically independent parameters U _{ k l }, Δ^{He} _{ k l }, and Δ^{H} _{ k l }.

The microwave spectrum and structure of the H_{2}O–SO_{2} complex
View Description Hide DescriptionThe microwave spectrum of H_{2}O–SO_{2} has been observed with a pulsed beam, Fabry–Perot cavity, Fourier‐transform microwave spectrometer. In addition to the normal isotopic form, we have observed the spectra of H_{2}O–^{3} ^{4}SO_{2}, HDO–SO_{2}, and D_{2}O–SO_{2}. For the normal and dideuterated forms we observe two states with a‐ and c‐type spectra which are split by internal rotation of the water unit, while for the ^{34} S and HDO species two states are observed but only the a‐type spectrum was assigned. Rotational analysis of each spectrum for H_{2}O–SO_{2} provides the constants A=8763.071(3) MHz, B=3819.655(2) MHz, and C=2900.899(2) MHz for the I=0 state and A=8734.268(4) MHz, B=3821.281(2) MHz, and C=2900.837(2) MHz for the I=l state, where I is the resultant protonnuclear spin.Stark effectmeasurements give electric dipole components μ_{ a } =1.984(2) D and μ_{ c } =0.488(4) D for H_{2}O–SO_{2}. The geometry obtained from fitting the derived moments of inertia has the planes of the two monomer units tilted approximately 45° from the parallel orientation with the oxygen atom of the water closest to the S atom of SO_{2}, giving an S–O distance of 2.824(16) Å and a center‐of‐mass distance R _{c.m.} =2.962(5) Å. The (SO_{2})_{2} species was also produced with the same nozzle expansion conditions as used for H_{2}O–SO_{2}. New measurements on (SO_{2})_{2} are reported and fitted with the measurements from Nelson, Fraser and Klemperer [J. Chem. Phys. 8 3, 945 (1985)], providing improved rotational and centrifugal distortion constants.

Dynamics of radiation damping in nuclear magnetic resonance
View Description Hide DescriptionThe time evolution of spins‐1/2 subject to radiation damping, which is commonly encountered for solvent peaks at high field, is examined in detail. The well‐known analytic results for rectangular pulses on undamped spin‐1/2 systems are extended to the radiation damped case, and reveal surprisingly complex dynamics. Explanations in terms of Bloch vectors are also presented, and composite pulse sequences which would also be insensitive to radiation damping are proposed. In addition, gradient optimization programs were developed to find shaped π and π/2 pulses insensitive to radiation damping. The optimized pulses compensate for radiation damping effects even when the characteristic damping time is shorter than the pulse length.

Ab initio determination of mode coupling in HSSH: The torsional splitting in the first excited S–S stretching state
View Description Hide DescriptionA mechanism for the enhanced splitting detected in the millimeter‐wave rotational spectra of the first excited S–S stretching state of HSSH (disulfane) has been studied. The mechanism, which involves a potential coupling between the first excited S–S stretching state and excited torsional states, has been investigated in part by the use of a b i n i t i o theory. Based on an a b i n i t i opotential surface, coupling matrix elements have been calculated, and the amount of splitting has then been estimated by second‐order perturbation theory. The result, while not in quantitative agreement with the measured splitting, lends plausibility to the assumed mechanism.

Vibrational spectra of Mg(OH)_{2} and Ca(OH)_{2} under pressure
View Description Hide DescriptionThe effects of compression on the structure and bonding of Mg(OH)_{2} brucite and Ca(OH)_{2} portlandite are documented to pressures of 34 and 24 GPa, respectively, by way of infrared spectroscopy of the O–H stretching vibration at 300 K. The frequency of the infrared‐active (A _{2u }) mode of both hydroxides decreases with pressure, by −0.6 cm^{−} ^{1}/GPa for Mg(OH)_{2} over the pressure range studied, and by −3.5 cm^{−} ^{1}/GPa for Ca(OH)_{2} to 10.6 GPa. An additional mode observed in Mg(OH)_{2} is due to an optically induced hot band (A _{2u } X A _{2u } overtone): with increasing pressure, its intensity grows and its frequency decreases (−3.7 cm^{−} ^{1}/GPa). The negative pressure dependencies of these modes (negative Grüneisen parameters) are explained in terms of compression increasing the hydrogen bond strength within the hydroxide structure. On increasing pressure above 12.8 GPa, the full width at half‐maximum of the O–H vibration in Ca(OH)_{2} rapidly broadens by a factor of 4. The infrared data suggest that Ca(OH)_{2} undergoes pressure‐induced amorphization at 11.7(±1.1) GPa at 300 K, consistent with independently collected x‐ray diffraction patterns. In the amorphous phase, a distribution of O–O bond lengths ranging between 2.8 and 3.3 Å is inferred from the breadth of the vibrational band. On decompression, the infrared spectra show a full return to the crystalline form at 2.8 (±1.4) GPa. Neither upon amorphization with increasing pressure nor upon recrystallization with decreasing pressure are there any prescursory phenomena evident in the spectra. In contrast with Ca(OH)_{2}, the isomorphous Mg(OH)_{2} appears to be stable throughout the pressure range of this study.

Line broadening, line shifting, and line coupling effects on N_{2}–H_{2}O stimulated Raman spectra
View Description Hide DescriptionIn order to understand the influence of H_{2}O on the stimulated Raman Q‐branch spectra of nitrogen in combusting media, an exhaustive theoretical and experimental study has been carried out. Starting from a semiclassical model, particularly convenient at high temperature, the Q‐line broadening and shifting coefficients have been calculated over a wide temperature range and for a large number of lines. Stimulated Raman Spectra(SRS) measurements have allowed us to test these calculated line broadening coefficients and thus establish the high accuracy of semiclassical values. The theoretical broadening coefficients have been inverted to deduce state‐to‐state rotational relaxation rates by using two types of fitting laws. A partial test of the resulting Q‐branch profiles has been realized at moderate pressures leading to a discrimination between these two laws. Furthermore, the effect of rotational energy transfers on collisionally narrowed profiles at higher densities has been simulated and compared with the pure N_{2} case.

Theoretical analysis of the vibrational structure of the T←N transition in ethylene
View Description Hide DescriptionA theoretical analysis is reported of the ground state‐to‐triplet absorptionspectrum of ethylene, observed as an electron‐loss spectrum. The analysis is based on a b i n i t i o MO calculations of the vibrational force fields in the ground state and in the two stationary configurations (planar and staggered) of the triplet state. The calculations use the gamess program with the 6‐31++G** basis set. The transition turns the double bond into a single bond and thus increases the CC distance. This affects modes ν_{2} and ν_{3} , corresponding to mixed CC stretching and CH scissoring, and leads to a long ν_{3} progression together with some ν_{2} structure. The transition also excites torsional levels (mode ν_{4} ), preferentially those just above and below the torsional barrier. This relatively narrow torsional spectrum is superimposed on each of the CC‐stretch peaks. The systematic change in this structure observed along the ν_{3} progression is due to anharmonic ν_{3} ν_{4} coupling. Because of this coupling, a meaningful assignment of ν_{4} quantum numbers to the spectrum is not possible. The significance of this interpretation for the enigmatic V←Nabsorptionspectrum is briefly indicated.

Microwave spectrum, quadrupole coupling constants, and structure of N–bromodifluoromethanimine
View Description Hide DescriptionA continuous flow system has been used to measure the microwave spectrum (26.5 to 40.0 GHz) of rapidly decomposing N–bromodifluoromethanimine, CF_{2}=NBr. The hyperfine structure of the spectrum caused by two nuclei with nuclear quadrupole moments (^{1} ^{4}N,^{7} ^{9}Br, or ^{8} ^{1}Br) has been analyzed and rotational, centrifugal distortion, and quadrupole coupling constants have been determined by a global least‐squares fit for both bromine isotopic species. Identification and assignment of R transitions in the b‐type spectrum have been facilitated considerably by the determination of the magnitude of the rotational constants from the hyperfine structure of the Q transitions alone. A structure compatible with the observed rotational constants has been determined for this planar molecule.

Mass spectrometric evidence for icosahedral structure in large rare gas clusters: Ar, Kr, Xe
View Description Hide DescriptionClusters of argon, krypton, and xenon are grown in a free jet and ionized by electron impact. The size of these clusters, (Rg)^{+} _{ n }, extends up to n≂1000. Individual cluster sizes are mass resolved up to n≂570 in the case of Ar^{+} _{ n }. The well known, but puzzling differences in the size distributions of Kr and Xe clusters disappear beyond n≂130, while those between Ar and Xe disappear beyond n≂220. The most pronounced ‘‘magic numbers’’ in the distributions of large cluster ions occur at n=147 (148 for Ar), 309, and 561, in striking agreement with the number of atoms required to build icosahedral clusters with 3, 4, and 5 complete coordination shells, respectively. Closure of the 6th icosahedral coordination shell is indicated by another strong intensity drop at n≂923 in the unresolved part of the spectra. Several additional intensity extrema are observed between major shell closures. A simple structural model, assuming an icosahedral core decorated by the additional atoms, accounts for these anomalies reasonably well up to n=561.

Theoretical study of the dipole moment function of OH(X ^{2}Π)
View Description Hide DescriptionWe present a theoretical study of the sensitivity of the dipole moment function (DMF) of the X ^{2}Π ground state of OH to basis set saturation and to refinements in the correlation treatment. Emphasis is placed on determining the slope of the DMF at r _{ e } and the r value at which the maximum occurs. We consider the effect of oxygen polarization functions up through h type, expansion of the active orbital space to include the O 3dδ orbital, the effect of higher excitations using the averaged coupled‐pair functional method, and the effect of evaluating the dipole moment as an energy derivative rather than as an expectation value. Our theoretical DMFs, which should be the most accurate to date, differ markedly from an empirical DMF of Turnbull and Lowe that is based on experimentally derived intensity ratios. The theoretical DMFs agree better with a recently published DMF of Nelson e t a l., but suggest that this empirical DMF is also inaccurate for r>2.3 a _{0}.

Theory of resonance secondary emission in femtosecond laser excitation: On the connection with wave packet dynamics
View Description Hide DescriptionThe resonance secondary emission (RSE) in femtosecond laser excitation is discussed in reference to the motion of the created wave packet moving on the excited statepotential surface. The density matrix of emitted light for the multi‐intermediate‐level system is outlined, from which the emission correlation function is derived. The correlation function is put into the theoretical expression of the time‐dependent ‘‘physical spectrum’’ for the Fabry–Perot interferometer (which is used in order to consider temporal and energetic resolution inherent in detection). The compact and practical expressions obtained connect the time‐ and frequency‐resolved spectrum with the time evolution of the wave packet. Numerical results for a displaced harmonic oscillator model indicate that the time‐ and frequency‐resolved spectrum can reveal how the wave packet created by a fs laser pulse travels on the excited potential surface if the response time 1/Γ_{ d } of the photodetector satisfies the relation that Ω<Γ_{ d } <∼ the Stokes shift (where Ω is the vibrational frequency). It is shown that the excited state wave function can be split into two terms, the one that adiabatically follows the temporal change in incident light (the adiabatic term) and the one that represents the effect of spectral broadening of light (the Fourier broadening term). It is only the Fourier broadening term that survives after the termination of incident light and reflects the motion of the created wave packet on the excited potential surface. In off‐resonance excitation, the adiabatic term produces Raman‐like emission and the Fourier broadening one produces fluorescence‐like emission. In resonance excitation, these two terms are indistinguishable from each other with respect to emission frequency: for the duration of incident light, the adiabatic term offsets the Fourier broadening one, leading to a slow buildup of intensity in the time‐ and frequency‐resolved spectrum (which is slower than the initial rise of the incident pulse profile).

A study of the singlet and triplet states of vinylidene by photoelectron spectroscopy of H_{2}C=C^{−}, D_{2}C=C^{−}, and HDC=C^{−}. Vinylidene–acetylene isomerization
View Description Hide DescriptionThe X̃ ^{1} A _{1}, ã ^{3} B _{2}, and b̃ ^{3} A _{2} states of vinylidene are observed in the ultraviolet (351.1–364.0 nm) photoelectron spectra of X̃ ^{2} B _{2} H_{2}CC^{−}, X̃ ^{2} B _{2} D_{2}CC^{−}, and X̃ ^{2} A’ HDCC^{−}. The X̃ ^{1} A _{1} state exhibits vibrational structure well above the barrier for isomerization to acetylene. A strict lower bound to the lifetime of the singlet state against rearrangement is τ>0.027 ps, with an estimate of τ≊0.04–0.2 ps based on a simulation of the line shapes including rotational broadening. A vibrational analysis of the singlet and lower triplet state bands provides vibrational frequencies and estimates of the changes of molecular geometries between the anion and the neutral species. A qualitative potential energy surface for the CH_{2} rock mode, which closely corresponds to the reaction coordinate for isomerization, is extracted from the experimental data. The adiabatic electron affinity is EA(X̃ ^{1} A _{1} H_{2}CC)=0.490±0.006 eV and the triplet term energies are T _{0}(ã ^{3} B _{2} H_{2}CC)=2.065±0.006 eV and T _{0}(b̃ ^{3} A _{2} H_{2}CC)=2.754±0.020 eV. Experimental values for the bond dissociation energy of vinyl radical, D _{0}(H_{2}CC–H)=80.0±5.0 kcal/mol, and the acetylene–vinylidene isomerization energy, ΔH _{ I }=46.4±5.5 kcal/mol, are derived. Combining the latter value with the upper limit of Field and co‐workers, ΔH _{ I }≤44.1–44.7 kcal/mol, yields ΔH _{ I }≊41–45 kcal/mol.

Nonradiative Rydberg↔valence relaxation of NO trapped in Ar, Kr, and Xe matrices
View Description Hide DescriptionElectronic and vibrational relaxation of NO in Ar, Kr and Xe matrices has been studied using excitation spectra and time and energy resolved emission spectra. In addition to the vibrationally relaxed emissions from a ^{4}Π(v=0), B ^{2}Π(v=0) and A ^{2}Σ^{+}(v=0) in Ar and Kr matrices, B(v=5 and 7) emissions are observed in Ar matrices. In Xe matrices, only RydbergA ^{2}Σ^{+}(v=0) fluorescence is observed. Nonradiative Rydberg–valence transitions are observed in all matrices and valence–Rydberg transitions only in Ar matrices. The intensity ratios I _{ A }/I _{ B }/I _{ a } are ∼3/9/88 in Ar, ∼9/3/88 in Kr, and ∼2–5/0/0 in Xe matrices. The quantum efficiency for total luminescence being near unity in Ar and Kr matrices. The branching ratios for intramolecular relaxation between Rydberg and valence states are described in terms of a model which combines the intramolecular Franck–Condon factors with the spectroscopically determined phonon Franck–Condon factors and solvent enhanced spin–orbit matrix elements. The latter increase from Ar to Xe matrices. This increase is rationalized in terms of a semiempirical model for the heavy‐atom effect on spin–orbit mixing. Weak Rydberg–Rydberg relaxation by a Δv=2 step in Ar and a Δv=1 step in Kr is also observed and interpreted in terms of a resonant Förster–Dexter‐type energy transfer. Finally in Xenon matrices, a strongly nonresonant energy transfer from the n=l exciton of solid xenon to the A(v=0) Rydberg level is observed.

State‐selective ionization of nitrogen in the X ^{2}Σ^{+} _{ g } v _{+}=0 and v _{+}=1 states by two‐color (1+ 1) photon excitation near threshold
View Description Hide DescriptionEfficient generation of N^{+} _{2} X ^{2}Σ^{+} _{ g }(v _{+}=0,1) is demonstrated by an extreme‐ultraviolet +visible (1+1) photon excitation scheme. The c ^{’} _{4} ^{1}Σ^{+} _{ u } (v’=0,1) Rydberg states are used as intermediates. The N_{2} molecules are ionized near the v _{+}=0 and v _{+}=1 ionization thresholds. The autoionizing n dδ_{ g } ^{1}Δ_{ g }Rydberg series with the rotational quantum numbers N=4, 5, and 6 (in the limit of l uncoupling) are observed for the first time. Extrapolation of the N=4 series yields an ionization energy of N_{2} of 125 666.959(67) cm^{−} ^{1}.

The S _{1}–S _{0} transition of indole and N‐deuterated indole: Spectroscopy and picosecond dynamics in the excited state
View Description Hide DescriptionA vibrational analysis for the excitation and fluorescence spectra of the S _{1}–S _{0} transition in jet‐cooled indole and N‐deuterated indole is presented. This analysis yields the frequencies for eight low‐lying S _{1} vibrational modes of both isotopic species. Single vibronic level fluorescence spectra also enable us to refine many of the S _{0} vibrational frequencies. Quantum interferenceeffects in the energy‐resolved fluorescence decays for one vibrational level of S _{1} indole are observed as a result of picosecond laser excitation. Through analysis of the fluorescence spectra and the modulated fluorescence decays, the coupling responsible for the quantum beats was found to result from a zero‐order in‐plane mode coupled to a level which is described by several zero‐order out‐of‐plane modes. The effect of the deuterium substitution on the quantum beats is also investigated.

Vacuum ultraviolet spectroscopy of the Cl_{2} molecule trapped in pure neon, pure argon, or mixed neon–argon matrices
View Description Hide DescriptionSynchrotron radiation excitation and emission spectra with lifetime measurements are reported for the first time in the VUV region for systems consisting of Cl_{2} molecules trapped in a neon matrix, an argon matrix, and mixed Ar/Ne matrices. In pure neon, the emission spectrum of the D’→A’ ‘‘laser’’ transition at 4.7 eV of the Cl_{2} molecule is vibrationally well resolved and constitutes an interesting example of UV spectroscopy of a matrix ‘‘isolated’’ molecule. In pure argon or mixed Ar/Ne matrices, new broad emissions at 4.1, 3.8, and 3.5 eV are clearly identified, which result from the specific interaction between Cl^{*} _{2} and Ar and are attributed to different charge–transfer states of the ArCl^{+}Cl^{−} entity. The Ar concentration dependence and the time‐gated spectra are shown to be especially useful in interpreting the large differences observed between the pure neon and the pure argon matrix case.

Use of high observing power in electron spin resonance saturation‐recovery experiments in spin‐labeled membranes
View Description Hide DescriptionBiomolecular collision rates of ^{1} ^{4}N‐containing nitroxide radical labeled stearic acid with similar ^{1} ^{5}N‐containing species in dimyristoylphosphatidylcholine (DMPC) liposomes have been determined using the saturation‐recovery electron spin resonance(ESR) method. It is shown that high microwave observing powers can be used to obtain these rates, thereby increasing the signal‐to‐noise ratio by about ten times relative to methods previously used. The data are analyzed using the rate‐equation approach with inclusion of the observing transition probability. A number of solutions to these equations for other experimental situations and for pulse and continuous wave electron–electron double resonance(ELDOR) are also presented. The bimolecular collision rate of labels at the C16 position was found to be 20% greater than for labels at the C12 position, suggesting that the interaction distance differs at these two positions. Even though the high observing power does not affect the ability to extract bimolecular collision rates, the effective spin‐lattice relaxation rates are altered. Plots of these latter rates versus power are linear and the slope can be used to determine the absolute microwave field at the sample for a given input power.

Fourier transform spectroscopy of the 1 ^{3}Σ^{+} _{ g }–a ^{3}Σ^{+} _{ u } transition of the ^{6}Li_{2} molecule
View Description Hide DescriptionThe 1 ^{3}Σ^{+} _{ g }–a ^{3}Σ^{+} _{ u } transition of ^{6}Li_{2} has been observed via collisionally induced fluorescence, excited by visible lines of an argon‐ion laser and detected at high resolution with a Fourier–transform spectrometer in the 8200–10 100 cm^{−} ^{1} region. By combining the results with previously obtained data on ^{7}Li_{2} [F. Martin, R. Bacis, J. Vergès, C. Linton, G. Bujin, C. H. Cheng, and E. Stad, Spectrochim. Acta Part A 4 4, 1369 (1988)], an accerate, isotopically consistent description of both states has been obtained for 1≤v’≤7 and 0≤v‘≤7. Equilibrium constants, Rydberg‐Klein‐Rees potential curves, and dissociation energies have been determined and found to be in good agreement with a b i n i t i o calculations. From the analysis, the following positions and dissociation energies of the two states were found. For 1 ^{3}Σ^{+} _{ g }, T _{ e } (cm^{−} ^{1}) is 16 328.8(1.7) and D _{ e } (cm^{−} ^{1}) is 7091.6(1.2). For a ^{3}Σ^{+} _{ u }, T _{ e } (cm^{−} ^{1}) is 8183.8(1.5) and D _{ e } (cm^{−} ^{1}) is 333(1).

Photoelectron spectroscopic studies of multiphoton processes in molecular chlorine involving the 2 ^{1}∏_{ g } Rydberg state
View Description Hide DescriptionKinetic energy resolved electron detection is used to study resonance enhanced multiphoton ionization and dissociation of molecular chlorine via the 2 ^{1}∏_{ g }Rydberg state. In the two‐photon energy region between 63 000 and 73 600 cm^{−} ^{1}, a long vibrational progression up to v’=15 associated with this intermediate gerade Rydberg state is observed. The regularity and magnitude of the vibrational spacing indicate that the 2 ^{1}∏_{ g } state, converging to the ionic ground state, has virtually unperturbed Rydberg character. However, the molecular signals in the photoelectron spectra show strong deviations from Franck–Condon behavior. In addition, intense electron signals arising from one‐photon ionization of excited chlorine atoms are observed. An interpretation in terms of a competition between electronic autoionization and dissociation from core‐excited molecular Rydberg states situated above the lowest ionizationenergy is presented. Some of the excited atomic states observed are indicative for Rydberg–Rydberg interactions at large internuclear distances in the dissociation channel.