Index of content:
Volume 101, Issue 5, 01 September 1994

Large amplitude skeletal isomerization as a promoter of intramolecular vibrational relaxation in CH stretch excited hydrocarbons
View Description Hide DescriptionThe high resolution, slit jet cooled infrared v=1←0 methyl asymmetric stretch spectra of trans‐2‐butene and 1‐butene are reported. Both of these molecules are singly unsaturated butene chains, have 30 vibrational degrees of freedom, and yield nearly equivalent vibrational state densities (ρ_{vib}≊200 states/cm^{−1}) at CH stretch levels of excitation. The key difference between these two molecules is the presence of a large amplitude C–C–C skeletal torsional coordinate in 1‐butene corresponding to a low barrier, internal isomerization pathway which is completely absent in trans‐2‐butene. The trans‐2‐butene asymmetric CH stretch (ν_{16}) spectrum is fully discrete at 0.002 cm^{−1} resolution, and the coarse structure readily assigned to zero order rovibrational transitions (J’_{ K’ a K’ c } ← J‘_{ K‘ a K‘ c }) in an asymmetric top. Fragmentation of these zero order transitions into spectral ‘‘clumps’’ of fine structure provides direct evidence for coupling of the CH stretch to vibrational bath states, but no evidence for loss of K _{ a } ^{’} and K _{ c } ^{’} as good quantum labels in the spectrum.
The average density of coupled states is found directly from the spectrum to be 114 states/cm^{−1}, i.e., on the order of 0.5 ρ_{vib}. In contrast to the behavior in trans‐2‐butene, the 1‐butene v=1←0 methyl asymmetric stretch spectrum exhibits an essentially continuous absorption contour even at T _{rot}=6 K and 0.002 cm^{−1} resolution. On closer inspection, the 1‐butene spectral envelope exhibits reproducible, intramolecular vibrational relaxation (IVR) induced fine structure limited by apparatus resolution and characteristic of highly congested IVR coupling. Analysis of this fine structure indicates a density of coupled states on the order of 1 000–10 000 states/cm^{−1}, i.e., 20–30‐fold in excess of ρ_{vib}, and 1–2 orders of magnitude larger than observed in trans‐2‐butene. In order to model the degree of fine structure observed in the spectrum, this level of spectral congestion essentially requires complete mixing of all ρ_{vib}⋅(2J’+1) rovibrational states consistent with conservation of total energy and angular momentum. The qualitatively dramatic differences between 1‐butene and trans‐2‐butene behavior support a simple model for strong vibration‐rotation (V‐R) coupling in the bath states due to large amplitude skeletal motion in the C–C–C torsional mode which greatly enhances the available state density for IVR. Hence, the presence of a low barrier, skeletal isomerization coordinate may prove to be a general, moiety specific promoter for IVR processes in CH stretch excited hydrocarbons.

Phonon relaxation processes in crystals (NaNO_{3}) at high pressure and low temperature
View Description Hide DescriptionNaNO_{3} is investigated in a diamond anvil cell in the pressure range of 0–9 GPa at 21 and 142 K by means of high‐resolution Raman spectroscopy (HRRS). The pressure dependent linewidth of ν_{1} (symmetric stretch) is determined and discussed in the framework of anharmonic lattice dynamics. The main relaxation pathways are depopulation processes which are influenced by anharmonic terms in the expansion of the crystal potential and by multiphonon densities of states. The interpretation is supported by numerical calculation of multiphonon densities of states.

Line shape asymmetries in Ar‐broadened HF(v=1–0) in the Dicke‐narrowing regime
View Description Hide DescriptionCollisional line shapes have been measured for the fundamental band transitions of HF in an Ar buffer gas at T=296 K using a tunable difference‐frequency laser spectrometer. The broadening and shift coefficients are in excellent agreement with recent close‐coupling scattering cross section calculations [Green and Hutson, J. Chem. Phys. 100, 891 (1994)] based on a realistic intermolecular potentialsurface determined by high‐resolution infrared and microwave spectroscopy of the Ar–HF van der Waals complex. Below atmospheric pressure, the line shapes exhibit strong collisional (Dicke) narrowing of the Doppler distribution and a slight asymmetry which we model with hard collision (Rautian) or soft collision (Galatry) profiles modified for partial correlation between velocity‐ and state‐changing collisions.

Dynamical structure of water in aqueous electrolyte solutions by low‐frequency Raman scattering
View Description Hide DescriptionDepolarized low‐frequency Raman spectra of aqueous electrolyte solutions (ACl and BCl_{2}; A=Li, Na, K, Rb and B=Mg, Ca) in the frequency region from −250 to 250 cm^{−1} have been investigated over a wide concentration range. The spectra were analyzed with one Cole–Cole type relaxation mode and two damped harmonic oscillators. A broad relaxation mode was found below 20 cm^{−1}. For the present aqueous electrolyte solutions, with increasing salt concentration each relaxation time τ becomes longer than that of liquid water τ_{water}. The relaxation time is considered to correspond to the duration time of the tetrahedral structure of bulk water. We have found that the concentration dependence of the ratio τ/τ_{water} is equivalent to the ratio η/η_{water}, where η is the viscosity of aqueous solutions.

On the measurement of superpositions of chiral amplitudes by polarized light scattering
View Description Hide DescriptionWe show that no experiment involving parity can measure the left–right coherence of a state ‖ψ〉=a _{ L }‖L〉+e ^{ iφ} a _{ R }‖R〉 if sin φ is zero and ‖L〉 and ‖R〉 are real mirrorimages of one another. We then exhibit a number of experiments in elastic linear light scattering which provide all‐or‐none measurements of left–right coherence.

Tunneling splittings in the S _{1} electronic states of symmetrically substituted 3,7‐dichlorotropolone, 3,5,7‐trichlorotropolone, and 3,7‐dibromotropolone
View Description Hide DescriptionThe S _{1}–S _{0} electronic spectra of 3,7‐dichlorotropolone, 3,5,7‐trichlorotropolone, and 3,7‐dibromotropolone have been measured in a supersonic free jet to investigate the effect on protontunneling of symmetrical substitution of Cl or Br atoms for H atoms of tropolone. The 0^{0} _{0}tunneling splittings in S _{1} of 3,7‐dichlorotropolone, 3,5,7‐trichlorotropolone, and 3,7‐dibromotropolone are 45, 31, and ≲1 cm^{−1}, respectively. The increase of the tunneling splitting for 3,7‐dichlorotropolone compared to that for tropolone (19 cm^{−1}) is attributed to a decrease in the O–H...O heavy atom separation, leading to smaller potential barrier height. The decrease of the 0^{0} _{0}tunneling splitting for 3,5,7‐trichlorotropolone compared to 3,7‐dichlorotropolone and remarkable decrease of the 0^{0} _{0}tunneling splitting for 3,7‐dibromotropolone are ascribed to significant changes of the normal coordinates compared to those of tropolone.

Monte Carlo simulations of the structures and optical absorption spectra of Na atoms in Ar clusters, surfaces, and solids
View Description Hide DescriptionOptical absorptionspectra of Na/Ar systems are calculated by combining the classical Monte Carlo simulation method with a quantum mechanical first‐order perturbation scheme [Balling and Wright, J. Chem. Phys. 79, 2941 (1983)] for estimating the energies of the Na* 3p(^{2} P) excited states. The model incorporates many drastic approximations, but contains no adjustable parameters. Our Na/Ar matrix simulations generated relaxed structures for several candidate trapping sites based on various sized vacancies in face‐centered‐cubic (fcc) solid Ar. Trapping sites for which the equilibrium structures belong to the O _{ h } or T _{ d } point groups yielded the experimentally well‐known ‘‘triplet’’ absorption line shape; for these cases, the splitting of the degeneracy of the excited Na* 3p(^{2} P) state is due solely to fluctuations away from the equilibrium structures. Simulations of Na/Ar clusters,surfaces, and matrix sites possessing a strong permanent axial asymmetry yielded a widely split ‘‘doublet plus singlet’’ absorption line shape. Despite our success at reproducing several qualitative aspects of the absorption spectroscopy of Na/Ar matrices, our simulations failed to quantitatively reproduce the experimental data. We discuss the major limitations of our model, as well as several possible improvements.

Coherent anti‐Stokes Raman spectroscopy of shock‐compressed liquid nitrogen/argon mixtures
View Description Hide DescriptionSingle‐pulse multiplex coherent anti‐Stokes Raman scattering(CARS) was used to obtain vibrational spectra of 20%/80% liquid nitrogen/argon mixtures, shock compressed to several high‐pressure/high‐temperature states. A semiclassical model for CARSspectra was used to extract best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths from the data. Up to a maximum single shock pressure of 17.1 GPa, the N_{2} vibrational frequency was found to increase monotonically with pressure. The vibrational frequencies measured in both the singly and doubly shocked N_{2}/Ar mixtures correspond within experimental error to those for pure nitrogen at equivalent pressures and temperatures, implying that the influence of the interaction potential on the N_{2} vibrational frequency for the N_{2}/Ar collision is not significantly different from that of a N_{2}/N_{2} collision at these conditions. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these shock pressures. Vibrational temperatures obtained were used to improve the potential function of argon used to calculate equation‐of‐state pressures and temperatures. The measuredlinewidths suggest that the nitrogen vibrational dephasing time decreased to about 1 ps at the highest pressure shock state.

On the lifetimes of Rydberg states probed by delayed pulsed field ionization
View Description Hide DescriptionWe present a simple model to evaluate the degree of l and m _{ l } mixing in high Rydberg states that results from perturbations caused by weak, homogeneous dcelectric fields and static ions. This model predicts the lifetime of these states qualitatively and explains several seemingly contradictory observations obtained using zero‐kinetic‐energy (ZEKE) photoelectron spectroscopy. The presence of a small homogeneous dcelectric field and a few ions in the sample volume causes m _{ l } mixing in general as well as l mixing, both of which contribute to the lengthening of the lifetimes. Consequently, the lifetime lengthening appears to be insensitive to the sample pressure. The effect of the dcelectric field on the lifetime is complex. Although the electric field results in l mixing, with increasing field strength it inhibits m _{ l } mixing, and, at still higher field strength, induces ionization. The variation of the lifetimes with ion concentration is also complicated. At low ion concentration, the m _{ l } mixing varies across the Stark manifold of Rydberg states that belong to the same principal quantum number, so that different states have different lifetimes. At higher ion concentration, l and m _{ l } mixing are more uniform, which lengthens the lifetimes and makes them more similar across the Stark manifold. At still higher concentrations, collisional ionization dominates, which shortens the lifetimes.

Optical absorption spectra of Au_{7}, Au_{9}, Au_{11}, and Au_{13}, and their cations: Gold clusters with 6, 7, 8, 9, 10, 11, 12, and 13 s‐electrons
View Description Hide DescriptionThe optical absorptionspectra of a series of small gold clusters and their cations have been measured, between 1.9 and 5.6 eV, using a method based upon the photodepletion of a molecular beam of their van der Waals complexes containing one and two xenon atoms. This method provides size‐specific information even though the molecular beam contains a wide range of cluster sizes. There is little difference between the spectra of complexes containing one or two xenon atoms. However there is a pronounced odd–even alternation in the spectra of gold clusters with differing numbers of valence selectrons. This alternation is described in terms of a simple electron pairing scheme. The spectrum for Au_{13} is in reasonable agreement with Dirac scattered‐wave molecular orbital considerations for icosahedral Au_{13} [A. F. Ramos, R. Arratia‐Perez, and G. L. Malli, Phys. Rev. B 35, 3790 (1987)]. This description of the molecular and electronic structure of small gold clusters in terms of localized molecular orbitals is contrasted with other models based upon jellium potentials and delocalized excitations that have been used to describe small clusters of alkali metals and silver. The bonding in gold clusters is influenced by relativistic effects that increase the degree of sd hybridization in the molecular orbitals. Even though gold clusters can be described in this way, some evidence for electron shells is also presented. Thus, it is concluded that structural motifs other than jellium potentials can lead to shell structure in cluster properties.

Hyperfine structure of pendular states and the sign of the dipole moment of ICl A state
View Description Hide DescriptionSince a first study by Cummings and Klemperer [J. Chem. Phys. 60, 2035 (1974)] and further works by Friedrich and Herschbach, the dipole moments of the X and A states of ICl are considered to have opposite signs. We have performed a high resolution study of the Stark effect of the A–X transition near the 21‐0 bandhead, and we have made a complete theoretical simulation of the spectrum, taking into account the hyperfine structure which is important. The comparison of the experimental spectrum with the simulation clearly prove that the dipole moments of the A and X state have the same sign. This result is in agreement with simple electronegativity arguments.

Electronic decay processes following the resonance excitation of the B 1s core electron in BF_{3}
View Description Hide DescriptionElectron spectroscopy has been carried out to investigate spectator and participant resonance Auger decay processes following the B 1s→2a _{2} ^{‘} excitation in gaseous BF_{3} molecules using monochromatized synchrotron radiation. The resonance‐enhanced satellite bands corresponding to the spectator Auger electron emission form six broad peaks showing good correspondence with the normal Auger bands. Resonance enhancement of the photolines corresponding to the participant Auger electron emission occurs only for the bonding orbitals having B character.

Rotational analyses and angular momentum assignments of [^{2}Π_{3/2}]4d Rydberg states of Br_{2}
View Description Hide DescriptionRotational structures and angular momenta of the two observed [^{2}Π_{3/2}]4d gerade Rydberg states of bromine in the region 68 800–72 000 cm^{−1} have been studied by high resolution resonance enhanced (2+1) multiphoton ionizationspectroscopy. The rotational constants have been obtained and the angular momenta of the two [^{2}Π_{3/2}]4d series have been assigned for the first time. These results are further determined and confirmed by computational spectral simulations. The angular momenta of the two observed [^{2}Π_{3/2}]4dRydberg states are concluded to be Ω=1. Comparison is also made for these two Rydberg states.

Algebraic approach to molecular rotation‐vibration spectra: Rotation‐vibration interactions
View Description Hide DescriptionWe study rotation‐vibration interactions of triatomic molecules within the framework of the vibron model. We use this method to analyze rotation‐vibration interactions in HCN. The method provides an alternative to the usual analysis.

9.6 GHz and 34 GHz electron paramagnetic resonance studies of chromium‐doped forsterite
View Description Hide DescriptionChromium‐doped forsterite single crystalsgrown under conditions that produce a high Cr^{4+}/Cr^{3+} ratio were examined by electron paramagnetic resonance(EPR) at 9.6 and 34 GHz. The crystals were grown in 2–3 atm of oxygen by the floating‐zone method starting from polycrystalline chromium‐doped forsterite powder synthesized via a sol–gel method. Three crystals with chromium concentrations of 110, 300, and 390 ppm were studied. At 34 GHz, transitions are observed for the laser‐active tetrahedral Cr^{4+} species that are not observable at 9.6 GHz, which improve the resolution and accuracy with which the magnetic parameters can be measured by EPR. In addition, peaks for a non‐Kramers species appear at 34 GHz that were not observed at 9.6 GHz. These peaks are not analyzed in detail, but are tentatively ascribed to Cr^{4+} in the octahedral substitution sites of the crystal. At the highest chromium concentration, the Cr^{3+} spectra show evidence of direct interaction with Cr^{4+}. A global least‐squares fit of the combined 9.6 and 34 GHz data for the 300 ppm crystal gives D=64.26±0.18 GHz, E=−4.619±0.009 GHz, g _{ x }=1.955±0.009, g _{ y }=2.005±0.040, g _{ z }=1.965±0.006, and places the magnetic z axis in the ab plane at an angle of 43.8±0.3° from the b crystallographic axis (in P _{ bnm }).
A method for accurately measuring the Cr^{4+}/Cr^{4+} ratio using EPR line intensities is given. The EPRlinewidth of the Cr^{4+} center exhibits a strong orientation dependence that is well‐modeled by including site variations in the D and E zero‐field splittings and in the orientation of the z magnetic axis. The linewidth analysis reveals a high degree of correlation between the distributions in D and E, and a somewhat weaker correlation between E and the z axis orientation. These results are interpreted to suggest that the tetrahedral Cr^{4+} sites vary mainly in the degree of compression of the tetrahedral cage along the a crystallographic axis. The Cr^{4+}EPRlinewidths increase significantly at higher chromium concentration, but maintain the same qualitative orientation dependence. The EPR data indicate that the major contribution to inhomogeneity in the tetrahedral site, which may be related to the tunable range of the Cr^{4+} laser center, is distortion induced by chromium substitution into the crystal lattice rather than direct chromium–chromium interactions.

Observation of the PF_{2} radical by resonance enhanced multiphoton ionization spectroscopy
View Description Hide DescriptionWe have observed nine new excited electronic states of the PF_{2} radical in the wave number range 50 000–71 000 cm^{−1} via the two photon resonance enhancements they provide in mass resolved multiphoton ionizationspectroscopy. We also report ab initio calculations which yield optimized energies, geometries, and vibrational frequencies for the ground states of PF_{2} and of the PF^{+} _{2} ion. Energetic considerations, and the observation that the frequencies of the ν_{1} ^{’} (symmetric stretch) and ν_{2} ^{’} (bending) modes in these excited states closely parallel those of the ground state ion, lead us to conclude that all nine excited states are Rydberg states belonging to series that converge to the ground stateionization limit. Polarization analyses and band contour simulations allow us to identify the electronic symmetries of the first two of these newly identified states (^{2} B _{1} and ^{2} A _{1}, respectively) and to provide an estimate of the excited state geometries [r(P–F)≊1.49 Å, ∠F–P–F≊109°].

Electron‐momentum‐specific valence‐shell electronic structure of and many‐body effects in cis‐dichloroethylene: Ionic‐state‐resolved orientationally averaged electron momentum density measurement by symmetric noncoplanar (e,2e) spectroscopy
View Description Hide DescriptionThe ionization energy (IE) spectra and electron momentum distributions (MDs) of the valence shell of cis‐dichloroethylene have been obtained using symmetric noncoplanar (e,2e) spectroscopy. The IE spectra were found to be in good accord with earlier photoelectron data and in qualitative agreement with a literature pole‐strength spectrumgenerated by a Green’s function (GF) calculation. In particular, extensive many‐body features were observed above ∼22 eV in the IE spectra and found to be consistent with the pole‐strength splitting of the three innermost valence states (7a _{1})^{−1}, (6b _{2})^{−1}, and (6a _{1})^{−1}, as predicted by the GF calculation. The measured MDs were compared with ab initio calculations using self‐consistent‐field wave functions of 4‐31G, 6‐31G, and 6‐31^{++}G** basis sets. The general lack of quantitative agreement between experiment and calculations indicated the need for improved wave functions beyond the 6‐31^{++}G** level for the outer‐valence orbitals. Moreover, the experimental MDs have provided an unambiguous assignment of the ordering of the D(8b _{2})^{−1} and E(2b _{1})^{−1} ionic states. A new satellite feature at ∼21 eV was found to have a MD similar to that of the H(7b _{2})^{−1} state, confirming the GF result. Furthermore, the many‐body features above 26 eV were dominated by s‐type angular dependence and could be ascribed predominantly to ionization of the innermost valence orbital 6a _{1}. Finally, the bonding morphology of the valence orbitals was found to be different from other related ethylene derivatives due to ligand substitution effects.

Spectral moments of collision‐induced absorption of CO_{2} pairs: The role of the intermolecular potential
View Description Hide DescriptionIn this paper we examine the role of the anisotropy of the intermolecular potential in the rototranslational collision‐induced absorption of the CO_{2} pairs. Using newly developed formulas [Borysow and Moraldi, Phys. Rev. Lett. 68, 3686 (1992); Moraldi, Phys. Rev. A (submitted)] that include the effects of anisotropy of the potential to all orders, we calculate the two lowest spectral moments γ_{1} and α_{1} for four different classes of CO_{2} pair potentials and compare the results with the experimental values. We assumed only multipolar induction in the process of forming the induced dipole, with the second‐order contributions included. Using a site–site LJ [Murthy et al., Mol. Phys. 50, 531 (1983)] and a site–site semi‐ab initio [Bohm, Mol. Phys. 56, 375 (1985)] intermolecular potentials we were able to reproduce the experimental values of γ_{1} and α_{1} moments over entire temperature range from 230 to 330 K. Also, the role of an electrostaticinteraction between two CO_{2} molecules and its impact on the spectral moments is thoroughly investigated. An isotropic core with a point quadrupole centered at each molecule is shown to be an inadequate representation of the CO_{2}–CO_{2} potential. Additionally, we show the results obtained with the first‐ and second‐order perturbation theory to be more than twice too small.

The doubly excited 1 ^{3}Σ^{−} _{ g } state of ^{7}Li_{2}
View Description Hide DescriptionThis paper reports the first experimental observation of the doubly excited valence (2p+2p)^{3}Σ^{−} _{ g } state of ^{7}Li_{2}. We used cw perturbation‐facilitated optical–optical double resonance (PFOODR) fluorescence excitation and resolved fluorescencespectroscopic techniques. All the observed levels have been detected through perturbations by the 2 ^{3}Π_{ g } state. The deperturbed primary molecular constants of this 1 ^{3}Σ^{−} _{ g } state are T _{ e }=34 045.354(43) cm^{−1}, ω_{ e }=216.820(37) cm^{−1}, B _{ e }=0.673 69(47) cm^{−1}, R _{ e }=2.670 81(94) Å, and D _{ e }=4279.306(43) cm^{−1}. The equilibrium internuclear distance of the 1 ^{3}Σ^{−} _{ g } state is smaller than that of the X ^{1}Σ^{+} _{ g } ground state.

Dipole moment function and equilibrium structure of methane in an analytical, anharmonic nine‐dimensional potential surface related to experimental rotational constants and transition moments by quantum Monte Carlo calculations
View Description Hide DescriptionThe pure rotational spectrum in the far‐infrared and its absolute intensity in the vibrational ground state of CHD_{3} and CH_{3}D, and the integrated band strength of the N=5 CH‐stretching overtone of CHD_{3} in the near infrared to visible were measured by high‐resolution interferometric Fourier transform techniques. The far‐infrared data result in permanent electric dipole moments (‖μ^{ z } _{0}‖=(5.69±0.14)×10^{−3} D for CHD_{3}, ‖μ^{ z } _{0}‖=(5.57±0.10)×10^{−3} D for CH_{3}D), consistent with previous experimental data. The integrated N=5 overtone cross section is found to be (0.828±0.068) fm^{2}. The overtone data are used, together with previous data, to derive a new, nine‐dimensional, isotopically invariant dipole moment function for CH_{4} within the chromophore model for the CH chromophore in CHD_{3}. With this function, the experimental data can be reproduced to an averaged factor of 1.2, in the best case. In the vibrational ground state, a nine‐dimensional calculation of expectation values on a new, fully anharmonic potential surface was performed using the solution of the rovibrational Schrödinger equation by diffusion quantum Monte Carlo methods. The results for the rotational constants of several isotopomers, which include significant contributions from rovibrational interactions, indicate that the equilibrium CH bond length of methane is r _{ e }=108.6 pm. The calculated value for the vibrationally averaged permanent dipole moment from these nine‐dimensional vibrational quantum calculations, using the dipole moment function consistent with the analysis of the overtone bands, is μ^{ z } _{0}=−(6.6±0.4)×10^{−3} D for CHD_{3} (with positive z coordinate for the H atom) and μ^{ z } _{0}=(6.8±0.5)×10^{−3} D for CH_{3}D (with positive z coordinate for the D atom) in essential agreement with the far‐infrared rotational intensities. The sign could be determined unambiguously by comparison with ab initio data. We predict the permanent dipole moment of several further methane isotopomers. The polarity of the CH bond in methane is C^{−}–H^{+}, within our simple bond dipole model, but is discussed to be a model dependent (not purely experimental) quantity.