Volume 87, Issue 11, 01 December 1987
Index of content:
Comparison of experiment and theory for the resonance Raman spectrum of I2 in solution. II. The Raman excitation and depolarization profiles in n‐hexane87(1987); http://dx.doi.org/10.1063/1.453452View Description Hide Description
The calculations for I2 in n‐hexane, which were started in paper I [J. Chem. Phys. 8 5, 3791 (1986)] are improved by considering the effect of the D state and of charge–tranfer states. Changes in intensities and positions of the B states and the effect of the rotation of I2 in the excited states are also considered. The experimental excitation profiles are recalibrated and compared to calculations. The depolarization profiles are measured and are also compared to calculations. The measured Raman cross sections are higher than calculated in the high frequency wing of the profile—a discrepancy for which we have not found an explanation. The depolarization ratios of the overtones may be explained by shifting the B and B‘ states about 200 cm− 1 further apart than in the gas phase and by taking into account the effect of rotation of the I2. The excited staterelaxation time,T 2, is still determined as about 0.3 ps.
Comparison of experiment and theory for the resonance Raman spectrum of I2 in solution. III. Perfluorohexane and chloroform87(1987); http://dx.doi.org/10.1063/1.453453View Description Hide Description
The absolute Raman excitation profiles (REP) of I2 in perfluorohexane and chloroform have been determined. The experimental results are compared to calculations and to our previous results in n‐hexane. Although there is good overall agreement between experiment and theory, certain discrepancies turn up in all the solvents. In particular, the experimental cross section is higher than that calculated in the high frequency wing of the REP. This discrepancy cannot be due to charge transfer states since it exists in all the solvents. The excited statelinewidth, Γ has been determined to be about 15 cm− 1 in all three solvents.
87(1987); http://dx.doi.org/10.1063/1.453733View Description Hide Description
Theoretical analysis of the time‐resolved impulsive stimulated light scattering (ISS) method is presented. A general theoretical framework is developed to describe ISS experiments on any type of material mode which is active in light scattering and conforms to linear response theory. ISS experiments permit time‐resolved observation of material motion through the dielectric response functionG εε(q,t). In the simplest case of ideal time and wave vector resolution, ISS signal gives ‖G εε(q,t)‖2 directly. Various consequences of limited t and q resolution are discussed in detail. ISS experiments on acoustic and optic phonons, Debye relaxational modes, and some combinations of modes are treated explicitly. A detailed comparison of time‐domain impulsive stimulated scattering and frequency‐domain spontaneous light‐scattering spectroscopy is presented in the companion paper.
Impulsive stimulated light scattering. II. Comparison to frequency‐domain light‐scattering spectroscopy87(1987); http://dx.doi.org/10.1063/1.453454View Description Hide Description
A comparison between time‐domain impulsive stimulated light‐scattering (ISS) and frequency‐domain, spontaneous light‐scattering (LS) spectroscopies is carried out in theoretical and practical terms. In some cases, the two experiments probe different material responses. In many cases the information content of ISS and LS data is identical in principle. The results can be related to each other through the time‐ and frequency‐dependent response functions G εε(q,t) and G εε(q,ω), or through the time‐correlation function C εε(q,t). Simulated ISS and LS data from vibrational and Debye relaxational modes are compared in view of experimental considerations, including wave vector and time or frequency resolution and range, and sources of ‘‘noise.’’ In many cases, one or the other experimental approache offers significant advantages in practice. The complementary nature of the techniques is illustrated.
87(1987); http://dx.doi.org/10.1063/1.453455View Description Hide Description
NBr in solid argon has been excited to the b 1Σ+ state with a pulsed tunable dye laser. In addition to the well‐known fluorescencespectrum of the b state, infrared emission from the vibrationally relaxed a 1Δ state was detected at 1086 nm. Three major trapping sites were resolved in absorption and fluorescence excitation spectra of the b–X system. Additional sites could be identified in the excitation spectrum of the a state, which is populated via the b state by internal conversion. There is a weak site effect on the lifetime of the vibrationally relaxed b state, which varies between 40 and 30 μs. Vibrational relaxation rates in the b state show a stronger site dependence. Internal conversion from b 1Σ+ to a 1Δ accounts for <0.1% to at least 17% of the decay rate of NBr b 1Σ+, v=0. Orbitally forbidden transitions to the X 10+ component of the ground state were identified 23.0 to 25.5 cm− 1 on the high‐energy side of the more intense a 1Δ→X 21± transition of several sites. The lifetime of the a state varies between 152±12 ms in the least perturbed site and 121±15 ms in the site with the strongest a 1Δ→X 10+ forbidden component. NBr trapped in double vacancies of pure fcc argon, of single stacking faults, and of multiple stacking faults (hcp pockets) in argon, can account for the sites.
87(1987); http://dx.doi.org/10.1063/1.453456View Description Hide Description
Ethylene clusters (C2H4) n are generated in a supersonic expansion with He and size selected by scattering from a helium beam. The clusters are dissociated upon absorption of a photon from a pulsed CO2 laser by exciting the ν7 mode of the monomer. During the collision about 30 meV of internal energy is transferred to the cluster so that the laser photons interact with internally hot clusters. The frequency and fluence dependences of the photodissociation cross sections are measured for (C2H4) n with n=2,3,4,5, and 6. Nearly all spectra exhibit structure which is most pronounced for the dimer. The overall width (FWHM) decreases from 31.2 cm− 1 for the dimer to 12.2 cm− 1 for the hexamer, while the maximum position is nearly the same (951.6 cm− 1). The structure of the dimer spectrum is attributed to hot bands. The large linewidth corresponds to a short lifetime in the ps range.
87(1987); http://dx.doi.org/10.1063/1.453457View Description Hide Description
We have studied the resonant two‐photon dissociation (TPD) of vibrationally excited HD+ by both 1sσ g →1sσ g vibrational and 1sσ g →2pσ u electronic transitions. Following Armstrong e t a l., a quantum mechanical description of the laser field enabled us to define the intermediate resonant state as a ‘‘pseudodissociating state’’ of the molecule–radiation system. This makes our theory formally similar to that of single‐transition (photon)autoionization problem as formulated by Fano. This similarity has been exploited to obtain the intensity dependent line shapes of the resonant TPD of HD+. Resonant TPD cross sections from the v i =6, J i =0 level of the ground 1sσ g state of HD+ are calculated with a l i n e a r l y polarized infrared laserradiation in the wavelength range 16 530–16 565 Å and at different intensities ranging from 1.53×108–3.83×101 3 W/cm2. Effects of laser intensity on the linewidth, line shift, and cross sections are found to be very different for the two types of transitions. Our previous results of nonresonant TPD cross sections are reproduced via this model.
87(1987); http://dx.doi.org/10.1063/1.453458View Description Hide Description
New measurements of ten K=1 lines, including six Q type and four R type, were made on the completely protonated species of the water dimer. For some of these lines, as well as for some K=0 transitions known from the literature, Stark coefficients were determined, and these Stark coefficients provide a confirmation of the assignments. The new K=1 measurements show that the splitting associated with the (HF)2‐like tunneling motion decreases from about 19.5 GHz for K=0 to about 16.2 GHz for K=1. To understand the fact that K=1 lines are populated in our 1 K beam, we must assume, in accordance with the results of beam studies on other molecules, that levels of different nuclear spin modification relax separately. In an attempt to gain information on tunneling splittings other than that caused by the (HF)2‐like motion, we have made new measurements on 1–0 and 2–1 transitions with K=0 for several partially deuterated species, in which the (HF)2‐like motion cannot occur. Small splittings ranging from 4 to 145 MHz were observed. Because of the nature of the tunneling motions involved, these new data yield only the difference of the tunneling splitting in the upper and lower states of the transition.
A vibrational study on a phase transition in tin(II) chloride dihydrate. The O–X stretchings (X=H or D)87(1987); http://dx.doi.org/10.1063/1.453459View Description Hide Description
The vibrational spectra of SnCl2⋅2H2O and its partially deuterated compound were measured in the O–X stretching region for both ordered and disordered phases. In the ordered phase some unexpected bands were found. They are explained in terms of hydrogen ion positional disorder present in the crystal. The Raman spectrum of SnCl2⋅2H2O was resolved in the disordered phase and direct information about disorder in hydrogen positions was obtained.
Vibrational spectra and assignments, normal coordinate analyses, a b i n i t i o calculations, and conformational stability of the propenoyl halides87(1987); http://dx.doi.org/10.1063/1.453460View Description Hide Description
The infrared and Raman (3500–30 cm− 1) spectra of gaseous and solid propenoyl bromide, CH2 CHCBrO, have been recorded. The Raman spectrum of the liquid has been obtained and qualitative depolarization values have been measured. The fundamental asymmetric torsion for both the s ‐ t r a n s and s ‐ c i s conformers has been observed along with several hot transitions. From these transitions the potential function governing the internal rotation has been determined and the s ‐ t r a n s to s ‐ c i s barrier is 1861 cm− 1 (5.32 kcal/mol), the t r a n s conformer being more stable by 158±20 cm− 1 (452±57 cal/mol). All normal modes have been assigned for the s ‐ t r a n s conformer and many of those for the s ‐ c i s conformer, based on band contours, depolarization values, and group frequencies. A normal coordinate calculation has been carried out by utilizing a modified valence force field to calculate the frequencies and the potential energy distribution for both conformers. Temperature studies of the Raman spectra of gaseous propenoyl fluoride and chloride and of the liquid phases of propenoyl chloride and bromide have been used to obtain the enthalpy differences between the s ‐ t r a n s and s ‐ c i s conformers for these compounds. For the bromide and chloride the s ‐ t r a n s conformer is clearly the more stable form but the data are inconclusive for the fluoride. Complete equilibrium geometries have been determined for both rotamers of the fluoride
and chloride by a b i n i t i o Hartree–Fock gradient calculations employing both 3‐21G and 6‐31G* basis sets. The structural parameters are compared to those suggested from microwave and/or electron diffraction studies of these molecules. Both calculations are consistent with the s ‐ t r a n sconformation being thermodynamically preferred for the fluoride molecule and the s ‐ c i sconformation for the chloride, which is contrary to the experimental data for the chloride. The 3‐21G basis set was used to obtain the a b i n i t i o force constants and frequencies for the s ‐ t r a n s and s ‐ c i s conformers of both the fluoride and chloride with fixed scaling, variable scaling, and without scaling factors. These results required several reassignments of fundamentals for both conformers of both molecules. Improved vibrational data for both the fluoride and chloride were very valuable for the reassignments. These results are compared with the corresponding quantities for some similar molecules.
Analysis of 2 7Al nuclear quadrupole interaction effects on electron spin echo modulation in disordered systems87(1987); http://dx.doi.org/10.1063/1.453461View Description Hide Description
The effects of 2 7Al (I=5/2) nuclear quadrupoleinteraction on the electron spin echo modulation (ESEM) in the case of S=1/2 in disordered systems are analyzed using exact diagonalization of the spin Hamiltonian. The Hamiltonian parameters used in the calculations are those expected in Cu+ 2‐exchanged zeolites. The ganisotropy is taken into account and the ESEM obtained using the spherical model approximation is compared with a trigonal correlated structure. It is found that in the 0.32–0.34 nm range of distances between the paramagnetic center and the Al nuclei all parameters related to the quadrupoletensor, namely the quadrupoleinteraction constant, the asymmetry parameter and the relative orientation with respect to the gtensor, have a substantial effect on the modulation depth.
87(1987); http://dx.doi.org/10.1063/1.453462View Description Hide Description
The infrared spectrum of a transient species HBNH was observed in the gas phase using a diode laserspectrometer. The HBNH molecule is formed in an ac discharge plasma of a diborane/ammonia or diborane/nitric oxide mixture. The ν3 band origin and the rotational constants of H1 1BNH were determined to be ν0=1786.193 08(72), B 0=1.099 335(81), and B 3=1.093 481(72) cm− 1, with three standard deviations in parentheses. The ν3 band of H1 0BNH was also observed and analyzed.
87(1987); http://dx.doi.org/10.1063/1.453463View Description Hide Description
The paper studies vibronic spectra (exciton+intramolecular vibrational quanta) of helical polymers in solutions. The theoretical model employed here makes it possible to describe the ultraviolet (UV) absorption and circular dichroism (CD) spectra in the conditions of linear and quadratic exciton–phonon interaction. On the basis of the expanded Moffitt exciton model the structure of degenerate and nondegenerate one‐phonon vibronic spectra containing one‐particle and two‐particle (unbound) exciton–phonon states is found. The frequencies and the shape of UV and CD spectral lines are calculated for typical values of the exciton–phonon interaction constants.
87(1987); http://dx.doi.org/10.1063/1.453464View Description Hide Description
FTIRspectra of various isotopic forms of formic and acetic acids in the gas phase have been recorded in the range 500–4000 cm− 1. The way for separating spectra due to monomers and to H‐bonded cyclic dimers is described. A careful measurement of pressure and a precise control of temperature make it possible to measure intensities related to one molecule for all bands appearing in this region. It allows to measure the effects of H bonds on intensities of all bands. The comparison of intensities of ν s (O–H↘⋅⋅⋅O) and νC=O bands between H‐bonded and D‐bonded dimers is particularly interesting as it confirms the existence of an anomalous isotope effect, which we propose to attribute, after analysis, to a nonadiabatic transfer of intensities between electronic and protonic transitions favored by the particular ring structure of these cyclic dimers (pseudo‐Jahn–Teller effect). It might explain why simple double‐well potentials have up to now failed to describe experimental results concerning transfers of protons through H bonds and it stresses the role that one may attribute to ring structures in describing dynamical properties of H bonds. The particular bandshape of ν s is analyzed using a peeling‐off procedure which allows, in a low resolution approximation, to eliminate features due to Fermi resonances. It allows to measure the magnitude of factor group splittings of ν s modes, which had been scarcely performed before. Peeled‐off spectra then appear as classical spectra of a rapid motion (ν s ) modulated by low‐frequency intermonomer modes of H bonds. Qualitative attributions of their submaxima is given in terms of a transition in ν s accompanied by transitions of intermonomer modes. Stretching as well as bending intermonomer modes are shown to have an active part in the modulation of ν s .
Structure of bimetallic clusters. Extended x‐ray absorption fine structure (EXAFS) of Pt–Re and Pd–Re clusters87(1987); http://dx.doi.org/10.1063/1.453465View Description Hide Description
Extended x‐ray absorption fine structure (EXAFS) studies were conducted on catalysts containing platinum and rhenium, or palladium and rhenium, on alumina. The atomic ratio of rhenium to either platinum or palladium was close to one in the catalysts investigated. The metallic entities in the catalysts were characterized by analyses of the EXAFS associated with the L Iabsorption edge of platinum, the L III edge of rhenium, and the Kabsorption edge of palladium. It was concluded that Pt–Re and Pd–Re bimetallic clusters are present in the catalysts. However, the clusters have regions rich in rhenium and other regions which are rich in either platinum or palladium. Exposure of Pt–Re clusters to sulfur has little influence on their structure.
87(1987); http://dx.doi.org/10.1063/1.453466View Description Hide Description
New experimental results on the structural and dynamical properties of NH3 dimer are reported in this work. J=1–0, K=0 transitions of 1 4NH3–1 5NH3, 1 5NH3–1 4NH3, ND3 dimer, and ND3–ND2H have been measured at high resolution and 1 4N electric quadrupole coupling constants are reported for each of these species. The NH3 subunits comprising the dimer are inequivalent. The quadrupole coupling constant associated with the first ammonia subunit e q Q 1 a a , is measured in 1 4NH3–1 5NH3 [−627(8)kHz], in ND3 dimer [−531(15) kHz], and in ND3–ND2H [−991(18) kHz]. For the other subunit, e q Q 2 a a is reported in 1 5NH3–1 4NH3 [892(8)kHz], in ND3 dimer [745(13) kHz], and in NH3–ND2H [1013(18) kHz]. These numbers can be used to estimate the vibrationally averaged polar angles of these isotopomers of NH3 dimer. The result is (including the primary isotopomer) θ1 for 1 4NH3–1 4NH3 is 48.6°, for 1 4NH3–1 5NH3 is 48.7°, for ND3 dimer is 49.6° and for ND3–ND2H is 45.3°; while θ2 for 1 4NH3–1 4NH3 is 64.5°, for 1 5NH3–1 4NH3 is 64.3°, for ND3 dimer is 62.6°, and for ND3–ND2H is 65.8°. The remarkable invariance of these values rules out the possibility of large vibrational averaging or tunneling averaging in this system and establishes that the angles θ1=49° and θ2=65° are near equilibrium. The isotope effect in the
component of the electric dipole moment along the a inertial axis μ a , is shown to correlate well with the trend in polar angles given by the quadrupole coupling constants. The absence of interchange tunneling effects in the observed states of NH3 dimer implies that these states are asymmetrically excited internal rotor states of the complex. These experimental structural results are in disagreement with all previous theoretically determined structures for NH3 dimer except one. A recent electronic structure calculation which incorporates correlation through the coupled pair functional approach (while systematically varying geometry) obtains a compact, asymmetric structure for the dimer in close accord to observations.
Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter‐wave spectroscopy87(1987); http://dx.doi.org/10.1063/1.453467View Description Hide Description
The temperature‐dependentstructure of crystalline methylammoniumtrihalogenoplumbates (II)—CH3NH+ 3PbX− 3 (X=Cl, Br, I)—as determined by x‐ray diffraction, is compared with measurements of the temperature‐dependent complex permittivity at frequencies of 50–150 GHz. The dielectricmeasurements reveal a picosecond relaxation process which corresponds to a dynamic disorder of the methylammonium group in the high‐temperature phases of the trihalogenoplumbates.
Triplet (T 1) state and radical cation resonance Raman spectroscopy of N,N,N′,N′‐tetramethyl‐ and N,N,N′,N′‐tetraethyl‐p‐phenylenediamines87(1987); http://dx.doi.org/10.1063/1.453468View Description Hide Description
The time‐resolved resonanceRaman spectra of the first triplet state and the cw resonanceRaman spectra of the radical cation are reported for various isotopic derivatives of the N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) and of the N,N,N′,N′‐tetraethyl‐p‐phenylenediamine (TEPD). Vibrational assignments and structural implications are discussed. The radical cation spectra are consistent with a semiquinoidal conformation of the ionized species. They are characterized by a significant frequency increase of the in‐phase ν(N–ring) mode with respect to the ground state spectra (Δν≥120 cm− 1) and by the high resonance enhancement of various bands assigned to vibrations of the N(alkyl)2 groups, resulting from couplings of these vibrations with modes of the π chromophore. The triplet spectra display almost only bands due to vibrations of the N(alkyl)2 groups, implying that similar couplings take place in the T 1 state. Two signals characterize the N–ring–N framework: one is assigned to the Wilson ring mode 9a (CH bending ); the second one, a strong signal around 1500 cm− 1, corresponds to a vibration involving a large contribution of the N–ring stretching motion and possibly also of the ring mode 8a.
87(1987); http://dx.doi.org/10.1063/1.453469View Description Hide Description
Intensity parameters for the luminescence transitions between crystal‐field levels of the 4f 6 configuration of Eu3 + in Y2O2S, LaOCl, LiYF4, Na5Eu(MoO4)4, and Na5Eu(WO4)4 are discussed. The superposition model is used to relate the parameters to parameters determined for other ions in both similar and different host crystals. The results confirm our earlier observations that mechanisms outside the point‐charge crystal‐field model are required in order to rationalize the s i g n s of the intensity parameters for lanthanide ions in solids.
The libration–vibration bands of the ammonium ion in ammonium tetraphenylborate: Evidence for nearly free rotation87(1987); http://dx.doi.org/10.1063/1.453470View Description Hide Description
The N–H stretching and H–N–H bending regions of the infrared spectrum of the title compound (ATPB) have been examined with care. The stretching bands show rotation‐like structure that we attribute to rotational motion of NH+ 4 in a very low barrier. The barrier has the symmetry of a tetrahedral molecule in a tetrahedral site and is characterized by the dimensionless parameter β=1. The J=0 and J=1 levels are below the barrier and the J=2 levels are above it. Slow interconversion of nuclear spin species is seen in keeping with the low barrier model. The spectra of the ATPB with some NH3D+, with a high concentration of K+ and with deuterated phenyl groups have been examined, and show spectra in keeping with the model.