Volume 91, Issue 2, 15 July 1989
Index of content:
91(1989); http://dx.doi.org/10.1063/1.457167View Description Hide Description
In Nd3 +‐doped CsCdBr3 crystals, most of the Nd3 + ions form pairs. A detailed analysis of the optical spectroscopy of these pairs has been carried out using both absorption and luminescencespectra. Due to the low value of the Debye cutoff frequency for the phonons, many excited levels are luminescent, thus giving a lot of spectroscopic information and allowing the assignment of 65 levels to their associated irreducible representations. These results have been used to deduce, through a fitting procedure, the free‐ion and crystal‐field parameters. The results are discussed and compared with those found in other compounds.
91(1989); http://dx.doi.org/10.1063/1.457168View Description Hide Description
The high‐resolution infrared spectrum of the K=1–0 subband of the H–F stretching vibrational band of the hydrogen‐bonded HF–DF complex has been recorded using a molecular‐beam electric resonance optothermal color‐center‐laser spectrometer. The spectrum exhibits minor perturbations and vibrational predissociationlinewidths of 23±2 MHz full width at half‐maximum for comparison to the 11±1 MHz widths found for the corresponding mode of the homonuclear HF–HF dimer.
91(1989); http://dx.doi.org/10.1063/1.457169View Description Hide Description
Microwave and infrared spectra of HF–HCl and HCl–HF have been obtained using a molecular‐beam electric‐resonance optothermal spectrometer, which operates by quadrupole‐field focusing of polar molecules onto a bolometerdetector. The HF–HCl microwave measurements extend to K a =1, the previous K a =0 results of Janda, Steed, Novick, and Klemperer, allowing the determination of the K a dependence and asymmetry of the Cl quadrupole coupling constant. For the metastable HCl–HF isomer no previous spectroscopic measurements have been reported. Here, microwave spectra are observed for the K a =0 and 1 states and interpreted in terms of an L‐shaped hydrogen‐bonded structure for the complex, with a 3.235 Å center‐of‐mass separation between the HF and HCl subunits. The D J distortion constant indicates that the harmonic stretching force constant for HCl–HF is ∼35% larger than that of HF–HCl. Infrared spectra of the K a =0–0 and 1–0 subbands of the H–F stretching band for HF–HCl and of the K a =0–0 subband of the H–F stretch for HCl–HF are also reported. The vibrational predissociation linewidths depend on vibration, K a state, isotopic species, and isomer excited.
91(1989); http://dx.doi.org/10.1063/1.457170View Description Hide Description
The infrared spectra of the ν3, ν4, and ν6 bands of formaldehyde in the region from 890 cm− 1 to 1580 cm− 1 have been obtained at high resolution using tunable diode laser (TDL) and Fourier transform‐infrared (FT‐IR) spectroscopy. The transition frequencies have been analyzed using a Hamiltonian including terms through sextic in centrifugal distortion and including five interstate vibration–rotation coupling terms. Excited state pure rotational transitions are also included in the data, and their frequencies are reproduced well. Individual measured line intensities are used to determine dipole derivatives and band strengths using the fully coupled, asymmetric top eigenvectors.
Elastic and dynamic properties of polymer electrolytes: A Brillouin scattering study of poly(propylene glycol)–NaCF3SO3 complexes91(1989); http://dx.doi.org/10.1063/1.457171View Description Hide Description
In the present study of ion conductingpolymers we demonstrate how light scattering can be used to obtain information about the elastic properties and the local intrachain mobility, which are essential characteristics of a high performance polymeric solid electrolyte. Brillouin scattering has been applied in studies of sodium triflate (NaCF3SO3)–poly(propylene glycol) (PPG) complexes of concentrations O:M=5:1, 8:1, and 16:1, where O:M is the backbone oxygen to sodium ratio. From the spectra sound velocity and absorption data were obtained. The acoustic data reveal structural relaxation processes associated with local translational (and orientational) intrachain motions of the polymer segments. The characteristic relaxations times τ were determined and found to be strongly dependent on the concentration of the dopant salt. This is because the dissociated ions act both as crosslinks betwen polymer chain segments and as ‘‘free’’ ions available for conduction. Therefore, as the concentration of dissociated ions increases the intrachain mobility is slowed down (τ increases) due to increased crosslinking. The effect is demonstrated in the hypersonic velocity dispersion curve and in the absorption peak which both shift to higher temperatures with increasing salt concentration. The shift correlates with the increase in the glass transition temperature as salt is added to the polymer. It also correlates, at least at low salt concentrations, with the concentration of dissocated ions and with the conductivity. It can be concluded that the motions of free ions responsible for charge transport are strongly coupled to the intrachain motions of the polymer.
Experimental measures of the electron distribution and bonding in bis(η5‐cyclopentadienyl) osmium from He(I) and He(II) valence photoelectron spectroscopy91(1989); http://dx.doi.org/10.1063/1.457172View Description Hide Description
The He(I) and He(II) high resolution photoelectron spectra of osmocene are reported. Vibrational fine structure is observed in all the valence metal‐based ionizations and in the cyclopentadienyl π ionizations which derive from the e 1g and e 1u symmetry combinations. Analysis of the vibrational progressions found in the metal‐based ionizations provides a measure of the force constants and vibrational frequencies for the metal–ring stretch in the positive ions. The vibrational analysis for the 2 E 2(5/2) and 2 E 2(3/2) states of the osmocene cation [derived from the spin–orbit split ionization of the metal e 2g (d x 2−y 2 , d x y ) set] indicates an 0.12 Å greater metal–ring bond length in the cation compared to the neutral molecule. The sharp 2 A 1(1/2)ionization [correlating with removal of an electron from the metal a 1g (d z 2 ) orbital] is observed to have a much shorter vibrational progression. The adiabatic ionization is the most intense (vertical) band of the series, indicating that there is no appreciable change in metal–ring bond distance upon ionization from the nonbonding a 1g (d z 2 ) orbital to produce the 2 A 1(1/2) cationic state. A large spin–orbit coupling is observed in the metal ionization region and a moderate amount in the cyclopentadienyl ring π ionization region. From evaluation of the spin–orbit coupling, the ionization band which correlates primarily with the e 1g combination of the cyclopentadienyl π orbitals is shown to contain about 27% metal character. The spin–orbit coupling effects are negligible in the ionization which corresponds to the e 1u combination of the ring orbitals, as expected from the lack of symmetry interaction with the metal d orbitals. The ionizations of this complex also provide experimental measures of relative metal‐based and carbon‐based ionization cross sections with He(I) and He(II) energy sources which are compared with theoretical calculations. All of the observations indicate that covalent bonding is more prevalent in osmocene
than in ferrocene.
91(1989); http://dx.doi.org/10.1063/1.457173View Description Hide Description
Visible laser excitation (460–725 nm) of dilute rare gas/I2 (2000:1) matrices resulted in emission from the I2 A 3Π(1 u ) state. Reanalysis of the A→Xspectra provided revised molecular constants for matrix isolated I2. A state lifetimes of 70±20, 80±20, and 110±30 μs were observed in Ar, Kr, and Xe hosts, respectively. Excitation spectra for the A state closely followed the I2 continuum absorptionspectrum, indicating that transfer from the B 3Π(0+ u ) and 1Π(1 u ) states was effective in populating I2(A). At dilution ratios of 600:1 or lower the I 2 P 1/2–2 P 3/2 transition was observed in conjunction with the A–X bands. Excitation studies showed that isolated I atoms, trapped during the deposition process, were excited by energy transfer from nearby I* 2 molecules. A vibronic progression, similar to the A–X bands, but shifted to longer wavelengths, was noted in concentrated Rg/I2(300:1) matrices. This system, which was emitted with a lifetime of about 10 ms, most probably originated from perturbed I2 A’ 3Π(2 u ). Intermolecular energy transfer was observed in matrices that contained I2 codeposited with O2. Electronic excitation of I2 resulted in a long‐lived emission from O2 a 1Δ g . Matrices containing high concentrations of iodine also exhibited O2(a)→I(2 P 1/2) transfer.
91(1989); http://dx.doi.org/10.1063/1.457174View Description Hide Description
Using a reduced equation of motion for the density matrix which accounts for spontaneous emission and superradiance, we analyze the fluorescence and transient grating (TG) decays from a dilute, optically thin distribution of molecular aggregates. We find that the fluorescence is a limited form of superradiance, where cooperativity is restricted to the number N of two level systems which make up the aggregate. The dependence of the linear aggregate decay rate on the amount of inhomogeneous broadening, which is randomly distributed according to Gaussian line shape of width (1/e)2σ, is calculated. It is shown that σ must be comparable in magnitude to the nearest neighbor dipole–dipole coupling V and not the superradiant decay rate, in order to quench the superradiance.
91(1989); http://dx.doi.org/10.1063/1.457175View Description Hide Description
We have employed direct two‐photon laser excitation of specific vibrational levels of N2(a 1 Π g ) to measure the lifetime of this state. Direct observation of emission from the a 1Π g –X 1∑+ g transition in a large cell was employed to follow fluorescence decays. Experiments were conducted to verify that the effects of collisional transfer and diffusion were not contributing to the observed lifetime. Our experiments showed that the radiative lifetime of vibrational levels 0–2 is 56±4 μs and is independent of vibrational level, within experimental error. The observed lifetimes are in good agreement with recently reported theoretical calculations.
91(1989); http://dx.doi.org/10.1063/1.457176View Description Hide Description
We show how the Kohn variational principle for the K matrix is free from anomalies when complex short‐range basis functions are used. Using only the real part of the K v matrix increases the accuracy of the calculation and guarantees the unitarity of the resulting S matrix.
91(1989); http://dx.doi.org/10.1063/1.457177View Description Hide Description
We have used a b i n i t i o methods to characterize the Ne–HF van der Waals complex. The interaction energy was determined using size consistent, correlated CEPA wave functions expanded in a Gaussian basis chosen to represent both intraatomic effects and the low order multipole moments and polarizabilities of Ne and HF. The calculated well depths are −65 cm− 1 for linear Ne–HF and −39 cm− 1 for linear Ne–FH, with an intervening saddle point at −27 cm− 1. The induction contribution to the energy is significantly greater for Ne–HF than for Ne–FH, but dispersion remains the dominant attraction over the region of interest. Converged variational and close‐coupling calculations using the a b i n i t i opotential surface reveal three bound levels of the Ne–HF stretch mode, and several metastable levels correlating asymptotically with rotationally excited HF( j=1). Though nearly degenerate, the lifetimes of the two metastable Π (body frame Λ=±1) bending levels differ markedly because of different rotational coupling strengths to the Σ (body frame Λ=0) bending state, which undergoes rapid rotational predissociation. From the calculated line positions, widths, and intensities we have synthesized far infrared and infrared spectra of Ne–HF and Ne–DF.
Slit jet infrared spectroscopy of NeHF complexes: Internal rotor and J‐dependent predissociation dynamics91(1989); http://dx.doi.org/10.1063/1.457178View Description Hide Description
Direct absorption tunable difference frequency IR spectroscopy in a slit jet supersonic expansion has been used to observe complexes of Ne with HF for the first time. Spectra of both the weak HF stretch fundamental (1000)←(0000) and the 10–20 fold more intense bend and stretch combination band (111e,f 0)←(0000) transitions are observed, and illustrate several interesting dynamical features. The large ratio of combination band to fundamental intensity is evidence for a highly isotropic potential with respect to HF rotation. The HF bend vibration is thus better thought of as nearly free internal rotor motion with a nearly good space fixed quantum number, j HF =1. Weak anisotropy in the potential permits the j HF=1 (Π e bend) levels to relax intramolecularly to j HF =0 (Σ) levels, leading to predissociative line broadening in the sub‐Doppler slit jet spectra. This observed dissociation of NeHF with 44 cm− 1 of internal excitation provides a rigorous upper limit to the binding energy. The Π f bend levels, on the other hand, have no lower energy internal rotor states of the correct parity to relax into, and thus the (111f 0) ←(0000) spectra exhibit linewidths limited by the apparatus resolution. The internal relaxation of the j HF=1 (Π e bend) levels results from Coriolis mixing with the Σbend states (1200) by overall rotation of the NeHF complex, and thus the lifetimes depend dramatically on J. The J‐dependent predissociation rates observed are in remarkably quantitative agreement with quantum scattering calculations on the CEPA a b i n i t i o surface of ONeil and co‐workers.
91(1989); http://dx.doi.org/10.1063/1.457125View Description Hide Description
Polarized resonance Raman measurements of highly oriented films of c i s polyacetylene have been carried out. The intensities of the three Raman fundamentals in the four different scattering configurations have been measured and corrected for the strong anisotropy of the optical properties. The experimental data are indicative of an almost perfect alignment of the c i s polyenic chains along the stretching direction. Preliminary measurements of the depolarization ratios carried out on isotropic samples in the resonance region appear to be inconsistent with the view that the visible band is dominated by a single transition polarized along the chain axis.
Conformational stability and barriers to internal rotation of 2‐methylpropanal by far infrared and microwave spectroscopy91(1989); http://dx.doi.org/10.1063/1.457126View Description Hide Description
From the far infrared spectrum of 2‐methylpropanal (isobutyraldehyde), (CH3)2CHCHO, in the gaseous state, the asymmetric torsion for the g a u c h e conformer was observed as a series of Q branches at 75.0, 80.9, 85.8, and 90.3 cm− 1 with similar transitions observed for (CD3)2CDCHO. Also from an investigation of the Raman spectrum of the gas a transition at ∼110 cm− 1 has been tentatively assigned as the ‘‘double jump’’ of the asymmetric torsion for the t r a n s conformer. The microwave spectrum of the excited vibrational states for the light molecule has been investigated in the region from 12.5 to 40 GHz and, from relative intensity measurements, the frequencies of the asymmetric torsions were determined to be 72±7 and 50±20 cm− 1 for the g a u c h e and t r a n s rotamers, respectively. Additionally, ‘‘tunneling’’ splitting of 2.0±0.4 and 60.0±0.3 MHz has been observed for the second and third excited states, respectively, of the asymmetric torsion of the g a u c h e conformer. From these data the asymmetric potential function has been calculated and the following potential constants have been evaluated: V 1=−131±35, V 2=−225±21, V 3=555±10, V 4=−70±5, V 5=95±5, and V 6=−140±10 cm− 1 where the 1←0 transition of the g a u c h e has been assigned at 75.0 cm− 1. This potential function leads to an enthalpy difference of 250±66 cm− 1 (715±189 cal/mol) which is in good agreement with the value of 248±35 cm− 1 determined from variable temperature studies of the Raman spectrum of the gas. Analysis of the transitions observed between 250 and 150 cm− 1 in the far infrared spectrum of the gas indicated that the two methyl rotors are coupled for the g a u c h e conformer and the barriers to internal rotation of the methyl tops have been calculated to be 987 cm− 1 (2.82 kcal/mol) and 1132 cm− 1 (3.24 kcal/mol).
Stretch–bend coupling between van der Waals modes in the S 1 state of substituted benzene–Ar1 complexes91(1989); http://dx.doi.org/10.1063/1.457127View Description Hide Description
The van der Waals vibrations of aniline–, phenol–, fluorobenzene–, and chlorobenzene–Ar1 complexes have been measured using one‐color resonance enhanced multiphoton ionizationspectroscopy, together with time‐of‐flight mass spectrometry, in a skimmed supersonic molecular beam. A delayed ionization extraction technique is used to suppress contributions to the spectra from dissociating complexes. The S 1–S 0 electronic origins for the van der Waals complexes are found to be shifted towards lower energy (red shift) relative to the parent molecule electronic origin for all the Ar1 complexes. The red shifts increase in magnitude in the order: fluorobenzene, chlorobenzene, phenol, aniline. Progressions, overtones and combination transitions involving the low frequency van der Waals vibrations, i.e., the symmetric bend (b x ), the asymmetric bend (b y ) and the stretch (s z ) are observed clearly in the S 1←S 0 excitation spectra. Intensity profiles are found to deviate substantially from those expected on the basis of harmonic Franck–Condon factors. A model involving stretch–bend anharmonic coupling via cubic terms in the vibrational potential is found to account for the observed spectral features and intensity anomalies.
91(1989); http://dx.doi.org/10.1063/1.457128View Description Hide Description
Laser‐induced fluorescence excitation spectra have been recorded for many vibronic transitions of the A(2Π)–X(2Π) and B(2Σ+)–X(2Π) systems of NCS under supersonic free jet expansion conditions. New assignments have been made for many of these bands and several of the assignments from previous work have been revised. Vibronic energies have been determined for levels involving excitation of all three vibrational modes in the ground electronic state and both excited electronic states. A detailed rotational and vibrational analysis has been carried out for levels involving excitations of the two stretching modes and the many rotational and potential function constants have been determined for the A and X states. The variation of the spin–orbit constant with vibrational level has also been investigated. Results of this paper provide the ground work for a detailed analysis of the Renner–Teller effect in the bending vibrational mode of this radical for both the X 2(Π) and A(2Π) states to appear in a forthcoming paper. They also provide a spectroscopic database for future dynamical and kinetics studies of processes involving this radical.
General phase modulation method for stored waveform inverse Fourier transform excitation for Fourier transform ion cyclotron resonance mass spectrometry91(1989); http://dx.doi.org/10.1063/1.457129View Description Hide Description
A general phase modulation algorithm has been developed for the stored waveform inverse Fourier transform (SWIFT) excitation method used in Fourier transformion cyclotron resonance mass spectrometry (FT‐ICR or FTMS). The algorithm, based on the time shifting theorem and the uncertainty principle, shows that the quadratic phase modulation is the theoretically optimal method for square magnitude spectral profiles. For more complicated magnitude spectral profiles, the corresponding phase functions can be generated through the algorithm by using a nonlinear grid on the frequency domain. The degree of dynamic range reduction can be estimated from a simple equation.
91(1989); http://dx.doi.org/10.1063/1.457130View Description Hide Description
Cyanoacetylene underwent polymerization reaction in a solid phase at pressures above 1.5 GPa. The Raman study of the reaction product showed that the polymer had a conjugated linear backbone with CN pendant groups. The Raman spectra for this substituted polyacetylene demonstrated a resonance behavior similar to that reported for t r a n s‐polyacetylene. The optical gap associated with the π–π* transition in the conjugated system was smaller than that of t r a n s‐polyacetylene, probably due to the resonance interactions between the CN triple bonds and the conjugated double bonds.
91(1989); http://dx.doi.org/10.1063/1.457131View Description Hide Description
The infrared spectrum of gaseous CH3I has been investigated in the range 500–16 500 cm− 1, up to six quanta of excitation in CH stretching. A total of 16 perpendicular bands lying above 3700 cm− 1 have been analyzed, some as Fermi resonance diads, in order to determine accurate vibrational wave numbers for the constituent levels. The local mode model has been applied to the CH stretching manifold to account for anharmonicity and the effects of Darling–Dennison resonances. Vibrations not involving CH stretching have been treated in a normal mode basis. The joint approach, which also takes into account known Fermi resonances, enables a complete set of 27 vibrational anharmonicity parameters to be determined for CH3I which fits 63 observed vibrational levels up to V=6 with a root‐mean square (rms) error of 2.75 cm− 1.
91(1989); http://dx.doi.org/10.1063/1.457132View Description Hide Description
An accurate and computationally tractable theoretical procedure for the calculation of the nonresonant, electronic components of the third‐order molecular polarizabilities, γ(0;0,0,0), γ(−3ω;ω,ω,ω), and γ(−2ω;ω,ω,0), can be constructed. This procedure partitions γ into a σ‐electron component (γσ) and a π‐electron component (γπ). The γσ term is evaluated using the bond‐additivity approximation; the γπ term is calculated using the semiempirical INDO all‐valence‐electron molecular orbital method combined with full single‐ and double‐excitation configuration interaction (SDCI) of singlet π‐electron configurations, and Orr and Ward’s sum‐over‐states expression for γ. The INDO‐SDCI method is also used to calculate one‐ and two‐photon spectroscopicproperties of the 1ππ* states salient to γπ for the molecules of interest. It is shown that single‐excitation CI alone is not sufficient for the calculation of γπ for linear polyenes and benzene. Calculations of the effect of chain length and conformation on the values of γ for ethylene, c i s and t r a n s linear polyenes, and benzene indicate that γ is strongly influenced by conjugation chain length. A simple relationship can be established between the calculated value of γπ(0;0,0,0) for the t r a n s linear polyenes investigated and that for ethylene, the molecule with the solitary π‐electron C–C bond: γπ(0;0,0,0)≂γπ(0;0,0,0)ethylene N C–C 3, where N C–C=1,3,5,7, and N C–C is the total number of C=C and C–C bonds in the given polyene, i.e., the length of the π‐bonding network. As γ increases with chain length, so does the ratio γπ/γ. Virtual electronic transitions involving excited π‐electron states with extensive charge separation and double excited configurational character are important contributors to γπ for the linear polyenes and benzene. An approximation of γπ(0;0,0,0) for the linear polyenes can be written in terms of the linear π‐electron polarizabilities for the ground state and 1 1 B u π‐electron excited state. Although this
approximation is strictly applicable to the centrosymmetric linear polyenes, it does s u g g e s t a very interesting criterion for the selection of organic molecules with large third‐order polarizabilities. Namely, the change in polarizability between the ground state and a strongly one‐photon absorbing excited state is an important factor to consider when selecting candidate molecules.