A general local-mode theory for high energy polyatomic overtone spectra and application to dichloromethane

A new model is developed to describe the overtone spectra of polyatomic molecules containing XH_n moieties. The infrared absorption is described as corresponding to excitation of localized modes of vibration rather than a set of symmetry allowed combinations of anharmonic normal modes. The possibility of assigning new bands in the more complex regions of the overtone spectra which involve combinations of both stretching and bending modes is examined and illustrated using the overtone spectrum of ammonia. The overtone spectrum of dichloromethane is recorded in the range λ = 9260 Å to λ = 5300 Å, and the CH overtone bands at 8850 Å, 7223 Å, and 6165 Å corresponding to the Δv_CH = 4, 5, and 6 overtones are identified. Specific features of these bands are discussed in detail in relation to the model. The common features and simplicity of the Δv_CH = 4 overtone bands of a series of halomethanes and ethanes are also interpreted in terms of the model.     

CH local mode overtone excitations in benzyl chloride—A conformational study

The CH local mode overtone spectrum of benzyl chloride in the visible and NIR regions studied by laser induced thermal lens and conventional NIR absorption is presented. The analysis shows that the −CH₂Cl group is symmetrically oriented with respect to the benzene ring, thus finalizing one of the two possible conformational models predicted by electron diffraction studies. The aryl CH bonds have a slightly larger force constant than that in benzene.     

Multiple inelastic electron scattering processes in thin molecular films: a simple model

Electron energy loss spectra of multilayer molecular films contain bands resulting from inelastic collisions at two or more sites within the film. The contributions of twofold loss processes are analysed for selected vibrational spectra of benzene and SF₆ using a simple model that was suggested previously and is extended here. It is shown that this analysis can be used not only to detect dipole scattering contributions to particular vibrational excitations. Combined with an estimate of the twofold loss spectrum based on the pattern of the fundamental bands the model also enables the identification of overtone and combination bands which is important for the detection of resonant processes.     

Reverse polarized highly intense Raman transitions in Jahn-Teller and non-Jahn-Teller modes

Intensity enhancement effects in vibrational Raman spectra of electronically degenerate states are investigated. It is shown that the fundamental and overtone Raman transitions in Jahn-Teller active modes are strongly intensified via enhancement effects involving the symmetric and anti-symmetric Raman polarizability tensors. Such enhancements ensue via intensity borrowing from the Rayleigh scattering process. It is explicitly shown that the isotropic Raman polarizability tensor associated with these transitions is not subjected to enhancement effects. Thus the intense fundamental and overtone Raman transitions in the Jahn-Teller active modes will be reverse polarized (or depolarized) rather than polarized. Moreover, fundamental and overtone Raman transitions associated with vibrations which couple with Jahn-Teller active modes via mixed cross quadratic nuclear potential interactions may also be intensified and reverse polarized. The present work modifies and extends the selection rules which were established long ago by M. S. Child and H. C. Longuet-Higgins [Philos. Trans. R. Soc. London A 254, 259–294 (1961)].     

Raman spectra of the orthorhombic semiconductor compound Cu2SnTe3

The optical phonon spectrum of the semiconductor Cu₂SnTe₃, that crystallizes in the orthorhombic structure with space group Imm2 (), have been studied by measuring unpolarized Raman scattering between 10 and 300 K. The experimental frequencies of the phonon modes observed were compared to those calculated by using simplified lattice dynamical models reported in the literature. From combined analysis of these results together with the factor group analysis of the zone-center vibrational modes, valuable information about these modes was obtained and their possible symmetry was assigned. A₁ modes at 71, 123, 167, 176 and 190 cm⁻¹; A₂ modes 115 and 131 cm⁻¹; B₁ modes at 76, 142 and 152 cm⁻¹; B₂ modes at 89, 100 and 206 cm⁻¹; a overtone at 246 cm⁻¹, and combinations at 218, 270 and 292 cm⁻¹; have been observed in this compound.     

Compact overtone band carbon monoxide laser

An electric-discharge carbon monoxide broadband overtone laser has been developed. The laser is capable of lasing on CO first overtone bands only (Δv = 2) and delivering up to 12 W cw power at efficiencies of up to 5%. The laser output consists of vibrational-rotational lines of the v = 9 → 7 to v = 35 → 33 overtone bands and covers the spectral range between 2.6 and 3.9 μm. Totally about 40-45 lines could be observed in the laser spectrum simultaneously. A kinetic modeling code has been developed to predict the power, efficiency, and the output spectrum of the CO overtone laser.     

The infrared emission spectrum of 14N16O in the overtone region and determination of Dunham coefficients for the ground state

Five bands of the 2-0 overtone sequence of the vibration-rotation spectrum of nitric oxide have been observed in emission. The results of the analysis have been combined with other vibration-rotation data and with some ultraviolet data to obtain ground electronic state constants of vibration-rotation levels with values of v up to 16.     

Overtone spectra of styrene and polystyrene in the visible and near infrared regions

Vibrational overtone spectra of styrene (liquid) and polystyrene (solid), studied by the laser-induced thermal lens (for ΔV=6) and the conventional near infrared absorption (for ΔV=3–5) techniques, are reported. For polystyrene, the overtone energy-bond length correlation predicts that the aryl CH bonds are ～0.0005 Å longer than that in benzene, while no such conclusions could be drawn on styrene. Thesp ³ CH overtones in polystyrene are observed on the low energy side of the aryl CH overtones.     

Ultrafast redistribution of vibrational excitation of CH-stretching modes probed via anti-Stokes Raman scattering

Resonant infrared femtosecond light pulses in the 3 μm region are used to excite specific CH-stretching modes of liquid 1,1,1-trichloroethane, cyclopentane, and cyclohexane. The ultrafast redistribution of vibrational excitation between different CH-stretching modes and the vibrational relaxation of these modes are monitored by spontaneous anti-Stokes Raman scattering of properly delayed and spectrally shaped probe pulses. The results are treated with a rate-equation system, yielding energy-redistribution and energy-relaxation times. Received: 16 December 1999 / Published online: 7 August 2000     

Near-infrared spectra of hydrogen-bonded cyclic dimers of some carboxylic acids

Near-infrared spectra in the 3900–11000 cm⁻¹ region were measured for formic and acetic acids and also for their deutero analogs in the gaseous state at various temperatures. Observed monomer and dimer bands were assigned to overtone and summation bands of various vibrational modes. Special attention was paid to the first overtone bands of the OH and OD stretching modes of the hydrogen-bonded cyclic dimers, which were observed at ∼5930 cm⁻¹ and ∼5870 cm⁻¹ for formic-d acid and acetic-d₃ acid, respectively, and also at ∼4370 cm⁻¹ for acetic acid-d. On the basis of these data the anharmonicity constants were calculated for the OH and OD stretching modes and were found to increase by hydrogen-bond formation in the gaseous state. Hydrogen-bonded overtone bands are broad and have submaxima with intervals nearly half of those of the OH and OD fundamental bands. The results concerning the band breadths of the OH and OD stretching vibrations of the dimers were interpreted on the line of the idea that they strongly couple with the O?O stretching vibration.     

Fine structure in the proton magnetic resonance spectrum of solid benzene and solid benzene-cyclohexane mixtures

The PMR spectrum of solid benzene in the vicinity of the melting point consists of broad and narrow lines. New narrow lines in the spectrum of benzene containing some impurity appear at the top of the broad line. Intensities of narrow lines increase with rising of temperature. The temperature dependence of line width and relaxation timesT ₁ andT ₂ have been measured in pure solid benzene an benzene-cyclohexane mixtures. It is assumed that narrow lines in spectra of pure benzene and its mixtures are caused by appearance of a phase with mobile molecules located in the crystallite joint or on its surface. The spectrum of imperfect monocrystalline benzene have been studied also. The narrow line of this sample is split into several narrow components. This is explained by demagnetization fields the distribution of which has a discrete character,     

Critical-point analysis of the two-phonon Raman spectrum of silicon

Two-phonon Raman spectrum of silicon has been measured in back-scattering configuration at room temperature using Argon ion laser. It reveals many critical points and consists mainly of two-phonon overtone states. The agreement between the calculated density of states from neutron diffraction data and the present Raman spectrum is excellent except the region where combination spectrum appears. Polarization measurement shows that the structure centered at 300 cm^?1 is not the one-phonon fine structure, predicted by Cowley and observed in part and identified as such by Wright and Mooradian, but the two-phonon overtone spectrum of acoustic phonons. Critical-point analysis has been done on the two-phonon overtone and combination spectra, and phonon frequencies of points of high symmetry in the Brillouin zone are determined and compared with other experimental results. In contrast to the Raman spectrum of diamond, no peak was observed near twice the frequency of zone-center optical phonon. This fact is explained by the difference of phonon dispersion relation between diamond and silicon.     

Additional information from the measurements by Hushfar,Rogers and Stair of the infrared chemiluminescence from the reaction N+O2 → NO+O

Hushfar et al. measured the chemiluminescent first-overtone radiation at 2.7 μm from the reaction N+O₂ → NO+O, which is followed by the fast quenching reaction N+O → N₂+O. From the rate of overtone photons per O₂ molecule and the spectrum approximated by a Boltzmann distribution, they computed ?₂, the number of overtone photons per NO-forming reaction in the low-density limit when no quenching occurs. We show ?₂ is essentially independent of the spectrum, infer the limits on the fundamental-band photon efficiency, ?₁, and obtain the initial population distribution of NO(2 ? χ ? 6).     

Two-phonon absorption in InP and GaP

Far-infrared absorption in crystals of GaP and InP was investigated using a high-resolution Fourier-transform spectrometer. The two-phonon absorption was found to be very similar in these two materials. Van Hove singularities on the GaP spectrum were identified with the aid of calculated two-phonon sum and overtone density-of-states curves. The analysis of the InP spectrum was based on its homology to GaP. Many of the prominent features of the absorption spectra are assigned to pairs of phonons on the hexagonal face of the Brillouin zone while phonon pairs at Γ, X and L generally contribute only minor features to the spectra.     

Calculation of dipole moment functions with density functional theory: application to vibrational band intensities

We have calculated fundamental and overtone XH stretching vibrational band intensities for H₂O, benzene, cyclohexane, 1,3-butadiene, and HCN. The band intensities were calculated with a simple harmonically coupled anharmonic oscillator local mode model and a series expanded dipole moment function. The dipole moment functions were obtained from local, non-local and hybrid density functional theory calculations with basis sets ranging from 6–31G(d) to 6–311++G(3df,3pd). The calculated band intensities have been compared with intensities calculated with conventional ab initio methods and with experimental results. Compared with conventional correlated ab initio methods, a carefully chosen density functional method and basis set seems to give better fundamental and overtone intensities with far less resources used. We have found that the density functional methods appear to be less sensitive to the choice of basis set, with little difference between the results obtained with a non-local or hybrid density functional method.     

Infrared combination and difference bands of the NO dimer

The mid-infrared (1500-3800 cm⁻¹) absorption spectrum of gaseous nitric oxide has been studied at low temperature (99 K) with a long absorption path (160 m) in order to observe weak combination, difference, and overtone bands of the NO dimer. About ten new bands were assigned with greater or lesser certainty. Combined with previous results, they lead to a set of 12 secure and 7 tentative vibrational term values for (NO)₂, essentially doubling our knowledge of NO dimer vibrational states. The strongest non-fundamental bands in this region, other than the ν₁ (symmetric N-O stretch) + ν₅ (asymmetric N-O stretch) overtone, involve combinations of ν₅ with ν₃ (intermolecular stretch). Excitation of ν₅ results in increased frequencies for the intermolecular modes ν₂, ν₃, and ν₄. A new value of 155.5 cm⁻¹ was obtained for ν₄, the elusive infrared-inactive out-of-plane fundamental vibration.     

l-type resonance in an overtone band. The 2ν4 spectrum of nitrogen trifluoride

l-type resonance in an overtone band is discussed. It is shown that if the anharmonic constant g_ll is small the resonance effect may be as large as or even larger than, the corresponding fundamental. Intensity perturbation may be manifested by the appearance of forbidden lines. This analysis is applied to the high frequency portion of the 2ν₄ spectrum of NF₃ and band constants are obtained.     

Intensity measurements of the CH4 bands in the region 4350 Å to 10,600 Å

The CH₄ overtone bands from 4410 to 9990 Å, long known in the spectra of the major planets, were studied at room temperature with a long-path, high-pressure White cell. Band intensities and profiles were measured, and are more complete than other recent laboratory measurements of these bands. Vibrational assignments of these bands are suggested, which are compatible with the assignments of the overtone bands longward of 1μ; these assignments imply that all the bands studied are combinations increasingly dominated at shorter wavelengths by v₁, the symmetric stretch fundamental. In this self-consistent analysis, the weak band at 6825 Å is assigned as 2v₁+3v₃, providing further laboratory evidence that it is probably not the 5v₃ pure overtone that planetary astronomers had hoped it to be.