Showing papers on "Fermi resonance published in 1998"

Vibrational Energy Redistribution in Polyatomic Liquids: Ultrafast IR−Raman Spectroscopy of Acetonitrile

Abstract: Ultrafast anti-Stokes Raman scattering after intense mid-IR excitation is used to study vibrational energy relaxation (VER) and vibrational cooling (VC) in neat liquid acetonitrile. The mid-IR pulse (3000 cm-1) excites a combination of C−H stretching fundamentals and C−H bending overtones, which are coupled by Fermi resonance. Vibrational excitation decays from the pumped C−H stretch in 5 ps. Almost no energy is transferred from C−H stretch to C⋮N stretch (2253 cm-1). The C⋮N stretch behaves as a VER “blocking group” which keeps vibrational energy in the CH3−C moiety. A 5 ps buildup, which mirrors the C−H stretch decay, is seen in the C−H bending modes (∼1500 cm-1) at about one-half the C−H stretch energy, and in the lowest energy vibration at 379 cm-1, a C−C⋮N bend. By diluting the acetonitrile with CCl4, it is shown that the buildup of population in the C−C⋮N bend mirrors the buildup of excitation of the bath. Monitoring the C−C⋮N bend allows us to track the instantaneous fraction of energy which has be...

IR spectral density of weak H-bonded complexes involving damped Fermi resonances. I. Quantum theory

Abstract: The IR spectral density of the high frequency stretching mode of weak H-bonded complexes involving Fermi resonances is studied within the linear response theory from a full quantum mechanical point of view: the anharmonic coupling between the high frequency X–H and the low frequency X–H⋯Y modes is treated inside the strong anharmonic coupling theory. Following Witkowski and Wojcik [A. Witkowski, M. Wojcik, Chem. Phys. 1 (1973) 9.], the Fermi resonance between the first excited state of the fast mode and the first harmonic of single or several bending modes is introduced. Besides, the direct relaxation involved by the fast and bending modes are incorporated, in the spirit of the reduced Green formalism, by aid of imaginary damping terms. The spectral density is obtained by the Fourier transform of the autocorrelation function of the dipole moment operator of the fast mode, in which time dependent terms appear that are solution of a set of coupled linear differential equations. It reduces in the special situation where the Fermi coupling is ignored to that obtained by Rosch and Ratner [N. Rosch, M. Ratner, J. Chem. Phys. 61 (1974) 3344.]. Furthermore, when the anharmonic coupling between the slow and fast modes is neglected, it reduces to the spectral density that may be obtained in the framework of the Giry et al. [M. Giry, B. Boulil, O. Henri-Rousseau, C.R. Acad. Sci. Paris 316 s.II (1993) 455; B. Boulil, M. Giry, O. Henri-Rousseau, Phys. status solidi (b) 158 (1990) 629.] approach. At last, it reduces to the Witkowski and Wojcik [A. Witkowski, M. Wojcik, Chem. Phys. 1 (1973) 9.] approach, when the relaxation disappears. A generalization to several Fermi resonances is also proposed. Numerical tests of the theory and physical discussions are reported in the following paper [D. Chamma, O. Henri-Rousseau, Chem. Phys. 229 (1998) 51].

Effect of guest–host interaction on Raman spectrum of a CO2 clathrate hydrate single crystal

Abstract: The polarized Raman spectra of an artificial CO2 clathrate hydrate single crystal have been measured in order to examine the crystal-orientation dependence of the Raman spectra. Since the crystal had crystallographic facets, the orientation of the crystal was determined by using the Miller indices of the facets. When the angle θ between the polarization plane of the incident laser beam and the direction of one of the 〈110〉 axes of the single crystal varied, it was observed that the intensities of the peaks, which were caused by the Fermi resonance of the symmetric stretching mode and the overtone of the bending mode of CO2, and the O–H symmetric stretching vibration mode, varied with θ. Since the tetrakaidecahedron cage in the CO2 clathrate hydrate is distorted along the 〈100〉 axis, the variations of the scattering intensities of the CO2 have been calculated by using a simple model that assumes that the CO2 rotates on the {100} plane in the tetrakaidecahedron cage. The results obtained from the experiment...

Infrared spectroscopy and ab initio potential energy surface for Ne–C2H2 and Ne–C2HD complexes

Abstract: The rotationally resolved spectra of Ne–C2H2 and Ne–C2HD were measured in the region of the asymmetric C–H stretch (ν3) band of the acetylene monomer. The transitions in the Ne–C2H2 spectrum are substantially broadened by vibrational predissociation, while those of Ne–C2HD are quite narrow. This difference is attributed to the fact that in the former dissociation proceeds through a “doorway” state, related to a Fermi resonance involving the bending vibrations of C2H2. In C2HD this Fermi resonance is absent. The potential energy surface (PES) for the Ne–acetylene complex has been computed using symmetry-adapted perturbation theory. This PES has been fit to an analytic form and applied in calculations of the rovibrational energy levels of Ne–C2H2 and Ne–C2HD. From these levels and calculated transition intensities we generated the near-infrared spectra of these complexes in the region of the ν3 band. These complexes may be considered as nearly free internal rotors. For Ne–C2H2 the results obtained from the ...

Fermi resonance distortion of the Ru-CO stretching mode of CO adsorbed on Ru(001)

Abstract: A Fermi resonance of the Ru–CO stretching mode and the combination band of the hindered rotation+hindered translation mode of adsorbed CO on Ru(001) has been detected using infrared absorption spectroscopy. The Fermi resonance has been unambiguously identified through isotopic substitution experiments at various CO coverages. Clear evidence for a delocalized nature of the Fermi resonance is found.

Pressure Dependent Vibrational Fermi Resonance in Liquid CH3OH and CH2Cl2

Abstract: The vibrational Fermi resonance of two liquids, methanol (CH3OH) and dichloromethane (CH2Cl2), is investigated by measuring changes in the position and intensity of Fermi-coupled Raman bands as a function of pressure, in a diamond anvil cell. The Fermi resonance of interest occurs in the 2900 cm-1 spectral region, where coupling between the CH symmetric stretch fundamental and a CH bend overtone gives rise to two prominent bands. The methanol results reveal a pressure induced transition through exact resonance at 1.25 GPa, where the two coupled states decompose into a pair of fully mixed hybrid bands. In dichloromethane, on the other hand, the two coupled states are driven farther apart and become less mixed with increasing pressure. The Fermi resonance coupling coefficient, W, is found to be constant in each liquid up to pressures exceeding 1 GPa (W ≈ 52.6 and 22.3 cm-1 in CH3OH and CH2Cl2, respectively). The anharmonic shift of the CH bend is about 10 cm-1 in both liquids, determined by comparing the fr...

A spectroscopic study of CaOCH3 using the pump/probe microwave and the molecular beam/optical Stark techniques

Abstract: The Stark effect on the qR22(0,0.5) (ν=17 682.9251 cm−1) and qP11(0,1.5) (ν=17 682.1966 cm−1) branch features of the (0,0) B 2A1–X 2A1 band system of calcium methoxide, CaOCH3, was measured and analyzed to give the magnitude of the permanent electronic dipole moments, |μ|, of 1.58(8) D and 1.21(5) D for the X 2A1 and B 2A1 states, respectively. The dipole moments are compared with other monovalent calcium compounds and those predicted from a simple electrostatic model. Pure rotational transitions in the X 2A1 state were recorded using the pump/probe microwave-optical double resonance technique. The proton magnetic hyperfine splitting pattern confirms a C3v symmetry of the ground electronic state. The determined Fermi contact and dipolar parameters are: aF=−0.421(27) MHz; Taa=1.070(45) MHz, and |Tbb−Tcc|=0.292(47) MHz. The latter parameter is associated with the lifting of the K-degeneracy of this symmetric top molecule and mixing the I0=1/2 and I0=3/2 levels.

Analysis of the highly excited vibrational dynamics of HCP using a high-order Fermi resonance Hamiltonian

Abstract: Based on an ab initio potential energy surface, the features of the quantum spectrum of HCP have been recently discussed in terms of the periodic orbits of the exact classical Hamiltonian [J. Chem. Phys. 107, 9818 (1997)]. In particular, it was shown that the abrupt change in the bending character of the states at the lower end of the Fermi polyads, at about 15 000 cm−1 above the origin, can be ascribed to a classical saddle node bifurcation. The purpose of the present article is to show that the use of a very accurate Fermi resonance Hamiltonian, which was derived very recently from high-order perturbation theory [J. Chem. Phys. 109, 2111 (1998)] can provide a still deeper insight into the highly excited vibrational motion. The principal advantages of the resonance Hamiltonian compared to the exact one rely on the remaining good quantum numbers and classical action integrals, which enable one to consider HCP as a formal one-dimensional system parametrized by the polyad number i and the number v3 of quant...

Dynamics of crystalline n-methylacetamide: temperature dependence of infrared and inelastic neutron scattering spectra

Abstract: N-methylacetamide may be viewed as a simple model for polypeptides. Unusual intensities have previously been observed in its amide vibrational spectra and were attributed to contradictory causes, namely, Fermi resonance, vibrational polarons, and dynamic proton transfer along the H bond. We report the first inelastic neutron scattering study of three specifically deuterated derivatives at different temperatures along with new infrared spectra. The analysis of the librations in the INS spectra indicates a degeneracy between the N−CH3 methyl torsional mode and another degree of freedom assigned to one component of the skeletal torsion. No direct evidence for a methyl−methyl coupling is observed. However γ(CO) appears strongly coupled to the motions of the neighboring C−CH3 group. A new absorption band (“X”) is observed at approximately 1590 cm-1 in the IR spectrum of the amide-1/amide-2 region. This calls into question the previous assignment of a band at ∼1600 cm-1 as the overtone of γ(NH). In addition, al...

Vibrational overtone spectroscopy and internal dynamics in gaseous nitromethane NO2CH2D

Abstract: The CH-stretching overtone spectra of the methyl group in gaseous nitromethane NO2CH2D have been recorded with conventional Fourier transform near-infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. All spectra exhibit a complex structure; they have been analyzed with a theoretical model which takes into account, within the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by Morse potentials, with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving methyl bending modes. Most of the parameters of this model, and their variation with the internal rotation coordinate (θ), are identical to ones used to account for the overtone spectra of the monohydrogenated and perhydrogenated species. Fermi resonance phenomena, also modeled with θ dependent parameters, lead to only weak intramolecular vibrational energy redistribution. This simple calculation s...

Vibrational spectroscopy, anharmonic resonances, and intramolecular vibrational redistribution in tetrafluoroiodoethane

Abstract: Vibrational spectra of tetrafluoroiodoethane (CF3CHFI) have been recorded from the far infrared (20cm-1) to the visible (14200cm-1) range at resolutions between 0.004cm-1 and 1.0 cm-1 by interferometric Fourier transform infrared and photoacoustic techniques. The band positions and integrated band strengths of all fundamentals, numerous combination bands, and overtone transitions are reported. Assignments of fundamentals are made by comparison with harmonic vibrational frequencies, calculated ab initio. The overtone transitions of the C-H chromophore are analysed by means of an effective model Hamiltonian which includes the C-H stretching-bending Fermi resonance effective coupling constants (k'saa = 56cm-1 and k'sbbb =43cm-1), the quartic Darling Dennison resonances (constants γ) and the chiral symmetry breaking coupling described by the anharmonic constant k'sab = 27 ± 10 cm-1. The femtosecond dynamics of highly excited vibrational states are derived from the spectroscopic results.

Dynamics of Fermi resonance solitary waves propagating along two interfaces

Abstract: The dynamics of Fermi resonance solitary waves propagating along two parallel interfaces in a layered organic semiconductor system is investigated both analytically and numerically. It is shown that the interaction between solitary waves leads to their attraction or repulsion, depending on their initial phase difference. In the case of attraction the solitary waves create a bound state, and their centers oscillate in time with respect to their common mass center. The corresponding period of oscillations is calculated. It is found that the amplitudes and widths of the solitary waves also oscillate in time. @S0163-1829~98!01604-X# The search for organic materials for nonlinear optics, photonics, and electronics promoted the development of methods for the preparation of a class of organic structures, namely, organic crystalline superlattices ~OCS!. The latest achievements in this field were demonstrated in a number of publications. 1‐5 At present, investigations in this direction are developing further, therefore the analysis of qualitatively new properties of OCS is very topical and important. The interaction of OCS with light is a fundamental physical problem, as well as of importance for future applications. Papers 6‐10 have been devoted to just such an analysis of these properties of OCS. In particular, different kinds of nonlinear excitations propagating through the superlattice have been discussed ~Fermi resonance interface modes, 7 Fermi resonance interface solitary waves 9,10 !. Here we want to consider the dynamics of two Fermi resonance solitary waves located on two different interfaces of a three-layer system. For convenience, instead of the term ‘‘solitary waves’’ in the following we use the shorter term ‘‘solitons,’’ as frequently done in the literature. These solitons interact with each other due to the penetration of the vibrational field of one of them into the location region of the other one. As we shall see, such ‘‘tunnel’’ coupling results in a considerable change of the dynamics of the solitons as compared to a single soliton. Let us consider a system consisting of three layers of organic semiconductors with two interfaces. We suppose that a film withN11 b-molecular layers lies between two ‘‘halfinfinite’’ crystals made of c molecules. The molecules are labeled as follows: sites ( nx ,n y ,nz<21) are occupied by c molecules, sites ( nx ,n y,0

Vibrational spectroscopic study of the thermal transition behavior in polyamide 11

Abstract: Three different sample preparations of polyamide 11, melt-crystallized, meta-cresol cast and trifluoroacetic acid cast, are examined in terms of variable temperature infrared analysis. Digital subtraction, second derivative spectroscopy, curve resolution and thermal transition behavior are used to distinguish between structural differences in the ordered and disordered regions. It is found that the N–H stretching region of the semicrystalline material containing α or α-like crystal forms displays a resolvable peak due to the amide I overtone in Fermi resonance with the N–H stretching fundamental. The N–H stretching region along with various other regions are used to follow the frequency and integrated area changes with increased temperature. The Brill and glass transitions which are detectable in frequency and area versus temperature plots are used to distinguish between the transition behavior in the crystal and amorphous regions of the three sample preparations. It is found that whereas a melt-crystallized film displays very obvious transition behavior, in the remaining two preparations the transitions are not detectable. The basis for the assignment of the frequency and area versus temperature response to specific transition is discussed.

Dynamic rheo-optical characterization of uniaxially oriented polyamide 11

Abstract: Dynamic mechanical analysis, coupled with polarized step-scan FTIR transmission spectroscopy, has been used to monitor the submolecular motional behavior of uniaxially oriented polyamide 11. The dynamic in-phase spectra depend upon the morphology of the samples as well as on the polarization direction of the infrared radiation. The lineshape features of the dynamic in-phase spectra and their relationship to sample deformation are analyzed on the basis of changes of the internal coordinates, the reorientation movement of several functional groups, and the thickness change of the film during the stretching cycle. Dynamic infrared spectra are helpful for deconvolution of overlapping bands on the basis of their different responses to the external perturbation, which sometimes cannot be resolved well by derivative spectroscopy or curve-fitting analysis. The lineshape features have been used to follow microstructural changes after isothermal heat treatment. Near the N-H stretching frequency, two bands at 3270 cm -1 and 3200 cm -1 are resolved and analyzed in terms of Fermi resonance between the N-H stretching fundamental mode and the overtone and combination modes of the amide I and II vibrations. The dynamic response of the N-H stretching mode correlates with the modulation of hydrogen bond strength in uniaxially oriented PA-11. After thermal treatment at the highest temperature (190°C), the dynamic response in this region is mainly caused by the modulation of crystals. In amide I region, three bands at 1680 cm -1 , 1648 cm -1 , and 1638 cm -1 are separated and assigned to hydrogen bond-free, hydrogen-bonded amorphous, and hydrogen-bonded crystalline regions, respectively. The dynamic responses of the hydrogen-bonded regions are more sensitive to external perturbation. Two components are found in the amide II region, and the band at 3080 cm -1 is assigned to the overtone resonance of the component with perpendicular polarization.

Resonance ionization spectroscopy of cyanogen

Abstract: Strong (1+2) resonance-enhanced multiphoton ionization (REMPI) spectra have been recorded for cyanogen resonantly excited by single photon absorption on vibronically allowed transitions to the A 1Σu− and B 1Δu electronic states at wavelengths from 205 to 220 nm. A previously unreported Fermi resonance between the ν2 and 4ν5 vibrational modes of the A state is observed for the rotationally resolved spectra of transitions that involve excitations of the ν2 mode. New measurements of vibrational and rotational spectroscopic constants are reported for the A and B electronic states. A detection limit of 120 ppb measured for (1+2) REMPI of cyanogen is an order of magnitude larger than typical (2+1) REMPI detection limits for other aliphatic compounds.

First-principles calculation of hydrogen vibrations of the H-P complex in silicon

Abstract: We present a theoretical investigation of the local vibrational modes of hydrogen in the H-P complex in silicon. The energy surface of the hydrogen is calculated by ab initio methods, fitted to a model potential, and the vibrational spectrum is investigated by perturbation theory. A Fermi resonance is found between the second hydrogen-wagging and fundamental stretching modes. The calculated vibrational frequencies compare very well to the experimental results, and confirm a recent assignment of the hydrogen vibrational bands of donor-hydrogen complexes in Si.