Showing papers on "Hot band published in 2000"

An introduction to the theory of local mode vibrations

Abstract: A description is given of highly excited rotational and/or vibrational states of molecules in terms of localized vibrations or local modes. It is a didactic paper, making an attempt to unify the ideas and notations of the key publications on the subject in a manner that treats the vibrational and rotational motions equally and that demonstrates the importance of molecular symmetry. Emphasis is put on explaining the intimate relationship between local mode vibrations and the formation of both vibrational and rovibrational energy level clusters. In rovibrational cluster states, a molecule can sustain localized vibrations for very many vibrational periods, even with low vibrational excitation. Local mode behaviour is induced by both vibrational and rotational excitation. An important application of local mode theory is the derivation of relations between the effective rotation-vibration parameters for highly excited vibrational states of a molecule. It is shown here, using the H2 80Se molecule as example, th...

Decoding the Dynamical Information Embedded in Highly Mixed Quantum States

Abstract: ▪ Abstract The standard description of the vibrational and rotational motion of polyatomic molecules, as expressed by the distortable rotor/harmonic oscillator approximation, provides an adequate description of the molecular quantum states only in regions of low total state density. When the total state density is large, exceeding 100 states/cm−1, the vibrational dynamics are “dissipative” and the fundamental process of intramolecular vibrational energy redistribution is operative. The presence of intramolecular vibrational energy redistribution leads to molecular quantum states of a qualitatively different nature. With respect to a normal-mode vibrational basis, these quantum states are “highly mixed” in their vibrational character and represent nuclear motion that is a combination of all the normal-mode motions. This review describes frequency domain spectroscopy techniques designed to probe the vibrational, rotational, and structural composition of these eigenstates. Recent work that investigates spect...

Ultrafast infrared–Raman studies of vibrational energy redistribution in polyatomic liquids

Abstract: Vibrational energy relaxation and vibrational cooling of polyatomic liquids were studied with the ultrafast infrared–Raman (IR–Raman) technique. In the IR–Raman technique, a type of two-dimensional vibrational spectroscopy, a vibrational transition is pumped with a mid-infrared pulse and the instantaneous populations of all Raman-active transitions are simultaneously probed via incoherent anti-Stokes Raman scattering of a time-delayed visible pulse. The theoretical framework for these measurements, including force–force correlation function methods and perturbative techniques, is reviewed. Experimental aspects of the IR–Raman technique are discussed, including laser instrumentation, experimental set-up, the nature of the pumping and probing processes, detection sensitivity and optical background, and the interpretation of results including spectroscopic artifacts. Then examples are provided from recent research by our group, focusing on timely problems such as the pseudo-vibrational cascade, the dynamics of doorway vibrations, dynamics of overtones with Fermi resonance, multiple vibrational excitations via combination band pumping and spectral evolution in associated liquids. Copyright © 2000 John Wiley & Sons, Ltd.

A theoretical study of P 4 O 6 : vibrational analysis and infrared and Raman spectra

Abstract: The normal mode frequencies and the corresponding vibrational assignments of tetraphosphorus hexoxide (P 4 O 6 ) in T d symmetry are examined theoretically using the gaussian 94 set of quantum chemistry codes at the HF/6-31G ∗ , MP2/6-31G ∗ and DFT/B3LYP/6-31G ∗ levels of theory. By comparison to experimental normal mode frequencies deduced by Chapman [A.C. Chapman, Spectrochim. Acta A 24 (1968) 1687–1696] correction factors for predominant vibrational motions are determined and compared. Normal modes were decomposed into three nonredundant motions {P–O–P wag, P–O–P bend, and P–O stretch}. Standard deviations found for the DFT and MP2 corrected frequencies compared to experiment are particularly noteworthy yielding values of 15 and 11 cm −1 , respectively.

Simulated structure, dynamics, and vibrational spectra of liquid benzene

Abstract: A classical molecular dynamics simulation of liquid benzene is performed, using a potential model which allows for full molecular flexibility. The short range intermolecular radial distribution function is on average reminiscent of the crystalline structure, although practically no preferential orientation can be found for the molecules in the first coordination shell. The average cage lifetime and its vibrational dynamics are obtained from appropriate time correlation functions. The intramolecular vibrations are investigated by calculating the vibrational density of states and the infrared and Raman spectra, achieving an excellent agreement with the experimental data. Finally, the dephasing of the ν1(A1g) ring breathing mode and of the ν6(E2g) in-plane bending mode is analyzed on the basis of the Kubo dephasing function. For ν1 mode the Kubo correlation time of 516 fs agrees with the experimental value, and is consistent with a relaxation mechanism involving the cage reorganization. In contrast, ν6 has a...

Tailoring of vibrational state populations with light-induced potentials in molecules

Abstract: We propose a method for achieving highly efficient transfer between the vibrational states in a diatomic molecule. The process is mediated by strong laser pulses and can be understood in terms of light-induced potentials and their vibrational eigenstates. In addition to describing a specific molecular system, our results show how, in general, one can manipulate the populations of the different quantum states in double-well systems.

Theoretical analysis of low-lying vibrational modes of free canonical 2-deoxyribonucleosides

Abstract: The low-lying vibrational mode frequencies of 2-deoxyribonucleosides (DNs) below 200 cm −1 were analyzed using the density functional theory. Three types of vibrational modes of DNs were observed: the motion of sugar and base with respect to each other as almost rigid fragments; the modes localized within the sugar or base fragments; and the collective vibrations involving deformation of both the sugar and the base. These modes clearly differ by order and frequencies. The population of the ground and excited vibrational levels was estimated. These values indicate that only 15–27% of the molecules possess a geometry close to equilibrium at every moment in time.

Ab initio calculation and spectroscopic analysis of the intramolecular vibrational redistribution in 1,1,1,2-tetrafluoroiodoethane CF3CHFI

Abstract: We report a new mechanism for intramolecular vibrational redistribution (IVR) in CF3CHFI which couples the CH chromophore vibrations through a strong Fermi resonance to the formal CF stretching normal mode (a heavy atom frame mode) involving the trans F-atom across the CC bond. The analysis is made possible by comparing spectroscopic results with extensive ab initio calculations of the vibrational fundamental and overtone spectra in the range extending to 12 000 cm−1. Potential energy and electric dipole moment hypersurfaces are calculated ab initio by second order Moller–Plesset perturbation theory (MP2) on a grid involving the CH stretching, two CH bending modes and one high frequency CF stretching normal mode. The potentials are scaled to obtain agreement between the experimental spectrum and the theoretical spectrum calculated by a discrete variable representation technique on this grid. Both spectra are then analyzed in terms of three-dimensional (3D) and four-dimensional (4D) effective vibrational H...

Nonlinear Two-Dimensional Vibrational Spectroscopy

Abstract: There is currently great interest in developing the vibrational analog to two-dimensional NMR spectroscopy. One approach to implementing two-dimensional vibrational spectroscopy is to use doubly vibrationally enhanced (DOVE) four-wave mixing (FWM). Non-linear signals occur because of correlations and mode coupling that are induced by interactions involving the driven modes. Since cross peaks do not occur between modes if interactions are absent, spectral congestion is removed and only the coupled modes remain. We describe the development of a two-dimensional doubly vibrationally enhanced four wave mixing method that extends the doubly resonant nonlinear spectroscopies to vibrational nonlinearities. We demonstrate the selective enhancements of coupled modes that are possible with the double resonances where the intensity of the enhancements reflects the strength of the interactions that are responsible for the vibrational mode coupling. We also demonstrate the capabilities for selectively enhancing specific sample components in an isotopic mixture. Since biological applications of DOVE require aqueous environments, we have examined the ability of DOVE methods to discriminate against the strong water absorption and have found that water has a small vibrational nonlinearity that allows DOVE of the solutes. Our results demonstrate the feasibility and features required to make DOVE methods practical for a wide range of scientific applications where identification of intra- and intermolecular interactions is important.

Vibrational analysis and excited-state geometric changes of betaine-30 derived from Raman and infrared spectra combined withab initio calculations

Abstract: We present a combined experimental and theoretical study of vibrational modes and excited-state geometry changes of betaine-30 (B-30). Infrared and Raman spectra were recorded under electronic off-resonant excitation conditions and also in resonance with the charge transfer transition of B-30. Comparing these spectra with the corresponding vibrational patterns calculated by Hartree–Fock methods we obtained the assignments of the vibrational modes. Excited-state geometry changes were calculated for a smaller model system of B-30 using configuration interaction with single excitations. The calculations predict that the central phenoxide and pyridinium rings move from a twisted conformation in the electronic ground state into a perpendicular position in the first excited state. In addition, the pyridinium ring tilts and the nitrogen atom becomes pyramidalized. In correspondence, we measured two strong low-wavenumber Raman bands with large origin shifts at 291 and 133 cm−1. They were assigned to N-inversion and torsional vibrational modes and are expected to mediate the excited state and back-electron transfer reaction of B-30. Copyright © 2000 John Wiley & Sons, Ltd.

Vibrational spectra and intramolecular vibrational redistribution in highly excited deuterobromochlorofluoromethane CDBrClF : experiment and theory

Abstract: The rovibrational spectra of deuterobromochlorofluoromethane (CDBrClF) were measured at intermediate (01 cm−1) and high resolution (00024 cm−1 full bandwidth, half-maximum) by interferometric Fourier transform infrared spectroscopy in the range from the far infrared at 200 cm−1 to the near infrared (12 000 cm−1) covering all the fundamentals and CD stretching overtones up to polyad N=5 The spectra are completely analyzed in terms of their vibrational assignments to fundamentals, combinations and overtones At high excitation the analysis reveals the dominant anharmonic coupling between four high frequency vibrational modes; the CD stretching (ν1), two CD bending (ν2,ν3), and the CF stretching mode (ν4) The analysis is carried out using effective model Hamiltonians including three and four vibrational degrees of freedom We also present vibrational variational calculations on a grid in a four-dimensional normal coordinate subspace The potential energy and the dipole moment function are calculated ab i

Nitric oxide excited under auroral conditions: Excited state densities and band emissions

Abstract: Electron impact excitation of vibrational levels in the ground electronic state and nine excited electronic states in NO has been simulated for an IBC II aurora (i.e., ∼10 kR in 3914 A radiation) in order to predict NO excited state number densities and band emission intensities. New integral electron impact excitation cross sections for NO were combined with a measured IBC II auroral secondary electron distribution, and the vibrational populations of 10 NO electronic states were determined under conditions of statistical equilibrium. This model predicts an extended vibrational distribution in the NO ground electronic state produced by radiative cascade from the seven higher-lying doublet excited electronic states populated by electron impact. In addition to significant energy storage in vibrational excitation of the ground electronic state, both the a 4Π and L2 Φ excited electronic states are predicted to have relatively high number densities because they are only weakly connected to lower electronic states by radiative decay. Fundamental mode radiative transitions involving the lowest nine excited vibrational levels in the ground electronic state are predicted to produce infrared (IR) radiation from 5.33 to 6.05 μm with greater intensity than any single NO electronic emission band. Fundamental mode radiative transitions within the a 4Π electronic state, in the 10.08–11.37 μm region, are predicted to have IR intensities comparable to individual electronic emission bands in the Heath and e band systems. Results from this model quantitatively predict the vibrational quantum number dependence of the NO IR measurements of Espy et al. [1988].

Mode-specific vibrational excitation and energy redistribution after ultrafast intramolecular electron transfer

Abstract: Vibrational relaxation in the electronic ground state initiated by intramolecular back-electron transfer (b-ET) of betaine-30 (B-30) is studied by picosecond time-resolved anti-Stokes Raman spectroscopy. Measurements were carried out with B-30 dissolved in slowly as well as in rapidly relaxing solvents. We observed a risetime of the Raman band with the highest frequency near 1600 cm−1 which is close to the b-ET time τb-ET of B-30. For B-30 dissolved in propylene carbonate (τb-ET∼1 ps), the population of this mode exhibits a rise time of 1 ps whereas vibrational populations between 400 and 1400 cm−1 increase substantially slower. In contrast, in glycerol triacetin (τb-ET∼3.5 ps) and in ethanol (τb-ET∼6 ps) rise times of all modes are close to the respective b-ET times. Within the first few picoseconds, direct vibrational excitation through b-ET is favored for modes with the highest frequencies and high Franck–Condon factors. Later on, indirect channels of population due to vibrational energy redistribution (IVR) become effective. Thermal equilibrium populations of the Raman active modes are established within 10 to 15 ps after optical excitation.

The vibrational structure of benzene adsorbed on Si(001)

Abstract: High resolution electron energy loss spectroscopy (HREELS) measurements and density functional model cluster calculations are presented to clarify the vibrational structure of the adsorption system C6H6/Si(001). All vibrational modes of the adsorption complex, which previously was identified to exhibit a cyclohexadiene-like structure, have been calculated and characterized according to the motion of the different atoms of the adsorption complex. Special emphasis is placed on the low-frequency modes. The coupling between the adsorbate and the substrate modes is analyzed with the help of a model that represents various limiting situations. Different coupling variants are found to apply to different collective modes of the adsorbate. The A1 and B1 modes can be described rather well by a model that only encompasses the adsorbate and the Si dimer underneath; for the A2 and B2 modes a frozen substrate description of the adsorption complex is more appropriate.

Redistribution and Relaxation of Vibrational Excitation of CH-Stretching Modes in 1,1-Dichloroethylene and 1,1,1-Trichloroethane

Abstract: Anti-Stokes Raman scattering after resonant infrared excitation with femtosecond light pulses was used to monitor the vibrational excitation of the CH-stretching modes of liquid CH2CCl2 and CH3CCl3. Selective excitation and probing allowed determination of the various time-constants for the redistribution between the CH-stretching modes and their relaxation. For the symmetric CH-stretching mode of CH2CCl2 relaxation times of 3.5 and 9.5 ps are determined for the transfer to overtones and to the asymmetric CH-stretching mode, respectively. For CH3CCl3 these time constants are 5.0 and 4.7 ps.

Lowest Excited Triplet State of Naphthalene by Transient Polarized Resonance Raman, Matrix-Isolation Infrared, and Density-Functional-Theory Methods

Abstract: Transient polarized resonance Raman spectra of naphthalene and its fully deuterated species in the lowest excited triplet state were measured. An analysis of the depolarization ratios showed that all the observed bands were assignable to the ag vibrational modes. Matrix-isolation infrared spectra of the excited state [J. Mol. Struct. 1999, 475, 253] were remeasured to observe further bands with weak intensities. The DFT (density functional theory) calculation was also performed to confirm the vibrational assignments, where the 6-31G* basis set was used to optimize the geometrical structure. The DFT calculation is found to be useful to predict the vibrational wavenumbers of the lowest excited triplet state as well as the ground singlet state of this molecule.

Calculation of the two-dimensional vibrational response function

Abstract: We present a complete analytical expression of the two-dimensional (2D) vibrational response function without invoking the factorization approximations based on the Wick’s theorem that have been used in most previously reported theories. Since the harmonic approximation to the vibrational degrees of freedom is not required in this new formulation, the vibrational-level dependencies of the transition frequencies and the vibrational relaxation rates are fully incorporated in the obtained formula for the 2D vibrational response function. Furthermore, the non-Markovian nature of the vibrational dephasing process in condensed phases is also fully taken into account by carrying out the resummation of the associated diagrams with the linked diagram theory. It is found that there exists an additional contribution to the 2D vibrational response function, which was completely ignored due to the approximations based on the Wick’s theorem.

Two-dimensional vibrational spectroscopy. V. Novel 2-dimensional surface vibrational spectroscopies of adsorbed molecules on surfaces or at interfaces

Abstract: Novel two-dimensional (2D) vibrational spectroscopies that are suitable for the investigation of the vibrational couplings of adsorbed molecules on surfaces or at interfaces are theoretically proposed. Depending on the sequence of the IR, Raman, and hyper-Raman transitions involved, there are seven distinctive nonlinear response functions associated with various 2D surface vibrational spectroscopies, which are either three- or five-wave-mixing processes. It is suggested that these novel techniques can be used to obtain information on the nonlinear coordinate dependencies of the dipole moment, polarizability, and hyperpolarizability as well as on the anharmonic couplings between any two vibrational degrees of freedom of adsorbed molecules.

Vibrational properties of Ca3Sc2Ge3O12, a garnet host crystal for laser applications

Abstract: The IR and the Raman spectra of the Ca3Sc2Ge3O12 (CaSGG) garnet crystal have been measured and discussed in terms of internal and external modes of the tetrahedral GeO4-4 moiety. Some important aspects of the electronic spectroscopy of these materials activated with luminescent ions, i.e. the multiphonon relaxation of the excited states and the vibrational structure of the optical bands, have been taken into consideration, and correspondences with the vibrational properties of the host lattice have been presented.