Showing papers on "Hot band published in 1982"

Vibrational energy transfer from highly excited anharmonic oscillators. Dependence on quantum state and interaction potential

Abstract: In order to elucidate the general features of vibrational deactivation of highly excited anharmonic oscillators, we present quasiclassical trajectory calculations on prototype collinear I2 (v)‐inert gas collision systems. The results for vibrational‐translational energy transfer reveal several interesting trends as a function of initial vibrational quantum state, projectile mass, and projectile–oscillator interaction potential. (1) Vibrational deactivation is inefficient from all quantum levels and for all projectile masses. The average energy transfer per collision ΔE is strongly peaked at intermediate vibrational levels (v≊80) and is observed to be at most ≊−kbT. Furthermore, when scaled to h/ω(E), the ’’local’’ oscillator energy spacing, ΔE can be accurately represented by a simple power law in vibrational quantum number over a wide range of bound states. (2) Energy transfer is progressively less efficient from levels in the neighborhood of and approaching dissociation. (3) Vibrational energy loss for ...

Photofragmentation dynamics of CH3I at 248 and 266 nm: Vibrational distributions in the CH3(ν2) ‘‘umbrella’’ mode

Abstract: Vibrational population distributions in the ν2 out‐of‐plane bending mode of CH3 when produced by photofragmentation of CH3I at 266 and 248 nm are analyzed by infrared emission spectroscopy. In both cases, there is excitation of the ’’umbrella’’ mode vibration up to 10 quanta, and the peak of the excitation occurs at ν2 = 2 for both wavelengths. The vibrational distribution produced at 266 nm agrees well with molecular beam time‐of‐flight results. The distribution at 248 nm is only moderately enhanced, in contradiction to theoretical predictions which suggest that the peak of the distribution should be shifted to ν2 = 5. The infrared emission spectra suggest that there is no additional rotational excitation in the radical over the ambient rotational temperature of the parent CH3I. In addition, there is no detectable excitation in other vibrational modes, produced either directly in the dissociation or upon collisions.

New results on vibrational population decay in simple liquids

Abstract: The population lifetimes of the symmetric CH2-stretching mode of several methylene halides were measured after infrared excitation by spontaneous anti-Stokes Raman scattering of delayed picosecond probe pulses. Data ranging from 6.5 to 45 ps are reported. The results give evidence for vibrational energy transfer via anharmonic coupling to neighbouring vibrational combination bands and overtones.

Torsional splittings and assignments of the Doppler-limited spectrum of ethane in the C-H stretching region

Abstract: The Doppler-limited absorption spectrum of the C-H stretching region of ethane has been recorded at T≃ 119 K with a tunable difference-frequency laser spectrometer. The strong torsional hot band structure at room temperature is eliminated at 119 K, and the enhanced resolution from the Doppler width reduction allows us to observe small torsional splittings. The two fundamentals in the region, v7, a perpendicular band and, v5, a parallel band have been essentially completely assigned as have a large number of transitions in the parallel component of the v8 + v11 combination band. A number of perturbations of both global and local nature have been observed. The complete spectrum and a listing of transition wavenumbers, intensities and assignments are presented here to facilitate identification and quantitative analysis of ethane in a variety of monitoring applications. Precise ground state rotational constants have been determined from combination differences.

Vibrational state of the chemisorbed molecule on metal surfaces: Role of electron‐hole pair excitation

Abstract: A theory is presented to study the vibrational state of a molecule chemisorbed on a metal surface. The dynamical interaction between localized vibrational state and continuum of electron‐hole pair excitations of the coupled molecule–metal system is found to be a dominant channel of vibrational energy dissipation of chemisorbed molecules, where the charge fluctuation in the molecular electronic states during a vibration plays an important role. The vibrational line shapes thus calculated are characterized by both large red shift in the frequency and very broad width (short lifetime) compared with those of free molecules, thereby showing a fairly good agreement with what was experimentally observed in conventional vibrational spectroscopy of chemisorbed molecules on transition metal surfaces.

A Raman spectroscopic temperature study of NH3+ torsional motion as related to hydrogen bonding in the L‐alanine crystal

Abstract: A Raman study of the NH3+ torsional mode in the amino acid L-alanine has been performed. The temperature dependence of this mode has been followed in the temperature region 110–350 K. The activation energy for NH3+ reorientation obtained from a linewidth vs temperature investigation, Ea = 3100±400 cm−1, is in excellent agreement with corresponding NMR results, thus demonstrating that Raman spectroscopy can serve as a complementary method to NMR in obtaining torsional barrier heights. Based on an X-ray investigation, crystal and hydrogen bond parameters have been calculated as a function of temperature. The increase of the NH3+ torsional barrier on going to lower temperatures is essentially attributed to the shortening of one specific hydrogen bond. The detection of the NH3+ torsional hot transition is important for the determination of the shape of the lower portion of the NH3+ torsional potential. The sixfold term V6 of the potential used, three-plus sixfold, is thus very sensitive to the frequency difference between the NH3+ torsional fundamental and hot band. The optimum frequency fit gives, however, too large a value for the barrier height V3. The reason for this is interpreted as arising from the severe distortion of the hydrogen bonds at large torsional angles. The results presented here support recent findings that a revision of the dynamic picture of the hydrogen bonds in L-alanine crystal, as reported in two earlier papers, is needed.

Higher excited vibrational states of polyatomic molecules

Abstract: The overtone bands in the vibrational specturm of methane can be accounted for on the same footing as the fundamentals by use of a simple vibrational Hamiltonian and the application of conventional symmetry considerations and radiative selection rules. This approach gives a rough map of all stretch vibrational states up to ∼24 000 cm−1, by fitting the states of the appropriate symmetry to the observed overtone spectrum. All vibrational states are described in terms of anharmonic symmetrized internal coordinate (SIC) states which are very close to the eigenstates of the Hamiltonian. The one‐quantum SIC states are identical to the normal modes while some multiquantum SICs present features analogous to the ’’local modes’’ conventionally used in the interpretation of overtone spectra. The discrete structure of the vibrational state–space obtained from this treatment is in conflict with the symmetryless quasicontinuous structure of state–space postulated to explain IR multiphoton absorption in polyatomic molecules. At the same time, it suggests that this process might take place mainly through an equidistant ladder of states, as is found in molecules with several identical bonds.

High resolution translational energy spectroscopy: Electronic transitions in Kr±, and I+ and vibronic transitions in N2±

Abstract: High resolution kinetic energy spectroscopy (0.1 eV fwhm) has been performed on 8 kV Kr±, Xe±, I+ and N2± scattered off of He at small angles. For Kr± and Xe± electronic transitions were observed between the 2P 3 2 and 2P 1 2 states while for I+ transitions between the 3P2, 3P1, 3P0 and 1D2 states were observed. In the Kr± and Xe± systems one peak due to energy loss (inelastic collisions) and one peak due to energy gain (superelastic collisions) were observed. The I+ ions exhibited two energy gain and two energy loss peaks. In the N2± studies, energy loss peaks between the X 2Σ2+ ground state and the A 2Πu and B 2Σu+ excited states were observed, as well as several energy gain peaks. Structure due to vibronic transitions into the A 2Πu and B 2Σu+ states was observed. The relative intensities of the vibronic transitions are Franck—Condon. In addition, the vibronic structure included a hot band from the ground X 2Σ 2+ state. Examples of how translational energy spectroscopy may be used in the study of low lying excited states as well as in the study of the chemistry of selected excited states are presented.

A theoretical investigation of the U.V. excited 1 A 1→2 A 1 photoelectron spectra of NH3 and ND3

Abstract: The vibronic spectra of the 1 A 1→2 A 1 U.V. photoelectron transitions in NH3 and ND3 are analysed theoretically. Optimum geometry and potential curves are obtained by means of ab initio configuration interaction calculations. A one-dimensional approach, using the decoupled planar upper state symmetry coordinates and a Manning type function for the double-minimum potentials, is found to account for frequency shifts, anharmonicity and isotope shifts in the 3a 1 electron ionization. The intensities of the photoelectron spectrum are redetermined through integration of the experimental vibronic bands and are furthermore computed within the Franck-Condon approximation. From calculations of hot band progressions the first adiabatic ionization potentials of NH3 and ND3 could be set to 10·073 and 10·12 eV, respectively.

Measurements and calculations of self-broadening coefficients of lines belonging to the 2ν2, ν1, and ν3 bands of H216O

Abstract: Using Fourier transform spectra (resolution ≈ 0.005 cm−1), the self-broadening coefficients of 340 lines belonging to the 2ν2, ν1 and ν3 bands, and to the ν2 + ν3 − ν2 hot band of H216O, have been measured. The average uncertainty is about 19% and varies from 15 to 28% depending on the line involved. The broadening coefficients, by natural water vapor, of 40 other lines belonging to the ν3 and ν1 bands of H217O and H218O have also been measured. Theoretical calculations of self-broadening coefficients are performed, using the Anderson–Tsao–Curnutte method, and taking into account the four intermolecular interactions: dipole–dipole, dipole–quadrupole, quadrupole–dipole, and quadrupole–quadrupole. In these calculations, accurate spectroscopic data have been used: precise energy levels, realistic wavefunctions, and a complete dipole moment operator expansion in order to compute the transition probabilities. Particularly, all resonances between the three interacting vibrational states (020), (100), and (001) ...

The three‐dimensional potential energy surface for the ring‐puckering, ring‐deformation, and SiH2 rocking vibrations of 1,3‐disilacyclobutane

Abstract: The mid‐infrared and Raman combination and hot band spectra involving the ring‐puckering (ν22), ring‐deformation (ν6), and SiH2 in‐phase rocking (ν21) vibrations have been analyzed for 1,3‐disilacyclobutane in order to determine the ring‐puckering energy levels for the ground and excited state of the other two vibrations. The puckering levels are only slightly perturbed in the rocking excited state but substantially altered in the ring‐deformation excited state. The parameters of the three‐dimensional potential energy function V = a1x41+b1x21+b2x22+ b3x23+c12x21x22+c13x21x2 3, where the subscripts 1, 2, and 3 refer to ν22, ν6, and ν21, respectively, were adjusted in order to obtain the best fit between the observed and calculated frequencies. For the energy level calculations, the Hamiltonian was symmetry factored into eight separate symmetry blocks. The basis functions for the three‐dimensional calculation were determined by first solving the one‐dimensional problem in x1 and multiplying the resulting ei...

On vibrational energy localization at high levels of excitation. Vibrational excitons

Abstract: This review discusses the properties of highly excited vibrational states of polyatomic molecules and molecular crystals. As we know, one can describe small vibrations of molecules with the concept of normal modes. In molecules having several identical valence bonds (C6H6, H2O, etc.) the normal modes that describe the vibrational excitations of these bonds amount to vibrations whose energy is more or less uniformly distributed over all the bonds, with a degree of delocalization of the energy over the bonds that increases with increasing level of excitation. On the other hand, an extensive set of physical phenomena exists (e.g., dissociation of molecules) in which local excitations, a considerable fraction of which are spatially localized, play an important role. A localized state corresponds to a complicated superposition of normal modes. Hence the concept of normal vibrations is inadequate for describing vibrations (or, better expressed, movements) of a highly excited molecule. One can conveniently describe such movements of the molecule in the representation of local modes (LM). As a rule, one takes an LM to mean simply the coordinate of a valence bond of the molecule, e. g., O–H, C–H, etc. A large number of papers has been published recently on the experimental study of LM and infrared spectra, relaxation experiments, selective photochemistry, etc. This review casts light on these experimental data on the basis of the theory of LM.

The vibrational spectra and normal co‐ordinate analysis of NaOX (X = H or D)

Abstract: The vibrational spectra of NaOX (X = H or D) at room temperature and 77 K are reported. A normal co-ordinate analysis has been carried out to assign the observed bands on the basis of the valence force field. The force constants were adjusted by considering a bond distance and reproduce well the observed frequencies. In the dispersion relations on k2 and kx the librational modes were found to be insensitive to k, whereas the translational modes except for the A1g and B1u modes vary considerably with k. Some bands were assigned to the combination band between a fundamental and an acoustic mode.

Vibrational excitation of SF6 in collisions with He atoms

Abstract: Vibrational excitation of SF 6 molecules in collisions with He atoms is studied using a vibrational close-coupling, rotational infinite-order-sudden method of calculation. Integral and differential cross sections for excitation of the triply degenerate ν 6 and ν 5 vibrational modes of SF 6 are reported for thermal collisional energies. Excitation of the ν 6 mode is found to be particularly efficieny. The cross sections are much larger than those calculated previously for the excitation of the bending mode in the He + CO 2 system. The differential cross sections are backward peaked.

Picosecond infrared double resonance studies on pentafluorobenzene

Abstract: Utilizing synchronized 50 ps pulses from two independently tunable CO2 lasers, a pump–probe experiment is performed on pentafluorobenzene The molecule, which has two infrared active modes accessible to the CO2 laser wavelengths, allows an extensive investigation into the statistical nature of the intramolecular vibrational energy relaxation process We find discrete state effects, indicated by oscillatory behavior in the time‐resolved absorption spectrum, disappear when the molecule is heated into the quasicontinuum Using the anharmonic redshifting and broadening of the normal mode absorption features in the infrared spectrum as a measure of local temperature, our results indicate rapid equilibration (<50 ps) of absorbed energy among modes, with the final distribution of energy consistent with thermal heating

Stimulated Raman investigation of CO2-laser-excited SF6☆

Abstract: Stimulated Raman spectroscopy is used to probe the photoexcitation dynamics of CO 2 -laser-excited SF 6 in a molecular free-expansion jet. Time-dependent depletion of rotational levels in the ground state is observed by monitoring the ν 1 Raman spectrum at various delay times after the CO 2 -laser excitation pulse. Optical Stark shifts are also seen, at short delay times, arising from resonances of the IR laser pulse with ν 3 transitions from both ground and ν 1 = 1 states. Molecules excited to the ν 3 = 1 level are detected by scanning the ν 1 + ν 3 − ν 3 hot band. A direct determination of the anharmonicity X 13 = −2.910 ± 0.002 cm −1 is thus obtained.

Vibrational frequencies of the ground and lowest excited triplet state of thioformaldehyde h2, d2, and hd

Abstract: Harmonic vibrational frequencies calculated by ab initio methods with double zeta plus polarization basis sets are reported for the ? and ? states of H2CS, D2CS, and HDCS. These frequencies are scaled to predict as yet unknown experimental values. For the triplet state, vibrational energy levels are given for a double minimum potential in the out‐of‐plane bending coordinate within a rigid bender approximation.

Vibrational Band Intensities of Hydrocarbons

Abstract: The experimental observables of a vibrational spectrum are depicted either in the form of their positions, i.e., frequency relating the energy required in a given quantum transition, or as the intensities of absorption and scattering related to their transition probabilities. Expressed in terms of molecular parameters, the frequencies depend on the geometry, atomic masses, and intramolecular forces [11 while the band intensities [2] reflect changes in dipole moment (infrared) or polarizabilities (Raman) which are caused during a vibrational displacement and are related to movement of electronic charges within the individual bonds. The mathematical basis for determining vibrational frequencies was well established as early as 1940 by Wilson [3] and others [4, 51. Applying the interpretation of vibrational spectra has become routine in the multitudinous disciplines of chemical literature [6–26]. Accounts of infrared and Raman spectra [27–29], collection of literature [30], and reasonable sets of in...