Overtone Transitions Research Articles

Overtone Transitions of12CO2beyond the Venus Bands

Combination overtone transitions of12CO2much weaker than the Venus bands in the near infrared (11 700–14 200 cm−1) have been observed for the first time by using photoacoustic spectroscopy. A total of six bands from two Fermi polyad groups were recorded and both the band origins and relative intensities have been determined and compared with the calculated values. Through a comparison of the relative absorption cross sections within the same type of polyads, intensity shifts have been observed and will be discussed.

Toward an accurate frequency standard at 1.5 μm based on the acetylene overtone band transition

We have made a frequency standard operating at the 1.5 /spl mu/m region using the saturated absorption of the v/sub 1/+v/sub 3/ band of /sup 13/C/sub 2/H/sub 2/. To enhance the moderate power from a laser diode and to saturate a very weak molecular overtone transition, we used a Fabry-Perot (FP) cavity and inside of it we put an acetylene cell. In order to obtain a longer life time for the cell, we designed a new scheme consisting of a cavity with a ULE (ultralow expansion) glass spacer and a baked and sealed-off cell with Brewster windows. We observed the saturated spectrum having a 1 MHz spectral linewidth. The signal obtained using the same cell ten months later had the same signal-to-noise ratio (SNR) and spectral linewidth. We made two versions of the frequency standard and observed a beat note between these two lasers from which we derived the frequency stability, in terms of the square root of the Allan variance /spl sigma/(/spl tau/)=2.4/spl times/10/sup -13/ at an integration time /spl tau/=1000 s.

Infrared absorption line strengths of the Na⋯FH van der Waals molecule

The dipole moment function of the Na⋯FH van der Waals molecule obtained in accurate multireference configuration interaction calculations and the recently published ab initio potential energy surface for NaFH [Topaler et al., J. Chem. Phys. 108, 5349 (1998)] have been used to predict the infrared absorption line strengths of Na⋯FH for several fundamental and overtone transitions. The fundamental excitation of the HF stretch mode in Na⋯FH is enhanced by a factor of 2.2 compared to the isolated HF molecule, and the first and second overtones are enhanced by factors of 21 and 53, respectively.

Absorption and Resonance Raman Study of the 2B1(X)−2A2(A) Transition of Chlorine Dioxide in the Gas Phase

The photochemical reaction dynamics of chlorine dioxide (OClO) are investigated using absorption and resonance Raman spectroscopy. The first Raman spectra of gaseous OClO obtained directly on resonance with the 2B1−2A2 electronic transition are reported. Significant scattering intensity is observed for all vibrational degrees of freedom (the symmetric stretch, bend, and asymmetric stretch), demonstrating that structural evolution occurs along all three normal coordinates following photoexcitation. The experimentally measured absorption and resonance Raman intensities are compared to the intensities predicted using both empirical and ab initio models for the optically active 2A2 surface. Comparison of the experimental and theoretical absorption spectra demonstrates that the frequencies and intensities of transitions involving the asymmetric stretch are well reproduced by the empirical model characterized by a double-minimum along the asymmetric stretch. However, the ab initio model is also found to reproduce a subset of the experimental intensities. In addition, the extremely large resonance Raman intensity of the asymmetric stretch overtone transition is predicted by both models. The results presented here taken in combination with the model for the 2A2 surface in condensed environments suggest that the phase-dependent photochemical reactivity of OClO is due to environment-dependent excited-state structural evolution along the asymmetric stretch coordinate.

On librational broadening of vibrational transitions in liquids: a simple model

Sensitive absorption measurements can expose irregularly shaped vibrational overtone bands from the near-IR well into the visible region for stretching modes involving light atoms—particularly hydrogen. For the fifth overtone transition of the C-H stretching mode of liquid benzene, the asymmetric shape has been modelled as Franck–Condon structure in the separate rolibrational space of the molecule. The two rolibrational potentials, one for the ground state, the other for the fifth overtone state, were approximated as truncated anharmonic wells. In the present work the rolibrational potentials are treated as periodic cosinusoids and the associated Franck–Condon structure is derived in which one can smoothly pass from hindered rotation to free rotation, which is particularly relevant to potential barriers that are on the order of kT or less. Application to benzene provides a fit to the shape of the asymmetric fifth overtone band that is essentially within experimental error. The symbolic capabilities of Mathematica (ver. 3.0) have greatly facilitated the calculations by providing a quasi-analytic approach valid over a large range of parameters.

State-to-state inelastic scattering from vibrationally activated OH–H2 complexes

State-selective infrared excitation of o-H2–OH via the pure OH overtone transition has been used to induce a half-collision inelastic scattering event between the OH radical and ortho-H2 under restricted initial orientation conditions. The time evolution and final state distribution of the OH products from vibrational predissociation have been evaluated by ultraviolet probe laser-induced fluorescence measurements. The half-collision scattering takes place with ∼3350 cm−1 of energy available to the OH (v=1)+o-H2 products, an energy that exceeds the classical barrier to reaction. The OH (v=1) products are preferentially populated in high rotational levels with a distribution that is consistent with an energy gap law. A significant fraction of the OH fragments are promoted to the excited spin–orbit state in the predissociation process. A strong lambda-doublet propensity is also found, indicating that the OH unpaired pπ orbital is preferentially aligned perpendicular to the rotational plane of the OH products. Finally, the OH rotational and fine structure distributions are compared with those obtained in previous full collision inelastic scattering studies at energies below the threshold for reaction.

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

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 ) 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.

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

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 ) 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.

Photodissociation of CH stretch overtone excited CH3Cl and CHD2Cl(vCH=5): Cl spin–orbit branching and atomic fragment yields

The photodissociation dynamics of CH3Cl and CHD2Cl excited through their respective fourth C–H stretch overtone transitions have been investigated by detection of ground state Cl(2P3/2), spin–orbit excited Cl(2P1/2), H, and D atomic fragments by 2+1 resonance enhanced multiphoton ionization in a time-of-flight mass spectrometer. The yield of atomic Cl fragments from the vibrationally mediated photodissociation of CHD2Cl is significantly less than for CH3Cl, more than can be accounted for from differences in the cross sections for overtone excitation. This suggests that state mixing leads to a greater amplitude of the wave function of the vibrationally excited level along the dissociation coordinate in CH3Cl than in CHD2Cl. The observed differences in the Cl spin–orbit branching in the photolysis of ground state versus vibrationally excited molecules are ascribed to differences in the nuclear dissociation dynamics, rather than access to other excited electronic states. The relative yields of H and D atoms from the photodissociation of ground and vibrationally excited CHD2Cl are also reported and discussed.

Integrated intensities of OH vibrational overtones in alcohols

Intensities for several OH vibrational overtone bands have been measured for vapor-phase methanol, ethanol, and isopropanol. The trends in the intensities as a function of excitation level have been modeled by two empirical approaches, yielding intensity predictions for the higher overtone transitions, up to 7 ν(OH). These methods were also applied to recent HNO 3 overtone measurements, resulting in new intensity predictions for higher, photochemically active overtone bands, which are compared with ab initio estimates.

Sub-Doppler optothermal overtone spectroscopy of ethylene and dichloroethylene

Sub-Doppler optothermal spectroscopy of ethylene has been performed in a collimated molecular beam with a single-mode color center laser tunable around 1.6 μm and a silicon bolometer cooled to 1.6 K. The spectra of the ν 5+ ν 9 overtone transitions and their perturbations are analyzed. With an enhancement cavity, overtone transitions of di- and trichloroethylene have also been measured.

Influence of the oxidiser gas composition on the overtone generation efficiency of a supersonic cw chemical HF laser

An experimental investigation was made of the influence of the chemical composition of the oxidiser gas in an atomic-fluorine generator on the efficiency of generation of radiation representing the first overtone of the HF molecule in a self-contained supersonic cw chemical HF laser with the active medium 70 cm long. The optimal chemical composition was different for the fundamental and overtone transitions. A specific output energy of 84 J g-1 at a specific mass flow rate of 0.13 g s-1 cm-2 through the nozzle array was achieved by optimisation of a linear three-mirror optical cavity at the 1.33 — 1.35 μm wavelengths. The overtone radiation power generated in the whole of the active medium was 7.5 kW, corresponding to a 41% efficiency of energy conversion to an overtone.

Mid- and near-IR study of the hydrogen bond interaction between N-methyldiacetamide and phenols

The hydrogen bond complexes between phenol derivatives (p K a = 10.20−8.20) and N-methyldiacetamide (NMD) are studied by mid- and near-IR spectroscopy in carbon tetrachloride solution. The equilibrium constants and enthalpies of complex formation are determined and the results suggest that the proton acceptor ability of NMD is lower than that of monocarbonyl bases. Complex formation results in decoupling of the ν(CO) vibration. The frequencies and intensities of the bonded ν(OH) vibration are determined for the fundamental and first overtone transition. Hydrogen bond formation results in a very weak increase of the mechanical anharmonicity and a strong decrease of the intensity of the first overtone of the OH stretching vibration. The first ( μ 1) and second ( μ 2) dipole moment derivatives are calculated. In this limited domain, a linear correlation is found between Δμ 1 and Δμ 2, the increase of the dipole moment derivatives resulting from hydrogen bond formation.

High-resolutionFTIR spectroscope to study isotopic substitutions ofOH - inLiF andNaF doped withMg 2+ andTi 3+

High-resolution (0.04 cm-1 FTIR spectroscopy in the range 9-300 K is applied to study the stretching modes of16OH- and of its isotopic substitutions, as18OH,16OD and16OT, perturbed by neighbouring defects induced by Mg2+ and Ti3+ in LiF and NaF. The spectra are analysed in the framework of the Morse anharmonic oscillator model. The anharmonicity parameter (2–3 x 10-2) is evaluated either by considering the isotopic replicas or the first overtone transition. The former is found slightly higher than the latter, since the hydroxyl ion vibrates as a “nearly” free diatomic molecule. The very weak isotopic replica due to18O and D present in the samples as a result of the isotope natural abundance are detected as well.

Accurate gas diagnostics for sealed-off CO/sub 2/ lasers using near-infrared DFB semiconductor lasers

The selective spectroscopic detection of CO/sub 2/ and H/sub 2/O in the gas mixture of a 400-W sealed-off CO/sub 2/ slab laser is reported. The detection is based on the acquisition of absorption Voigt profiles of two vibrational overtone transitions of CO/sub 2/ and H/sub 2/O, obtained using two InGaAsP distributed feedback (DFB) semiconductor lasers. This spectroscopic method enables a direct absolute measurement of the temporal evolution for the concentrations inside the laser active volume. Monitoring takes place during laser operation without gas extraction. The experiments carried out on CO/sub 2/ and H/sub 2/O abundances confirm the paramount importance of wall desorbance phenomena in all-metal radio frequency (RF) excited lasers.

Anharmonicity and cross section for absorption of radiation by water dimer

We calculate the absorption cross section of water dimer molecules in thermal equilibrium at temperatures typical of the lower atmosphere using quantum mechanical coupled nonlinear equations of motion. Empirical Morse-oscillator potentials are used to describe the local modes of water monomer, and the RWK2 potential is employed for the interaction between atoms of different water monomers. The strong anharmonicity is taken into account by an extension to molecular dimers of methods originally developed for the lattice dynamics of solid helium. Approximations based on exploiting the hierarchy of energy scales in the dynamics of the weakly hydrogen-bonded water dimer allow the determination of the absorption spectrum over the range of significant solar radiation, up to 20 000 cm−1, including the important contributions of overtone and combination transitions. This approach can tackle the complicated task of mixing of vibrational fundamentals and overtones. We have found that the absorption by these vibrational overtones, within the solar energy range, is quite significant due to the anharmonicity of Morse-oscillator potentials and the large vibrational amplitude of hydrogen atoms. These overtones may play a role in the solar energy absorption of the atmosphere.

Near-infrared cavity ringdown spectroscopy of water vapor in an atmospheric flame

We have used cavity ringdown spectroscopy (CRDS) to measure near-infrared overtone transitions of water in atmospheric flames (propane premixed jet and laminar methane-air flat flame burner). The strong signal output with a well-defined laser beam direction and the insensitivity to strong background emission present in hostile environments make CRDS ideal for the study of combustion environments. We have obtained spectra of water vapor from within a flame and extracted a profile of the rotational temperature and concentration of water vapor as a function of distance from the plane burner surface.

Inversion of absorption spectral data for relaxation matrix determination. I. Application to line mixing in the 106←000 overtone transition of HCN

A new method of extracting the relaxation matrix directly from absorption spectral data is formulated and applied to R-branch line mixing in HCN. The formulation makes use of a general iterative inversion algorithm based upon first-order sensitivity analysis and Tikhonov regularization. The recovered relaxation matrices describe line mixing much better than those derived from the fitting laws currently in use, and the inversion algorithm usually converges within just three iterations. This formulation presents the first known method for extracting the imaginary, off-diagonal elements of the relaxation matrix.

Variation of intermolecular interaction and local lattice distortion of parahydrogen crystals upon vibrational excitation

When a hydrogen molecule in a parahydrogen crystal is excited to a high vibrational overtone state, its electronic properties vary significantly since the energy of excitation is a sizable fraction of the energies of the excited electronic states. Thus the vibrational excitation leads to a significant variation of the intermolecular potential and resultant local distortion of the crystal lattice. Such an effect is sensed by the variation of the splitting of $M$ sublevels of the $J=1$ ${\mathrm{H}}_{2}$ impurity that causes the overtone transition. In this paper we attempt to theoretically explain the observed large variations of the pair and crystal-field splitting parameters $\ensuremath{\Delta}B$ and ${\ensuremath{\varepsilon}}_{2c}$ upon the $v=3\ensuremath{\leftarrow}0$ vibrational excitation. Using the ab initio anisotropic intermolecular pair potential and simplified models on the vibrational dependence of the potential and crystal distortion, observed values are well explained.

OH stretch overtone spectroscopy and transition dipole alignment of HOD

Photoacoustic spectroscopy in room-temperature H2O/HOD/D2O cells has been used to probe the 3νOH and 4νOH overtone bands in HOD via excitation with a single-mode (160 MHz) injection-seeded optical parametric oscillator (OPO) and pulsed dye laser, respectively. Transitions corresponding to HOD are isolated by comparison between H2O/HOD/D2O and pure H2O overtone spectra and analyzed with a Watson asymmetric top Hamiltonian. Least-squares fits yield the vibrational dependence of Av, Bv, and Cv rotational constants as well as a Birge–Sponer analysis of the vibrational overtone series for up to vOH=4. Relative intensities of a-type and b-type bands are reported for 3νOH and 4νOH which indicate that the transition dipole moment vector tilts away from the OH bond with increasing νOH excitation. This observation is qualitatively inconsistent with simple local-mode, bond–dipole model predictions of the vibrational transition moments. However, the data are in excellent agreement with full 3D quantum variational calculations based on the H2O potential of Polyansky et al. [J. Chem. Phys. 105, 6490 (1996)] and dipole moment surface of Gabriel et al. [J. Chem. Phys. 99, 897 (1993)]. These results highlight the relative importance of electrical versus mechanical anharmonicity effects in preparing highly excited vibrational states of HOD via laser overtone pumping.