Vibration-rotation Transitions Research Articles

FTIR Spectroscopy of Three Fundamental Bands of H2F+

The ν1, ν2, and ν3 bands of H2F(+) were observed with a Fourier transform absorption spectroscopic technique in the 3 and 7 μm regions. The ion was produced with a hollow cathode discharge in a F2, He, and H2 gas mixture. A simultaneous analysis of FT data combined with laser spectroscopic data was carried out using the Watson's A-reduced Hamiltonian to determine molecular constants in vibrationally excited states. The effect of the vibration-rotation interaction between the ν1 and ν3 states was found to be small compared with the case of H2O. The vibration-rotation transitions of the ν2 band were first identified and analyzed to obtain molecular constants in the ν2 state, and the band origin was determined to be 1370.5236 (7) cm(-1) with one standard deviation in parentheses. Determined molecular constants can be used to derive the re structure of H2F(+) as re(H-F) = 0.9608(6) Å, ∠e(H-F-H) = 112.2(2)° with the error corresponding to the uncertainty of the assumed vibration rotation constant γ2(a) and the range of the values derived from three pairs of rotational constants.

A new approach to understand liquid injection into atmospheric plasma jets

Infrared images are used to characterize the atmospheric plasma and the influence of liquid injection into it. In the infrared spectral range low energies of rotational vibrational transitions can be captured which are emitted by hot liquid vapors. These vapor streams enable the qualification of the injection depth. High speed shadowgraphy is used to prove the correlations between vapor cloud and drop atomization behavior. In addition the combustion of organic liquids like ethanol and pentanol can be seen directly with an increase of emitted radiation. A direct correlation between vapor cone and liquid properties can be made. Lower Ohnesorge numbers lead to more focused vapor beams.

Hierarchies of intramolecular vibration–rotation dynamical processes in acetylene up to 13,000 cm−1

The vibration–rotation dynamics of acetylene are computed from a spectroscopic Hamiltonian with 468 parameters fit to 19,582 vibration–rotation transitions up to 13,000 cm−1 of vibrational energy. In this energy range, both the bending and the CH stretching vibrations can reach large amplitudes, but the maximum energy remains below the threshold for isomerization to vinylidene. In contrast to the behavior at energies below 5000 cm−1 [Mol. Phys. 108, 1115 (2010)], excitation of single bright states leads, in almost all cases, to computed intramolecular vibrational redistribution (IVR) that is irreversible on the timescales investigated. Hierarchies of IVR processes on timescales ranging from 20 fs to 20 ps result when different bright states are excited. Different parts of the vibrational quantum number space are explored as a result of the four different classes of coupling terms: vibrational l-type resonance, anharmonic resonances, the rotational l-type resonance, and Coriolis couplings. The initial IVR rates are very different depending on whether the bright states are bending states or stretching states, normal modes or local modes, edge states or interior states. However, the rates of the rotationally mediated couplings do not depend substantially on these distinctions.

IUPAC critical evaluation of the rotational–vibrational spectra of water vapor, Part III: Energy levels and transition wavenumbers for H216O

This is the third of a series of articles reporting critically evaluated rotational–vibrational line positions, transition intensities, and energy levels, with associated critically reviewed labels and uncertainties, for all the main isotopologues of water. This paper presents experimental line positions, experimental-quality energy levels, and validated labels for rotational–vibrational transitions of the most abundant isotopologue of water, H216O. The latest version of the MARVEL (Measured Active Rotational–Vibrational Energy Levels) line-inversion procedure is used to determine the rovibrational energy levels of the electronic ground state of H216O from experimentally measured lines, together with their self-consistent uncertainties, for the spectral region up to the first dissociation limit. The spectroscopic network of H216O containstwo components, an ortho (o) and a para (p) one. For o-H216O and p-H216O, experimentally measured, assigned, and labeled transitions were analyzed from more than 100 sources. The measured lines come from one-photon spectra recorded at room temperature in absorption, from hot samples with temperatures up to 3000K recorded in emission, and from multiresonance excitation spectra which sample levels up to dissociation. The total number of transitions considered is 184667 of which 182156 are validated: 68027 between para states and 114129 ortho ones. These transitions give rise to 18486 validated energy levels, of which 10446 and 8040 belong to o-H216O and p-H216O, respectively. The energy levels, including their labeling with approximate normal-mode and rigid-rotor quantum numbers, have been checked against ones determined from accurate variational nuclear motion computations employing exact kinetic energy operators as well as against previous compilations of energy levels. The extensive list of MARVEL lines and levels obtained are deposited in the supplementary data of this paper, as well as in a distributed information system applied to water, W@DIS, where they can easily be retrieved.

ExoMol line lists - I. The rovibrational spectrum of BeH, MgH and CaH in theX 2Σ+state

Accurate line lists for three molecules, BeH, MgH and CaH, in their ground electronic states are presented. These line lists are suitable for temperatures relevant to exoplanetary atmospheres and cool stars (up to 2000 K). A combination of empirical and ab initio methods is used. The rovibrational energy levels of BeH, MgH and CaH are computed using the programs level and dpotfit in conjunction with ‘spectroscopic’ potential energy curves (PECs). The PEC of BeH is taken from the literature, while the PECs of CaH and MgH are generated by fitting to the experimental transition energy levels. Both spin-rotation interactions (except for BeH, for which it is negligible) and non-adiabatic corrections are explicitly taken into account. Accurate line intensities are generated using newly computed ab initio dipole moment curves for each molecule using high levels of theory. Full line lists of rotation–vibration transitions for 9BeH, 24MgH, 25MgH, 26MgH and 40CaH are made available in an electronic form as supporting information to this paper and at www.exomol.com.

The infrared spectrum of 13C2H2 in the 60–2600 cm−1 region: bending states up to v 4 + v 5 = 4

The vibration–rotation spectra of 13C substituted acetylene, 13C2H2, have been recorded in the region between 60 and 2600 cm−1 at an effective resolution ranging from 0.001 to 0.006 cm−1. Three different instruments were used to collect the experimental data in the extended spectral interval investigated. In total 9529 rotation vibration transitions have been assigned to 101 bands involving the bending states up to v tot = v 4 + v 5 = 4, allowing the characterization of the ground state and of 33 vibrationally excited states. All the bands involving states up to v tot = 3 have been analyzed simultaneously by adopting a model Hamiltonian which takes into account the vibration and rotation l-type resonances. The derived spectroscopic parameters reproduce the transition wavenumbers with a RMS value of the order of the experimental uncertainty. Using the same model, larger discrepancies between observed and calculated values have been obtained for transitions involving states with v tot = 4. These could be satisfactorily reproduced only by adopting a set of effective constants for each vibrational manifold, in addition to the previously determined parameters, which were constrained in the analysis.

Speed dependence of collision parameters in theH218Onear-IR spectrum: Experimental test of the quadratic approximation

A laser absorption technique, based upon a pair of offset-frequency-locked extended-cavity diode lasers, was employed to perform line-shape measurements for ${\mathrm{H}}_{2}{}^{18}\mathrm{O}$ vibration-rotation transitions at 1.38 $\ensuremath{\mu}$m, with an extremely high spectral fidelity. This important feature was mostly ensured by the highly accurate, absolute, and repeatable frequency axis characterizing each acquired spectrum. We present the outcomes of a specific study of the speed dependence of collision parameters influencing the shape of the ${\mathrm{H}}_{2}{}^{18}\mathrm{O}$ absorption lines. More particularly, the performance of the quadratic approximation, either used alone or considered in conjunction with the Dicke narrowing effect, was investigated and compared to that of the confluent hypergeometric dependence. Hence, experimental evidences of the partial failure of the quadratic approximation are given and discussed. Finally, we investigate the possible influence of the choice of the speed-dependent line-shape model on the retrieved collisional parameters.

Spectroscopic measurement of the vapour pressure of ice

We present a laser absorption technique to measure the saturation vapour pressure of hexagonal ice. This method is referenced to the triple-point state of water and uses frequency-stabilized cavity ring-down spectroscopy to probe four rotation-vibration transitions of at wavenumbers near 7180 cm(-1). Laser measurements are made at the output of a temperature-regulated standard humidity generator, which contains ice. The dynamic range of the technique is extended by measuring the relative intensities of three weak/strong transition pairs at fixed ice temperature and humidity concentration. Our results agree with a widely used thermodynamically derived ice vapour pressure correlation over the temperature range 0°C to -70°C to within 0.35 per cent.

Infrared-terahertz double-resonance spectroscopy of CH3F and CH3Cl at atmospheric pressure

A method for highly selective remote sensing of atmospheric trace polar molecular gases is described. Based on infrared-terahertz double-resonance spectroscopic techniques, the molecule-specific coincidence between the lines of a CO${}_{2}$ laser and rotational-vibrational molecular absorption transitions provide two dimensions of recognition specificity: infrared coincidence frequency and the corresponding terahertz frequency whose absorption strength is modulated by the laser. Atmospheric pressure broadening expands the molecular recognition ``specificity matrix'' by simultaneously relaxing the infrared coincidence requirement and strengthening the corresponding terahertz signature. Representative double-resonance spectra are calculated for prototypical molecules CH${}_{3}$F and CH${}_{3}$Cl and their principal isotopomers from which a heuristic model is developed to estimate the specificity matrix and double-resonance signature strength for any polar molecule.

The A2Δ–X2Π band system of the CD radical

The A2Δ–X2Π, Δv=0 sequence of the CD radical was observed in the 22800–24000cm−1 spectral region using a conventional spectroscopic technique. The CD molecules were formed and excited in a stainless steel hollow-cathode lamp with two anodes and filled with a mixture of He buffer gas and CD4. The emission from the discharge was observed with a plane-grating spectrograph and recorded using a photomultiplier tube. The observed lines were assigned to the 0–0, 1–1, 2–2, and 3–3 bands. In total, 1189 transition wavenumbers were precisely measured, with an estimated accuracy of 0.003cm−1, and rotationally analyzed. In the final global fit, the present data were combined with available high-resolution measurements of the vibration–rotation transitions [Morino et al., J. Mol. Spectrosc. 174 (1995) 123–131] and pure rotational transitions [Brown and Evenson, J. Mol. Spectrosc. 136 (1989) 68–85; Halfen et al., Astrophys. J. 687 (2008) 731–736]. This procedure enabled the extraction of molecular constants for the A2Δ and X2Π states of CD. The equilibrium parameters were compared with the calculations performed using the Born–Oppenheimer approximation, and a slight difference was observed in some cases due to the partial breakdown of the B–O approximation. The electronic isotopic shift, Δνe, for the A–X transition was determined to be 32.105cm-1, and the shift independent of the nuclear mass coefficient, ΔU=64.762(38)cm-1, was also calculated.

Synthesis of nitrogen-doped diamond films using vibrational excitation of ammonia molecules in laser-assisted combustion flames

Nitrogen-doped diamond was synthesized in open air using laser-assisted combustion flame method. A wavelength-tunable CO2 laser was used to resonantly excite the vibration modes of ammonia molecules, which were added into the diamond forming combustion flame. The wavelength of the CO2 laser was tuned to match frequencies of the NH wagging mode of the ammonia molecules. High efficiency energy coupling was achieved at laser wavelengths of 9.219, 10.35, and 10.719 μm, which are related to a rotational–vibrational transition (1084.63 cm−1), and splitting of the NH wagging mode (υ2+, 932.51 cm−1 and υ2−, 968.32 cm−1). Vibrational excitations of the ammonia molecules under these wavelengths actively intervenes the reaction courses, which steers the chemical reaction in the combustion flame and eventually promotes nitrogen concentration in the deposited diamond films. Concentration of the doped nitrogen atoms reaches up to 1.5 × 1020 atoms/cm3 in the diamond films deposited with a laser wavelength of 9.219 μm. Optical emission spectroscopy and mass spectrometry were used to study the evolution of chemical reactions with and without laser excitations.

The near infrared cavity-enhanced absorption spectrum of methyl cyanide

The absorption spectrum of methyl cyanide (CH3CN) has been measured in the near IR between 6000 and 8000cm−1 with a resolution of 0.12cm−1 using Fourier transform incoherent broadband cavity-enhanced absorption spectroscopy. The spectrum contains several weakly perturbed spectral regions; potential vibrational combination bands contributing to the spectrum are outlined. Line positions and cross-sections of CH3CN between 6814 and 7067cm−1 have been measured at high-resolution of 0.001cm−1 using diode laser based off-axis cavity-enhanced absorption spectroscopy. A total of 4630 new absorption lines of CH3CN are identified in this region. A value for the self-broadening coefficient has determined to be (3.3±0.2)×10−3cm−1 mbar−1 for one isolated line at 7034.171cm−1. Several line series have been identified in these regions and an autocorrelation analysis performed with a view to aiding future assignments of the rotational-vibrational transitions.

Collisional broadening of CO2 lines by N2O pressure

Carbon dioxide line broadening coefficients by N2O pressure, as well as their temperature exponents, were calculated for a wide range of rotational quantum numbers in the 0001–1000 band. A semiempiric method that is based on the semiclassical broadening theory was used. It showed good results in calculations of half-widths and shifts of lines by pressure for H2O-N2(O2), CO2-N2(O2), and O3-N2(O2) systems. The calculated collisional half-widths were compared with the measured data obtained with a frequency-stabilized tunable CO2 laser for 11 vibrational-rotational transitions with rotational quantum numbers of the lower state to 38 at room temperature and for three transitions R(10), R(22), and R(32) in the temperature interval of 300 ≤ T ≤ 700 K in the 0001–1000 band. The calculated and measured data are in a good agreement.

Interaction-induced dipoles of hydrogen molecules colliding with helium atoms: A new ab initio dipole surface for high-temperature applications

We report new ab initio results for the interaction-induced dipole moments Δμ of hydrogen molecules colliding with helium atoms. These results are needed in order to calculate collision-induced absorption spectra at high temperatures; applications include modeling the radiative profiles of very cool white dwarf stars, with temperatures from 3500 K to 9000 K. We have evaluated the dipoles based on finite-field calculations, with coupled cluster methods in MOLPRO 2006 and aug-cc-pV5Z (spdfg) basis sets for both the H and He centers. We have obtained values of Δμ for eight H(2) bond lengths ranging from 0.942 a.u. to 2.801 a.u., for 15 intermolecular separations R ranging from 2.0 a.u. to 10.0 a.u., and for 19 different relative orientations. In general, our values agree well with earlier ab initio results, for the geometrical configurations that are treated in common, but we have determined more points on the collision-induced dipole surface by an order of magnitude. These results make it possible to calculate transition probabilities for molecules in excited vibrational states, overtones, and rotational transitions with ΔJ > 4. We have cast our results in the symmetry-adapted form needed for absorption line shape calculations, by expressing Δμ as a series in the spherical harmonics of the orientation angles of the intermolecular vector and of a unit vector along the H(2) bond axis. The expansion coefficients depend on the H(2) bond length and the intermolecular distance R. For large separations R, we show that the ab initio values of the leading coefficients converge to the predictions from perturbation theory, including both classical multipole polarization and dispersion effects.

Local modes in vibration–rotation spectroscopy

AbstractThe present paper is concerned with the description of highly excited rotational and/or vibrational states of molecules in terms of localized vibrations or local modes. Local mode effects are most common in molecules with multiple equivalent H–X bonds, which give rise to equivalent H–X local mode stretching vibrations, and the theory is outlined for the simplest such case, that of an H2X molecule. In the local mode picture of molecular vibration, experimentally observed, initially unexpected near‐degeneracies of vibrational states at high vibrational excitation, and of rotation–vibration states at high rotational excitation, can be explained in a relatively straightforward manner. The local mode theory predicts relations between the conventional rotation–vibration parameters whose values are determined in least‐squares fittings to observed rotation–vibration transition frequencies or wavenumbers. It should be emphasized, however, that such relations are valid only for particular forms of the effective rotation–vibration Hamiltonian used in the spectral analysis. We illustrate the theory with examples of experimental spectroscopic work where local mode effects play an important role in the interpretation of the experimental findings. The fact that local mode vibrations not only cause clustering of highly excited vibrational energy levels, but also of highly rotationally excited rotation–vibration energy levels, has been understood fairly recently. We outline the theoretical background for this phenomenon and relate it to the existing experimental work. © 2012 John Wiley & Sons, Ltd.This article is categorized under: Theoretical and Physical Chemistry > Spectroscopy

Absolute frequency list of the ν_3-band transitions of methane at a relative uncertainty level of 10^−11

We determine the absolute frequencies of 56 rotation-vibration transitions of the ν(3) band of CH(4) from 88.2 to 90.5 THz with a typical uncertainty of 2 kHz corresponding to a relative uncertainty of 2.2 × 10(-11) over an average time of a few hundred seconds. Saturated absorption lines are observed using a difference-frequency-generation source and a cavity-enhanced absorption cell, and the transition frequencies are measured with a fiber-laser-based optical frequency comb referenced to a rubidium atomic clock linked to the international atomic time. The determined value of the P(7) F(2)((2)) line is consistent with the International Committee for Weights and Measures recommendation within the uncertainty.

Lamp mapping technique for independent determination of the water vapor mixing ratio calibration factor for a Raman lidar system

We have investigated a technique that allows for the independent determination of the water vapor mixing ratio calibration factor for a Raman lidar system. This technique utilizes a procedure whereby a light source of known spectral characteristics is scanned across the aperture of the lidar system's telescope and the overall optical efficiency of the system is determined. Direct analysis of the temperature-dependent differential scattering cross sections for vibration and vibration-rotation transitions (convolved with narrowband filters) along with the measured efficiency of the system, leads to a theoretical determination of the water vapor mixing ratio calibration factor. A calibration factor was also obtained experimentally from lidar measurements and radiosonde data. A comparison of the theoretical and experimentally determined values agrees within 5%. We report on the sensitivity of the water vapor mixing ratio calibration factor to uncertainties in parameters that characterize the narrowband transmission filters, the temperature-dependent differential scattering cross section, and the variability of the system efficiency ratios as the lamp is scanned across the aperture of the telescope used in the Howard University Raman Lidar system.

Analytical Research on Rotation-Vibration Multiphoton Absorption of Diatomic Molecules in Infrared Laser Fields

The multiphoton rotation-vibration energy absorption of diatomic molecules in infrared laser fields is analytically studied using the algebraic approach. The analytical expression of the rotation-vibration transition probability is given. The long-time average absorbed energy spectra and the average number of photons absorbed by the molecule are discussed. The results show that both molecular orientation and molecular anharmonicity are important factors in the rotation-vibration multiphoton absorption.

Pulsed H2 – F2 laser with simultaneous lasing on rotational and vibrational—rotational transitions

The possibility of operation of a pulsed chemical H2 – F2 laser on purely rotational transitions with simultaneous lasing on vibrational – rotational transitions is theoretically investigated. The lasing of a hydrogen—fluorine laser (mixture pressure 1.1 atm) on both v,j – 1 → v – 1,j (j=2 – 20) and v,j → v,j – 1 (j = 12 – 19) transitions has been modelled taking into account the resonant one- and two-quanta vibrational – rotational processes. The calculated specific energy extraction for purely rotational transitions reaches 0.3–0.5 J L-1 at j=18 and 19 (wavelength 15.5–14 μm) and 0.4–0.7 J L-1 at j=13 and 14 (wavelength 21–19 μm).

Extremely sensitive detection of NO_2 employing off-axis integrated cavity output spectroscopy coupled with multiple-line integrated absorption spectroscopy

We report on the development of a new sensor for NO₂ with ultrahigh sensitivity of detection. This has been accomplished by combining off-axis integrated cavity output spectroscopy (OA-ICOS) (which can provide large path lengths of the order of several kilometers in a small volume cell) with multiple-line integrated absorption spectroscopy (MLIAS) (where we integrate the absorption spectra over a large number of rotational-vibrational transitions of the molecular species to further improve the sensitivity). Employing an external cavity quantum cascade laser operating in the 1601-1670 cm⁻¹ range and a high-finesse optical cavity, the absorption spectra of NO₂ over 100 transitions in the R band have been recorded. From the observed linear relationship between the integrated absorption versus concentration of NO₂ and the standard deviation of the integrated absorption signal, we report an effective sensitivity of detection of approximately 28 ppt (parts in 10¹²) for NO₂ To the best of our knowledge, this is among the most sensitive levels of detection of NO₂ to date.