Anharmonic Resonances Research Articles

High sensitivity cavity ring down spectroscopy of NO2 between 7760 and 7917 cm−1

The very weak absorption spectrum of the main isotopologue of nitrogen dioxide, 14N16O2, is investigated for the first time between 7760 and 7917cm−1. The studied region corresponds to the highest energy range of the vibrational spectra of 14N16O2 investigated so far at high spectral resolution. The absorption spectra were recorded by very high sensitivity Continuous Wave-Cavity Ring Down Spectroscopy with a noise equivalent absorption of αmin≈5×10−11cm−1. The spectrum results from the superposition of the rovibrational transitions of the 2ν1+5ν2+ν3, 2ν1+ν2+3ν3 and 5ν1+ν3 bands at 7790.9, 7888.2 and 7904.3cm−1, respectively. The spectrum assignment and modeling were performed using the effective Hamiltonian approach, which involves altogether three bright – (2,5,1), (2,1,3) and (5,0,1) and three dark – (2,7,0), (2,3,2) and (5,2,0) states. As a result, 3020 rovibrational transitions were assigned including 51 extra lines of the 2ν1+3ν2+2ν3 and 5ν1+2ν3 bands. In this way, the overall set of 1494 spin-rotation energy levels were reproduced with an rms of 4.9×10−3cm−1 for the (obs.−calc) deviations, leading to the determination of 66 fitted parameters. The effective Hamiltonian for the {(5,2,0), (2,3,2), (2,7,0), (2,5,1), (2,1,3), (5,0,1)} interacting states takes into account both the spin–rotation interactions within each vibrational state and C-type Coriolis and anharmonic resonances between different vibrational states, according to symmetry considerations. Indeed for NO2 the (v,1v2±2,v3∓1)↔(v,1v2,v3) spin rotation energy levels are usually coupled through C-type Coriolis resonances, and accordingly the (2,7,0)↔(2,5,1)↔(2,3,2)↔(2,1,3) and (5,2,0)↔(5,0,1) interactions were included in the effective Hamiltonian model. Furthermore, these two blocks of interacting states are coupled by an additional C-type Coriolis and anharmonic resonances. Using the fitted values of the Hamiltonian parameters and the values of the 2ν1+5ν2+ν3, 2ν1+ν2+3ν3 and 5ν1+ν3 bands transition dipole moment operators determined from a fit of a selected set of experimental line intensities, a synthetic spectrum was generated for the entire investigated region and is provided as Supplementary material.

New Analysis of the ν3 and ν4 Bands of HNO3 in the 7.6 μm Region

A new line position analysis of the ν3 and ν4 bands of nitric acid (HNO3) at 1326.186 and 1303.072 cm(-1) together with its associated interacting bands is presented. The 3(1) and 4(1) energy levels were obtained from an extended analysis of high-resolution Fourier transform spectra recorded at Giessen in the 7.6 μm region. The energy levels of 3(1) and 4(1) upper states of nitric acid are strongly interacting with those of the 9(3), 6(2), 5(1)9(1), and 7(1)8(1) dark states centered at 1288.899, 1289.46, 1341.05, and 1343.78 cm(-1), respectively. Informations on these perturbing dark states were achieved through previous partial investigations of hot bands in high-resolution Fourier transform spectra recorded at 22 μm in Giessen (for 3ν9-2ν9 and 3ν9-ν5), at 12 μm in Denver (for 3ν9-ν9), and at 11 μm in Orsay (for ν5+ν9-ν9). The energy levels calculation accounts for the various Fermi, anharmonic, A-type, B-type, and C-type Coriolis resonances, which couple together the {6(2),9(3),4(1),3(1),5(1)9(1),7(1)8(1)} interacting energy levels. For nitric acid, the ν9 mode (OH torsion relative to the -NO2 moiety) is a large amplitude motion. The theoretical model used in this work accounts also for large amplitude effects in the 9(3) dark state, which lead to a splitting of the 9(3) energy levels of about 0.060 cm(-1). In this way, the existence of torsional splittings for several ν4 perturbed lines was explained by the occurrence of local A-type and B-type Coriolis resonances coupling the 4(1) energy levels with those of 9(3). Because four dark bands had to be accounted for in the model, the results of the energy level calculations are reasonable, although not perfect. However, a very significant improvement was achieved in terms of understanding the 7.6 μm absorbing bands of nitric acid as compared to the analysis of the ν3 and ν4 bands performed several years ago [Perrin, A.; Lado-Bordowski, O.; Valentin, A. Mol. Phys. 1989, 67, 249-267]. Finally, the present analysis also features, for the first time, the ν3+ν9-ν9 hot band located at 1331.09 cm(-1). This study will help to improve HNO3 measurements by satellites. This will be indeed the case for the "Infrared Atmospheric Sounding Interferometer" (IASI) experiment.

Electronic states, conical intersections, and spin-rovibronic spectroscopy of the nitrogen oxide sulfide radical

Highly correlated ab initio methods are used to investigate the lowest electronic states of doublet and quartet spin multiplicities for SNO. One-dimensional cuts of the three-dimensional potential energy surfaces (3D-PESs) of these electronic states along the stretch and bend coordinate are calculated. Several avoided crossings and conical intersections are located for bent and linear configurations. The dynamics on the excited electronic states of SNO are very complex, and suggest that multi-step mechanisms should occur to populate the ground state via radiationless processes or lead to predissociation. In addition, our calculations show that the ground (X̃(2)A(')) and the first excited (Ã(2)A(")(Π)) states of this radical form a linear-bent Renner-Teller system. They correlate to the SNO(1(2)Π) state at linearity. Systematic studies of both components are performed using standard coupled cluster approaches, explicitly correlated coupled cluster technique, and multi-configurational methods in connection with large basis sets. Core-valence and scalar relativistic effects are examined. For both electronic states, the 3D-PESs are mapped in internal coordinates at the RCCSD(T)-F12b∕cc-pVTZ-F12 level. The analytical representations of these potential energy surfaces are incorporated later into perturbative and variational treatments of the nuclear motions. A set of spectroscopic parameters and spin-rovibronic levels calculated variationally are presented. Strong anharmonic resonances are found. These new results allow for the reassignment of earlier experimental IR bands of SNO trapped in cooled argon matrices.

High-resolution Raman spectroscopy of ν2and associated hot bands of13CH12CH: global analysis of the anharmonic resonances

The Raman spectrum of the 13CH12CH molecule in the region of its ν2 vibrational band was recorded at a resolution of 2.6 × 10−3 cm−1 and analyzed. Several Q branches were observed and identified, corresponding to the ν2 fundamental and the associated hot bands ν2 + ν4 – ν4, ν2 + ν5 – ν5, ν2 + 2ν4 – 2ν4 and 2ν2 – ν2. The usual normal modes of vibration in acetylene, with the symmetric (ν1) and antisymmetric (ν3) CH stretchings, the CC stretching (ν2), the degenerate trans (ν4) and cis (ν5) bendings were adopted. All the spectra were obtained using the experimental technique of quasi-continuous stimulated Raman spectroscopy. The fundamental ν2 band and the ν2 + ν4 – ν4 and ν2 + ν5 – ν5 hot bands were recorded at room temperature, while ν2 + 2ν4 – 2ν4 was recorded at a temperature of 355 K. Finally, the spectrum of the 2ν2 – ν2 band was recorded at 170 K using a Raman-Raman double resonance technique in which a stimulated Raman pumping process is used to populate a single rotational level of the v2 = 1 state, prior to the spectroscopy stage. A 15 ns delay between pump and spectroscopy is introduced to allow partial collisional relaxation of the rotational population of v2 = 1 among rotational levels. The assigned Raman transitions and several previously observed infrared bands have been simultaneously fitted in order to derive a set of deperturbed molecular parameters, including the quartic anharmonic coupling coefficients K1,255 and K3,245. The ro-vibrational pattern of the stretching and stretching-bending combination states and an accurate description of the anharmonic resonances in this molecule have been obtained. Copyright © 2013 John Wiley & Sons, Ltd.

The CW-CRDS spectra of the 16O/18O isotopologues of ozone between 5930 and 6340 cm−1—Part 1: 16O16O18O

The first investigation of the very sensitive CW-Cavity Ring Down Spectra (noise equivalent absorption αmin≈1×10−10cm−1) of 18O enriched ozone isotopologues is reported. The ozone generation from 18O enriched oxygen molecules resulted in six isotopic species. The contributions of the 16O16O18O/16O18O16O and of the 18O18O16O/18O16O18O isotopomers were separated using two partial pressures of 16O2/18O2 mixtures. In the 5930–6340cm−1 range, the four dominant bands of 16O16O18O were vibrationally assigned as 2ν1+2ν2+3ν3_1, 2ν2+5ν3, 5ν1+ν3 and 2ν1+2ν2+3ν3_2. They correspond to the highest vibration excitations observed so far in an asymmetric CS ozone isotopologue. The rovibrational spectra for 18O enriched CS ozone species show specific features related to the nuclear spin statistics and to simultaneous symmetry allowed Coriolis and anharmonic accidental resonances, which are discussed. Altogether for the four new bands, 2155 rovibrational transitions were assigned up to Jmax=32. Observed line positions were fitted with rms deviations ranging from 0.005 to 0.01cm−1 using effective Hamiltonian models accounting for dark state perturbations. The derived band centers and rotational constants are in good agreement with new theoretical calculations from the molecular potential function. The corresponding line lists are provided as Supplementary Material.

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.

Experimental and theoretical studies of the vibrational spectra of CHD2Br

The dideuterated form of methyl bromide, CHD2Br, has been synthesized and the gas-phase infrared spectra investigated in the range 400–10,000 cm–1 using a medium-resolution FTIR spectrometer. The nine fundamental bands have been characterized in detail. Six of them, i.e. ν 1, ν 4, ν 5, ν 7, ν 8 and ν 9, have been rotationally analysed through the assignment of the partially resolved structure of the PQK and RQK cluster of lines and the spectroscopic parameters have been derived in the symmetric top limit approximation. Among the fundamental levels, anharmonic resonance occurs between ν 7/ν 4 + ν 8 and ν 8/ν 6 + ν 9. An isotopic 79/81Br shift was found for ν 6 and in the more complex region of the ν 8 fundamental. High-quality ab initio calculations – carried out at coupled cluster level [CCSD(T)] employing the correlation-consistent basis set of Dunning (cc-pVTZ) – were performed to determine quadratic, cubic and quartic (semidiagonal) force constants. Using these constants and applying second-order vibrational perturbation theory (VPT2), with allowance for resonances (when necessary), permitted us to identify and assign, in addition to the fundamentals, about 70 overtones and combination bands up to three quanta.

Analysis of the Brunel model and resulting hot electron spectra

Among the various attempts to model collisionless absorption of intense and superintense ultrashort laser pulses, the so-called Brunel mechanism plays an eminent role. A detailed analysis reveals essential aspects of collisionless absorption: Splitting of the electron energy spectrum into two groups under p-polarization, prompt generation of fast electrons during one laser cycle or a fraction of it, insensitivity of absorption with respect to target density well above nc, robustness, simplicity, and logical coherence. Such positive aspects contrast with a non-Maxwellian tail of the hot electrons, too low energy cut off, excessively high fraction of fast electrons, and inefficient absorption at moderate angles of single beam incidence and intensities. Brunel’s pioneering idea has been the recognition of the role of the space charges induced by the electron motion perpendicular to the target surface that make irreversibility possible. By setting the electrostatic fields inside the overdense target equal to zero, anharmonic resonance and mixing of layers leading to Maxwellianization are excluded. To what extent the real electron spectra and their scaling on laser intensity are the product of the interplay between Brunel’s mechanism and anharmonic resonance is still an open question.

Calculation of vibrational energy levels of triatomic molecules with the C 2v and C s symmetries by summing divergent series of the Rayleigh-Schrödinger perturbation theory

The Rayleigh-Schrodinger perturbation theory is applied to calculation of vibrational energy levels of triatomic molecules with the C2v and Cs symmetries: SO2, H2S, F2O, HOF, HOCl, and DOCl. Particular attention is given to the states coupled by anharmonic resonances; for such states, the perturbation theory series diverge. To sum these series, the known methods of Pade, Pade-Borel, and Pade-Hermite and the method of power moments are used. For low-lying levels, all the summation methods give satisfactory results, while the method of quadratic Pade-Hermite approximants appears to be more efficient for high-excited states. Using these approximants, the structure of singularities of the vibrational energy, as a function in the complex plane, is studied.

High resolution FTIR spectrum of cyanogen iodide between 200 and 5000 cm−1. Global rovibrational analysis of IR and MW data for I12C14N

The infrared absorption spectrum of ICN has been recorded in the region 200–5000cm−1 with a Bruker 120HR interferometer, and several tenthousands of lines have been measured with a precision between 0.0001 and 0.001cm−1. Most of these lines have been assigned. These new data were combined with transitions previously measured in the microwave and millimeterwave ranges. After a band-by-band analysis including 292 subbands a rovibrational global analysis has been performed using all available data. The main resonances are the rotational ℓ-type resonance and the strong Fermi resonance coupling the states ν1=1 and ν2=2 via the cubic anharmonic constant k122. In addition, many local resonances became evident by the observation of the corresponding avoided crossings. In particular, there is a direct coupling between the states (400) and (240) due to the fourth-order resonance k112222. There is also a fourth-order Coriolis resonance governed by C11222 responsible for interactions between the (200) and (030) states. All these interactions have been taken into account in one-step diagonalization procedures. A statistical agreement with the 6344 selected pieces of data has been obtained, with an estimated standard deviation σˆ=0.660. All those data and their deviations are given in the first depository. A set of 109 parameters including 24 off-diagonal anharmonic and Coriolis resonance parameters has been derived. They have been used to calculate effective rovibrational parameters for all substates up to more than 6000cm−1, also listed in the second depository.

A laser-induced fluorescence study of the jet-cooled nitrous oxide cation (N2O+)

Laser-induced fluorescence and wavelength resolved emission spectra of the à (2)Σ(+) - X̃ (2)Π(i) electronic transition of the jet-cooled nitrous oxide cation have been recorded. The ions were produced in a pulsed electric discharge at the exit of a supersonic expansion using a precursor mixture of N(2)O in high pressure argon. Both spin-orbit components of the 0(0) (0) band were studied at high resolution and rotationally analyzed to provide precise molecular constants for the combining states. Emission spectra were obtained by laser excitation of the 0(0) (0), 2(0) (1), 3(0) (1), and 2(0) (2) absorption bands, providing extensive data on the ground state bending, stretching, and combination vibrational levels. These data were fitted to a Renner-Teller model including spin-orbit, anharmonic, and Fermi resonance terms. The observed energy levels and fitted parameters were found to be comparable to those in the literature predicted from an ab initio potential energy surface.

Detection and analysis of four new bands in CRDS 16O3 spectra between 7300 and 7600 cm−1

The absorption spectrum of the 16O3 isotopologue of ozone was recorded in the 7000–7920cm−1 region by using high sensitivity CW-Cavity Ring Down Spectroscopy (αmin∼10−10cm−1). This report is devoted to the analysis of the 7300–7600cm−1 region dominated by four A-type bands: 6ν1+ν3 centred around 7395cm−1, 3ν1+5ν2+ν3 and 2ν1+4ν2+3ν3 lying in the 7450cm−1 region and 5ν1+2ν2+ν3 centred around 7579cm−1. 213 transitions of the 6ν1+ν3 band were assigned and the corresponding line positions were modeled using an effective Hamiltonian including a Coriolis resonance interaction between the (601) upper state and a A-type dark state. The two very close 3ν1+5ν2+ν3 and 2ν1+4ν2+3ν3 bands were analysed using a similar effective Hamiltonian scheme involving the anharmonic resonance coupling between the (351) and (243) states. For these two bands, 304 transitions were assigned. The modelling also includes a first Coriolis resonance interaction between the (351) bright state and the (530) dark state, and a second one between the (243) bright state and the (144) dark state. In the 7579cm−1 region, 205 transitions of the 5ν1+2ν2+ν3 band were assigned and modelled taking into account the Coriolis resonance interactions between the (521) upper state and the (700), (342) and (280) dark states.The dipole transition moment parameters of the four analysed bands were determined by a least-squares fit to the measured line intensities. For the studied band systems, the effective Hamiltonian and transition moment operator parameters were used to generate line lists provided as Supplementary Materials.

On the nature of highly vibrationally excited states of thiophosgene#

In this work an analysis of the highly vibrationally excited states of thiophosgene (SCCl2) is made in order to gain insights into some of the experimental observations and spectral features. The states analysed here lie in a spectrally complex region where strong mode mixings are expected due to the overlap of several strong anharmonic Fermi resonances. Two recent techniques, a semiclassical angle space representation of the eigenstates and the parametric variation of the eigenvalues (level-velocities) are used to identify eigenstate sequences exhibiting common localization characteristics. Preliminary results on the influence of highly excited out-of-plane bending modes on the nature of the eigenstates suggest a possible bifurcation in the system. Dynamical assignment of highly vibrationally excited states of thiophosgene is made using two recent techniques based on classical-quantum correspondence. The analysis give insights into eigenstate sequences and predicts possible bifurcations at energies far above the ground state.

Nonlinear spectroscopy of superconducting anharmonic resonators

We formulate a model for the steady state response of a nonlinear quantum oscillator structure, such as those used in a variety of superconducting qubit experiments, when excited by a steady, but not necessarily small, ac tone. We show that this model can be derived directly from a circuit description of some recent qubit experiments in which the state of the qubit is read out directly, without a superconducting quantum interference device (SQUID) magnetometer. The excitation profile has a rich structure depending on the detuning of the tone from the small-signal resonant frequency, on the degree of damping and on the excitation amplitude. We explore two regions in detail. Firstly, at high damping there is a trough in the excitation response as a function of detuning, near where the classical Duffing bifurcation occurs. This trough has been understood as a classical interference between two metastable responses with opposite phase. We use Wigner function studies to show that while this picture is roughly correct, there are also more quantum mechanical aspects to this feature. Secondly, at low damping we study the emergence of sharp, discrete spectral features from a continuum response. We show that these the structures, associated with discrete transitions between different excited-state eigenstates of the oscillator, provide an interesting example of a quantum Fano resonance. The trough in the Fano response evolves continuously from the ‘classical’ trough at high damping.

Modelling the anharmonic and Coriolis resonances within the six level polyad involving the ν4 fundamental in the ro-vibrational spectrum of vinyl fluoride

The Fourier transform infrared (FTIR) spectrum of vinyl fluoride, H2CCHF, has been deeply investigated in the ν4 band region around 6μm at a resolution of 0.002cm−1. This normal mode, of A′ symmetry species and corresponding to the CC stretching motion, yields an a/b hybrid band with a prevalent a-type character. In the present contribution, a systematic investigation is carried out by explicitly taking into account the strong and invasive anharmonic and Coriolis resonances, which perturb all of the ν4 ro-vibrational states. Indeed, the ν4=1 state is involved in a six-level resonant polyad, which, besides ν4 (1656.0cm−1), comprises the A′ symmetry vibrational states ν7+ν9 (1635.5cm−1), ν10+ν12 (1641.8cm−1) and 2ν11 (1733.6cm−1), as well as the ν8+ν12 (1638.7cm−1) and 2ν9+ν12 (1683.8cm−1) combination bands of A″ symmetry. The ro-vibrational analysis led to the assignment of more than 2100 transitions of ν4 with J″≤57 and Ka″≤16, and about 1000 ro-vibrational transitions (J″≤47 and Ka″≤11) reaching the ν7=ν9=1 state. 505 transitions of these belong to the ν7+ν9−ν9 hot band, which is located at about 1052.4cm−1 within the atmospheric window. No features are detected for the remaining vibrational states involved in the polyad, which therefore behave as dark states. By adopting an interaction model involving fourteen different anharmonic and Coriolis resonances, the assigned transitions are simultaneously fitted within the Watson's A-reduction Hamiltonian in the Ir representation up to the sixth power of angular momentum operators. As demonstrated by spectral simulations, the obtained ro-vibrational Hamiltonian well reproduces the ro-vibrational spectrum of vinyl fluoride in the 6μm region.

High resolution infrared and Raman spectroscopy of ν 2 and associated combination and hot bands of 13C12CD2

Infrared and Raman spectra of dideuterated acetylene containing one 13C atom, 13C12CD2, have been recorded and analysed to obtain detailed information on the fundamental ν 2 band and associated combination and hot bands. Infrared spectra were recorded at 4 × 10−3 cm−1 resolution in the region 1150−2900 cm−1, which contains combination and hot bands from the ground and the bending v 4 = 1 and v 5 = 1 states. The Q-branches of the ν 2 fundamental and associated hot bands (ν 2 + ν 4 − ν 4, ν 2 + ν 5 − ν 5, ν 2 + 2ν 4 − 2ν 4, ν 2 + 2ν 5 − 2ν 5 and ν 2 + ν 4 + ν 5 − (ν 4 + ν 5)) were recorded using inverse Raman spectroscopy, with an instrumental resolution of about 3 × 10−3 cm−1. In addition, the observation of the 2ν 2 − ν 2 Raman band was carried out populating the v 2 = 1 state by stimulated Raman pumping. In total, 11 Raman and 9 infrared bands were analysed, involving all the l-vibrational components of the excited stretching−bending manifolds up to v t = v 4 + v 5 = 2. A simultaneous analysis of all infrared and Raman assigned transitions has been performed on the basis of a theoretical model which takes into account the rotation and vibration l-type resonances within each vibrational manifold and the Darling−Dennison anharmonic resonance between the ν 2 + 2ν 4 and ν 2 + 2ν 5 states. The parameters obtained reproduce the assigned transition wavenumbers with a standard deviation of the same order of magnitude as the experimental uncertainty.

Confinement-induced resonances in anharmonic waveguides

We develop the theory of anharmonic confinement-induced resonances (ACIR). These are caused by anharmonic excitation of the transverse motion of the center of mass (COM) of two bound atoms in a waveguide. As the transverse confinement becomes anisotropic, we find that the COM resonant solutions split for a quasi-1D system, in agreement with recent experiments. This is not found in harmonic confinement theories. A new resonance appears for repulsive couplings ($a_{3D}>0$) for a quasi-2D system, which is also not seen with harmonic confinement. After inclusion of anharmonic energy corrections within perturbation theory, we find that these ACIR resonances agree extremely well with anomalous 1D and 2D confinement induced resonance positions observed in recent experiments. Multiple even and odd order transverse ACIR resonances are identified in experimental data, including up to N=4 transverse COM quantum numbers.

Anharmonic resonances in the CH chromophore overtone spectra of CHBrF2

The gas-phase Fourier transform infrared spectra of bromodifluoromethane, CHBrF2, have been recorded up to 10,100 cm−1 with a resolution of 0.5 cm−1. All the most relevant absorptions in the overtone region have been ascribed to the CH chromophore. Twenty-six bands involving either the CH-stretching (ν 1) and bending (ν 2, ν 7) modes have been assigned and their relative band strengths have been determined. Vibrational analysis of the corresponding polyads (up to N = 7/2) has been performed by means of an effective Hamiltonian that takes into account anharmonic resonances, thus leading to the determination of Fermi and Darling–Dennison coupling terms. The obtained effective spectroscopic parameters were able to quantitatively describe both the position and intensity of all the absorption bands, and also to rationalize the effects and the magnitudes of the various resonances in each polyad. The results obtained using this approach have been compared with those computed using high-quality ab initio electronic structure calculations employing second-order vibrational perturbation theory (VPT2). Overall, satisfactory agreement between the observed and the ab initio computed values was obtained by properly taking into account in the final calculations the relative magnitude of the Fermi and Darling–Dennison resonance effects in each polyad.

The ν 2 + ν 4 and 2ν 2 + ν 5 high-resolution infrared bands of FClO3

The high-resolution infrared spectra of the mono isotopic species F35Cl16O3, F37Cl16O3, F35Cl18O3 and F37Cl18O3 have been analysed in the region of the ν 2 + ν 4 and 2ν 2 + ν 5 perpendicular combination bands, from 1920 to 2060 cm−1. Both bands are perturbed by the anharmonic interaction effective between the v 2 = v 4 = 1 and v 2 = 2, v 5 = 1 excited states of E symmetry. The ν 2 + ν 4 bands are of medium–strong intensity, while the 2ν 2 + ν 5 bands are very weak. Since the origins of the two bands differ by about 10 cm−1, the 2ν 2 + ν 5 bands at lower wavenumbers are almost completely hidden by ν 2 + ν 4. In total, about 3400, 2400, 2800 and 3200 transitions have been assigned for F35Cl16O3, F37Cl16O3, F35Cl18O3 and F37Cl18O3, respectively. The assignments of each dyad have been analysed simultaneously with the inclusion of the anharmonic resonance term in the model Hamiltonian. The ro-vibration parameters of the v 2 = v 4 = 1 and v 2 = 2, v 5 = 1 excited states have been obtained, together with the deperturbed band origins and the anharmonic resonance coefficients W 245. Combining the ν 2 + ν 4 origins with those of the ν 2 and ν 4 fundamentals, the anharmonicity constants x 24 have been derived for the four molecules and compared with the ab initio values calculated taking into account the Fermi resonance.

Rotational and rovibrational spectroscopy of the v 8 = 1 and 2 vibrational states of CH3NC

The lowest frequency vibration–rotation band ν 8, corresponding to the degenerate CNC bending vibration of methyl isocyanide (CH3NC) with band center at 267.3 cm−1, was studied by FTIR spectroscopy between 215 and 320 cm−1 and rotational spectroscopy in the range from 201 to 383 GHz in order to describe the vibration–rotation dynamics with the inclusion of all important anharmonic and vibration–rotation interactions. Assignments extended to higher-K rotational states in both vibration–rotation and rotational spectra reveal a resonance crossing with the v 8 = 2 overtone level due to a local anharmonic Fermi resonance. The simultaneous analysis of both types of data provides a fully quantitative reproduction of the experimental data as well as a preliminary characterization of the v 8 = 2 level. The parallel component of the overtone band with band center at 524.6 cm−1 was assigned for the first time from spectra in the region from 480 to 585 cm−1 together with the hot bands in the region of the fundamental band.