Anharmonic Resonances Research Articles

High-resolution spectroscopy of the C–N stretching band of methylamine

The C-N stretching infrared fundamental of CH(3)NH(2) has been investigated by high-resolution laser sideband and Fourier transform synchrotron spectroscopy to explore the energy level structure and to look for possible interactions with high-lying torsional levels of the ground state and other vibrational modes. The spectrum is complicated by two coupled large-amplitude motions in the molecule, the CH(3) torsion and the NH(2) inversion, which lead to rich spectral structure with a wide range of energy level splittings and relative line intensities. Numerous sub-bands have been assigned for K values ranging up to 12 for the stronger a inversion species for the v(t) = 0 torsional state, along with many of the weaker sub-bands of the s species. The C-N stretching sub-state origins have been determined by fitting the upper-state term values to J(J + 1) power-series expansions. For comparison with the ground-state behaviour, both ground and C-N stretch origins have been fitted to a phenomenological Fourier series model that produces an interesting pattern with the differing periodicities of the torsional and inversion energies. The amplitude of the torsional energy oscillation increases substantially for the C-N stretch, while the amplitude of the inversion energy oscillation is relatively unchanged. Independent inertial scale factors ρ were fitted for the torsion and the inversion and differ significantly in the upper state. The C-N stretching vibrational energy is determined to be 1044.817 cm(-1), while the effective upper state B-value is 0.7318 cm(-1). Several anharmonic resonances with v(t) = 4 ground-state levels have been observed and partially characterized. A variety of J-localized level-crossing resonances have also been seen, five of which display forbidden transitions arising from intensity borrowing that allow determination of the interaction coupling constants.

Explicitly correlated treatment of H2NSi and H2SiN radicals: Electronic structure calculations and rovibrational spectra

Various ab initio methods are used to compute the six dimensional potential energy surfaces (6D-PESs) of the ground states of the H(2)NSi and H(2)SiN radicals. They include standard coupled cluster (RCCSD(T)) techniques and the newly developed explicitly correlated RCCSD(T)-F12 methods. For H(2)NSi, the explicitly correlated techniques are viewed to provide data as accurate as the standard coupled cluster techniques, whereas small differences are noticed for H(2)SiN. These PESs are found to be very flat along the out-of-plane and some in-plane bending coordinates. Then, the analytic representations of these PESs are used to solve the nuclear motions by standard perturbation theory and variational calculations. For both isomers, a set of accurate spectroscopic parameters and the vibrational spectrum up to 4000 cm(-1) are predicted. In particular, the analysis of our results shows the occurrence of anharmonic resonances for H(2)SiN even at low energies.

Anharmonic resonances with recursive delay feedback

We consider application of time-delayed feedback with infinite recursion for control of anharmonic (nonlinear) oscillators subject to noise. In contrast to the case of a single delay feedback, recursive delay feedback exhibits resonances between feedback and nonlinear harmonics, leading to a resonantly strong or weak oscillation coherence even for a small anharmonicity. Remarkably, these small-anharmonicity induced resonances can be stronger than the harmonic ones. Analytical results are confirmed numerically for van der Pol and van der Pol–Duffing oscillators.

Explicitly correlated treatment of the Ar–NO+ cation

We present an application of the recently developed explicitly correlated coupled cluster method to the generation of the three-dimensional potential energy surface (PES) of the Ar-NO(+) cationic complex. A good overall agreement is found with the standard coupled clusters techniques employing correlation consistent atomic basis sets (aug-cc-pVnZ, n= D, T, Q) of Wright et al. This PES is then used in quantum close-coupling scattering and variational calculations to treat the nuclear motions. The bound states energies of the Ar-NO(+) complex obtained by both approaches are in good agreement with the available experimental results. The analysis of the vibrational wavefunctions shows strong anharmonic resonances between the low frequency modes (intermonomer bending and stretching modes) and the wavefunctions exhibit large amplitude motions.

Detection and analysis of new bands of 16O 3 by CRDS between 6500 and 7300 cm −1

The absorption spectrum of the 16O 3 isotopologue of ozone has been recorded in the 7000–7920 cm −1 region by high sensitivity CW-Cavity Ring Down Spectroscopy. This report is devoted to the analyses of the 7065–7300 cm −1 region dominated by the ν 1 + 2 ν 2 + 5 ν 3 and ν 1 + 5 ν 2 + 3 ν 3 A-type bands at 7130.8 and 7286.8 cm −1 respectively. 289 transitions were assigned to the ν 1 + 2 ν 2 + 5 ν 3 band. The corresponding line positions were modeled with an effective Hamiltonian involving Coriolis resonance interactions between the (1 2 5) upper state and the (4 4 0), (0 2 6) and (6 1 0) dark states, and an anharmonic resonance interaction with the (2 0 5) state. The very strong interaction (up to 50% mixing of the wavefunctions) between the (1 2 5) and (6 1 0) states leads to the observation of two extra lines of the 6 ν 1 + ν 2 band due to a resonance intensity transfer. 213 transitions of the ν 1 + 5 ν 2 + 3 ν 3 band were assigned and modeled taking into account a Coriolis resonance interaction with the (3 6 0) state. We take the opportunity of the present work to report the analysis of the very weak 4 ν 2 + 4 ν 3 B-type band at 6506.1 cm −1 which was assigned from previously recorded CRDS spectra. 286 transitions were modeled using the effective Hamiltonian approach. The dipole transition moment parameters of the three analyzed bands were determined by a least-squares fit to the measured line intensities. For the three studied band systems, the effective Hamiltonian and transition moment operator parameters were used to generate line lists provided as Supplementary Materials.

Fourier transform synchrotron spectroscopy of torsional and CO-stretching bands of CH 317OH

The Fourier transform spectrum of the CH 3 17OH isotopologue of methanol has been recorded in the 65–1200 cm −1 spectral region at a resolution of 0.00096 cm −1 using synchrotron source radiation at the Canadian Light Source. Here we present an extension to higher torsional states of our investigation of the torsion–rotation transitions within the small-amplitude vibrational ground state, now including assignments of more than 16 500 lines involving quantum numbers in the ranges v t ⩽ 3, J ⩽ 30 and | K| ⩽ 12, as well as a study of the strong CO-stretching band centered at 1020 cm −1. Energy term values have been determined for assigned ground and CO-stretching levels by use of the Ritz program, and have been fitted to series expansions in powers of J( J + 1) to determine substate origins and effective B values. Several Fermi anharmonic and Coriolis level-crossing resonances coupling the CO stretch with high torsional ground-state levels have been identified and characterized. The study is motivated by astrophysical applications, with a principal aim being the compilation of an extensive set of energy term values to permit prediction of astronomically observable sub-millimetre transitions to within an uncertainty of a few MHz.

Rotational and Rovibrational Spectroscopy of CH3NC of the Ground and ν4 = 1 Vibrational States

The parallel vibration-rotation band ν(4) of methyl isocyanide (CH(3)NC), with a band center at 944.9 cm(-1), was studied by FTIR spectroscopy between 890 and 980 cm(-1) in order to improve the ground-state rotational constants. Such improvement is essential for the scheduled studies of excited vibrational levels and their mutual anharmonic resonances occurring at higher values of the K rotational number. Ground-state combination differences generated from this band, spanning values of J/K from 0 to 85/13, were combined with rotational data from the literature and newly measured rotational transitions, extending the J/K range from 3/0 up to 31/14, and fitted simultaneously with a fully quantitative reproduction of the data. The infrared data of the ν(4) band were analyzed together with rotational data of the ν(4) = 1 level, spanning values of J/K from 4/0 to 14/12. The fit in the approximation of an isolated vibrational state, with the transitions perturbed by weak local resonances excluded, yields reproduction of the data within experimental uncertainties.

Submillimetre-wave spectrum of diacetylene and diacetylene-d 2

Rovibrational bands between closely lying bending vibrational states of normal diacetylene and its bideuterated variant have been recorded in the submillimetre-wave region using a source-modulation spectrometer. The two isotopologues were produced by pyrolysis (1200○C) of benzene or benzene-d 6 vapour in a flow reactor. Measurements of the ν8 − ν6 band and of its ν9-associated hot band have been extended up to 615 and 420 GHz, respectively. Rotational and vibrational l-type resonance were taken into account in the analysis of the ν8 + ν9 − ν6 − ν9 band; in addition the cubic anharmonic resonance existing between the v 8 = v 9 = 1 state and the v 3 = 1 stretching state has been considered explicitly in the least-squares fits to the observed frequencies. Very accurate values of the band origins and of numerous spectroscopic constants have been determined for both isotopologues.

Laser excitation spectrum of C 3 in the region 26 000–30 700 cm −1

The vibrational structure of the A ˜ 1 Π u electronic state of C 3 in the region 26 000–30 775 cm −1 has been re-examined, using laser excitation spectra of jet-cooled molecules. Rotational constants and vibrational energies have been determined for over 60 previously-unreported vibronic levels; a number of other levels have been re-assigned. The vibrational structure is complicated by interactions between levels of the upper and lower Born–Oppenheimer components of the A ˜ 1 Π u state, and by the effects of the double minimum potential in the Q 3 coordinate, recognized by Izuha and Yamanouchi [16]. The present work shows that there is also strong anharmonic resonance between the overtones of the ν 1 and ν 3 vibrations. For instance, the levels 2 1 + 1 and 0 1 + 3 are nearly degenerate in zero order, but as a result of the resonance they give rise to two levels 139 cm −1 apart, centered about the expected position of the 2 1 + 1 level. With these irregularities recognized, every observed vibrational level up to 30 000 cm −1 (a vibrational energy of over 5000 cm −1) can now be assigned. A Σ u + vibronic level at 30181.4 cm −1, which has a much lower B′ rotational constant than nearby levels of the A ˜ 1 Π u state, possibly represents the onset of vibronic perturbations by the B ˜ ′ 1 Δ u electronic state; this state is so far unknown, but is predicted by the ab initio calculations of Ahmed et al. [36].

Theoretical spectroscopy of acetylene dication and its deuterated species

We mapped the six-dimensional potential energy surface of the electronic ground state of HCCH(++)(X (3)Sigma(g) (-)) dication using the coupled cluster approach. This potential energy surface is incorporated later into perturbative and full variational treatments to solve the nuclear motions. We derived a set of spectroscopic data for HCCH(++), HCCD(++), and DCCD(++). Our calculations reveal the presence of anharmonic resonances even at low energies, which complicates their assignment by vibrational quantum numbers. In light of our theoretical vibrational spectra, we propose an assignment of the experimental vibrationally resolved valence double ionization spectra of HCCH, HCCD, and DCCD. These spectra are viewed to be mostly composed by a pure vibrational progression involving the CC stretching mode together with a second progression involving both the CC stretching and the bendings.

High resolution spectroscopy of DOBr and molecular properties of hypobromous acid

The ν 1 and ν 2 bands of DOBr centered near 2668.8 and 852.0 cm - 1 respectively have been observed at 0.006 cm - 1 resolution. The ν 1 band is perturbed by a quintic anharmonic resonance ( Δ K a = 0 ) with 3 ν 3 + ν 2 . In addition, the millimeter wave spectra arising from the v 2 = 1 and v 3 = 1 states have been observed. All rotational and vibrational spectra from both bromine isotopologs have been fitted with a single calculation. The perturbation in the ν 1 band has been well described. Equilibrium rotational and centrifugal distortion constants and changes in quadrupole coupling with the BrO stretch and DOBr bend have been determined. The equilibrium structure has been derived from the DOBr and HOBr rotational constants. The harmonic force field has been calculated and compared with those of related molecules as well as with those derived from ab initio calculations.

High sensitivity CW-Cavity Ring Down Spectroscopy of five 13CO 2 isotopologues of carbon dioxide in the 1.26–1.44 μm region (I): Line positions

The absorption spectrum of highly enriched 13C carbon dioxide has been investigated by CW-Cavity Ring Down Spectroscopy with a setup based on fibered distributed feedback (DFB) laser diodes. By using a series of 30 DFB lasers, the CO 2 spectrum was recorded in the 7029–7917 cm −1 region with a typical sensitivity of 3×10 −10 cm −1. The uncertainty on the determined line positions is on the order of 8×10 −4 cm −1. More than 3800 transitions with intensities as low as 1×10 −29 cm/molecule were detected and assigned to the 13C 16O 2, 16O 13C 17O, 16O 13C 18O, 17O 13C 18O and 13C 18O 2 isotopologues. For comparison, only 104 line positions of 13C 16O 2 were previously reported in the literature in the considered region. The band-by-band analysis has led to the determination of the rovibrational parameters of a total of 83 bands including 56 bands of the 13C 16O 2 species. The measured line positions of 13C 16O 2 and 16O 13C 18O were found in good agreement with the predictions of the respective effective Hamiltonian (EH) models but the agreement degrades for the minor isotopologues. Several cases of resonance interactions were found and discussed. In the 20033-10002 band of 13C 16O 2, an anharmonic resonance interaction leads to deviations on the order of 0.05 cm −1 compared to the EH predictions. The existence of interpolyad interactions affecting the non-symmetric isotopologues of carbon dioxide is confirmed by the observation of two occurrences in 16O 13C 17O and 16O 13C 18O. The obtained results improve significantly the knowledge of the spectroscopy of the 13C isotopologues of carbon dioxide. They will be valuable to refine the sets of effective Hamiltonian parameters used to generate the CDSD database.

Ангармонические колебательные резонансы в малых водных ассоциатах

Ангармонические колебательные резонансы в малых водных ассоциатах

Vibration−Rotation Energy Pattern in Acetylene:13CH12CH up to 10 120 cm−1

All 18,219 vibration-rotation absorption lines of (13)CH(12)CH published in the literature, accessing substates up to 9400 cm(-1) and including some newly assigned, were simultaneously fitted to J-dependent Hamiltonian matrices exploiting the well-known vibrational polyad or cluster block-diagonalization, in terms of the pseudo quantum numbers N(s) = v(1) + v(2) + v(3) and N(r) = 5v(1) + 3v(2) + 5v(3) + v(4) + v(5), also accounting for k = l(4) + l(5) parity and e/f symmetry properties. Some 1761 of these lines were excluded from the fit, corresponding either to blended lines, for about 30% of them, or probably to lines perturbed by Coriolis for the remaining ones. The dimensionless standard deviation of the fit is 1.10, and 317 vibration-rotation parameters are determined. These results significantly extend those of a previous report considering levels below only 6750 cm(-1) [Fayt, A.; et al. J. Chem. Phys. 2007, 126, 114303]. Unexpected problems are reported when inserting in the global fit the information available on higher-energy polyads, extending from 9300 to 10 120 cm(-1). They are tentatively interpreted as resulting from a combination of the relative evolution of the two effective bending frequencies and long-range interpolyad low-order anharmonic resonances. The complete database, made of 18,865 vibration-rotation lines accessing levels up to 10 120 cm(-1), is made available as Supporting Information.

Theoretical spectroscopy of trans-HNNH+ and isotopomers

The six-dimensional potential energy surface of the electronic ground state of trans-HNNH(+) (X (2)A(g)) is mapped at the RCCSD(T)/aug-cc-pV5Z level of theory. This potential energy surface is incorporated later into perturbative and variational treatments to solve the nuclear motion and to derive a set of spectroscopic data for trans-HNNH(+), trans-HNND(+), and trans-DNND(+). Our vibrational spectra are compared with those deduced from the earlier photoelectron spectra by Frost et al. [J. Chem. Phys. 64, 4719 (1976)], for which a good agreement between the theoretical and experimental results is found. Our calculations reveal the presence of strong anharmonic resonances between the vibrational levels of these cations even at low energies, thus complicating even more their assignment by vibrational quantum numbers. These resonances should participate in the transfer of intensities between the active modes during the direct photoionization of the neutral molecule and the combination modes and overtones of the inactive modes belonging to the totally symmetric irreducible representation.

Infrared Spectrum and Anharmonic Force Field of CH2DBr

The gas-phase infrared spectrum of monodeuteromethyl bromide, CH2DBr, has been examined at medium resolution in the range 400-10000 cm(-1), leading to the identification of 70 vibrational transitions. The assignment of the absorptions in terms of fundamentals, overtones, combinations, and hot bands, assisted by quantum chemical calculations, is consistent all over the region investigated. The (79/81)Br isotopic splitting for the lowest fundamental nu6 and the value for the v8 = 1 level have been now precisely determined. Anharmonic resonances are very marginal for all fundamentals and the Coriolis interaction effects are clearly evident in the nu4/nu8 band system, in the nu2 and nu7 fundamentals. Spectroscopic parameters, obtained from the analysis of partially resolved rotational structure, have been derived in the symmetric tops limit approximation. High-quality ab initio calculations have been performed, and harmonic and anharmonic force fields have been predicted from coupled cluster CCSD(T) calculations employing the cc-pVTZ basis set. A good agreement between computed and experimental data, also including the C-H stretching overtones at 6000 and 9000 cm(-1), has been obtained.

Rotational transitions within the 21, 31, 41, and 61 states of formaldehyde H212C16O This article is part of a Special Issue on Spectroscopy at the University of New Brunswick in honour of Colan Linton and Ron Lees.

This work, besides its fundamental interest, is motivated by the atmospheric and astrophysical importance of formaldehyde (H2CO). The goal of this study is to complete the already existing list of rotational transitions within the ground vibration state by a list of transitions within the four first excited 21, 31, 41, and 61 vibrational states, to help the detection of this species by microwave or millimetre wave techniques. For this purpose, the rotational spectra of H2CO in the 21, 31, 41, and 61 excited vibrational states have been investigated in Lille and Cologne in the millimetre region at 160–600 GHz and 850–903 GHz, respectively. The results of these millimetre wave measurements were combined with the 21, 31, 41, and 61 infrared energy levels, which were obtained from previous analysis of FTS spectra of the ν4 (out of plane bending mode), ν6 (CH2 rock mode), and ν3 (CH2 bending mode) bands recorded in the 10 µm region (D.C. Reuter, S. Nadler, S.J. Daunt, and J.W.C. Johns. J. Chem. Phys. 91, 646 (1989)) and more recently for the ν2 fundamental band (C=O stretching, located at 1746.009 cm–1) (F. Kwabia Tchana, A. Perrin, and N. Lacome. J. Mol. Spectrosc. 245, 141, (2007)). The energy level calculation of the 21, 31, 41, and 61 interacting states accounts for the various Coriolis-type resonances that perturb the energy levels of the 21, 31, 41, and 61 vibrational states as well as for the anharmonic resonances coupling the 21 and 31 energy levels, and in this way the microwave and infrared data could be reproduced within their associated experimental uncertainty. However, it is clear that the theoretical model used to account for the very large A-type Coriolis resonance linking the 41 and 61 energy levels of H2CO is only effective with poor physical meaning.

Interactions between vibrational polyads of propyne, H 3C [sbnd]C [tbnd]CH: Rotational and rovibrational spectroscopy of the levels around 1000 cm −1

The study of vibration resonance physics in propyne is based on experimental measurements of about 600 new rotational transitions between 495–590 and 700–760 GHz in excited vibrational levels v 5 = 1, v 8 = 1, v 10 = 3 and v 9 = v 10 = 1 with vibrational energies around 1000 cm −1. The limits to the assignments and analysis were imposed by as yet unresolved anharmonic resonances with the states of the next higher polyad of levels lying above 1200 cm −1, which affect the rotational states involved in transitions that would be measurable with non-vanishing intensities. Vibration–rotation spectra pertaining to the levels in question were studied in the regions 880–1150 cm −1 (the ν 5 and ν 8 fundamental bands), 550–750 cm −1 (the v 9 = v 10 = 1 ← v 10 = 1 hot bands) and 250–400 cm −1 (the v 10 = 3 ← v 10 = 2 “superhot” bands). A simultaneous least-squares fit of both types of data provides their reliable but in the case of accurate rotational data not always fully quantitative reproduction.

Collisionless Laser-Energy Conversion by Anharmonic Resonance

After two decades of experiments with intense fs laser pulses the physical mechanism of collisionless absorption in overdense matter is still not understood. We show that anharmonic resonance in the self-generated plasma potential at a steep ion density profile may represent the leading physical absorption mechanism. Resonance provides for the phase shift of the free electron current which is compulsory for laser beam energy transfer to any medium and is capable of explaining the prompt generation of fast electrons with maximum energies exceeding many times their quiver energy, and the polarization dependence.

Theoretical Spectroscopy of the N2HAr+ Complex

The six-dimensional potential energy surface of the electronic ground state of N2HAr+ is determined by ab initio computations at the CCSD(T) level of theory. The potential energy surface is used to derive a set of spectroscopic data for N2HAr+ and N2DAr+ using second order perturbation theory. Full six-dimensional (6-D) rotation-vibration computations are also carried out using an analytical representation of the surface for J=0 and 1, in order to deduce the rovibrational spectra of N2HAr+ and its deuterated isotopomer. Our variationally determined anharmonic wavenumbers differ by less than 15 cm(-1) from the most accurate experimental values. Strong anharmonic resonances are found between the rovibrational levels of both cations even at low energies.