Lowest Vibrational Levels Research Articles

Bichromatic Photoassociation Spectroscopy for the Determination of Rotational Constants of Cs2 0 u + Long-Range State below the 6S1/2 + 6P1/2 Asymptote.

This article demonstrates new observation of the high-resolution ro-vibrational bichromatic photoassociation spectra (BPAS) of Cs2 in the long-range state below the asymptotes 6S1/2 + 6P1/2. By combining with a modulation spectroscopic technique, precise references of the frequency differences have been engineered through the BPAS, with which the rotational constants of low-lying vibrational levels of the Cs2 long-range state have been accurately determined by fitting the frequency differences to the non-rigid-rotor model. The rotational constants for the newly observed seven ro-vibrational levels are summarized and disagreement for the level ῦ = 498 is clarified. The rotational constants of different vibrational levels demonstrate strong perturbations of the related energy structures. A simple analysis is performed and shows good agreement with experimental results.

Radiative lifetimes of the 11Σ–, 11Δ, 21Σ+, a3Π, 13Σ+, d3Δ, and e3Σ– states of carbon monosulfide

Numerous experimental and theoretical investigations studied the transition properties of the A1Π – X1Σ+ system of carbon monosulfide. However, only a few transition properties are currently available except for the A1Π state. Therefore, this study calculated the properties of the dipole-allowed transitions arising from the 11Σ–, 11Δ, 21Σ+, a3Π, 13Σ+, d3Δ, and e3Σ– states and the spin–forbidden transition from the a3Π to the X1Σ+ state. The radiative lifetimes were approximately 10 ns for the 21Σ+ state, 10 μs for the e3Σ– state, 10 – 100 μs for the d3Δ state, and 10 – 1000 μs for the 13Σ+ state. The emissions from the 21Σ+ – X1Σ+ system were strong. The radiative lifetimes were of the order of 0.1 – 1000 ms for the 11Δ state and 0.1 – 105 ms for the 11Σ– state. The lower the vibrational level of the 11Δ and 11Σ– states was, the longer the radiative lifetime became, suggesting that the spontaneous emissions arising from the lower vibrational levels of the two states were difficult to be measured through spectroscopy. The radiative lifetimes were of the order of 1 – 10 and 1 ms for the a3Π1 and a3Π0+ states, respectively; whereas those of the a3Π2 and a3Π0– states were extremely long. The radiative–lifetime distribution varying with the rotational angular quantum number J was investigated at J ≤ 70 for a particular vibrational level of the 11Σ–, 11Δ, 21Σ+, a3Π, 13Σ+, d3Δ, e3Σ–, a3Π1, and a3Π0+ states.

Complete characterization of the 3p Rydberg complex of a molecular ion: MgAr+. I. Observation of the Mg(3pσ)Ar+ B+ state and determination of its structure and dynamics.

We report on the experimental observation of the B+ 2Σ+ state of MgAr+ located below the Mg+(3p 2P3/2) + Ar(1S0) dissociation asymptote. Using the technique of isolated-core multiphoton Rydberg-dissociation spectroscopy, we have recorded rotationally resolved spectra of the B+ 2Σ+(v') ← X+ 2Σ+(v″ = 7) transitions, which extend from the vibrational ground state (v' = 0) to the dissociation continuum above the Mg+(3p 2P3/2) + Ar(1S0) dissociation threshold. The analysis of the rotational structure reveals a transition from Hund's angular-momentum-coupling case (b) at low v' values to case (c) at high v' values caused by the spin-orbit interaction. Measurements of the kinetic-energy release and the angular distribution of the Mg+ fragments detected in the experiments enabled the characterization of the dissociation mechanisms. The vibrational levels of the B+ state above v' = 6 are subject to predissociation into the Mg+(3p 2P1/2) + Ar(1S0) continuum, and the fragment angular distributions exhibit anisotropy β parameters around 0.5, whereas direct dissociation into the continuum above the Mg+(3p 2P3/2) + Ar(1S0) asymptote is characterized by β parameters approaching 2. Molecular ions excited to the B+ state with v' = 0-6 efficiently absorb a second photon to the repulsive part of the 2Σ+ state associated with the Mg+(3d 2D3/2,5/2) + Ar(1S0) continua. The interpretation of the data is validated by the results of ab initio calculations of the low-lying electronic states of MgAr+, which provided initial evidence for the existence of bound vibrational levels of the B+ state and for the photodissociation mechanisms of its low vibrational levels.

Modelling of vibrational nonequilibrium effects on the H2–air mixture ignition under shock wave conditions in the state-to-state and mode approximations

Kinetics of physicochemical processes occurring during combustion of a hydrogen–air mixture initiated by a shock wave was studied using state-to-state and mode models that take into account nonequilibrium vibrational excitation of N2, O2, H2, and OH molecules. The effect of vibrational–translational relaxation on the induction time was considered: it was shown that the delay in establishing thermodynamic equilibrium between vibrational and translational degrees of freedom slows down the initiation of combustion, while the intensity of this effect depends on the temperature and pressure of the gas behind the shock front. An analysis of the peculiarities of vibrational distributions showed that populations of the lower vibrational levels of molecules in the course of chain reactions of combustion behind the shock wave are close to the Boltzmann ones. Therefore, it is sufficient to use the mode approximation. Calculations of the induction time using models of mode approximation showed a significant dependence of the results on the choice of the nonequilibrium factor model. If the nonequilibrium factors are calculated based on the summation of level rate constants of physicochemical processes, the accuracy of the mode model is not inferior to the accuracy of the state-to-state approximation.

Photoswitchable Photon Upconversion from Turn-on Mode Fluorescent Diarylethenes

Photon upconversion (UC) is one kind of anti-Stokes shift process, which generally requires a combination of two or more low-energy photons to produce one high-energy photon. However, another inter...

Study of the Kinetics of Metastable Molecular Nitrogen in the Atmospheres of the Earth, Triton, Titan, and Pluto

In the interaction of high-energy electrons with gases of planetary atmospheres where the primary component is molecular nitrogen, a significant fraction of particle energy is spent on the excitation of electronically excited triplet states of N2. The processes of energy transfer from metastable molecular nitrogen $${{{\text{N}}}_{{\text{2}}}}\left( {{{{\text{A}}}^{{\text{3}}}}\Sigma _{{\text{u}}}^{ + }} \right)$$ to other components in the atmosphere of the Earth (a mixture of N2–O2–O gases), as well as in the atmospheres of Titan, Triton, and Pluto (a mixture of N2–CH4–CO gases), are considered. The paper discusses the processes in which metastable molecular nitrogen $${{{\text{N}}}_{{\text{2}}}}\left( {{{{\text{A}}}^{{\text{3}}}}\Sigma _{{\text{u}}}^{ + }} \right)$$ affects the kinetics of electronically excited atomic and molecular oxygen in the auroral ionosphere of the Earth. In addition, it is shown numerically for the first time that the contribution of $${{{\text{N}}}_{{\text{2}}}}\left( {{{{\text{A}}}^{{\text{3}}}}\Sigma _{{\text{u}}}^{ + }} \right)$$ to the formation of electronically excited carbon monoxide CO(a3Π) increases significantly with increasing density in the atmospheres of Titan, Triton, and Pluto, and becomes predominant for the lower vibrational levels of CO(a3Π).

Rotational and rovibrational transitions in the a 1Δ2 and b 1Σ+0+ states of sulfur monoxide radical

ABSTRACTSulfur monoxide radical has widely been detected in outer space using ground-state spectroscopy. The a 1Δ2 and b 1Σ+0+ states of this radical have low excitation energies, and they possibly exist in outer space. In this work, the potential energy curves and dipole moment functions of the two states were evaluated using the complete active space self- consistent field method, followed by the valence internally contracted multireference configuration interaction approach. The transition line positions, oscillator strengths, band transition dipole matrix elements, Einstein A coefficients, and Franck–Condon factors of all transitions were calculated for lower vibrational levels at rotational angular momentum quantum number J up to 150. The transition line positions calculated in this study are in good agreement with the experimental results. The rovibrational transition became noticeably weak at Δυ > 5. Comparing the results of a 1Δ2 and b 1Σ+0+ states reported in this paper with the previous values, we conclude that these results are the most accurate and complete to date.

Electronic Structure Calculations with the Spin Orbit Effect of the Low-Lying Electronic States of the YbBr Molecule.

This work presents an electronic structure study employing multireference configuration interaction MRCI calculations with Davidson correction (+Q) of the ytterbium monobromide YbBr molecule. Adiabatic potential energy curves (PECs), dipole moment curves, and spectroscopic constants (such as Re, ωe, Be, De, Te, and μe) of the low-lying bound electronic states are determined. The ionic character of the YbBr molecule at the equilibrium position is also discussed. With spin–orbit effects, 30 low-lying states in Ω = 1/2, 3/2, 5/2, 7/2 representation are probed. The electronic transition dipole moment is calculated between the investigated states and then used to determine transition coefficients, for example, the Einstein coefficient of spontaneous emission Aij and emission oscillator strength fij. Vibrational parameters such as Eν, Bν, Dν, Rmin, and Rmax of the low vibrational levels of different bound states in both Λ and Ω representations are also calculated. Upon calculating the Franck–Condon factors, they are found to be perfectly diagonal between three couples of low-lying excited states. Vibrational Einstein coefficients and radiative lifetimes are computed as well for the lowest vibrational transitions. Most of the data reported in this work are presented here for the first time in the literature. Very good accordance is obtained in comparison with the previously reported constants by means of experimental methods.

Dissociative Recombination of CH+ Molecular Ion Induced by Very Low Energy Electrons

We used the multichannel quantum defect theory to compute cross sections and rate coefficients for the dissociative recombination of CH + initially in its lowest vibrational level v i + = 0 with electrons of incident energy below 0.2 eV. We have focused on the contribution of the 2 2 Π state which is the main dissociative recombination route at low collision energies. The final cross section is obtained by averaging the relevant initial rotational states ( N i + = 0 , ⋯ , 10 ) with a 300 K Boltzmann distribution. The Maxwell isotropic rate coefficients for dissociative recombination are also calculated for different initial rotational states and for electronic temperatures up to a few hundred Kelvins. Our results are compared to storage-ring measurements.

Multivalued property of Rayleigh–Schrödinger perturbation series for vibrational energy levels of molecules

We have calculated 200 lower vibrational energy levels of the formaldehyde molecule using ab initio quartic force field, high-order Rayleigh–Schrödinger perturbation theory (up to 200th order) and algebraic Hermit–Padé approximants (up to sixth degree) to sum up divergent perturbation series, corresponding to each vibrational state. Both energy levels and algebraic Hermit–Padé approximants are, in fact, multivalued functions and consist of several branches. We find that branches of the approximant constructed using coefficients of the perturbation series for a particular vibrational state can reproduce not only the energy of this state, but also of other states which are close in energy to the considered one and are coupled with them by anharmonic resonances. The multivalued property of algebraic approximants has been previously demonstrated by Jordan in the case of two-state avoided crossings of the potential energy curves of diatomic molecules (Jordan 1975 Int. J. Quantum Chem. 9 325) and by Fernandez and Diaz in the case of one-dimensional periodic eigenvalue problems (Fernandéz and Diaz 2001 Eur. Phys. J. D 15 41). This paper extends the study of this nontrivial property of perturbation theory to the vibrational states of polyatomic molecules, a problem with many dimensions. Additionally, we have found that this property is useful in some special cases when perturbation series diverges so fast that most high-order approximants do not reproduce the correct value of the energy with the required accuracy.

Resonant Two-Photon Photoelectron Imaging and Intersystem Crossing from Excited Dipole-Bound States of Cold Anions.

We report the observation of a dipole-bound state (DBS) 659 cm-1 below the electron detachment threshold of cryogenically cooled deprotonated 4,4'-biphenol anion (bPh-) and 19 of its lowest vibrational levels. Resonant two-photon photoelectron imaging (R2P-PEI) via the vibrational levels of the DBS displays a sharp peak with a constant binding energy. This observation indicates vertical detachment from the vibrational levels of the DBS to the corresponding neutral levels with the conservation of the vibrational energy, suggesting that the highly diffuse electron in the DBS has little effect on the neutral core. The R2P-PEI spectra also exhibit two features at lower binding energies, which come from intersystem crossings from the DBS to two lower-lying valence-bound triplet excited states of bPh-. The current study discloses the first R2P-PEI spectra from vibrational excited states of a DBS and direct spectroscopic evidence of transitions from a DBS to valence-bound states of anions.

Quantum mechanical study of the high-temperature H+ + HD → D+ + H2 reaction for the primordial universe chemistry

ABSTRACT We use the time-independent quantum-mechanical formulation of reactive collisions in order to investigate the state-to-state H+ + HD → D+ + H2 chemical reaction. We compute cross-sections for collision energies up to 1.8 eV and rate coefficients for temperatures up to 10 000 K. We consider HD in the lowest vibrational level v = 0 and rotational levels j = 0–6, and H2 in vibrational levels v′ = 0–3 and rotational levels j′ = 0–9. For temperatures below 4000 K, the rate coefficients strongly vary with the initial rotational level j, depending on whether the reaction is endothermic (j ≤ 2) or exothermic (j ≥ 3). The reaction is also found less and less probable as the final vibrational quantum number v′ increases. Our results illustrate the importance of studying state-to-state reactions, in the context of the chemistry of the primordial universe.

An accurate ab initio electronic structure calculation for interstellar argonium

A high level electronic structure calculation of the Born–Oppenheimer (BO) potential, scalar-relativistic effect and adiabatic correction is performed for the X1Σ+ ground state of astronomically important isotopologues of the ArH+ cation. Particular attention is paid to both short and long distances, where experimental spectroscopic data is not yet available. The BO energy is obtained over a wide range of interatomic distances (R ∈ [0.5, 20] Å) in the framework of multi-reference coupled pair functional (MR-ACPF) and single-reference coupled-cluster (CCSD(T)) methods. The all-electrons basis sets of the def2-nZVPP (n=3,4) and nZaPa-NR, aug-cc-pCVnZ (n=3−6) quality are used to monitor energy convergence. The BO energies are corrected for the basis set superposition error (BSSE) via the counterpoise method and are reduced to the complete basis set (CBS) limit using alternative extrapolating schemes. The permanent dipole moment d(R) and static dipole polarizability α(R) are calculated using a finite-field method. The molecular structure parameters coincide with their empirical counterparts for low vibrational levels to within 0.1%. Implementation of the ab initio results to circumscribe our understanding of the chemical networks in interstellar space is discussed.

Ab initio spectroscopic characterization of the radical CH3OCH2 at low temperatures.

Spectroscopic and structural properties of methoxymethyl radical (CH3OCH2, RDME) are determined using explicitly correlated ab initio methods. This radical of astrophysical and atmospheric relevance has not been fully characterized at low temperatures, which has delayed astrophysical research. We provide rovibrational parameters, excitations to the low energy electronic states, torsional and inversion barriers, and low vibrational energy levels. In the electronic ground state (X2A), which appears "clean" from nonadiabatic effects, the minimum energy structure is an asymmetric geometry whose rotational constants and dipole moment have been determined to be A0 = 46 718.67 MHz, B0 = 10 748.42 MHz, and C0 = 9272.51 MHz, and 1.432D (μA = 0.695D, µB = 1.215D, µC = 0.302D), respectively. A variational procedure has been applied to determine torsion-inversion energy levels. Each level splits into 3 subcomponents (A1/A2 and E) corresponding to the three methyl torsion minima. Although the potential energy surface presents 12 minima, at low temperatures, the infrared band shapes correspond to a surface with only three minima because the top of the inversion Vα barrier at α = 0° (109 cm-1) stands below the zero point vibrational energy and the CH2 torsional barrier is relatively high (∼2000 cm-1). The methyl torsion barrier was computed to be ∼500 cm-1 and produces a splitting of 0.01 cm-1 of the ground vibrational state.

Nanosecond Pulsed Discharge for CO2 Conversion: Kinetic Modeling To Elucidate the Chemistry and Improve the Performance

We study the mechanisms of CO 2 conversion in a nanosecond repetitively pulsed (NRP) discharge, by means of a chemical kinetics model. The calculated conversions and energy efficiencies are in reasonable agreement with experimental results over a wide range of specific energy input values, and the same applies to the evolution of gas temperature and CO 2 conversion as a function of time in the afterglow, indicating that our model provides a realistic picture of the underlying mechanisms in the NRP discharge and can be used to identify its limitations and thus to suggest further improvements. Our model predicts that vibrational excitation is very important in the NRP discharge, explaining why this type of plasma yields energy-efficient CO 2 conversion. A significant part of the CO 2 dissociation occurs by electronic excitation from the lower vibrational levels toward repulsive electronic states, thus resulting in dissociation. However, vibration-translation (VT) relaxation (depopulating the higher vibrational levels) and CO + O recombination (CO + O + M → CO 2 + M), as well as mixing of the converted gas with fresh gas entering the plasma in between the pulses, are limiting factors for the conversion and energy efficiency. Our model predicts that extra cooling, slowing down the rate of VT relaxation and of the above recombination reaction, thus enhancing the contribution of the highest vibrational levels to the overall CO 2 dissociation, can further improve the performance of the NRP discharge for energy-efficient CO 2 conversion.

The CRDS spectrum of acetylene near 1.73 µm

The high-resolution absorption spectrum of acetylene has been recorded at room temperature by high sensitivity cavity ring down spectroscopy (CRDS) in the 5693–5882 cm–1 region. The noise level of the recordings corresponds to a typical minimum detectable absorption, αmin, below 10-10 cm−1. The positions and intensities of about 4000 lines were determined and analyzed together with about 3900 lines left unassigned in a previous CRDS study in the 5850–6340 cm−1 region. In total, about 2620 12C2H2 and 250 12C13CH2 absorption lines are assigned in the present work, most of them being newly reported. The 12C2H2 lines belong to 53 bands and include 24 bands newly identified or improved in the region of the previous study. Altogether, 107 bands in the entire 5693–6340 cm−1 while the HITRAN database provides line parameters of only nine 12C2H2 bands in the same region. The lines of 12C13CH2 in normal abundance (2.2%) belong to eight bands whose intensities are reported for the first time. Spectroscopic parameters of the upper vibrational levels were derived from standard band-by-band fits of the line positions (typical rms values of the (obs.-calc.) deviations are better than 0.002 cm–1). The vibrational transition dipole moment squared and Herman–Wallis coefficients of each band were derived from a fit of the measured intensity values. A recommended line list is provided for the bands newly assigned with positions calculated using empirical spectroscopic parameters of the lower and upper energy vibrational levels and intensities computed using the derived Herman–Wallis coefficients. As a result, the constructed line list includes a total of 4471 transitions belonging to 53 bands of 12C2H2 and 8 bands of 12C13CH2. This list of acetylene transitions above 5693 cm−1 extends to lower energy and completes our empirical spectroscopic database for acetylene in the 5850–9415 cm−1 range.

Understanding propyl cyanide and its isomers formation: ab initio study of the spectroscopy and reaction mechanisms.

Recent detection of propyl cyanide (C3H7CN) with both linear and branched structures has stimulated many experimental and theoretical studies. In this theoretical work, we present the spectroscopic properties of the far infrared spectra of these species and we investigate their different paths of formation in the gas phase. Our spectroscopic study concerns the far infrared spectra of iso, anti and gauche propyl cyanide isomers. The equilibrium structures and the potential energy surfaces are calculated using explicitly correlated cluster ab initio methods (CCSD(T)-F12) and a variational procedure designed for non-rigid species and large amplitude motions. Accurate rotational constants, centrifugal distortion constants, potential energy barriers and surfaces are provided. The rovibrational parameters in the ground vibrational states compare very well with experimental data. The low energy vibrational levels correspond to torsional modes. Far infrared energies are calculated up to 500 cm-1 using the variational approach and the vibrational second order theory (VPT2), and a good agreement with previous experimental values is found. We have also investigated the gas phase formation of the different C3H7CN isomers. After several trials of reacting gaseous species, we considered that a possible formation route of the C3H7CN isomers can be from the bimolecular reaction of HCN with propene. At the UMP2(full)/aug-cc-pVTZ level of theory, this reaction involves two steps for each isomer; the first one corresponds to the association of the two radicals while the second one corresponds to H transfer. From highly correlated ab initio calculations by means of CCSD(T)/aug-cc-pVTZ//UMP2(full)/aug-cc-pVTZ, the geometries, energetics and minimum energy paths of the reactions are obtained. Also, the first step's transition state disappears, as the diradical minimum is much less stabilized with UCCSD(T) than by UMP2(full). We employ the zero curvature tunneling and canonical variational (CVT/ZCT) semiclassical method to predict rate constants for propyl cyanide isomers formation in the gas phase. However, due to the presence of a significant barrier, this reaction leads to very small rate constants. Very recently, we probed reaction mechanisms involving radical addition and we found that these reactions are barrierless and highly exothermic leading to expected fast reactive processes for the formation of propyl cyanide products. The kinetics of such diradical reactions is under study.

Ab initio study of the low-lying electronic states of YbCl molecule including spin-obit effects

Ab initio quantum chemistry calculations for the low-lying electronic states of YbCl molecule, including the spin-orbit effects, have been performed via the CASSCF/MRCI (single and double excitations with Davidson correction) method. Adiabatic potential energy curves have been investigated for the low-lying electronic states in the 2s+1Λ(+/−) and Ω representations. Static dipole moment curves of the most investigated states are also computed. The spectroscopic constants and the percentage ionic character fionic of the lowest doublet and quartet bound states are calculated. The transition dipole moments, the spontaneous radiative lifetime and some emission coefficients are determined for the lowest electronic transitions. By using the canonical functions approach, the ro-vibrational parameters are also determined for different bound states. New theoretical data is studied in this work. The Franck–Condon factors (FCFs) for transitions involving the low vibrational levels of the (2)1/2, (1)3/2 and (3)1/2 states are highly diagonally distributed, unlike the other investigated electronic transitions. The off-diagonal FCFs of the transitions between the lowest-excited states and the ground state, indicate that direct laser cooling of the studied molecule is experimentally difficult.

Time-dependent quantum description of molecular double-core-hole-state formation: Impact of the nuclear dynamics on sequential two-photon processes

We present a theoretical model to investigate double-core-hole-state formation in molecules through sequential absorption of two x-ray photons from a femtosecond laser pulse. A complete time-dependent quantum description taking into account x-ray absorption and nuclear dynamics explicitly and Auger decay phenomenologically is established within the local approximation. Using this model, we assess the impact of the nuclear dynamics on double-core photoionization processes in the case of the carbon monoxide molecule. We show that sequential absorption of two x-ray photons modifies significantly the vibrational distribution of the photoelectron spectra of the double-core-hole states compared to direct single x-ray photon absorption. Depending on the shape of the potential energy curves involved in the sequential absorption processes, lower or higher vibrational levels may be favored. Furthermore, in the case where the final state is dissociative, the electron spectrum is further broaden and blue-shifted in the two-photon process.

New insights for mesospheric OH: multi-quantum vibrational relaxation as a driver for non-local thermodynamic equilibrium.

The question of whether mesospheric OH(υ) rotational population distributions are in equilibrium with the local kinetic temperature has been debated over several decades. Despite several indications for the existence of non-equilibrium effects, the general consensus has been that emissions originating from low rotational levels are thermalized. Sky spectra simultaneously observing several vibrational levels demonstrated reproducible trends in the extracted OH(υ) rotational temperatures as a function of vibrational excitation. Laboratory experiments provided information on rotational energy transfer and direct evidence for fast multi-quantum OH(high-υ) vibrational relaxation by O atoms. We examine the relationship of the new relaxation pathways with the behavior exhibited by OH(υ) rotational population distributions. Rapid OH(high-υ) + O multi-quantum vibrational relaxation connects high and low vibrational levels and enhances the hot tail of the OH(low-υ) rotational distributions. The effective rotational temperatures of mesospheric OH(υ) are found to deviate from local thermodynamic equilibrium for all observed vibrational levels.