Lowest Rotational Levels Research Articles

Dissociative recombination of rotationally cold ArH+

We have experimentally studied dissociative recombination (DR) of electronically and vibrationally relaxed ArH+ in its lowest rotational levels, using an electron-ion merged-beams setup at the Cryogenic Storage Ring. We report measurements for the merged-beams rate coefficient of ArH+ and compare it to published experimental and theoretical results. In addition, by measuring the kinetic energy released to the DR fragments, we have determined the internal state of the DR products after dissociation. At low collision energies, we find that the atomic products are in their respective ground states, which are only accessible via nonadiabatic couplings to neutral Rydberg states. Published theoretical results for ArH+ have not included this DR pathway. From our measurements, we have also derived a kinetic temperature rate coefficient for use in astrochemical models. Published by the American Physical Society 2024

Mechanism of Ultraviolet-Induced CO Desorption from CO Ice: Role of Vibrational Relaxation Highlighted.

Although UV photon-induced CO ice desorption is clearly observed in many cold regions of the Universe as well as in the laboratory, the fundamental question of the mechanisms involved at the molecular scale remains debated. In particular, the exact nature of the involved energy transfers in the indirect desorption pathway highlighted in previous experiments is not explained. Using abinitio molecular dynamics simulations, we explore a new indirect desorption mechanism in which a highly vibrationally excited CO (v=40) within an aggregate of 50 CO molecules triggers the desorption of molecules at the surface. The desorption originates first from a mutual attraction between the excited molecule and the surrounding molecule(s), followed by a cascade of energy transfers, ultimately resulting in the desorption of vibrationally cold CO (∼95% in v=0). The theoretical vibrational distribution, along with the kinetic energy one, which peaks around 25meV for CO with low rotational levels (v=0, J<7), is in excellent agreement with the results obtained from VUV laser induced desorption (157nm) of CO (v=0, 1) probed using REMPI.

Dissociative Recombination of Rotationally Cold OH+ and Its Implications for the Cosmic Ray Ionization Rate in Diffuse Clouds

Observations of OH+ are used to infer the interstellar cosmic ray ionization rate in diffuse atomic clouds, thereby constraining the propagation of cosmic rays through and the shielding by interstellar clouds, as well as the low energy cosmic ray spectrum. In regions where the H2-to-H number density ratio is low, dissociative recombination (DR) is the dominant destruction process for OH+ and the DR rate coefficient is important for predicting the OH+ abundance and inferring the cosmic ray ionization rate. We have experimentally studied DR of electronically and vibrationally relaxed OH+ in its lowest rotational levels, using an electron–ion merged-beams setup at the Cryogenic Storage Ring. From these measurements, we have derived a kinetic temperature rate coefficient applicable to diffuse cloud chemical models, i.e., for OH+ in its electronic, vibrational, and rotational ground level. At typical diffuse cloud temperatures, our kinetic temperature rate coefficient is a factor of ∼5 times larger than the previous experimentally derived value and a factor of ∼33 times larger than the value calculated by theory. Our combined experimental and modeling results point to a significant increase for the cosmic ray ionization rate inferred from observations of OH+ and H2O+, corresponding to a geometric mean of (6.6 ± 1.0) × 10−16 s−1, which is more than a factor of 2 larger than the previously inferred values of the cosmic ray ionization rate in diffuse atomic clouds. Combined with observations of diffuse and dense molecular clouds, these findings indicate a greater degree of cosmic ray shielding in interstellar clouds than has been previously inferred.

Exhaustive state-specific dissociation study of the N2(Σg+1)+N(S4) system using QCT combined with a neural network method.

This work studies the exhaustive rovibrational state-specific collision-induced dissociation properties of the N2+N system by QCT (quasi-classical trajectory) combined with a neural network method based on the abinitio PES recently published by Varga et al. [Phys. Chem. Chem. Phys. 23, 26273 (2021)]. The QCT combined with a neural network for state-specific dissociation (QCT-NN-SSD) model is developed and used to predict the dissociation cross sections and their energy dependence on the thermal range from a sparsely sampled noisy dataset. It is conservatively estimated that using this method can reduce the cost of the calculation by 96.5%. The rate coefficient of thermal non-equilibrium between different energy modes is obtained by combining the QCT-NN-SSD model with the multi-temperature model. The results show that, for the equilibrium state, dissociation mainly occurs at high vibrational and moderately low rotational levels. When the system is in non-equilibrium, there is no obvious vibrational level preference and highly rotationally excited molecules play a major role in promoting the dissociation by compensating for the lack of vibrational energy. The use of neural network training to generate complete datasets based on limited and discrete data provides an economical and reliable way to obtain a complete kinetic database needed to accurately simulate non-equilibrium flows.

Collision excitation of nitrous acid (HONO) by helium: isomerization effect

ABSTRACTWe generated new 3D-potential energy surfaces (3D-PESs) for the cis-HONO–He and trans-HONO–He weakly bound complexes along the intermonomer coordinates. We used the explicitly correlated Coupled Clusters with single, double, and perturbative triple excitations (CCSD(T)-F12) approach for the electronic structure computations, where the atoms were described using the aug-cc-pVTZ basis set. Then, we derived analytical forms for each PES. These PESs exhibit different shapes and present strong anisotropies. After quantum close-coupling scattering calculations for the lower rotational levels (up to ${9}_{2,7}$), and the coupled-states approximation for higher levels (up to ${22}_{1,22}$) using these PESs, we derived the collisional excitation cross sections of cis-HONO and trans-HONO by He for total energies 0.1 ≤ E ≤ 900 cm−1 and the rate coefficients for kinetic temperatures T ≤ 100 K. Our work shows that the collision data of cis-HONO and trans-HONO are different mainly because of the different 3D-PESs since the rotational energy structures of both isomers are very similar. Also, computations show that the data of the non-detected cis-HONO are as large as those of the detected trans-HONO isomer. They confirm the large values for the detected transition 52,4 → 41,3 of trans-HONO. Therefore, our work strongly suggests revisiting radiative transfer calculations to determine accurately the population of the rotational levels of these isomers. Our work should help astrophysicists for the detectability of such nitrogen oxide molecules and for the possible formation mechanisms and isomerization pathways specificities.

Microscopic theory of Raman scattering for the rotational organic cation in metal halide perovskites

A gap exists in the microscopic understanding the dynamic properties of the rotational organic cation (ROC) in the inorganic framework of the metal halide perovskites (MHP) to date. Herein, we develop a microscopic theory of Raman scattering for the ROC in MHP based on the angular momentum of a ROC exchanging with that of the photon and phonon. We systematically present the selection rules for the angular momentum transfer among the three lowest rotational levels. We find that the phonon angular momentum that arises from the inorganic framework and its specific values could be directly manifested by Stokes (or anti-Stokes) shift. Moreover, the initial orientation of the ROC and its preferentially rotational directions could be judged in Raman spectra. This study lays the theoretical foundation for the high-precision resolution and manipulation of molecular rotation immersed in the many-body environment by Raman technique.

Disilicon carbide (Si2C) in the interstellar medium

The Si2C and SiC2 both are considered to play key role in the formation of the SiC dust grains in the atmosphere of the carbon-rich stars. The molecule of our interest Si2C has been detected in the envelope of the red supergiant star IRC+10216 first time. It is an asymmetric top molecule having electric dipole moment of 1 Debye along the b-axis of inertia. Because of zero nuclear spin of both the Carbon and Silicon atoms, it has only paratransitions. Using the given spectroscopic data (rotational and centrifugal distortion constants and electric dipole moment), for the para-Si2C, we have calculated energies of 200 lower rotational levels (having energy up to 217.8 cm-1) and the Einstein A and B coefficients for 867 radiative transitions between the levels. We have solved a set of 200 statistical equilibrium equations coupled with 867 equations of radiative transfer (Sobolev analysis), where the collisional rate coefficients are taken from a scaling law. Out of 867 radiative transitions, 13 transitions have been found showing weak MASER action, and 19 transitions showing anomalous absorption. One transition 808-717 is found to show both the MASER action as well as the anomalous absorption. These transitions in addition to the observed transitions may play important role in the identification of Si2C in the cosmic objects.

Observation of rotationally dependent fine-structure branching ratios near the predissociation threshold N(2D5/2,3/2) + N(2D5/2,3/2) of 14N2.

Photofragment spin-orbit fine-structure branching ratios have long been predicted to depend on the rotational quantum number J' by theory near the dissociation thresholds of several diatomic molecules, while this has rarely been observed in any photodissociation experiments yet. Here, we measured the fine-structure branching ratios N(2D5/2)/N(2D3/2) produced in the N(2D5/2,3/2) + N(2D5/2,3/2) channel at the b'1Σu +(v = 20) state of 14N2 by using our vacuum ultraviolet (VUV)-pump-VUV-probe time-sliced velocity-mapped ion imaging setup. It is found that 14N2 almost exclusively dissociates into the spin-orbit channel N(2D5/2) + N(2D3/2) at low rotational levels and gradually approaches the statistical or diabatic limit by distributing all possible spin-orbit channels at higher rotational levels. The strongly rotationally dependent fine-structure branching ratios should be due to the increasing strength of nonadiabatic Coriolis interaction among various dissociative states in the so-called "recoupling zone" as J' increases. They are supposed to provide unprecedented information on the near threshold photodissociation dynamics of 14N2.

Collisional excitation of isotopologs of H2S by molecular hydrogen: D2S and HDS

ABSTRACT Rate coefficients for transitions between the lower rotational levels of two isotopologs of hydrogen sulphide, specifically HDS and D2S, induced by collisions with para-H2 and ortho-H2 are presented in this work. The availability of these rate coefficients will allow accurate estimates to be made of the abundance of these species in the interstellar medium to be made from astronomical observations. The rate coefficients were computed in close-coupling calculations using potential energy surfaces (PESs) obtained by transformation of the H2S–H2 PES previously calculated with the explicitly correlated CCSD(T)-F12a coupled-cluster method. Rate coefficients for transitions between rotational levels with energies less than 196 cm−1 and temperatures from 5 to 400 K have been calculated for HDS, ortho-D2S, and para-D2S. The rate coefficients for the H2S–H2, HDS–H2, and D2S–H2 systems are compared

The measurements of the CMB temperature in diffuse interstellar medium of the Milky-Way and high redshift galaxies based on excitation of CI fine-structure and H2 rotational levels

Evolution of the cosmic microwave background (CMB) temperature with redshift is predicted by the standard ΛCDM cosmological model and has been confirmed by measurements of the Sunyaev-Zel’dovich effect in Plank data (at z ≤ 1) and excitation of CO rotational levels in quasar spectra (at 1.7 ≤ z ≤ 2.7). Excitation of the fine-structure levels of neutral carbon (CI) is also sensitive to the temperature of the CMB radiation. However collisions and UV pumping lead to a significant degeneracy of the fitting parameters, since poor data on physical conditions. We found that a joint fit to excitation of low H2 rotational and CI fine-structure levels can break this degeneracy and provide a tighter constraints on the T CMB. We present estimates of the T CMB derived from excitation of CI fine-structure levels in the Milky Way clouds and high redshift absorption systems.

Transition 110−111 of methanimine in interstellar medium

Being precursor of glycine (NH2CH2COOH)—the simplest amino acid, the methanimine (CH2NH) in gas phase is of great importance for spectroscopists and astronomers both. Using accurate rate coefficients for collisional transitions between 15 lower rotational levels of methanimine, we have performed Sobolev large velocity gradient analysis and have discussed about weak MASER action of 110−111 transition of methanimine. We have done analytical analysis also for understanding the MASER action of 110−111 transition of methanimine. It is found that contribution of collisional transitions is less significant as compared to that of radiative ones.

Physical conditions in the diffuse interstellar medium of local and high-redshift galaxies: measurements based on the excitation of H2 rotational and C i fine-structure levels

ABSTRACT We present the results of an analysis of the physical conditions (number density, intensity of UV field, kinetic temperature) in the cold H2-bearing interstellar medium of local and high-redshift galaxies. Our measurements are based on the fit to the observed population of H2 rotational levels and C i fine-structure levels with the help of grids of numerical models calculated with the photon-dominated region (PDR) Meudon code. A joint analysis of low H2 rotational levels and C i fine-structure levels breaks the degeneracy in the IUV−nH plane and provides significantly tighter constraints on the number density and intensity of the UV field. Using archive data from the VLT/UVES, KECK/HIRES, HST/STIS and FUSE telescopes, we selected 12 high-redshift damped Lyα systems (DLAs) in quasar spectra and 14 H2 absorption systems along the lines of sight towards stars in the Milky Way and the Magellanic Cloud galaxies. These systems have strong H2 components, with a column density log N(H2)/[cm−2] &amp;gt; 18 and associated C i absorptions. We find that H2-bearing media in high-redshift DLAs and in local galaxies have similar values of the kinetic temperatures Tkin ∼ 100 K and number density 10−500 cm−3. However, the intensity of incident UV radiation in DLAs varies in a wide range (0.1−100 units of the Mathis field), while it is ∼0.1−3 units of the Mathis field for H2 systems in the Milky Way and Large and Small Magellanic Cloud galaxies. The large dispersion of measured UV flux in DLAs is probably a consequence of the fact that the DLA sample probes galaxies selected from the overall galaxy population at high redshift, and therefore corresponds to a wide range of physical conditions.

Suggestion for Search of Malononitrile (CH2(CN)2) in a Cosmic Object: Potential Spectral Lines

Though accurate laboratory studies of malononitrile (CH2(CN)2), a dinitrile molecule, have been carried out from time to time, and it has large electric dipole moment $$\mu = 3.735$$ D, its detection in a cosmic object is still awaited. Because of two equivalent hydrogen atoms, it has ortho and para species. In order to obtain information about some potential spectral lines of malononitrile, which may play important role for its identification in a cosmic object, we have carried out Sobolev large velocity gradient (LVG) analysis, using spectroscopic data, for each of the species. We have calculated energies of 250 lower rotational levels (up to 98 cm–1) for each species and the Einstein $$A$$ and $$B$$ coefficients for 1231 (ortho) and 1229 (para) radiative transitions between the levels. The collisional rate coefficients, required for the analysis, are calculated with the help of a scaling law. Very exciting results are obtained; we have analyzed 6 potential transitions for each species of CH2(CN)2, which may play important role in the identification of malononitrile in a cosmic object.

Rotational excitation of cyanogen ion, CN+ (X1Σ+) by He collisions

Modeling the physical conditions of interstellar medium requires knowledge of accurate rate coefficients for collisional excitation of molecules by the abundant chemical species like He and H2. The present paper aims to study transitions in the low rotational levels in the ground vibrational state of CN+(X1Σ+) by collisions with helium atoms. We computed ab initio two-dimensional (rigid-rotor) potential energy surface for the He-CN+ van der Waals collision complex using multi-reference configuration interaction method employing augmented correlation consistent polarized valence quadruple-ζ basis set. The bound-states of the van der Waals complex are obtained by coupled channel approach. The state-to-state rotational excitation cross sections are computed by exact close-coupling quantum mechanical formalism up to center of mass collision energy of 2000 cm−1. The corresponding rate coefficients are obtained up to 300 K temperature.

Field-free, Stark, and Zeeman spectroscopy of the Ã2Π1/2−X̃2Σ+ transition of ytterbium monohydroxide

The 000A2Π1/2−X2Σ+, 110A2Π1/2−X2Σ+, and 101A2Π1/2−X2Σ+ bands of an internally cold molecular beam sample of ytterbium monohydroxide, YbOH, have been recorded at the near natural linewidth limit and analyzed to determine the fine structure parameters. Numerous lines in the 000A2Π1/2−X2Σ+ band associated with the lowest rotational levels were recorded in the presence of a static electric field and analyzed to determine the magnitude of the molecular frame permanent electric dipole moment, |μel|, for the X2Σ+(0,0,0) and A2Π1/2(0,0,0) states of 1.9(2) and 0.43(10) D, respectively. An electrostatic polarizability model is used to predict the μel for the X2Σ+(0,0,0) state. The 000A2Π1/2−X2Σ+ band is recorded in the presence of a weak static magnetic field to observe the electric dipole allowed transitions having ΔJ=−2, which are used to assist in the rotational quantum number assignment and confirm the identity of the excited state. An expression for the A2Π1/2(0,0,0) magnetic g factor is derived and shown to correctly model the observed magnetic tuning. A comparison with YbF is made and implications for the use of YbOH as a venue for symmetry violation measurements are discussed.

Low-Energy Water-Hydrogen Inelastic Collisions.

New molecular beam scattering experiments are reported for the water-hydrogen system. Integral cross sections of the first rotational excitations of para- and ortho-H2O by inelastic collisions with normal-H2 were determined by crossing a beam of H2O seeded in He with a beam of H2. H2O and H2 were cooled in the supersonic expansion down to their lowest rotational levels. Crossed-beam scattering experiments were performed at collision energies from 15 cm-1 (below the threshold for the excitation to the lowest excited rotational state of H2O: 18.6 cm-1) up to 105 cm-1 by varying the beam crossing angle. The measured state-to-state cross-sections were compared to the theoretical cross-sections (close-coupling quantum scattering calculations): the good agreement found further validates both the employed potential energy surface describing the H2O-H2 van der Waals interaction and the state-to-state rate coefficients calculated with this potential in the very low temperature range needed for the modeling of interstellar media.

Roaming Dynamics in the Photodissociation of Formic Acid at 230 nm.

Roaming dynamics is observed in the photodissociation of formic acid (HCOOH) at 230 nm by using the slice imaging method. In combination with rotational state selective (2 + 1) resonance-enhanced multiphoton ionization of the CO fragments, the speed distributions of the CO fragments exhibit a low recoil velocity at low rotational levels of J = 9 and 20, while the velocity distributions of CO at high rotational levels of J = 30 and 48 show a relatively large recoil velocity. The experimental results indicate that the roaming of OH radical should be related with the formation of CO + H2O channel at the present photolysis energy. Unlike the roaming pathways occurring in H2CO that can be described by loose flat potential, our CO speed distribution analysis suggests the presence of a "tight" flat potential in the roaming dynamics of HCOOH molecules.

Continuous Loading of Ultracold Ground-State ^{85}Rb_{2} Molecules in a Dipole Trap Using a Single Light Beam.

We have developed an approach to continuously load ultracold ^{85}Rb_{2} vibrational ground-state molecules into a crossed optical dipole trap from a magneto-optical trap. The technique relies on a single high-power light beam with a broad spectrum superimposed onto a narrow peak at an energy of about 9400 cm^{-1}. This single laser source performs all the required steps: the short-range photoassociation creating ground-state molecules after radiative emission, the cooling of the molecular vibrational population down to the lowest vibrational level v_{X}=0, and the optical trapping of these molecules. Furthermore, we probe by depletion spectroscopy and determine that 75% of the v_{X}=0 ground-state molecules are in the three lowest rotational levels J_{X}=0, 1, 2. The lifetime of the ultracold molecules in the optical dipole trap is limited to about 70ms by off-resonant light scattering. The proposed technique opens perspectives for the formation of new molecular species in the ultracold domain, which are not yet accessible by well-established approaches.

Anomalous Intensities in the 2+1 REMPI Spectrum of the E 1Π-X 1Σ+ Transition of CO.

We report on one-color experiments near 214 nm involving the photodissociation of jet-cooled OCS to produce high rotational states (40 < J < 80) of CO (X 1Σ+, v = 0, 1) which were then ionized by 2+1 resonance-enhanced multiphoton ionization via the E 1Π state. The nominally forbidden Q-branch of the two-photon E 1Π-X 1Σ+ transition is observed with intensity comparable to the allowed R-branch. The bright character of the high- J Q-branch lines can be described quantitatively as intensity borrowing due to mixing of the E 1Π and C 1Σ+ states, using J-dependent mixing coefficients extrapolated from the observed Λ-doubling in the lower rotational levels of the E state. In addition to the significant enhancement of Q-branch intensities above the values predicted by conventional two-photon line strengths for a 1Π-1Σ+ transition, the high- J lines of the R- and P-branches appear to be suppressed in intensity by approximately a factor of 3 compared to the unperturbed low- J line strengths, most likely due to perturbations associated with a 1Σ- state. The E-state rotational term values for J < 80, v = 0 derived from the present spectra agree within our measurement and calibration uncertainties with the extrapolations based on the molecular constants previously derived from rotational levels with J < 50. The E-X transition is attractive for future application to photodissociation dynamics and rotational polarization measurements of CO photofragments, with convenient access to state-selective probing on multiple rotational branches, which exhibit different sensitivity to fragment alignment.

Collisional Quantum Dynamics for MgH- (1Σ+) With He as a Buffer Gas: Ionic State-Changing Reactions in Cold Traps.

We present in this paper a detailed theoretical and computational analysis of the quantum inelastic dynamics involving the lower rotational levels of the MgH− (X1Σ+) molecular anion in collision with He atoms which provide the buffer gas in a cold trap. The interaction potential between the molecular partner and the He (1S) gaseous atoms is obtained from accurate quantum chemical calculations at the post-Hartree-Fock level as described in this paper. The spatial features and the interaction strength of the present potential energy surface (PES) are analyzed in detail and in comparison with similar, earlier results involving the MgH+ (1Σ) cation interacting with He atoms. The quantum, multichannel dynamics is then carried out using the newly obtained PES and the final inelastic rats constants, over the range of temperatures which are expected to be present in a cold ion trap experiment, are obtained to generate the multichannel kinetics of population changes observed for the molecular ion during the collisional cooling process. The rotational populations finally achieved at specific temperatures are linked to state-selective laser photo-detachment experiments to be carried out in our laboratory.All intermediate steps of the quantum modeling are also compared with the behavior of the corresponding MgH+ cation in the trap and the marked differences which exist between the collisional dynamics of the two systems are dicussed and explained. The feasibility of the present anion to be involved in state-selective photo-detachment experiments is fully analyzed and suggestions are made for the best performing conditions to be selected during trap experiments.