Single-photon ionization of SiC in the gas phase: experimental and ab initio characterization of SiC. Phys Chem Chem Phys 2023;25:23568-23578. [PMID: 37656136 DOI: 10.1039/d3cp02775a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We report the first experimental observation of single-photon ionization transitions of the SiC radical between 8.0 and 11.0 eV performed on the DESIRS beamline at the SOLEIL synchrotron facility. The SiC radical, very difficult to synthesize in the gas phase, was produced through chemical reactions between CHx (x = 0-3) and SiHy (y = 0-3) in a continuous microwave discharge flow tube, the CHx and SiHy species being formed by successive hydrogen-atom abstractions induced by fluorine atoms on methane and silane, respectively. Mass-selected ion yield and photoelectron spectra were recorded as a function of photon energy using a double imaging photoelectron/photoion coincidence spectrometer. The photoelectron spectrum enables the first direct experimental determinations of the X+ 4Σ- ← X 3Π and 1+ 2Π ← X 3Π adiabatic ionization energies of SiC (8.978(10) eV and 10.216(24) eV, respectively). Calculated spectra based on Franck-Condon factors are compared with the experimental spectra. These spectra were obtained by solving the rovibrational Hamiltonian, using the potential energy curves calculated at the multireference single and double configuration interaction level with Davidson correction (MRCI + Q) and the aug-cc-pV5Z basis set. MRCI + Q calculations including the core and core-valence electron correlation were performed using the aug-cc-pCV6Z basis set to predict the spectroscopic properties of the six lowest electronic states of SiC+. Complete basis set extrapolations and relativistic energy corrections were also included in the determination of the energy differences characterizing the photoionization process. Using our experimental and theoretical results, we derived semi-experimental values for the five lowest ionization energies of SiC.

Strong ortho/para effects in the vibrational spectrum of Cl-(H2)

The predissociation spectrum of the Cl-35(H₂) complex is measured between 450 and 800 cm^-1 in a multipole radiofrequency ion trap at different temperatures using the FELIX infrared free electron laser. Above a certain temperature, the removal of the Cl^-(p-H₂) para nuclear spin isomer by ligand exchange to the Cl^-(o-H₂) ortho isomer is suppressed effectively, thereby making it possible to detect the spectrum of this more weakly bound complex. At trap temperatures of 30.5 and 41.5 K, we detect two vibrational bands of Cl^-(p-H₂) at 510(1) and 606(1) cm^-1. Using accurate quantum calculations, these bands are assigned to transitions to the inter-monomer vibrational modes (v₁,v₂ ^l₂ ) = (0, 2⁰) and (1, 2⁰), respectively.

Correction: Quantum tunneling dynamical behaviour on weakly bound complexes: the case of a CO2-N2 dimer

Correction for 'Quantum tunneling dynamical behaviour on weakly bound complexes: the case of a CO2-N2 dimer' by Miguel Lara-Moreno et al., Phys. Chem. Chem. Phys., 2019, 21, 3550-3557, DOI: 10.1039/c8cp04465a.

Predissociation spectra of the 35Cl-(H2) complex and its isotopologue 35Cl-(D2)

The predissociation spectra of the 35Cl-(H2) and 35Cl-(D2) complexes are determined within an accurate quantum approach and compared to those recently measured in an ionic trap at 8 K and 22 K. The calculations are performed using an existing three-dimensional potential energy surface. A variational approach is used for the accurate quantum calculations of the rovibrational bound states. Several methods are compared for the search and the characterization of the resonant states. A good agreement between the calculated and measured spectra is obtained, despite a slight shift to the red of the calculated spectra. The comparison shows that only the ortho or para contribution is observed in the measured 35Cl-(H2) or 35Cl-(D2) spectrum, respectively. Quantum numbers are assigned to the rovibrational resonant states. It demonstrates that the main features observed in the measured predissociation spectra correspond to a progression in the intermonomer vibrational stretching mode.

Potential energy surface and rovibrational bound states of the H2-C3N- van der Waals complex

Since their recent detection in the interstellar medium, anions have raised the question of their possible mechanisms of formation, destruction and excitation. This requires knowledge of their interaction with the most abundant interstellar species. In the present work, a four dimensional rigid rotor model of the potential energy surface is developed for the collision of C3N- with H2. Ab initio calculations are performed with explicitly-correlated coupled-cluster theory via CCSD(T)-F12/aug-cc-pVTZ. Two linear equilibrium structures are found, different in the orientation of C3N-. Two more equilibrium structures, symmetrically equivalent, are obtained by the permutation of H atoms. The vibrational dynamics is mainly controlled by the considerable difference between the two bending frequencies that correspond to the hindered rotations of C3N- and H2. This arises from the potential energy surface which is soft for rotation of C3N- and stiff for rotation of H2, and also from the large difference in mass between both monomers. Although a high potential barrier prevents the rotation of H2, a significant tunneling effect is observed which causes a splitting in the degenerate energy levels. On the contrary, the rotation of C3N- is allowed since the energy of the saddle points is lower than the energy of the bound states, but the wavefunctions remain localized around each linear structure unless a large excitation energy is available.

Quantum tunneling dynamical behaviour on weakly bound complexes: the case of a CO2–N2 dimer

The dynamics induced by quantum tunneling is found to be closely connected to the shape of the potentials for weakly bound complexes.

On the gas-phase formation of the HCO- anion: accurate quantum study of the H- + CO radiative association and HCO radiative electron attachment

The hydrogen anion has never been observed in the interstellar medium, but it is most likely present in some interstellar regions. Since direct detection appears especially difficult, improving the knowledge of the astrochemical processes involving this anion should be valuable in defining a way of indirect detection. We present the first study of the radiative association of H- and CO to form the HCO- anion within a quantum time-independent approach. We use a state-of-the-art potential energy surface which has been calculated for the present study. The calculated radiative association rate coefficient is monotonically decreasing from 6 × 10-16 to 5 × 10-19 cm3 per molecule per s across the 0.01-1000 K temperature range. At the typical temperature of the cold interstellar medium, ∼10 K, the radiative association rate is ∼2 × 10-17 cm3 per molecule per s. On the other hand, the plane wave approximation is used to calculate the HCO radiative electron attachment rate coefficient. It is found to be almost constant and also equal to 2 × 10-17 cm3 per molecule per s. Setting aside the question of the abundances of the reactants of both processes, these results demonstrate that among the two gas-phase modes of production of the HCO- anion in cold interstellar medium considered in this study, the H- + CO radiative association is dominating below 10 K while the radiative electron attachment rate is larger above 10 K.

Unveiling the Ionization Energy of the CN Radical

The cyano radical is a ubiquitous molecule and was, for instance, one of the first species detected in astrophysical media such as comets or diffuse clouds. In photodissociation regions, the reaction rate of CN⁺ + CO → CN + CO⁺ is one of the critical parameters defining nitrile chemistry. The enthalpy of this charge transfer reaction is defined as the difference of ionization energies (E_I) between CN and CO. Although E_I(CO) is known accurately, the E_I(CN) values are more dispersed and deduced indirectly from thermodynamic thresholds only, all above E_I(CO), leading to the assumption that the reaction is fast even at low temperature. Using a combination of synchrotron radiation, electron/ion imaging coincidence techniques, and supporting ab initio calculations, we directly determine the first adiabatic ionization energy of CN at 13.956(7) eV, and we demonstrate that E_I(CN) < E_I(CO). The findings suggest a very slow reaction in the cold regions of interstellar media.

State-to-state chemistry and rotational excitation of CH+ in photon-dominated regions

We present a detailed theoretical study of the rotational excitation of CH+ due to reactive and nonreactive collisions involving C+(2P), H2, CH+, H and free electrons. Specifically, the formation of CH+ proceeds through the reaction between C+(2P) and H2(νH2 = 1, 2), while the collisional (de)excitation and destruction of CH+ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10-3000 K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH+ with H atoms at kinetic temperatures below 50 K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our Non-LTE calculations confirm that the formation pumping due to vibrationally excited H2 has a substantial effect on the excitation of CH+ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH+ toward the Orion Bar and the planetary nebula NGC 7027. Our results further suggest that the population of νH2 = 2 might be significant in the photon-dominated region of NGC 7027.

Full-Dimensional Theory of Pair-Correlated HNCO Photofragmentation

Full-dimensional semiclassical dynamical calculations combining classical paths and Bohr quantization of product internal motions are reported for the prototype photofragmentation of isocyanic acid in the S₁ state. These calculations allow one to closely reproduce for the first time key features of state-of-the-art imaging measurements at photolysis wavelengths of 201 and 210 nm while providing insight into the underlying dissociation mechanism. Quantum scattering calculations being beyond reach for most polyatomic fissions, pair-correlated data on these processes are much more often measured than predicted. Our theoretical approach can be used to fill this gap.

Explanation of efficient quenching of molecular ion vibrational motion by ultracold atoms

Buffer gas cooling of molecules to cold and ultracold temperatures is a promising technique for realizing a host of scientific and technological opportunities. Unfortunately, experiments using cryogenic buffer gases have found that although the molecular motion and rotation are quickly cooled, the molecular vibration relaxes at impractically long timescales. Here, we theoretically explain the recently observed exception to this rule: efficient vibrational cooling of BaCl(+) by a laser-cooled Ca buffer gas. We perform intense close-coupling calculations that agree with the experimental result, and use both quantum defect theory and a statistical capture model to provide an intuitive understanding of the system. This result establishes that, in contrast to the commonly held opinion, there exists a large class of systems that exhibit efficient vibrational cooling and therefore supports a new route to realize the long-sought opportunities offered by molecular structure.

Rotational Excitation of the OH(+) Radical by Collision with H at Low Temperature

A ro-vibrationally inelastic close coupling study of the rotational excitation of OH(+)(X(3)Σ(-)) by collisions with H((2)S) is presented. The two lowest potential energy surfaces of doublet and quadruplet spin multiplicity are involved. The former is the one we developed recently, and the latter is a modified version of the quadruplet surface of Martinez et al. to include the long-range charge-induced-dipole potential. The details of the modification of this surface are presented as well as the comparison of the rotational excitation resulting from collisions with hydrogen on these two surfaces. The effect of the coupling between vibration and rotation on the rotational excitation rate is also discussed, as the potential well depth of the doublet surface is quite large and allows the coupling between many vibrational channels of OH(+). As the hydrogen exchange reaction can occur for both potential energy surfaces, we discuss the reliability of the approximation made by the calculation of the cross sections with a quantum dynamics limited to the inelastic process. The relative importance of the collisions on the doublet or quadruplet surface within a given rotational transition is also discussed.

Low temperature rate coefficients of the H + CH(+) → C(+) + H2 reaction: New potential energy surface and time-independent quantum scattering

The observed abundances of the methylidyne cation, CH(+), in diffuse molecular clouds can be two orders of magnitude higher than the prediction of the standard gas-phase models which, in turn, predict rather well the abundances of neutral CH. It is therefore necessary to investigate all the possible formation and destruction processes of CH(+) in the interstellar medium with the most abundant species H, H2, and e(-). In this work, we address the destruction process of CH(+) by hydrogen abstraction. We report a new calculation of the low temperature rate coefficients for the abstraction reaction, using accurate time-independent quantum scattering and a new high-level ab initio global potential energy surface including a realistic model of the long-range interaction between the reactants H and CH(+). The calculated thermal rate coefficient is in good agreement with the experimental data in the range 50 K-800 K. However, at lower temperatures, the experimental rate coefficient takes exceedingly small values which are not reproduced by the calculated rate coefficient. Instead, the latter rate coefficient is close to the one given by the Langevin capture model, as expected for a reaction involving an ion and a neutral species. Several recent theoretical works have reported a seemingly good agreement with the experiment below 50 K, but an analysis of these works show that they are based on potential energy surfaces with incorrect long-range behavior. The experimental results were explained by a loss of reactivity of the lowest rotational states of the reactant; however, the quantum scattering calculations show the opposite, namely, a reactivity enhancement with rotational excitation.

Potential energy surface of the CO2-N2 van der Waals complex

Four-dimensional potential energy surface (4D-PES) of the atmospherically relevant CO2-N2 van der Waals complex is generated using the explicitly correlated coupled cluster with single, double, and perturbative triple excitation (CCSD(T)-F12) method in conjunction with the augmented correlation consistent triple zeta (aug-cc-pVTZ) basis set. This 4D-PES is mapped along the intermonomer coordinates. An analytic fit of this 4D-PES is performed. Our extensive computations confirm that the most stable form corresponds to a T-shape structure where the nitrogen molecule points towards the carbon atom of CO2. In addition, we located a second isomer and two transition states in the ground state PES of CO2-N2. All of them lay below the CO2 + N2 dissociation limit. This 4D-PES is flat and strongly anisotropic along the intermonomer coordinates. This results in the possibility of the occurrence of large amplitude motions within the complex, such as the inversion of N2, as suggested in the recent spectroscopic experiments. Finally, we show that the experimentally established deviations from the C2v structure at equilibrium for the most stable isomer are due to the zero-point out-of-plane vibration correction.

Rotational relaxation of CS by collision with ortho- and para-H2 molecules

Quantum mechanical investigation of the rotationally inelastic collisions of CS with ortho- and para-H2 molecules is reported. The new global four-dimensional potential energy surface presented in our recent work is used. Close coupling scattering calculations are performed in the rigid rotor approximation for ortho- and para-H2 colliding with CS in the j = 0-15 rotational levels and for collision energies ranging from 10(-2) to 10(3) cm(-1). The cross sections and rate coefficients for selected rotational transitions of CS are compared with the ones previously reported for the collision of CS with He. The largest discrepancies are observed at low collision energy, below 1 cm(-1). Above 10 cm(-1), the approximation using the square root of the relative mass of the colliders to calculate the cross sections between a molecule and H2 from the data available with (4)He is found to be a good qualitative approximation. The rate coefficients calculated with the electron gas model for the He-CS system show more discrepancy with our accurate results. However, scaling up these rates by a factor of 2 gives a qualitative agreement.

Semiclassical and Quantum Mechanical Calculations of Isotopic Kinetic Branching Ratios for the Reactionof O(3P) with HD

Tunneling probabilities for the reactions O + HD → OH + D and O + DH → OD + H have been calculated by semiclassical dynamical methods and compared to accurate quantal calculations for the same potential energy surface. The results are used to test the reliability of variational transition state theory with the least-action semiclassical method for tunneling probabilities for the prediction of intramolecular kinetic isotope effects.