M. Hochlaf

A study of the mode-selective trans-cis isomerization in HONO using ab initio methodology

Ab initio calculations on the six-dimensional cis--trans double minimum potential energy surface of the electronic ground state of the HONO molecule were performed using a coupled cluster approach. An analytic fit to the data points was established. The interconversion barrier was calculated to be 4105 cm(-1). The nuclear motion problem was solved variationally using a full six-dimensional Hamiltonian in internal coordinates. The eigenstates up to about 3650 cm(-1) were tentatively assigned by harmonic quantum numbers. The assignment was based on the mean values of the internal coordinates of the six-dimensional eigenfunctions and on a comparison of the eigenenergies with those calculated by second-order perturbation theory from a full quartic force field in dimensionless normal coordinates. In cold matrices the trans- and the cis-OH nu(1) stretching modes and the first trans- and cis-NO 2nu(2) stretching overtones lead to isomerization. In the isolated molecule these modes (J=0) were found to be entirely localized. However, several overtones of the nu(4) ONO bending and nu(5) N-O stretching, which are close in energy to the OH stretch and combined with the torsional mode, were found to be strongly cis-trans delocalized.

Photoionization of 2-pyridone and 2-hydroxypyridine

We studied the photoionization of 2-pyridone and its tautomer, 2-hydroxypyridine by means of VUV synchrotron radiation coupled to a velocity map imaging electron/ion coincidence spectrometer. The photoionization efficiency (PIE) spectrum is composed of steps. The state energies of the [2-pyridone](+) cation in the X[combining tilde] ground and A excited electronic states, as well as of the [2-hydroxypyridine](+) cation in the electronic ground state, are determined. The slow photoelectron spectra (SPES) are dominated by the 0(0)(0) transitions to the corresponding electronic states together with several weaker bands corresponding to the population of the pure or combination vibrational bands of the cations. These vibrationally-resolved spectra compare very well with state-of-the-art calculations. Close to the ionization thresholds, the photoionization of these molecules is found to be mainly dominated by a direct process whereas the indirect route (autoionization) may contribute at higher energies.

Benchmarks for the generation of interaction potentials for scattering calculations: applications to rotationally inelastic collisions of C4 (X3Σ−g) with He

We present an application of recently developed, explicitly correlated, partially spin-restricted coupled-cluster RCCSD(T)-F12x (x = A/B) methods [G. Knizia, T. B. Adler, and H.-J. Werner, J. Chem. Phys., 2009, 130, 054104] for the generation of multi-dimensional potential energy surfaces (PESs) for scattering calculations. We test the method on the O(2)-He van der Waals model system by a comparison with standard orbital-based coupled-cluster techniques, employing correlation-consistent atomic basis sets (aug-cc-pVXZ, X = T, Q, 5, 6) and a complete basis set. From this comparison, it is obvious that the RCCSD(T)-F12/aug-cc-pVTZ approach is accurate enough for the description of short and long-range interactions with low computational effort. We apply this new method in studies of the interaction of the carbon-rich interstellar species C(4)(X(3)Σ) with atomic He. This PES is subsequently used in quantum close-coupling scattering calculations. The collisional excitation cross-sections of the fine-structure levels of C(4) by He are calculated at low collisional energies. The thermal dependence of rate coefficients is calculated up to 50 K. The propensity rules between fine-structure levels are studied, and it is shown that F-conserving cross sections are much larger, especially for high-N rotational levels rather than F-changing cross sections, as expected from theoretical considerations. This is the first report on the collisional rate coefficients for this system and may have important implications for the astrophysical detection of C4 and modeling of carbon-rich media.

Unimolecular decay pathways of state-selected CO2+ in the internal energy range of 5.2–6.2 eV: An experimental and theoretical study

The vacuum ultraviolet pulsed field ionization (PFI)-photoelectron (PFI-PE) spectrum of CO2 has been measured in the energy region of 19.0–20.0 eV. The PFI-PE vibrational bands resolved for CO2+(C 2Σg+) are overwhelmingly dominated by the origin band along with weak vibrational bands corresponding to excitations of the ν1+ (symmetric stretching), ν2+ (bending), and ν3+ (antisymmetric stretching) modes. The simulation of the rotational contour resolved in the origin PFI-PE band yields a value of 19.3911±0.0005 eV for the ionization energy of CO2 to form CO2+(C 2Σg+). A PFI-PE peak is found to coincide with each of the 0 K dissociation thresholds for the formation of O+(4S)+CO(X 1Σ+) and CO+(X 2Σ+)+O(3P). This observation is tentatively interpreted to result from the lifetime switching effect, arising from the prompt dissociation of excited CO2 in high-n (n⩾100) Rydberg states prior to PFI. We have also examined the decay pathways for state-selected CO2+ in the internal energy range of 5.2–6.2 eV using the ...

Photo double ionization spectra of CO: comparison of theory with experiment

High level ab initio calculations have been undertaken of potential energy curves of CO2+ (and for the CO neutral ground state). The accuracy of the potentials was tested by a synthesis of the available vibrationally resolved threshold photoelectrons in coincidence (TPEsCO) and time of flight, photo electron photo electron coincidence (TOF-PEPECO) spectra of CO2+. Good agreement was found between experimental and theoretical spectra once relative energies of the calculated double ionization energies were slightly adjusted (by approximately 1%) to match experiment. Vibrational separations within individual electronic states are very well reproduced (the worst error is 0.07%).

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.

Theoretical study of the electronic states of

Three-dimensional potential energy functions (PEFs) have been generated for the X, a, and b states of using the internally contracted multireference configuration interaction approach. Analytic forms of the PEFs were employed in calculations of the vibrational energy levels, vibrational wavefunctions and Franck-Condon factors for the hypothetical direct ionization process . For the state the Renner-Teller problem has been solved and the pattern of the bending levels analysed. The collinear charge separation path yielding has been calculated for 14 electronic states. The electronic ground state of was found to have a barrier height of 1.4 eV, in good agreement with the experimentally detected onset of this charge separation process. The shapes of the close-lying potential energy functions indicate that for energies higher than about 4 eV above the electronic ground state, dissociation processes from these states will be accompanied by complicated coupling effects.

Electronic structure calculations on the C4 cluster

The ground and the electronically excited states of the C4 radical are studied using interaction configuration methods and large basis sets. Apart from the known isomers [l-C4(X(3)Sigmag (-)) and r-C4(X(1)Ag)], it is found that the ground singlet surface has two other stationary points: s-C4(X(1)Ag) and d-C4(X(1)A1). The d-C4 form is the third isomer of this cluster. The isomerization pathways from one form to the other show that deep potential wells are separating each minimum. Multireference configuration interaction studies of the electronic excited states reveal a high density of electronic states of these species in the 0-2 eV energy ranges. The high rovibrational levels of l-C4((3)Sigmau (-)) undergo predissociation processes via spin-orbit interactions with the neighboring (5)Sigmag + state.

Titan's ionic species: theoretical treatment of N2H+ and related ions.

We use different ab initio methods to compute the three-dimensional potential energy surface (3D-PES) of the ground state of N(2)H(+). This includes the standard coupled cluster, the complete active space self-consistent field, the internally contacted multi reference configuration interaction, and the newly developed CCSD(T)-F12 methods. For the description of H and N atoms, several basis sets are tested. Then, we incorporate the 3D-PES analytical representations into variational calculations of the rovibrational spectrum of N(2)H(+)(X(1)Sigma(+)) up to 7200 cm(-1) above the zero point vibrational energy. Our data show that the CCSD(T)-F12/aug-cc-pVTZ approach represents a compromise for good description of the PES and computation cost. This technique is recommended for full dimensional PES generation of atmospheric and astrophysical relevant polyatomic systems. We applied this method to derive the rovibrational spectra of N(2)H(+)(X(1)Sigma(+)) and of N(2)H(++)(X(2)Sigma(+)). Finally, we discuss the existence of the N(2)H(++)(X(2)Sigma(+)) in Titans atmosphere.

High resolution pulsed field ionization–photoelectron study of CO2+(X 2Πg) in the energy range of 13.6–14.7 eV

The vacuum ultraviolet pulsed field ionization–photoelectron (PFI–PE) spectra for CO2 have been measured in the energy range of 13.6–14.7 eV, revealing complex vibronic structures for the ground CO2+(X 2Πg) state. Many vibronic bands for CO2+(X 2Πg), which were not resolved in previous photoelectron studies, are identified in the present measurement based on comparison with available optical data and theoretical predictions. As observed in the HeI photoelectron spectrum of CO2, the PFI–PE spectrum is dominated by the symmetry allowed ν1+ (symmetric stretch) vibrational progression for CO2+(X 2Πg). However, PFI–PE vibronic bands due to excitation of the symmetry disallowed ν2+ (bending) and ν3+ (asymmetric stretch) modes with both odd quanta, together with the symmetry allowed even quanta excitations, are clearly discernible. The simulation of rotational contours resolved in PFI–PE vibronic bands associated with excitation to the (ν1+=0–1, ν2+=0–2, ν3+=0) vibrational levels has yielded accurate ionization ...