Ab initio study of the vibronic and spin–orbit structure in the X 2Π electronic state of CCCH

A global CHIPR potential energy surface for ground-state C3H and exploratory dynamics studies of reaction C2 + CH → C3 + H

A full-dimensional global potential-energy surface (PES) is first reported for ground-state doublet C₃H using the combined-hyperbolic-inverse-power-representation (CHIPR) method and accurate ab initio energies extrapolated to the complete basis set limit. The PES is based on a many-body expansion-type development where the two-body and three-body energy terms are from our previously reported analytic potentials for C₂H(²A') and C₃(¹A',³A'), while the effective four-body one is calibrated using an extension of the CHIPR formalism for tetratomics. The final form is shown to accurately reproduce all known stationary structures of the PES, some of which are unreported thus far, and their interconversion pathways. Moreover, it warrants by built-in construction the appropriate permutational symmetry and describes in a physically reasonable manner all long-range features and the correct asymptotic behavior at dissociation. Exploratory quasi-classical trajectory calculations for the reaction C₂ + CH → C₃ + H are also performed, yielding thermalized rate coefficients for temperatures up to 4000 K.

Slow electron velocity-map imaging of negative ions: applications to spectroscopy and dynamics

Anion photoelectron spectroscopy (PES) has become one of the most versatile techniques in chemical physics. This article briefly reviews the history of anion PES and some of its applications. It describes efforts to improve the resolution of this technique, including anion zero electron kinetic energy (ZEKE) and the recently developed method of slow electron velocity-map imaging (SEVI). Applications of SEVI to studies of vibronic coupling in open-shell systems and the spectroscopy of prereactive van der Waals complexes are then discussed.

Spin-orbit vibronic coupling in Π3 states of linear triatomic molecules

The Renner-Teller vibronic-coupling problem of a 3Pi electronic state of a linear molecule is analyzed with the inclusion of the spin-orbit coupling of the 3Pi electronic state, employing the microscopic (Breit-Pauli) spin-orbit coupling operator for the two unpaired electrons. The 6x6 Hamiltonian matrix in a diabatic spin-electronic basis is obtained by an expansion of the molecular Hamiltonian in powers of the bending amplitude. The symmetry properties of the Hamiltonian with respect to the time-reversal operator and the relativistic vibronic angular momentum operator are analyzed. It is shown that there exists a linear vibronic-coupling term of spin-orbit origin, which has not been considered so far in the Renner-Teller theory of 3Pi electronic states. While two of the six adiabatic electronic wave functions do not exhibit a geometric phase, the other four carry nontrivial topological phases which depend on the radius of the integration contour. The spectroscopic effects of the linear spin-orbit vibronic-coupling mechanism have been analyzed by numerical calculations of the vibronic spectrum for selected model examples.

Calculation of the vibronic structure of the X̃Π2 photoelectron spectra of XCN,X=F, Cl, and Br

The vibronic structure of the photoelectron spectra of the X (2)Pi state of XCN(+) (X=F, Cl, and Br) has been calculated, assuming that the X (2)Pi state can be considered as an isolated electronic state. The Renner-Teller coupling of the two components of the (2)Pi state via the degenerate bending mode as well as spin-orbit coupling effects are taken into account. The two stretching modes are treated within the so-called linear vibronic-coupling model. The vibronic and spin-orbit parameters have been determined by accurate ab initio electronic-structure calculations. While spin-orbit effects are small in FCN(+), the large spin-orbit splitting of the X (2)Pi state of the BrCN(+) leads to a complete quenching of the Renner-Teller effect. The X (2)Pi state of the ClCN(+) is shown to be of particular interest: here the resonance condition for linear-relativistic Renner-Teller coupling is approximately fulfilled. This coupling mechanism leads to a significant intensity transfer to vibronic levels with odd quanta of the bending mode. The calculated spectrum indicates that this novel relativistic vibronic-coupling effect should be observable in high-resolution (electron energy resolution of the order of a few meV) photoelectron spectra of ClCN.

Theoretical investigation of the hyperfine structure in spatially and spin degenerate electronic states of triatomic and tetra-atomic molecules

The present paper reviews the results of ab initio studies on the magnetic hyperfine structure in spectra of spatially and spin degenerate electronic states of triatomic and tetra-atomic molecules. The main goal of the present paper is to show that such theoretical investigations can be used to reliably reproduce, explain and predict the results of the corresponding measurements.

Is the HCCS radical linear in the excited state?

The A (2)Pi-X (2)Pi 415 nm band system of the linear HCCS radical has been known since 1978, but the vibronic structure in this complex spectrum, which has both spin-orbit and Renner-Teller complications, has never been satisfactorily assigned, despite serious experimental and theoretical efforts. In a further attempt to understand the spectrum, we have studied the laser-induced fluorescence spectra of jet-cooled HCCS and DCCS, produced from thiophene precursors using the discharge jet technique. The 0(0) (0) bands of HCCS and DCCS have been rotationally analyzed, providing precise ground and excited state spin-orbit splittings. The energy levels of the v(')=0 (2)Pi(3/2) component of DCCS are found to be perturbed by a very low-lying (2)Sigma vibronic level, indicating that the HCC bending mode Renner-Teller effect is much larger than predicted by ab initio calculations with a linear excited state geometry. With this observation, the vibronic bands in the spectra of both isotopomers have been consistently assigned for the first time. Model calculations show that the large Renner-Teller effect and substantially different HCCS and DCCS excited state zero-point spin-orbit splittings can be explained with the assumption of a quasilinear excited state geometry.

An ab initio calculation of the anisotropic hyperfine coupling constants in the low-lying vibronic levels of the X [sup 2]Π electronic state of CCCH

The results of ab initio calculations of the vibronically averaged components of the anisotropic magnetic hyperfine tensor in the low-lying vibronic species of the X (2)Pi electronic state of CCCH and CCCD are reported. The electronically averaged hyperfine coupling constants for hydrogen and (13)C in (12)C (12)C (12)CH, (13)C (12)C (12)CH, (12)C (13)C (12)CH, (12)C (12)C (13)CH, and (12)C (12)C (12)CD are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with help of a variational approach which takes into account the Renner-Teller effect and spin-orbit coupling. The results of the present study help to reliably interpret the experimental data previously published and predict the yet unobserved hyperfine structure in excited vibronic states of CCCH and CCCD.

An ab initio study of the hyperfine structure in the X [sup 2]Π electronic state of CCCH

The results of an ab initio study of the magnetic hyperfine structure in the X (2)Pi electronic state of CCCH are reported. The potential surfaces for two components of the X (2)Pi electronic state were computed by means of an extensive configuration interaction approach. The electronically averaged hyperfine coupling constants of H and (13)C for (12)C (12)C (12)CH, (13)C (12)C (12)CH, (12)C (13)C (12)CH, and (12)C (12)C (13)CH are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with the help of a variational approach which takes into account the Renner-Teller effect and spin-orbit coupling. The model Hamiltonian is expressed in terms of the normal bending coordinates. It is found that, due to the generally strong geometry dependence of the hyperfine coupling constants, it is necessary to carry out the vibronic averaging of the corresponding functions in order to obtain the values which can be compared to the results of the measurements. The results of the present study help to reliably interpret the experimental data previously published. They also predict the yet unobserved hyperfine structure in excited vibronic states.