N. Shimakura

Conical intersections involving the dissociative 1πσ* state in 9H-adenine: a quantum chemical ab initio study

The conical intersections of the dissociative 1πσ* excited state with the lowest 1ππ* excited state and the electronic ground state of 9H-adenine have been investigated with multireference electronic structure calculations. Adiabatic and quasidiabatic potential energy surfaces and coupling elements were calculated as a function of the NH stretch coordinate of the azine group and the out-of-plane angle of the hydrogen atom, employing MultiReference Configuration-Interaction (MRCI) as well as Complete-Active-Space Self-Consistent-Field (CASSCF) methods. Characteristic properties of the 1ππ*–1πσ* and 1πσ*–S0 conical intersections, such as the diabatic-to-adiabatic mixing angle, the geometric phase of the adiabatic electronic wavefunctions, the derivative coupling, as well as adiabatic and diabatic transition dipole moment surfaces were investigated in detail. These data are a prerequisite for future quantum wavepacket simulations of the photodissociation and internal-conversion dynamics of adenine.

Ultrafast radiationless transition pathways through conical intersections in photo-excited 9H-adenine

We performed CASSCF and MRCI calculations for determination of the effective pathways of ultrafast radiationless transitions from the optically allowed ππ* 1La state to the ground state S0 of 9H-adenine. The nπ*, πσ*, and two ππ* states were taken into account as states involved in the radiationless process. Optimized geometry and conical intersections were searched in the full dimensional space for the vibrational degrees of freedom by using the suite of quantum chemistry codes MOLPRO. The MRCI transition energies to excited states are in good agreement with the experimental values. The mechanisms of three competing pathways, two indirect pathways via the πσ* and nπ* states, 1La→πσ*→S0 and 1La→nπ*→ S0, and a direct pathway 1La→S0, were examined on the basis of the structures and energies of conical intersections involved in ultrafast radiationless transitions from 1La to S0. Any conical intersection between the πσ* and nπ* states was not found. This suggests that the two indirect pathways are independent of each other. The ππ* 1La-πσ* conical intersection lies higher than the ππ* 1La state at the Franck-Condon geometry by 0.19 eV according to the present MRCI calculation, which is consistent with the experimental observation that a new channel is open at the excess energy of 0.2 eV above the band origin of the ππ* 1La state. It is concluded that relaxation from the ππ* 1La-πσ* conical intersection to S0 occurs mainly through the πσ*-S0 conical intersection. The ππ* 1La-nπ* conical intersection lies higher by 0.1 eV (MRCI value) than the ππ* 1La state at the Franck-Condon geometry. The fast decay component in time-resolved spectra of 9H-adenine is attributed to rapid radiationless transitions to the nπ* state via this conical intersection followed by the transition to S0 via the nπ*-S0 (or ππ* 1La-S0) conical intersection. The ππ* 1La-S0 conical intersection of large out-of-plane distortion has the lowest energy among the conical intersections found in this study. We identified the transition state between the ππ* 1La at the Franck-Condon geometry and the ππ* 1La-S0 conical intersection. The MRCI energy of the transition state on the 1La potential surface is higher by 0.21 eV than the vertical excitation energy. The possibility of strong coupling between the two close-lying states 1La and nπ* indicates that, besides this direct pathway, radiationless transitions to S0 via the ππ* 1La-S0 conical intersection can also occur after rapid relaxations between 1La and nπ*. The analysis of the h-vector for each conical intersection has shown that the active coupling for the πσ* pathway is dominated by the out-of-plane normal mode ν10, while the active coupling for the nπ* pathway is distributed among many normal modes. Control of the branching ratio of the two indirect pathways can be achieved by selective excitation of single vibronic levels involving active coupling modes such as the mode ν10.

Spin-orbit coupling effects in dihydrides of third-row transition elements. II. Interplay of nonadiabatic coupling in the dissociation path of rhenium dihydride

This is the second paper in a series of investigations on spin-orbit coupling (SOC) effects in dihydrides of third-row transition elements. The dissociation path of rhenium dihydride was explored using the multiconfiguration self-consistent-field method followed by diagonalization of SOC matrices, in which the Stevens-Basch-Krauss-Jasien-Cundari (SBKJC) basis sets were employed after adding one set of polarization functions for each atom. The most stable rhenium dihydride has a linear structure and its ground state is (6)Sigma(g)(+). Both C(2v) and C(s) dissociation paths into a Re atom and a hydrogen molecule (Re((6)S) + H(2)((1)Sigma(g)(+))) were explored on the potential energy curves of low-lying states. A relatively high energy barrier was obtained along the C(2v) path and two conical intersections were found at the H-Re-H angles of 29.8 degrees and 96.1 degrees along the C(2v) path. Since it was revealed that the geometrical deformation to C(s) symmetry at the H-Re-H angle of 29.8 degrees does not provide explicit lowering of the energy barrier for the dissociation, even after considering nonadiabatic couplings (NACs) in the neighborhood of the conical intersections, it can be concluded that the most feasible path is hopping from the lowest (6)A(1) state to the lowest (6)B(2) state at the H-Re-H angle of 96.1 degrees followed by hopping from the lowest (6)B(2) state back to the lowest (6)A(1) state at the H-Re-H angle of 29.8 degrees, where the latter crossing point is the highest in energy along this path. Thus, when the molecular system can reach the areas of these crossing points, the molecular system hops from one of the states to another owing to NAC or SOC effects; especially, SOC effects become important at the crossing point with C(2v) symmetry.

Charge-overlap effect on the electronic transitions in moderate-energy collisions between closed-shell particles with rare-gas structure

A potential model, which is represented by the overlap of electron clouds of interacting particles, has been applied to discuss the diabatic potential crossings which lead to one- and two-electron transitions in moderate-energy collisions of closed-shell particles. The crossings evaluated with the potential model interpret reasonably well both differential scattering experiments and ab initio calculations. In the model, the potential crossings originate from the hole(s) produced by the promotion of electron(s) in the outermost shell(s) of incident particle(s).

Neutral-Fragmentation Paths of Methane Induced by Intense Ultrashort IR Laser Pulses: Ab Initio Molecular Orbital Approach

Instantaneous (laser-field-dependent) potential energy curves leading to neutral fragmentations of methane were calculated at several laser intensities from 1.4 × 10(13) to 1.2 × 10(14) W/cm(2) (from 1.0 × 10(10) to 3.0 × 10(10) V/m) using ab initio molecular orbital (MO) methods to validate the observation of neutral fragmentations induced by intense femtosecond IR pulses (Kong et al. J. Chem. Phys. 2006, 125, 133320). Two fragmentation paths, CH(2) + 2H and CH(2) + H(2), in (1)T(2) superexcited states that are located in the energy range of 12-16 eV were considered as the reaction paths because these states are responsible for Jahn-Teller distortion opening up reaction paths during ultrashort pulses. As field intensity increased, the low-lying excited (1)A(1) states originated from the Jahn-Teller (1)T(2) states were substantially stabilized along the neutral-fragment path CH(4) → CH(2) + 2H and were located below the ionization threshold. On the other hand, the low-lying excited (1)B(1) states, which also originate from the Jahn-Teller (1)T(2) states, were embedded on the ionized state along the dissociation path to CH(2) + H(2). This indicates that ionic fragments, rather than neutral ones, are produced along the CH(2) + H(2) path. The computational results support neutral fragmentations through superexcited states proposed by Kong et al.

Interaction potential for Li+–He at small distances

Abstract Repulsive potential for Li+–He at the small distances of 0.09⩽R⩽0.95 A has been determined from the summed differential-cross-sections measured at 500⩽E lab ⩽1500 eV . The experimental potential at R>0.3 A is well represented by a single exponential function, while at R 0.25 A and R

Electron capture processes in collisions between Beq + ions and He atoms

Theoretical calculations of the electron capture cross sections in collisions of beryllium ions with helium atoms are performed in a close-coupling method based on molecular-state expansion for the Beq + (q = 2,4) ion impacts below 35 keV amu-1, and in the continuum distorted-wave method for the Beq + (q = 1-4) ion impacts above 100 keV amu-1. The present results are discussed in comparison with other available theoretical calculations.

Molecular-state treatment of electron capture in collisions of ions with He atoms below

Cross sections of electron-capture processes are calculated by using a molecular-state expansion method in collisions of ions with helium atoms. Semiclassical and quantum-mechanical close-coupling equations with ten molecular channels for the former and with three channels for the latter are solved numerically at collision energies from to and from to , respectively, to obtain scattering amplitude. The total cross sections for the triplet ion formation have a broad maximum at about and decrease below, reaching a minimum around . But they begin to increase again below . At collision energies lower than the contribution of the state to the electron-capture process is found to be larger than that of the state, but at intermediate collision energies between and the order of the contributions from these two states reverses. The total cross sections for the singlet ion formation have a broad minimum at about and they increase monotonically with decreasing collision energy. At most of the collision energies studied here, the contribution of the state is found to be dominant. In both triplet and singlet ion formation, at lower collision energies below , the cross sections begin to increase rapidly with decreasing collision energy due to an orbiting effect, and several resonance-type peaks are also seen. The position of these resonances can be assigned with the rovibrational level of a quasimolecule formed during collision.

Molecular-orbital treatment of electron capture in an external magnetic field: collisions of ions with H atoms

A semiclassical close-coupling equation is formulated for the electron-capture process in ion - atom collisions under the influence of an external magnetic field. The direction of the magnetic field is chosen to be parallel to an initial velocity vector of a projectile. As we deal with the case for the magnetic field strength less than the critical field strength , the second- and higher-order terms for the magnetic field are neglected. Eigenfunctions in zero magnetic field are employed as a basis set for expansion. The equation obtained is the same in structure as that at zero magnetic field except that the relative velocity v is replaced by the scaled velocity . Effects due to the magnetic field are: (i) dynamical couplings, similar to rotational couplings and (ii) the Zeeman splitting of energies. The phase factor due to the gauge transformation is similar in form to the electron-translation factor. We apply this close-coupling equation to a study of electron-capture processes in a singlet system at , for which the zero-field results have been reported earlier (Shimakura et al 1993). Calculated results show that the total electron-capture cross section increases by about 30% at the magnetic field T compared to that at zero magnetic field at collision energy E = 122.5 eV/amu. At low collision energies below 500 eV/amu, the effect of the magnetic field becomes dominant over that of rotational couplings at . Because the magnetic field removes the degeneracy of electronic energies for (and ) states by the Zeeman effect, transition probabilities for and (and also and ) states differ significantly. The transition probabilities oscillate with time even if the collision pair separates considerably, particularly for and states because of the magnetic field.

Magnetic field effects on electron capture processes by highly charged ions: collisions of B4+ ions with H atoms

The electron capture processes in collision of highly charged ions with neutral atoms in a strong magnetic field are investigated in the case of arbitrary field direction. A semiclassical molecular-orbital close-coupling method is employed. The phase factors arising from the electron translation factor and the gauge transformation are included. We apply this method to the singlet (B4+ + H) system which have been investigated previously by us only for the field direction being along with the incident velocity vector. The total cross section averaged over field direction increases with field strength. When the field direction is perpendicular to the collision plane, the field effects become maximum.