Numerical generation of hyperspherical harmonics for tetra-atomic systems

A numerical generation method of hyperspherical harmonics for tetra-atomic systems, in terms of row-orthonormal hyperspherical coordinates—a hyper-radius and eight angles—is presented. The nine-dimensional coordinate space is split into three three-dimensional spaces, the physical rotation, kinematic rotation, and kinematic invariant spaces. The eight-angle principal-axes-of-inertia hyperspherical harmonics are expanded in Wigner rotation matrices for the physical and kinematic rotation angles. The remaining two-angle harmonics defined in kinematic invariant space are expanded in a basis of trigonometric functions, and the diagonalization of the kinetic energy operator in this basis provides highly accurate harmonics. This trigonometric basis is chosen to provide a mathematically exact and finite expansion for the harmonics. Individually, each basis function does not satisfy appropriate boundary conditions at the poles of the kinetic energy operator; however, the numerically generated linear combination of these functions which constitutes the harmonic does. The size of this basis is minimized using the symmetries of the system, in particular, internal symmetries, involving different sets of coordinates in nine-dimensional space corresponding to the same physical configuration

Geometric phase effects in H3 predissociation

We model the predissociation of H3 in the electronic state corresponding to the upper sheet of the conically intersecting 1 2A[prime] and 2 2A[prime] states, and we show that product-state rovibrational distributions are strongly influenced by the geometric phase. Similarly, the differences in the product-state energy distributions in recent three-body dissociation experiments for the 2s,2A1[prime] and 2p,2A2[double-prime] states of H3 are shown to result from the presence of the geometric phase in this system, and thus provide experimental evidence of the influence of this phase in a molecular dynamical process

Electronic field emission models beyond the Fowler-Nordheim one

International audienceWe propose several quantum mechanical models to describe electronic field emission from first principles. These models allow to correlate quantitatively the electronic emission current to the electrode surface details at the atomic scale. They all rely on electronic potential energie surfaces obtained from three dimensional density functional theory calculations. They differ by the various quantum mechanical methods (exact or perturbative, time dependent or time independent) which are used to describe tunneling through the electronic potential energy barrier. Comparison of these models between them and with the standard Fowler-Nordheim one in the context of one dimensional tunneling allows to assess the impact on the accuracy of the computed current of the approximations made in each model. Among these methods, the time dependent perturbative one provides a well balanced trade-off between accuracy and computational cost

A three dimensional numerical quantum mechanical model of field electron emission from metallic surfaces covered with carbon adsorbates

International audienceThe effect of metallic surface contamination on field electron emission is investigated for the first time using a three dimensional quantum mechanical model. The plane wave periodic version of the density functional theory is used to obtain wavefunctions and potentials. Local and averaged emitted current densities are obtained from them using time dependent perturbation theory. This method is used to study the effect of the presence of carbon adsorbates on emission from tungsten surfaces. Fowler-Nordheim plots which provide the dependence of the emitted current with the external electric field show that carbon contamination inhibits emission. Significant differences with the results of the analytical Fowler-Nordheim model are observed. Emissions images (i. e. the spatial dependence of the emitted current density) are presented to identify the important emission spots. These images are significantly different from the electronic density plots usually presented to model constant height scanning tunnelling microscope images. Analysis of the emitted current density energy distributions in the light of the projected local density of states provides a deeper understanding of the emission process

Electronic and vibrational predissociation in Ari2 photodissociation dynamics

A quantum dynamical study of the ArI2 predissociation where both vibrational and electronic processes can take place was performed. A set of 5 coupled diatomics-in-molecules (DIM) electronic potentials was used. Both perpendicular and linear initial ArI2(X) isomers were considered. Only the a′ state had non-negligible effect on photodissociation dynamics for the linear isomer. Decay rates oscillated as a function of the vibrational excitation of I2(B) but the intramolecular vibrational energy was the main source of energy which occurred in vibrational predissociation.This work has been supported by DGICYT @Ministerio de Educacio´n y Ciencia ~MEC!, Spain# under Grant No. PB95-0071, INTAS under Grant No. 97-31573, and the Spanish–French PICASSO Project No. HF1999-0132. A.A.B. also thanks MEC for sabbatical fellowship.Peer Reviewe

Non-adiabatic molecular dynamics investigation of the size dependence of the electronic relaxation in polyacenes

International audienceThe Tully's fewest switches surface hopping algorithm is implemented within the framework of the time-dependent density functional based tight binding method (TD-DFTB) to simulate the energy relaxation following absorption of a UV photon by polycyclic aromatic hydrocarbons (PAHs). This approach is used to study the size effect on the ultrafast dynamics in excited states for a special class of PAH species called polyacenes. We determine the dynamical relaxation times and discuss the underlying mechanisms. Our results show that there is a striking alternation in decay times of the brightest singlet state for neutral polyacenes with 3 to 6 aromatic cycles. The alternation corresponds to an order-of-magnitude variation between roughly 10 and 100 fs and is correlated with a qualitatively similar alternation of energy gaps between the brightest state and the state lying just below in energy

Photoabsorption spectrum of helium trimer cation-Theoretical modeling

The photoabsorption spectrum of He+3 is calculated for two semiempirical models of intracluster interactions and compared with available experimental data reported in the middle UV range [H. Haberland and B. von Issendorff, J. Chem. Phys.102, 8773 (1995)]. Nuclear delocalization effects are investigated via several approaches comprising quantum samplings using either exact or approximate (harmonic) nuclear wavefunctions, as well as classical samplings based on the Monte Carlo methodology. Good agreement with the experiment is achieved for the model by Knowles et al. , [Mol. Phys.85, 243 (1995); Knowles et al. , Mol. Phys.87, 827 (1996)] whereas the model by Calvo et al. , [J. Chem. Phys.135, 124308 (2011)] exhibits non-negligible deviations from the experiment. Predictions of far UV absorption spectrum of He+3 , for which no experimental data are presently available, are reported for both models and compared to each other as well as to the photoabsorption spectrum of He+2 . A simple semiempirical point-charge approximation for calculating transition probabilities is shown to perform well for He+3 .Web of Science13920art. no. 20431

Vibrational spectrum of Ar-3(+) and relative importance of linear and perpendicular isomers in its photodissociation

The photodissociation dynamics of the argon ionized trimer Ar 3+ is revisited in the light of recent experimental results of Lepère et al. [J. Chem. Phys. 134, 194301 (2009)], which show that the fragment with little kinetic energy is always a neutral one, thus the available energy is shared by a neutral and ionic fragments as in Ar 2+. We show that these results can be interpreted as the photodissociation of the linear isomer of the system. We perform a 3D quantum computation of the vibrational spectrum of the system and study the relative populations of the linear (trimer-core) and perpendicular (dimer-core) isomers. We then show that the charge initially located on the central atom in the ground electronic state of the linear isomer migrates toward the extreme ones in the photoexcitation process such that photodissociation of the linear isomer produces a neutral central atom at rest in agreement with measured product state distributions