Light absorption during alkali atom-noble gas atom interactions at thermal energies: a quantum dynamics treatment

The absorption of light during atomic collisions is treated by coupling electronic excitations, treated quantum mechanically, to the motion of the nuclei described within a short de Broglie wavelength approximation, using a density matrix approach. The time-dependent electric dipole of the system provides the intensity of light absorption in a treatment valid for transient phenomena, and the Fourier transform of time-dependent intensities gives absorption spectra that are very sensitive to details of the interaction potentials of excited diatomic states. We consider several sets of atomic expansion functions and atomic pseudopotentials, and introduce new parametrizations to provide light absorption spectra in good agreement with experimentally measured and ab initio calculated spectra. To this end, we describe the electronic excitation of the valence electron of excited alkali atoms in collisions with noble gas atoms with a procedure that combines l-dependent atomic pseudopotentials, including two- and three-body polarization terms, and a treatment of the dynamics based on the eikonal approximation of atomic motions and time-dependent molecular orbitals. We present results for the collision induced absorption spectra in the Li-He system at 720 K, which display both atomic and molecular transition intensities.     

Real-time study of the adiabatic energy loss in an atomic collision with a metal cluster

Gas-phase hydrogen atoms are accelerated towards metallic surfaces in their vicinity. As it approaches the surface, the velocity of an atom increases and this motion excites the metallic electrons, causing energy loss to the atom. This dissipative dynamics is frequently described as atomic motion under friction, where the friction coefficient is obtained from ab initio calculations assuming a weak interaction and slow atom. This paper tests the aforementioned approach by comparing to a real-time Ehrenfest molecular dynamics simulation of such a process. The electrons are treated realistically using standard approximations to time-dependent density functional theory. We find indeed that the electronic excitations produce a friction-like force on the atom. However, the friction coefficient strongly depends on the direction of the motion of the atom: it is large when the atom is moving towards the cluster and much smaller when the atom is moving away. It is concluded that a revision of the model for energy dissipation at metallic surfaces, at least for clusters, may be necessary.     

A molecular dynamics simulation of rebound,capture and diffusion in collisions at thermal energies between a rare gas atom and an argon cluster (n=125)

Molecular dynamics calculations have been performed to simulate the low energy collision (0.2 eV) of a rare gas atom (He, Ar, Xe) with a cluster of 125 argon atoms. Depending on its relative mass to argon, the projectile is either deflected (He) or captured (Ar, Xe) by the argon cluster. We have determined the deflection function of the He projectile that is scattered, and for Xe we have determined wether it stays near the surface of the cluster or migrates inside. These results have been discussed in the light of very simple models.     

Frequency distribution of radiation emitted from excited atomic systems

General expressions for the time-dependent probability amplitudes of the quantum states of two arbitrary, interacting atoms are calculated when one atom is initially in an excited p state and the other atom is in an s ground state. The lifetimes of the excited states and the line shape of the emitted radiation are obtained as functions of both the atomic separation and the energy difference between the excited states of the two atoms. The emission line shape is shown to be doubly peaked and to agree with the line shape of the radiation scattered by a system of two interacting atoms. The expressions for the lifetimes of the excited states are found to be identical to those obtained for the radiation scattering situation.     

A theoretical study of He2ICl van der Waals cluster

The structure, energetics, and dynamics of He2ICl complex in its ground state are studied by means of ab initio electronic structure and quantum-mechanical calculations. Interaction energies for selected He2ICl configurations are calculated at the coupled-cluster [CCSD(T)] level of theory using a large-core pseudopotential for the I atom and the aug-cc-pVTZ and aug-cc-pV5Z basis sets for the Cl and He atoms, respectively. The surface is characterized around its lower five minima and the minimum energy pathways through them. The global minimum of the potential corresponds to a "police-nightstick (1)" configuration, the second one to a linear, the next one to tetrahedral configuration, and the following two to "bifork" and "police-nightstick (2)" structures, with well depths of -99.12, -97.42, -88.32, -85.84, and -78.54 cm(-1), respectively. An analytical form based on the sum of the three-body parametrized HeICl interactions plus the He-He interaction is found to represent very well the tetra-atomic CSSD(T) results. The present potential expression is employed to perform variational five-dimensional quantum-mechanical calculations to study the vibrational bound states of the van der Waals He2ICl complex. Results for total angular momentum J = 0 provide the binding energy D0 and the corresponding vibrationally averaged structure for different isomers of the cluster. Comparison of these results with recent experimental observations further justifies the potential used in this work.     

Cs*He(n) exciplexes in solid 4He

We present a theoretical and experimental study of the laser-induced formation process and of the emission spectra of Cs*He(n) exciplexes in the hcp and bcc phases of solid helium. Two different exciplex molecules are detected: a linear triatomic Cs*He2, which can exist in two electronic states: APi(1/2) and BPi(3/2), and a larger complex, where six or seven He atoms form a ring around a single cesium atom in the 6P(1/2) state. A theoretical model is presented, which allows the interpretation of the experimentally observed spectra.     

Fragmentation of ionized doped helium nanodroplets: theoretical evidence for a dopant ejection mechanism

We report a theoretical study of the effect induced by a helium nanodroplet environment on the fragmentation dynamics of a dopant. The dopant is an ionized neon cluster Ne(n) (+) (n=4-6) surrounded by a helium nanodroplet composed of 100 atoms. A newly designed mixed quantum/classical approach is used to take into account both the large helium cluster zero-point energy due to the light mass of the helium atoms and all the nonadiabatic couplings between the Ne(n) (+) potential-energy surfaces. The results reveal that the intermediate ionic dopant can be ejected from the droplet, possibly with some helium atoms still attached, thereby reducing the cooling power of the droplet. Energy relaxation by helium atom evaporation and dissociation, the other mechanism which has been used in most interpretations of doped helium cluster dynamics, also exhibits new features. The kinetic energy distribution of the neutral monomer fragments can be fitted to the sum of two Boltzmann distributions, one with a low kinetic energy and the other with a higher kinetic energy. This indicates that cooling by helium atom evaporation is more efficient than was believed so far, as suggested by recent experiments. The results also reveal the predominance of Ne(2) (+) and He(q)Ne(2) (+) fragments and the absence of bare Ne(+) fragments, in agreement with available experimental data (obtained for larger helium nanodroplets). Moreover, the abundance in fragments with a trimeric neon core is found to increase with the increase in dopant size. Most of the fragmentation is achieved within 10 ps and the only subsequent dynamical process is the relaxation of hot intermediate He(q)Ne(2) (+) species to Ne(2) (+) by helium atom evaporation. The dependence of the ionic fragment distribution on the parent ion electronic state reached by ionization is also investigated. It reveals that He(q)Ne(+) fragments are produced only from the highest electronic state, whereas He(q)Ne(2) (+) fragments originate from all the electronic states. Surprisingly, the highest electronic states also lead to fragments that still contain the original ionic dopant species. A mechanism is conjectured to explain this fragmentation inhibition.     

Spectroscopy of alkali atoms (Li,Na, K) attached to large helium clusters

Large liquid helium clusters (He_n, n ≈ 10⁴) produced in a supersonic jet are doped with alkali atoms (Li, Na, K) and characterized by means of laser induced fluorescence. Each cluster contains, on average, less than one dopant atom. Both excitation and emission spectra have been recorded. The observed excitation spectra are analyzed, calculating the transitions within an approach based on the hypothesis that the chromophores are trapped in a dimple on the cluster’s surface as predicted by the theoretical calculations of Ancilotto et al. [9]. The results of the model calculations are in good qualitative agreement with the experimental findings. In spite of the very weak binding energy (a few cm^?1), some of the excited atoms remain bound to the surface, provided the excitation occurs at frequencies not too far from the alkali’s gas phase absorptions. These bound-bound excitations produce very broad, red shifted emission spectra. At other, blue shifted frequencies, the excited atoms desorb from the cluster’s surface, giving rise to unshifted, free atom, emission spectra. The heavier alkali metals (Na, K) show, compared to the calculations, an additional broadening which is attributed to surface excitations on the helium droplet.     

氢原子在平坦金属锂(100)面上表面扩散行为的ab initio研究

本文用Li7(4,3)-H和Li9(5,4)-H小原子簇模拟氢原子在平坦金属锂(100)面吸附体系, 取小基组作了各吸附位吸附势能曲线及相应分子轨道能级图、吸附和表面扩散势能面的ab initio研究。结果表明, 氢原子优先吸附在配位数较高的吸附位上, 并倾向于由高配位数吸附位向低配位数吸附位迁移, 表面扩散各向异性, 扩散跳跃步长与锂单晶晶格原子间距数量级相同。从吸附和表面扩散势能确定了最低能量表面扩散途径, 分析了原子在平坦金属表面上迁移的微观过程。     

Size-dependent optical spectroscopy of a homologous series of CdSe cluster molecules.

The optical properties and electronic structure of a homologous series of CdSe cluster molecules covering a size range between 0.7 and 2 nm are investigated. CdSe cluster molecules with 4, 8 10, 17, and 32 Cd atoms, capped by selenophenol ligands, were crystallized from solution and their structures determined by single-crystal X-ray diffraction. The cluster molecules are composed of a combination of adamanthane and barylene-like cages, the building blocks of the zinc blende and the wurtzite structures of the bulk CdSe. The onset of the room temperature absorption and low-temperature photoluminescence excitation spectra exhibit a systematic blue shift with reduced cluster size manifesting the quantum confinement effect down to the molecular limit of the bulk semiconductor. Blue-green emission, shifted substantially to lower energy from the absorption onset, is observed only at low temperature and its position is nearly independent of cluster size. The wavelength dependence of both photoluminescence and photoluminescence excitation was measured. The emission is assigned to forbidden transitions involving the cluster-molecule surface-capping ligands. This assignment is supported by the emission decay which exhibits distributed kinetics with microsecond time scale. The temperature dependence of the emission intensity is quantitatively explained by multiphonon-induced nonradiative relaxation mediated by low-frequency vibrations of the selenophenol capping ligands. Upon irradiation, the emission of all cluster molecules is quenched. Warming up and recooling leads to recovery of the emission (partial or complete) for all but the cluster molecule with 10 Cd atoms. This temporary darkening is assigned to the photoinduced charging of the cluster-molecule surface ligands, resembling the reversible on-off blinking of the emission observed for larger CdSe nanocrystals.     

Ionic dimers in He droplets: interaction potentials for Li2(+)-He,Na2(+)-He, and K2(+)-He and stability of the smaller clusters

We present post Hartree-Fock calculations of the potential energy surfaces (PESs) for the ground electronic states of the three alkali dimer ions Li(2) (+),Na(2) (+), and K(2) (+) interacting with neutral helium. The calculations were carried out for the frozen molecular equilibrium geometries and for an extensive range of the remaining two Jacobi coordinates, R and theta, for which a total of about 1000 points is generated for each surface. The corresponding raw data were then fitted numerically to produce analytic expressions for the three PESs, which were in turn employed to evaluate the bound states of the three trimers for their J=0 configurations: The final spatial features of such bound states are also discussed in detail. The possible behavior of additional systems with more helium atoms surrounding the ionic dopants is gleaned from further calculations on the structural stability of aggregates with up to six He atoms. The validity of a sum-of-potential approximation to yield realistic total energies of the smaller cluster is briefly discussed vis-a-vis the results from many-body calculations.     

The structure of the phenol-nitrogen cluster: a joint experimental and ab initio study

The rotationally resolved LIF spectra of four different isotopomers of the phenol--nitrogen cluster have been measured to elucidate the structural parameters of the cluster in ground and electronically excited (S1) state. The fit of the rotational constants has been performed by a genetic algorithm and by an assigned fit to the line frequencies. The results of both methods are compared. The intermolecular structures are fit to the inertial parameters and are compared to the results of ab initio calculations for both states. This fit was performed under the restriction that the geometry of the monomer moieties do not change upon complexation. Of the remaining five intermolecular parameters two dihedral angles were fixed due to the planarity of the complex, which was inferred from the inertial defects of all isotopomers. The distance of the nearest nitrogen atom to the hydrogen atom of the phenolic hydroxy group is found to decrease upon electronic excitation of the chromophore considerably more than predicted from ab initio calculations. This deviation between theory and experiment can be traced back to the absence of electron-electron correlation in the performed complete active space self-consistent field calculations. The shortening of the OH...NN "hydrogen" bond upon electronic excitation is in agreement with the increased dipole moment of phenol in the S1-state.     

Path integral Monte Carlo simulation of the absorption spectra of an Al atom embedded in helium

We use a multilevel path integral Monte-Carlo (PIMC) method to simulate the arrangement of He atoms around a single Al atom doped in a He cluster. High-level ab initio Al-He pair potentials and a Balling and Wright pairwise Hamiltonian model are used to describe the full potential and the electronic asymmetry arising from the open-shell character of the Al atom in its ground and excited electronic states. Our calculations show that the doping of the Al 3p electron strongly influences the He packing. The results of the PIMC simulation are used to predict the electronic excitation spectrum of an Al atom embedded in He clusters. With inclusion of tail corrections for the ground and excited states potentials, the calculated 3d<--3p spectrum agrees reasonably well with the experimental spectrum. The blueshift of the calculated spectrum associated with the 4s<--3p transition of solvated Al is about 25 nm (2000 cm-1) larger than seen in experiments on Al embedded in bulk liquid He. We predict that the spectrum associated with the 4p<--3p transition will be blueshifted by approximately 7000 cm-1 (nearly 1 eV).     

Electronic structures of size-selected single-layered platinum clusters on silicon(111)-7x7 surface at a single cluster level by tunneling spectroscopy

Tunneling spectra of size-selected single-layered platinum clusters (size range of 5-40) deposited on a silicon(111)-7x7 surface were measured individually at a temperature of 77 K by means of a scanning tunneling microscope (STM), and the local electronic densities of states of individual clusters were derived from their tunneling spectra measured by placing an STM tip on the clusters. In a bias-voltage (V(s)) range from -3 to 3 V, each tunneling spectrum exhibits several peaks assignable to electronic states associated with 5d states of a constituent platinum atom and an energy gap of 0.1-0.6 eV in the vicinity of V(s)=0. Even when platinum cluster ions having the same size were deposited on the silicon(111)-7x7 surface, the tunneling spectra and the energy gaps of the deposited clusters are not all the same but can be classified in shape into several different groups; this finding is consistent with the observation of the geometrical structures of platinum clusters on the silicon(111)-7x7 surface. The mean energy gap of approximately 0.4 eV drops to approximately 0.25 eV at the size of 20 and then decreases gradually as the size increases, consistent with our previous finding that the cluster diameter remains unchanged, but the number density of Pt atoms increases below the size of 20 while the diameter increases, but the density does not change above it. It is concluded that the mean energy gap tends to decrease gradually with the mean cluster diameter. The dependence of the mean energy gap on the mean Pt-Pt distance shows that the mean energy gap decreases sharply when the mean Pt-Pt distance exceeds that of a platinum metal (0.28 nm).