Chemical Article

A QUANTUM DYNAMIC APPROACH TO THE CONDENSATION PROCESSES OF ZINC ATOMS BY THE INNER-CORE EXCITATION DUE TO ION RECOMBINATION 

Mitsugi Hamasaki1*, Masumi Obara1 , Kozo Obara1 , Hirotaka Manaka1 1 Graduate School of Science and Engineering Kagoshima University, Korimoto 1-21-40, Kagoshima, 8900065, Japan

ABSTRACT     

    Isolated atoms in group II-B such as zinc (Zn), cadmium (Cd), and mercury (Hg) are chemically stable. These atoms are important in the formation of excimer. Zinc in particular has been investigated by many researchers, as Zn2 excimer holds promise because of its long lifetime and its potential as an energy-storage system. However, excimer’s benefits are based on excitation of the outermost electron. Our study confirmed the quantum dynamical condensation processes in which inner-core excitation arises due to ion-recombination between the vapor phase and the solid phase. The X-ray diffraction of the condensed structure of zinc film had included strong diffuse scattering depending on the incident energies. In this research, we produced the excited state of zinc excimer characterized by an extremely long lifetime. Intriguingly, a feature of the zinc film is that it transforms from metallic to insulative. It is thought that such a structure with this characteristic has been affected by electron spin and atomic distortion by inner-core excitation. The structure obtained in our experiment is expected to prove promising in engineering applications, such as electronics, spintronics, and batteries. 

 1. INTRODUCTION 

In the vapor phase growth processes, condensation is a most important process in which the translational kinetic energy of incident atoms in the gas phase is dissipated at the crystal surface (Ehrlich & Hudda, 1966). So far, incident energy has been treated as independent from the surface atom system because the surface of the atom system is neutral (Bendavid et al., 2000). In this paper, we propose a method to analyze the quantum dynamic processes in the condensation process from vapor phase to solid phase. We respect the parity of the incident particles. Since the analysis of the particles with the same parity is impossible, we adopted particle systems having different parities. The simplest way to adopt the particles having different parities is obtained by the ionizing in opposite polarities. Normally the lifetime of negative ions is much shorter than that of positive ones (Herzberg, 1944); a unique approach is needed to elongate the lifetime of the negative ions. For this research, a negatively charged crystal surface with electron irradiation was used.

The condensation of a positive ion and a negative ion is called the ion-recombination process. The Coulomb force between two ions with opposite polarities produces a strong cohesive force. The translational kinetic energy of the incident ions dissipates at the first collision of these two ions. The most effective process for energy dissipation in this process is the energy transfer from translational energy to e internal energy in the ion’s electron system. In order to detect these quantum dynamic processes, we used X-ray diffraction techniques for detecting the spatial correlation between the two atoms. X-ray diffraction intensities clearly depended on the energies of electron irradiation. We found strong diffuse scattering of X-rays on the thin films of zinc deposited by electron irradiation. The strong diffuse scattering indicates the existence of a lattice defect in the thin film observed at the discrete energies of electron irradiation. Further strong Bragg reflection intensities were observed at the discrete energies. These energies correspond to the binding energies of a zinc atom: 3d(10eV), 3p(90 eV), and 3s(140 eV). These experimental data show the evidence of inner-electron excitation of zinc ions. These electron energies have been dominated by the selection rule of electron transition, Δl = ±1, where l is the orbital angular quantum number. We finally confirmed the existence of unpaired spins in excited zinc atoms by Electron Spin Resonance (ESR). This signal suggests that ion-recombination produces excited states of zinc that appear following ion-recombination and which are characterized by long lifetimes.

2. EXPERIMENTAL

2.1. Experimental system All experimental procedures were conducted in a vacuum. The vacuum system consisted of conventional equipment including a turbo molecular pump, in which ultimate pressure was 10-5 Pa order. Figure 1 provides a schematic illustration of incident electrons, and substrate and zinc ions. The electronic states on the substrates were controlled by the energies of electron irradiation emitted from a cathode in an electron gun. The magnitude of electron emissions was adjusted by the filament current. The kinetic energy of incident electrons was controlled by a potential between the anode of gold thin film deposited around the area of sapphire substrate and the hot filament of cathode in an electron gun. The incident angle of electrons was 45o from the normal substrate surface. Zinc atoms were deposited from the effusion cell, in which the purity of zinc was 99.999%. The zinc atoms were deposited on the insulative area, which measured 6.5 mm in diameter at the center of the substrate.

 

After the anode potential was adjusted to an appropriate level, the first stage was designed to control the potential energy of the insulative area of substrate surface by irradiation of electrons. Electrification in this area is monitored by the transmission electron current measured as the anode current (Obara et al., 2000). This method is one application of “angle resolved transmission electron current spectroscopy” (Obara et al., 1999). After 10 hours from irradiation of the electrons, the electron current became stable at 0.1 μA. The goal of the next stage was to deposit zinc atoms on the substrate surface. The incident angle of the zinc atom beam coming from the effusion cell was held to normal on the substrate surface. As a guide, a typical deposition time of zinc is about 1000 seconds at 600o C in the effusion cell.

2.2. Formation of boundary conditions

The microscopic understanding of growth processes is due to two different concepts of movements: the individual movement of incident atoms and the movement of surface atom systems as a group. The Surface Phase is considered to be the region where the two concepts merge (Bird, 1995). A key to controlling the reaction processes in the Surface Phase is the electron states of the surface atom system. The electron states near Fermi energy are complex because the incident atoms in condensation processes are a mix of many electron states that form a new band (Jones et al., 1934). We adopted an approach to the area of the surface phase from the gas phase side, as shown in Figure 1, because it is easy to describe the movement of individual atoms and to make the image of collisions between Zn+ in the gas phase and Zn- in the surface phase. In controllable two-body collision processes, changing of spatial and temporal parameters is a fundamental viewpoint for the generation of new reaction fields. We proposed to decrease the spatial parameters of the reaction field to increase the acceleration of incident Zn+ just before collision. The magnitude of the acceleration of Zn+ dominated the interaction with electromagnetic waves in the ion-recombination process. It should be noted that an electromagnetic wave is transformed to interaction energy with electric dipole moments (Ni et al., 2010). As shown in Figure 1, the Sapphire substrate had a gold electrode for applying the bias voltage for incident electrons. Charged electrons on the insulative area form the negative field and the magnitude of the surface potential, eVB. This value was equal to the bias potential of the gold electrode. Charged electron density depends on the distance from the center of the circular substrate, and charged electron density near the edge of the circular area was much larger than that at the center of the substrate because of creating homogeneous potential of the substrate.

2.3. Reaction processes 

In the Figure 1, Zn+ is controlled by the energy of reaction field and reach to Zn- on the substrate surface. In the early stages, the interaction of both ions is possible to describe as like the dynamic and electromagnetic model which is called direct collision, because the distances between both ions are long. The direct collision process is possible to transfer by high efficiency from total energy to inner energy (Kawazoe et al., 2005). If the distance between ions approaches at the atomic diameter level, the interelectronic interaction increases in the both ion. In this situation, the linear combination with wave function of Zn+ and Zn- make possible to formation of molecular orbital, which is represented by the following equation in a quantum dynamic manner. 

3. RESULTS 

3.1. X-ray diffraction 

Figure 2 shows the energy dependence of X-ray diffraction intensities scattered from the zinc films deposited under electron irradiation with the energy, eVB. Strong diffuse scattering of Xrays was observed at inherent electron energies, 10 eV, 90 eV, 100 eV, and 230 eV, which corresponded to the electron binding energies, 3d(10 eV), 3p(90 eV), 3d+3p(100 eV), and 3p+3s(230 eV) of the zinc atoms. Peak profiles at 90 eV, 100 eV, and 230 eV broadened as electron energy increased.

4. CONCLUSION 

4.1. Contribution of inner-core excitation The electron excitation process depends on the initial and the final states in ions. The initial electron states of both ions are [Ar]3d104s1 for Zn+ , and [Ar]3d104s 2 4p 1 for Zn- . The excitation model from the inner-core electron states to the 4s-state for Zn+ , and 4p-state for Zn- 

The column marked with “--” has no transited ion state. The term “(Zn+ )*” means the existence of excitation from 3d-state to 4p- state in Zn+ . The column of “Transition probability” of each ion state is written by numeric “1” for possible cases or “0” for impossible scenarios. The column, “Product of transition probability,” means the product of two ions’ probabilities. For this reason, numeric “1” shows the strong intensity of diffuse scattering or Bragg reflection and both strong intensities, and “0” shows the weak intensities. The column, “Intensity of diffraction,” shows the intensities for diffuse scattering and Bragg reflection, in which characters “H” and “L” mean, respectively, High and Low intensities. The column, “Contribution of excitation,” indicates that the “Intensity of diffraction” is affected by excitation of either ion species. The double excitations at 100 eV and 230 eV showed very strong diffuse scattering and Bragg reflection intensity. In the 3 single excitations at 10 eV, 90 eV and 140 eV, both 90 eV and 140 eV showed completely different characteristics.

4.2. Condensation model From the discussions of subsections 3.2 and 4.1, authors of this research proposed a model for the condensation process due to the ion-recombination process, as shown in Figure 6. The boundary condition was in the space where Zn- is located in the surface phase and in the space where Zn+ was located in the gas phase above the surface phase. As shown in Figure 6(a), the excitation in Zn+ had an influence on the nearest neighbor, Zn- . However this excitation was not available to bonding to the Zn atom in the solid phase because of the long distance between Zn+ in the gas phase and Zn atoms in the solid phase. When the excitation at the Zn+ site, the bonding with the solid phase became weak. Therefore, Zn+ and Zn- have a broad distribution function of lattice spacing for Zn in solid phase, and strong diffuse scattering occur.

5. CONCLUSION 

The quantum dynamic processes for the first step of the condensation process from gas phase to solid phase were investigated by using the ion-recombination process. The electron energy dependencies of the derived crystals showed very strong diffuse scattering at discreet energies, which corresponded to the binding energies of zinc atoms. Strong Bragg reflections were also observed at discreet energies. From the comparison between these two series of experimental data, we proposed a model for the excitation of ions in which the excitation in Zn+ located at the  gas phase induces strong diffuse scattering while excitation in Zn- located at the surface phase induces the strong Bragg reflection. This model demonstrates that the inner-core excitation process occurs before the process of charge exchange. We also confirmed the characteristic transformation of zinc film from a metallic to an insulative quality.

6. ACKNOWLEDGEMENT The authors thank Professor Yosihiko Hatano of the Advanced Science Research Center of Japan Atomic Energy Agency, and Professor Noriaki Itho of Nagoya University for continuous encouragement and helpful discussions.

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