Ion current generation in the field ion microscope: II. quasi-static approach

The field ion current is calculated for helium on tungsten, for various fields, and for various values of tip temperature and gas temperature on the basis of the balance equation developed by Van Eekelen. The expression of ion current by rate constants for ionization and for escape, the total supply and the capture probability is derived. The behaviour of ion current as a function of other parameters is discussed in the light of these equilibrium properties of the system. Field adsorption effects are also considered. The increase of the mass ratio of the gas atom to the metal atom causes the increase of ion current even if ionization probability is decreased by the field adsorption. Anomalous features of the field ion image at 4.2 K are discussed.     

Calculations of field ionization in the field ion microscope

The ionization of a gas atom as it occurs in the field ion microscope is discussed. A wide range of values for the electric field intensity at the metal surface are considered in calculating the ionization occurring both far away from and close to the tip. Ionization distribution curves are calculated and electric fields strengths are determined at points where the distributions peak. Calculations of ionization zone widths and best image conditions are made and a new interpretation of best image conditions is considered. A new field calibration method is suggested.     

On the field penetration into semiconductors in the field ion microscope

The calculation of field penetration in semiconductors and consequent band bending during field ionization/evaporation is discussed. The shielding by surface states is also taken into account. The Si(111) face example demonstrates that neglection of surface states may give unrealistic high band bending values. Because of the lack of reliable data for the density of surface states, a possible maximal band bending has been calculated for GaAs. Its value in the case of an external applied field of $\text{1}\text{V}\text{A}\text{?}$ may be such smaller as formerly assumed in recent works.     

Direct observation of reconstructed surfaces in the field ion microscope

Both the Ir(110)1×2 and the Ir(100)1×5 reconstructed surfaces have been imaged in the field ion microscope. The observations agree with results obtained on macroscopic single crystal surfaces. Evidence for a possible 1×2 reconstruction of the Ir(113) surface is shown.     

Neon gas imaging of gold in the field ion microscope

An improved field ion microscope (FIM) technique has been developed for the neon gas imaging of gold specimens. The technique produces images which are stable at best image voltage at a tip temperature (T_T) of 30 K or less. The first stage of the technique consisted of the development of an end form at 55 K in the presence of a partial pressure of air (～ 2 × 10^?8 Torr gauge pressure) and neon gas (～ 3 × 10^?5 Torr gauge pressure) followed by further field evaporation at 28 K. The second stage involved neon gas imaging of the previously developed end form in a baked FIM in a background pressure of (0.5 to 3) × 10^?9 Torr. The FIM images obtained in conjunction with the field ionization characteristic curves showed that there is a working range (in the sense defined by Southon and Brandon). A detailed study was made of artifact vacancies detected in the {203}, {321}, {315}, {421}, {671} and {731} planes, and it was found that at 28 K their concentration was < 2.5 × 10^?3 at.fr. Approximately 191,000 atomic sites were examined for artifact vacancies. The artifact vacancy concentrations measured in the present study were a factor of 13 to 60 lower than those measured earlier by Schmid and Balluffi who employed a background pressure of ～ 5 × 10^?8 Torr in their FIM. Hence, the artifact vacancy concentrations detected in gold are dependent upon the background partial pressure employed in the FIM. This latter result plus the result that the images are only stable in ultra-high vacuum (UHV) conditions indicates the need for UHV conditions for the successful imaging of gold surfaces.     

The in situ cleaning of specimens in the field ion microscope by argon ion bombardment

The extensive adoption of argon bombardment cleaning techniques for specimens used in LEED and Auger studies, and the frequent, and often difficult, requirement of preparing field ion emitters, and their supports, free from contamination, led to the investigation of in situ argon ion bombardment of specimens in the field of ion microscope, both from the point of view of the efficiency of the cleaning process, and the investigation of ensuing surface and lattice damage, a task to which the technique of field ion microscopy is particularly appropriate.Experiments were carried out in some detail for tungsten specimens, and subsequently extended to the hexagonal metal, rhenium, with a view to obtain information on the thermally annealed end forms of such metals. In both cases, very small radius clean thermally annealed specimens could be prepared.     

Hydrogen promoted corrosion of tungsten by oxygen in an electric field: A field ion microscope study

The extensive changes in surface topography observed to occur on tungsten field ion emitters as a result of exposure to oxygen in presence of hydrogen at 78°K and at fields of 2 V/Å have been studied in detail. Field promoted diffusion of gas from the low field region of the emitter shank over the imaged area of the surface removes kink site metal atoms and subsequently deposits them on either side of the well defined diffusion paths along close packed zones; field evaporation of such atoms may also occur when the change in local surface contour causes sufficient field enhancement. Although oxygen is primarily responsible for the corrosion effects, its diffusion at such temperatures is promoted by the presence of hydrogen, and the rate of reaction is dependent, inter alia, on hydrogen pressure.     

Ion current characteristics of an Ar field ionization source

Ion current-voltage (I–V) characteristics and ion current-pressure (I–P) characteristics of an Ar ionization source on a W tip at 80 K have been studied by simultaneous measurements for the center region of the tip where no contact with the shank exists and the region surrounded by the shank. The characteristics of these regions are found to be different. Further, the gradients of the I–P characteristics are shown to depend on the I–V characteristics.     

Kinetics of microplasma atmospheric ion generation correlated with discharge current

The conditions necessary for achieving a stable bipolar ion generation (in the order of 10⁶ ion/cm³) and lower ozone concentration (less than 50 ppb) using a surface discharge microplasma device (SMD) by adjusting the applied voltage and frequency were experimentally determined and investigated. Measurements of the discharge current characteristics of the SMD revealed saturation against the frequency (1.5–2.5 kHz, depending on the applied voltage). The ion and ozone concentrations both increased in step with the discharge current in the lower frequency region. The ion concentration reached equilibrium in the frequency range of 200–500 Hz, and the point of equilibrium within that range depended on the applied voltage. The ozone concentration did not reach equilibrium under our experimental conditions (ozone concentration < 100 ppb). The kinetics of the ion/ozone generation rate with a focus on the plasma reaction and recombination of bipolar ions is discussed.     

The Bohm approach to cavity quantum scalar field dynamics. Part I: The free field

Bohm 's approach to quantum field theory is illustrated through its application to cavity quantum scalar field dynamics. Specific calculations demonstrate how the evolution of the well-defined scalar field is governed by the nature of its quantum state. The implications of the nonlocality inherent in quantum mechanics and the meaning of the classical limit are discussed in this context.     

Growth of well ordered Rh overlayers on Si surfaces: A field ion microscope study

Well ordered Rh overlayers have been grown on Si tip surfaces in ultrahigh vacuum. The field ion images of the overlayers grown on the Si{012} plane reveal two distinctive atomic structures. One has a diamond structure with an angle of 76° ± 2° and one has a rhombic structure with an angle of 66° + 2°. The former fits well to the atomic structure of the Rh layer of the RhSi(110) plane and the latter fits well to the structure of the Si(012) plane.     

Influence of the current density longitudinal distribution in current sheets on their magnetic field structure and formation dynamics. I: Calculation of the magnetic field of current sheets with different configurations

The structure of the current sheet magnetic fields are numerically calculated for different distributions of the current density across the current sheet width. The structural features of the magnetic fields of current sheets with current density double-humped distributions are revealed.     

Probing dynamic interference in high-order harmonic generation from long-range molecular ion:Bohmian trajectory investigation

Using Bohmlan trajectory （151） method, we investigate the dynamic interference in nlgn-orcter narmonlc generauon from diatomic molecular ions. It is demonstrated that the main characteristics of the molecular harmonic spectrum can be well reproduced by only two BTs which are located at the two ions. This haiapens because these two localized trajectories can receive and store the whole collision information coming from all of the other re-collision trajectories. Therefore, the amplitudes and frequencies of these two trajectories represent the intensity and frequency distribution of the harmonic generation. Moreover, the interference between these two trajectories shows a dip in the harmonic spectrum, which reveals the molecular structure information.     

不均匀场中氢分子离子高次谐波产生的理论研究

在非Born-Oppenheimer近似下,通过求解含时薛定谔方程的方法,对氢分子离子在不均匀场中高次谐波的产生进行了理论研究.计算结果显示,同均匀场相比,电离的电子在不均匀场中加速会获得更多的能量,从而更有利于得到宽频谱.此外,通过优化不均匀场中的空间不均匀度,长量子路径被明显的抑制,最终通过叠加110阶到150阶谐波,获得一个60as的孤立阿秒脉冲.同时,通过库仑势和激光场的相互作用势以及时频分布图解释了其中的物理机制.     

不均匀场中氢分子离子高次谐波产生的理论研究

在非Born-Oppenheimer近似下,通过求解含时薛定谔方程的方法,对氢分子离子在不均匀场中高次谐波的产生进行了理论研究.计算结果显示,同均匀场相比,电离的电子在不均匀场中加速会获得更多的能量,从而更有利于得到宽频谱.此外,通过优化不均匀场中的空间不均匀度,长量子路径被明显的抑制,最终通过叠加110阶到150阶谐波,获得一个60as的孤立阿秒脉冲.同时,通过库仑势和激光场的相互作用势以及时频分布图解释了其中的物理机制.     

Scanning helium ion microscope: Distribution of secondary electrons and ion channeling

The principles and features of operation of a scanning helium microscope are reviewed briefly. The measurement data on the energy distribution of secondary electrons excited by the ion beam in an Au film and on the angular dependence of the backscattered ion yield are obtained and presented for the first time. The effect of ion channeling in silicon single crystal with the (110) orientation is demonstrated.