Measurement of the charge state distribution of field evaporated ions: Evidence for post-ionization

Charge state distributions of field evaporated Si, Ni, Mo, Rh, W, Re, Ir and Pt ions have been measured as a function of electric field strength using the pulsed laser atom-probe. The results are compared to previously published theoretical calculations based on the post-ionization model of field evaporation. The agreement between theory and experiment is sufficient to establish the general validity of the post-ionization model. Measurements of the charge state distributions as a function of evaporation rate at constant temperature (increasing field) and constant field (increasing temperature) are also presented for W, Mo and Si. The observation that the fractional abundances of different charge states for the same material do not change with changing temperature indicates that the activation energies of desorption are the same for the different charge states and provides further support for the post-ionization model. The anomalous field evaporation behavior observed at high temperatures (e.g., desorption from localized areas on the surface and the occurrence of ionic clusters) is also discussed.     

Charge state of ions in liquid metal field ion sources

The post-ionization model of field evaporation is shown to be consistent with observations of singly and doubly charged ions in liquid metal field ion sources. The model can be used to estimate the field strength at the apex of the Taylor cone which is found to be 1.9–2.0 V/Å for a Ga source. Experiments to test the post-ionization model and to determine the apex field strength more accurately are suggested. A possible method of obtaining ～μA currents of highly charged ions, e.g. Zr⁴⁺, Ta⁴⁺, Ga³⁺, As³⁺, is proposed.     

Field evaporation of impurity ions from a liquid gallium ion source

The charge states of ions emitted from a gallium liquid metal field ion (LMI) source contaminated with tin and copper have been measured. The results for tin show that the proportion of Sn²⁺ to Sn⁺ is much larger than is found for a LMI source of pure tin. A model in which Sn²⁺ is assumed to be produced by post-ionization of Sn⁺ is used to set an upper limit to the electric field at the point of emission. Its value is approximately equal to that predicted by field evaporation theory for a pure gallium source. Consequently the charge states of emitted impurity ions are determined by field strengths imposed by the main component.     

Layer-by-layer analysis of charge state distribution of field evaporated ions from the W(011) plane

A charge state distribution of the field evaporated ions is layer-by-layer analyzed in a W(011) plane by using an atom-probe field ion microscope. A specimen temperature is varied from ～ 20 to ～ 250 K. The results indicate that at any temperature, the surface tungsten atoms field evaporate as triply and quadruply ions from the (011) plane, and the quadruply charged ions are always detected during the final collapse of the plane. They are also discussed on the basis of the post-ionization model.     

Effect of tunneling ionization on dynamic alignment and post-ionization alignment of nitrogen molecules irradiated by different laser intensities

The dynamic alignment and post-ionization alignment of nitrogen molecules are investigated while considering the effect of tunneling ionization. The effects of tunneling ionization on the angular distribution are calculated when the molecules are irradiated by different laser intensities. The results show that laser intensity directly affects the time and extent of dynamic alignment. Furthermore, the extent of post-ionization alignment is not only determined by laser intensity but also affected by the final extent of dynamic alignment. The post-ionization alignment will dominate during the process of molecular (or molecular ion) rotational alignment for femtosecond laser pulse. The time of tunneling ionization is a significant factor to the final ensemble angular distribution of molecular ions when laser intensity is low.     

Theoretical investigation of liquid metal ion sources: Field and temperature dependence of ion emission

We investigate the field and temperature dependence of the rate of ion formation by important mechanisms in Liquid Metal Ion Sources (LMIS). In addition to field evaporation and field ionization of thermally evaporated neutrals we identify a third mechanism, thermal-field evaporation, which is intermediate between the other two mechanisms. Field (or thermal-field) evaporation is found to be dominant in the normal operating regime of LMIS in agreement with the conclusions of Prewett et al. A jet-like protrusion model of LMIS shape, which is consistent with direct observations, allows high temperatures to be reached with a reasonable power input to the source. Thus a small, but sometimes important, contribution to the total ion current from field ionization of thermally evaporated atoms is expected.     

An examination of field evaporation theory

Contrary to opinions expressed by earlier investigators, thorough examination of the existing models for field evaporation indicates that they are capable of predicting changes in the relative abundance of ionic species as temperature is changed and evaporation rate is held constant, without assuming atomic tunneling or slowness of electronic transition. However, this need not imply that atomic tunneling and slowness of electronic transition do not exist. Furthermore, the inclusion of energy level shifts in field evaporation theory results in the prediction of higher charge states than would be otherwise expected. This is in agreement with the observations of various investigators on copper and other metals. The examination further reveals that very significant differences in the predicted variation of evaporation rate with field exist between the image-hump and intersection models. This suggests a means by which one can attempt to determine experimentally the “correct” model to apply in a given situation. Furthermore, rigorous interpretation of the intersection model in conjunction with the experimental data of Tsong and Müller can result in significantly different values for the distance of the equilibrium position of the surface atom from the metal surface and the polarizability of the surface atom, than obtained using linear approximations.     

Possibilities and limitations of high-resolution mass spectrometry in life sciences

We have applied time-of-flight secondary ion mass spectrometry (ToF-SIMS) and laser post-ionization secondary neutral mass spectrometry (laser-SNMS) to examine the immobilization process of PNA and its hybridization capability to unlabeled complementary DNA fragments, to characterize immobilized proteins, and to image intrinsic elements and molecules with subcellular spatial resolution in different types of frozen non-dehydrated biological samples.The possibilities and limitations of ToF-SIMS and laser-SNMS for imaging elements and molecules in biological samples are discussed. Furthermore possibilities for enhancing the detection sensitivity by using polyatomic and cluster primary ions and different laser post-ionization schemes, as well as ways of obtaining 3D molecular images from biological samples are described.The data shows that both ToF-SIMS and laser-SNMS are capable of imaging elements and molecules in complex biological samples and that they are very valuable tools in advancing applications in life sciences. It was found that cluster-ion bombardment is very useful for enhancing the molecular yield, while laser-SNMS resulted in much higher detection sensitivity for elements and specific molecules and is particularly well suited for imaging ultra-trace element concentrations in biological samples.     

Field evaporation of doubly charged ions from a polar liquid

The effect of charge on field evaporation of ions from polar liquids is considered. Using the electromembrane ion source, we performed mass-spectral analysis of field evaporation of ions from the solution of sodium sulfate in a water-glycerol mixture. The composition of doubly charged cluster ions in the field evaporation from glycerol is determined. The rates of the field evaporation of doubly charged ions and singly charged ions are compared. It is shown that the ion charge as well as its localization considerably influences the efficiency of field evaporation of ions from polar liquids.     

High-temperature field evaporation and its connection with surface ionization

High-temperature field evaporation of metals and alloys and its connection with surface ionization are considered. The main parameters of the evaporation process (dependence of the evaporation rate on the emitter temperature and on the electric field at the emitter surface, the charge of the ions being evaporated and its temperature dependence, kinetic parameters of the evaporation process, as well as the state of the emitter surface under simultaneous action of high fields and temperatures) are analyzed. The similarity and the difference between field evaporation at high temperatures and surface ionization in a strong electric field are determined.     

The properties of the nonequilibrium LaB6 surface resulting from field evaporation

The nonequilibrium surface of single-crystal lanthanum hexaboride needles and its modifications are studied with a time-of-flight atomic probe. The surface is obtained by room-temperature field evaporation. The mass spectra of field evaporation shed light on the surface composition at the needle tip immediately after tip etching, corrosion in residual gases, intense cleaning by field evaporation, and the relaxation of the nonequilibrium surface by heating to 1250 K. Conditions for the breakdown of an oxide film on the tip surface and for obtaining the mass spectra of field evaporation for stoichiometric or lanthanum-enriched pure LaB₆ single crystals are discussed.     

Salient features of the analysis of layered structures by means of ion sputtering

Results are presented from an experimental investigation of layered structures made up of elements with strongly different atomic masses, using ion sputtering with post-ionization of secondary neutrals in a gas discharge plasma. An anomalous increase of the signal at boundaries between layers is noted. It is proposed that the phenomenon of selectivity in etching affects the measurement of the profile of secondary particle yield. Zh. Tekh. Fiz. 67, 120–122 (September 1997)     

Isotopic ratio of evaporated ions,critical ionization distances,and ionization regions in the process of the field evaporation of molybdenum at high temperatures

A magnetic mass spectrometer with a field ion source has been used to study the steady-state field evaporation of molybdenum at a temperature of 1000–2000 K. Ions of all seven molybdenum isotopes have been observed in the process of evaporation; only low-charge ions Mo⁺² and Mo⁺ have been detected. The critical ionization distances and ionization regions for single- and double-charge Mo ions have been identified based on the measured ion energies and the experimentally determined intensity of the evaporation field. It has been demonstrated that ions are produced in the process of field evaporation of surface atoms at certain distances from the emitter surface in a very narrow spatial region.     

碳纳米管薄膜场蒸发效应

在超高真空系统中对基于丝网印刷方法制备的碳纳米管薄膜的场蒸发效应进行实验研究. 实验发现, 碳纳米管薄膜样品存在场蒸发现象, 蒸发阈值场在10.0-12.6 V/nm之间, 蒸发离子流可以达到百皮安量级; 扫描电子显微镜分析和场致电子发射测量结果表明, 场蒸发会使碳纳米管分布变得更加不均匀, 会导致薄膜的场致电子发射开启电压上升(240→300V)、场增强因子下降(8300→4200)、蒸发阈值场上升(10→12.6V/nm), 同时使得薄膜场致电子发射的可重复性明显变好. 场蒸发也是薄膜自身电场一致性修复的表现, 这种修复并非表现在形貌上, 而是不同区域场增强因子之间的差距会越来越小, 这样薄膜场致电子发射的可重复性和稳定性自然会得到改善.     

3D atom probe assisted by femtosecond laser pulses

The 3DAP allows to image a material in 3D on a nearly atomic scale. It is based on the field evaporation occurring at the surface of a biased tip like shape specimen with an end radius of 50 nm. Surface atoms are removed one by one from the tip by means of fs laser pulses so that the physical process involved in this laser enhanced field evaporation might correspond to the very early stages of the ablation process. This technique makes possible to distinguish between different regimes of material removal such as thermal evaporation or in the case of metals or semiconductors an evaporation assisted by the rectification of the optical field at the surface. In this paper the principle of the 3DAP is presented and the underlying physics involved in the field evaporation assisted by femtosecond laser pulses is discussed.     

A determination of the rate and heat of evaporation by measuring the radius variations of a metal tip

A new material loss technique for the experimental determination of the free evaporation rate of a metal is presented. This technique is based on the study of tip profile changes. The tip radius variation is a consequence of the simultaneous action of free evaporation and surface self-diffusion. The rate of free evaporation is related to the tip radius evolution, so the vapor pressure and heat of evaporation can be determined by measuring in situ the radius changes with time. The method permits an easy control of the cleanliness of the surface by field electron microscopy, and should enable one to study the influence of adsorption on evaporation. The method is tested with molybdenum tips. The variation of the radius of the tips, heated in ultra-high vacuum, is determined by field electron and scanning microscope measurements. Vapor pressures and heat of evaporation values obtained are in agreement with values obtained by other techniques.     

Field evaporation of silicon surfaces

Quantitative measurements on field evaporation of Si(111) surfaces in hydrogen imaging gas have been carried out by field ion microscopy. The field evaporation rate is found to increase exponentially with increase of the reciprocal of tip temperature in the range 80–103 K. The evaporation field strength increases with increase of tip temperature in the investigated range, 80–300 K. Within the applied pressure range, 5× 10^?6 to 2 × 10^?4 Torr of hydrogen gas, the evaporation rate linearly increases with the gas pressure. Similar effects of temperature and gas pressure on field evaporation of Si(111) surfaces have been observed also in silane imaging gas. A model, based on a field-induced formation of surface hydrides as a rate-determining step, is proposed, which accounts for all the experimental results of the field evaporation process.     

Comparison of vibrational population of excited ions formed in penning ionization determined by electron spectroscopy and fluorescence measurement

The electron spectroscopical determination of the primary distribution of vibrational levels of the HCl⁺ and HBr⁺ ions in the excited A²Σ⁺ states, formed in Penning ionization by helium metastables, has shown that the vibrational distribution determined by fluorescence measurement is altered by post-ionization relaxation effects.     

Influence of field evaporation treatment on the field emission properties of carbon nanotubes array

Field evaporation was used in the post-fabrication treatment of a carbon nanotubes (CNTs) array and effectively modified the CNTs morphology in favor of the field emission under a moderate field. After the field evaporation treatment, the uniformity of the emission site distribution improved but the onset voltage rose. Using the Fowler-Nordheim theory, the actual onset field and the evaporation field around the CNT were calculated to be −4.6-5 and 9-12 V/nm, respectively. These values are close to those obtained from the individual CNT samples. The above results have provided an alternative to modify the configuration of an array sample and demonstrated the feasibility of tackling the problem of the disparity in the field emission capability of different CNTs in an array.     

Effects of an intense electric field on metal surface geometry

The effects of a strong positive electric field on the surface geometry of a single crystal, smooth metal surface are discussed. Explicit density functional calculations are presented for the field induced relaxation and evaporation of the Al(111) surface layer of atoms. The results are then explained in terms of a simple model and approximately extended to other metal surfaces. Our model determines the critical field for surface evaporation if there is no reconstruction. Finally, we will briefly mention the possibility of field induced reconstruction and its role in determining the critical field for stripping the surface layer of atoms.