Residual Gas Molecules Research Articles

Kinetics of residual gas adsorption on Ge(111) surface in low-energy electron backscattering

Low-energy (0.4–1.2 eV) electron backscattering is applied for the investigation of kinetics of residual gas adsorption effect on the concentration and energy positions of surface electron states of Ge(1 1 1) surface. Chemosorption of residual gas molecules on Ge(1 1 1) at P ∼ 10 −7 Pa and room temperature is shown to be most active during the first 48 h. Low concentration of dangling valence bonds on the reconstructed Ge(1 1 1) (2 × 8) surface is shown to determine its low activity to chemosorption.

Size and energy distribution of gas cluster ion beam measured by energy resolved time of flight mass spectroscopy

Energy resolved time of flight (ERTOF) mass spectroscopy has been developed to measure the energy and size distributions in a gas cluster ion beam (GCIB). Gas cluster ions, being aggregates of a few to several thousands of atoms, were generated by the ionization of neutral clusters, which were created by the adiabatic expansion of a high pressure gas into a vacuum. As a result of the large cross section of cluster ions, the cluster size can be easily reduced by collision with fast neutrals and monomer ions. An ERTOF system consisted of a retarding grid in front of a drift tube with a pair of deflection electrodes that acted as a gate. This system was attached to an ionization chamber, where Ar gas clusters, generated by a nozzle in a source chamber, were ionized by electron impact. Current waveforms were measured with ERTOF as an increasing voltage was applied to the retarding grid. The size distribution of clusters having a particular kinetic energy was calculated from the difference in the current waveform obtained at a corresponding retarding voltage to that at a larger voltage. Using ERTOF, it was found that the GCIB consisted of two components; small sized clusters with low kinetic energy and large sized clusters with high kinetic energy. It was also noted that the depth to which a substrate, on which the cluster beam impinged, was etched increased with the proportion of large clusters to the GCIB, since the small clusters, resulting from disintegration of large sized clusters by collisions with monomer ions, fast neutrals and residual gas molecules, have too low a kinetic energy to remove atoms from the surface. Note that the etched depth was affected by both the energy and size distribution of cluster ions. Therefore, ERTOF measurement could be utilized to assure the reproducibility of processes for the manufacture of semiconductors and magnetic recording heads.

Observation of an Intact Noncovalent Homotrimer of Detergent-solubilized Rat Microsomal Glutathione Transferase-1 by Electrospray Mass Spectrometry

Microsomal glutathione transferase-1 (MGST1) is a membrane-bound enzyme involved in the detoxification of xenobiotics and the protection of cells against oxidative stress. The proposed active form of the enzyme is a noncovalently associated homotrimer that binds one substrate glutathione molecule/trimer. In this study, this complex has been directly observed by electrospray mass spectrometry analysis of active rat liver MGST1 reconstituted in a minimum amount of detergent. The measured mass of the homotrimer is 53 kDa, allowing for the mass of three MGST molecules in complex with one glutathione molecule. Collision-induced dissociation of the trimer complex resulted in the formation of monomer and homodimer ion species. Two distinct species of homodimer were observed, one unliganded and one identified as a homodimer.glutathione complex. Activation of the enzyme by N-ethylmaleimide through modification of Cys(49) (Svensson, R., Rinaldi, R., Swedmark, S., and Morgenstern, R. (2000) Biochemistry 39, 15144-15149) was monitored by the observation of an appropriate increase in mass in both the denatured monomeric and native trimeric forms of MGST1. Together, the data correspond well with the proposed functional organization of MGST1. These results also represent the first example of direct electrospray mass spectrometry analysis of a detergent-solubilized multimeric membrane protein complex in its native state.

High-energy negative ions from expansion of high-temperature femtosecond laser plasma

The atomic, charge and energy distributions of ions in expanding femtosecond laser plasmas have been studied by means of time-of-flight mass–charge spectroscopy. High-energy negative and positive ions with energies of up to 35 keV have been detected during the interaction of 200-fs laser pulses (I∼2×1016 W/cm2) with silicon, titanium and other solid targets. A high correlation between negative and positive single-charged ions of the identical atomic number is shown. Negative ions are produced as a result of collisions of fast single-charged ions and neutrals with molecules of residual gas.

Electron attachment to negative fullerene ions: a Fourier transform mass spectrometric study

Fullerene dianions were prepared by low-energy electron attachment to negatively charged fullerene ions C 76 − and C 86 − stored in a commercial Penning trap spectrometer under UHV conditions. Up to 85% of trapped monoanions can be converted to dianions upon long-term irradiation with electrons having kinetic energies averaging 6 eV corresponding to an attachment cross-section on the order of 100 Å 2 for C 76 −. The lifetime of the resulting C 76 2− was determined to be >10 4 s—limited by collisions with residual gas molecules. A model for statistical electron emission based on Klots’ theory is used to provide an upper limit for the dianion temperature.

Time variation of the work function of field emitter tip surface and the development of adsorption of residual gas molecules studied by sawtoothlike emission current method

The sawtoothlike emission current (STEC) method and its theory are described, and this method is shown to make possible the simultaneous measurement of the time variation of the work function φ(t) and its derivative dφ/dt. Application of the STEC method to the field emission (FE) current from a tungsten emitter tip in an ultra-high vacuum of 2×10−8 Pa shows that the time variation of φ(t) and dφ/dt can be divided into four stages. In addition, it gives the effective tip radius αr0=1.1 μm, resulting in α=6.9 for the observed tip radius r0=0.16 μm. The observed time variation of φ(t) and dφ/dt is analyzed from the viewpoint of the development of the adsorption of residual gas molecules on the emitter tip surface, and each stage is successfully identified as follows: The first stage is the formation stage of adsorption nuclei and clusters, the second stage is the free growth stage of adsorption islands, the third stage is their coalescence growth stage, and the fourth stage is their Langmuir growth stage. The mode of adsorption development is similar to the mode of crystal growth taking place in three-dimensional space and on surface.

Recent Developments of Field-Emission Research. Carbon Nanotube Field Emitters.

Carbon nanotubes (CNTs) possess unique geometrical, electrical, mechanical and chemical properties that make them excellent field emitters. We have investigated the field emission of electrons from CNTs by using field emission microscopy (FEM). In this article, our recent observations of adsorption and desorption of residual gas molecules on clean CNT surfaces and their effects on the emission current are first reported, and then the electron energy distributions are described. Adsorbed gas molecules were observed as bright spots in FEM patterns, giving rise to an abrupt increase in the emission current. The energy spectra clearly exhibited a shoulder at about 500 meV below the main peak, and the full width at half-maximum (FWHM) was about 330 meV. Finally, the methods for preparing CNT cathodes for field emission displays (FEDs) are described and recent development of FEDs with triode-type structures are reviewed.

Field emission microscopy of adsorption and desorption of residual gas molecules on a carbon nanotube tip

Field emission of electrons from multiwall carbon nanotubes has been investigated by field emission microscopy (FEM) in ultra-high vacuum. A carbon nanotube, at the end of which at least six pentagons exist to make a closed cap, gives an FEM pattern consisting of bright pentagonal rings if the nanotube surface is clean. Adsorption of residual gas molecules is observed as bright spots in the FEM pattern, giving rise to an abrupt increase in the emission current. Adsorbed molecules seem to reside preferentially on the pentagonal sites where the strong electric field is concentrated. A heat cleaning of the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers its original clean surface. It has been revealed that the adsorption and desorption of gas molecules are responsible for stepwise fluctuation of the emission current.

Development of a Field Emission VP-SEM and Some Initial Applications

The use of variable pressure SEMs (VP-SEMs) is increasing in various fields of science and industry, allowing microscopy in a variable pressure environment of 1 ∼ 270 Pa utilizing backscattered electrons for imaging. The VP-SEM allows microscopy of insulated samples without the need for sample preparation. Charging artifacts can be minimized as well. When the VP-SEM is operated with a cooling stage, which allows cooling of samples at −20° and above, vaporization of water from samples is reduced. This permits microscopy of wet samples at close to the natural state for extended periods of time.Poor S/N ratio and deterioration of resolution, both of which are due to collisions among residual gas molecules and primary/backscattered electrons, have limited the performance of VP-SEMs. For resolving these limitations, we have completed the development of a new field emission VP-SEM which operates with a stable Schottky field emission source, a new environmental secondary electron detector (ESED), and a multi-stage differential pumping system. Fig. 1 shows a sectional view of the column with the differential pumping system. This design allows stable gun vacuum conditions with variable specimen chamber pressure 10 through 3,000 Pa, permitting a pressure difference from the gun by 1011 Pa without problems.

The sp 3 bond fraction in carbon films prepared by mass-separated ion beam deposition

Amorphous carbon films were prepared by mass-separated ion beam deposition (MSIBD), and the sp 3 bond fraction was examined by electron energy loss spectroscopy. In previous studies, the carbon ion current was low, and the vacuum pressure during deposition was insufficient. There seems to be an influence of residual hydrogen gas, and damage to the film by the high-speed neutral species produced by the charge exchanging with residual gas molecules during ion transportation. In this study, an MSIBD system was developed with a high ion current density under ultra-high vacuum conditions. Typical carbon ion beam density was 0.2 mA/cm 2 at an energy of 100 eV under a vacuum pressure of 6×10 −7 Pa. The sp 3 bond fraction of the carbon film, which was prepared under a vacuum pressure of 10 −5 Pa during deposition, was 34%. However, that of the film prepared under 10 −7 Pa reached 83%. It was thought that the impact of the high-speed neutral carbon species on the films accounts for the degradation of the sp 3 bond fraction. The dependence of the sp 3 bond fraction on the carbon ion energy was examined. The maximum sp 3 bond fraction of 84% is obtained at an ion energy between 75 and 100 eV.

The ultra-high vacuum system of HIRFL-CSR

HIRFL-CSR, a new accelerator project planned at the Heavy Ion Research Facility in Lanzhou (HIRFL), is a multipurpose cooling storage ring (CSR). To minimize the beam loss due to charge exchange of very heavy ions with the residual gas molecules, ultra-high vacuum of 3×10 −9 Pa is required for the HIRFL-CSR facility, which is the lowest pressure in a large vacuum system in China up to now. In this article we describe: (1) design of the system; (2) vacuum chambers; (3) reduction of the outgassing rate; (4) vacuum equipment; (5) pressure distribution; (6) future programme

Computer simulation of the pressure distribution for cyclotron’s vacuum chamber and ion beam guide line

The main approaches of the simulation method for an estimation of the pressure distribution inside the vacuum chamber in the different configurations of the vacuum chamber as well as different geometry of the ion beam transport lines are considered. The results of the simulation program codes tested on the base of the data obtained at the FLNR heavy ion cyclotrons are presented. The developed simulation method practically allows optimizing any configuration of the vacuum system from the point of view of the pumping speed and pump’s location. In conjunction with another program code, developed by the authors for an evaluation of the ion charge changing cross-section, this simulation method allows to forecast the ion beam loss due to the charge exchange with the residual gas molecules.

A two-dimensional beam profile monitor based on residual gas ionization

A three-dimensional beam profile monitor based on tracking the ionization of the residual gas molecules in the evacuated beam pipe is described. Tracking in position and time of the ions and electrons produced in the ionization enables simultaneous position sampling in three dimensions. Special features which make it possible to sample very low beam currents were employed.

Measurement of gas bremsstrahlung at the SPring-8 insertion device beamline using PWO scintillator

Gas bremsstrahlung, generated by the interaction of stored electrons with residual gas molecules in a storage ring vacuum chamber, was measured at the insertion device beamlines of SPring-8 by using a new-type scintillator PWO (PbWO 4). The fine energy resolution for the bremsstrahlung photons was obtained, which is about three times as good as that of lead glass. The measurements were made at the two beamlines, BL11XU (high-betatron function beamline) and BL46XU (low-betatron function beamline), in relation to the characteristics of the stored electron beam. The gas bremsstrahlung was measured as a function of the aperture size of the slits, showing the apparent dependence on the conditions of the stored electron beam, as well as the beam current and the vacuum pressure. The measurements of the aperture size dependency were compared with the calculations by using the Monte Carlo code EGS4, to show a fairly good agreement. The total power of the gas bremsstrahlung was estimated as 25.8±0.8 nW/10 −8 Pa/mA and the corresponding equivalent dose rates for a scoring radius of 0.023 cm were calculated to be 15.8 nSv/s/10 −8 Pa/mA for BL11XU, 9.22 nSv/s/10 −8 Pa/mA for BL46XU. Calculations were also made to obtain 25.9 nSV/s/10 −8 Pa/mA neglecting the stored beam conditions at 40 m from the center of the straight section of SPring-8.

Vacuum microelectronics devices based on the controlled electron motion in electric and magnetic fields

Several novel field emission devices are analyzed in a unitary way. The devices involve the use of field emitter arrays in a special configuration where the emitted electrons are subject to crossed electric and magnetic fields. Due to the acting electric and (magnetic) Lorentz forces, the electrons are either deviated or have a cycloid-like confined motion. The angular deviation of the electron trajectory can be used for measuring magnetic fields. The pulsed electron current produced by a cold cathode can be used to generate electromagnetic radiation. Because the electrons have a confined motion with a long trajectory length, their chance of hitting a residual gas molecule is increased. They can either excite gas molecules, which in turn emit UV-radiation when relaxing back to the stable state, or ionize them, the ionic current being correlated with the gas pressure. Accordingly, devices for generating UV-radiation and vacuum gauges can be devised. Furthermore, if the ion generation takes place in a narrow and well-defined region, then their angular deviation can be correlated with their mass. A mass spectrometer (with miniaturized dimensions and field emission electron source) can be devised on this principle.

Absorbed Dose from 7-GeV Bremsstrahlung in a PMMA Phantom

Electron storage rings generate energetic bremsstrahlung photons through radiative interaction of the particle beam with the residual gas molecules and other components inside the storage ring. At the Advanced Photon Source (APS), where the stored beam energy is 7 GeV, bremsstrahlung generated in the straight sections of the insertion devices comes down through the beamlines. The resulting absorbed dose distributions by this radiation in a 300 mm × 300 mm × 300 mm tissue substitute phantom were measured with LiF:Mg, Ti (TLD-700) thermoluminescent dosimeters. The average normalized absorbed dose, in a cross sectional area of 100 mm2 at a depth of 150 mm of the PMMA phantom, was measured as 3.3 × 106 mGy h−1W−1 for a 7-GeV bremsstrahlung spectrum.

Field electron emission from W covered with In

We have investigated a liquid metal electron source that is expected to give a very bright electron beam and also to resist damage due to ion bombardment of the residual gas molecule by restoring the destruction. Although liquid metal ion sources are in use, very few experimental reports on liquid metal electron sources have been published. Here we study the fundamental characteristics of liquid metal electron sources, with emphasis on beam brightness and emission stability.

Mechanistic studies of multipole storage assisted dissociation

The degree and onset of fragmentation in multipole storage assisted dissociation (MSAD) have been investigated as functions of several hexapole parameters. Strict studies of hexapole charge density (number of ions injected) and hexapole storage time were made possible by placing a pulsed shutter in front of the entrance to the mass spectrometer. The results obtained show that the charge density is the most critical parameter, but also dependencies on storage time, radio-frequency (rf) -amplitude, and pressure are seen. From these data, and from simulations of the ion trajectories inside the hexapole, a mechanism for MSAD, similar to the ones for sustained off-resonance irradiation (SORI), and for low energy collisionally induced dissociation in the collision multipole of a triple quadrupole mass spectrometer, is proposed. It is believed that, at higher charge densities, ions are pushed to larger hexapole radii where the electric potential created by the rf field is higher, forcing the ions to oscillate radially to higher amplitudes and thereby reach higher (but still relatively low) kinetic energies. Multiple collisions with residual gas molecules at these elevated energies then heat up the molecules to their dissociation threshold. Further support for this mechanism is obtained from a comparison of MSAD and SORI spectra which are almost identical in appearance.

Energy analysis of negative hydrogen ions using detached electrons

An electron of a negative hydrogen ion (H−) can be easily detached by collisions with an external laser beam or residual gas molecules in the beam line, because its electron affinity is small (0.754 eV). The detached electrons provide information about the original negative hydrogen ions and should have the same velocity as the original hydrogen ions. By measuring the energy of the detached electrons, the energy of a negative hydrogen-ion beam can be estimated. A preliminary experimental result of an energy analysis of a H− beam using detached electrons is presented.

Electron Emission from Pentagons on a Carbon Nanotube Tip Revealed by Field Emission Microscopy

AbstractField emission of electrons from multiwall carbon nanotubes (MWCNTs) has been investigated by field emission microscopy (FEM) in an ultra-high vacuum chamber. An MWCNT whose tip is capped by curved graphite layers gives a FEM pattern consisting of 6 bright pentagons when the surface of the nanotube tip is clean. Even in the ultra-high vacuum with a base pressure of about 10-10 Torr, residual gas molecules, attracted by polarization forces, adsorb on the nanotube tips. The adsorbed molecules reside preferentially on the pentagonal sites, giving bright spots in the FEM pattern. A flash heating the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers the original clean surfaces. The adsorption and desorption of gas molecules are responsible for stepwise increases and decreases in the emission current, respectively.