Molecular Beam Intensity Research Articles

Molecular beam epitaxy of ZnSe xTe 1− x (0 ≲ x ≲ 1)

Single-crystalline films of ZnSe x Te 1− x (0 ≲ x ≲ 1) have been successfully grown by molecular beam epitaxy. Evaluation of the epitaxial films shows the growth of good-crystallinity films with flat surfaces. The dependence of the composition ratio x on the molecular beam intensities of Zn, Se, and Te was studied and the following features became clear: (1) the incorporation rate of Se is much higher than that of Te; (2) the control of the composition ratio can be best achieved by maintaining the arrival rate ratio of Te/Se above unity; (3) the composition ratio depends mainly on the arrival rate ratio of Se/Zn.

Surface segregation of Sn during MBE of n-type GaAs established by SIMS and AES

Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) have been used to study in situ the surface segregation of Sn, which occurred during molecular-beam epitaxy (MBE) of Sn-doped GaAs films. With both surface diagnostic techniques, a significant amount of Sn, about three orders of magnitude larger than the portion being incorporated into the growing film was detected in the outermost atomic layers (∠10–20 Å thick) of the prepared film. The surface segregation of Sn started at a substrate temperature as low as 490° C and increased with temperature and−to some extent−with increasing dopant concentration. Thus, the measured doping profiles of Sn-doped GaAs layers, when grown above 490° C, deviated considerably from the ideal profile calculated from the molecular beam intensity of the dopant. The surface segregation of Sn during epitaxial growth was also found in Sn-doped GaAs films prepared by liquid-phase epitaxy (LPE) from Ga solutions.

Molecular beam intensities and collision cross-sections of small Ne, H 2, N 2, NO and O 2 clusters

The temperature- and pressure-dependence of cluster intensities were determined in a molecular beam from a supersonic nozzle source. The ratio σ 2/σ 1 for the total cross-section of the monomeric and dimeric systems investigated was found to be between 1.34 and 1.55 with a heavy inert gas as scattering partner. The magnetic deflection properties of H 2 −, O 2 −, and NO-dimer beams are discussed.

The interaction of an O 2 molecular beam with an Fe(110) surface

The initial interaction between an O 2 molecular beam and a cleaned Fe(110) surface has been studied by a combination of Auger electron spectrometric (AES) and mass spectrometric techniques. The incident molecular beam intensity was calibrated using a stagnation detector, and the initial sticking coefficient for chemisorption was determined by mass spectrometric measurement of the transient in molecular scattering behavior observed when the cleaned surface was exposed to the molecular beam. This permitted an absolute calibration of the AES system for oxygen, and allowed comparison of the kinetic measurements of the oxygen adsorption process by the two techniques. Results indicate that the initial sticking coefficient is 0.2 ± 0.01. Oxygen is initially chemisorbed up to a coverage of 1.6 ± 0.16 × 10 15 cm −2, by a process following Langmuir kinetics. Beyond this point AES studies indicate a slower rate of oxygen uptake which is independent of gas-phase oxygen pressure. The mass spectrometric studies further indicate that for a cleaned, annealed surface those oxygen molecules which are not chemisorbed are scattered in a non-diffuse manner.

317. Investigation of distribution of intensity of molecular beam emitted from a conical ring source: T S Tsulaya, Zh Tekh Fiz, 43 (6), 1973, 1290–1296 ( in Russian)

317. Investigation of distribution of intensity of molecular beam emitted from a conical ring source: T S Tsulaya, Zh Tekh Fiz, 43 (6), 1973, 1290–1296 ( in Russian)

High-Precision Measurements of Condensation Coefficients. Results for Carbon Dioxide and Water Molecules

A molecular beam at temperature TG was impinged onto the surface of a copper block held at temperature TS. The fraction of molecules reflected was 1−γ, where γ is the condensation coefficient. Of the reflected molecules, a fixed fraction was condensed on the surface of a quartz crystal microbalance. Precision measurements of the mass flux onto the microbalance gave a direct determination of the rate of reflection of molecules from the surface of the copper block. This information, combined with the known flux of molecules onto the copper block, gave high precision (∼0.01%) values of γ. For CO2 condensing on a film of solid CO2, γ>0.96 when TS≤83 K and TS=274 K. Water molecules condensing on water ice films give γ≥0.99 when TS≤150 K and TG=273 K. Values of γ for CO2 were found to depend on TS, TG and the molecular beam intensity ṅCB. The range through which ṅCB varied was 1.5×1014 to 1.5×1016 molecules cm−2⋅sec−1.

Measurement of very small pressure variations

The possibility of measuring small pressure variations of the order of 1.33 �9 10 "~ N/m s on a background of 1.33.10 -3 N/m s by using the thermoelectrical principle has been shown in [1] and [2]. The sensitivity threshold of thermoelectric vacuum gauges can be lowered still further by raising the temperature sensitivity of the thermoelectric system and by reducing the electrical thermal fluctuations, as well as the fluctuations of the background pressure. The thermoelectric vacuum gauge designed by us has a sensitivity threshold of 1.33 �9 10 -8 N/m 2 with a background pressure of a nitrogen atmosphere in the range of 13.3-0.133 N/m s. As distinct from the existing thermoelectric vacuum gauges used for the same purpose [1, 2], our instrument can register reliably relative pressure variations of the order of 10-7-10 -5 Pb, where Pb is the background pressure. The sensing thermoelectric system (Fig. la) consists of the heated filament a-b, whose temperature is measured with the thermistor Tr, which is in a thermal and electrical contact with the filament. The filament is made of a 0.02-ram platinum wire. The miniature thermistor T r has the shape of a pearl 0.1 mm in diameter. The element which provides the heat exchange with the rarified gas consists of the filament a-b heated with an electrical current. The thermistor is used only for measuring the filament temperature, but it is not a thermally-active element as in the existing semiconductor vacuum gauges [3]. Therefore, the system shown in Fig. la can be considered as a semiconductor analog of the classical thermoelectric system used in measuring low pressures. The laboratory technology for making such a system is similar to the one described in [4]. Figure lb shows the version of this system which has been adopted in our instrument shown schematically in Fig. 2. The part a-Tr of the heated filament and the thermistor form the two adjacent arms 1 and 2 of a balanced bridge and are located in a hot-wire gauge bulb. The remaining two bridge arms 3 and 4 consist of set resistances of two decade boxes. The bridge is symmetrical and, at room temperature and normal atmospheric pressure, R 1 = R 4 = 18 2 and R z = R 3 = 3,7003. If the decade resistance boxes in the arms 3 and 4 are replaced by another system simiIar to the one shown in Fig. lb, a differential vacuum gauge will be obtained for measuring pressure differences. Such a vacuum gauge was used as a monometric transducer for measuring the intensity of molecular beams in light gases [5]. The particular feature of the bridge shown in Fig. 2 consists of the fact that two temperature-sens itive elements with reversed-sign temperature coefficients of resistance constitute its two adjacent arms. The metal filament cools with increasing pressure, and its resistance is reduced. The thermistor which is in thermal and electrical contact with the filament is also cooled, but its resistance increases. Thus, the bridge is embalanced owing to a simultaneous variation in opposite directions of the bridge arms 1 and 2 resistances. The temperature coefficient of platinum is apt = +0.039/~ and that of the thermistor is aTr = -0.30/~ Therefore, an increased sensitivity of the bridge depends above all on the large temperature coefficient of the thermistor. It has been shown in [5] that the sensitivity of such a bridge, with the remaining conditions being equal, is (apt + aTr)/2 aTr times higher than that of a bridge whose opposite arms consist of two metal resistors. The bridge is fed with a direct current from a high-resistance source with an adequately large stabilization factor. The constant-current bridge supply ensures a stable operation of the vacuum gauge and reduces the effect of the fluctuations of the current and of the ambient temperature as compared with constant voltage or constant power bridge supplies [6]. In existing vacuum gauges [3, 7, 8] the thermistors are connected to the bridge as active elements which are heated by the current flowing through them and, according to the heat-exchange conditions, their resistance is changed as well as the electrical condition of the bridge. This method leads to the following difficulties.

Supersonic molecular beam intensities

This paper attempts to obtain agreement between theoretical and experimental supersonic molecular beam intensities. Measurements of absolute beam intensities are presented, corrected for the effect of background scattering in the expansion chamber. It is found that these intensities are considerably lower than those predicted by existing theories. This is true even when losses due to imperfect skimming are negligible. A new mechanism is therefore proposed which postulates a considerable attenuation of the beam due to "self-scattering" collisions upstream of the skimmer and downstream of the onset of translational freezing. Using this mechanism it is possible to reasonably reproduce the experimental intensities.

Measurement of Atomic and Molecular Beam Intensities with a Torsion Balance

Beam intensities of 1014 particles·cm−2·sec−1 are measured by a torsion balance with an accuracy of 3%. The sensitivity of the balance is 21.1 rad/dyne. In a dynamic mode the sensitivity can be increased by a factor of about 50. Accommodation coefficients need not be known.

The performance of a mass spectrometric modulated beam detector for gas-surface interaction measurements

This paper describes the performance of an instrument designed to detect molecular flow properties in the presence of the relatively high background gas noise of a space environment chamber. Two modes of operation are possible: (1) as a molecular flow rate detector, and (2) as a molecular density detector. The technique of molecular beam modulation is employed, which consists of three primary elements: (1) the mechanical beam modulator, (2) a quadrupole mass spectrometer, and (3) a lock-in amplifier. Molecular beam intensities of about 1 × 10 10 molecules/sec-cm 2 at a background pressure of 2 × 10 −7 torr which corresponds to about 1 × 10 14 molecules/sec-cm 2. The detector performance for molecular beams of He, A, N 2 and CO 2 is reported. This high sensitivity makes possible the use of molecular beam techniques for direct measurements of gas condensation rates.

A modulated beam technique for the measurement of very low molecular beam intensities: J B Hudsonet al, Trans 10 AVS Nat Vac Symp, MacMillan & Co, 1963, p 411.

A modulated beam technique for the measurement of very low molecular beam intensities: J B Hudsonet al, Trans 10 AVS Nat Vac Symp, MacMillan & Co, 1963, p 411.

315. The measurement of the intensity of high velocity molecular beams

315. The measurement of the intensity of high velocity molecular beams

Mass-spectrometric sampling of 1-ATM flames

As part of a program aimed at mass-spectrometric sampling of the burned gas region of 1-atm flames for thermochemical studies, we have investigated the phenomena occurring in forming molecular beams from high-pressure sources. A simple picture of beam formation involving isentropic expansion of gas through the first orifice and into a supersonic molecular beam is justified. Two effects observed in high-pressure beam formation, mass separation, and polymer formation, are consistent with this picture, and their pertinence to the interpretation of high-pressure sampling results is discussed. A semiquantitative description of the time, temperature, and pressure history of a typical sampling situation is given showing that conversion to molecular flow occurs in several microseconds. Our sampling system, which gives molecular beam intensities of 1017 molecules/cm2/sec at 10 cm from the first orifice, was tested by measuring the stable products in CH4−O2−Ar flames and the dissociation of Cl2 in CO−O2−Ar flames. Satisfactory agreement with calculated composition was obtained in both cases. The condensable species HBO2 was successfully quenched from a H2−O2−BCl3 flame.

Multiple Aperture Slits for Molecular Beam Sources

The intensity of molecular beams can be increased by the use of multiple aperture slits. A crinkly foil slit was developed by Zacharias and Haun, but the beam coming from it did not have a maxwellian velocity distribution because the individual sources were canals rather than ideal slits. For those cases where a maxwellian velocity distribution is desirable, a multiple aperture slit may be made from commercially available electroformed nickel mesh. (M.C.G.)

The Scattering of Beams of Alkali Atoms in Various Gases. I. Sodium and Argon

ABS>An apparatus is described for measuring molecular beam intensity in vacuo and in a scattering gas. The collision radius is calculated. Data are given for sodium and argon and discussed.

A High Intensity Source for the Molecular Beam. Part II. Experimental

The ordinary effusion (``oven'') slit of a conventional molecular beam apparatus was replaced by a slit of special design facing into a supersonic jet from a miniature nozzle. Using ammonia as a test gas, it was observed that at inlet pressures above about 170 mm the molecular beam intensity began to rise far more rapidly than the pressure. The maximum observed intensity exceeded by more than a factor of twenty that obtainable from the effusion slits under optimum conditions and continued to rise rapidly at the highest inlet pressures possible with the available equipment. Certain imperfections of the present apparatus restricted observations to ammonia; their presence indicates that still greater enhancement of molecular beam intensity is possible with an apparatus of refined design.

Measuring the Intensity of Molecular Beams

A Pirani gauge sensitive to pressure changes of the order of ${10}^{\ensuremath{-}8}$ mm of mercury has been developed and used to measure the intensity of molecular beams defined by two circular openings, one 0.4 and the other 0.6 mm in diameter placed 25 mm apart. The gauge is situated 40 mm from the last opening. A beam of air gives a galvanometer deflection equivalent to 600 cm with a pressure of 0.5 mm of mercury behind the first opening. Under similar conditions ${\mathrm{H}}_{2}$ produces a deflection of 1800 cm. The galvanometer sensitivity is 11.6 mm per microvolt. The time lag of the gauge is less than the period of the galvanometer (7 seconds). The limits within which the beam-forming system may be expected to give a Maxwellian distribution of velocities are discussed. The final pressure in the gauge due to a given beam depends upon the dimensions of the gauge opening as kinetic theory predicts. The reduction in intensity of a beam of ${\mathrm{H}}_{2}$ by collision with either air or ${\mathrm{H}}_{2}$ present in the experimental chamber at various pressures has been measured. The data show that the gauge may be used to measure mean free paths and to obtain more detailed information than has previously been possible as to the probability of scattering through various angles by collision.