Hydrogen Atom Beam Source
Some technical features are:
- thermal dissociation of hydrogen in a hot tungsten capillary heated by DC current (I < 15 A, U < 15 V),
- thermal energy, no high-energy particles,
- efficient thermal radiation shielding, low power consumption (N < 200 W),
- encapsulating water cooling, low thermal load on other experimental equipment,
- sensitive temperature monitoring (optional),
- efficient shutter (optional),
- customized aperture (optional).
The hydrogen atom beam source is thoroughly characterized. The intensity of the source can be controlled by the flow rate of hydrogen and the heating power. The heating power determines the temperature of the capillary. With respect to control of these operational parameters we suggest different procedures for high and low intensity runs.
In case of high intensity runs the gas feed is preferably maintained by means of a mass flow controller installed in the gas feed line. Mass flow controllers do not require hands-on control of the gas feed thus making unattended long-term runs feasible. They are appropriate for higher gas flow rates. When the flow rate has been preset the intensity can be adjusted by the heating power. Figure 1 shows the on-axis hydrogen atom flux density at a sample positioned 6 cm in front of the capillary. The flux density can reach as much as some monolayers per second.
Figure 1. Hydrogen atom flux density at a sample positioned 6 cm in front of the capillary. The parameter is the flow rate of the hydrogen feed gas, which is adjusted by a mass flow controller.
Low intensities result from low gas flow rate and/or low heating power. Low flow rates can be adjusted by means of a leak valve installed in the gas feed line. In this configuration the heating power can be preset and the atom beam intensity varied by manipulating the leak valve. By measuring the pressure where the gas line is connected to the source, the flow rate can be evaluated as the product of this pressure and the flow conductance of the source. The conductance has been measured and is 6.1 cm3/s when the capillary is hot. Figure 2 shows the on-axis hydrogen atom flux density at a sample positioned, as before, 6 cm in front of the capillary. Flux densities as low as a tenth of a monolayer per second were measured.
Figure 2. Hydrogen atom flux density at a sample positioned 6 cm in front of the capillary. The parameter is the heating power determining the capillary temperature. The flow rate of the hydrogen feed gas is varied by means of a leak valve.
The in-vacuum length of the atomic hydrogen beam source can be fabricated to the customer’s specification.
The following options are available:
The source can be equipped with a thermocouple attached to the radiation shielding. The temperature indicated is not that of the capillary, but it is strongly related to it and is therefore useful to independently monitor the temperature level of the source, especially during long-term operation.
When the heating power is increased from standby to operation, the atom flux reaches 90 % of the stationary flux in less than 1 minute, which is fast enough to switch long-term runs on and off. Short exposures can be terminated by a shutter which attenuates the hydrogen atom flux down to the detection level of our quadrupole mass analyzer (QMA).
Hydrogen atoms passing the sample and hitting the chamber walls can induce reactions detrimental to the experiment. A customized aperture is available which limits the solid angle of hydrogen atom emission. Outside this angle no beam atoms could be detected by the QMA.
Further details and papers dealing with the development and characterization of the source are found here.
Although tested so far with hydrogen only, the source can be used as well as a radical beam source by decomposing molecules at temperatures up to 2200 K.
For further details please contact:
| Dr. Karl Georg Tschersich | |
| Institut für Schichten und Grenzflächen Forschungszentrum Jülich GmbH D-52425 Jülich |
 Dr. Karl Georg TschersichTel.: ( 49) 24 61/61-65 26 Fax: ( 49) 24 61/61-39 07 |
last change 14.11.2003 | | Print