Common Procedures and UHV Components

2.4.1 Introducing Gases into UHV Chamber

Reactant gases are introduced into the UHV chamber in a controlled manner

through leak valves attached to gas-handling lines (for introducing gases or liquids of

moderate to high vapor pressure) and/or a Knudsen source (for introducing solids or

liquids of very low or very high vapor pressure). The following section presents a typical

2.4.1.1 Gas-handling Line

A gas-handling manifold is attached to each chamber for introducing gases. A

typical diagram for a gas line is given in Figure 2-5. The gas line is constructed from

Pyrex glass. The low reactivity of Pyrex glass minimizes the decomposition of introduced

gases in the gas line and reduces the risk of contamination and adhesion into the walls of

the glass material. The gas lines are usually pumped by a combination of mechanical

pump and diffusion pump. The pressure is measured by a diaphragm monometer attached

to it. The gas line can reach a pressure of 10-7 Torr without baking. Gases are introduced into the gas line directly from a gas cylinder which is connected to the gas line by a

flexible hose and 1/4" Swagelok fitting. If the sample to be introduced is in the liquid

form then the sample is transferred to a glass vial and stored in the gas line by Swagelok

fittings and purified by several freeze-pump-thaw cycles before introduction to the UHV

chamber. The gas line is connected to leak valves (which are attached to the UHV

chamber) by glass-to-metal seals. The gas line is usually divided into two or three

separate sections. Each section is isolated from others by gas valves with Teflon

stopcocks. Each arm connects to a leak valve which is attached to the vacuum chamber.

This allows the gas line to be filled with multiple gases at the same time to be introduced

Figure 2-5 Schematic diagram of a typical diffusion pumped gas line 2 2.4.1.2 Knudsen source

There are some compounds used in this study which are either solids or liquids

having very low vapor pressures and cannot be introduced from the gas line or are very

reactive and require differential pumping while dosing. For these molecules a homebuilt

Knudsen source is used. The Knudsen source is made up of two 2 ¾" Conflat flanges

welded to the ends of a ¼" stainless-steel tubing. A “T” is introduced in the middle to

accommodate the sample to be introduced either by a blank Swagelok fitting or a glass

vial. A turbo molecular pump is mounted at one end of the Knudsen source on a Conflat

flange for differential pumping and a dosing tube is welded on the chamber side to the

flange for direct dosing into the sample. The dosing tube is either made of stainless steel

or glass tubing. For dosing compounds with very high vapor pressure, the compound is

pressure of the sample, the compound is either dosed for a short period of time at room

temperature or the compound is immersed in a cryostatic bath such as ethylene glycol-dry

ice mixture or ice water - salt etc. while dosing. For compounds with very low vapor

pressure and require heat for dosing, a small amount was placed in the source by a 1/4"

blank stainless steel Swagelok fitting. The whole source was uniformly wrapped with

resistive heating tapes and connected to a variable transformer. The whole Knudsen

source assembly is wrapped up uniformly with two layers of aluminum foil for uniform

heat distribution. If the temperature is not even throughout the source, then the vapor will

condense on the cold spots and problems will arise when dosing the sample. A

thermocouple junction is spot welded to the Swagelok fitting of the Knudsen source

assembly to monitor the temperature of the source.

2.4.2 Sample cleaning

The Pd(111) crystal was initially bulk cleaned by several cycles of argon ion

bombardment at an energy of 2 kV with a background argon pressure of ~3×10-5 Torr measuring a sample current of ~2 microamps, and then annealing to ~1100 K in vacuum

after each bombardment cycle. The next cleaning steps consisted of one cycle of argon

ion bombardment, annealing to ~1100 K in vacuum, followed by heating the sample to

~800 K in ~4 ×10-8 Torr of oxygen for ~15 minutes (oxygen roasting) and then briefly annealing to ~1100 K in vacuum. These steps are repeated several times and then the

sample cleanliness is checked by Auger spectroscopy. The primary contaminants were

found to be sulfur and carbon. While the amount of sulfur on the surface can be easily

seen by Auger spectroscopy, it is difficult to gauge the amount of carbon because one of

present on the surface using Auger spectroscopy, a ratio of height of carbon peak to the

most intense palladium peak was measured. A ratio of C: Pd peak of 1:5 indicates that the

sample is almost free of carbon contamination. Sulfur contamination can be removed

easily by a few cycles of Ar sputtering. Finally Temperature-Programmed desorption

spectroscopy (TPD) was carried out to check the cleanliness of the sample, where the

sample is dosed with molecular oxygen at room temperature and then heated to high

temperature while monitoring desorption products, mostly molecular oxygen and carbon

monoxide in the mass spectrometer. If the sample is contaminated with carbon, it will

react with atomic oxygen and desorb form the surface as CO at ~800 K. However, if the

surface is clean, the atomic oxygen will recombine and desorb as molecular oxygen

without any CO desorption. The sample is usually briefly argon ion bombarded and

cleaned with oxygen as a part of normal cleaning procedure before carrying out any

experiments. Periodically the sample cleanliness is checked by Auger spectroscopy and

by oxygen TPD.