THE GAS HANDLING LINES

S p ecim en gas d o sin g w a s ach iev ed through a g lass g as h and ling line. T his c o n siste d o f a set o f gas b ottles, each attached to a m a n ifo ld , separated from the m ain ch am b er by a leak va lv e. G as d o sin g w as a ch iev ed b y back fillin g the ch am b er to a set pressure in the 5 x 10'^ - 10'^ torr range. A cap illary d oser w a s attached to the leak v a lv e to in crease the flu x o f the adsorbate gas at the surface o f the sam p le. In order to reduce the p rob lem o f an u neven flu x distribution b ein g p rod uced at the fa ce o f the sam p le, the d ista n ce b etw een the gas d oser and the sam p le w a s rela tiv ely lo n g (~ 50m m ).

A separate stain less steel line w as u sed to introduce o x y g e n and argon to the cham ber for clean in g.

T he gas h and ling lin es co u ld be p um ped to pressures <10'^ torr by the auxiliary p u m p in g system before b ein g filled with the required gases.

2.4

CRYSTAL PREPARATION

2.4.1

The Crystals

T h e initial preparation o f the P t( llO ) and P t(3 3 1 ) sam p les w as carried out at the M aterial S c ie n c e C entre o f the U n iv ersity o f B irm in gham . A 99 .99% pure sin g le crystal rod (M eta ls R esearch L td.) w as m ou n ted on an x-ray g on io m eter, oriented to w ith in 0 .2 5 ° o f the (1 1 0 ) direction and a d isc w as cut o f f in a spark cu ttin g m achine. T his p rocedure w as repeated in the (3 3 1 ) direction. T h e d iscs are ap proxim ately 9m m in diam eter and 1m m thick. In preparation for this w ork, the crystals w ere p o lish ed m an u ally w ith a - A I2O3 (elem en tary particle siz e ~ 0 .3 |i) on a fin e em ery board.

2.4.2 Sample Mounting

In the TPD /LEED system the platinum crystal under consideration was spot- w elded between two 0.25 mm tantalum wires, positioned opposite each other on the edges of the crystal (fig. 2.4). A chrom el/alum el therm ocouple o f 0.05m m diam eter was also spot-w elded to the edge of the crystal. This assem bly was treated in an ultrasonic acetone bath to rem ove any residues o f grease that m ight be a source of contam ination.

The tantalum wires were then clam ped betw een the tw o plates o f a copper holder. The arms of the copper holder were screw ed to a ceram ic block to provide the connections for the heating wires and a channel for the electrical feedthroughs to the therm ocouple wires. A steel driving rod, connected to the sam ple m anipulator, was attached to the top of this ceram ic block. The sam ple m anipulator allow ed the crystal to be m anipulated in the x, y, z plane.

The sam ple was heated by passing an a.c. current through the copper holder and tantalum wires and its tem perature was m onitored by the chrom el/alum el therm ocouple.

Drive Rod

C eram ic Blocks \

•Copper P la te s Mica S h e e t s

C hrom el/A lum el T h erm ocou p le Tantalum H e a tin g , , Wire V V 'C o p p e r H older

F ig. 2 .4 D iagram o f crystal m ounting in the L E E D /T P D system .

M ounting the crystals in the ARUPS system also involved spot-w elding tantalum wires to the edge of the crystal (fig. 2.5). These tantalum wires were then spot-w elded to two tungsten rods, one of which was insulated from earth via a sheet of mica. These rods were in turn attached to the end of a probe. A chrom el/alum el therm ocouple was spot-w elded to the edge of the crystal and channeled by a ceramic tw in-capillary to the electrical feedthroughs. The crystal could be cooled to 85K by a liquid nitrogen well at the centre of the probe.

Ceramic Twin Capillary

To H eater Power

f Supply

T u n gsten Rod

Fig. 2.5 D iagram o f crystal m ounting in the A R U P S system .

2.4.3 Sample Cleaning in Vacuo

Exposure of the crystals to atm osphere results in considerable surface contam ination. Once the sam ple is m ounted in the UHV cham ber the contam ination must be removed. The crystal was first flashed to lOOOK to rem ove any contam inants w hich could be desorbed into the vacuum as volatile oxides, sulphides or carbides. M ore stubborn contam inants such as carbon (which form s strongly bound com pounds with the substrate m aterial) were rem oved by cycles of heating at lOOOK in 10'^ torr of oxygen and bom barding the crystal with argon ions. Argon bom bardm ent was also necessary after each amine TDS experim ent as som e C-N bond breakage occurred leaving C on the surface. Follow ing bom bardm ent the crystal was always annealed at 1300K to restore order to the surface which was left in a heavily dam aged state, with em bedded argon atoms.

An unm odified, open ended, Bayard-A lpert type ionization gauge (Leisk IG 32N ) was the source of ions for cleaning in this work. The sample, turned toward the gauge, was polarized by -5 0 0 V (fig. 2.6). The cham ber was then isolated from the

ion pum p and back filled to a partial pressure of 5 x torr with argon. W ith a 1mA em ission from a standard ion gauge controller (IGP3) an ion current o f 6.5|xA is obtained at the sample. The ion current could be varied by altering the argon pressure. T he gauge is connected as a standard ion gauge, so it could be used to m easure pressure during bom bardm ent.

Leak