UHV (Ultra High Vacuum) and electron spectroscopy, Part 3

Concerning: Ion getter pumps and Ti sublimation pumps Metal getters (like Ti) had been used a long time ago to maintain and improve the vacuum inside tubes used in radios, TV, high voltage power suuply etc.. The glass tubes were carefully evacuated, the glass tubes sealed and then, from the outside by induction heating, the getter material (inside) is evaporated. The 8068 pentode stems from a Kepco high voltage (-2000 to + 2000V) differential Amplifier.

UHV (Ultra High Vacuum) and electron spectroscopy, Part 3 Calculating the effective pumping speed.

L2 = 50l/s

L1 = 30l/s

21

1111

LLSSeff

S eff = 17.5 l/s If we change to a smaller Turbo pump 50l/s: Seff = 13.6 l/s The large pump is in this case not very effective!

Pumping speed S = dV/dt m3/s or l/s Pumping through put

Conductivity s

Pamp

dt

dVpSqpv

3

pLqpv

Compare to Ohm’s Law:

pLqpv

UR

q1

UHV (Ultra High Vacuum) and electron spectroscopy, Part 3

• For the calculation of vapour pressure and sublimation pressure we use the chemical potential

• Concentration dependence

• This is exact fow low concentrations c

• And usually a good approximation for higher c.

• Pressure dependance:

• Solid and liquid:

• Gases:

• Temperature dependance:

• Linear approximation (for example):

0

0 lnc

cRT

mVp

0

0 ln)()(p

pRTpp

mST

TSpVpppT mm),15.298(),15.298( 000

UHV (Ultra High Vacuum) and electron spectroscopy, Part 3

• Sublimation of Titanium by direct heating through an electrical current

• The equilibrium changes with temperature )()( gTisTi

RT

TS

p

p

p

pRTTSTS

pTT

m

ggsms

gs

)15.298(exp

)ln()15.298()15.298()15.298()15.298(

),()(

0

0

0

000

UHV (Ultra High Vacuum) and electron spectroscopy, Part 3

• Phase transitions: solid-liquid- gas described with the chemical potential ( example H2O):

TST m0)15.298()15.298(

Equilibrium between H2O(liq) and H2O(gas) at f(T)

Thermionic emission of electrons

• Metal like tungsten (W) can emitt at higher temperatutes (2500-300K) electrons

• Electrons. To understand this process we assume a simple metal with free electrons. Following the Pauli principle , energy levels can only be occupied by two electrons (with different spin orientation). Therefor electrons in metal occupy in the conducting band a high number of energetic lecvels. The ocupancy of these levels can be desribed by a continuos function: the Fermi Dirac distribution function:

kTATJ eexp2

1exp

1)(

kt

EEEf

f

Fermi-Dirac distribution function Richardson equation For the current density f(T).

Thermionic emission of electrons

A typical value of the Fermi level EF is 4.5 eV

= 520 K =5200K =26000 K =52000K

The work function

Contact potential: If we bring two metals with different Fermi levels into electrical contact, electron flows from the higher to the lower level. We will obtain a measurerable electrical potential difference V

tsCPDV

Production of electrons by field emission

Production of electrons by field emission