Desorption from Solids

Question 1

Temperature programmed desorption (TPD)

Answer 1

measures rate of evolution of a material into gas phase from surface as the surface is heated, follows Polyani-Wigner equation

Question 2

Limiting behaviors in TPD

Answer 2

low pumping speed wrt heat gives accumulation of desorbed material
high pumping speed wrt heat gives rapid removal of desorbed material from chamber

Question 3

contributions to the total pressure in TPD measurements

Answer 3

thermal desorption of adsorbate, vacuum walls leaking and removal of gas by pumping

Question 4

Zero order TPD profile

Answer 4

multilayer desorption, exponential increase with matching rising edges, max T increases as coverage increases

Question 5

1st order TPD profile

Answer 5

monolayer desorption and same max T with increasing pressure

Question 6

2nd order TPD profile

Answer 6

recombinative desorption, max T decreases with increasing coverage, same trailing edges

Question 7

redhead analysis

Answer 7

assume Ea,des and Ades are independent of coverage sub T = T0 +Bt then sub into polyani-wigner eqn

Question 8

leading edge analysis

Answer 8

Ades and Ea,des independent of coverage, assume well known initial coverage and rearrange polyani-wigner to give linear eqn

Question 9

collision induced desorption

Answer 9

collision between adsorbate and fast rare gas gives desorption