## I. Evaporation Basics

### The rate of free evaporation of vapor atoms from a clean surface per unit area in vacuum, Mc, is given by Langmuir's equation:

Mc=5.85*10^(-2)*P*(M/T)^0.5 g*cm^(-2)*s^(-1)
Where P=vapor pressure of evaporant under saturated vapor conditions at temperature T, torr or mmHg.
M=molecular weight of evaporant material
T=absolute temperature, K

### Alternatively, we may write the evaporation rate as
Ne=3.51*10^22*P*(MT)^(-.5) molecules*cm^(-2)*s^(-1)

where Ne=rate of evaporation of vapor molecules per unit area in vacuum

#### (a)Profile of vapor material condensed at different angles. (b) Schematic of a surface element receiving coating from a small-area source

### The directionality of the evaporant molecules leaving the source is given by the well-known cosine law :

### Small source surface area dA1 emitting a vapor flux M. Then the material passing through a solid angle *phi* at a direction of angle *psi* with the normal to the surface dA1 per unit time is:

dM=(M/PI)*cos(phi)*cos(psi)

### Material arrived at a small substrate area dA2 inclined at an angel *theta* to the vapor stream direction at a distance *r* from the source is given by:

dM=M*cos(phi)*cos(theta)*dA2/(PI*r*r)

For a point source,

dM/dA2=M*cos(phi)/(PI*r*r)