Surface impact rate

The surface impact rate or impact rate is a term from the kinetic gas theory . It describes the number of molecules in a closed volume of gas that hit the surface of a vessel wall per time : ${\displaystyle j}$ ${\displaystyle N}$ ${\displaystyle t}$ ${\displaystyle A}$

${\displaystyle j={\frac {N}{A\cdot t}}={\frac {n\cdot {\bar {v}}}{4}}}$

It is

• ${\displaystyle n={\frac {N}{V}}}$the particle density , d. H. the number of particles per volume of gas
• ${\displaystyle {\bar {v}}}$ the mean thermal velocity.

The surface impact rate comes from the microscopic model conception of the kinetic gas theory and is able to describe many macroscopic properties of gases such as pressure , viscosity and friction .

The surface impact rate is also of great importance in surface physics and surface chemistry because it indicates the number of gas molecules with which the surface interacts in a given time and thus also the maximum number of molecules that can adsorb on a surface . The impact rate is only so low at a very low particle density, i.e. a gas pressure in the ultra-high vacuum range (10 ⁻¹⁰  mbar and below), that investigations on clean surfaces are possible.