Exponental Tunnelling Factor (Electron Microscope)

[ZedCar]

I'm reading a little bit about electron microscopes.

It states, "the exponential tunnelling factor is proportional to the square root of the mass. So, the lower mass means lower exponential decay of the wavefunction".

Would anyone be able to let me know what this exponential tunnelling factor is?

I presume it looks something like exp() proportional to (mass)^0.5

Thanks you!

 

[ZedCar]

Actually, does it look something like:

P is proportional to exp(-alpha x a)

If so, what does alpha and a represent?