Hi, researchers,
I confronted some difficulty in understanding the convergence semi-angle on the condition of over-focus.
The convergence angle in the condition of focus like that Fig.1 shows. however, the over-focus condition seems different, as Fig.2 shows.
The defination of canvergence angle under focus/overfocus is the same? How can we explain or understand that?
Dear Xu,
the angular distributions of the electron trajectories is the same in both cases. The electrons do not change their directions of propagation when passing the 'cross-over' section indicated in fig. 2. This will be only the case for very 'strong' electron currents when the electron density is high enough, so that the relative distances of the electrons are quite small there, and therefore a significant amount of electrons is able to repell each other. But here in your case the electron beam cone angles remain the same (at least in your sketch /fig.2).
So please calculate the semi angle ß according to fig. 1 in both cases.
Hi Xu,
I can´t be sure if we are right when talking about convergence angle in overfocus condition. In my opinion we may talk about convergent beam up to crossover. Overcoming crossover on Fig.2, the electron beam become divergent.
The main reason why the 2nd condenser lens is set up into overfocus, is that we would like to reach the most parallel electron beam with optic axis at specimen plane as possible. The focused or underfocused 2nd condenser lens in TEM gives undesired convergence. It is not so clear from Fig.2, but in real TEM after 2nd condenser lens there is upper objective lens, which acts as convex lens and produce (almost) parallel beam from incoming divergent beam. The specimen is then illuminated by parallel beam.
Overfocused 2nd condenser lens create source image above image plane. So the convergence (or divergence) angle is defined by diameter of this crossover above image plane and its distance from specimen. In underfocused case the convergence angle is defined by condenser lens aperture.
- Badges
- Science topic
- Similar topics
- Chemistry
- Nanotechnology
- Nanostructures
- Thin Films