What gives an optimum value for electron energy to ionise atom?

[paul_iow]

TL;DR Summary

electron energy to ionise an atom

There is an optimum energy which gives the greatest probability of ionisation of a particular element.
This is said to align with the wavelength of the electron being close to resonances in the atom.
Looking at this in a different way as particles, would it be correct to say that the optimum electron energy is due to a compromise between having enough momentum to be dominant in its trajectory over forces exerted by electrons in the atom; and being in the proximity of the atom for a sufficently long time?
Time will of course reduce as energy is increased.

Paul

 

[Baluncore]

Welcome to PF.

Look up what energy is needed to ionise a particular electron of the element.
https://en.wikipedia.org/wiki/Ionization_energy

Next, illuminate the atom with photons having that energy.
The photon wavelength required will be determined by the Planck-Einstein relation.
https://en.wikipedia.org/wiki/Planck–Einstein_relation

Since heat will widen the energy distribution, only half the photons will have sufficient energy. If you increase the photon energy slightly you can get a greater energy transfer.

 

[paul_iow]

Thankyou for the reply baluncore, but it is not really what I was getting at. I am talking about using EI to ionise a sample. Thinking about the electron passing through the space ocupied by the atom as a particle: is it possible to understand the concept of the electron having a frequency that is matched to the structure that we want to destabilise? For example one electron passing another electron will have a force frequency spectrum that increases in freq. as electron speed is increased - is this something connected?
Or alternatively is this something completely different to what determines the optimum electron energy to ionise a sample?

 

[Baluncore]

Ionisation involves removing an electron from an atom. If you deliver kinetic energy with an electron, then what will happen to the electron used to deliver the energy?

 

[paul_iow]

The question is quite simple, but I don't think I initially phrased it well.
When a stream of electrons is used for ionisation, the optimum value for electron energy is normally attributed to something in the frequency domain. Question:
Is it possible to visualise why there is an optimum value from looking at the trajectory of the electron in relation to the atom?

 

[Bystander]

Is it possible to visualise why there is an optimum value from looking at the trajectory of the electron in relation to the atom?

No.

 

[PeroK]

The question is quite simple, but I don't think I initially phrased it well.
When a stream of electrons is used for ionisation, the optimum value for electron energy is normally attributed to something in the frequency domain. Question:
Is it possible to visualise why there is an optimum value from looking at the trajectory of the electron in relation to the atom?

I think I understand your question, although sadly I don't know the answer. Classically, you would not expect this "resonant" energy. It sounds like a similar phenomenon to quantum tunnelling, whereby electrons with certain resonant energies can tunnel though an effectively infinite potential barrier. Transforming the electron energies to a nominal wavelength that corresponds to the "length" of bonds in the molecule sounds like a heuristic to me. The full calculations ultimately would be quantum-electrodynamical, where the amplitude for ionisation would turn out to be optimised at some energy. It's possible that those calculations would be excruciatingly difficult in order to confirm the correctness of the heuristic wavelength calculations!

That's my guess, anyway!

 

[paul_iow]

Thanks PeroK and Bystander. It is often said that the de Broglie wavelength should be close to the physical dimensions of the structure. But thinking about the electron and atom in just a Newtonian view, and of course accepting that the electron will never make contact with another particle, it can be seen that if the electron is accelerated to too high a velocity its disturbane to the atom will be reduced due to the time for which it is close to the atom being reduced. Are these things related, or is the Newtonian suggestion of how electron energy may effect things not able to relate to what is really going on ?

 

[PeroK]

Thanks PeroK and Bystander. It is often said that the de Broglie wavelength should be close to the physical dimensions of the structure. But thinking about the electron and atom in just a Newtonian view, and of course accepting that the electron will never make contact with another particle, it can be seen that if the electron is accelerated to too high a velocity its disturbane to the atom will be reduced due to the time for which it is close to the atom being reduced. Are these things related, or is the Newtonian suggestion of how electron energy may effect things not able to relate to what is really going on ?

Newtonian mechanics simply does not apply to atomic and molecular structures. If it did, there would be no atoms, molecules, chemistry or chemistry students to study them.

 

[paul_iow]

I understand that we can't go back and try to understand Atoms in Newtonian terms, not that I really understand the way they do work. When the electron is part of an atom it is bound by laws beyond my understanding, but for the free electron before and after it is close to the atom it obeys Newton's laws well enough for our purposes, so the duration of the destabilising pulse that the atom is subject to does depend on the (admittedly rather fuzzy) size of the atom and the speed of the electron, I believe. If the resonant frequency of the atom is related to its size and C (it is said EI works best when de Broglie wavelength same size as atom) then from just a Newtonian view the electron would need to move at a speed in the region of C to produce this frequency, and we know it is vastly slower than this. So I see these things are unrelated. Many thanks all.