B How are the mercury atoms in a fluorescent lamp ionised?

Summary
I understand when the current flows through the lamp, the electrons collide with the mercury atom and causes them to be ionised. But how does this work? In terms of the energy transfer between energy levels?
I think I vaguely understand how it works but not completely. So far, I understand that fluorescent lamps have an electrode at both ends and are filled with an unreactive, low-pressure gas such as mercury. When the current passes through, the electrons are accelerated and they collide with the mercury atoms and causes them to be ionised. (But how are they ionised here?) I also understand that when the electron collides with the mercury atom, it can knock an electron out and thus causing it to be ionised. If the electron came from a lower energy level, an electron falls from a higher level to fill this gap causing the emission of a photon. However, when does the electron move to a higher energy level? How will the photon be emitted here?
 

anorlunda

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If I interpret the question correctly, the basic information is here:

https://en.wikipedia.org/wiki/Ionization#Production_of_ions said:
Negatively charged ions are produced when a free electron collides with an atom and is subsequently trapped inside the electric potential barrier, releasing any excess energy. The process is known as electron capture ionization.

Positively charged ions are produced by transferring an amount of energy to a bound electron in a collision with charged particles (e.g. ions, electrons or positrons) or with photons. The threshold amount of the required energy is known as ionization potential. The study of such collisions is of fundamental importance with regard to the few-body problem (see article on few-body systems), which is one of the major unsolved problems in physics. Kinematically complete experiments,[2] i.e. experiments in which the complete momentum vector of all collision fragments (the scattered projectile, the recoiling target-ion, and the ejected electron) are determined, have contributed to major advances in the theoretical understanding of the few-body problem in recent years.

Adiabatic ionization is a form of ionization in which an electron is removed from or added to an atom or molecule in its lowest energy state to form an ion in its lowest energy state.[3]

The Townsend discharge is a good example of the creation of positive ions and free electrons due to ion impact. It is a cascade reaction involving electrons in a region with a sufficiently high electric field in a gaseous medium that can be ionized, such as air. Following an original ionization event, due to such as ionizing radiation, the positive ion drifts towards the cathode, while the free electron drifts towards the anode of the device. If the electric field is strong enough, the free electron gains sufficient energy to liberate a further electron when it next collides with another molecule. The two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause impact ionization when the next collisions occur; and so on. This is effectively a chain reaction of electron generation, and is dependent on the free electrons gaining sufficient energy between collisions to sustain the avalanche.[4]
 

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