Aydin Ashrafi here I am trying to explain you each process one by one.

Electron impact ionization: It is the collision process in which electrons are used as a projectile to collide with some target be it solid liquid or gas ( depends on application, what you want to investigate e.g. mass spectrometry, Characterization of a sample for material science etc.) and depending on the research interest, one look for particular e.g. fragments ( in mass spectrometry) , characteristics x-rays for material characterization or auger electrons or energy loss measurements for potential well studies.

Highly charged ion Ionization: In this type of ionization highly charged ions like ( Xe10+, C3+ etc. or Bare ions like U92+) are produced from particle accelerators like ECR ion source based accelerators, palletron accelerators, Superconducting Linac and Synchrotrons. And these HCI’s are used as projectile to collide with thin solid/thick solid targets, or targets in gas phase or in condensed phase to investigate various associated phenomena. In General, what happens is when projectile collides with the target, there is certain probability (depending on projectile energy as well) of charge exchange, ionization of target, emission of electrons and characteristic x-rays from projectile and target. And by recording this type of data, people do X-ray production cross section measurements, impact parameter dependent investigations and many more.

X-ray Ionization: In this x rays (photons) are used as projectile to investigate the targets some examples are X-ray Fluorescence, Photo-electron Spectroscopy etc.

femtosecond pulsed laser ionization: In this laser pulses are used to investigate the dynamics of the system with respect to time, laser pulses used are sometimes also referred as probe and system under investigation as pump, the so-called Pump-probe experiments. Electronic structure, time dependent excitations and relaxations are the things that are usually investigated in this kind of experiments.

Depending on the energy of projectile, one can investigate outer shell ionization processes (Low projectile energy) Inner shell ionization process ( higher projectile energy/projectile velocity comparable to velocity of K shell electron of the target).

I hope you got your point. If you go deep in each one of these processes you will find your interest.

Dear Aydin Ashrafi

Electron impact ionization

It is the process in which high kinetic energetic electron bombarded with atom the secondary electron will emit making the atom into ion (secondary electron emission).

Highly charged ion ionization

When the atom is subjected to high electric filed the matter starts emitting electrons (Field emission)

X-ray ionization

When the X-ray is incident on matter the photo electric emission takes place making the matter into ionized (Photoelectric emission)

femtosecond pulsed laser ionization

It is the instantaneous process when the pulse(quantity of photons) of laser incident on matter again the PEE takes place

Femtosecond pulsed laser ionization takes place through multi photon ionization process. Even any focused laser produces ionization in the material through multi photon ionization. This is the dominant process along with other linear processes. Femtosecond laser has various applications as has been discussed by Sarvesh Kumar.