Glossary help!

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Hi everyone,

I am a beginner in the field of laser science and I have been reading a few things about techniques like velocity mapping and charged particle imaging. I understand most of it, but there are certain terms/words that needs clarification. I am going to point them out and explain what I understand from then if any of you can explain if i am correct then please say so and if i am not, please correct me. Ok so first:

" In the case of neutral atoms, radicals or molecules, a quantum-state-specific resonance enhanced multi-photon ionisation (REMPI) scheme is generally used to produce positively charged species with unchanged velocities; the recoil of a very light electron typically causes a negligible change to the velocity of the neutral species when it is ionised " (Taken from literature)

Ok so the first term resonance. How I understand it is this. If a molecule/atom absorbs a photon at a frequency that corresponds to the Delta E and hence satisfies the Bohr frequency condition the molecule makes a transition to a high energy state (lets call this the excited state) and such a process is called (resonant) absorption. Ok so that would mean that for the molecule to be resonantly ionised the photon must have energy that correspond to/higher than the ionisation potential for that molecule? And in the case of REMPI do we mean resonance enchaned because we go through a (resonant) intermediate state before ionisation?

What I understand from recoil electron is this: the motion of electron( here ejection) initiated by collision. In the same text at other places, it talks about the recoil velocity or ion recoil. Does that mean in the same context?

any clarification will be most appreciated.

Edited by ChemSiddiqui
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A resonance is a real transition, as you say. Any multi-photon process that involves a real transition will be much more likely; simplistically speaking, it has a certain lifetime so the chances of absorbing another photon while in that state is enhanced, assuming selection rules allow the transition.

Any time an atom absorbs a photon or emits one, or emits an electron, momentum has to be conserved. The recoil is small because the photon momenta are small, and the ejected electron will also typically have a small momentum, as compared to the thermal values of the atom.

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