In non-thermal plasmas, If the electron temperature is in the range of 1-2 eV, can species with a higher excitation threshold energy (e.g., 5eV) be formed through direct electron impact? or can reactions with a higher activation energy proceed?
Is it true that one species can obtain more and more energy from the accumulation of collisions with two, three, four, five..or more energetic electrons?
Thank you very much!
Theoretically yes, as your electrons allways have an energy distribution, which means that you will allways find electrons with much higher energy in your plasma. However, the number is comparably small, if your Te is in the range of 2 eV.
To your second question: yes, also this is possible if the relaxation time of your quantum state is larger than the collision time. If this is the case your atom/molecule can be excited to a higher energy with the first collision and pushed upwards the energy "ladder" by subsequent collisions.
Thanks very much, Johannes.
So, even no electron has higher energy than the excitation energy of one species, this species is also possible to form through a number of collisions with low-energy electrons? because the energy can be accumulated.
But the precondition is that the relaxation time of the quantum state is larger than the collision time.
The propability that no electron has an energy higher than the excitation threshold is very unlikely due to the energy distribution.
The rest is true, provided that each of the low energy electrons has at least enough energy to excite the particle in an inelastic collision to the next higher energetic state.
That is usual situation in low temperature plasma. The electron temperature 1-2 eV and the high energetic tail of electron energy distribution excites energy levels of 10-13 eV by direct electron impact (one two-body collision). However the energy of incident electron should be higher then threshold, large amount of slow electrons could not overcome threshold of the lowest excited state.
Just to add one comment about your second question: Very often in non-thermal plasma ionization is not a single step process but at least a two step process. A good example is ionization of Xe e.g. in pulsed high pressure plasma driving XeCl excimer laser. Xe has metastable levels at 8.3 eV and 9.4 eV. At "electron temperatures" (I avoid using this term because "temperature" implicates thermal equilibrium and Maxwellian electron energy distribution) in the range of 1-3 eV these levels are excited with much higher rate than direct ionization (12.13 eV). Ionization out of these excited levels (threshold energy ~ 3.8 eV) is very efficient. Thus it is a question of electron number density and neutral collision quenching of the excited levels deciding about the importance of two step ionization as compared to single step ionization.
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