Is high tempurature/ thermal radiation a requirement for ionization of gas to plasma using different frequencies of photons?

Question

I am wondering if thermal radiation or heat energy is fundamental to ionizing a gas into a plasma when using lasers or different frequencies of light. I know that gas discharge tubes or Geissler tubes require a high voltage(usually by a step-up transformer to rectifier) to ionize different noble gasses to plasma. But the thermal energy given off by the gas discharge tube is only based on the resistance of the tube from the high voltage. I also know that thermal radiation exists based on the black body spectrum and lasers that use ultraviolet radiation could eject electrons of an atom or by the photoelectric effect, but a bit of the UV radiation contains thermal energy which I am not sure if that requires high temperatures. I also know that lasers could have a negative temperature to slow down the kinetic energy of electrons to nearly 0K. I am wondering if instead lasers can be used to increase the kinetic energy of electrons without requiring high temperature/high thermal energy? Did I miss something from my understanding that should involve temperature to ionize a gas to plasma?

Edit: A more straightforward question is whether I can create plasma by photon energy alone without high temperatures.

Answer 1

I think that you might have misunderstood "thermal energy". The energy contained in light is simply photon or electromagnetic energy. There is no "thermal energy" in light per se. You could, however, have light with a spectrum that corresponds to thermal/black-body radiation, which only means that the distribution of photon energies follow Planck's law. Also note that one of the defining features of laser light is that it is essentially monochromatic, i.e. only has one wavelength/photon energy. Laser light can therefore be said to not have any thermal radiation component to it, since you can't fit the single peak of the laser spectrum to any black-body spectrum. Therefore I would say that laser light do not contain any thermal energy.

I would also add that "thermal energy" does not exist on the microscopic level. You ask for increasing the electrons kinetic energy without increasing their "thermal energy", but the problem with that is that higher kinetic energy (almost) automatically leads to higher temperature (depending on how that kinetic energy is distributed).

With that said, you can of course use lasers to create plasmas. Some methods are:

• "Stone-age method" -- Simply heating the material until they are hot enough to be ionized. In this method the laser energy is transferred to the material which gains kinetic energy, but when the electrons and atoms collide with each other the energy will be distributed in a thermal distribution, i.e. the laser energy has been converted to thermal energy (what you wanted to avoid).
• "Quantum method" -- With high enough photon energy (i.e., wavelength of $\lesssim100\rm\,nm$). With this method the energy transferred to the electron is simply the photon energy.
• "Strong field method" -- This lies somewhat in between the stone-age and the quantum methods, but if you have a strong enough laser intensity ($\gtrsim10^{14}\rm\,W/cm^2$) the strength of the electric field of the laser light is enough to rip out the electrons from their bound states in the atom. Again no "thermal energy" is in play here.

However, the plasma that you create with the last two methods will rapidly recombine to form neutral atoms again, unless you supply more energy than needed to ionize the atoms (see for instance this question). If you supply more energy than needed, then that excess energy will eventually (if you also manage to confine the plasma) be distributed among the particles, via particles collisions, so that the electrons and ions are thermally distributed. That is, the second law of thermodynamics make it really hard to not end up with something that is "non-thermal", i.e. not thermally distributed.