|MadSci Network: Physics|
You are quite right that light can influence plasmas. The interaction of plasma and light is a pretty complex field that is a little bit too broad to cover here, but I can describe a few common mechanisms in which light can influence plasma. Some of those would work on liquids or solids, too.
Your example of a storm cloud is a good one. In a storm cloud, positive and negative charges are separated. This situation is not very stable: if a conductive path were to exist between the positive and negative charge, a current would start to flow that would eliminate the difference. This dissipates the energy stored in the electric field that exists between the positive and negative charge. This rapid current flow is the lightning1 we see.
Lightning is an example of a plasma2. A plasma is a hot, partially ionized gas. Such a partially ionized gas is a reasonable to very good (depending on plasma properties) conductor of energy, while a "normal" gas does not conduct electricity at all.
This difference in conductivity is what drives lightning. While the exact mechanism of lightning is extremely complicated, in a nutshell, it works as follows. There is a potential (voltage) difference between two clouds or between the ground and a cloud. Sometimes, for instance, a cosmic particle can cause some ionization in the air. This means that neutral air molecules get split in free electrons and ions. Normally, these ions will recombine, and the normal composition of air is restored.
The electric field in the air can accelerate the electrons. If they are sufficiently accelerated, they can knock electrons from molecules. If this generation is faster than the recombination, there is an avalance (Townsend Avalanche) 3. This avalanche generates a plasma. This plasma fills a little bit of the distance between the cloud and the Earth. As the plasma grows, it closes the distance between the cloud and the Earth. When the gap is closed, a conductive path is formed and the current can start flowing. This current flow causes the lightning we see.
Light can create a plasma with several mechanisms. Which mechanism is dominant depends on the wavelength (color), the intensity, and the duration of the light. In the case of the thunderstorm, this little "seed" plasma takes the place of the cosmic particle in "triggering" the lightning. Hence, when a small bit of plasma is made, the thunderstorm takes care of the rest, so comparatively little power is needed.
The first method involves using light with a very specific wavelength. Each atom or molecule has a certain energy by which it retains its electrons. If the wavelength of the light exactly matches this energy, an efficient transfer is possible. This leads to ionization 4. In practice, this can be achieved by using a laser that has a wavelength that matches the ionization energy. However, most ionization energies require light that is in the far UV, and such light cannot be handled by common optics. It is possible, however, to use two-photon processes (in this case, two photons striking an atom simultaneously produce the required energy). This requires a far higher light intensity to be efficient, however. Schemes involving excited states are possible, too.
The second method involves heating the sample into the plasma state 5,6. By focusing a laser with sufficient power, matter can be heated to several 1000 K, a temperature at which ionization becomes significant. Of course, large amounts of laser power are needed to heat the sample to these temperatures. Short, intense laser bursts do keep the heating quite localized.
The third method is only possible using the most powerful lasers. The binding of electrons to their core is essentially caused by a strong electric field. If a laser of sufficient strength is used, it is possible to have an electric field that is so strong it (nearly) cancels out the binding field. Electrons can then leak away by quantum mechanical tunneling. Using this process, it is possible to turn matter into plasma in a matter of picoseconds (10-12 s)5,6.
In principle, all of these methods would work on water, too. Molecules are molecules, and it does not matter (too much) how tightly they are packed. However, in water the density and recombination rate is much higher. Hence, it would be much more difficult to maintain the plasma. Furthermore, the second and third method will turn the water into steam (or rather, into steam and ripped-up water molecules). Steam has a density that is about 1000 times lower than that of water, so an explosion is likely to occur.
In summary, it is theoretically quite possible to use light to enhance the conductivity of water. However, in practice, it is extremely energy-inefficient, so it would be of little use to enhance electrolysis.
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