"Of all the radiation - chemical reactions that have been studied in aqueous solution, the most complex and bewildering is the decomposition (dissociation) of pure water itself. Early workers had shown that water decomposed to hydrogen and oxygen, with some hydrogen peroxide, under bombardment from X-rays. A detailed study of the reaction was made in 1913 by Duane and Scheuer and their results were confirmed twenty-five years later by Lanning and Lind. Meanwhile Risse and then Fricke had shown that water under X-rays in a closed vessel appeared to decompose practially not at all. Fricke indeed found traces of gas resulting from irradiation, but on analysis this turned out to be composed of hydrogen and carbon dioxide, obviously arising from organic impurities in the water. When the water was more carefully purified the carbon dioxide disappeared, but a trace of hydrogen gas was always found even from irradiation of the most highly purified water. On the other hand, Guenther and Holzapfel irradiated water with X-rays in contact with a large free volume in a vacuum system and found large continuing yields of hydrogen gas. The experimental situation on water radiolysis in 1940 was indeed confusing. This confusion is reflected in the ideas about water radiolysis expressed in D. E. Lea's otherwise excellent book "Actions of Radiations on Living Cells" published in 1946. A better understanding of the subject had already been obtained within the U.S. atomic energy project, but this material could not be published at that time." [The Radiation Chemistry of Water and Aqueous Solutions, D. Van Nostrand Co, Princeton, NY, 1961]

Dissociating Water with X-rays - Using the Linac Coherent Light Source (LCLS), SLAC's X-ray laser, the researchers blasted tiny jets of water with short pulses of powerful X-rays. They learned that when the X-ray laser hit the jet, it vaporized the water around it and produced a shockwave. As this shockwave traveled through the jet, it created copies of itself, which formed a "shockwave train" that alternated between high and low pressures. Once the intensity of underwater sound crosses a certain threshold, the water breaks apart into small vapor-filled bubbles that immediately collapse. The pressure created by the shockwaves was just below this breaking point, suggesting it was at the limit of how loud sound can get underwater. https://phys.org/news/2019-05-record-shattering-underwater.html

Dissociating Water with Radio Waves (Radiolysis)
http://www.popsci.com/scitech/article/2007-11/turning-water-fuel