in solid matrix Irradiation (UV/VIS); heating of water, Ar (excess) and Ca at 465 - 550°C for 1 or 2 hin steel crucible in Ta furnace and cocondensation on mirrored Cu held at 15 K; photolysis of formed matrices with Xe or Hg lamp for 15 min to 2.5 h; reaction mechanisms described; not isolated; detn. by IR; mixture of HBaOH, Ba(OH)2 and Ba(OH) obtained;