Small wavenumber shift Raman spectroscopy of water and heavy water under negative pressure

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A metal Berthelot tube having 3 sapphire windows was made for Raman spectroscopy of water under negative pressure, but it yielded oniy -30 bar which was too low. The causes were investigated and three key findings were established. (1) An electrochemical criterion for selecting metals in a water Berthelot system is that metals which are corroded with water should not be used. Aluminum reacts with water to produce hydrogen gas which inhibited even generation of negative pressure. This criterion, however, was not applied to organic liquids. (2) The smaller the area of specimen chamber wall, the faster the rate of increase in negative pressure with temperature cycles. This is because the number of cavitation nuclei is proportionally smaller. (3) Pre-treatment for metals under supercritical water yielded negative pressure of around-120 bar in initial a few cycles. Without this it used to take 2 weeks for carrying out ca.1000 cycles. Further rise in negative pressure would be attained if the autoclave itself was degassed before the pre-treatment. The last statement is still being investigated.