High pressure optical cell for synthesis and in situ Raman spectroscopy of hydrogen clathrate hydrates

We report the design, realization, and test of a high-pressure optical cell that we have used to measure the Raman spectra of hydrogen clathrate hydrates, synthesized in situ by the application of 200-300 MPa of gas pressure on solid water. The optical apparatus is mounted on a cryogenic system so to allow measurements and sample treatment at any temperature between 300 and 20 K. A capillary pipe is connected to the inside of the cell to allow the gas flow into and out of the cell, and to regulate the internal pressure at any value from 0 to 300 MPa. In the experimental test described in this paper, the cell has been partly filled, at room temperature, with a small amount of water, then frozen at 263 K before injecting hydrogen gas, at pressure of 150 MPa, into the cell. This procedure has permitted to study hydrogen clathrate formation, by measuring Raman spectra as a function of time.