Sound velocity, heat conduction and virial coefficients of gaseous mixtures at high pressure from NIR laser-induced grating experiments

Using near-infrared laser radiation at 1064 and 1890 nm, respectively, lase r-induced gratings (LIG) were generated in gaseous mixtures of nitrogen wit h small amounts (less than 1% mole fraction) of methanol (CH4O) or 1,2-ethy lenediamine (C2H8N2). The variation of the electrostrictive and thermal LIG signal amplitude with laser energy and concentration of absorbing species was analysed. At a pump wavelength of 1064 nm electrostrictive as well as t hermal gratings could be observed, whereas at 1890 nm the electrostrictive contribution to the LIG signal was negligible. The speed of sound and the t hermal heat conduction in the medium were derived from the observed acousti c oscillations and the long term decay of the LIG signal amplitude, respect ively. Second and third order acoustic virial coefficients were determined in the pressure range between 1 and 50 MPa. Although absorption at the two laser wavelengths was extremely small the detection limit for both absorbin g species was below 20 ppm at 50 MPa.