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

Abstract

Using near-infrared laser radiation at 1064 and 1890 nm, respectively, laser-induced gratings (LIG) were generated in gaseous mixtures of nitrogen with small amounts (less than 1% mole fraction) of methanol (CH4O) or 1,2-ethylenediamine (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 thermal gratings could be observed, whereas at 1890 nm the electrostrictive contribution to the LIG signal was negligible. The speed of sound and the thermal heat conduction in the medium were derived from the observed acoustic oscillations and the long term decay of the LIG signal amplitude, respectively. 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 absorbing species was below 20 ppm at 50 MPa.