Application of the diffraction theory for photothermal deflection to the measurement of the temperature coefficient of the refractive index of a binary gas mixture

• Published in: Journal of applied physics Vol. 99; no. 10
• Main Authors: Rohling, Jurandir H., Shen, Jun, Zhou, Jianqin, Gu, Caikang Elton, Medina, Antonio N., Baesso, Mauro L.
• Format: Journal Article
• Language: English
• Published: 15.05.2006
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.2198988

The application of recently developed diffraction theory for continuous wave photothermal deflection (PD) spectroscopy in the measurement of the temperature coefficient of the refractive index, dn∕dT, of a gas as a deflecting medium was presented. Both amplitude and phase signals were elucidated for a better interpretation and a simultaneous measurement of the thermal diffusivity αg and dn∕dT. The probe-beam size effect (PBSE) on the PD phase signal was also quantitatively analyzed to clearly show its physical meaning. A distance-scan method to measure the distance x between the probe beam and sample, which was difficult to determine in the PD, but a necessary parameter for measuring dn∕dT, was proposed. The consistent values of x measured respectively from the amplitude and the phase could not be obtained without considering the PBSE. αg and dn∕dT of pure gases (O2, N2, and CO2) and binary gas mixtures (O2-CO2 and N2-CO2) were precisely measured with the measured x. The measured αg and dn∕dT of pure gases were in very good agreement with the literature values, and furthermore the measured dn∕dT values of the pure gases had one more significant figure than the literature ones, demonstrating the high precision of this method. It was found the behavior of αg and dn∕dT of a binary gas mixture versus concentration could be described by a thermodynamic theory and the Lorentz-Lorenz formula, respectively.