Abstract: Based on the R-Gtheory of the photoacoustic effect, a new formula on the PAphase was presented while the phase caused by non-radiative relaxation processes was considered. The phase is associated with the absorption coefficient(β), thermal diffusion length of sample(_s), the delay time of relaxation(τ) and the ratio of the heat component(R) of rapid relaxation to that of slow one. The phase spectra of PrF₃ and chlorophyll b were explained well by using of this formula. The phase of PrF₃ is determined by the absorption coefficient for the f-f transition. The phase difference between the two bands of chlorophyll b is caused by the different heat component ratios of rapid relaxation to slow one.

Key words: Photoacoustic phase spectrum, Non-radiative relaxation processes, Praseodymium fluoride, Chlorophyll b