The mechanism why extended X-ray absorption fine structure (EXAFS) is observed in X-ray photoacoustic spectra, has been discussed from various points of view, however, it is still unclear. We continued to gather many aspects of data of X-ray photoacoustic phenomena in order to solve this question. Recently, we found EXAFS also in phase spectra of X-ray photoacoustic signal and related it with the signal shape. This phenomenon reflects the thermal depth depending on the absorption coefficients,

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... owever, there are left still unresolved aspects. All aspects of the data are shown and discussed in relation to its mechanism. Since X-ray photoacoustic(PA) phenomena and its spectra were observed'-4', the mechanism why the extended X-ray absorption fine structure (EXAFS) is observed in these spectra, has been discussed from many points of view, not only from experimental evidence but also from theoretical simulation5). However, we are far from the clearcut explanation. For example, as seen in Fig.1 which shows the parallel changes between absorption spectrum and X-ray PA spectrum4), the abrupt change at the K-edge is same, however, the changing width of the fine structure is smaller in the PA spectrum than that for absorption. This means that the PA signal amplitude does not linearly correlate with the dependence of the absorption coefficient on the wavelength. It seems to show that the mechanism of heat generation is different in fine structure region. Of course, as seen in Fig. 2 , the X-ray PA signal amplitude depends on not only the absorption coefficient but also on the heat conductivity and heat capacity of the sample2). However, this is not the reason for the previous inconsistency. Recently, we have found that EXAFS is also found in the phase spectrum as shown in Fig. 3 . In this case, EXAFS is hardly seen in thinner foils and is clearly and strongly observed for thicker foils. Judging from the evidence that the amplitude of the changing width in the fine structure region is same as the absorption spectrum, the fine structure of the phase spectra seems to reflect the heat generation depth depending on the absorption coefficients. Fig. 4 shows the difference in the signal amplitude and phase at various thickness of sample. The Article published online by EDP Sciences and available at http://dx.