Atomic background and EXAFS of gaseous hydrides of Ge, As, Se and Br

K edge absorption spectra of elements from Ge to Br in gaseous hydrides wer e measured at the BM 29 station of ESRF with noise level as low as 2x10(-5) , so far achievable only on noble gases. The dominant feature of the spectr a is the atomic background similar to that of Kr and comparable to it in th e quality of detail. Onto the background, a smooth weak EXAFS signal due to the hydrogen neighbors is superposed. For the very simple molecules such a s these, its contribution can be calculated ab initio, with exact treatment of the effect of molecular vibrations, so that no best-fit adjustment to t he experimental data is necessary. In comparison to the main constituent of the atomic background, i.e. the shake-up absorption edges, the EXAFS signa l is of minor importance in HBr with a single H neighbor, but of the same o rder of magnitude in GeH4.