Sensitivity of extended x-ray-absorption fine structure to thermal expansion

The sensitivity of extended x-ray-absorption tine structure (EXAFS) to ther mal expansion has been studied by temperature-dependent measurements on ger manium. The first cumulant does not reproduce the thermal expansion owing t o vibrations normal to the bond. The perpendicular relative displacement [D elta u(perpendicular to)(2)] has been for the first time experimentally obt ained; the ratio [Delta u(perpendicular to)(2)]/[Delta u(parallel to)(2)] i s in agreement with vibrational model calculations. Low-temperature quantum effects on the 3rd cumulant have been for the first time observed. The pos sibility of measuring thermal expansion from the 3rd cumulant is demonstrat ed, provided that quantum effects are taken into account. [S0031-9007(99)09 215-7].