Development of apparatus for x-ray absorption spectroscopy of free clusters.

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We have developed a new apparatus for X-ray absorption spectroscopy of free clusters and a new method for size selective EXAFS measurement of free clusters.The new apparatus mainly consists of two vacuum chambers, vacuum pumping system and cluster source. Each vacuum chamber is independently evacuated by turbo molecular pump to achieve enough pumping speed required for producing large clusters. In addition, we have developed a new cluster source which produces a cluster beam by an adiabatic expansion of gas or gas mixture into a vacuum chamber. The sample gas is ejected from the interchangeable nozzle which is mounted on top of a continuous flow cryostat. The nozzle temperature can be adjusted to values between 77 to 300 K and kept constant with an accuracy of ±1K when the cryostat is operated with liquid nitrogen. The performance of the apparatus have been evaluated by producing large argon clusters (up to Ar_<20> clusters are detected under typical condition).We have suggested a new … More method for size selective EXAFS measurement of free clusters by using the information of de-excitation process of clusters subsequent to the x-ray absorption. In order to examine our suggestion, we have developed a new data acquisition system for EXAFS-PEPICO synchronous measurement and carried out EXAFS-PEPICO synchronous measurements for Se cluster beam at the X-ray undulator beamline BL10XU in SPring-8. Although the number density of Se atoms in the cluster beam is smaller than that of bulk Se samples by more than ten orders of magnitude, the X- ray absorption spectrum which has been taken by total ion yield (TIY) mode indicates clear EXAFS oscillation. The partial ion yield (PIY) spectra which are obtained by integrating the peaks of the PEPICO spectra also show clear EXAFS oscillation. We have deduced EXAFS spectra of Se_2 and "Se_5" clusters according to the size selective EXAFS method. The EXAFS oscillation of "Se_5" differs from that of Se_2 in characteristic manner and the spacing of the EXAFS oscillation clearly demonstrates that the bond length of "Se_5" is longer than that of Se_2. More precisely, the former proves to be 2.35A and the latter 2.16A, which is fully consistent with a pile of EXAFS data for various forms of Se. Therefore, we may conclude that the proposed size-selective XAFS is a reliable and realistic method. Less