MOLECULAR DEFORMATION OF B 1s CORE-EXCITED STATES OF BCl_3 PROBED BY ANGLE-RESOLVED ION YIELD SPECTROSCOPY Y. Shimizu$^\ast$, K. Ueda$^\ast$, H. Chiba$^\ast$, Y. Sato$^\ast$, M. Izumisawa$^\dagger$, T. Hayaishi$^\dagger$, M. Kitajima$^\ddagger$ and H. Tanaka$^\ddagger$ $^\ast$Research Institute for Scientific Measurements, Tohoku University, Sendai 980-8577, Japan $^\dagger$Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8577, Japan $^\ddagger$Department of Physics, Sophia University, Tokyo 102-8554, Japan We have measured the angle-resolved ion yield spectra in the vicinity of the B 1s -> 2a_2'' and B 1s -> 4e' resonances in the D_{3h} planar molecule BCl_3 and find that the B atom is pushed out in the B 1s^{-1}2a_2'' state while the molecule deforms mainly within the molecular plane in B 1s^{-1}4e'.