Spin flip transitions in /sup 13/C and /sup 19/F probed with the (. pi. /sup -/,. gamma. ) reaction

Photon spectra from radiative capture of stopped negative pions captured by /sup 13/C and /sup 19/F nuclei have been measured with a high resolution pair spectrometer. In the bound region transitions have been observed to /sup 13/B(g.s.), /sup 13/B(3.5 MeV), /sup 19/O(g.s.), /sup 19/O(4.9 MeV), and /sup 19/O(6.3 MeV). In addition, strong transitions to narrow states above the neutron separation threshold were observed in both cases. The results for /sup 13/C are in agreement with detailed shell model calculations of this process. Major strength is observed for states formed by coupling extra nucleons or holes to the spin-quadrupole d/sub 5/2/ and d/sub 3/2/, J/sup ..pi../ = 2/sup -/ states in /sup 16/O. The /sup 13/C(..pi../sup -/,..gamma..)/sup 13/B/sub g.s./ branching ratio is shown to be consistent with beta decay and electroexcitation data, through a combined phenomenological analysis. The observed transition strengths are accounted for by matrix elements of the spin-density operators (sigma x Y/sub 0/)/sup 1/t/sup +/ and (sigma x Y/sub 1/)t/sup +/.