Optical isomer shift for the spontaneous-fission isomer /sup 240/Am/sup m/

The optical isomer shift in the /sup 8/S/sub 7/2/ ..-->.. /sup 10/P/sub 7/2/ atomic transition in neutral americium has been measured for the approx. 1-ms spontaneously fissioning isomer, /sup 240/Am/sup m/(SF). With use of a laser-excited optical pumping technique, this transition is shifted by +2.6 +- 0.2 A to 6407.2 +- 0.2 A implying a difference in nuclear mean square radii, delta, of 5.1 +- 0.2 fm/sup 2/ between the fission isomer and the normal ground state. These experiments provide the first direct experimental proof for the large deformations expected for fission isomers.