 Franck 's first paper there , co @-@ authored with Edward Teller , was on photochemical processes in crystals . Hans Gaffron became his collaborator . They were joined by Pringsheim , who escaped from Belgium after the German invasion . Franck arranged a position for Pringsheim at his laboratory . Both his daughters and their families moved to the United States , and he was also able to bring out his elderly mother and aunt . He became a naturalised United States citizen on 21 July 1941 , so he was not an enemy alien when the United States declared war on Germany on 11 December 1941 . His daughters still were , though , so they were restricted from travelling , and could not take care of their mother when she fell ill and died on 10 January 1942 , although they were permitted to attend her funeral . 
