Rasmus Christenson from Skien, Telemark, Norway

Rasmus was born in Gjerpen, Telemark around 1840 and was married to Martena, whose maiden name was Christianson ( dotter?- father was P.C. Christiansen) and was born in Kongsberg, Buskerud. They were residing in Skien with their two sons, Christen (1863-1894) and Petter Louis (2/16/1865-6/9/1945) for the 1865 census, emigrating sometime after 1/1/1866 and May, 1867, when their next son, Antono, was born in Wisconsin. I cannot find any passenger ship lists with them listed. I would appreciate any help in finding when and how they emigrated to the US and who their ancestors in Norway were. ( My great grandfather is Petter Louis)