 To pursue an academic career in Germany , having a doctorate was not enough ; one needed a <unk> <unk> , or habilitation . This could be achieved with either another major thesis or by producing a substantial body of published work . Franck chose the latter route . There were many unsolved problems in physics at the time , and by 1914 he had published 34 articles . He was the sole author of some , but generally preferred working in collaboration with Eva von Bahr , Lise Meitner , Robert Pohl , Peter Pringsheim , Robert W. Wood , Arthur <unk> or Wilhelm Westphal . His most fruitful collaboration was with Gustav Hertz , with whom he wrote 19 articles . He received his habilitation on 20 May 1911 . 
