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{{Short description|German–American atomic physicist (1882–1964)}}
{{Distinguish|James Frank}}
{{Good article}}
{{Use dmy dates|date=August 2024}}
{{Infobox scientist
| name              = James Franck
| image             = James Franck 1925.jpg
| caption           = Franck in 1925
| birth_date        = {{Birth date|1882|08|26|df=yes}}
| birth_place       = [[Hamburg]], [[German Empire]]
| death_date        = {{Death date and age|1964|05|21|1882|08|26|df=yes}}
| death_place       = [[Göttingen]], [[Lower Saxony]], <br/> [[West Germany]]
| citizenship       = {{Plain list|
* Germany
* United States (from 1941)
}}
| education         = [[Wilhelm-Gymnasium (Hamburg)|Wilhelm-Gymnasium]]
| alma_mater        = [[University of Berlin]] ([[Doctor of Philosophy|Dr. phil.]], [[Habilitation|Dr. habil.]])
| known_for         = {{Plain list|
* [[Franck–Hertz experiment]]
* [[Franck–Condon principle]]
* [[Franck Report]]
}}
| spouses           = {{Plain list|
* {{Marriage|Ingrid Josefson|1911|1942|end=died}}
* {{Marriage|[[Hertha Sponer]]|1946}}
}}
| children          = 2
| awards            = {{Plain list|
* [[Nobel Prize in Physics]] (1925)
* [[Max Planck Medal]] (1951)
* [[Rumford Prize]] (1955)
* [[Fellow of the Royal Society#Foreign member|ForMemRS]] (1964)
}}
| fields            = [[Atomic physics]]
| work_institutions = {{Plain list|
* University of Berlin (1911–18)
* [[Kaiser Wilhelm Society]] <br/> (1918–20)
* [[University of Göttingen]] <br/> (1920–33)
* [[Niels Bohr Institute]] (1933–35)
* [[Johns Hopkins University]] (1935–38)
* [[University of Chicago]] <br/> (1938–47)
}}
| doctoral_advisor  = [[Emil Warburg]]
| thesis_title      = Über die Beweglichkeit der Ladungsträger der Spitzenentladung
| thesis_url        = https://ui.adsabs.harvard.edu/abs/1906AnP...326..972F/abstract
| thesis_year       = 1906
| doctoral_students = {{Collapsible list|title={{Nobold|''See list''}}
 | [[John Torrence Tate Sr.|John Tate Sr.]] (1914)<ref name=MGP>{{Cite web|title=James Franck - The Mathematics Genealogy Project|url=https://www.genealogy.math.ndsu.nodak.edu/id.php?id=126428|website= genealogy.math.ndsu.nodak.edu|access-date=2025-06-01}}</ref>
 | [[Wilhelm Hanle]] (1924)
 | [[Arthur R. von Hippel|Arthur von Hippel]] (1924)
 | [[Hans Kopfermann]] (1925)
 | [[Herbert Arthur Stuart]] (1925)<ref name=MGP/>
 | [[Heinrich Gerhard Kuhn]] (1926)
 | [[Fritz Houtermans]] (1927)
 | [[Robert d'Escourt Atkinson]] (1928)<ref name=MGP/>
 | [[Heinz Maier-Leibnitz]] (1935)
 | [[Theodore Puck]] (1940)<ref name=MGP/>
}}
{{Infobox military person
| embed             = yes
| allegiance        = {{Flagu|German Empire|size=20px}}
| branch_label      = Branch
| branch            = [[Imperial German Army|German Army]]
| serviceyears      = 1914–1918
| rank_label        = Ranks
| rank              = {{Plain list|
* ''[[Offizierstellvertreter]]''
* ''[[Leutnant]]''
}}
| battles_label     = Wars
| battles           = {{Tree list}}
* [[World War I]]
** [[Western Front (World War I)|Western Front]]
** [[Eastern Front (World War I)|Eastern Front]]
{{Tree list/end}}
| awards            = {{Plain list|
* [[Iron Cross]] (2nd Class, 1915; 1st Class, 1918)
* [[Hanseatic Cross]] (1916)
}}
}}
}}

'''James Franck''' ({{IPA|de|ˈdʒɛɪ̯ms ˈfʁaŋk|lang|De-James Franck.ogg}}; 26 August 1882 – 21 May 1964) was a German–American [[atomic physicist]] who shared the 1925 [[Nobel Prize in Physics]] with [[Gustav Hertz]] "for their discovery of the laws governing the impact of an [[electron]] upon an [[atom]]".<ref name="Nobel Prize">{{cite web |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1925/ |title=The Nobel Prize in Physics 1925 |publisher=The Nobel Foundation |access-date=16 June 2015 }}</ref>

Franck completed his [[doctorate]] in 1906 and his [[habilitation]] in 1911 at the [[Humboldt University of Berlin|Frederick William University]] in Berlin, where he lectured and taught until 1918, having reached the position of [[Academic ranks in Germany|professor extraordinarius]]. He served as a volunteer in the German Army during [[World War I]]. He was seriously injured in 1917 in a gas attack and was awarded the [[Iron Cross]] 1st Class.

Franck became the head of the physics division of the [[Kaiser Wilhelm Institute|Kaiser Wilhelm Gesellschaft]] for Physical Chemistry. In 1920, Franck became [[professor ordinarius]] of experimental physics and Director of the Second Institute for Experimental Physics at the [[University of Göttingen]]. While there he worked on [[quantum physics]] with [[Max Born]], who was Director of the Institute of Theoretical Physics. His work included the [[Franck–Hertz experiment]], an important confirmation of the [[Bohr model]] of the [[atom]]. He promoted the careers of women in physics, notably [[Lise Meitner]], [[Hertha Sponer]], and [[Hilde Levi]].

After the [[Nazi Party]] came to power in Germany in 1933, Franck resigned from his position at the [[University of Göttingen]] in protest against the dismissal of fellow colleagues. He assisted [[Frederick Lindemann]] in helping dismissed Jewish scientists find work overseas, before he left Germany in November. After a year at the [[Niels Bohr Institute]] in Denmark, he moved to the United States, where he worked at [[Johns Hopkins University]] in [[Baltimore]] and then the [[University of Chicago]]. During this period he became interested in [[photosynthesis]].

Franck participated in the [[Manhattan Project]] during [[World War II]] as director of the chemistry division of the [[Metallurgical Laboratory]]. He was also the Chairman of the Committee on Political and Social Problems regarding the [[atomic bomb]], which is best known for the compilation of the [[Franck Report]], which recommended that the atomic bombs not be used on the Japanese cities without warning.

== Education ==
James Franck was born on 26 August 1882 in [[Hamburg]], Germany, into a [[Jewish]] family, the second child and first son of Jacob Franck, a banker, and Rebecca Nachum Drucker.{{sfn|Rice|Jortner|2010|p=4}} He had an older sister, Paula, and a younger brother, Robert.{{sfn|Lemmerich|2011|pp=8–11}} His father was a devout and religious man, while his mother came from a family of [[rabbi]]s.{{sfn|Rice|Jortner|2010|p=4}}

In 1891, Franck enrolled at the [[Wilhelm-Gymnasium (Hamburg)|Wilhelm-Gymnasium]] in Hamburg, which was then a boys-only school.{{sfn|Lemmerich|2011|pp=8–11}} Hamburg had no university then, so prospective students had to attend one of the 22 universities elsewhere in Germany. Intending to study law and economics, In 1901, Franck entered the [[University of Heidelberg]], as it had a renowned law school.{{sfn|Lemmerich|2011|pp=12–15}} He attended lectures on law, but was far more interested in those on science. While there, he met [[Max Born]], who would become a lifelong friend. With Born's help, he was able to persuade his parents to allow him to switch to studying physics and chemistry.{{sfn|Kuhn|1965|pp=53–54}} He attended mathematics lectures by [[Leo Königsberger]] and [[Georg Cantor]], but Heidelberg was not strong on the physical sciences, so he decided to go to the [[University of Berlin]].{{sfn|Lemmerich|2011|pp=12–15}}

At Berlin, Franck attended lectures by [[Max Planck]] and [[Emil Warburg]].{{sfn|Lemmerich|2011|pp=16–17}} For his [[Ph.D.]],{{sfn|Rice|Jortner|2010|p=5}} Warburg suggested that he study [[corona discharge]]s. He found this topic too complex, so he changed the focus of his thesis.{{sfn|Kuhn|1965|pp=54–55}} Titled ''Über die Beweglichkeit der Ladungsträger der Spitzenentladung'' (On the mobility of [[ion]]s),{{sfn|Lemmerich|2011|p=331}} it would subsequently be published in ''[[Annalen der Physik]]''.<ref>{{cite journal
|last=Franck|first= J.|title=Über die Beweglichkeit der Ladungsträger der Spitzenentladung|journal=Annalen der Physik|volume=326|pages= 972–1000|number=15
|issn=1521-3889|doi=10.1002/andp.19063261508|year=1906|language=de|bibcode = 1906AnP...326..972F |hdl= 2027/mdp.39015064464335|url=https://zenodo.org/record/1424075}}</ref>

With his thesis completed, Franck had to perform his deferred military service. He was called up on 1 October 1906 and joined the 1st Telegraph Battalion. He suffered a minor horse riding accident in December and was discharged as unfit for duty. He took up an assistantship at the Physikalische Verein in [[Frankfurt]] in 1907, but did not enjoy it, and soon returned to the University of Berlin.{{sfn|Lemmerich|2011|pp=24–26}} At a concert, he met Ingrid Josefson, a Swedish pianist. They were married in a Swedish ceremony in [[Gothenburg]] on 23 December 1907. They had two daughters: Dagmar "Daggie", who was born in 1909; and Elisabeth "Lisa", who was born in 1912.{{sfn|Lemmerich|2011|pp=34–35}}

To pursue an academic career in Germany, having a doctorate was not enough; one needed a ''[[venia legendi]]'', 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 (physicist)|Eva von Bahr]], [[Lise Meitner]], [[Robert Pohl]], Peter Pringsheim, [[Robert W. Wood]], [[Arthur Wehnelt]], and [[Wilhelm Westphal]]. His most fruitful collaboration was with [[Gustav Hertz]], with whom he wrote 19 articles. He received his habilitation on 20 May 1911.{{sfn|Lemmerich|2011|pp=24–31}}

== Franck–Hertz experiment ==
{{main|Franck–Hertz experiment}}
[[Image:Franck-Hertz en.svg|thumb|right|Anode current (arbitrary units) versus grid voltage (relative to the cathode). This graph is based on the original 1914 paper by Franck and Hertz. |alt=Graph. The vertical axis is labelled "current", and ranges from 0 to 300 in arbitrary units. The horizontal axis is labelled "voltage", and ranges from 0 to 15&nbsp;volts.]]
In 1914, Franck teamed up with Hertz to perform an experiment to investigate [[fluorescence]]. They designed a [[vacuum tube]] for studying energetic [[electron]]s that flew through a thin vapour of [[mercury (element)|mercury]] atoms. They discovered that when an electron collided with a mercury atom it could lose only a specific quantity (4.9&nbsp;[[electronvolt]]s) of its [[kinetic energy]] before flying away. A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons just bounce off mercury atoms without losing any significant speed or kinetic energy.{{sfn|Kuhn|1965|pp=55–56}}<ref name=FH1>{{cite journal | first1=J. |last1=Franck |first2=G. |last2=Hertz |author-link2=Gustav Hertz |title = Über Zusammenstöße zwischen Elektronen und Molekülen des Quecksilberdampfes und die Ionisierungsspannung desselben |language=de |trans-title=On the collisions between electrons and molecules of mercury vapor and the ionization potential of the same |journal = Verhandlungen der Deutschen Physikalischen Gesellschaft | year = 1914 | volume = 16 | pages = 457–467}}</ref>

These experimental results provided confirmation of [[Albert Einstein]]'s [[photoelectric effect]] and Planck's relation ({{nowrap|1=''E'' = ''fh''}}) linking energy (''E'') and frequency (''f'') arising from quantisation of energy with the [[Planck constant]] (''h''). But they also provided evidence supporting the [[Bohr model|model of the atom]] that had been proposed the previous year by [[Niels Bohr]]. Its key feature was that an electron inside an atom occupies one of the atom's "quantum energy levels". Before a collision, an electron inside the mercury atom occupies its lowest available energy level. After the collision, the electron inside occupies a higher energy level with 4.9&nbsp;eV more energy. This means that the electron is more loosely bound to the mercury atom. There were no intermediate levels or possibilities.{{sfn|Kuhn|1965|pp=55–56}}<ref>{{cite journal |title=Franck and Hertz versus Townsend: A Study of Two Types of Experimental Error |first=Giora |last=Hon |journal=[[Historical Studies in the Physical and Biological Sciences]] |volume=20 |issue=1 |pages=79–106 |year=1989 |jstor=27757636 |doi=10.2307/27757636 }}</ref>

In a second paper presented in May 1914, Franck and Hertz reported on the light emission by the mercury atoms that had absorbed energy from collisions. They showed that the [[wavelength]] of this [[ultraviolet]] light corresponded exactly to the 4.9&nbsp;eV of energy that the flying electron had lost. The relationship of energy and wavelength had also been predicted by Bohr.{{sfn|Kuhn|1965|pp=55–56}}<ref name=FH2>{{cite journal |first1=J. |last1=Franck |first2=G. |last2=Hertz |author-link2=Gustav Hertz |title = Über die Erregung der Quecksilberresonanzlinie 253,6&nbsp;μμ durch Elektronenstöße |language=de |trans-title=On the excitation of mercury resonance lines at 253.6&nbsp;nm by electron collisions |journal = Verhandlungen der Deutschen Physikalischen Gesellschaft |year = 1914 |volume=16 |pages=512–517}}</ref> Franck and Hertz completed their last paper together in December 1918. In it, they reconciled the discrepancies between their results and Bohr's theory, which they now acknowledged.{{sfn|Lemmerich|2011|p=61}}<ref>{{cite journal |first1=J. |last1=Franck |first2=G. |last2=Hertz |author-link2=Gustav Hertz |journal=Physikalische Zeitschrift |language= de |title=Die Bestätigung der Bohrschen Atomtheorie im optimalen Spektrum durch Untersuchungen der unelastischen Zusammenstöße |year=1919 |volume=20 |pages=132–143 }}</ref> In his Nobel lecture, Franck admitted that it was "completely incomprehensible that we had failed to recognise the fundamental significance of Bohr's theory, so much so, that we never even mentioned it once".<ref>{{cite web |first=James |last=Franck |work= Nobel Lecture |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1925/franck-lecture.pdf |title=Transformations of Kinetic Energy of Free Electrons into Excitation Energy of Atoms by Impacts Nobel Lecture |date=11 December 1926 |publisher=The Nobel Foundation |access-date=16 June 2015 }}</ref> On 10 December 1926, Franck and Hertz were awarded the 1925 [[Nobel Prize in Physics]] "for their discovery of the laws governing the impact of an electron upon an atom".<ref name="Nobel Prize" />
 
== World War I ==
Franck enlisted in the [[German Army (German Empire)|German Army]] soon after the outbreak of the [[First World War]] in August 1914. In December, he was sent to the [[Picardy]] sector of the [[Western Front (World War I)|Western Front]]. He became a deputy officer (''[[offizierstellvertreter]]''), and then a lieutenant (''[[leutnant]]'') in 1915.{{sfn|Lemmerich|2011|pp=52–58}} In early 1915, he was transferred to [[Fritz Haber]]'s new unit that would introduce clouds of chlorine gas as a weapon.<ref>{{cite journal|last1=Van der Kloot|first1=W.|title=April 1918: Five Future Nobel prize-winners inaugurate weapons of mass destruction and the academic-industrial-military complex|journal=Notes Rec. R. Soc. Lond.|date=2004|volume=58|issue=2|pages=149–160|doi=10.1098/rsnr.2004.0053|s2cid=145243958}}</ref> With [[Otto Hahn]], he was responsible for locating sites for the attacks.    He was awarded the [[Iron Cross]], Second Class, on 30 March 1915,<ref name="Papers">{{cite web |url=https://www.lib.uchicago.edu/e/scrc/findingaids/view.php?eadid=ICU.SPCL.FRANCK |title=Guide to the James Franck Papers 1882–1966 |publisher=[[University of Chicago]] |access-date=18 June 2015 }}</ref> and the city of Hamburg awarded him the [[Hanseatic Cross]] on 11 January 1916.<ref name="Papers" /> While in hospital with [[pleurisy]], he co-wrote yet another scientific paper with Hertz, and he was appointed an assistant professor in his absence by Frederick William University on 19 September 1916.  Sent to the [[Eastern Front (World War I)|Russian front]], he came down with [[dysentery]]. He returned to Berlin, where he joined Hertz, Westphal, [[Hans Geiger]], Hahn, and others at Haber's [[Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry]], working on the development of [[gas masks]].{{sfn|Lemmerich|2011|pp=52–58}} He was awarded the Iron Cross, First Class, on 23 February 1918. He was discharged from the army on 25 November 1918, soon after the war ended.<ref name="Papers" />

With the war over, Haber's Kaiser Wilhelm Institute now returned to research, and Haber offered Franck a job. His new post came with more pay, but was not a tenured position. It did however allow him to pursue his research as he wished. Working with new, younger collaborators such as [[Walter Grotrian]], Paul Knipping, Thea Krüger, [[Fritz Reiche]], and [[Hertha Sponer]], his first papers at the Kaiser Wilhelm Institute examined atomic electrons in their excited state, results that would later prove important in the development of the [[laser]].{{sfn|Lemmerich|2011|pp=61–64}} They coined the term ''[[metastable]]'' for atoms spending an extended time in a state other than [[ground state|that of least energy]].{{sfn|Kuhn|1965|pp=57–58}} When Niels Bohr visited Berlin in 1920, Meitner and Franck arranged for him to come to the Kaiser Wilhelm Institute to talk with the younger staff without the presence of the ''bonzen'' (bigwigs).{{sfn|Lemmerich|2011|pp=70–71}}

== Career in Göttingen ==
[[File:Franckfeier 1923 Die Bonzen.jpg|thumb|left|Left to right: {{Interlanguage link|Max Reich|de}}, [[Max Born]], James Franck, and [[Robert Pohl]] (the ''bonzen'') in 1923]]
In 1920, the [[University of Göttingen]] offered [[Max Born]] its Chair of Theoretical Physics, which had recently been vacated by [[Peter Debye]]. Göttingen was an important centre for mathematics, thanks to [[David Hilbert]], [[Felix Klein]], [[Hermann Minkowski]], and [[Carl Runge]], but not so much for physics. This would change. As part of his price for coming to Göttingen, Born wanted Franck to head experimental physics there. On 15 November 1920, he became Professor of Experimental Physics and Director of the Second Institute for Experimental Physics, a fully tenured ''professor ordinarius''. He was allowed two assistants, so he brought [[Hertha Sponer]] with him from Berlin to fill one of the positions, and Pohl, a gifted teacher, headed the First Institute and handled the lectures.{{sfn|Lemmerich|2011|pp=75–79}}<ref name="Nobel bio">{{cite web |url=http://nobelprize.org/physics/laureates/1925/franck-bio.html |title= James Franck – Biographical |publisher=The Nobel Foundation |access-date=16 June 2015 }}</ref> He refurbished the laboratory with the latest equipment using funds from his own pocket.{{sfn|Rice|Jortner|2010|p=7}}

Under Born and Franck, Göttingen was one of the world's great centres for physics from 1920 to 1933.<ref name="Nobel bio"/>{{sfn|Rice|Jortner|2010|p=7}} Although they published only three papers together, Born and Franck discussed every one of their papers with each other. Gaining admittance to Franck's laboratory became highly competitive. His doctoral students included [[Hans Kopfermann]], [[Arthur R. von Hippel]], [[Wilhelm Hanle]], [[Fritz Houtermans]], [[Heinrich Gerhard Kuhn|Heinrich Kuhn]], and [[Heinz Maier-Leibnitz]].{{sfn|Kuhn|1965|pp=58–59}} In supervising doctoral candidates, he had to ensure that thesis topics were well-defined, and would teach the candidate how to conduct original research, while still staying within the limits of the candidate's ability, the laboratory's equipment and the institute's budget.{{sfn|Lemmerich|2011|p=90}} Under his direction, research was carried out into the structure of atoms and molecules.{{sfn|Kuhn|1965|pp=62–63}}

In his own research, Franck developed what became known as the [[Franck–Condon principle]], a rule in [[spectroscopy]] and [[quantum chemistry]] that explains the intensity of [[vibronic transition]]s, simultaneous changes in electronic and vibrational energy levels of a molecule due to the absorption or emission of a [[photon]] of the appropriate energy. The principle states that during an [[electronic transition]], a change from one [[quantum vibration|vibrational]] [[energy level]] to another will be more likely to happen if the two vibrational [[wave functions]] overlap more significantly.{{sfn|Rice|Jortner|2010|pp=8–9}}<ref>{{cite journal | last= Franck |first=J. | date = 1926 | title = Elementary processes of photochemical reactions | journal = Transactions of the Faraday Society | volume = 21 | pages = 536–542 | doi = 10.1039/tf9262100536
 }}</ref> The principle has since been applied to a wide variety of related phenomena.{{sfn|Rice|Jortner|2010|pp=9–11}}

For his work during this time period, Franck was elected to the [[American Academy of Arts and Sciences]] in 1929.<ref>{{Cite web |date=9 February 2023 |title=James Franck |url=https://www.amacad.org/person/james-franck |access-date=22 May 2023 |website=American Academy of Arts & Sciences |language=en}}</ref>

== Middle life ==
This period came to an end when the [[Nazi Party]] won power in Germany in [[March 1933 German federal election|an election]] on 2 March 1933. The following month, it enacted the [[Law for the Restoration of the Professional Civil Service]], which provided for the retirement or dismissal of all Jewish civil servants, along with political opponents of the government. As a veteran of the First World War, Franck was exempt, but he submitted his resignation anyway on 17 April 1933.{{sfn|Lemmerich|2011|pp=188–194}} He once commented that science was his god and nature was his religion. He did not require his daughters to attend religious instruction classes at school,<ref name="David Nachmansohn 1979 62">{{harvnb|Nachmansohn|1979|p=62|quote=James Franck was born in Hamburg ... As he said, science was his God and nature his religion. He did not insist that his daughters attend religious instruction classes (Religionsunterricht) in school. But he was very proud of his Jewish heritage ...}}</ref> and even let them have a decorated tree at Christmas;{{sfn|Lemmerich|2011|p=132}} but he was proud of his Jewish heritage all the same.<ref name="David Nachmansohn 1979 62"/> He was the first academic to resign in protest over the law.{{sfn|Rice|Jortner|2010|p=12}} Newspapers around the world reported it, but no government or university protested.{{sfn|Lemmerich|2011|p=197}}

Franck assisted [[Frederick Lindemann]] in helping dismissed Jewish scientists find work overseas, before he left Germany in November 1933.{{sfn|Lemmerich|2011|pp=203–204}}  After a brief visit to the United States, where he measured the absorption of light in [[heavy water]] with Wood at [[Johns Hopkins University]], he took up a position at the [[Niels Bohr Institute]] in [[Copenhagen]].{{sfn|Lemmerich|2011|pp=209–214}} He needed a new collaborator, so he took on [[Hilde Levi]], whose recent thesis had impressed him.{{sfn|Schweber|2012|p=276}} His original intention was to continue his research into the fluorescence of vapours and liquids, but under Bohr's influence they began to take an interest in biological aspects of these reactions, particularly [[photosynthesis]], the process by which plants use light to convert [[carbon dioxide]] and water into more organic compounds. Biological processes turned out to be far more complicated than simple reactions in atoms and molecules. He co-authored two papers with Levi on the subject, which he would return to over the following years.{{sfn|Lemmerich|2011|pp=209–214}}<ref>
{{cite journal
 |year=1935
 |issn=0028-1042
 |journal=[[Naturwissenschaften]]
 |volume=23
 |issue=14
 |pages=229–230
 |doi=10.1007/BF01497533
 |title=Zum Mechanismus der Sauerstoff-Aktivierung durch fluoreszenzfähige Farbstoffe
 |trans-title= On the mechanism of oxygen activation by fluorescence dyes
 |last1=Franck |first1=J.
 |last2=Levi |first2= Hilde |author-link2=Hilde Levi
 |language=de
 |bibcode = 1935NW.....23..229F |s2cid=12659462
}}</ref><ref>
{{cite journal
 |last1=Franck |first1= J.
 |last2=Levi |first2= Hilde |author-link2=Hilde Levi
 |title=Beitrag zur Untersuchung der Fluoreszenz in Flüssigkeiten
 |trans-title=Contribution to the study of fluorescence in liquids
 |journal=[[Zeitschrift für Physikalische Chemie]]
 |issn=0942-9352
 |volume=B27
 |pages=409–420
 |year=1935
 |language=de
}}</ref>

Franck found a position at the ''[[Technical University of Denmark|Polytekniske Læreanstalt]]'' in Copenhagen for Arthur von Hippel, who was now his son in law, having married his daughter Dagmar. He decided to provide financial security for his children by dividing his Nobel Prize money between them. The gold medal itself was entrusted to Niels Bohr for safekeeping.{{sfn|Lemmerich|2011|pp=218–219}} When Germany [[German invasion of Denmark (1940)|invaded Denmark]] on 9 April 1940, the Hungarian chemist [[George de Hevesy]] dissolved the gold medal, along with that of [[Max von Laue]] in [[aqua regia]] to prevent the Germans from taking them. He placed the resulting solution on a shelf in his laboratory at the Niels Bohr Institute. After the war, he returned to find the solution undisturbed and precipitated the gold out of the acid. The Nobel Society then recast the Nobel Prize medals.{{sfn|de Hevesy|1962|pp=27–28}}<ref>{{cite web |url=http://www.gamma.nbi.dk/Galleri/gamma150/stephanschwarz.pdf |title=The Case of the Bottled Nobel medals |first=Stephan |last=Schwartz |publisher=[[Niels Bohr Institute]] |access-date=20 June 2015 |archive-url=https://web.archive.org/web/20140116141809/http://www.gamma.nbi.dk/Galleri/gamma150/stephanschwarz.pdf |archive-date=16 January 2014 |url-status=dead }}</ref>

In 1935, Franck moved to the United States, where he had accepted a professorship at Johns Hopkins University.{{sfn|Kuhn|1965|pp=63–64}} The laboratory there was poorly equipped compared to the one in Göttingen, but he received $10,000 for equipment from the [[Rockefeller Foundation]]. A more intractable problem was that the university had no money to hire skilled staff. Franck was concerned about his family members remaining in Germany, and needed money to help them emigrate. He therefore accepted an offer from the [[University of Chicago]], where his work on photosynthesis had attracted interest, in 1938.{{sfn|Lemmerich|2011|pp=223–224}}

Franck's first paper there, co-authored with [[Edward Teller]], was on photochemical processes in crystals.{{sfn|Lemmerich|2011|p=229}} [[Hans Gaffron]] became his collaborator.{{sfn|Lemmerich|2011|pp=233–235}} They were joined by Pringsheim, who escaped from Belgium after the [[Battle of Belgium|German invasion]]. Franck arranged a position for Pringsheim at his laboratory.{{sfn|Lemmerich|2011|p=238}} Both his daughters and their families moved to the United States, and he was also able to bring out his elderly mother and aunt.{{sfn|Lemmerich|2011|p=233}} He became a [[naturalization|naturalized]] US citizen on 21 July 1941,<ref name="Papers"/> so he was not an [[enemy alien]] when the United States [[United States declaration of war upon Germany (1941)|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.{{sfn|Lemmerich|2011|p=237}}

In February 1942, [[Arthur Compton]] established its [[Metallurgical Laboratory]] at the University of Chicago. As part of the [[Manhattan Project]], its mission was to build [[nuclear reactor]]s to create [[plutonium]] that would be used in [[atomic bomb]]s.{{sfn|Compton|1956|pp=82–83}} The Metallurgical Laboratory's Chemistry Division was initially headed by [[Frank Spedding]], but he preferred hands on work to administration. Compton then turned to Franck, with some trepidation owing to his German background.{{sfn|Compton|1956|pp=123–124}}  Compton later wrote:

{{blockquote|How Franck welcomed an invitation to join our project! It was a vote of confidence that far exceeded his hopes, and it gave him a chance to do his part for the cause of freedom. "It's not the German people I'm fighting", he explained. "It's the Nazis. They have a stranglehold over Germany. The German people are helpless until we can break the strength of their Nazi masters." The chemists welcomed Franck as an elder scientific statesman whose guidance they were glad to follow.{{sfn|Compton|1956|p=124}} }}

In addition to heading the Chemistry Division, Franck was also the chairman of the Metallurgical Laboratory's Committee on Political and Social Problems regarding the atomic bomb, which consisted of himself and [[Donald J. Hughes]], [[J. J. Nickson]], [[Eugene Rabinowitch]], [[Glenn T. Seaborg]], J. C. Stearns and [[Leó Szilárd]].<ref name="Nina Byers" /> In 1945, Franck warned [[Henry A. Wallace]] of their fears that "mankind has learned to unleash atomic power without being ethically and politically prepared to use it wisely."{{sfn|Rice|Jortner|2010|p=16}} The committee compiled what became known as the [[Franck Report]]. Finished on 11 June 1945, it recommended that the [[atomic bomb]]s not be used on the Japanese cities without warning.<ref name="Nina Byers">{{cite journal |journal=CERN Courier |date=13 October 2002 |url=https://arxiv.org/html/physics/0210058 |title=Physicists and the 1945 Decision to Drop the Bomb |first=Nina |last=Byers |author-link=Nina Byers |access-date=20 June 2015 |bibcode=2002physics..10058B |arxiv=physics/0210058 }}</ref> In any event, the [[Interim Committee]] decided otherwise.{{sfn|Compton|1956|pp=235–236}}

== Later life ==
[[File:Portrait of Albert Einstein, Niels Bohr, James Franck and Rabi.jpg|thumb|right|Left to right: [[Niels Bohr]], James Franck, [[Albert Einstein]], and [[Isidor Rabi]] in 1954]]

Franck married [[Hertha Sponer]] in a civil ceremony on 29 June 1946,{{sfn|Lemmerich|2011|p=259}} after his first wife, Ingrid, died in 1942. In 1947, he became professor emeritus at the [[University of Chicago]], but continued to work there to tackle the problem of explaining the mechanism of [[photosynthesis]]. Meitner saw no break between his early and later work. She recalled that:

{{blockquote|Franck enjoyed talking about his problems, not so much to explain them to others as to satisfy his own mind. Once a problem had aroused his interest he was completely captivated, indeed obsessed by it. Common sense and straight logic were his main tools, together with simple apparatus. His research followed an almost straight line, from his early studies of ion mobilities to his last work on photosynthesis; it was always the energy exchange between atoms or molecules that fascinated him.{{sfn|Rice|Jortner|2010|pp=17–18}} }}

In addition to the Nobel Prize, Franck was awarded the [[Max Planck Medal]] of the [[German Physical Society]] in 1951, and the [[Rumford Prize]] of the [[American Academy of Arts and Sciences]] in 1955 "for his studies in the investigation of photosynthesis". He became an honorary citizen of [[Göttingen]] in 1953,{{sfn|Rice|Jortner|2010|p=20}} was elected a [[Member of the National Academy of Sciences]] in 1944,<ref>{{Cite web |title=James Franck |url=http://www.nasonline.org/member-directory/deceased-members/20001816.html |access-date=22 May 2023 |website=www.nasonline.org}}</ref> and elected a [[List of Fellows of the Royal Society elected in 1964|Foreign Member of the Royal Society (ForMemRS) in 1964]].{{sfn|Kuhn|1965|pp=67–68}} He was also an International Member of the [[American Philosophical Society]].<ref>{{Cite web |title=APS Member History |url=https://search.amphilsoc.org/memhist/search?creator=Franck&title=&subject=&subdiv=&mem=&year=&year-max=&dead=&keyword=&smode=advanced |access-date=22 May 2023 |website=search.amphilsoc.org}}</ref>

Franck died suddenly of a heart attack while visiting [[Göttingen]] on 21 May 1964 at the age of 81.{{sfn|Kuhn|1965|pp=67–68}} He is buried in [[Chicago]] with his first wife.{{sfn|Lemmerich|2011|p=309}}

In 1967, the [[University of Chicago]] named the [[James Franck Institute]] after him.<ref>{{cite web |url=http://jfi.uchicago.edu/aboutinstitute/index.shtml |title=About the Institute |publisher=James Franck Institute |access-date=20 June 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150620070031/http://jfi.uchicago.edu/aboutinstitute/index.shtml |archive-date=20 June 2015 }}</ref> [[Franck (crater)|A lunar crater]] has also been named in his honour.<ref>{{cite web |url=https://planetarynames.wr.usgs.gov/Feature/2012?__fsk=-1273938748 |title=Planetary Names: Crater, craters: Franck on Moon |publisher=[[USGS]] and [[NASA]] |access-date=20 June 2015 }}</ref> His papers and recast Nobel Prize medal are located in the [[University of Chicago Library]].<ref name="Papers"/>

== See also ==
* [[List of Jewish Nobel laureates]]

== Notes ==
{{Reflist|30em}}

== References ==
* {{cite book |last=Compton |first=Arthur |author-link=Arthur Compton |year=1956 |title=Atomic Quest |url=https://archive.org/details/atomicquestperso0000comp |url-access=registration |location=New York |publisher=Oxford University Press |oclc=173307 }}
* {{cite book |last=de Hevesy |first=Georg |author-link=George de Hevesy |year=1962 |title=Adventures in Radioisotope Research |location=New York |publisher=Pergamon Press |oclc=12162216 |url=https://archive.org/stream/adventuresinradi01heve#page/27/mode/1up/search/medals |access-date=20 June 2015 }}
* {{cite journal |last=Kuhn |first=Heinrich Gerhard |author-link=Heinrich Gerhard Kuhn |title=James Franck. 1882–1964 |journal=[[Biographical Memoirs of Fellows of the Royal Society]] |volume=11 |pages=53–74 |date=November 1965 |jstor=769261 |doi=10.1098/rsbm.1965.0004 |doi-access= |s2cid=71347884 }}
* {{cite book |last=Lemmerich |first=Jost |others=Translated by Ann M. Hentschel |year=2011 |title=Science and Conscience: The life of James Franck |location=Stanford, California |publisher=Stanford University Press |isbn=978-0-8047-7909-8 }}
* {{cite book|last=Nachmansohn|first=David |author-link=David Nachmansohn |title=German-Jewish pioneers in science, 1900–1933: highlights in atomic physics, chemistry, and biochemistry|date=1979|location=New York|publisher=Springer-Verlag|isbn=978-0-38790-402-3|url-access=registration|url=https://archive.org/details/germanjewishpion0000nach}}
* {{cite web |last1=Rice |first1=Stuart A. |author-link1=Stuart A. Rice |last2=Jortner |first2=Joshua |author-link2=Joshua Jortner |year=2010 |url=http://www.nasonline.org/publications/biographical-memoirs/memoir-pdfs/franck-james.pdf |title=James Franck 1882–1964: A Biographical Memoir |publisher=U. S. National Academy of Sciences |access-date=16 June 2015 }}
* {{cite book |last=Schweber |first=Silvan S. |author-link=Silvan S. Schweber |title=Nuclear Forces: The Making of the Physicist Hans Bethe |publisher=Harvard University Press |location=Cambridge, Massachusetts |year=2012 |oclc=758383322 |isbn=978-0-674-06587-1 }}

== Further reading ==
* {{cite book |last1=Medawar |first1=Jean |author-link1=Jean Medawar |last2=Pyke |first2=David |title=Hitler's Gift: The True Story of the Scientists Expelled by the Nazi Regime |location=New York |publisher=Arcade Publishing |date=2012 |isbn=978-1-61145-709-4 |ref=none}}

== External links ==
* [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4609-1 Oral history interview transcript with James Franck on 9 July 1962, American Institute of Physics, Niels Bohr Library & Archives]—Session I
* [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4609-2 Oral history interview transcript with James Franck on 10 July 1962, American Institute of Physics, Niels Bohr Library & Archives]—Session II
* [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4609-3 Oral history interview transcript with James Franck on 11 July 1962, American Institute of Physics, Niels Bohr Library & Archives]—Session III
* [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4609-4 Oral history interview transcript with James Franck and Hertha Sponer Franck on 12 July 1962, American Institute of Physics, Niels Bohr Library & Archives]—Session IV
* [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4609-5 Oral history interview transcript with James Franck and Hertha Sponer Franck on 13 July 1962, American Institute of Physics, Niels Bohr Library & Archives]—Session V
* [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4609-6 Oral history interview transcript with James Franck and Hertha Sponer Franck on 14 July 1962, American Institute of Physics, Niels Bohr Library & Archives]—Session VI
* {{Nobelprize}} including the Nobel Lecture, 11 December 1926: "Transformation of Kinetic Energy of Free Electrons into Excitation Energy of Atoms by Impacts"

{{Nobel Prize in Physics Laureates 1901–1925}}
{{1925 Nobel Prize winners}}
{{Manhattan Project}}
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{{DEFAULTSORT:Franck, James}}
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