# Challenge to Dirac: Journal of Chemical Physics Editor's Pick

- Dec 4, 2018
- Katharine Hunt

A recent paper by Professor Katharine L. C. Hunt and post-doc Dr. Anirban Mandal in the Department of Chemistry has been chosen as an EDITOR’S PICK in the Journal of Chemical Physics. The paper appeared online on November 29, 2018.

What is the probability of a transition, when an external electromagnetic field acts on a molecule? For more than 90 years, the theory developed by P. A. M. Dirac in 1926 and 1927 has been nearly universally accepted. Expanding on an approach suggested by Landau and Lifshitz, Hunt and Mandal have challenged Dirac’s theory. In place of Dirac’s use of the entire excited-state coefficient in the wave function as the transition probability amplitude, Hunt and Mandal concluded that only the nonadiabatic part of the coefficient characterizes actual transitions. They examined the response of a quantum system to a perturbing “plateau pulse,” which rises to a level value and remains constant for a period.

While the perturbation is constant, no transitions can occur, so the transition probability should also be constant. Hunt and Mandal obtained the results in the figure for the transition probability. At the top of the figure, Dirac’s form of the transition probability oscillates during the plateau, while in the lower part, the nonadiabatic transition probability given by Hunt and Mandal is constant, as required physically.

The paper selected as an Editor’s Pick by the Journal of Chemical Physics: A. Mandal and K. L. C. Hunt, J. Chem. Phys. 149, 204110 (2018).