For an electron (mass , charge ) bound by a harmonic potential and acted upon by a constant external electric field , the Schrödinger equation can be written as . An exact solution can be obtained by completing the square in the potential energy [1]: . Introducing the new variable , the Schrödinger equation can be written as , , making use of the known solution of the standard harmonicoscillator problem, expressed in terms of . The perturbed energies are shifted downward by a constant term: . The graphic shows the potential energy and energy levels for the unperturbed (in black) and perturbed (in red) oscillator, for selected values of and . For simplicity, atomic units, , are used. If the electric field is turned on during a time interval that is short compared to the oscillation period , the sudden approximation in perturbation theory can be applied [2]. Accordingly, the transition probability from state to a state is given by . These results can be seen by selecting "show transition probabilities" and the initial state .
The unperturbed harmonic oscillator has the eigenfunctions (in atomic units): , , where is a Hermite polynomial. The perturbed oscillator has an analogous form, with a shifted argument: , . The transition probability according to the sudden approximation is given by , where . [1] C. CohenTannoudji, B. Diu and F. Laloë, Quantum Mechanics, Vol. I (S. R. Hemley, N. Ostrowsky and D. Ostrowsky, trans.), New York: Wiley, 1977 pp. 552–554. [2] L. D. Landau and E. M. Lifshitz, Quantum Mechanics Nonrelativistic Theory, 2nd ed. (J. B. Sykes and J. S. Bell, trans.), New York: Pergamon Press, 1965 pp. 142–143.
