The problem that the strong-field approximation (SFA) in the length gauge (LG) cannot give the correct molecular harmonic generation spectrum when the internuclear distance is large is studied. By introducing the first excited state in addition to the ground state in the SFA and a procedure to resum translation invariance, the problem is resolved and the superiority of the LG to the velocity gauge in this case is discussed.

• Received 25 December 2006

DOI:https://doi.org/10.1103/PhysRevA.75.041402