It is shown that destructive interference between two coherent excitation pathways from different optical harmonics, a fundamental and a third harmonic, inhibits resonance-enhanced multiphoton ionization. Once the role of the third harmonic is realized, the interference follows naturally from simple considerations of Fermi's "golden rule" and Maxwell's equations.

• Received 17 June 1982

DOI:https://doi.org/10.1103/PhysRevLett.49.543