Observation of field phase dependent autoionization

We report on the observation of a held phase dependent autoionization rate of calcium in the region of the 4p7s[1/2](1)(0) doubly excited state. Excit ation of the autoionizing state occurs from the atomic ground state through two phase related and hence interfering channels, namely a three photon ch annel (3 (H) over bar omega) and a single photon channel ((H) over bar omeg a(3)) omega(3) being the third harmonic of omega. The autoionization rate e xhibits a sinusoidal modulation as a function of the relative phase of the two excitation fields. Both ionizing Fields me not focused in the interacti on region, thus demonstrating the possibility of phase control in a large i nteraction volume and free of phase shift effects associated with focused g eometries.