We observed well-resolved spectral features in the stepwise ionization of Na in an applied electric field near the zero-field ionization threshold. A 5890-A pulse excites atoms to the $3{}_{}{}^2{}_{}{}^{}P$ state and a~4085-A pulse, swept through threshold ionizes them. Using the method of Luc-Koenig, we calculated the oscillator strength density versus blue laser frequency and compared it with our data. The agreement is excellent. Parabolic quantum numbers have been assigned to the spectrum and the Fano profiles have been analyzed.

• Received 6 April 1981

DOI:https://doi.org/10.1103/PhysRevLett.47.83