Two new experimental techniques have been developed and used to measure the differential Stark shifts between the Zeeman sublevels in excited atomic states. For states whose differential Stark shifts, in uniform electric fields attainable in the laboratory, are comparable to the hfs separation, the method of pure electricfield level crossing may be used. This method has been applied to the 6p 2 P 3 /2 state of rubidium and the lp 2 P 3 /2 state of cesium. The differential shifts are E(dbi)-JE(=fci)=0.521±0.021 Mc/(kV/cm) 2 in Rb, and 1.077±0.043 Mc/(kV/cm) 2 in Cs. For isotopes with no hyperfine structure (1 = 0) or states whose Stark shifts are small compared to their hfs, we have used the level-crossing technique with parallel electric and magnetic fields. This technique has been employed to measure the differential shifts in the 5s5p 3 Pi state of cadmium and the 6s6p 3 Pj state of mercury. The results are E(dbl) -E(0) = -2.550±0.105 kc/(kV/cm) 2 in Cd, and -2.355±0.090 kc/(kV/cm) 2 in Hg. The theory of quadratic Stark shifts in terms of scalar and tensor operators is presented, and the shifts in these four elements are calculated using the Coulomb approximation for the potential of the outer electron. The agreement between the experimental and the theoretical values is satisfactory.

Measurements have been made of the hyperfine structure (hfs) constants and lifetimes of the first and second P&~2 states of the stable alkali atoms, except lithium, using the pure magnetic field level-crossing technique. The results for Na are State 4 Py2 a {MHg) 18.9 +0.3 6. 2+0. 2 b (MHz) 2. 4+ 0.3 1.0 +0.1 &(nsec) 16.0+0.5 95 +4 The results for K have been reported previously. For Rb and Cs, hfs constants were obtained that agreed with, but were less accurate than, previous work. The measured lifetimes are Element Rb" State 5 P3i2 6'P3 6 P3/2 2 7'PW2 &{nsec) 25. 5+0.5 118 +4 32. 7+1.5 134.5 +2. 8The error limits in these measurements are two standard deviations. The above results were obtained by fitting the observed level-crossing signal to a theoretical line shape computed from the Breit formula, using a, b, and 7 as parameters.

New YorkIn this Letter we report on the first observation of pure electric-field level crossings at finite electric field and the use of these observations to determine the differential Stark effect in the second excited state of the alkali metal atoms. In previous work Stark parameters have been measured by level crossing in combined electric and magnetic fields. ^?^ This work is the first experimental demonstration that the application of an electric field will cause certain hyperfine structure levels to intersect at other than zero electric field. These experiments are similar to the magnetic field level-crossing experiments in that if levels with m^ differing by one or two can be made to intersect by an electric field, then a change in the angular distribution and the polarization of the scattered resonance radiation will occur. Since our technique compares the shift of the levels due to an electric field with the hfs, the Ms of the excited state must be known in order to deduce the magnitude of the differential Stark effect.