Stark ionization of high-lying Rydberg states of sodium

We have used stepwise excitation in an atomic beam to excite slow-moving atoms to pure high-lying quantum states at densities low enough to eliminate collisional effects. The atoms were detected with high efficiency by Stark ionization. Results are given of a study of the threshold field ~ for ionization of s-states of sodium with principal quantum numbern from 26 to 37. The sodium atoms in an atomic beam were excited stepwise by two pulsed dye lasers pumped by a common nitrogen laser. The first dye laser was tuned to the D I line (5890 ~), while the second laser (~4100 ~) caused transitions from the p-state to high-lying s or d states. The highly excited atoms were detected by direct ionization in an applied Stark field. The laser beams intersected the atomic beam between electric field plates. A pulsed ionizing field was applied after laser excitation, and the resulting ions were observed with a channel electron multiplier. In addition to avoiding the problem of signal loss due to long radiative lifetime, this method provides close to 100% detection efficiency and very low background. The approximate ionization field required for s and d stated with principal quantum number n was (16n4) -I a.u. (~390 V/cm for n=30). Resolved s and d levels up to n=60 have been observed. We have studied the ionization probability as a function of electric field for levels n=26 to n=37. For each s and d state, a greater value of applied field was required to ionize the atoms, than that obtained from the simple result Ecrit = (16n'4) -I, where n* is the effective quantum number . This difference is attributed to the Stark effect at ionization. The present problem has generated great interest over the years as it represents the extreme case of distortion of a free atom by an electric field 0] . For s levels, where the onset of ionization is a sharp function of applied field, we could derive values for the Stark shift at ionization. A simple semi-empirical analysis gives AW(Stark) = 5.6 x 10 -5 a.u. for the 30s level. We have also used the fact that optical selection rules for stepwise two-photon processes are strongly affected by nuclear coupling in the intermediate state. The first laser pulse creates a coherent superposition state of the P3~ level hyperfine states since it is short compared to~i~ I, where the Aw~,s are the characteristic hyperfine splittings of the P3/2 level. T~e time evolution of this superposition state can be ~robed by means of resonant absorption from the second pulsed (~4100 A) laser having a variable delay with respect to the pulse from laser I. If both lasers are circularly polarized in the same sense, for example, these oscillations can be monitored by measuring the population of a high nsl/2 state as a function of the delay. This enables one to measure the hyperfine structure in the intermediate state, and provides an example for a general spectroscopic technique. One aspect of this phenomenon was used to excite selectively high-lying d states. If the lower laser pulses occur in rapid succession the selection rules for dipole transition are those for the case of no nuclear spin. Excitation of high-lying nsl/2 levels is then suppressed if both lasers are circularly polarized in the same sense.