Controlling translational motion of atoms and molecules is an interesting subject for study in recent years. The Stark effect in Rydberg states is applicable to such a purpose as proposed by Breeden and Metcalf [1] and as indeed demonstrated for H₂ molecules by Softley and co-workers [2]. Although the DC Stark effect has been extensively studied for rare-gas atoms, experimental knowledge is limited to high-n Rydberg states (39≤n in ref 3 and the recurrence property). This paper reports spectroscopic results on the Stark effect of Rydberg states in the range 15≤n≤40 for triplet helium atoms, and describes field ionization dynamics on the Stark map (energy level diagram as a function of electric field) when a pulsed electric field of 6000 V/cm is applied. Magnetic quantum number dependence on spectroscopic features in the Stark spectra is discussed in connection with avoided-crossed Stark levels for m =0 and |m|=1, respectively.
The helium atoms are first excited from the ground state to the intermediate 2³S state by DC electric discharge, and then photo-excited with an ultraviolet nano-second pulsed laser (λ=260-263 nm). A signal is detected through pulsed field ionization by the electric field. Stark manifolds are clearly observed in the region between the classical saddle point energy of the Stark field and the adiabatic field ionization limit by the pulsed field. The experimental spectra are fairly well reproduced by a simple calculation based on matrix diagonalization of the Hamiltonian for both m =0 and |m|=1 states.
Possibility for the diabatic ionization at the avoided crossings is negligibly small for the ionizing pulsed field of a slew rate S=4.0×10¹⁰ Vcm^-1s^-1. Rydberg states n≥16 are ionized by a pulsed electric field of 6000 V/cm adiabatically, whereas state n=15 is considered to be ionized through tunnelling ionization. Some of the sub-levels of the n=16 manifold are not observed in the |m|=1 case. The unobserved sub-levels represent a cut-off by the adiabatic ionization threshold; the difference of the position of the classical saddle point energy is caused by the centrifugal barrier. The high-n Rydberg triplet states studied here radiatively decay to the metastable 2³S state of which lifetime is 4200 s. Hence controlling the triplet Rydberg He atoms could lead to new experimental studies with He (2³S) such as collisions of a controlled beam of the metastable atoms.

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