Quantum Beats in the Electric-Field Mixing of the n = 2 States of Atomic Hydrogen

Abstract

The Lyman-alpha “Stark beats” produced by the coherent decay of the perturbed 2s and 2p states has been extensively studied, for example in collisions of electrons with atoms,¹ in heavy particle collisions^2,3 and beam-foil^4,5,6 or beam-gas⁷ excitation. The 2s_1/2 states of hydrogen is metastable with a field-free lifetime of 0.14 sec. It has four hyperfine states which will mix in an electric field with twelve possible 2p_1/2 and 2p_3/2 states with lifetimes of 1.6 nsec which can radiatively decay to the 1s_1/2 ground state with the emission of a Lyman-alpha photon. If the mixing field is applied suddenly compared to the response time of the atom, the decay radiation shows quantum beats. The pattern of the beats contains large oscillations near the Stark shifted 2s_1/2 - 2p_1/2 frequency (about 1,000 MHz) and the envelope of these oscillations is modulated at the hyperfine splitting of the 2s_1/2 state (about 180 MHz)and each peak contains rapid oscillations near the 2s_1/2 - 2p_1/2 frequency (about 10,000 MHz). Each frequency is a group of frequencies arising from the Stark and hyperfine splitting of the states. The above types of measurements provide information on the coherence of the excitation process and on the fine and hyperfine beat structure but they are limited in providing only the products of excitation amplitudes averaged over all scattering angles and the initial state amplitudes and their phases in the beam emerging from the foil are not adequately known.