Spectroscopic study and multichannel quantum defect theory analysis of the Stark effect in Rydberg states of neon

An experimental and theoretical investigation of the Stark effect in bound and autoionizing Rydberg states of 20Ne is presented. A narrow-band VUV laser source was used to excite Rydberg states with n = 23–26 in a resonance-enhanced two-photon excitation sequence via either the 3s[3/2]1 or the 3s'[1/2]1 intermediate states. Adjusting the relative polarization of the laser beams with respect to the applied electric fields (F = 0 − 250 V cm−1) made the selective excitation of MJ = 0 and MJ = 1 Stark states possible. Multichannel quantum defect theory (MQDT) calculations including the effects of the external electric fields were performed in order to identify all transitions in the experimental spectra and to determine the eigenquantum defects μα of the np(J = 1, 2) Rydberg series. The high resolution of the experiment in combination with the knowledge of the total magnetic quantum number MJ of the final Stark states allowed a detailed comparison of the experimental results and those calculated by MQDT. The agreement is generally good but some differences between the observed and calculated quadratic Stark shifts of low-l states point to a need for re-examination of the MQDT approximations.