New measurements of Rydberg spectrum of singlet Mg I in the 120–150 GHz frequency range

Several dozens of Rydberg recombination lines of Mg I were detected in emission spectrum of the Sun [1]. The lines were observed in far infrared and were assigned to the transitions between n = 5 - 8 and L <; 8 states but the assignment was tentative. This inspired a number of works on microwave spectroscopy of states with high L. In laboratory conditions Lyon et al. [2] observed and measured frequencies of Rydberg microwave transitions of Mg I for n = 17 - 23 between f-,g-,h-,i- states also determining dipole polarizability of Mg+. Snow et al. [3] for n = 17 measured transition frequencies between 6 ≤ L ≤ 11 states. There was a number of spectroscopic investigations of Mg in optic range published after analysis of Mg I spectrum performed by Martin and Zalubas for NIST database in 1980 [4]. Beigang and Schmidt performed an optic spectroscopic investigation of D state of Mg I for principal quantum number n range from 15 to 43 [5]. In the works of Ficher [6] and Lu [7] it was found that 3snd¹D₂ term is highly perturbed by doubly excited configuration 3p²¹D₂3. The largest interaction is observed for n = 3 and the perturbation decreases with increase of principal quantum number. Using methods of high precision infrared laser spectroscopy Lemoine et al. have recorded 23 Rydberg transitions of Mg I in the infrared range 740 ÷ 1126 cm^-1 [8]. It was found that fine structure of f-states with n = 5 - 7 is inverted. In addition this work as well as the calculations performed in [1] gave an opportunity to explain some previously unassigned lines in the Sun emission. Beigang et al. [9] have undertaken investigation of Rydberg F state of Mg using method of Doppler free laser spectroscopy that resulted in determination of isotopic shifts, magnitude of singlet-triplet splitting and hyperfine structure splitting of ²⁵Mg isotope in the range of principal quantum number n= 14-84. In this ra- ge of principal quantum number the inversion of the fine structure was not observed anymore. Rafiq et al. [10] have determined quantum defects for S,P,D and F states measuring in the course of optic range investigation also the energies of S, P, D, F terms up to n = 24,61,62,66 respectively. Nevertheless the calculations of microwave transition frequencies using results from [10] and comparison with our experimental results reveals differences up to several GHz. Aim of our work is to obtain reliable accurate data on microwave Rydberg spectra of Mg I.