J Dependence of Isotope Shifts at High-Lying Levels of Gd I

Abstract

In the past decades, gadolinium has been the subject of much interest to spectroscopists as it possesses rich optical transitions with various atomic configurations associated with 4f, 5d, 6s, and 6p open electrons. The higher-order effect of the isotope shift (IS), i.e., the crossed-second-order (CSO) effect, has been an interesting subject, which results in the J and term dependences of ISs. From the point view of nuclear laser spectroscopy, understanding of such a CSO effect is indispensable in deriving nuclear information from measured ISs. On the other hand, recent theoretical calculations of ISs using the configuration interaction method and many-body perturbation theory have reached high precision even for heavy elements with a few valence electrons such as Mg, Na, K, and Ca. High-order effects such as the quantum electrodynamic correction and the nuclear polarizability have also been calculated very recently for Li and He. The CSO effects in the ground configurations of Gd I 4f5d6s and Gd II 4f5d6s were studied early and those in the 4f5d6s and 4f6s6p configurations of Gd I with lower excited energies were reported later. The J dependences of ISs in the D and F terms of 4f5d6s6p at energies of about 18000 cm 1 were also measured. These experiments were performed in the visible and nearinfared regions. Recently, IS measurements at wavelengths of about 405 nm have been reported. However, the J dependences of ISs at high-lying levels such as at energies of about 26000 cm 1 have not yet been reported. Such J dependences, particularly for high-lying levels with a possibly strong configuration mixing, will provide a challenge for atomic theoretical calculation. In this paper, we report the high-resolution atomic-beam laser spectroscopy of Gd I in the ultraviolet (UV) region. ISs are measured for seven UV transitions including the previously reported three transitions. The J dependences of ISs in the three atomic configurations, related to high-lying levels, are obtained and discussed. The present experiment was performed using a UV laser beam and an atomic beam. Using a cw frequency doubler (Spectra-Physics WAVETRAIN), a UV laser beam with a wavelength of about 394 nm was obtained by the frequency doubling of a diode-laser beam produced from a commercial tunable diode laser (Newport 2010M). An atomic beam was produced by heating a molybdenum oven using an electronbombardment method and was made to intersect with a laser beam perpendicularly. Fluorescence from the atomic beam was detected with a cooled photon-counting photomultiplier (Hamamatsu R2257P). A confocal Fabry–Perot interferometer (FPI) with a free spectral range of 300MHz was used for relative frequency calibration. The experimental setup is essentially identical to that used in our previous work. Seven transitions in Gd I were studied in this experiment. Figure 1 shows the wavelengths of studied transitions together with atomic configurations, terms and total electronic angular momentums J of their lower and upper levels. These transitions are all from the ground D term of the 4f5d6s configuration, and their upper levels, with energies of about 26000 cm , relate to three configurations of 4f5d6s6p, 4f5d6p and 4f5d6s with four different terms of D, G, G, and G. Figure 2 shows the observed fluorescence spectrum of the 4f 5d6s D3–4f 5d6p G4 transition at 394.263 nm. It can be seen from Fig. 2 that all peaks are clearly observed for even-mass isotopes, including the lowest abundance (0.20%) isotope Gd. For the 395.868, 394.554, 395.337, and 394.180 nm transitions, no peaks of Gd could be observed owing to their weak transition intensities. The full width at half maximum (FWHM) of the peaks is about 23MHz, which is mainly due to the natural width of the upper level of transition and the residual Doppler broadening of the atomic beam. For measured spectra, peak centers were determined from a least-squares fit with a Lorentz function and calibrated with the FPI spectrum. For each transition, measurement was performed about 20 times. Thus, the ISs between evenmass isotopes were obtained and are presented in Table I for the seven transitions studied; the ISs of three transitions were reported in our previous paper. The uncertainties of the measured ISs, 1–6MHz, include the error of peak-center determination, the error of the free spectral range of the FPI (0.046MHz), and the error of linearity correction for frequency scanning. Hyperfine structures of the odd-mass isotopes Gd and Gd are discussed elsewhere. The IS difference between different transitions with identical lower levels yields the IS difference between different upper levels, i.e., the residual IS Tres. For example, the IS difference between the 394.324 and 394.557 nm transitions yields the IS difference between the upper D1 D2 D3 D5 D4 D6 D3 D2 D1 G4 G6 G4 G6