High-Resolution Ultraviolet Laser Spectroscopy in Atomic Erbium

Abstract

High-resolution laser spectroscopy has been proved to be a powerful technique to study the isotope shift (IS) and hyperfine structure (hfs). From such study, one can obtain atomic and nuclear information such as atomic configuration, hfs constants, electron density, nuclear moments and charge distribution; those are indispensable for understanding atomic and nuclear structures. Up to now, many studies have been reported in the visible and near-infrared regions. Spectroscopic measurement in the ultraviolet (UV) region has been scarcely performed. The IS, hfs, and even the atomic configuration are not yet well understood for the UV region which corresponds to high-lying atomic levels at about 26000 cm . Spectroscopic data for such high-lying levels would lead to a challenge for the theoretical calculations of many-electron atoms such as Er as successful calculations of the IS have been recently reported for atoms with a few valence electrons such as Li, He, and Mg. The hfs constants of the Er ground state have been precisely measured by using the laser-rf double resonance technique. Isotope shifts and hyperfine structures of some lower-lying levels in the visible region have been measured by means of atomic-beam laser spectroscopy. However, no experiment has been reported yet in the UV region for Er. In this paper, we report high-resolution atomic-beam laser spectroscopy of Er I in the UV region. ISs are measured for four UV transitions and the Er hfs constants are newly determined for two high-lying levels. From the IS, atomic configuration of the energy level at 25159.143 cm 1 has been assigned. The present experiment was performed by using an atomic beam together with a UV laser beam. An atomic beam was produced by using an electron-bombardment heating method. A molybdenum oven with a 1-mm-diameter hole was heated to about 1400 C and the atomic beam was collimated by a 2-mm-diameter aperture at a distance of 30 cm from the oven. A laser beam with a wavelength range of 780–810 nm was produced by using a commercial tunable diode laser with an external cavity system (Newport 2010M). A UV beam with a wavelength range of 390–405 nm was obtained by the frequency doubling of the diode-laser beam using a cw frequency doubler (Spectra-Physics WAVETRAIN). The power of the UV laser beam was about 30 mW for the 20mW diode-laser beam. The laser beam was collimated and crossed the atomic beam perpendicularly. Fluorescence induced by the incident laser beam was collected with a spherical mirror and detected with a cooled photon-counting photomultiplier (Hamamatsu R2257P). An interference filter (40 nm width at 400 nm) was used in front of the photomultiplier and, therefore, the background from the intense oven light was greatly reduced. For relative frequency calibration, the laser light transmitted through a confocal Fabry–Perot interferometer (FPI) with a free spectral range of 300MHz was recorded simultaneously with the fluorescence from the atomic beam. The experimental setup has been partially described elsewhere. Four transitions in Er I were studied in this experiment. Wavelengths of the transitions, atomic configurations and energies of the lower and upper levels are listed in Table I. The 393.701-nm transition is from the ground configuration 4 f 6s to the upper 4 f 5d6s configuration. The 394.442and 395.642-nm transitions are also from the ground configuration 4 f 6s to the upper 4 f 5d6s configuration. For the 397.358-nm transition, however, the configuration of the upper level is not yet known. A typical observed fluorescence spectrum is shown in Fig. 1 for the 393.701-nm transition in Er I. For stable Er, there are five even-mass isotopes, Er, Er, Er, Er, and Er, and one odd-mass isotope, Er. It can be seen from Fig. 1 that peaks are all clearly observed for the evenmass isotopes, including the lowest abundance (0.14%) isotope Er. The full width at half maximum (FWHM) of peaks is about 25MHz. This width is considered to be mainly due to the natural width of the upper level of the transition and the residual Doppler broadening of the atomic beam; the linewidth of the laser is less than 1MHz and its contribution is negligible. A least-squares fit with a Lorentz function was performed for the measured spectra. Thus, peak centers were determined and calibrated with the FPI spectrum. Relative frequencies between peaks, i.e., the ISs, were obtained for the studied transitions. Hfs constants were determined from derived hfs splittings of the level after the assignment of hfs peaks. The uncertainty of the measured IS includes 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. For each transition, measurements are performed about 20 times and the final uncertainty of the IS is, therefore, about 1–2MHz. Hfs constants A and B of Er were determined for the ground state and the two upper levels of the 393.701and 395.642-nm transitions. For the other two transitions, the spectrum intensities were not strong enough to determine hfs constants. Determined hfs constants are presented in Table II together with the previously reported values for comparison. For the ground state, the present values of constants A and B are in good agreement with the previously reported values and those of the two upper levels are newly determined. Obtained ISs are presented in Table III. For the 395.642nm transition, only one strong peak for the even-mass isotopes was observed and, therefore, the IS was considered to be 0 and its uncertainty to be the FWHM of peaks of about 25MHz. For the 4 f 6s–4 f 5d6s transition at 393.701 nm, the IS was found to be large positive value. For the 4 f 6s–4 f 5d6s transition, the IS at 394.442 nm is Journal of the Physical Society of Japan Vol. 78, No. 1, January, 2009, 015001 #2009 The Physical Society of Japan