VOLUME 64, NUMBER 25 PHYSICAL REVIEW LETTERS 18 JUNE 1990 Hyperfine Structure of the Metastable sS2 State of ' 0 Using an AlGaAs Diode Laser at 777 nm G. M. Tino, ' L. Hollberg, ' A. Sasso, ' and M. Inguscio ' European Laboratory of Nonlinear Spectroscopy (LENS), Largo E Ferm. i 2, I 501-25 Firenze, Italy M. Barsanti Scuola Normale Superiore, Piazza dei Cavalieri 7, 1-56100 Pisa, Italy (Received 12 February 1990) By exploiting a narrow-linewidth diode-laser source we measure the hyperflne structure of the S2 state of "O. Nuclear parameters can be calculated from the measured hyperfine structure. Recorded collision-free linewidths allow an estimate of the lifetime of the levels involved in a new scheme proposed for the cooling of atomic oxygen. PACS numbers: 35, 10. Fk, 32, 30. Jc Nuclear eff'ects in the spectrum of atomic oxygen are of great interest. Recently, a possible nuclear volume effect on the isotope shift of optical transitions was ob- served. This effect is usually negligible in light ele- ments but it is enhanced in this case by the "doubly magic" structure of the ' 0 nucleus which has both pro- ton and neutron closed shells. A deeper understanding of these effects requires more detailed information about the atomic structure. For example, information is re- quired about the electron density at the nucleus which can be obtained from the analysis of the hyperfine struc- ture. ' 0 is the only stable oxygen isotope which has hyperfine structure since it has one neutron in the d5/2 orbital outside the doubly closed ' 0 nucleus; this pro- duces a nuclear magnetic moment of — 1. 894Ittv. In this paper we report the first measurement of the hyperfine structure of atomic oxygen performed by high-resolution laser spectroscopy. In fact, the only ex- isting data on the hyperfine structure for this atom were obtained in a paramagnetic-resonance experiment on the ground state. The level we have investigated is the excited 3 S2 level (Fig. 1). The interest of this particular level is twofold. First, it is the lowest in energy (9.14 eV) for the excited configuration 1s 2s 2p 3s, so stronger nuclear effects can be expected. Since optical transitions start from this level, combined measurements of the isotope shift and the hyperfine structure represent a stringent test for a priori Hartree-Fock calculations, which are not easy for this multielectron atom. Second, the 3 S2 state is meta- stable (lifetime r =180 Its) and is connected to the 3 P~ 2 3 states by three strong transitions at k =777 nm; as we discuss below, radiative cooling of atomic oxygen could be achieved using these transitions and the subse- quent decay of the cooled atoms into the ground state would produce very cold ground-state oxygen. This rnetastable cooling is the only realistic cooling scheme for oxygen since no allowed transition at visible wave- lengths is available from the ground state. A critical pa- rameter for cooling is the lifetime of the upper 3 P state; it determines the time required for the deceleration of TRIPLET QUINTET S P D S 5p D 5d I , 605nm, '596nm 3p 5s 4d '. 645nm, '616nm / 7nm 80 ps) SINGLET g (e-(s) 558nm ll, (e-(00 630, --. 636nm 2. 1D FIG. l. A simplified energy-level scheme of Ot showing the optical transition investigated in this work (solid line) and oth- er transitions of spectroscopic interest (dashed lines). the atoms which must be shorter than the lifetime of the 3 S2 state. From the present results an estimate for the 3 P-state lifetime is obtained, which supports this pro- posed cooling scheme. In addition, GaAlAs/GaAs het- erostructure diode lasers emitting at A, -780 nm are now available with enough power to perform nonlinear high- resolution spectroscopy. Interest in diode-laser spectros- copy has increased since the development of several tech- niques which improve these lasers' spectral purity and tu- nability making them suitable for laser-cooling exper- iments. In this work a diode laser mounted in an extend- ed cavity configuration is used to perform saturation spectroscopy on the 3s S2-3p P3 transition at 777. 1 nm. The experimental arrangement is shown in Fig. 2. The laser we used was a commercial A1GaAs/GaAs diode laser emitting at 780 nm at room temperature. Coarse tuning over a range of more than 10 nm was achieved using optical feedback from a 1200-lines/mm 1990 The American Physical Society 2999