Magnetic field effects on the 1 083 nm atomic line of helium

Abstract

Abstract. The structure of the excited 2 3 S and 2 3 P triplet states of 3 He and 4 He in an applied magnetic field B is studied using different approximations of the atomic Hamiltonian. All optical transitions (line positions and intensities) of the 1083 nm 2 3 S-2 3 P transition are computed as a function of B. The effect of metastability exchange collisions between atoms in the ground state and in the 2 3 S metastable state is studied, and rate equations are derived, for the populations these states in the general case of an isotopic mixture in an arbitrary field B. It is shown that the usual spin-temperature description remains valid. A simple optical pumping model based on these rate equations is used to study the B-dependence of the population couplings which result from the exchange collisions. Simple spectroscopy measurements are performed using a single-frequency laser diode on the 1083 nm transition. The accuracy of frequency scans and of measurements of transition intensities is studied. Systematic experimental verifications are made for B=0 to 1.5 T. Optical pumping effects resulting from hyperfine decoupling in high field are observed to be in good agreement with the predictions of the simple model. Based on adequately chosen absorption measurements at 1083 nm, a general optical method to measure the nuclear polarisation of the atoms in the ground state in an arbitrary field is described. It is demonstrated at B ∼0

Fundamentals of metastability exchange optical pumping in helium

“…Level structure of the 2 3 S and 2 3 P states We use the notations of reference [24] where the structure and energies of the sublevels of the 2 3 S and 2 3 P states are derived for both isotopes in arbitrary magnetic field. For simplicity we only discuss two experimentally relevant limiting cases: low and high magnetic fields.…”

“…The magnetic sublevels can be written using the decoupled basis states |m S , m I . A 1 =|−1, − and A 4 =|1, + are pure states of maximum |m F |=3/2 but the other states involve large mixing parameters θ − and θ + [24]:…”

“…The resulting spectra of the 1083 nm transition (upper graphs, computed with 300 K Doppler widths and no collisional broadening) extend over ∼70 GHz when the isotope shift is considered. Optical transition frequencies are referenced to that of the C 1 line at zero field [24]. b: C 8 and C 9 lines, which are the most efficient in usual OP conditions, connect the 2 3 S 1 , F =1/2 and F =3/2 levels of 3 He, respectively, to the 2 3 P 0 level.…”

“…Energy references and state labelling are identical to those of [24]. Optical transition frequencies are referenced to that of the C 1 line in zero field [24]. Pump and probe lines relevant for high field MEOP are described in text.…”

“…This model has been also used to discuss OP experiments with a broadband laser [20,21]. Extensions to specific situations have been proposed, but not fully developed, for OP in 3 He- 4 He isotopic mixtures 1 [23] and for OP in high magnetic fields [12,13,24]. Recently, we have developed a more comprehensive model of MEOP.…”