Absolute frequency measurements on the 2S→3S transition of lithium-6,7

Abstract

Abstract. The frequencies of the 2S-3S two-photon transition for the stable lithium isotopes were measured by cavity-enhanced Doppler-free laser excitation that was controlled by a femtosecond frequency comb. The resulting values of 815 618 181.57(18) and 815 606 727.59(18) MHz, respectively, for 7 Li and 6 Li are in agreement with previous measurements but are more accurate by an order of magnitude. There is still a discrepancy of about 11.6 and 10.6 MHz from the latest theoretical values. This is comparable to the uncertainty in the theoretical calculations, while uncertainty in our experimental values is more than a hundred-fold smaller. More accurate theoretical calculation of the transition frequencies would allow extraction of the absolute charge radii for these stable isotopes, which in turn could improve nuclear charge radii values for the unstable lithium isotopes.

Uncertainty estimates for theoretical atomic and molecular data

“…For the two-and three-electron cases of helium-like and lithium-like atoms complete calculations in Hylleraas coordinates have been performed. Recent high-precision measurements [173,174] and theory [175][176][177] for the 1s 2 2s 2 S − 1s 2 3s 2 S two-photon transition in lithium provide an excellent example of what can be achieved. Table I lists the various contributions to the transition frequency, expressed as a double power series in powers of µ/M and α, where µ/M is the ratio of the reduced electron mass to the nuclear mass, and α is the fine structure constant (see table caption for numerical values).…”

Dynamic Stark shift of the7Li(2s→3s) transition

“…The first stage in the Li(2s → 3s) transition frequency experiment [1,3] involves the two-photon excitation to the 3s level. The laser photon with λ = 735.1 nm, emitted by a Ti:sapphire laser, has an energy of 0.061 98 a.u..…”

“…The third stage requires a single-photon excitation from the 2p to 3d level using a dye laser with a photon wavelength of λ = 610.5 nm corresponding to an energy of 0.074 63 a.u. [1,3].…”

“…The present calculation is primarily motivated by the need to generate a better description of the Li(2s → 3s) transition enabling a more reliable estimate of the isotope shift and thus the dimension of the 7 Li nuclei [1,2]. …”

“…High precision transition frequencies for the Li(2s → 3s) transition have been measured for the 6 Li and 7 Li atoms [1][2][3]. The experimental procedure involves two-photon excitation from the 2s ground state to the 3s level, a decay to the 2p level, a single-photon excitation to the 3d level, and finally photoionization driven by the laser fields used to excite the atom.…”