Forbidden transitions between the ground state and the metastable levels of Calcium and Magnesium have a large metrological interest for frequency standards working in the visible and near-infrared regions. For example, much work has been done on the Calcium 1 S 0 3 P 1 intercombination line at 657.3 nm, and some works are in progress on the corresponding 1 S 0 3 P 1 in magnesium at 457.1 nm.

transition

The 1 S 0 1 D 2 transition in Calcium seems very attractive too, due to its narrow linewidth (< 10 Hz). The 1 D 2 metastable level can be excited either through an electrical quadrupole (

E2

) absorption at 457.5

nm, or through a dipole-allowed two-photon absorption at 915 nm.

The two-photons transition appears particularly interesting for metrological applications, because of its intrinsic Doppler-free properties.

Calcium energy levels

An experiment is now in progress in in our laboratory in order to observe the two-photons transition on a cooled sample of Calcium atoms, that we have already developed [1].

The experimental values available in literature for the E2 transition rate are affected by a large error (20 %), and the theoretical estimations are not univocal [2]. Thus, as a first step we have measured the quadrupole transition probability on an absorption cell.

Typical recorded profiles

1.0

0.8

0.6

0.4

0.2

0.0

-3 657.3 nm -2 -1 457.5 nm 0 1 laser detuning (GHz) 2

1.0

0.8

0.6

0.4

0.2

0.0

-3

Isotopic structure of the intercombination line

42 Ca 44 Ca 40 Ca recorded signal -2 -1 0 1 laser detuning (GHz) 2 3

Summary of our results

Each point represents a group of measurements, taken in different conditions of pressure, temperature and optical alignment.

56 54 52 50 48 680 700 720 740 760 Doppler temperature (°C) 780

We measured the ratio between the two transition rates as 53.6

± 1.3.

Considering the most accurate experimental estimation available in literature [3] for the transition rate at 657.3 nm, this gives a value of 54.9

± 3.5 sec -1 for the

E2

transition rate.

References [1] N. Beveriniet al., J. Opt. Soc. Am. B

59

(1989), 2188-2193 [2] K. Fukuda, K. Ueda, J. Phys. Chem.

86

(1982), 676-677 [3] R. Drozdovski et al, Z. Phys. D

41

(1997), 125-131