Neubestimmung der absoluten Oszillatorenstärke der Eisen I-Resonanzlinieλ=3720 Å durch optische Doppelresonanz

The natural width of the strong Fe I spectral lineλ=3719,9 Å was determined in a double resonance experiment with an atomic beam as resonance cell and a hollow cathode as light source by the observation ofrf-induced magnetic dipole transitions between the Zeeman sublevels of the excited state in a weak static magnetic fieldH ₀. The optical line was isolated from disturbing neighbours in the same multiplet either roughly by an interference filter or strictly by a monochromator. From the widthΔν=5,35(14) MHz of the double resonance signal (monochromator value, including a small correction of 1.5% for the hyperfine structure of the 2.2% natural abundance of iron-57) the lifetimeτ of the (z ⁵ F ₅ ⁰ )-level is found to beτ=5,95 (16)×10^?8 sec. Thef-value of the transition to the ground state (a ⁵ D ₄) is calculated, by using the known relativef-values for the weak alternative transition possibilities to the (a ⁴ F ₅) and (a ⁵ F ₄) levels (λ=5012 and 5127 Å), to bef _λ3720=4,21 (11)×10^?2.