The FERRUM project: radiative lifetimes of intermediate-excitation states of Fe II measured in a fluorescence signal induced by laser pumping from a metastable state

A new collaborative program, the FERRUM project, aims at extending and improving the database of oscillator strengths for the astrophysical important Fe II spectrum. In this paper we report on the experimental method applied for measurements of radiative lifetimes of Fe II levels at intermediate excitation (5-10 eV) that can be populated by laser excitation from metastable states. The technique involves a laser-produced plasma as a source for metastable ions, a stimulated Brillouin scattering cell for shortening the laser pulse, and fast time resolved detection. We have applied the technique on two Fe II levels at about 7.5 eV and obtained the lifetimes: $\tau (z^4S^0_{3/2}) = 3.8 (2)\;{\rm ns}$; $\tau (y^4P^0_{5/2}) = 3.6 (3)\;{\rm ns}$