  Externally applied electromagnetic fields in general have an influence on the
width of atomic spectral lines. The decay rates of atomic states can also be
affected by the geometry of an applied field configuration giving rise to an
imaginary geometric phase. A specific chiral electromagnetic field
configuration is presented which geometrically modifies the lifetimes of
metastable states of hydrogen. We propose to extract the relevant observables
in a realistic longitudinal atomic beam spin-echo apparatus which allows the
initial and final fluxes of the metastable atoms to be compared with each other
interferometrically. A geometry-induced change in lifetimes at the 5%-level is
found, an effect large enough to be observed in an available experiment.

Atomic Physics