  We propose a new spectroscopic method for measuring weak transitions in cold
and trapped atoms, which exploits the long interaction times and tight
confinement offered by dark optical traps together with an electron shelving
technique to achieve extremely high sensitivity. We demonstrate our scheme by
measuring a 5S_{1/2}-> 5D_{5/2} two-photon transition in cold Rb atoms trapped
in a new single-beam dark optical trap, using an extremely weak probe laser
power of 25 micro-Watt. We were able to measure transitions with as small
excitation rate as 0.09 sec^(-1).
