  We propose and experimentally investigate a scheme for observing Feshbach
resonances in atomic quantum gases in situ and with a high temporal resolution
of several ten nanoseconds. The method is based on the detection of molecular
ions, which are optically generated from atom pairs at small interatomic
distances. As test system we use a standard rubidium gas (87Rb) with well known
magnetically tunable Feshbach resonances. The fast speed and the high
sensitivity of our detection scheme allows to observe a complete Feshbach
resonance within one millisecond and without destroying the gas.
