  A new type of double-resonance spectroscopy of a quantum gas based on
interaction-induced frequency modulation of a probe transition has been
considered. Interstate interaction of multilevel atoms causes a
coherence-dependent collisional shift of the transition between the atomic
states |1> and |2> due to a nonzero population of the state |3>. Thus, the
frequency of the probe transition |1>-|2> experiences oscillations associated
with the Rabi oscillations between the states |1> and |3> under continuous
excitation of the drive resonance |1>-|3>. Such a dynamic frequency shift leads
to a change in the electromagnetic absorption at the probe frequency and,
consequently, greatly enhances the sensitivity of double-resonance spectroscopy
as compared to traditional "hole burning", which is solely due to a decrease in
the population of the initial state |1>. In particular, it has been shown that
the resonance linewidth is determined by the magnitude of the contact shift and
the amplitude of the drive field and does not depend on the static field
gradient. The calculated line shape and width agree with the low-temperature
electron-nuclear double-resonance spectra of two-dimensional atomic hydrogen.
