We have measured the emission spectra of two-level-like atoms driven by a s
patially inhomogeneous bichromatic field comprised of one resonant and one
off-resonant component. Under these excitation conditions, observed spectra
consist of several narrow peaks appearing on top of a broad continuumlike
structure. This experimental approach provides for selective observation of
those spectral features having a Rabi-frequency-independent emission frequ
ency. Such emission processes involve multiphoton transitions: with one leg
being spontaneous, between dressed-state levels differing only in their ph
otonic quantum numbers. i.e., between equivalent dressed-state-doublet subl
evels. Related vacuum-mediated transitions have recently been invoked as an
underlying mechanism of inversionless gain.