  Frequency comb based multidimensional coherent spectroscopy is a novel
optical method that enables high resolution measurement in a short acquisition
time. The method's resolution makes multidimensional coherent spectroscopy
relevant for atomic systems that have narrow resonances. We use double-quantum
multidimensional coherent spectroscopy to reveal collective hyperfine
resonances in rubidium vapor at 100 C induced by dipole-dipole interactions. We
observe tilted lineshapes in the double-quantum 2D spectra, which has never
been reported for Doppler-broadened systems. The tilted lineshapes suggest that
the signal is predominately from the interacting atoms that have near zero
relative velocity.
