▁See ▁also ▁the ▁figure ▁with ▁the ▁original ▁dataset ▁on ▁the ▁right ▁and ▁the ▁corresponding ▁ 2 D ▁spectrum ▁in ▁the ▁figure ▁below . ▁ ▁Pres ence ▁of ▁ 2 D ▁spect ra ▁ 2 D ▁synchron ous ▁and ▁asynchronous ▁spect ra ▁are ▁basically ▁ 3 D - datas ets ▁and ▁are ▁generally ▁represented ▁by ▁cont our ▁plots . ▁X - ▁and ▁y - ax es ▁are ▁identical ▁to ▁the ▁x - axis ▁of ▁the ▁original ▁dataset , ▁whereas ▁the ▁different ▁cont ours ▁represent ▁the ▁magnitude ▁of ▁correlation ▁between ▁the ▁spectral ▁intens ities . ▁The ▁ 2 D ▁synchron ous ▁spectrum ▁is ▁symmetric ▁relative ▁to ▁the ▁main ▁diagonal . ▁The ▁main ▁diagonal ▁thus ▁contains ▁positive ▁pe aks . ▁As ▁the ▁pe aks ▁at ▁( x , y ) ▁in ▁the ▁ 2 D ▁synchron ous ▁spectrum ▁are ▁a ▁measure ▁for ▁the ▁correlation ▁between ▁the ▁intensity ▁changes ▁at ▁x ▁and ▁y ▁in ▁the ▁original ▁data , ▁these ▁main ▁diagonal ▁pe aks ▁are ▁also ▁called ▁aut ope aks ▁and ▁the ▁main ▁diagonal ▁signal ▁is ▁referred ▁to ▁as ▁aut oc or relation ▁signal . ▁The ▁off - di ag onal ▁cross - pe aks ▁can ▁be ▁either ▁positive ▁or ▁negative . ▁On ▁the ▁other ▁hand , ▁the ▁asynchronous ▁spectrum ▁is ▁asym metric ▁and ▁never ▁has ▁pe aks ▁on ▁the ▁main ▁diagonal . ▁ ▁Gener ally ▁cont our ▁plots ▁of ▁ 2 D ▁spect ra ▁are ▁orient ed ▁with ▁rising ▁axes ▁from ▁left ▁to ▁right ▁and ▁top ▁to ▁down . ▁Other ▁orient ations ▁are ▁possible , ▁but ▁interpretation