A single transition is described in the <Processes><Radiative> table as a <RadiativeTransition> block. For example (truncated for clarity):
<RadiativeTransition id="PCDMS-R7452690" process="excitation"> <EnergyWavelength> <Frequency methodRef="MCDMS-6648"> <SourceRef>BCDMS-344</SourceRef> <Value units="MHz">104711.4035</Value> <Accuracy>0.0057</Accuracy> </Frequency> <Frequency methodRef="MCDMS-6646"> <Value units="MHz">104711.3656</Value> <Accuracy>0.0003</Accuracy> </Frequency> </EnergyWavelength> <UpperStateRef>SCDMS-9240239</UpperStateRef> <LowerStateRef>SCDMS-9240238</LowerStateRef> <SpeciesRef>XCDMS-132</SpeciesRef> <Probability> <TransitionProbabilityA> <Value units="1/cm">5.45485286998e-08</Value> </TransitionProbabilityA> </Probability> </RadiativeTransition>
Note that the species (SpeciesRef), the lower level ( LowerStateRef), and the upper level (UpperStateRef) are identified by their unique identifiers which backpoint to the given species in the <Molecules> table. The measured frequency is described in a <Frequency> block. There can be several frequencies for a single transition, measured by several sources.