Figure 2.

Experimental spectral pattern observed for transitions of a-type (top) and of c-type (bottom). Markers of the same color indicate the doublets arising from the C₂ internal rotation of the water molecule. Markers of the same shape (circles or triangles) indicate the doublets arising from the tunneling motion of the water molecule between the two equivalent isomers of the Phe–H₂O complex: far-left and far-right. The two transitions are labeled following the notation J_Ka,Kc ← J_K′a,K′c^′. For each component of the tunneling splitting, the lower and upper tunneling states are indicated as 0⁺, 0^–, 1⁺, and 1^–. A schematic of the interstate a- and intrastate c-type transitions is also shown at the bottom of the figure. The schematic of the transitions is not drawn to scale.