(a1), (a2) Temperature-dependent intensities of the isotope-shifted peaksυisoin the linear absorption spectrum of different isotopologs. Both the curves of Mid-D and Turn-6 are shifted for better comparison. Temperature-dependent intensities of the isotope-shifted peaksυisoin the 2DIR spectra for Trpzip2 (b1) and Trpzip4 (b2). The curves of Mid-D and Turn-6 are shifted for better comparison.