Figure 2.

(A) Transient spectra showing the absorption change as a function of probe frequency for a range of mixing times. The pump frequency is resonant to the higher frequency band (1,673 cm⁻¹), corresponding to the dotted line in Fig. 1B. The data have been scaled to eliminate the effect of T₁ relaxation. (B) Population ratio as a function of mixing time for Ala-Ala*-Ala (open points) and Ala-Ala-Ala (solid points), as obtained from the ratio of the cross and diagonal peaks observed in transient spectra recorded at different mixing times. The solid lines represent fits to the data, with cross-relaxation rates of 0.07 ps⁻¹ and 0.19 ps⁻¹ for Ala-Ala*-Ala and Ala-Ala-Ala, respectively.