The high resolution vibronic spectra of the S 1 ← S 0 transition (ν ≈ 34 900-35 300 cm −1 ) of indole and its derivatives 3-methyl indole, tryptamine and N-acetyl tryptophan amide (NATA) embedded in large (N 10 3 − 10 4 ) helium droplets exhibit an unexpected splitting of the zero phonon lines. Up to four components are observed, also for the different conformers in tryptamine, with spacings of about ∆ν ≈ 1-2 cm −1 not seen in previous studies of the embedded amino acids tryptophan and tyrosine and other organic molecules. The split levels are attributed to different arrangements of two He atoms from the bath which are localized at the two different sites on the two sides of the indole chromophore. The relative intensities of the different components depend on the particular "side chain" attached to a common indole chromophore.
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