Splitting of the zero phonon lines of indole, 3-methyl indole, typtamine and N-acetyl tryptophan amide in helium droplets

The high resolution vibronic spectra of the S-1 <-- S-0 transition (<nu> ap proximate to 34 900-35 300 cm(-1)) of indole and its derivatives 3-methyl i ndole, tryptamine and N-acetyl tryptophan amide (NATA) embedded in large (N similar or equal to 10(3)-10(4)) helium droplets exhibit an unexpected spl itting of the zero phonon lines. Up to four components are observed, also f or the different conformers in tryptamine. with spacings of about by Delta nu approximate to 1-2 cm(-1) not seen in previous studies of the embedded a mino acids tryptophan and tyrosine and other organic molecules. The split l evels are attributed to different arrangements of two He atoms from the bat h which are localized at the two different sites on the two sides of the in dole chromophore. The relative intensities of the different components depe nd on the particular "side chain" attached to a common indole chromophore.