J. Edler, R. Pfister, V. Pouthier, C. Falvo and P. Hamm

Physikalisch-Chemisches Institut, Universitet Zurich, Wintherthurerstr. 190, 8057 Zurich, Switzerland and
Laboratoire de Physique Moleculaire, UMR CNRS 6624, Facult des Sciences, La Boeloie, Universit de Franche-Comt, 25030 Besancon Cedex, France

Femtosecond infrared pump-probe spectroscopy of the N-H mode of a stable alpha - helix reveals two excited state absorption bands which disappear upon unfolding of the helix. Numerous test experiments prove that these bands originate from the N-H band solely. A quantitative comparison with polaron theory shows that these two bands reflect two types of two-vibron bound states connected to the trapping of two vibrons at the same site and at nearest neighbor sites, respectively. The latter states originate from an acoustic phonon in the helix, which correlates adjacent sites.