Welcome to the webpages of molecular spectroscopy group,
Laser spectroscopy division of  Prof. T. W. Hänsch

Since more than a decade ago, extremely high light intensities can be realized by lasers producing pulses with energies of several milijoules and durations smaller than 100 femtoseconds. When atoms and molecules are exposed to the focused light of such sources, they feel intensities of up to 10¹⁵ W/cm². The light can no longer be considered as a small perturbation, but has to be treated equally as the molecule, since the both terms in Hamiltonian are of the same magnitude now. This molecule-photon system exhibits so far unknown effects, which became known as molecular bond-softening, level-shifting and trapping of the molecules in new formed potentials, which leads to a stabilization of the molecule in the light fields.

In our experiment small molecules like hydrogen molecular ion H₂⁺ and its isotopic variants D₂⁺ and HD⁺ are formed in an ion beam and exposed to intense laser pulses. The fragments produced in photodissociation and Coulomb explosion of these molecules are projected on a 2-dimensional detector, where their velocity distribution is imaged. Due to the high velocity resolution, the fate of single vibrational levels could be investigated for the first time. In the Coulomb explosion spectra, we have recently discovered a peak structure, which is related to ionization at so-called "critical" internuclear distances. The isotopic variant HD⁺ with its different symmetry is of special interest: during photodissociation the electron can join the proton or the deuteron. Theoretical calculations predict that it favours the deuteron, what would be a severe symmetry breakdown of the Born-Oppenheimer approximation. Our experiment allows to test this prediction. The evaluation of the results is underway.