Ultrafast laser studies of molecules in helium clusters

We show that rotational line spectra of molecular clusters with near zero permanent dipole moments can be observed using impulsive alignment. Aligned rotational wave packets were generated by non-resonant interaction with intense femtosecond laser pump pulses and then probed using Coulomb explosion by a second, time-delayed femtosecond laser pulse. By means of a Fourier transform a rich spectrum of rotational eigenstates was derived. Guided by ab initio calculations for the smallest cluster comprising of a single acetylene molecule and a helium atom, essentially all rotational eigenstates up to the dissociation threshold could be established, providing a comprehensive picture of the rotational energy level structure. The C2H2-He complex is found to exhibit distinct features of large amplitude motion and very early onset of free internal rotor energy level structure.