VIBRATIONAL SPECTROSCOPY OF SMALL WATER COMPLEXES EMBEDDED IN LARGE LIQUID-HELIUM CLUSTERS

Infrared molecular beam depletion spectroscopy has been employed to st udy the vibrational spectroscopy of water molecules and small water po lymers [(H2O)(n), n=2,3,4] embedded in large liquid helium clusters (H e-N, 100 less than or equal to(N)less than or equal to 5000). The spec tral region between 3500 and 3800 cm(-1) was covered with an injection -seeded optical parametric oscillator. The following vibrational bands could be located and investigated: for the monomer the v(3) asymmetri c stretch, for the dimer the asymmetric stretch of the proton acceptor molecule and the free and bonded O-H stretches of the donor molecule, for the trimer both free and bonded O-H stretches, and for the tetram er the free O-H stretch. By comparison with the data on foe gas phase water complexes, it was found that the helium host clusters induce onl y minor perturbations in form of small frequency redshifts and that th ey constitute an ideal nano-matrix. Furthermore, it was found that bot h the water molecule and the dimer rotate almost freely in the host cl uster and that the internal-rotationlike tunneling motion of the water dimer is not quenched. Due to the extremely low internal temperatures , a splitting of the trimer band associated with the O-H ring vibratio n could be resolved for the first time. This splitting indicates that the trimer structure is asymmetric, as has been predicted by theoretic al calculations. (C) 1996 American Institute of Physics.