CO2-LASER SPECTROSCOPY OF AMMONIA MOLECULES AND COMPLEXES ADSORBED ONLARGE ARGON HOST CLUSTERS

The pick-up technique has been employed to deposit single ammonia mole cules as chromophores on the surface of large ATN clusters and to crea te small ammonia complexes attached to host clusters. Mass spectrometr ic investigations have been carried out to characterize the pressure d ependence of the guest cluster formation. The technique is then combin ed with molecular beam depletion spectroscopy employing the infrared r adiation of a pulsed CO2 laser. In the region between 920 and 1060 cm( -1) in which the nu(2) bending vibration (umbrella mode) can be excite d, absorption spectra have been measured for the ammonia monomer as we ll as for the ammonia dimer, trimer, and tetramer adsorbed on large AT N host clusters. For the monomer a significant blue shift of 14.74 cm( -1) is observed. Furthermore, it is found that the inversion motion as sociated with the umbrella mode is appreciably perturbed, resulting in a reduction of the inversion splitting by a factor of 0.3. As in the argon matrix, the rotation is only slightly hindered. The absorption s pectra measured for ammonia dimers, trimers, and tetramers resemble ve ry much the gas phase spectra of these complexes, except that they are blue shifted by 17 (dimer) and 6 cm(-1) (trimer and tetramer). From t his close resemblance it is concluded that the structures of the small ammonia complexes are not affected by the argon host cluster. (C) 199 7 American Institute of Physics.