Microsolvation of the water cation in argon: II. Infrared photodissociation spectra of H2O+-Ar-n (n=1-14)

Mid-infrared photodissociation spectra of H2O+-Ar-n (n = 1-14) complexes ha ve been recorded in the vicinity of the O-H stretch vibrations of the water cation. The rovibrational structure of the transitions in the dimer spectr um (n = 1) are consistent with a planar, proton (H)-bound H-O-H-Ar equilibr ium geometry. The slightly translinear intermolecular bond in the ground vi brational state is characterized by a bond angle phi (0) = 175(5)degrees, a n interatomic H-Ar separation R-0 = 1.929(15) Angstrom, and an intermolecul ar stretching force constant k(s) similar to 29 N/m. The assignment of the vibrational transitions is confirmed by spectra of partly deuterated specie s. The relaxation dynamics depend strongly on the excited vibrational state and do not obey statistical theories. Analysis of the spin-rotation consta nts indicates that the electromagnetic properties of the H2O+ cation in its B-2(1) ground electronic state are not significantly affected by the forma tion of the intermolecular bond to Ar. The vibrational bands in the spectra of larger clusters tn = 2-14) are assigned to O-H stretch fundamentals and their combination bands with the intermolecular H-Ar stretch modes. The ob served systematic band shifts as a function of cluster size provide informa tion about the cluster's geometries and the occurrence of structural isomer s. The most stable trimer (n = 2) geometry has two equivalent intermolecula r H-bonds. This stable trimer core is further solvated by two Ar ligands at tached to opposite sites of the 2p(y) orbital of oxygen (n = 3,4) and subse quently by less strongly bound Ar ligands (n = 5-14) to form an kr solvatio n shell, probably around an interior H2O+ ion.