Spectroscopic investigations of hydrogen-bonding and van der Waals' interactions
m molecular clusters were studied by the techniques of infrared predissociation and
resonance-enhanced multiphoton ionization spectroscopies (REMPI). Ab initio
calculations were applied in conjunction for data interpretation.
The infrared predissociation spectroscopy of CN^-•(H_2O)_n (n = 2 - 6) clusters was
reported in the region of 2950 - 3850 cm^(-1).
The hydrogen bondings for the C-site and N-site
binding, and among the water molecules were identified for n = 2 to 4. A spectral
transition was observed for n = 5 and 6, implying that the anion was surface-bound onto
the water aggregates in larger clusters.
The infrared predissociation spectroscopy of Br^-•(NH_3) and I^-•(NH_3)_n (n =1-3)
clusters was reported in the region of 3050-3450 cm^(-1).
For the Br^-•(NH_3) complex, a
dominating ionic NH stretch appeared at 3175 cm^(-1), and the weaker free NH stretch
appeared at 3348 cm^(-1). The observed spectrum was consistent to the structure in which
there was one nearly linear hydrogen bond between Br^- and the NH_3 moiety. For the I^-
•(NH_3) complex, five distinct IR absorption bands were observed in the spectrum. The
spectrum was not consistent with basic frequency patterns of three geometries considered
in the ab initio calculations - complex with one, two and three hydrogen bondings
between I^- and the NH_3 moiety. Substantial inhomogenous broadening were displayed in
the spectra for I^-•(NH_3)_n (n =2-3), suggesting the presence of multiple isomers.
The REMPI spectroscopy of the bound 4p ^2П 1/2 and ^2П 3/2 states, and the
dissociative 3d ^2Σ^+ 1/2 state in the Al•Ar complex was reported. The dissociative spectrum
at Al^+ channel suggested the coupling of the 4p ^2П 1/2,3/2 states to the repulsive 3d ^2Σ^+ 1/2
state. The spin-electronic coupling was further manifested in the dissociative Al^+
spectrum of the 3d ^2Σ^+ 1/2 state. Using the potential energy curves obtained from ab initio
calculations, a bound → continuum Franck-Condon-intensity simulation was performed
and compared with the one-photon 3d ^2Σ^+ 1/2 profile. The agreement provided evidence
for the petturbation above the Al(3d)Ar dissociation limit, and the repulsive character of
the 3d ^2Σ^+ 1/2 state. </p
