HIGH RESOLUTION INFRARED ABSORPTION SPECTROSCOPY OF PROTONATED WATER CLUSTERS IN A FAST ION BEAM

Experimental information on the structure, dynamics, and quantum mechanical properties of nonrigid molecular ions and weakly bound cluster ions is limited because of the difficulty of spectroscopic studies in all frequency regions, with only partial resolution available in even the best $cases.^{1}$ The Direct Laser Absorption Spectroscopy in Fast Ion Beams technique (DLASFIB) has been developed specifically to directly measure vibration-rotation-tunneling (VRT) spectra of weakly bound cluster ions with sub-Doppler resolution and with accurate determination of the mass and number density of the carriers of all spectral $features.^{2}$ Absolute vibrational band intensities for ions can also be measured, as demonstrated for $HCO^{+}, HNN^{+}, H_{3}O^{+}$, and $NH_{4}^{+3}$. We are currently searching for two OH stretches for the protonated water cluster dimer ion $H_{5}O_{2}^{+}$ in the region near $3700 cm^{-1}$, guided by ab initio calculations and multiphoton dissociation experiments performed by Yeh $at al.^{1}$ In this search, we have measured several previously unobserved transitions of $H_{3}O^{+}$, possibly belonging to the totally symmetric OH stretching mode of the monomer ion. These transitions have been unobservable in direct absorption experiments in plasmas due to neutral water absorptions. We are now experimenting with a Corona ion source with differential pumping and gas dynamic focusing in order to be able to produce ion beams with large numbers of internally cold cluster ions.