Vibrational spectroscopy of large water clusters of known size

The structure of large water clusters of known size distributions (n) = 20-2000 is investigated by vibrational spectroscopy of the OH stretch mode. The water clusters are predissociated by a pulsed tunable infrared optical parametrical oscillator (OPO) in the frequency range 2800-3800 cm(-1). Their fragments are detected off-axis by electron impact, ionization and mass analyzed by a quadrupole mass spectrometer. The suggest ion signal stems from the neutral water hexamer fragment. The ion Yield is investigated at certain wavelengths while the size of the clusters is varied, and for certain sizes complete absorption spectra of the OH stretch motion are measured. Fingerprints of the different coordination types of the water molecules in the clusters are found and it turns out that our method is especially sensitive to amorphous structures with frequencies shifting in the range of 3300-3400 cm(-1).