Vibrational spectroscopy of large water clusters of known size

The structure of large water clusters of known sizedistributions 〈n〉 = 20-2000is investigated by vibrational spectroscopy of the OH stretchmode. The water clusters are predissociated by a pulsed tunableinfrared optical parametrical oscillator (OPO) in the frequencyrange 2800-3800 cm^-1. Their fragmentsare detected off-axis by electron impact ionization and massanalyzed by a quadrupole mass spectrometer. The largest ionsignal stems from the neutral water hexamer fragment. The ionyield is investigated at certain wavelengths while the size ofthe clusters is varied, and for certain sizes completeabsorption spectra of the OH stretch motion are measured.Fingerprints of the different coordination types of the watermolecules in the clusters are found and it turns out that ourmethod is especially sensitive to amorphous structures withfrequencies shifting in the range of 3300-3400cm^-1.