Infrared Spectroscopy of Neutral Water Dimer Based on a Tunable Vacuum Ultraviolet Free Electron Laser

Infrared (IR) spectroscopy provides detailed structural and dynamical information on clusters at the fingerprint level. Herein, we demonstrate the capability of a tunable vacuum ultraviolet free electron laser (VUV-FEL) for selective detection of a wide variety of neutral water clusters and for recording the size-dependent IR spectra. The present technique does not require the presence of an ultraviolet chromophore or a dipole moment and is generally applicable for IR spectroscopy of neutral clusters free from confinement. To show the features of our technique, we report here the IR spectra of neutral water dimer in the OH stretch region, providing benchmarks for theoretical study of the accurate description of hydrogen bonding structures involved in liquid water and ice. Quantum mechanical calculations on a 12-dimensional ab initio potential energy surface are utilized to simulate the anharmonic vibrational spectra of water dimer. These results help to resolve the controversy of the exact vibrational assignment of each band feature of the water dimer.