Excited states and vibrational spectroscopy of ice and adsorbed biomolecules

The photodesorption of water molecules from amorphous solid water (ASW) by 157-nm irradiation has been examined using resonance-enhanced multiphoton ionization (REMPI). The rotational temperature has been determined, by comparison with simulations, to be 425 ± 75 K. The time-of-flight (TOF) spectrum of H2O (v = 0) has been fit with a Maxwell-Boltzmann distribution with a translational temperatures of 700 ± 200 K (0.12 ± 0.03 eV). H+ and OH+ fragment ions have been detected with non-resonant multiphoton ionization, indicating vibrationally excited parent water molecules with translational energies of 0.24 ± 0.08 eV. The cross section for water removal from ASW by 7.9-eV photons near 100 K is (6.9 ± 1.8) x 10-20 cm2 for > 10 L H2O exposure. Electronic structure computations have also probed the excited states of water and the mechanisms of desorption. Calculated electron attachment and detachment densities show that exciton delocalization leads to a dipole reversal state in the first singlet excited state of a model system of hexagonal water ice. Ab Initio Molecular Dynamics (AIMD) simulations show possible desorption of a photo-excited water molecule from this cluster, though the non-hydrogen bonded OH bond is stretched significantly before desorption. Potential energy curves of this OH stretch in the electronic excited state show a barrier to dissociation, lending credence to the dipole reversal mechanism.M.S