INTENSE CO2-LASER IONIZATION OF D2O AND (D2O)2

Experimental results of the ionization of heavy water vapour in the tunnelling regime, at two different CO2 laser frequencies, are presented. The ionization mechanism appears to have a similar dependence on laser intensity as that for atoms, although the ionization probability seems to be higher in the case of the molecules in our experiment. It is proposed that vibrational excitation and strong field dressing of the potential energy surfaces of the molecule help in reducing its ionization potential. Numerical fits of ion number versus peak laser intensity for the ionization products D2O+ and (D2O)2+ using atomic tunnelling theory provide an estimation for the probability of multiphoton excitation and strong field dressing prior to ionization. The possibility of the proposed ionization process occurring in other molecules subject to similar CO2 laser pulses is also discussed.