New prospects for molecular post-ionisation using femtosecond IR lasers

We have investigated the response of a number of small organic molecules to intense infrared femtosecond laser pulses to explore the potential for strong-field post-ionisation in bio-imaging applications. The experimental system has been optimised and characterised using the well-documented ionisation behaviour of xenon. Preliminary work has involved a systematic study of the ionisation–dissociation characteristic of gas phase molecules, to optimise the ionisation step itself. The efficiency of molecular ion and fragment ion production as a function of laser wavelength and intensity has been studied. Here, we report on the behaviour of the toluene molecule, which is typical of several other small organics we have studied. With 800-nm irradiation, a clear transition in behaviour with increasing laser power is observed, favouring molecular ion production. At longer laser wavelengths, the ratio of parent molecular ion to fragment ions is increased by more than one order of magnitude. The molecular ion signal for toluene is also significantly increased on switching from 800-nm to 1300-nm irradiation. We discuss these observations in the context of the underlying ionisation mechanisms.