Abstract

It is now possible to desorb a variety of organic molecules from surfaces using a tightly focused energetic ion beam. The molecules are detected as secondary ions using time-of-flight mass spectrometry, and may be spatially resolved by rastoring the ion beam over a larger area. A chemically resolved image is then acquired by examining the intensity of a particular mass as a function of x,y position. Presently, liquid metal ion sources using 25 KeV Ga⁺ ion projectiles can be focused to a spot of less than 20 nm in diameter. The limiting factor for molecule-specific imaging is sensitivity. The ion dose must be kept less than 1% of the total number of surface molecules to prevent chemical damage. Moreover, the ionization probability of desorbed molecules is generally less than 1 in 10⁴. Since the molecules desorb from the first layer and since there are at most 4 × 10⁶ molecules per square micron (depending on size of course), the signal rapidly approaches zero as the spatial resolution or beam probe size is reduced below 1 micron.1 Here we investigate the use of high intensity 100 fs laser pulses to photoionize the desorbed neutral molecules in an attempt to increase the measurement efficiency of this type of experiment. Our model system is dopamine, an important neurotransmitter that has aromatic character, but is subject to significant fragmentation using ns laser pulses. The results suggest that this approach can indeed expand the performance of mass-spectrometry based imaging experiments and can open new applications in bioimaging.

© 1998 Optical Society of America