Detection of molecular surface adsorbates at the 10−6 monolayer level and below

Ultrathin molecular surface overlayers were deposited in an UHV chamber and were detected using pulsed infrared laser desorption followed by resonant laser post-ionization. A single-shot detection limit in the 10−6 monolayer range was found for aniline adsorbed on silica, a system whose adsorption, laser desorption and ionization properties have been well studied. An excellent signal-to-noise ratio, as well as some simple modifications that will improve the detection efficiency suggest ultimate detection limits in the 10−9 to 10−10 ML range with this scheme.