Laser-Stimulated Field Desorption of Molecular Ions from a Tungsten

We report here on a experimental observation ofphoton-stimulated field emission of molecular anthracene ions from the surface of a layer adsorbed on a tungsten field-emitter tip. When the tip is irradiated with laser pulses 249, 308, and 400 nm in wavelength falling within the absorption bands of anthracene, the stimulated ion signal is proportional to the pulse fluence. The efficiency of the process decreases with the increasing laser wavelength. Photon stimulation is believed to be due to the resonance excitation of the anthracene molecules, followed by the field ionization of the excited molecules. PACS: 33,79.20.Ds, 79.70. + q Electrostatic fields with strength as high as 107 V/cm are observed to give rise to field desorption of ions of molecules adsorbed on metal surfaces. The process can be stimulated if the field emitter is irradiated with light. The stimulation of field desorption is, in most cases, due to the radiation heating of the surface [1, 2]. That the quantum stimulation Of the process is possible was indicated by the results of [33 in which the flux of molecular ions from a silver surface was observed to increase under the effect of a nonresonant laser radiation, the increase being proportional to the photon flux in the laser pulse. In the works reported in [4, 5], subject to investigation for the first time was the possibility of stimulating field desorption by resonant photoexcitation and ionization of the adspecies. They observed experimentally two-photon stimulation of the field desorption of molecular tetracene ions by KrF laser radiation. This paper reports on the observation of laser stimulation of the field desorption of molecular anthracene ions from the surface of a molecular layer adsorbed on a tungsten tip under the effect of onephoton excitation, and analyzes the possible mechanisms responsible for this phenomenon. It appears to hold much promise for laser field-ion biomolecular microscopy. 1. Experimental The experimental setup (Fig. 1) is a field-ion microscope equipped with quartz windows to let laser radiation in and out. The electric fields 107 to 108 V/cm is strength necessary to observe field desorption were produced on the apex of a tungsten tip with a radius from a few hundred to a few thousand •. The tips used in our experiments were prepared by the standard electrochemical polishing technique, and cleaned from surface impurities prior to the experiment through controlled field evaporation of the tip material at field strength around 6 V/A. The surface morphology was monitored on an atomic-scale resolution by helium field-ion imaging at a pressure of 10 -5 Torr. The absolute field strength at the tip surface and the tip radius were found accurate to within 15 %-20 % from the best image voltage (BIV). Measured to the same degree of accuracy best image field for tungsten EBwne =4.5 V/A and EBIVAr =2.2 V/A [6]. The residual gas pressure in the chamber did not exceed 10-8 Torr. Anthracene molecules were adsorbed on the tungsten tip surface from the gas phase at a partial vapor pressure of 10-7 Torr. At field strengths below 0.7 V/A used to observe the field desorption of anthracene, the field-desorption signal was easy to 54 S.E. Egorov et al.