ON THE FULL-WIDTH-AT-HALF-MAXIMUM OF FIELD-ION ENERGY-DISTRIBUTIONS

The full-width-at-half-maximum, FWHM, of rare-gas (He, Ne) field ion energy distributions was measured in three different probe-hole FIMs, all combined with retarding potential analysis. The transmission functions of the spectrometers were analyzed through measurements of the total field electron energy distributions. The FWHM resolution for ion energy spectroscopy ranged between 80 and 280 meV depending on pass energies. Differential distribution curves were submitted to deconvolution procedures involving direct and inverse Fourier transformations. Experimental FWHM data, derived from deconvolved ion energy distributions were compared with results of model calculations. At best image field-strengths and at temperatures above 150 K, field ionization of non-accommodated He and Ne appears to cause relatively broad FWHMs (between 0.6 and 1.0 eV). The FWHMs decrease typically by a factor of two, as the temperature of the emitter is decreased below 79 K. For He+ emission from an individual W[100] zone line decoration atom at T(W) = 45 K, a 287 meV FWHM was determined. In the low temperature case, the postulated explanation is the field ionization of He and Ne being approximately at rest in the instant of ionization. Our measurements indicate that selection of different emission sites and variation of the tip radius affect the FWHMs.