Methods for a precision measurement of ionic masses and appearance energies using the pulsed-laser time-of-flight atom probe

Abstract

An attempt has been made to use the pulsed-laser time-of-flight atom probe for a precision measurement of ionic masses and the critical energy deficits, or the appearance energies, of field emitted ions. For this purpose methods for determining accurately the flight-path constant and the time-delay constant of the atom probe have been devised. Using 300-ps laser pulses and a digital electronic timer of 1-ns resolution and a flight path of slightly over 4 m, ionic masses of pulsed-laser field desorbed gaseous ions can be measured with an accuracy of better than 0.0005 to 0.003 amu for ions of low to intermediate masses. For multiply charged field evaporated heavy metal ions, the accuracy is not as good due mainly to the very limited number of ions which can be collected at a given low emitter voltage. The accuracy of measuring the critical energy deficit depends on the emitter voltage, and the mass and the charge of the ions. It ranges from ∼0.3 eV for singly charged, heavy gas ions at a low emitter voltage to a few electron volts for multiply charged metal ions at a high emitter voltage. The precision of this instrument in these measurements can be further improved by over a factor of 10 simply by using laser pulses of shorter duration and an electronic timer of better resolution. These units are now commercially available. Using this pulsed-laser field ion emission technique, those solids which can be formed into a field ion emitter and those gases which can be field adsorbed on an emitter surface can be studied. The technique also has a very high sensitivity.