Ion microscope, also known as a field-ion microscope, was one of the most powerful magnifying instruments used in science from about the 1950’s to the 1980’s. It magnified with such sharpness that individual atoms could be seen. Scientists used it to learn how metal atoms are arranged to form crystals and how gases and radiation affect such crystals. For most uses, scanning probe microscopes and other magnifying devices have replaced ion microscopes (see Scanning probe microscope ).
The main part of an ion microscope is a thin needle made of the metal to be examined. This needle is 1,000 times sharper than the tip of an ordinary pin. It points toward a fluorescent screen positioned nearby. When the microscope is in operation, the screen shows an enlarged image of the tip of the needle. This image is a pattern of bright dots. These dots show the arrangement of the metal atoms that form the needle’s tip.
The ion microscope works on the principle of electrical attraction and repulsion. The needle and screen are enclosed in an airless glass tube that contains a small amount of helium, neon, or argon gas. An electric field of up to 30,000 volts is established between the needle and the screen. The needle receives a positive charge, and the screen becomes negatively charged. The needle attracts electrons, which are negatively charged, from gas atoms that drift near its tip. When the gas atoms lose electrons, they become positively charged ions. Because of their charge, the ions are repelled from the atoms that form the tip of the needle. The ions fly directly to the screen. Wherever they strike the screen, they cause it to glow. As the ions stream from the needle, they fan out to cover the screen. They create a magnified image of the needle tip, showing the arrangement of the atoms in the crystal.
The ion microscope was invented in 1951 by Erwin W. Mϋller (also spelled Mueller), a German physicist who moved to the United States that year. He developed it from the field-emission microscope, which he invented in 1936. In this instrument, a high negative voltage is applied to a metal needle and electrons ejected from the needle are attracted to a positively charged screen. The images displayed on the screen are too fuzzy to reveal individual atoms. But they can give information about the ways electrons stream from different metals. In 1951, Mϋller used an ion microscope to take the first picture of the arrangement of atoms on a metal’s surface.
In 1954, Mϋller developed a modified version of the ion microscope, the atom-probe field-ion microscope. It can analyze a single atom on a specimen’s surface. It can also remove atoms from a metal surface and send them to an instrument called a mass spectrometer (see Mass spectrometry ).
