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Various metal targets bombarded by swiftly moving ions emit from 7 to 20 electrons per incident ion. The majority of these secondary electrons have energies less than 10 electron volts. The commonly used metals such as nickel, copper, molybdenum, aluminum, tungsten, silver, tin, magnesium and cadmium differ but little from one another in emission of electrons. The emission is an increasing function of the ionic energy in the range 0.7 to 2.35 mv. Targets of molybdenum and tungsten heated in vacuum have shown a decreased emission but the effect is temporary. Freshly formed films of the alkali metals show a steadily increasing emission for some time after formation. In the case of lithium the effect is extremely rapid and the emission reaches values as high as 50 electrons per incident ion.
Leo H. Linford (1935) studied this question.