LeRoy W. Apker (June 11, 1915 – July 5, 1970) was an American

experimental physicist Experimental physics is the category of disciplines and sub-disciplines in the field of physics that are concerned with the observation of physical phenomena and experiments. Methods vary from discipline to discipline, from simple experiments and o ...

. Along with his colleagues E. A. Taft and Jean Dickey, he studied the

photoelectric The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation such as ultraviolet light. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physic ...

emission of

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s from

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

s and discovered the phenomenon of exciton-induced photoemission in

potassium iodide Potassium iodide is a chemical compound, medication, and dietary supplement. It is a medication used for treating hyperthyroidism, in radiation emergencies, and for protecting the thyroid gland when certain types of radiopharmaceuticals are u ...

. In 1955, he received the

Oliver E. Buckley Condensed Matter Prize The Oliver E. Buckley Condensed Matter Prize is an annual award given by the American Physical Society "to recognize and encourage outstanding theoretical or experimental contributions to condensed matter physics." It was endowed by AT&T Bell Labor ...

of the

American Physical Society The American Physical Society (APS) is a not-for-profit membership organization of professionals in physics and related disciplines, comprising nearly fifty divisions, sections, and other units. Its mission is the advancement and diffusion of ...

for his work.

Biography

Born in

Rochester, New York Rochester is a city in and the county seat, seat of government of Monroe County, New York, United States. It is the List of municipalities in New York, fourth-most populous city and 10th most-populated municipality in New York, with a populati ...

on June 11, 1915, Apker attended the

University of Rochester The University of Rochester is a private university, private research university in Rochester, New York, United States. It was founded in 1850 and moved into its current campus, next to the Genesee River in 1930. With approximately 30,000 full ...

, receiving a

Bachelor of Arts A Bachelor of Arts (abbreviated B.A., BA, A.B. or AB; from the Latin ', ', or ') is the holder of a bachelor's degree awarded for an undergraduate program in the liberal arts, or, in some cases, other disciplines. A Bachelor of Arts deg ...

degree in 1937. He then commenced graduate studies there under Lee Alvin DuBridge, along with fellow graduate students Ernest Courant, Esther M. Conwell, Robert H. Dicke, and others. He received his Ph.D. in

physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

in 1941. Also in 1941, he began working for the General Electric Research Laboratory in

Schenectady Schenectady ( ) is a City (New York), city in Schenectady County, New York, United States, of which it is the county seat. As of the United States Census 2020, 2020 census, the city's population of 67,047 made it the state's ninth-most populo ...

New York New York most commonly refers to: * New York (state), a state in the northeastern United States * New York City, the most populous city in the United States, located in the state of New York New York may also refer to: Places United Kingdom * ...

. On July 5, 1970, he was found by his wife, suffering from a gunshot wound to the head on the driveway of his home. He was taken to a hospital in Schenectady, where he later died.

Research

Photoelectric effect in semiconductors

While at General Electric, he began to research the

photoelectric effect The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation such as ultraviolet light. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physi ...

, which causes matter to emit

s when exposed to some types of

electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...

. In 1916

Robert Andrews Millikan Robert Andrews Millikan ( ; March 22, 1868 – December 19, 1953) was an American physicist who received the Nobel Prize in Physics in 1923 "for his work on the elementary charge of electricity and on the photoelectric effect". Millikan gradua ...

, while verifying the photoelectric equations of

Albert Einstein Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...

, had proposed the idea that photoelectrons emitted from

s should behave in a different way than those emitted from other types of matter, and a very similar theory was advanced by

Edward Condon Edward Uhler Condon (March 2, 1902 – March 26, 1974) was an American nuclear physicist, a pioneer in quantum mechanics, and a participant during World War II in the development of radar and, very briefly, of nuclear weapons as part of the Ma ...

in 1938. In 1948 Apker, working with E. A. Taft and J. E. Dickey, he completed experiments that confirmed Condon's theory. The main discovery made was that photoelectrons from some semiconductors moved much slower than photoelectrons from metals with the same

work function In solid-state physics, the work function (sometimes spelled workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" ...

, an unexpected result which was used to increase understanding of the electronic structure of semiconductors.

Flash filament method

Apker was also active in the field of

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

science. In 1948 he developed the flash filament method for measuring very low pressures, which was the first widely used method for measuring pressures less than

10^-8

Torr The torr (symbol: Torr) is a Pressure#Units, unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (unit), atmosphere (101325 Pa). Thus one torr is exactly (≈ ). Historically, one torr was intended to be ...

. In this method, a gas is allowed to

adsorb Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which ...

onto a clean

tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...

filament for a set amount of time, and the filament is then rapidly heated. The gas adsorbed onto the filament is released, and the resulting pressure burst can be measured. Though very time-consuming, the flash filament method was later used for thermal desorption spectroscopy.

Potassium iodide

Apker followed up his work on the photoelectric effect with an investigation of the photoelectric properties of the alkali halides, particularly

. In potassium iodide, an ionic crystal, some

iodide An iodide ion is I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine deficiency ...

ions can be removed and their vacant places will be filled by electrons. Called "

F-Center An F-center or color center or Farbe center (from the original German ''Farbzentrum'', where ''Farbe'' means ''color'' and ''zentrum'' means ''center'') is a type of crystallographic defect in which an anionic Vacancy defect, vacancy in a crystal ...

s," these defects absorb visible and

ultraviolet light Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of th ...

, coloring the crystals at photon energies where they are usually transparent. Additionally, the absorption of visible radiation can free trapped electrons inside the crystal and produce photoconductivity. Apker found that in addition to visible radiation, near-ultraviolet radiation also produces photoconductivity. Deeper into the ultraviolet spectrum, however, potassium iodide has a strong

absorption line Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption (electromagnetic radiation), absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. Th ...

due to the formation of chargeless particles called

exciton An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb's law, Coulomb force resulting from their opposite charges. It is an electrically neutral quasiparticle regarded as ...

s. These excitons transfer energy to the electrons in the

s with remarkably high efficiency, and these excited electrons are excited from the crystals in exciton-induced photoemission. Apker observed the same sort of behavior in other crystals such as

barium oxide Barium oxide, also known as baria, is a white hygroscopic non-flammable chemical compound, compound with the formula BaO. It has a Cubic crystal system, cubic structure and is used in cathode-ray tubes, crown glass, and Catalysis, catalysts. It ...

Legacy

In 1978, Apker's wife and colleague Jean Dickey Apker established the LeRoy Apker Award of the

in memory of Apker. The award is presented to two college undergraduates each year.

Bibliography

* * * * *

References

{{DEFAULTSORT:Apker, LeRoy 1915 births 1970 suicides 1970 deaths American experimental physicists Scientists from Rochester, New York University of Rochester alumni Oliver E. Buckley Condensed Matter Prize winners Suicides by firearm in New York (state)