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

American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the "United States" or "America" ** Americans, citizens and nationals of the United States of America ** American ancestry, pe ...

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 Jean O'Brien Dickey (31 October 1945 – 9 May 2018) was an American scientist. Dickey was a pioneering geodesist and particle physicist with expertise in Earth rotation. After receiving a PhD from Rutgers University, she spent much of her caree ...

, he studied the

photoelectric The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...

emission of

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s from

semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...

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 us ...

. 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 Lab ...

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 k ...

for his work.

Biography

Born in

Rochester, New York Rochester () is a City (New York), city in the U.S. state of New York (state), New York, the county seat, seat of Monroe County, New York, Monroe County, and the fourth-most populous in the state after New York City, Buffalo, New York, Buffalo, ...

on June 11, 1915, Apker attended the

University of Rochester The University of Rochester (U of R, UR, or U of Rochester) is a private research university in Rochester, New York. The university grants undergraduate and graduate degrees, including doctoral and professional degrees. The University of Roc ...

, receiving a

Bachelor of Arts Bachelor of arts (BA or AB; from the Latin ', ', or ') is a bachelor's degree awarded for an undergraduate program in the arts, or, in some cases, other disciplines. A Bachelor of Arts degree course is generally completed in three or four years ...

degree in 1937. He then commenced graduate studies there under

Lee Alvin DuBridge Lee Alvin DuBridge () was an American educator and physicist, best known as president of the California Institute of Technology from 1946–1969. Background Lee Alvin DuBridge was born on , in Terre Haute, Indiana. His father was Fred DuBridge, ...

, along with fellow graduate students

Ernest Courant Ernest Courant (March 26, 1920 – April 21, 2020) was an American accelerator physicist and a fundamental contributor to modern large-scale particle accelerator concepts. His most notable discovery was his 1952 work with Milton S. Livingston a ...

Esther M. Conwell Esther Marley Conwell (May 23, 1922 – November 16, 2014) was a pioneering American chemist and physicist, best known for the Conwell-Weisskopf theory that describes how electrons travel through semiconductors, a breakthrough that helped revolut ...

Robert H. Dicke Robert Henry Dicke (; May 6, 1916 – March 4, 1997) was an American astronomer and physicist who made important contributions to the fields of astrophysics, atomic physics, physical cosmology, cosmology and gravity. He was the Albert Einstein ...

, and others. He received his

Ph.D. A Doctor of Philosophy (PhD, Ph.D., or DPhil; Latin: or ') is the most common degree at the highest academic level awarded following a course of study. PhDs are awarded for programs across the whole breadth of academic fields. Because it is a ...

physics Physics is the natural science that studies matter, its 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 which r ...

in 1941. Also in 1941, he began working for the

General Electric Research Laboratory General Electric Research Laboratory was the first industrial research facility in the United States. Established in 1900, the lab was home to the early technological breakthroughs of General Electric and created a research and development environm ...

Schenectady Schenectady () is a city in Schenectady County, New York, United States, of which it is the county seat. As of the 2020 census, the city's population of 67,047 made it the state's ninth-largest city by population. The city is in eastern New Y ...

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

. 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 when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid st ...

, which causes matter to emit

s when exposed to some types of

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...

. In 1916

Robert Andrews Millikan Robert Andrews Millikan (March 22, 1868 – December 19, 1953) was an American experimental physicist honored with the Nobel Prize for Physics in 1923 for the measurement of the elementary electric charge and for his work on the photoelectric e ...

, while verifying the photoelectric equations of

Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...

, 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 ...

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 spelt 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" m ...

, 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 is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...

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 unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ). Historically, one torr was intended to be the same as one "millimeter of mercury ...

. 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 a fl ...

onto a clean

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...

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 halide In chemistry, alkali metal halides, or alkali halides, are the family of inorganic compounds with the chemical formula MX, where M is an alkali metal and X is a halogen. These compounds are the often commercially significant sources of these metal ...

s, particularly

. In potassium iodide, an ionic crystal, some

iodide An iodide ion is the ion 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 defic ...

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

F-Center An F 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 in a crystal lattice is occupied by one or more un ...

s," these defects absorb

visible Visibility, in meteorology, is a measure of the distance at which an object or light can be seen. Visibility may also refer to: * A measure of turbidity in water quality control * Interferometric visibility, which quantifies interference contrast ...

and

ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

, 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 photoconductitivty. Deeper into the ultraviolet spectrum, however, potassium iodide has a strong

absorption line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

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 force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. The ...

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 compound with the formula BaO. It has a cubic structure and is used in cathode ray tubes, crown glass, and catalysts. It is harmful to human skin and if swallowed in large q ...

Legacy

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

LeRoy Apker Award The LeRoy Apker Award is a prize that has been awarded annually by the American Physical Society (APS) since 1978, named after the experimental physicist LeRoy Apker. The recipients are undergraduate students chosen for "outstanding achievements i ...

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 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)