LeRoy Apker
Encyclopedia
LeRoy W. Apker was an American
experimental physicist
. Along with his colleagues E. A. Taft and Jean Dickey, he studied the photoelectric
emission of electron
s from semiconductor
s and discovered the phenomenon of exciton-induced photoemission in potassium iodide
. In 1955, he received the Oliver E. Buckley Condensed Matter Prize of the American Physical Society
for his work.
on 11 June 1915, Apeker attended the University of Rochester
, receiving a Bachelor of Arts
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
in 1941. Also in 1941, he began working for the General Electric Research Laboratory
in Schenectady, New York
.
, which causes matter to emit electron
s when exposed to some types of electromagnetic radiation
. In 1916 Robert Andrews Millikan, while verifying the photoelectric equations of Albert Einstein
, had proposed the idea that photoelectrons emitted from semiconductor
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
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 photelectrons from metals with the same work function
, an unexpected result which was used to increase understanding of the electronic structure of semiconductors.
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
Torr
. In this method, a gas is allowed to adsorb
onto a clean tungsten
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
.
s, particularly potassium iodide
. In potassium iodide, an ionic crystal, some iodide
ions can be removed and their vacant places will be filled by electrons. Called "F-Center
s," these defects absorb visible and ultraviolet light, 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 due to the formation of chargeless particles called exciton
s. These excitons transfer energy to the electrons in the F-Center
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
.
of the American Physical Society
in memory of Apker. The award is presented to two college undergraduates each year.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
experimental physicist
Experimental physics
Within the field of physics, experimental physics is the category of disciplines and sub-disciplines concerned with the observation of physical phenomena in order to gather data about the universe...
. Along with his colleagues E. A. Taft and Jean Dickey, he studied the photoelectric
Photoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
emission of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s from semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
s and discovered the phenomenon of exciton-induced photoemission in potassium iodide
Potassium iodide
Potassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...
. In 1955, he received the Oliver E. Buckley Condensed Matter Prize of the American Physical Society
American Physical Society
The American Physical Society is the world's second largest organization of physicists, behind the Deutsche Physikalische Gesellschaft. The Society publishes more than a dozen scientific journals, including the world renowned Physical Review and Physical Review Letters, and organizes more than 20...
for his work.
Biography
Born in Rochester, New YorkRochester, New York
Rochester is a city in Monroe County, New York, south of Lake Ontario in the United States. Known as The World's Image Centre, it was also once known as The Flour City, and more recently as The Flower City...
on 11 June 1915, Apeker attended the University of Rochester
University of Rochester
The University of Rochester is a private, nonsectarian, research university in Rochester, New York, United States. The university grants undergraduate and graduate degrees, including doctoral and professional degrees. The university has six schools and various interdisciplinary programs.The...
, receiving a Bachelor of Arts
Bachelor of Arts
A Bachelor of Arts , from the Latin artium baccalaureus, is a bachelor's degree awarded for an undergraduate course or program in either the liberal arts, the sciences, or both...
degree in 1937. He then commenced graduate studies there under Lee Alvin DuBridge
Lee Alvin DuBridge
Lee Alvin DuBridge was an American educator and physicist.DuBridge was born in Terre Haute, Indiana, and graduated from Cornell College in 1922, and then began a teaching assignment at the University of Wisconsin–Madison, from which he received an M.A. degree in 1924 and a Ph.D. in 1926...
, along with fellow graduate students Ernest Courant
Ernest Courant
Ernest Courant has been called the "father of modern particle accelerators". A fundamental contributor to the field, he has also been mentor to several generations of students...
, Esther M. Conwell
Esther M. Conwell
Esther Marley Conwell is a physicist who studied properties of semiconductors and organic conductors, especially transport.-Biography:...
, Robert H. Dicke
Robert H. Dicke
Robert Henry Dicke was an American physicist who made important contributions to the fields of astrophysics, atomic physics, cosmology and gravity.-Biography:...
, and others. He received his Ph.D.
Doctor of Philosophy
Doctor of Philosophy, abbreviated as Ph.D., PhD, D.Phil., or DPhil , in English-speaking countries, is a postgraduate academic degree awarded by universities...
in physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
in 1941. Also in 1941, he began working for the General Electric Research Laboratory
General Electric Research Laboratory
General Electric Research Laboratory, the first industrial research facility in the United States, was established in 1900. This lab was home to the early technological breakthroughs of General Electric and created a research and development environment that set the standard for industrial...
in Schenectady, New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
.
Photoelectric effect in semiconductors
While at General Electric, he began to research the photoelectric effectPhotoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
, which causes matter to emit electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s when exposed to some types of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
. In 1916 Robert Andrews Millikan, while verifying the photoelectric equations of Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
, had proposed the idea that photoelectrons emitted from semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
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 Condon
Edward Uhler Condon was a distinguished American nuclear physicist, a pioneer in quantum mechanics, and a participant in the development of radar and nuclear weapons during World War II.-Early life and career:...
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 photelectrons from metals with the same work function
Work function
In solid-state physics, the work function is the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface...
, 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 vacuumVacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
science. In 1948 he developed the flash filament method for measuring very low pressures
Pressure measurement
Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges....
, which was the first widely used method for measuring pressures less than
TorrTorr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
. In this method, a gas is allowed to adsorb
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
onto a clean tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
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
Thermal desorption spectroscopy
Thermal desorption spectroscopy , also known as temperature programmed desorption is a Buzzword for the method of observing desorbed molecules from a surface when the surface temperature is increased...
.
Potassium iodide
Apker followed up his work on the photoelectric effect with an investigation of the photoelectric properties of the alkali halideAlkali halide
The alkali halides are the family of ionic compounds with simple chemical formula X+Y- or XY, where X is an alkali metal and Y is a halogen. One of the most well known of these is sodium chloride or common table salt....
s, particularly potassium iodide
Potassium iodide
Potassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...
. In potassium iodide, an ionic crystal, some iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
ions can be removed and their vacant places will be filled by electrons. Called "F-Center
F-Center
An F-Center or Farbe center is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more electrons, depending on the charge of the missing ion in the crystal. Electrons in such a vacancy tend to absorb light in the visible spectrum such that a material...
s," these defects absorb visible and ultraviolet light, 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 due to the formation of chargeless particles called exciton
Exciton
An exciton is a bound state of an electron and 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...
s. These excitons transfer energy to the electrons in the F-Center
F-Center
An F-Center or Farbe center is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more electrons, depending on the charge of the missing ion in the crystal. Electrons in such a vacancy tend to absorb light in the visible spectrum such that a material...
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
Barium oxide, BaO, is a white hygroscopic compound formed by the burning of barium in oxygen, although it is often formed through the decomposition of other barium salts.It reacts with water to form barium hydroxide.-Uses:...
.
Legacy
In 1978, Apker's wife and colleague Jean Dickey Apker established the LeRoy Apker AwardLeRoy Apker Award (APS)
The LeRoy Apker Award is a prize that has been awarded annually by the American Physical Society since 1978, named after the experimental physicist LeRoy Apker...
of the American Physical Society
American Physical Society
The American Physical Society is the world's second largest organization of physicists, behind the Deutsche Physikalische Gesellschaft. The Society publishes more than a dozen scientific journals, including the world renowned Physical Review and Physical Review Letters, and organizes more than 20...
in memory of Apker. The award is presented to two college undergraduates each year.