Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter threshold wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and reemits it for a much longer time after the radiation source is removed. There is no distinct boundary between the emission times of fluorescence and phosphorescence (i.e. if a substance glows under a black light it is generally considered fluorescent, and if it glows in the dark it is often simply called phosphorescent). The phenomena can usually be classified by the three different mechanisms that produce the light and the typical timescales at which they emit light: fluorescence, triplet phosphorescence, and persistent phosphorescence. Fluorescent materials stop emitting light within nanoseconds (billionths of a second) after the excitation radiation is removed, while phosphorescent materials may continue to emit an afterglow ranging from a few microseconds to many hours after the excitation is removed. There are two separate mechanisms that may produce phosphorescence, called triplet phosphorescence (or simply phosphorescence) and persistent phosphorescence (or persistent luminescence):
Triplet phosphorescence occurs when an atom absorbs a high-energy photon, and the energy becomes locked in the spin multiplicity of the electrons, generally changing from a fluorescent singlet state to a slower emitting triplet state. The slower timescales of the reemission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur relatively slowly in certain materials, absorbed radiation is reemitted at a lower intensity, ranging from a few microseconds to as much as one second after the excitation is removed. Persistent phosphorescence occurs when an atom absorbs a high-energy photon and an electron becomes trapped in a defect in the lattice of the crystalline or amorphous material. A defect such as a missing atom (vacancy defect) can trap an electron like a pitfall, storing that electron's energy until released by a random spike of thermal (vibrational) energy. Such a substance will then emit light of gradually decreasing intensity, ranging from a few seconds to up to several hours after the original excitation. Everyday phosphorescent materials include glow-in-the-dark toys, stickers, paint, and clock dials that glow after being charged with a bright light such as in any normal reading or room light. Typically, the glow slowly fades out, sometimes within a few minutes or up to a few hours in a dark room. The study of phosphorescent materials led to the discovery of radioactive decay. Uranium salts are phosphorescent and fog photographic plates sensitive to x-rays. For years it was thought that phosphorescence was the cause of the fogging. In 1896 Henri Becquerel left uranium salts in a closed drawer with photographic plates and later discovered the plates had fogged without a light source to excite the salts. Becquerel's discovery that the uranium salts emitted radiation inspired the work of Marie Curie and yielded both a Nobel Prize in 1903.
Etymology The term phosphorescence comes from the Ancient Greek word φῶς (phos), meaning "light", and the Greek suffix -φόρος (-phoros), meaning "to bear", combined with the Latin suffix -escentem, meaning "beginning, becoming, tending to be". Thus, phosphorescence literally means "having a tendency to bear light". It was first recorded in 1766. The term phosphor had been used since the Middle Ages to describe minerals that glowed in the dark. One of the most famous, was Bolognian phosphor, or lapis solaris, discovered around 1604 by Vincenzo Casciarolo near Bologna, Italy. After being heated in an oxygen-rich furnace, it absorbed sunlight and glowed in the dark. In 1677, Hennig Brand isolated a new element that glowed due to a chemiluminescent reaction when exposed to air, and named it "phosphorus". The term luminescence (from the Latin lumen for "light"), was coined by Eilhardt Wiedemann in 1888 as a term to refer to "light without heat", while Sir George Stokes coined florescence in 1852, when he noticed that a solution of quinine sulfate exposed to light refracted through a prism glowed when exposed to invisible-light (now known to be UV light) beyond the violet end of the spectrum. Stokes formed the term from a combination of fluorspar and opalescence (preferring to use a mineral instead of a solution). It was later discovered that fluorspar glows due to phosphorescence. There was much confusion between the meanings of these terms throughout the late nineteenth to mid-twentieth centuries. Florescence mostly referred to luminescence that, to the eye, ceased immediately when removed from excitation. Phosphorescence referred to most substances that glowed for in darkness, sometimes including chemiluminescence. After the 1950s and 1960s, advances in quantum electronics, spectroscopy, and lasers made it possible to distinguish processes, although colloquially the terms are often used interchangeably.
Introduction
… excerpt ends here. Continue reading the full article.
