Luminescence physics

Mechanism of luminescence

The emission of visible light (that is, light of wavelengths between about 690 nanometres and 400 nanometres, corresponding to the region between deep red and deep violet) requires excitation energies the minimum of which is given by Einstein’s law stating that the energy (E) is equal to Planck’s constant (h) times the frequency of light (ν), or Planck’s constant times the velocity of light (c) in a vacuum divided by its wavelength (λ); that is,

The energy required for excitation therefore ranges between 40 kilocalories (for red light), about 60 kilocalories (for yellow light), and about 80 kilocalories (for violet light) per mole of substance. Instead of expressing these energies in kilocalories, electron volt units (one electron volt = 1.6 × 10⁻¹² erg; the erg is an extremely small unit of energy) may be used, and the photon energy thus required in the visible region ranges from 1.8 to 3.1 electron volts.

The excitation energy is transferred to the electrons responsible for luminescence, which jump from their ground-state energy level to a level of higher energy. The energy levels that electrons can assume are specified by quantum mechanical laws. The different excitation mechanisms considered below depend on whether or not the excitation of electrons occurs in single atoms, in single molecules, in combinations of molecules, or in a crystal. They are initiated by the means of excitation described above: impact of accelerated particles such as electrons, positive ions, or photons. Often, the excitation energies are considerably higher than those necessary to lift electrons to a radiative level; for example, the luminescence produced by the phosphor crystals in television screens is excited by cathode-ray electrons with average energies of 25,000 electron volts. Nevertheless, the colour of the luminescent light is nearly independent of the energy of the exciting particles, depending chiefly on the excited-state energy level of the crystal centres.

Electrons taking part in the luminescence process are the outermost electrons of atoms or molecules. In fluorescent lamps, for example, a mercury atom is excited by the impact of an electron having an energy of 6.7 electron volts or more, raising one of the two outermost electrons of the mercury atom in the ground state to a higher level. Upon the electron’s return to the ground state, an energy difference is emitted as ultraviolet light of a wavelength of 185 nanometres. A radiative transition between another excited state and the ground-state level of the mercury atom produces the important ultraviolet emission of 254-nanometre wavelength, which, in turn, can excite other phosphors to emit visible light. (One such phosphor frequently used is a calcium halophosphate incorporating a heavy-metal activator.)

This 254-nanometre mercury radiation is particularly intensive at low mercury vapour pressures (around 10⁻⁵ atmosphere) used in low-pressure discharge lamps. About 60 percent of the input electron energy may thus be transformed into near-monochromatic ultraviolet light—i.e., ultraviolet light of practically one single wavelength.

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chemical analysis: Luminescence

Whereas at low pressure there are relatively few collisions of mercury atoms with each other, the collision frequency increases enormously if mercury gas is excited under high pressure (e.g., eight atmospheres or more). Such excitation leads not only to collisional de-excitation of excited atoms but also to additional excitation of excited atoms. As a consequence, the spectrum of the emitted radiation no longer consists of practically one single, sharp spectral line at 254 nanometres, but the radiation energy is distributed over various broadened spectral lines corresponding to different electronic energy levels of the mercury atom, the strongest emissions lying at 303, 313, 334, 366, 405, 436, 546, and 578 nanometres. High-pressure mercury lamps can be used for illumination purposes because the emissions from 405 to 546 nanometres are visible light of bluish green colour; by transforming a part of the mercury line emission to red light by means of a phosphor, white light is obtained.

When gaseous molecules are excited, their luminescence spectra show broad bands; not only are electrons lifted to levels of higher energy but vibrational and rotational motions of the atoms as a whole are excited simultaneously. This is because vibrational and rotational energies of molecules are only about 10⁻² and 10⁻⁴, respectively, those of the electronic transition energies, and these many energies can be added to the energy of a single electronic transition, which is represented by a multitude of slightly different wavelengths making up one band. In larger molecules, several overlapping bands, one for each kind of electronic transition, can be emitted. Emission from molecules in solution is predominantly bandlike caused by interactions of a relatively great number of excited molecules with molecules of the solvent. In molecules, as in atoms, the excited electrons generally are outermost electrons of the molecular orbitals.

The terms fluorescence and phosphorescence can be used here, on the basis not only of the persistence of luminescence but also of the way in which the luminescence is produced. When an electron is excited to what is called, in spectroscopy, an excited singlet state, the state will have a lifetime of about 10⁻⁸ second, from which the excited electron can easily return to its ground state (which normally is a singlet state, too), emitting its excitation energy as fluorescence. During this electronic transition the spin of the electron is not altered; the singlet ground state and the excited singlet state have like multiplicity (number of subdivisions into which a level can be split). An electron, however, may also be lifted, under reversal of its spin, to a higher energy level, called an excited triplet state. Singlet ground states and excited triplet states are levels of different multiplicity. For quantum mechanical reasons, transitions from triplet states to singlet states are “forbidden,” and, therefore, the lifetime of triplet states is considerably longer than that of singlet states. This means that luminescence originating in triplet states has a far longer duration than that originating in singlet states: phosphorescence is observed.

The interactions of a large number of atoms, ions, or molecules are greater still in solution and in solids; to obtain a narrowing of the spectral band, subzero temperatures (down to that of liquid helium) are applied in order to reduce vibrational motions. The electronic energy levels of crystals such as zinc sulfide and other host crystals used in phosphors form bands: in the ground state practically all electrons are to be found on the valence band, whereas they reach the conduction band after sufficient excitation. The energy difference between the valence band and the conduction band corresponds to photons in the ultraviolet or still shorter wavelength region. Additional energy levels are introduced by activator ions or centres bridging the energy gap between valence band and conduction band, and, when an electron is transferred from the valence band to such an additional energy level by excitation energy, it can produce visible light on return to the ground state. A rather close analogy exists between the forbidden transitions of certain excited molecular electronic states (triplet–singlet, leading to phosphorescence) and the transition of an electron of an inorganic phosphor kept in a trap: traps (certain distortions in the crystal lattice) are places in the crystal lattice where the energy level is lower than that of the conduction band, and from which the direct return of an electron to the ground state is also forbidden.

When a solid is bombarded by photons or particles, the excitation of the centres can occur directly or by energy transfer. In the latter case, excited but nonluminescing states are produced at some distance from the centre, with the energy moving through the crystal in the form of excitons (ion-electron pairs) until it approaches a centre where the excitation process can occur. This energy transfer can also be realized by radiation in inorganic phosphors containing two activators, as well as in solutions of organic molecules.

Spontaneous and stimulated emission

The radiative return of excited electrons to their ground state occurs spontaneously, and when there exists an assembly of excited electrons their individual spontaneous radiative transitions are independent of each other. Therefore, the luminescence light is incoherent (the emitted waves are not in phase with each other) in this case. Sometimes the emission of luminescence can be stimulated by irradiation with photons of the same frequency as that of the emitted light; such stimulated transitions are used in lasers, which produce very intensive beams of coherent monochromatic light.

The spontaneous luminescent emission follows an exponential law that expresses the rate of intensity decay and is similar to the equation for the decay of radioactivity and some chemical reactions. It states that the intensity of luminescent emission is equal to an exponential value of minus the time of decay divided by the decay time, or L = L₀ exp (−t/τ), in which L is the intensity of emission at a time t after an initial intensity L₀, and τ is the decay time of the luminescence; that is, the time in which the assembly of the excited atoms would decrease in luminescence intensity to a value of 0.368 L₀.

When excited atoms of the centres are in contact with other atoms, as is the case in condensed phases (liquids, solids, in gases of not-too-low pressure), part of the excitation energy will be transformed into heat by collisional deactivation (thermal quenching). The decay time, therefore, has to be replaced by an effective excited-state lifetime, resulting in a more complicated exponential decay law that depends on the collision frequency, the energy imparted to the excited atoms of the centre that causes the transfer of excitation energy into heat (activation energy), a constant, and the temperature of the luminescent material. This law describes the actual luminescence decay of a great number of luminescent materials—e.g., calcium tungstate.

Increase of activation energy for nonradiative deactivation of excited-centres luminescence decay can be achieved by changing the host crystal or by electron traps. The traps are imperfections in the crystal lattice where electrons are captured after they have been ejected from a luminescent centre by excitation energy. That the luminescent properties of phosphor centres are strongly dependent on the chemical nature of the host crystal may be seen in the Table, showing that the same activator ions (manganese ions with two positive charges, indicated as Mn²⁺, or Mn[II]), in different host crystals yield remarkably different-coloured emissions and decay times (measured in fractions of a second).

Prolonging the emission time of phosphors up to days or even longer (production of phosphorescence of the phosphors) is possible by inserting traps into the host crystal. Trapped electrons cannot return directly to the centre. In order to be released from the traps they must first obtain additional thermal energy—in this case, thermal energy stimulates luminescence—after which they recombine with a centre and undergo radiative transition. Trapping in crystals has its analogy to forbidden transitions in molecules (triplet–singlet transitions) or in radiation processes from metastable atomic energy levels.

An example of a practical application of stimulated emission of a phosphor with trapped electrons is cubic strontium sulfide/selenide activated with samarium and europium ions, the coactivators being strontium sulfate and calcium fluoride. This phosphor has been used in devices for viewing scenes at night by reflected infrared light emitted by infrared lamps. The traps in this phosphor have been identified as samarium ions, whereas europium ions are the active ions in the centres. The phosphor is first excited by photons of about three electron volts (blue light), which results in an ejection of an electron from a europium ion (Eu²⁺) centre. This excited electron is trapped by a triply charged samarium ion (Sm³⁺), which is transferred to a doubly charged samarium ion (Sm²⁺). Heat or irradiation by infrared photons releases one electron from the doubly charged samarium ion (Sm²⁺). The electron is then recaptured from a triply charged europium ion (Eu³⁺), yielding an excited doubly charged europium ion (Eu²⁺), which returns to its ground state by emitting a photon of 2.2 electron volts energy (yellow light). The trap depth of this phosphor (i.e., the energy required for release of an electron from it) is large compared with the thermal energy of the lattice of the host crystal, and, therefore, the lifetime of the traps at room temperature is many months long. Bombarding this phosphor with photons of energy higher than that of infrared photons but not sufficient for excitation can lead to photoquenching: the traps are emptied far more rapidly, and thermal deactivation of the centres is enhanced.

When iron, cobalt, or nickel ions are present in a phosphor, an excited electron can be captured by these ions. The excitation energy is then emitted as infrared photons, not as visible light, so that luminescence is quenched. These ions, therefore, are called killers—the killing process being opposite to stimulation.

In chemiluminescence, such as the oxidation of luminol, light emission depends not only on radiative and quenching or intramolecular deactivation processes but also on the efficiency of the chemical reaction leading to molecules in an electronically excited state.

In bioluminescence reactions, the production of electronically excited molecules, as well as their radiative transitions back to their ground state, is efficiently catalyzed by the enzymes acting here, and bioluminescence light output is therefore high.

The luminescence photons emitted by one kind of excited atom, molecule, or phosphor can excite another to emit its specific luminescence: this type of energy transfer is observed with inorganic as well as organic substances. Thus, excited benzene molecules can excite naphthalene molecules by radiative energy transfer. The radiation produced by the luminol chemiluminescence can produce fluorescence when fluorescein is added to the reaction mixture. In most of these cases the acceptor molecules have luminescent electrons with energy levels lower than those of the primary excited molecules, and emitted secondary luminescence is therefore of longer wavelength than the primary. Practical application of this phenomenon, called cascading, is used in radar kinescopes, which have composite fluorescent screens consisting of a layer of blue-emitting zinc sulfide/silver (chloride) phosphor—the hexagonal crystal, ZnS/Ag(Cl) deposited on a layer of yellow-emitting zinc or cadmium sulfide/copper [chloride] phosphor [the hexagonal crystal, (Zn,Cd)S/Cu (Cl)].

The cathode-ray electrons excite the blue-emitting phosphor, whose photons, in turn, excite the yellow-emitting phosphor, which has traps with a decay time of about 10 seconds. Excitation of the blue-emitting phosphor alone would be unfavourable, as the sharply focussed cathode rays are absorbed by the blue phosphor to a small extent only, and its decay time is too short; also, direct excitation of the yellow-emitting phosphor alone would yield poor efficiency because the traps are emptied too rapidly by the heat produced by the relatively high-energetic electron impact.

Another energy-transfer mechanism is referred to as sensitization: a calcium carbonate phosphor (rhombohedral CaCO₃/Mn), for example, emits orange light under cathode-ray irradiation but is not excited by the 254-nanometre emission of mercury atoms, whereas this emission produces the same orange light with calcium carbonate (rhombohedral CaCO₃) activated by manganese and lead ions. This is not cascade luminescence: a mechanical mixture of a manganese and a lead-activated calcium carbonate exhibits no emission under ultraviolet radiation. In a phosphor containing both activators, the lead ions act as sensitizers in introducing an additional excitation band into the system from which the manganese ions get their excitation energy in a nonradiative energy transfer. Similar sensitization is observed in gases and in liquids.

Solid-state energy states

The complicated problems concerning the energy states in solids of a luminescence centre are commonly visualized by adapting the energy-level diagram used in describing energy transitions in an isolated diatomic molecule (Figure 1).

In this diagram, the potential energy of a centre is plotted as a function of the average distance (x̄) between the atoms: x̄* represents the ground state and x̄_o represents the lowest excited state of the centre. In a tetrahedral permanganate-ion centre (MnO₄), for example x̄ would be the average distance between the central manganese ion and an oxygen ion in any of the corners of the tetrahedron.

At a temperature of absolute zero the ground-state energy level is near the bottom of curve I at the minimum amplitude of atomic vibration. At room temperature (300 K [81° F]) the ground state lies higher, at a, where the centre has considerable vibrational energy. When an electron of the centre is excited, it is lifted to the higher energy level at b in curve II. This electronic transition occurs far more rapidly than the readjustment of the atoms of the centre, which then occurs within a time of about 10⁻¹² second to reach the minimum vibrational level at c. The energy difference (b − c) is dissipated as heat in the host-crystal lattice. From the excited-state level c, the electron can return to the ground-state level d shown in Curve I, the liberated energy being emitted as a photon.

The last step is a readjustment of the centre to a, the energy difference (d − a) again being dissipated as heat. Nonradiative transition of the excited electron back to its ground state occurs when the electron is excited to an energy level above the intersection point f of the ground-state and the excited-state energy curve. This is caused mainly by increasing the vibrations of the lattice by application of higher temperatures. The energy difference (f − c) is equal to the activation energy already mentioned, and therefore most centres become increasingly nonradiative at higher temperatures. In trap-type phosphors the temperature must be sufficiently high, of course, to eject the electron from the traps.

In some phosphors—calcium tungstate (CaWO₄), for example—absorption and emission of the exciting energy appear to take place mainly in the same centre; the excited electron remains near the centre. Such phosphors do not exhibit photoconductivity because only a few excited electrons succeed in reaching the conduction band where they are freely mobile. The luminescence decay is exponential.

Zinc sulfide phosphors, however, are photoconducting, which means that many excited electrons are lifted to the conduction band of the host crystal. The energy levels of different centres and of the host-crystal lattice have to be taken into account simultaneously.

The relative levels of the zinc sulfide valence band (ground state of the host-crystal lattice) and the conduction band (excited state of the host-crystal lattice), of activator levels and of trap levels are shown in Figure 2. Points 1, 2, 3, and 4 represent one situation in a host crystal, and points 5, 6, 7, 8, 9, and 10 represent another situation.

The activator ions introduce additional ground-state levels and excited-state levels of energies between those of the valence and the conduction band of the zinc sulfide. When the excitation energy is sufficiently high, an electron is raised to the conduction band (1 → 2, 5 → 6, corresponding to the ionization continuum in a gas). It moves away from the centre (2 → 3, 6 → 8) and may either be trapped by an imperfection of the lattice (8) or return to an ionized centre (activator), in which it first occupies an excited level (3 → 4) and then drops to the ground state of the activator centre by emitting a photon. An activator centre that captures such an excited electron has already lost one of its own electrons to a positive hole (electron vacancy) in the host-crystal lattice.

The energetic level of the traps is about 0.25 electron volt beneath the conduction-band level. A trapped electron (8) must be raised to the conduction band by thermal energy before a recombination with an ionized activator centre can occur. The green emission (530 nanometres) of the zinc sulfide phosphor (ZnS/Cu) is explained by the recombination of an electron from the conduction band and a copper ion in an activator centre (7 → 9); the blue emission (463 nanometres) is due to recombination of the excited electron and a copper ion in an interstitial place.

Direct excitation of the activator centres is also possible. When an electron recombines with a killer ion (10), no visible emission occurs.

In solid-state electroluminescence, the radiative processes occurring in a phosphor under irradiation are produced by applying external electric fields of several hundred volts, alternating at several thousand cycles per second. Special preparations of zinc sulfide (hexagonal ZnS), with an iodine coactivator and high concentrations of a copper activator, are embedded in a thin layer of about 0.01 centimetre (0.004 inch) of insulating organic material or glass, which is mounted between the electrodes.

High luminescence efficiencies result. Application of a direct-current field yields luminescence in crystals of gallium arsenide (GaAs), silicon carbide (SiC), cadmium sulfide (CdS), and zinc monocrystals of sulfide with copper activator (ZnS/Cu); the cathode injects electrons into the conduction band, whereas the anode removes electrons.

Efficiency of luminescence; luminance

The efficiency of luminescence emission must be regarded on an energy and a quantum basis. When every exciting photon yields an emitted photon of the same energy (as is the case for resonance excitation—i.e., excitation of fluorescence by a monochromatic light of exactly the same wavelengths as the resulting fluorescence—and radiation of isolated atoms in dilute gases), the luminescence efficiency is 100 percent with respect to input energy as well as to the number of quanta. When the number of secondary photons is equal to that of the primary but their energy is less because some energy is dissipated as heat, the quantum efficiency is 100 percent but the luminescence efficiency is less than 100 percent. The quantum efficiency of most luminescences is far lower than 100 percent; zinc sulfide phosphors have about 20 percent efficiency, and solid-state electroluminescence is less than 10 percent efficient.

In chemiluminescence the quantum efficiency is about 1 percent in “brilliant” reactions, such as the oxidation of luminol, and up to 23 percent in the oxalate chemiluminescence. Solid-state electroluminescence, or electroluminescence of gases excited by high-frequency electric fields, is usually less than 10 percent.

The light intensity of luminescent processes depends chiefly on the excitation intensity, the density, and the lifetime of the radiative atoms, molecules, or centres. For practical purposes this luminous intensity per unit area is called photometric brightness or luminance of a material and is measured in lambert or millilambert (0.001 lambert) units (one lambert is equal to one candle per square centimetre divided by π).