THE FUNDAMENTALS OF FLUORESCENCE ARNET L. POWELL Office of Naval Research Boston, Massachusetts
INTRODUCTION
lies just below the visible region of 4000-7800 a.u. (see With the increase in general interest in fluorescence Figure 1). and phosphorescence in recent years as the result of the 1850 3000 4000 7800 advent of the fluorescent lamp and the increasing appliSchumann Far Near cations of the so-called "black-light" type of luminesRegion Ultraviolet Ultraviolet Visible Infrared cence, it seems desirable a t the present time to review Figure 1. Electromagnetic Spectrum some of the fundamentals of these phenomena. This ultraviolet region extends from about 1850Fluorescence and the closely related property of phosphorescence both come under the general designa- 4000 a x . and is usually subdivided into two regions: tion of luminescence (or photoluminescence) which may 185Lb3000 a x . , known as the far ultraviolet, and 3000be defined as the production of light by a substance un- 4000 a.u., the near ultraviolet. Energy in the far ultrader excitation by energy of another wave length. The violet is utilized for excitation of the fluorescent powders exciting radiation must have a wave length which falls used in the commercial fluorescent lamp and as a source within a critical absorption band and, in general, the of bacteria killing energy in germicidal lamps of various emitted light is of a longer wave length than that of the kinds. Radiation in the near ultraviolet produces tanning and burning of the skin and is used to excite a exciting energy (Stokes' law). The difference between fluorescence and pbosphores- large number of organic and inorganic substances to cence lies entirely in the time lag involved in the.re- fluorescence and phosphorescence. Fluorescence can be produced in a large number of emission of light after the exciting source has been removed. With fluorescence, emission .of light by the common organic substances, such as oils, grease, dairy substance ceases immediately when the energizing products, seeds, plants, woods, paper, etc. It is true, radiation is extinguished. For this case the duration of however, that in most instances the emitted radiation afterglow has been found by a number of investigators is of a low order of brightness. Fluorescence can also be t o be of the order of lo-@seconds. A phosphorescent excited in many minerals and artificially prepared submaterial, on the other hand, continues to emit light for a stances, both solid and in solution. Examples may be period of time after the energizing source has been re- found in the case of such dyestuffs as rbodamine B, moved. The duration of the afterglow varies with the eosin, and fluorescein and such solid inorganic subsubstance and may be anything from 10W2seconds to stances as sulfides, oxides, borates, tungstates, and silicates. several days. The phenomenon of fluorescence is attributed to the Another phenomenon known as chemiluminescence involves the production of light by chemical excitation displacement of an electron from the outer shell of an of the molecules. Luminescence of this type is com- atom to a higher energy level; on its return to its normal monly observed in decaying wood when certain types level it may emit light in the visible region in passing of bacteria are present, the firefly, luminous marine or- through various transmission I'evels. I n the case of ganisms, and yellow phosphorous. In all instances, an phosphorescence the same type of electron shift occurs oxidation process is involved in the evolution of the but a delay in the return of the electron t o its normal light. An easily demonstrated example is afforded by energy level enables the light emission t o persist after 3-aminophthalhydrazide, or L'luminol," which emits a the exciting source has been removed. brilliant blue luminescence in alkaline solution in the FLUORESCENCE OF GASES presence of an oxidizing agent. Fluorescence may oecur in gases and in this case the Generally, as already pointed out, fluorescence is produced by excitation of a substance with light of short phenomenon is known as resonance radiation. The exn-ave length which is absorbed and converted t o an citing energy must have a wave length corresponding t o emitted radiation of longer wave length. This follows an absorption band of the gas. Excitation of the atoms the classical formula laid down by Stokes' law. There or molecules of the gas by absorption of energy is sufficiare, however, a few exceptions which are generally de- ent to cause an electron to be driven out to an outer orbit. scribed as "Anti-Stokes' Radiation." The luminescence is caused by the return of the exA convenient source of the short wave-length energy required for fluorescence excitation is provided by the cited electron to its normal orbit. This electron return ultraviolet region of the electromagnetic spectrum which is accompanied by emission of energy of the same fre-
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the luminescence first increasing and then decreasing as the concentration of the fluorescent compound is increased. As the concentration of the dissolved substance increases in solution, the probability of collisions between excited and unexcited molecules of the same kind also E,= E, E, = hv increases. As a result of this the total number of excited where E, represents the total energy of the atom in the molecules is reduced by collisions of the second k i d and higher (excited) state and En the total energy in the the fluorescence is thus diminished. lower (normal) state. This difference may be equated Collisions with solvent molecules apparently do not to hv, where his Planck's constant and v the frequency of quench the fluorescence appreciably. This is quite well the emitted energy. established by the fact that there is not much diierence To illustrate this, let us use mercury vapor as an ex- in intensity of fluorescence of the same substance in ample. From Figure 2, which is the energy level diagram different solvents. With higher concentrations in solution, the fluorescence is quenched by association of the molecules. The I SINGLETS TRIPLETS larger, associated molecules which are formed under I these conditions do not fluoresce. In the case of solids, the conditions under which fluorescence will occur are somewhat more complex. Some organic substances will fluoresce in the dry state but more often it is necessary to dissolve them in solvents. Some dyes exhibit fluorescence on the dyed fabric but neither in solution nor in powdered form. In such instances the nature of the fabric as well as the concentration of dyestuff used has a marked effect on the intensity of the fluorescence. In the past a number of complicated theories wereadvanced to explain fluorescence in solution. A study of the substances which fluoresce showed that all the fluorescing molecules which had been observed had a complex chemical structure. An attempt was made to connect visible color and fluorescent color with the groups within such a molecule. "Chromophore" groups were considered as producing the color, "auxochromes" as enhancing the color, and "fluorophor" groups as producing fluorescence. In the light of present-day information these considerations are of little importance inasmuch as the appearance of color is due to a special case in which the absorption band is situated within the visible part of the spectrum and because it is now known that some substances of simple constitution have been found to have a t least a low order of fluorescence. About the for mercury, it can be seen that only the 1849-a.u. and only generalization that can be made is that substances 2537-a.u. lines, corresponding to the 6'SF6'P1 and which have a rather complex chemical structure, i. e., 6 lSr6 3P1transitions, can be absorbed according to the presence of unsaturated rings, etc., are the most likely selection rules. These lines are emitted as fluorescent to emit a strong fluorescence. The presence of the ring light and are known as resonance lines because they structure alone is not sufficient. Evidently the groups have the same wave length as the energy irradiating the must be well protected in the molecule, for pyridine mercury vapor.
quency, and hence the same wave length, as the exciting radi&on. The energy E which is evolved may be represented as the difference in enerw of the initial and final electronic states and may be inzcated by the equation
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FLUORESCENCE OF LIQUIDS AND SOLUTIONS
Fluorescence also occurs in the liquid state; actually there are two kinds of liquid fluorescence, in the pure state and in solution. In fact, the best known example of fluorescence is that of certain organic substances in solution and it was this type that led to the discovery of the phenomenon itself. In general, fluorescence of dissolved substances is most marked in dilute solutions,
does not fluoresce while acridine
C a
exhibits a blue fluorescence.
SEPTEMBER, 1947
TABLE 1 Fluorescence of Rhodamine B a t Various Concentrations i n Cellulose Acetate Films Molar concentration
Peak in fluorescence ( A . U.)
Yield of fluorescence, Per c a t of radiant enwgy
A number of investigators, including Jennes, Speas, and Levsbm (8), have studied the effect of temperature and concentration on the absorption and fluorescence bands. With rhodamine B, an increase in concentration or a decrease in temperature shifts the peaks of the bands to longer wave lengths. Table 1shows the effect of increased concentration of rhodamine B on peak wave length of fluorescence in cellulose acetate films
w.In the case of fluorescein, on the other hand, a de-
crease in temperature shifts the position of the bands to Examples of compounds which fluoresce in the shorter wave lengths. The shift in the fluorescence liquid state are paraffins, olefins, benzene, and various bands with temperature is not the same for different other types of hydrocarbons and cyclic compounds. substances in solution, but for a given dissolved comThe fluorescence of most of these substances is not very pound the change is always in the same direction for the intense, benzene being the only one with a reasonably fluorescence as for the absorption bands. Certain types of fluorescent compounds which disbright luminescence. sociate a t least to some extent in solution can be used Ketones and aldehydes exhibit luminescence when dissolved in suitable solvents but the most extensive as indicators for the hydrogen-ion' concentration of a studies of fluorescence have been made with solutions of solution. The intensity of the fluorescence of some of dyestuffs, such as fluorescein, eosin, rhodamine B, these substances increases with the pH of the solution; halogen-substituted fluoresceins, @-naphthol, phthali- in others a change of color also occurs. In Figure 4 the intensity of fluorescence of riboflavin is plotted as a . mide, quinine, esculine, and the like. Rhodamine affords a good example of a dyestuff function of pH (4). Table 2 shows the effect of change of hydrogen-ion conwhich fluoresces brilliantly in solution but which is centration in solution upon the fluorescent color of a completely inactive in the pure solid state. In solution number of indicators. rhodamine may be excited to fluorescence by ultr* violet or by radiation lying within the range of its absorption band in the visible spectrum. In Figure 3 the relative positions of this absorption band and the fluorescenceband may be observed (1). It is easily seen that each band is nearly the mirror image of the other, a relationship first pointed out by Levshin (2). QUENCHING OF FLUORESCENCE IN SOLUTIONS
Fluorescence in gases is quenched by collisions with molecules or atoms of the same kind or of added gases. Similar phenomena occur in liquids and in solutions. Concentration quenching in solutions has already been discussed in another part of this paper and it was pointed out that increasing concentration promotes the probability of collisions between excited and unexcited molecules. When this occurs, energy is transferred to the unexcited molecule; after this, neither of the two molecules has sufficient energy to reach the higher energy level necessary to originate emission of fluorescence. A decrease in viscosity reduces the intensity of the fluorescence. Evidently the lower viscosity enhances the motion of the particles, thus making it easier for collisions of excited and unexcited molecules to take place. The effect of increase of temperature in most cases is to enhance the quenching effect as the number of collisions is increased. Where associated molecules are present in the liquid, the higher temperature will tend to remove the association and the fluorescent intensity will be increased. This is particularly noticeable in aqueous solution, for water has a strong tendency to form asaociated molecules.
Figure 3.
Adso~ption end
nuoreacence s.sctra
A I C O ~ O Isolution (I)
of Rhod.rnin.
in
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In most cases fluorescence can he produced in inorganic substances only when a small smount of some foreign compound is present. Such compounds are termed activators and are usually metallic in nature. Apparently, the tungstates and molybdates provide an exception to this rule inasmuch as they are capable of fluorescing without the presence of an activator, but it is possible that the fluorescence is produced by a nonhomogeneous arrangement of some of the component atoms in the crystal lattice. Fluorescence appears in the alkaline halide phosphors when a trace of thallium is added. Evidently the fluorescence is produced by excitation of the thallium. This is borne out by the appearance of two new ahsorption bands when thallium is present and the fact that fluorescence results from radiation of any wave length lying within these bands (8,9). As in the case of solutions where an optimum concentration of dyestuff existed, there is an optimum content of activator for phosphors of this type. The fact that the fluorescent intensity reaches a maximum with increasing activator content and then decreases with larger amounts of the added impurity suggests that an excited activator atom can lose energy when an atom of the same kind approaches it too closely. Table 3 shows the variation in the relative intensity of the fluorescence of cesium iodide activated with different percentage8 of thallium (3).
Thus fluorescent indicatoton may he used to indicate pH changes in solution in an analogous manner to other types of indicators which produce color changes in various hydrogen-ion concentration ranges. Fluorescent indicators, such as eosin, fluorescein, and dichlorofluorescein, have been used in precipitation titrations to indicate the end point by the appearance of an adsorbed color on the surface of the precipitate suspended in the solution (5). Fluorescence in solution may also be quenched by adding another dyestuff or salt (fluorescing or nonTABLE 3 fluorescing) with an absorption band which is close to Fluorescence of Cesium Iodide Activated with Various the emission band of the fluorescent substance. For exPeroentages of Thallium ample uranine may he quenched by eosin or rhodamine Percentages of thallium Relative B by potassium permanganate. The quenching takes b2/ weight ,fluorescenee place by transfer of energy from the excited molecule to 0.07 88 the added molecule by a collision of the second kind and 0.22 100 isaresonant effect,forthedegreeof deactivationincreases the closer together are the energy levels of the colliding molecules (6,7). FLUORESCENCE OF SOLID INORGANIC SUBSTANCES
Solid inorganic fluorescent materials, commonly called phosphors, usually fluoresce only when a small amount of impurity is present. It h a s been demonstrated that a low degree of blue florescence can he produced in fired zinc sulfide which has been previously carefully purified. However, this fluorescence might have been caused by some small amount of residual impurity which the chemical purification had not been capable of completely removing. TABLE 2 Indicator
Colw chanae
OH ranae
Bensaflavin Aesoulin Salicylic acid Eosin a-Naphthylamine Fluorescein @-Naphthol a-haphthol sulfonie acid #-Naphthol sulfonic acid
Yellow to green Colorless to blue Colorless to dwk blue Colorless to green Colorless to blue Colorless to green Colorless to blue Blue to violet Blue to violet
0.3-1.7 1.5-2.0 2.5-3.5 2.5-4.5 3.4-4.8 4.0-4.5 6 -8 8 -9 9 -10
Activators for the alkaline earth sulfides and zinc sulfide include copper, silver, manganese, antimony, lead, and bismuth (10). The color of the fluorescence produced varies with the activator used. For example, the color of the fluorescence of zinc sulfide can be changed over a wide range of wave lengths from blue to red, depending upon the nature of the foreign metal impurity added. Manganese is extensively used as an activator for the silicate class of phosphors (11). In the case of zinc silicate, the fluorescence spectrum is displaced toward the red with increase of manganese content in the phosphor as can be seen by examination of Table 4 (3). The tungstates apparently require no foreign activator but it is quite possible that an excess of the metal may be present in the crystal lattice after firing. The light emitted by a phosphor usually covers a wide range of wave lengths in the visible spectrum and as a result pastel shades are produced. For example, Caw04 fluoresces pale blue, Cd3B1O9pink, and Zn2Si0,. Mn green. The spectral distribution of six different
SEPTEMBER, 1941 TABLE 4 Effect of Manganese Concentration upon the Color of Fluorescence in Zinc Silioate Phosphors Percentage of manoanese
Peak in fluorescence (A.U.)
commercial phosphors used in fluorescent lamps is given in Figure 5 (18). Phosphors are prepared by firing the basic salt or mixture of salts, usually with a small amount of activator, a t a temperature a little below the melting point. The efficiency of a phosphor varies with the amount of activator added. The efficiency increases with increasing concentration of the added element up to a maximum, which for sulfides is reached a t about one activator atom in every lo4 molecules of the base material. For silicates the optimum is reached a t a higher conceutration-approximately one atom ont of every 200 should be manganese (13). Extreme care must be taken in the preparation of these phosphors to exclude unwanted foreign elements, as some of these may act as poisons or fluorescence inhibitors. In Table 5 is presented a survey of the properties of several d i e r e n t types of phosphors. In this summarization phosphors which require anactivator are written with the chemical symbol of the activator following the molecular formula of the phosphor, i. e., ZnS.Cu, ZnBeSiO4.Mn. Quantum efficiencies as measured by Thayer and Barnes (14) are included for some of the fluorescent powders in the table.
tinguish the individual molecules which rqake up the whole; the appearance is in general as a continuous hunch of closely packed atoms. The arrangement of the atoms in the crystal lattice leaves a distribution of positiye and negative charges a t fixed distances apart. In fact, in the case of inorganic salts it is well established that the ions which appear in solution actually exist in this state in the crystal lattice and hence present an alternation of positive and negative charges in the solid. This definite arrangement of opposite charges produces a regularly varying electrical field. If we assume the crystal to he only a single row of atoms, it is possible to compute the allowable energies that a single negative electric charge may have in the associated electric field. These calculations show the energies to be discontinuous. Allomable energy levels for an electron in this electric field lie in certain discrete '
THEORY OF PHOSPHORS
Any theory which attempts to explain the cause of fluorescence in inorganic substances must take into account the present-day picture of the behavior of electrons in crystalline solids (18, 16, 16). The stmcture of crystals is such that i t is not possible to dis-
Figure 5. sp=trsl E~~~~~ ~ i ~ t . i b of ~ tF i ~ ~I ~ x . h t~ From ~ Six Different Phosphors Used i n Commsrsial Lamps: A. "Gmen"; B. "Cream Whit.": C. "Warm White": D. "Whit."; E. "D~ylipht'.: F, "soft Whit." (12).
TABLE 5 Properties of Phosphors Approximate peak Quanta ojfluoreseent wave length of Fluorescent light per incident cob7 quantum of $537 A.U. Class Phosphor Excitation ( A .U.) Sulfides ZnS.Cu 3200-3900. Green ZnS. Ag 3200-3900' Blue ZnS. CdS. Ag 3200-3900' Blue to red denendine on mixture CaS. Bi 3200-3900* Blue Borate CdsBdOs 0.66 2530 Pink Silicates Zn$iO.. Mn 2600 Green 0.74 CdBiO,. Mn 2600 Pink 0.55 ZnBeSiO,. Mu 2600 Pink 0.53 Tungstates CaWO, 2700 Blue 0.70 . Blue CaWO,.Ph 2700 MgWO, 2800 Blue-white 0.70 The sulfides a s s group are most efficientlyexcited by radiation between 3200 A.U. and 3900 A.U., the peak wave length varying with the type of sulfide and the amount and kind of activator present.
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bands. These bands are separated by forbidden regions which become narrower in the higher energy regions but never quite disappear. These levels apply to the crystal as a whole; they are not atomic energy levels. To explain fluorescence, i t is necessary to take into account the higher energy bands. It is known in the case of phosphors that the top band is unoccupied. The next lower band is "forbidden" and the one below this is normally filled with electrons (see Figure 6 A ) . a
a
EMPTY
b
FORBIDDEN
C
NORMPiLLY OCCUPIED BY ELECTRONS
I
If exciting radiation such as ultraviolet light falls on the type of crystal represented in Figure 6A and the forbidden band is wide, nothing will happen. If it is narrow enough, electrons will shift from c to a. When these electrons return to their normal energy level c, emission of fluorescent light takes place. This explanation applies only to fluorescence of a pure substance. In most instances the forbidden band b is too wide for the electrons to jump across. In order to make fluorescence possible, it is necessary to introduce a foreign substance (activator) into the crystal. These stranger atoms will set up new impurity levels a t various localities in the crystal lattice. If the nature of the added compound is such as to put new localized energy levels in the forbidden band b, an intermediate step is now provided for electrons to jump to when they leave band c. A local energy level of this nature is indicated as 1in Figure 6 B and electrons may jump from c to 1 and from 1to a. When the electrons return to their normal states, fluorescence emission results. PHOSPHORESCENCE
Phosphorescence has already been described as being similar to fluorescence except that light emission continues for a period of time after the exciting source has been extinguished. Both phenomena involve atomic excitation after absorption of energy. According to Lenard (lo), fluorescence involves an instantaneous process whereby the excited electron returns immedi-
ately to its normal level, while in phosphorescence the return of the excited electron is delayed enabling light emission to continue for a time after excitation has ceased. With all phosphorescent substances under excitation, both fluorescence and phosphorescence occur simultaneously but as soon as the energy source is turned off, only the latter process continues. Careful measurements have shown that for fluorepr cent substances light emission persists for about seconds after removal of the energizing source. When emission lasts longer than seconds after the illuminating energy has been removed, phosphorescence is said to occur. After removal of the exciting source, a gradual decay in the intensity of the phosphorescent light is manifested until extinction occurs. It is then only necessary to expose the substance again to the energizing radiation in order to restore the phosphorescence. A number of the zinc sulfide type phosphors exhibit phosphorescence when properly prepared. If a radioactive substance is incorporated into the mixture, the afterglow may be made to persist indefinitely. Phosphors of this type find extensive application in illuminated watch dials, indicators for light switches and cords, "blackout" tape, and the like. LITERATURE CITED (1) POR~STKY, A,, J . Franklin Inst., 197, 527 (1924). W . , 2.Physik, 72, 368, 382 (1931); Acla Physico (2) LEVSKIN, chim. U.S. S . R. 1,685 (1931); ibid., 2,221 (1931); ibid., 7.213 (1937). G. R., Elec. Eng., 57, 677 (1938). (3) FONDA, R., AND G. MARUZZI, Ber., 67. 888 (1934). (4) KUHN, H. B., AND A. L. POWELL, Ind. Eng. Chem.,Anal. (5) FELDMAN, Ed., 11,89 (1939). (6) Scnxm~,G. C., Ann. Physik, 65,247 (1921); R.K A ~ S K Y , A. HIRSCH, AND W. BAUMEISTER, Ber., 64B,2053 (1931). (7) PERRIN, J., Compt. rend., 184, 1097 (1927). F., J. Chem. Phys., 6, 454 (1938). (8) SEITZ, R., Z . angm. Chem., 49, 69 (1936); W . VON (9) HILSCH, MEYEREN, 2.Physzk, 61,321 (1930). P., "Handbook of ExperimentalPhysiysioo," Volume (10) LENARD, 23 Part I, Akademie Verlagsgesellsehoft, Leiprig, 1928. i l l ) ANDREWS.W . S.. Am. Mzneral.. 7. 19 11922). ~cnumak,J. H., J . Applied ~ h ; s . ,i7, 902, (1946). JOHNSON, R. P., Am. J. Phys., 8, 143 (1940). THAYER, R. N.,AND B. T.BARNES, J. Optical Sac. Am., 29,
CHEMICAL ENGINEERING ECONOMY COURSE AT BROOKLYN POLYTECHNIC
THE graduate course "Chemical Engineering EoonomJ;" will again be offered during the first semester of the academic year 191748 at the Polytechnic Institute of Brooklyn, N. Y. It will consist of a 15-lecture symposium given on Wednesday evenings from 8:00 to 10:OO P.M. Interested persons should apply to Dean R. E. Kirk, Graduate School, Polytechnic Institute, Borough Hall, Brooklyn, N. Y.
