FLUORESCENCE AND
`PHOSPHORES:CENCE
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`b.y PETER PRINGSHEIM .
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`· Argonne National Laboratory, Chicago, Illinois
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`1949(/B)
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`INTERSCIENCE PUB;LH3HERS, INC., NEW YORK
`INTERSCIENCE PUBLlSHER.S LTD., LO-NDON
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`VIZIO 1010
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`Q.C415
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`All Rights Reserved
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`FEB ~·1 1950
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`PRIN. TED IN THE NETHERLANDS BY
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`NOV- 3 1949
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`VIZIO 1010
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`INTRODUCTION
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`A. General Theory
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`1. Postulates of Bohr's Quantum Theory. By ~bsorption of light
`'the energy of the absorbing system is increased. According to the laws
`of thermodynamics the inverse process, emission of energy in the
`form of radiation, must be possible. This inver~e process must occur; if
`no other way of returning the system to its initial state of lower·
`energy is available. Light emission excited by' light absorption is called
`photoluminescence. For a long time photoluminescence was supposed
`to be an exceptional phenomenon characteristic of relat~vely few
`substances. The real pro~lem is, however, to 1.1-nderstand why so many
`substancesare not photoluminescent.
`Bohr's theory! first developed for interpretation of the spectra of
`the H at?m and later adapted to more and more complicated systems,
`postulates that energy can be taken up by such a system only in
`certain definite steps; the system is stable only in discrete, more or less
`sharply defined energy levels. The lowest of these levels is the ground
`level.or the ground state of the system. For all atoms and for many
`diatomic· molecules the energy levels are perfectly. known. For poly-:
`. atomic molecules and for still more complicated systems like crystals,
`knowledge of the energy ·levels is still far from complete. Even for
`these systems however, the assumption of the existence of such energy
`levels has proved itself very fertile in developing an understanding of
`all processes connected with _light absorption, and light emission.
`Oniy if the en~rgy absorbed by a molecule is so large that one
`parf of the system is completely separated from the' remainde~, as in
`a process of ion~zation or dissociation, can the SC1parating particles
`take up ulidetermined amounts of kinetic energy, so that no discrete
`enetgy levels ex:ist for the system as a whole.
`·
`. In quantum mechaniCs a system in a given state is characterized
`by a "wave function" y; whiGh is the prodUct_ of the wave functions y;i
`of all individual particles composing the ·system. These functiO!lS lj;i
`determine the probability with which a particle is found ~t a point
`irt space.
`Apart from the ·introduction of discrete energy levels; Bohr's
`1
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`VIZIO 1010
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`2
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`GENERAL THEORY
`
`theory postulated the following relation for a .transition between-two
`levels Nand F with the energies E; and EF:
`·
`
`( 1)
`
`h being Planck's constant = 6.63 · I0-27 erg sec and "FN the frequency
`of the radiation which is emitted or absorbed by the transition.
`In general, the wave number v = 1/A. is used instead of the fre(cid:173)
`quency v, which has the dimension of sec-1. iJ is measured in cm-1 and
`is related to the frequency by the equation v = vjc. Hence a "term''
`T, which is ·characterized b)r'its wave number v, has the energy vhc,.
`but for the sake of brevity energies. are frequently expressed in cm-1 .
`On the other hand, it is customary to measure energies in electron
`volts ( e V), one electron volt being the energy which an electron
`acquires under the acceleration produced by a potential difference of.
`one volt.
`1 eV ~ 8.11·103 cm-1 ~ 1.59·10-12 ergs~ 23 _){cal/mole
`
`--~~----~~+-r7--r-.-- c
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`3
`. 2
`I
`· FIG. 1. Energy • level diagrp.m for
`the representation of fluorescence·
`and phosphorescence,
`1 : resonance radiation.
`2 ; phosphorescence.
`3.: fluorescence.
`4 and 5: al}ti-Stokes fluorescence.
`
`2. Energy Levels. In the diagram of Figure 1 several ertel'gy levels·
`of an atom or a more complicated system are represented·by horizontal
`lines. The vertical distance between two of these lines is proportional
`to the corresponding difference fu
`F'
`energy; the level N represents the
`--r--.-.-.--x-r--r-r-r-.-r--t--.-- F
`--+-1-f-L~+-+-1-l-+-t--1--li- M ground state. By absorption. of
`light of frequency vFN the atorrds
`raised to the level F and if no other
`energy levels exist between N and .
`F, the a tom can return to N only
`---+--1-1--H---''-t--t--t--- N' by re-emission of light o.f the same
`frequency vNF :. theoretically this
`is. t~e- simplest case,· of photo(cid:173)
`lumin~cence ;. it is .called · "reso(cid:173)
`nance radiation.'.' In · the ·diag.ram
`of Figure 1, however; several levels
`C,D .... are located between Nand
`F. Under. these conditions other
`transitions from F .to C; D. ~ . can
`occur, resulting in the emission of spectral lines of frequency 'vFc,
`"F.n , .. These frequencies are smaller than "NF: The law a GOrding to ·
`which the wavelength of fluorescence is always g1·eater than, or in the
`liniiting c~e equal to, the wavelength of the exciting light was · first
`found empirically by Stokes (r585); the quantum theor~tical e-xpla-
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`VIZIO 1010
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`DURATION OF LUMINESCENCE PROCESS
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`3
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`nation was given by Einstein more than fifty years.later (344). Small
`deviation-s from Stokes' law are p0ssible if other energy levels N' or F'
`are located immediately above Nor F :respectively,_so that the system
`can be raised by transfer of thermal energy either into N' before the
`exciting light is absorbed, or into F' durirtg the lifetime of the system
`in the excited state F. Under these condl.tions the frequency of the
`exciting light vFN' is smaller than the frequency of the fluorescence
`vpN, or the .frequency of the ~bsorbed light vFN is· smaller than the
`frequency of the fluorescence vF'N: anti-Stokes fluorescence.
`Such deviations from Stokes' law, by which additional energy is
`supplied by a pody oHow temperature to the radiation from a source
`of much higher temperature, of course in rio way invalidates the second
`law of thermodynamics, as was suggested ~rroneously by Lenard
`(I284,I29]b,I726,I762b,t762e).
`3. Duration of the Luminescence ·Process. In the classical Lorentz(cid:173)
`Drude theory the emission· of monochromatic light by an q.tom· or
`m~lecule ~r.igina tes ~fr.o:n ~he oscillation .of an ~lectron which is bou~d
`to us position of eqmlibnum by a quasi-elastic force. The decrease m
`energy ofthe osdllating electric dipole which is caused by the emission
`'of radiation, and the corr.esponding ·decrease in intensity of the radi(cid:173)
`ation' itself, follow an exponential law. The averag~ duration of the
`emission, or the time after which the intensity has dropped from its
`initial yalue / 0 to (1/ e)I0, is:
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`(2)
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`For visible light, with v R:;j 5 ·1014sec-1, T is of the order of w-s sec. •
`In· the absence of all extemal perturbations the ~ifetime of an
`excited state is determi,ned, ac~ording to quantum theory, by the total
`probability of all possible· transitions to· lower energy levels. These
`transition probabilities AFK can-.be calculated if the wave f~ctions rf/
`·and ifl" of the coml;:>ining'levels EF and EK are known:
`
`AFK ~ { l ~,~.*rdv}'
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`The lifetime of a molecule in the eiccited state F is then:
`:- 1
`r = .EA FK
`•K
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`As in radioactive decay, the number· of transitions per -unit of time
`is at every instant proportional to the numher.of excited molecules and
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`( 2a.)
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`_ ........... _ ... ~ -"-~ ... ~~~-., --··· .... -- -·.:. ....... -·-------
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`VIZIO 1010
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`4
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`GENERAL THEORY
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`thus ·· the decay of the luminescence intensity again follows an ex(cid:173)
`ponential law, exactly as in the older th,eory.
`The transition probabilities between various levels of one and the
`same molecule are 'of widely different magnitudes. While the Lorentz(cid:173)
`Drude theory dealt only with electric dipole radiation, with a mean
`lifetime depending exclusively on the frequency of the oscillator, much
`weaker radiation of much longer duration can also be explained on _ the
`ground of classical electrodynamics by assuming electric quadrupoles
`or multipoles. or magnetic dipoles or multi poles as sources of radiation.
`The emission by an electric quadrupole or a magnetic dipole lasts
`about 106 time9longer than that of an electdc dipole. In the quantum(cid:173)
`mechanical models, however, an electric dipole can have a much
`smaller moment than the oscillating electron of the Lorentz theory and
`thus the decay of its radiation also can be much slower. Several
`e:xperimental methods have been found which allow a discrimination
`between the radiation of electric and maghetic dipoles and multipoles
`.
`(28oa,435,I 49I,i492 ,I76Ia).
`Transitions wliich have a very . small probability because they
`correspond to the radiation of an electric dipole of small moment ot
`of an electric multipole or a magnetic pole are called "forbidden" and
`the corresponding spectral lines are "forbidden lines." If no "allowed
`transition" from an excited state M to any lower energy level exists,
`the system~ once brought into this state, mu;t remain in it for a
`relatively long period .'. Such states are termed "metastable.'" If the
`.system is absolutely unperturbed (as, for instance, in the highly rare(cid:173)
`fied atmospheres of'stellat nebulas) light emission nevertheless occurs,
`·but with very small intensity''and slow decay. On the other hand, the
`transition from the ground state to the state M is also forbidden and
`the corresponding absorption line, if at all observable, is very weak.
`However, M can be reached indirectly; in the level scheme of Figure 1,
`this can occur by absorption of the line corresponding to the transition
`F ~ N, *.and by the subsequ~nt transition F -+ M. If M is separated ·
`by only .3: small amount .of energy from F and if the excited system is
`in · thermal equilibrium with the surroUnding molecules a sufficii:mt
`amount of energy can be provided "to the system so that .it can return
`to F :from there the .emission of the lines corresponding to th~ tran(cid:173)
`sitions F -+N, F-+ C, etc~, may again take place. A photoluminescence
`* The symbol for the higher level a\_way"s _precedes the symbol for i:lie lower
`level; the directio~ of t h e t ransition is indicated by" t he arrow. This prib.cipl e,
`which is gen~rally used for t he deScription ·of t h e sp ectra of Oiatotn,ic molecules,
`. is applied here similarly t o t he represent ation of atomic spect ra.
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`EFFECTS OF PERTURBATIONS
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`5
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`process of thi~ type, involving the passage through a metastable level,
`is called Phosphor.escence.
`In the older literature fluorescence and phosphorescence were
`distinguished only by the c~iterion of an observable afterglow: if the
`luminescence did not last longer than the irradiation, it was· called
`fluorescence; if it was visible for an appreciable length of time after
`the e:rid of the excitation, it was called phosphorescence. Modern
`experimental technique however, permits the measurement of the
`finite duration of any emission process, even if it is as short as I0-9
`sec, and, on the other hand, th~spontaneous transition probabilities,
`even in atomic processes, correspond to lifetimes which vary continu(cid:173)
`ously from 1 o-s sec to several seconds. Therefore, it is no longer
`possible to define some arbitrary dur.ation of the emission process as
`the boundary between fluorescence and phosphorescence. (For a more
`complete defini tion.of fluorescence and pJ'wsphorescence, see chapter IV.)
`While, according to the definition given above, fluorescence and
`· phosphorescence are first-order processes and follow exponential laws
`of decay, another kind of luminescence is a typical bimolecular reaction.
`If an electron is complete}y separated from its molecule by photo(cid:173)
`electric ionization and if its recombination ·with any other ion produces
`the emission of light, th~rocess is of the second order and decays,
`therefore, according ~o a ~~rbolicallaw. Luminescence cc:used by
`recombination is observed in electrical discharge. through gases or
`metal vapors under especially favorable COijditions. These cannot be
`. achieved in the case of ·excitation by light absorption. However, a
`phenomenon of ·the same kind. occurs in . certain phosphorescent
`crystals; it will be called "recombination afterglow" in t~e following
`treatment.
`4. Effects of Perturbations. An excited system can be transferred
`to neighboring energy levels by outside perturbations, for instance by
`collisions or' other.interactioiis wi.fh stiiiol.Ulding.molecules, and from
`these new levels transitions occur which produee ·emi~sion lines n~t
`contained in the primary fhi9rescence spectrum. Furthermore, such
`perturbations may cause a momentary displacement of an energy level
`and if these displacements fluctuate with time an.d in space, broad and
`di_ffuse bands appe,ar ins~ead of sharp lines in the absorption and
`fluorescence spectra. This''is true in particular for all condensed systerps
`· which are capable of lp.minescence (with the exception of certain
`crystals, especially at low temperatures). Under such co:qditions the
`peak of the emission ba;nd must be shifted with respect to the peak of
`the corresponding absorption band in the direction ·of greater
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`VIZIO 1010
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`6
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`GENERAL THEORY
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`wavelengths. This consequence of Stokes' law will be dealt with in
`more detail in a later chapter.
`If the whole energy of excited molecl.\.les can be lost as the result
`of collisions or other perturbations, the mean life of all excited mole(cid:173)
`cules is shortened and the fluorescence yield is decrease~.
`· The quantum yield of fluorescence is:
`Q = 1/A
`(3)
`where the fluorescence int~nsity I and the radiant energy A absorbed
`per unit time are measured by the number of light quanta contained
`in the emitted and absorbed radiat!on. In the case of resonance
`radl.ation, the quantum yield Q can be replaced by the "energy
`yield" ([> (I and A being givf;n in ergs or' calories), since in this case
`Q = ([> (compare Section 105). [In general (namely in the case of
`Stokes fluorescence), the energy yield is smaller than the quantum
`yield, while in the case of anti~Stokes fluorescence cJ> is slightly larger
`than Q.]
`If no perturbations occur, the fluorescence intensity '!0 is equal to
`A. when · equilibriUfll is reached during the irradiation, and Q' = 1.
`Under these cop.ditions, the spontaneous transition probability ao ·
`alone determines the lifetime T0, and the numbern0 of excited molecules
`in equilibrium is given by the ' equation:
`.
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`(4)
`'A = 1o = aono = ncl•o with To= 1/ao and no:_ Afao
`If the excitation energy ~an be lost by a second c6mpetirig process.
`wit? a probability a1, Equation ( 4) is replaced by:
`A 1 = n 1 (a0 + a1) = nif'r1 where T 1 = 1/(a0 + a 1) and n 1 = A 1/(a0 + a1}
`( 5}
`n1 is smaller than n0 • Supposing that the absorbing p<>wer of the
`molecules is not altered by the perturbations (f1 1 = A 0 = I 0), .the
`fluorescence· intensity becomes:
`
`. 4
`
`(6}
`
`and tl}.e yield :
`
`Ql ,..,- 11/A = aonl/aono
`·
`·
`The "quenching constant" a 1 is: a 1 =
`
`(7}
`
`(7a)
`
`. •dr.o .
`a 11-Q)
`01
`1
`• Ql
`In the same way -the yield- is reduced by another· perturbation with
`.
`the jxobabili ty a 2 to:
`.
`Q~ _ 7:2/7:0 where 7:2 = 1/(ao + a 2 )
`.
`Hence the general relation: ·
`
`(7b}
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`COHERENCE OF THE SECONDARY RADIATION
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`T
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`The fluorescence yield of a given system is directly proportional to the
`actual lifetime of the excited stq.te (I22I,I57I).
`If several processes compete simultaneously with the radiating
`transition and if their probabilities are av a2 • •• an, the lifetime of the
`excited state becomes:
`
`1
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`and the intensity of theluminescence :
`
`(9)
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`( 10)
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`According to the classical 'theory, the width of a spectral line is
`proportional to the damping coefficient or inversely proportional to the
`mean life of the excited .state. Qualitatively the same law holds ih
`quantum theory. All perturbations by . which the fluorescence is
`quenched increase the width of the corresponding emission line. The ·
`latter is determined, however, not only by the' actio_n of the pertur(cid:173)
`bation on the excited state from which the erp.ission processoriginates,
`but also by the action on the final s.tate to _which the system is
`· transferred.
`5. Coherence of the Secondary Radiation. The only photolumi(cid:173)
`nescence process for which the. classical wave theory of light could
`account without the introd11ction of rather artificial hypotheses was
`the excitation of resonance radiation. Existence of this phenomenon
`had been predicted on theoretical grounds by Lord Rayleigh long
`before· its discovery by R. W. Wood. Resonance radiation was under(cid:173)
`stood, as. special case of light ·scattering in which . the ·scattering
`resonators were exactly in tune with the frequency of the primary
`radiation.
`According to the original quantum theory only the average
`lifetime 7 of an excited state could be determined, while for the .indi(cid:173)
`vidual molecule the time elapsing between the absorption and the
`re-emission of light obeyed the laws of statistics. Hence it. seemed
`impossible that a definite phase relation could exist between the
`wave trains of. the. primary ' and the. secondary light. Resonance
`radiation was considered to be incoherent. On the other hand Ra'yleigh
`scattering, which was knowri to be coherent, was ascribed to forced
`vibrations of " _virtual oscillators"' within the molecules. There was no.
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`- - - - - - - - - - · ·-
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`8
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`GENERAL THEORY
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`connection between the two phenomena. It was even assumed, for a
`time, that for radiation in resonance with the characteristic frequency
`of the scattering oscillators both processes might occur simultaneously
`and that it might. be possible to separate them xperimentally (e.g.,
`by observing a different decay period for either of them) (I29).
`In the quantum-mechanical treatment, however, the
`lectro(cid:173)
`magnetic field produced by the interaction of the primary radiation
`and the virtual os illators of a molecule is exactly the same as that of
`the classical wave theory, and thus the steady transition from
`Rayleigh' scattering_ to resonance radiation is r~stored. If various
`energy levels C, D , .. exist in the molecule between the ground state
`Nand the level F to which the molecule is raised by the absorption of
`the primary light, the resulting electromagnetic field sUI.~ro:undi:n'g
`the excited molecule is the same as if it. contained a number. of
`vibrating oscillators of frequency vpc, . vFv ... in addition to the
`vFN· The "strength" of each··
`absorbing oscillator of ' frequency
`6·scillator is proportional to one ·of the transition probabilities F -+.C,
`F -+ D . . . F -+ N. All phenomena related to the wave nature of the
`secondary radiation (coherence, interference, polarization) are to be
`derived from thjs model. However, the energy of the radiation-is no
`longer spread continuously over the whole-wave field and' proportional
`everywhere to the sqnare of .the wave amplitude. The square of the
`amplitnde rletermi:n:es. only the probability for a pho.to.U of the corre- · ·
`sponding frequency to be found at a given instant at a partic1-1lar point.
`The absorption and emission of radiant energy occurs exactly as in
`Bohr's original theory, in quanta with'in p1·actically infinitely short
`periods of time (I8IJ).
`· Insofar 3,$ the problem of col;lerence is determined by the phase
`reLation between pri.Ipary and t secondary. radiatipn, on:1y Rayleigh
`scattering· and resona11ce radiation 'can be cop.sidered ·(I529f The
`existence of such coherence is ·;not revealed by any experimental facts.
`Resonance radiation is obsenr~d exclusively in gases and vapors at
`low pressures. Under the's.e conditions the coheren~e of the ''classical"
`Rayleigh scattering with the primary radiation cannot be proved
`either, because of the random distribution of the ~molecules in . a
`perfect gas.
`If the fluorescence spectrum of ·a monatomic gas contains lines
`of wavel.~ngths different from that of 'the exciting iight, a constant
`phase relation between the waves of the primary and the secondary
`radiation is out of the question. However, . a· constant phase relation
`.might still ~xist between the secondary wave trains·"originating at
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`VIZIO 1010
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`OTHER EXCITATION PROCESSES
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`different atoms; this kind of coherence could again only be observed
`by t.he angular distribution of the fluorescence intensity if the fluo(cid:173)
`rescing atoms were fixed in space in a regular lattice.
`. Every kind of coherence between the radiation coming from
`different molecules must disappear in the fluorescence of molecules in
`which nuclear vibrations and rotations occur simultaneously with the
`electronic transition and independently in each individual molecule.
`The same is true if the molecules und~rgo external perturbations
`during their lifetime in the excited state. This is the case for the
`fluorescence of all liquids and solids. On the other hand, it has been
`proved by wide-angle interference experiments that the radiation
`emanating in.different directions from an individual molecule of a
`liquid solution is.coherent, exactly as the radiation emitted by a dipole
`as a spherical wave is coherent in itself according to the classical wave·
`theory (435,I489-I492,I8~7).
`'
`• 6. Comparison with Other Excitation Processes. The radi~tion
`emitted ~y an atom or amolecule<iepe;ndsonly on its state of excitation
`and on the prooabilities of transitions from this state to those•of lower
`energy. It does not depend on .the mode of excitation by which the
`system has been brought into the excited state. In this sense ther is
`no difference between fluorescence ap.d any other kind . qi light
`emission by the same atorns or molecules caused by <;ollisions with
`elections, by chemical processes, or by thermal agitation. The charac(cid:173)
`teristic properties. of a spectral line or -~ band (for instance, the
`dependence on temperatUre and pressure or the sensitivity to magnetic
`and electric .fields) must be the same in every case.
`A system .eniitti.:Q.g luminescence is not, however, in a stat of
`thermal equilibrium; some of its molecules contain a much higher
`elec~ronic energy t.llan that conesponding t9 the actual temperature
`of the sys.tem and. this is the essential feature of every luminescence
`process. It follows that the ''excited" mol_ecule~ can lose their excessive
`energy by collisions with otl_:ler molecules: luminescence, for instance
`the photoluminescence of iodine · vapor, can be suppressed or
`"quenched" by the addition of relatively small quantities of oxygen.
`I:(the sani.e quantity of 9xygen is added to iodine vapor heated in ;:t
`q~a;rtz tube to a temperature of 1000° C at T;vhich it emits its cha;rac(cid:173)
`. tetistic bands as temperature radiation according to Kirchhoff's law,
`. no a_ppreciable change in the emjssion occurs, because now, in thermal
`equilibrium, the quenching collisions must be compensated by an
`equal number of exciting collisions (I284) . .
`Photoluminescepce is disting:ui,shed furthermore by a:n almost
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`VIZIO 1010
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`10
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`EXPERIMENTAL TECHNIQUE
`
`complete control of the excitation process, sine~, among all atoms or
`molecules which are present, only those in a weil-defi.ned initial state
`are transferred into an equally defined e~cited state by the absorption
`of light of a given frequency. The complete spectrum of all atoms and
`molecules, modified within certain limits by the temperature, is
`emitted by a flame or an arc. By means of electron collisions in a gas at
`low pressure it is possible to exclude from the speCtrum all lines which
`require an excitation energy surpassing the energy of the electrons
`under the applied voltage, but all levels lying below this energy are
`excited simultaneously. Besides, the accuracy of th.e method is not
`great enough to differentiate between the excitation of closely adjacent
`.
`lines. On the other hand, the possibility of separately exciting neigh-
`boring energy sta.tes of a molecule by the absorption of monochrq(cid:173)
`·matic light is limited only by th~ degree to which the primary light
`can be made monochromatic. Even the state of polarization which is
`characteristic for a certain transition c.an be determined by this
`, method of excitation; thus itbeco·mespossible toascertaih the ~xis~ence
`of the various Zeeman levels in very· weak magnetic fields which are
`separated by s~ch small intervals that they cannot b~· dist~nguished
`by other spectroscopic methods.
`The ?arne is still true, although to a smaller degree, for condensed
`systems;. there, also, a much ·fuier differentiation is' obtained in··the
`excitation of .individual emission processes by light absdrptign "th.an
`by any pther mode of excitation.
`
`.
`
`B. Experimental Technique
`
`7. Phosphot·oscopes and Fluorometers. The experimental methods
`applied to tb,e investigatipn of photo~'Umiuescence are, in general, very
`simple. The most important types of apparatus wP,ich have lJeen
`especially designed for this purpose are th~ phosphoroscopes and
`fluorometer~. These serve for measuring the duration of short or
`almost instantaneously decaying emission processes. All of these
`instruments are bCl:sed on ~he principle of permitting the observation pf
`the lumi;nesce'nce a short, and if desired, a variable tin:ie after the e:q.d
`of the excitation pe:riod.
`.
`The fust phosphoroscope was in'\Cented by E. ·Becquerel' (A,78);
`in a ''Becquerel phosphotoscope''.the luminescent substance is placed
`between two .discs M and N (Figure 2), which are ··mounted on a
`common axis and have sector-shaped aperttll'es .tl and D shifted with
`
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`VIZIO 1010
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`VIZIO 1010
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`VIZIO 1010
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`CHAPTER IV
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`GENERAL SURVEY'
`
`A~- Nature of Luminescent Substances
`
`95. Con~itions for Occurrence of Photoluminescence. Unperturbed
`fluorescence of gases and vapors 'is noted only at lowest pressures. At
`pressures at which collisions of excited' mole.cules become sufficiently
`probable, either the secondary radiation is changed in frequency, or
`its intensity is weakened ·or even completely quenched. Polyatomic
`moleGules seem to be Jess sensitive to quenching ,by collisions, in
`generalr than monatomic vapors. Jn condensed states (pure liquid or
`solid, liquid or solid solution) the ability to fluore)l-ce is lost, however,
`even in the majority of polyatomic compounds. -The reasons for the
`absence of fluorescence due to -the interaction of excited molecules
`with other molecules are, in principle, the same in condensed systems
`as in vapors: ·inducedpredissociation, chemical reactions, and "internal
`conversion." It is easily u;nderstood that the first of these processes
`has a greater chance of realization iJ?. condensed systems, where the
`excited molec~es are _in a . constant state of . collision.* In most
`instances no chemical reactions are produced by the absorption of
`light, and, especially if the nature of the surrolinqing mole'cules (of
`the solvent, for instance) has no' m'arked influence on the optical·
`properties of the absorbing...,substance, the· re-emission of radiation
`·
`must be suppressed by the third type of process (424).
`The probabil~ty of internal conversion is greatly· enhanced in
`condensed systems for two reasons. It, 'in a polyatomic vapor, the
`electronic excitation. energy of an isolated molecule is converted to·
`high vibrational energy of the el~honic' ground state, .the inverse
`process must occur after some time. This fluctuation of energy from
`one form to the other may be repeated more than once;, but as long as
`no collision takes place, the absorbed energy must eventually be re(cid:173)
`·emitted as radiation. Wheneve! ·a molecule has acquired a hig!I
`
`*·It has already' been pointed out that, on the other hand, the probability.
`of spontaneous predissociation can be reduced by'. the stabilizing effect of
`·
`collisions (see Section 83). '
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`285
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`286
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`CONDENSED SYSTEMS
`
`vibrational energy in a condensed system, this energy is almost
`immediately dissipated into thermal agitation of the surrounding
`medium and is never restored to the initially excited molecule.
`Also a molecule in a condensed system, especially in a solution, can
`never be treated as an isolated entity: in nearly every case it forms
`· some sort of complex; witll surrounding molecules, e.g., of the solvent.
`Many metal ions exhibit, in aqueous solutions, absorption bands of
`much lower frequencies than those of the resonance lines in the vapor
`state and these may be due at least partially to electronic transitions
`between the dissolved molecules and the solvation envelope by which
`they are surrounded. In other instances the influence of the solvents on
`the absorption spectra of the dissolved molecules is' rel~tively small,
`but none -the less an interaction between the latter and the solvatiol1,
`envelope takes place and can greatly influence the fluorescence yield.
`Since the existence of narrow absorption bands proves the corre(cid:173)
`sponding electronic transitions to be . w~ll protected against pertur(cid:173)
`bations from outside, one might assume that molecules exhibiting such
`bands should have a greater chance to be fluorescent than others. This
`is correct up to a point; among compounds with nar:row absorption
`bands the number of fluorescing spbstances is relatively great, al(cid:173)
`though fluorescence -is by no means a g~neral property of such com(cid:173)
`pounds. For instance, the chromium alums and the uranous S(!.lts are
`not fluorescent. On the other hand, the absorption bands of inany
`strongly fluorescent dye solutions are no less diffuse and broad than
`those ofnonfluorescent dyes. Very small changes ~n the constitution of
`a mol~cule can have a great influence on the ,probability of interim!
`· conversion and, thus, on the occurrence or nonoccurrence of fluo(cid:173)
`rescence without appreciably affecting the power of absorption.
`96. Most Important Types of Luminescent Substances. If photo(cid:173)
`luminescence is a characteristic property .of a compound as such, the
`molecilles of this compound must be fluotescent urider variOU$ con(cid:173)
`ditions -. for instance, when the compound is in the crystaUine state,
`· in a liquid solution, and in the vapor state. Prac.tically all molecules
`which are photoluminescent -in condensed states are more or less
`complex. The only exceptions are the positive ions of some rare-earth .
`metals, the optical properties of which are so little perturbed by the
`surro'unding medium that, even in crystals or in aqueous solutions,
`they behave almost like the atoms of a vapor. Among the complex
`inorganic molecules the positive ions uo++ are, with f.ew exceptions,
`• fluorescent in crystalline uranyl salts and in liquid sqlutions of such
`salts. A few other metallic ions (fl+, Pb++, and Sn++) are able, in
`•
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`ENERGY TRANSFER FROM ABSORBING TO EMITTING MECHANISM
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`287
`
`aqueous solutions, to form complexes which can be excited. to fluo(cid:173)
`rescence (52I:522,6I7,I3D4). In addition the cyi'l:noplatinites are
`to be mentioned and, finally, some derivatives of silo:x:ene which,
`owing to their ring structure, have much in col11mon with aromatic
`compounds. It is doubtful whethe;r the tt~ngstates, molybdates, and
`some similar salts should be included in this class for, although many
`crystals containing these ions are strongly photoluminescent ·without
`being appreciably contaminated by an impurity, nothing' is lmown
`about their fiuorescencein C>ther than the crystalline state. Thus, they
`may be classified as belonging to the mineral crystal phosphors.
`Although only a relatively small number