S E C O S D REPORT O F T H E C O l I l I I T T E E O X PHOTOCHEMISTRT, DIYISION O F CHE:kIISTIIT A S D C H E X I C h L TECHNOLOGY T\'ATIOSXL RESEARCH COUNCIL B Y H U G H S. TAYLOR
I n the First Report of the Committee on Photochemistry' six papers were presented dealing with the quantitative technique of photo-reactions, the classical point of view in respect to these processes, the relation between photoreactions and those produced by ions, and finally the relationship between the physical concept of quantised absorption and the chemical processes which succeed such absorption. I n the interim, the most active developments in the field of photochemistry have been associated with this last phase of the subject. The researches of the physicist with respect to absorption spectra have clarified much that was tentative a t the time of writing the preceding report. The cases in which absorption spectra have been studied in detail have been mutiplied and the more penetrating analysis of band absorption spectra has yielded a n additional type of band spectrum, the predissociation spectrum, which has assumed a considerable importance recently in the discussion of several photochemical processes. I t has therefore seemed desirable to the writer of this Second Report to attempt a systematic analysis of the total photo-process in terms (a) of the initial process of quantised absorption, the primary absorption act and (b) the secondary processes of chemical change which succeed the primary process. This method of approach automatically limits the analysis to photo-reactions in gaseous systems since only in such systems has the physicist yet provided the broad systematisation of the absorption act. The report will include much of the older material acciimulated with respect to such gaseous reactions. This, however, has been orgsnised with a view to illustrating the mode of treatment of photo-reactions from the t\yo standpoints of primary process of absorption and subsequent chemical reactions. This recapitulation of the older material should, however, serve to exhibit whatever advantages or defects inhere in this method of treatment in contrast to those previously employed. In liquid systpms, the absorption is manifested normally as a broad region of general absorption, frequently without structure, and from which all fine structure is missing. Similar behavior is manifest with vapors at high pressure or in the presence of high pressures of foreign inert constituents, RS has recently been found by Tevrs with benzene vapor in presence of high pressures of nitrogen.? The structure of the benzene vapor spectrum becomes increasingly diffuse with increase of nitrogen pressure. This environmental influence is a determining influence in absorption by liquid systems. I n the J. Phys. Chem., 32, 481
* 2. P h p i k , 48, 244
(1928).
(1928).
HUGH S. TAYLOR
2050
absence of a solution to the physical problems involved in the interpretation of spectra in liquid systems, the photochemist must, content himself with a study of promising examples of phot,o-reactions in liquid systems from the kinetic and energetic standpoints and must endeavor to obtain an understanding of the mechanism of such processes with the aid which such studies yield. With dilute gaseous systems, on the other hand, the physical studies of spectra have been more fruitful. In atomic systems they have contributed materially to a more comprehensive classification of the elements,' their electronic structure and properties. More recently, the study of band spectra of molecules, based upon the fundamentals developed in the study of atomic spectra, has rapidly come to the fore.2 The complexity of the subject simultaneously increases with increase in the number of atoms present in the molecule and, as yet', only diatomic molecules have received adequate study and elucidation. Severtheless, even in these relatively simpler molecular systems, certain types of absorption spectra have been distinguished and associated with processes occurring as a consequence of the absorption. I t will not be possible in this report to present a detailed survey of the field, nor is this necessary. A differentiation of the types of absorption and an identification of the process associated with a given type suffice to give us a working basis upon which to base a discussion of mechanism in a variety of photo-active systems.
The Primary Absorption Process The energy, E, obtained in the absorption by a gram molecule of an absorbing reactant, when each molecule receives a quantum, hv, of frequency v is given by the equation E = Shv, where i Y is the Avogadro constant. The following table gives the magnitude of hv in ergs and of E in calories for several typical wave lengths in the visible and ultraviolet spectrum.
TABLE I Energy corresponding to Various Wave Lengths of Light Color of Light
TTave length in Angstroms Red . . . . . . . . . . . . . . 7 j o o - 6 j o o 6joo-j900 Orange, . . , , , , , Tellow. . . . . . . . . . j 9 o o - j 7 j o Green.. . . . . . . . . . . 57 50-4900 Blue. . . . . . . . . . . . . . 4900-4 j j o Yiolet. . . . . . . . . . . . . . 45 50-3950 Vltraviolet . . . . . . . . . . 2 0 0 0
ha 2.62-3.02 X 1 0 - l ~ 3 . 0 2 - 3 . 3 3 x I0-l' 3 . 3 3 - 3 . 4 2 x IO-'' 3.42-4.01 X 10-l~ 4.01-5.32 x I0-l' 4.32-4.97 X IO-" 9 . 9 x IO-"
E
=
Shu
37,800-43r630 43,630-48,060 48,060-49,3 2 0 49,320-57,880 j 7,880-62,330 62,330-7 1,800 142,000
Following a suggestion of Bodenstein and Wagner3 v e may call this quantity E, one Einstein or Light-equivalent, the energy of quantised absorp. See Noyes: Chem. Rev., 5 , 85;Dushman: 109 (1928).
* See hlulliken: Chem. Rev., 6,503 3 Z . physik. Chem., JB,456 (1929).
(1929).
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2051
tion which would be obtained by one gram atom or molecule of the absorbing system were each atom or molecule in the system to absorb one quantum of energy of the given frequency v. I t is a magnitude in photochemistry akin to the Faraday or electrochemical equivalent in electrochemistry, the amount of electricity associated with one gram equivalent, that is to say 6.06X 1oZ3 electrons or elementary charges per gram equivalent. With this convention we can substitute in reaction equations for hv (which refers to one atom or molecule) the symbol E, since, as ordinarily understood, chemical equations refer not to single molecules but to mols of material. Thus, we would write for the elementary process in the case of molecular bromine Brz El = Br2’ or Brz E2 = Br Br’ where the prime refers to an excitation and the two types of product are dependent (as will be discussed later) on the nature of the absorption and, therefore, on the magnitudes of Et and E*. Bodenstein and Wagner further propose that the number of Einsteins be designated by L and the intensity of absorbed light, Iebs, be stated in Einqteins per second. The total amount of light absorbed [Isba] might then be expressed as the number of Einsteins absorbed per litre per second. I t is a t once evident that the absorption of light brings to a reaction system considerable energy quantities, whereby secondary processes of change may be secured. Table I shows, qualitatively at least, why, in general, photoreactions are more frequently initiated by ultraviolet light than by visible light. Energy is accumulated in larger units with the shorter wave lengths of light. The primary process occurring when an atom or a molecule absorbs a quantum of light energy of the magnitudes just indicated has been the objective of a large amount of research in physical science in recent years. From this work, a fairly concrete picture may now be presented of the elementary act in processes of absorption. The following presentation summarises the present position in this rapidly developing field, the historical development being subordinated to an ordered and logically arranged treatment. We may subdivide the whole field according to the nature of the absorbing system whether (a) atomic or (b) molecular. The latter may be further subdivided according as the absorption manifests itself as ( I ) a continuous absorption ( 2 ) a fine structure discontinuous or hand absorption and (3) a diffusestructured discontinuous or band absorption.
+ +
+
Absorption of Light by Atomic Systems On the quantum theory, the absorption of light by atoms produces a change from a stationary state of lower energy to one of higher energy. These changes are revealed in the form of absorption line spectra each line corresponding to the change from a lower to a higher energy state. The line spectrum of absorption by atoms is much simpler than the emission spectrum produced by transitions from higher to lower energy states. For, in an unilluminated atomic system, the great majority of the atoms are present in
HUGH S. TAYLOR
20j2
the stationary state of lowest energy, the normal state, and such atoms can only undergo a few transitions from that state to those of higher energy. Thus, in the case of sodium as an example, normal s!dium atoms can absorb light of the two wave lengths 5897.76 and 5891.78A. respectively, the first resonance lines of the sodium atom. The energy gained is approximately 2.09 volts or 48200 calories per gram atom. Sodium atoms in these excited states, so-called exczted atoms, may re-emit their energies as fluorescent light of the frequencies absorbed, yielding therefore the same two lines in emission, the well-known D-sodium lines. K o p a l sodium atoms may also absorb the second resonance lines (3306-3302A) corresponding to 86000 calories of excitation energy; still further in the ultraviolet the absorption of light of wave length 2412.8A. produces ionisation of the atom, the energy being 5.11 volts or 117,7jocalories, the ionization potential of sodium vapor. Beyond this wave length, on the shorter side, the absorption is continuous, though in this case weak, the energy in excess of the ionisation energy being utilised as kinetic energy of the two fragments, ion and electron, produced. For mercury, the corresponding absorptions are at 2536.7 the first resonance line (4.865volts = 112000 calories), at 1849.6A.,Jhe second resonthe ionisation ance line (6.674volts = 1j3joo calories) and at 1187.96 energy equal to 10.392 volts or approximately 240000 calories. At low pressures of mercury vapor and in the absence of other gases, the light absorbed by the mercury will be re-emitted as fluorescent radiation. By an analysis of the fluorescence of the gas as a function of concentration and admixed gases the conclusion has been reached that the excited state of the mercury atom may endure for about IO-’ second unless robbed of its energy by collision with other atoms or molecules. De-activation of Excited Atoms Wood’ observed the extinction of mercury fluorescence by adding air to the illuminated vapor. Other gases behave similarly. Recently, Stuart? has demonstrated that the quenching of mercury fluorescence in presence of foreign gases cou!d be accounted for on the basis of collisions between excited atoms and gas molecules resulting in a transfer of energy from the excited atom. Different gas molecules, however, show widely varying efficiencies in the quenching of the fluorescence. This may be illustrated by Table I1 which gives Stuart’s values for the pressures of different gases required to reduce the mercury florescence to half its value in absence of the gas. The third line of the table gives Stuart’s estimate of the efficiency of the collisions, being the fraction of the total collisions TThich result in loss of the excitation energy of the mercury. TABLE I1 Quenching of Mercury Fluorescence Gas 0 2 HJ CO CO? H10 Y2 .Ir He Pressure in mm. forHalfExtinction 0.3j 0 . 2 0 0 . 4 0 2 . 0 4.0 30.0 240 j60 Collision Efficiency I I 0 . 8 0 0 . 2 0 0 . 1 0 0.013 0 . 0 0 2 o.ooo,j
K.,
Physik. Z., 13, 353 2 Z . Physik., 32, 262
(1912). (192j).
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Gaviolal has made a more penetrating analysis of the data of Stuart, introducing a correction for the fact that a quantum of resonance light may be reabsorbed and re-emitted several times before its escape from the reaction vessel. The data are also extrapolated to zero vapor pressure in order t o avoid the complicating factor that mercury atoms which have been transferred to the meta-stable z3POstate? by collisions, may also be returned to the resonance state z3P1by collisions of the first kind with swift moving molecules. Ciaviola's calculations indicate that the effective radius of the excited niercury atom is a function of the gas with which collision occurs, an important result theoretically. Thus, if the efficiency of quenching by hydrogen be taken as unity one must assume an effective radius of the excited atom 1.62 times the gas kinetic radius of the normal atom. With this radius, however, for excited mercury the collisions with carbon monoxide would have a 2 5 0 per cent efficiency. Gaviola therefore concludes that for hydrogen one must assume a radius of 5 . 5 x cm. and for carbon monoxide 2 . 9 X 10-3 giving in both cases a quenching efficiency of unity. He assigns the normal gas kinetic radius, 1.8 x IO-^ cm., to those cases in which this assumption does not lead to quenching efficiencies greater than unity. On this basis, he concludes that Rater vapor, nitrogen, argon and helium show quenching efficiencies of 0.4, 0 . 2 , 0 . 0 ; and 0.006 respectively. I t is of interest to note that XIannkopf3 has performed similar experiments Kith sodium vapor, but that a different order of collision efficiencies for the different gases is obtained from that found by Stuart for mercury vapor. It is the energy absorbed by the actinic atoms not re-emitted as fluorescence which is the source of the photochemical action produced in systems containing excited atoms. The production of the excited atom is the primary process to which the Einstein concept of quantised absorption applies. The secondary processes in such systems are the resultant of the energy obtained from the excited atoms by the specific collision processes just indicated. I t will be well therefore to outline the various possibilities that may occur as a result of such collisions. We owe our knowledge of such possibilities largely to the pioneering investigations of Franck and Cario. There are several types of process possible. ( a ) The energy m a y be zitz'lised in exciting other atomic systems electronically: This was shown by Cario and Franck4 who found that illumination of a mixtureo of mercury and thallium vapors by the resonance line of mercury, 2 j36.7 A, produced excited thallium atoms which then emitted characteristic thallium lines. The energies of the thallium atoms were normally less than that of the exciting mercury atom the excess energy being converted into kinetic energy of both the mercury and the thallium atoms, this change revealing itself in a broadening of the thallium lines. TVe may formulate this process by the equation Phys. Rev., (2) 33, 309 (1929). See Section (c) page 2054. 8 Z . Physik, 36, 315 (1926). '2. Physik, 17, 202 (1923).
HUGH S. TAYLOR
2054
Hg’
+ T1-
T1’
+ Hg + K.E.
the prime (’) indicating an excited atom, the excess energy (kinetic) being indicated by K.E. This formulation permits us to anticipate the possibility of the reverse process T1’ Hg K.E. + TI Hg’
+
+
+
whereby kinetic energy and the excitation energy of thallium may combine to produce the more highly excited state of mercury. That such reversal is possible is evident from the demonstration by Cario and Franck, that, at high temperature!, Le., high kinetic energies of the colliding atoms, excited mercury ( 2 j36.7 A,) in presence of thallium can yield excited states of thallium with higher energies than that of the excited mercury, emitting light therefore in the shorter wave length region. Katurally, however, the occurrence of such collisions is not frequent and is exponentially temperature-sensitive since the kinetic energy arises from the temperature condition. ( b ) The energy m a y be utilised in chemical interaction uith colliding gases: This method of utilisation of the excitation energy may be exemplified by means of excited mercury. When hydrogen is present in mercury systems illuminated by the resonance radiation of mercury it has been shown that the bands of mercury hydride are present in the emission spectrum.‘ This indicates the possibility of a reaction in the sense of the equation: Hg’
+
H2 =
HgH
+ H.
The energy of the excited mercury ( I I Z O O O cals.) is more than sufficient to dissociate hydrogen ( D H 2 =101000 cals.). The heat of formation of mercury hydride is small (0.369 volts = 8.5 Kg.Cals.). This represents the simplest type of photochemical reaction succeeding the primary absorption process. ( c ) T h e energy m a y be utilised wholly b y the colliding gas for a reaction in which case lhe excited atom does not directly participate: The simplest case of this kind‘may again be illustrated with hydrogen as the quenching agent for excited mercury. As Cario and Franck2 showed, an active form of hydrogen is produced by such collisions and it is usual to formulate the product as atomic hydrogen according to the equation, Hg’
+ Hz
Hg
+ zH.
The clean-up of the gas on the walls of the vessel and the rapid reduction of metallic oxides at room temperatures were employed as criteria of such a product. I t is to be noted that, in this case, the excited atom does not participate in the chemical reaction; I t merely acts as the agent for the transfer of light of wave length 2 j36.7 A . to hydrogen gas, to which light hydrogen is itself transparent. The mercury is said t o sensitise the hydrogen t o the given ’E.g.,Gaviola and Wood: Phil. Map., 6, I 191 (19281,who, however, suggest an alternative process for the production of the HgH. See also, Beutler and Rabinowitsch: Z. Elektrochemie, 35, 623 (1929). 2 2 .Physik, 11, 161 (1922).
S E C O S D REPORT O F COMMITTEE O S PHOTOCHEMISTRY
2055
wave-length and the process is known as photosensitisation. I n the example cited, the excess energy available is distributed as kinetic energy among the three resulting atoms. The utilisation of the excitation energy in this and the preceding case does not necessarily need to occur in one collision. That this is so is known from the behavior of excited mercury and nitrogen. At a small fraction of a volt below the first resonance state of mercury there is another state z3P, from which the atom can change to higher states by the absorption of radiation; but, once in it, the atom cannot revert to the normal state by radiating, neither can it be brought to this state from the normal one by direct absorption of radiation. Such a metastable state can only be reached from states of still higher energy content. I t was shoown by Cario and Franck that collisions between excited mercury ( 2 j36.7 A) and nitrogen were especial11 fruitful in producing this metastable state of mercury, in which, for the reasons given with respect to restriction of transition, a relatively long life period is possible. It is thus that a molecule requiring large energies for reaction may receive such by successive collisions with one excited and one metastable atom. There is some evidence that in this manner nitrogen may accumulate' enough energy to dissociate. I t is evident, however, that such processes of activation involving several stages must be infrequent as compared with the transfers of energy in a single collision. Photo-ionisation and Photochemical Reaction Paralleling the cases just discussed in which excited atoms are the sources of energy for photochemical and photosensitised reaction we may imagine cases in which the energy supply arises from a recombination of ion and electron produced by absorption of radiation at above the ionisation frequency. We may illustrate the possibility in the case of sodium vapor. As alreadyepointed out, sodium vapor xhen it absorbs light of wave length 2 4 1 2 . 8 A. or less is dissociated into sodium ions and electrons Sa
+ E = S a + + 0.
If, now, the recombination of these products occur as a three-body collision, two processes are possible analogous in many respects to the photochemical and photosensitised reactions discussed in (b) and (c) of the preceding sections. Thus, if hydrogen were the third body, we might expect the two processes, ( I ) the photochemical reaction, Ka+ and
(2)
+ 0 + H,
=
SaH
+H
the photosensitised reaction
+ +
+
+
Sa+ 0 Hz S a 2H. The ionisation energy (5.11 volts) is probably adequate for both processes. The continuous absorption by atomic sodium and potassi1,imhas been observed Gaviola: Phil. Mag., 6, 1191 (1928).
2056
HUGH S. TAYLOR
by Harrison’ and the predicted photoionisation has been studied by various workers.* The absorption is, however, very weak and requires refined methods for the detection of ionisation. Hence, the photochemical effects, though possible, may be expected to be small in amount in such systems and therefore difficult of experimental demonstration.
Absorption of Light by Molecular Systems With molecules, the phenomena resulting from light absorption are very much more complex than in the case of atoms. This arises from the multiplication within the molecule of effects that can succeed the absorption act. For, with molecules, in addition to the electronic excitation (changing in the limit to ionisation) that obtains with atoms, there are also possible changes in the rotational and vibrational energies of the molecule. The general method of approach to the problem is, however, similar to that used in the atomic systems. Corresponding to the several excited atomic states we find various states of electronically excited molecules. Thus, with a diatomic molecule AB, we may find electronic excitations which we may particulsrisc by the formulae A’B, A”B, AB‘, etc., in which the primes indicate the atom whose electronic system has been excited and the degree of excitation. There is, however, not one single energy condition of the molecule with a given state of electronic excitation. T o each such state as well as to the normal molecule without electronic excitation there belongs a whole spectrum of vibrational excitations which manifest themselves in the absorption spectrum as discrete bands. That these vibrational energies are revealed as band abeorption systems and not as single lines, in contrast to the conditions obtaining with atoms, is due to the fact that to each vibrational state of the molecule there corresponds a whole series of separate rotational states. These rotational states manifest themselves with s vibration band, with high resolution of the band, as a series of fine lines converging to n definite limit, the head of the particular vibrational band. In certain cases, to be discussed in detail below, certain vibration bands disclose, not a fine line structure, but a diffuse structure capable of resolution even with the highest dispersions yet available. The significance of such diffuse bands will occupy our special attention later. The normal molecule and each state of electronic excitation possess, in addition to the discontinuous band absorptions already mentioned, a region of continuous absorption in which neither vibrational nor rotational structure is rewaled. I n most cases, these several band systems are so complex and overlap onc another so markedly in the absorption regions that their resolution iiit n individual band systems is a laborious exercise even for the espcrt. &Such resolutions of absorption systenis is now the objective of a consideriiblc :iinount of physical research and a degree of success has attended the effort, espPcinlly Phys. Rev., ( 2 ) 24, 466 (1924). E. 0. Lawrence: Phil. Mag., 5 0 , 34j (1925); Mohler, Foote and Chenault: Piiys. Rev., (2) 27, 37 (1926). 2
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7
in the simpler molecular systems. With triatomic and still more with tetraatomic molecules the complexity is already so great that only in special cases has elucidation of the individual bands been partially achieved. Fortunately, for the application to photochemistry, it is not so much the resolution of the bands that is necessary but rather a definition of the nature of the band in the particular absorption region. For, the progress already made in band mnlysis has resulted in particular processes being associated with particular types of absorption. The character of the absorption process reveals itself in the type of absorption spectrum produced. IT-e may esamine the several possibilities in reference to a diatomic molecule -1I3\Those absorption spectra in the normal and electronically excited states may lie separately indicated AB-A'tB
AB-A'tB
AB-A.8
I /fU%
I
m
in the accompanying diagram, Figure I . These represent the absorption spectra respectively of the normal molecule AB and of the electronically e x i t e d states AB', A'B and A"R. The lowest horizontal line, ne. represents the energy level of the normal niolecule without electronic, vibrational or rotational energies. The successive levels, n l , n, and nl, represent the three states of electronic excitation, without vibrational and rotational energy. The band systems, I , 2, 3, etc., represent individual vibrational levels of a given molecule in one or another electronic state, the structure of the band corresponding to variations in the rotational energies, with vibrational and electronic energies constant. For clarity in the diagram the band structures in question have not been indicated. The diagram indicates that, in each band system, the individual vibrationrotation bands converge towards a region of continuous absorption indicated in the schematic representation by parallel vertical lines. already stated, these continuous regions do not possess any structure. There is complete absence of vibrational and rotational states. This is significant and suggests that we inay open our discussion of the processes occurring in each separate region of the spectrum, with these regions of continuous absorption.
HUGH S. TAYLOR
2058
Continuous Absorption by Atomically-bound, Homopolar, Molecules Franck,' in 1924, postulated that, with atomically-bound or homopolar molecules such for example as CI?, R r ? and I?, absorption in the continuous region involves the dissociation of the molecule into two atoms in the normal state, the other in an electronically excited state. At the convergence limit, the two atoms separate with negligibly small kinetic energies. Within the continuous region the energy in excess of that required for the dissociation in question emerges as kinetic energies of the two atoms. The postulate rested originally on observations of the change of electronic energy coupled with changes in vibrational energy when light absorption occurs in such systems. I t gained for itself a high degree of credence and acceptability as soon as the exact spect'roscopic data for the three molecules, CI2, Brz and I ? were available to test the postulate. These data were forthcoming from the researches of Turner' and of &hns on the lowest excitation energies of the three atoms in question. The data are collected in the following Table IT1 in such a manner as to indicate the truth of Franck's postulate.
TABLE I11 Spectroscopic Data and Heats of Dissociation of Halogens Gas Convergence Limit (A) (2)
(1)
Clz 4785 Br? j r o 7 4995
12
A.
E in Kg Cal for (3)
5 9 . 4 I
