QUAKTUM PROCESSES I N PHOTOCH.EMISTRY BY HUGH S. TAYLOR
The modern theory of absorption and emission of radiation by quanta is a t once both an amplification and limitation of the fundamental law of photochemistry formulated by Grotthuss and restated in its application to the hydrogen-chlorine reaction by Draper. According to this law only light which is absorbed by a chemical system is active in producing chemical change. Quantum theory amplifies this law by defining more particularly the absorption act. The progress which has been made in this definition is indicated in a preceding communication of this series by Turner.' I n so far as the quantum theory of absorption sets a limit to the energy available in a single absorption act it also sets a limit to the nature of chemical change that can result from such absorption. Thus, for example, if the absorbing system be composed of a single type of diatomic molecules and the light energy absorbed per quantum is less than the energy of dissociation of such molecules, it follows that such dissociation cannot occur as the result of absorption of a single quantum. Whether such dissociation may occur when the energy of a quantum exceeds the dissociation energy has already been discussed in some of its bearings by Turner. The relationship laid down by quantum theory between the energy of a quantum and the frequency of the radiation absorbed is given by the equation Q =Khv (1) where Q is expressed in calories per mol, N is the Avogadro constant ( =6.06 X IO*^), h is the Planck constant (=6.554 X IO-?'), v is the frequency (=c/X where cis the velocityof light and X itswave length and equals 1.048X xolOQ). During the past two decades, largely as a result of ideas first put forward by Stark2 and made definite by E i n ~ t e i nthe , ~ relationship between absorbed light energy and chemical reaction produced has been intensively studied. The attempt has been made to show that a simple numerical relationship exists between the number of absorbed quanta and the number of molecules brought to reaction as a result of the light absorption. I n the most definite form, sometimes known as the Einstein Law of the PhotocLemical Equivalent, an exact equivalence between absorbed quanta and reacting molecules was postulated. Abundant evidence is now available to show that this exact equivalence only holds in very exceptional circumstances and that, in the majority of photo-chemical reactions, the relationship obtained deviates from equivalence to an extent characteristic of the system in question. The deviation may be in both directions. Many systems show manyfold fewer reacting molecules than absorbed quanta. At the other extreme are reactions in which an enormous number of molecules react for every absorbed quantum. I t might therefore be asked what advantage there could be in retaining the con-
QUANTUM PROCESSES I N PHOTOCHEMISTRY
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cept of a relationship between quanta and reacting molecules. I t is the aim of the present article to set forth considerations which bear upon this problem. The Analogy between Electrochemical and Photochemical Processes. Since the time of Grotthuss, and especially in recent times by Bancroft4 and by Baur,; an analogy between electrochemical and photochemical processes has been stressed. The modern theory of quanta applied to photochemical systems is very much more definite than any such process of analogy. For the idea that the action of light is analogous to that of a voltaic cell or that ‘photolysis is a molecular electrolysis,’ quantum theory substitutes a perfectly definite statement as to the actual stages in the whole process. It postulates that the absorbing constituent of the system is changed by absorption of light, from its normal energy state in the dark, to an energy-rich form, the magnitude of the energy increase per unit absorbent, atom or molecule, being exactly that of the quantum, hv, of absorbed radiation. When the absorbing system is atomic, the nature of the energy change can be stated in terms of the quantum theory of atomic structure. Thu!, in the case of mercury, the absorption of light of wave length X = 2 j 3 6 . 7 A, is accompanied by the passage of the atom from the normal atom state (IS) to the excited state known as the 2 3 P 1 state. The energy associated with this change is given by Equation ( I ) as equivalent to 1 1 2 0 0 0 calories per gram atom of mercury in the excited state. When the absorbing system is polyatomic the energy absorbed is not necessarily wholly associated with an electronic change, but may also be distributed between the vibrational and rotational degrees of freedom of the absorbent. The total increase of all such types of energy is, however, given by Equation I. The quantum theory of photochemical processes, therefore, accounts for the existence of photochemical reactions by the presence in the reaction system of an energy-rich species and also of species with which such energy-rich atoms or molecules may react. The nature of the chemical change brought about by the agency of the light will depend on the normal chemical reactivity of the system and on the available energy. I t is thus possible for all such chemical reactions as occur in the dark to occur with one of the constituents activated by light. It is, however, possible that reactions may occur in the light, which occur only to a negligible extent in the dark, by reason of the larger units of activating energy involved. Thus, the chance that a molecule in a system at temperature T shall possess, thermally, a quantity of energy equal to 100,ooo calories is given by the Boltzmann relationship e - iooooo/RT which, for the ordinary temperature range, is very small. Molecules possessing such energy unitsaare created every time a quantum of light of wave length approximately 2800 A is absorbed. Depolarizers.-In stressing the analogy between photochemical and electrochemical processes Grotthuss and others have referred to the non-absorbing components of a photochemical system as depolarisers, and thus associated them with the electrochemical depolarisers which take part in electrode reactions on the discharge of ions. The quantum concept of photochemistry
518
HUGH S. TAYLOR
applied to such components sees in them merely reacting constituents of the system which may or may not partake in the reaction process depending on the energy quantities available. I n electrochemical systems the reactions are mainly oxidation and reduction since they are generally processes involving electron transfers. I n photochemical systems not only do oxidation and reduction occur but every other type of chemical process is possible, halogenation, hydration, hydrolysis, polymerisation or condensation, depolymerisation. I n general, the photo-reaction will not be associable with a transfer of electrons but will be associated with the definite utilisation of activating energy. The approach to an examination of this energy utilisation may be made through consideration of thermal reactions. If we heat up a vessel containing hydrogen gas, there will be negligible chemical change in the system until extremely high temperatures are reached. That is because the chemical reaction which can occur in such a system is the dissociation of molecules into atoms.(’) Now, in order to dissociate, a molecule must receive 5s activating energy a t least the dissociation energy of 100,ooo calories and this, as the Boltzmann expression above shows, is a seldom occurrence. Marked concentrations of atomic hydrogen are only obtained thermally above zooo°C. If, however, we introduce into the hydrogen gas some other reacting gas, reaction may occur long before such temperatures are reached. Thus, with iodine, reaction occurs a t a few hundred degrees of temperature since, as is now known,6 the activating energy demanded of such a system is that the colliding molecules of hydrogen and iodine shall jointly possess some 40,000 calories. The iodine may be regarded as a thermal depolariser of the hydrogen. I n the reverse reaction, the dissociation of hydrogen iodide, the substance may be thought of as its own depolariser. If unimolecularly dissociated, an activating energy not less than 68,000 calories (the dissociation energy) is required, whereas in the ‘depolarized process,’ the bimolecular reaction, only some 44,000 calories of activating energy are required by a colliding pair of molecules.6 Bromine acts in an entirely different manner as a ‘depolariser’ of hydrogen. On the basis of the experimental data of Rodenstein and Lind,’ Christiansen,s Polanyig and HerzfeldlO all independently showed that bromine atoms were the immediate agents of ‘depolarisation’ as is indicated by the reaction scheme : Brz = zBr Br Hz = H B r + H H Brz = H B r + B r H HBr = HZ Br. Oxygen is actually an extremely poor ‘depolariser’ for hydrogen gas. There is no experimental evidence that the homogeneous thermal gas reaction occurs to any reasonable extent when studied in closed systems.6 The bulk of the reaction is confined to the walls of the containing vessel.
+ + +
+
Excluding the posaible formation of polymers Ha, H,, etc.
519
QUANTCM PROCESSES IN PHOTOCHEMISTRY
This varying reactivity of systems under the influence of thermal activation is exactly paralleled by photochemical systems. As recorded by Bancroft in a following paper, copper sulphate solutions are stable of themselves in light, but are readily reducible when white phosphorus is present in the system. Potassium permanganate solutions show a certain degree of in&ability under suitable illumination but this instability is enormously increased by the addition of sodium oxalate.lI There does not, however, appear to be any essential point of difference between this and the thermal hydrogen and hydrogen-iodine systems just discussed. What is plain is that the reactivity of normal thermal systems of potassium permanganate and sodium oxalate is less frequent than the reactivity of photochemically activated permanganate with sodium oxalate. Such systems are, however, quite complex and it is desirable first to exemplify the quantum concept of photochemical reaction with much simpler systems, returning later to the more complex. Hydrogen Iodide.-The simplest reaction system for which the most comprehensive data are available is hydrogen iodide. I n this system the photodecomposition is unimolecular, indicating that single molecules only are involved in the decomposition process or, in the older mode of expression, the molecules do not act themselves as 'depolarisers' for the activated molecules. The reaction is proportional to the absorbed light energy over a wide variation in the state of the reactant. Thus, Warburg demonstrated this with gaseous pressures varying between 80 and 350 mm.12 Bodenstein and Lieneweg13extended this observation to liquid hydrogen iodide and quite recently Lewis" has reached the same conclusion with gas pressures as low as 0.1mm. Temperature has no influence on this ratio of absorbed energy to reaction rate. Warburg studied the decompositio? with monochromatic radiation of wave lengths X = 2 0 7 0 , 2 5 3 0 and 2 8 2 0 A. These measurements yielded results which are decisive from the standpoint of quantum theory. If we express the results obtained in terms of the number of gram atoms of iodine produced per gram calorie of light energy employed the following values are obtained:
x
=
20f0
2530
2820
Mol. I. x 1 0 5 = I .4'$ 1.85 2.09 These results show that, per gram calorie of light energy, the extreme ultraviolet is less efficient than the longer wave length light. This surprising result, completely inexplicable without quantum theory, becomes, however, immediately understandable, when the quantum relationship is introduced. The calculation then shows that, a t each wave length, the number of molecules decomposed per quantum of absorbed energy is 2.0 with a maximum deviation in all of the experiments of not more than 5 per cent, a deviation well within the experimental error. This ratio is also obtained by Bodenstein and Lieneweg for liquid hydrogen iodide a t room temperature, for the gas a t 150-17 j°C and by Lewis for the gas a t 0.1 mm. pressure. These several results, as well as similar studies with hydrogen bromide, exhibit strikingly, therefore, that the relation between light energy absorbed and chemical reaction produced is a quantum relationship.
520
HUGH S. TAYLOR
The quantum absorbed by a hydrogen iodide molecule secures its decomposition. Physical evidence by Tingey and Gerke’j and by Bonhoeffer and SteineF support this conclusion. But direct experimental evidence by showing the independence of the quantum yield of molecular collisions, is decisive that the process of decomposition does not involve a simultaneous reaction of two hydrogen iodide molecules. The measurements already discussed indicate that the photochemical yield corresponds to two molecules of hydrogen iodide decomposed per quantum. It is evident therefore that the primary photo-decomposition must be followed by a series of ‘dark’ reactions which raise the yield to twice that of the molecules activated by absorption of a quantum. These supervening reactions arc not photoreactions. They are purely thermal reactions possible in the given system whether produced initially by photo-action or other means. Their effect is to raise the yield beyond that anticipated upon the basis of absorbed quanta. They offera fruitful yield for the speculative chemist and have led accordingly to further experimental effort. In the hydrogen iodide case, the simplest of such suggestions which agrees with the experimental facts is that, after the truephotochemicalreaction, HI hv = H I (1) the following sequence of reactions occurs:
+
+
Bonhoeffer” has demonstrated that reaction ( 2 ) actually occurs as a purely thermal reaction a t ordinary temperatures. The Hydrogen-Bromine Combination.-When light of a wave length of j I Oj -1or less is absorbed by moist bromine, the physical evidence now available shows that the molecule is dissociated into one excited atom p d one normal atom. The energy corresponding to a wave length of 5107 A is approximately 5 j,j o o calories of which 4 j,zoo calories is the energy of dissociation and the residual IO,joo calories is the energy of excitation of the bromine atom.’ Shorter wave length light corresponding to larger units of energy would produce the same dissociation and the same excited atom, the excess energy being distributed, however, between the two as kinetic energy. It is well known, also, that all attempts to measure directly the concentration of bromine atoms in illuminated bromine vapor have failed to show any marked concentration. It therefore follows that, equating the light energy absorbed to atoms present in the illuminated system would give a ratio in which many millions of absorbed quanta correspond to unit dissociation of bromine. This is not in disagreement with the physical evidence. It only means that the reverse combination of bromine atoms occurs rapidly and that the measurement of atom concentration would only give a stationary state concentration under the given illumination and dependent upon this latter. If we introduce hydrogen, as an acceptor (or ‘depolariser’) for bromine atoms produced, we find that the efficiency of the acceptor is dependent on the temperature of the system. It increases with increasing temperature. This means that the thermal processes which succeed the initial photo-process
QUANTUM PROCESSES IN PHOTOCHEMISTRY
521
are dependent upon temperature. Measurements of the quantum yield over such a temperature range will obviously show a remarkable variation. It has been estimated that, a t room temperature, more than IOO quanta are absorbed per unit of hydrogen bromide formed. This ratio will also obviously decrease with temperature. At some one temperature it will be unity. Above this temperature it will be greater than unity. This in no way invalidates the physical evidence that one quantum of light energy produces one excited and one normal atom. It only means that the succeeding thermal processes result, with increasing temperature, in increasing production of hydrogen bromide and decreasing recombination to form molecular bromine. I n the view of Christiansen, Herzfeld and Polanyi, the following sequence of reactions occurs: firstly, the light reaction is, Br2 hv = zBr. (1)
+
This is then followed by the following purely thermal processes: Br
H H
+ H2 = H B r + H + Brz = HBr + Br + HBr = H2 + Br
(2)
( 31 (4) (5)
+
and Br Br = Brz Every one of these reactions is known to occur thermally. The investigations of Bodenstein and Lutkemeyer'* indicated that it is reaction ( 2 ) which is temperature-sensitive. The reaction only occurs thermally when the bromine atom and hydrogen molecule together possess an activation energy of about 17,000 calories. Reaction (4) accounts for the inhibiting action of hydrogen bromide. It is not possible here to enter into the extraordinarily interesting correlation by Bodenstein, of his thermal study with Lind7 on the formation of hydrogen bromide with that of Bodenstein and Lutkemeyer on the photoprocess. It is sufficient if it has been indicated that the evidence from the purely physical side concerning the light-absorption process and its resulting bromine-atom formation supplies a reaction system with which, by purely thermal processes, the known facts of the photo-reaction are completely interpretable. T h e Hydrogen-Chlorine Combination.-The same mode of interpretation is applicable to the historical hydrogen-chlorine reaction. Theoabsorption by moist chlorine of light of wave length shorter than X = 4785 A results in the production of a normal chlorine atom and an excited atom with an energy of excitation corresponding to some z 500 calories. With hydrogen present in the illuminated system, reactions similar to those occurring in the hydrogenbromine system are possible. Among others we may have: C1 Hz = HC1 H (I) H C1 = HCl (4) H Cls = HC1 C1 (2) c1+ C1 = Clz (5) HC1 H = Hz C1 (3) H H = Ht. (6) The essential differencebetween this and the bromine reaction is that, a t room temperatures and upwards, Reaction ( I ) of the above sequence does not require any activating energy. The thermal reactions are not temperature-
+
+ +
+ + +
+ +
522
HUGH S. TAYLOR
sensitive. This conclusion is in accord with the thermal data, for, in Reaction ( I ) above, the process involves practically zero heat of reaction whereas the corresponding reaction between bromine atom and hydrogen molecule is endothermic to the extent of 17,000 calories and thus requires activation energy of similar amount. Why the hydrogen chloride does not inhibit the reaction in contradistinction to the action of hydrogen bromide is also explainable by the same set of facts as well as by the direct observation of BonhoeffeP that hydrogen atoms led into hydrogen chloride produce a marked thermal effect, but, the product is only hydrogen and hydrogen chloride without any final production of chlorine. The extraordinary photochemical yield in the hydrogen-chlorine reaction (ca. 106 molecules per quantum) is therefore to be attributed to the facility with which a sequence of thermal reactions can follow the setting up of a reaction system containing atomic chlorine by the quantised process of light absorption. Photo-sensitised Processes The same attitude may be adopted towards photo-sensitised processes and, in such cases also, there is enough experimental evidence in simple cases to justify the procedure. The photosensitised processes differ from the photochemical processes in that the light-absorbing constituent is not a reactant in the chemical reaction occurring. Ezcited Mercury as Senstliser.-Undoubtedly the simplest of the photosensitised reaction processes are those involving excited mercury and hydrogen. For, in these cases, the light quanta involved are monochromatic, of exactly known energy content, producing physical and chemical effects the steps in which have been definitely ascertained. The normal mercury atom absorbs a qu5ntum of mercury resonance radiation corresponding to the wave length 2536 A and is thereby raised to the z3P1state, in which i t possesses energy equivalent to IIZ,OOO calories of energy. I n the absence of foreign gases this energy is re-emitted as fluorescent radiation of the same wave length. With foreign gases present it may lose this energy by collision with the gas molecules without any radiation. If hydrogen be present, the energy transfer results in a photosensitised process, the conversion of molecular into atomic hydrogen with the absorption of IOO,OOO calories of dissociation energy, the excess 12,000 calories being distributed among the resultants (Hg, PH) as kinetic energy. The presence of atomic hydrogen may be demonstrated by the ‘clean-up’ eff ect,I8by physical means such as the thermal conductivity of the illuminated system20 or by the chemical reactivity of the atomic hydrogen.21 Were the experiment possible, the ratio between the number of quanta absorbed by the mercury atoms and the number of hydrogen atoms produced by subsequent collision might be ascertained. This would be the photochemical yield of the process. We know it would depend on the hydrogen pressure, since the extinction of the fluorescence is dependent on this pressure and increases with increasing pressure. With sufficiently high hydrogen
QUANTUhI PROCESSES IN PHOTOCHEMISTRY
523
pressures we can conclude that every excited mercury atom should lead to the production of two hydrogen atoms. Any attempt to ascertain the ratio of quanta absorbed to atoms produced either by the ‘clean-up’ effect or by means of thermal conductivity would certainly yield a result less than two atoms per quantum since either method would fail to correct for the thermal recombination of hydrogen atoms formed. Attempts by chemical means to ascertain the number of atoms formed per quantum absorbed would be subject to the same limitations that the hydrogen-halogen photo-reactions, already discussed, suffer. The thermal processes possible would determine the yield obtained. This has already been demonstrated experimentally in a number of cases. Thus, if ethylene is introduced as the acceptor for atomic hydrogen, the following reactions are possible H +CZH4 = CZHS CzHs Hz = C A H C2Ha C1H4 = CiHa Cd% HZ = CCHlo H etc.,
+ + +
+ +
and the yield in terms of saturated hydrocarbons formed will be determined by the length of such sequences. The same is true with oxygen as acceptor where hydrogen peroxide is produced and with carbon monoxide where formaldehyde and glyoxalZ2are identified products, probably by such sequences as
H
+CO = HCO HCO + H z =HCHO+H HCO HCHO = (HC0)z H zHCO = (HC0)z.
+
+
I n all these cases the yield is not determined by the light quanta absorbed but by the reactivity of the system produced by the absorption process. The Photo-sensitised Decomposition of Ozone.-The decomposition of ozone may be photosensitised to visible light by introducing chlorine or bromine into the gas. I n either case the initial light absorption process by the halogen molecule must yield one excited and one normal atom. But, the subsequent yield of decomposed ozone is quite different in the two cases. As BonhoeffeF has shown, only two molecules of ozone are decomposed per pair of chlorine atoms produced, whereas as many as thirty ozone molecules decompose per pair of bromine atoms resulting from light absorption. The explanation of this difference must lie in the different reactivities of the chlorine atom-ozone system and the bromine atom-ozone system. No one has yet indicated what these differences are or attempted other methods than photochemical of studying them. It is possible that some progress might be made in this regard by a study of the effect of temperature on the photo-sensitised processes. The Stationary State Several examples have already been discussed in which the absorption of light by a reactant results in the displacement of the system from its normal ‘dark’ equilibrium. The physical evidence indicates that in both illuminated
524
HUGH S. TAYLOR
chlorine and bromine there exists a greater concentration of atoms than is present in the unilluminated system a t the same temperature. The penetrating analysis of the hydrogen-bromine combination by Bodenstein and Lutkemeyer’* permitted a calculation of the respective ‘dark’ and ‘illuminated’ bromine atom concentrations a t 218’C. The results showed that, for the intensity employed, approximately 300 times the concentration of atoms was present in the illuminated system. The stationary concentration was determined in part by the recombination of atoms to form molecules and, on the assumption that each quantum absorbed yielded two atoms, only about 0.0013 of all atomic collisions led to the formation of molecules. Had the quantum efficiency been calculated from the stationary state concentration the efficiency of the absorbed quanta would have been negligibly small. There are a number of other reactions in which ir is experimentally possible to determine the rate of the photo-reaction resulting in the displacement of equilibrium, independently of the dark reactions occurring simultaneously. These include the decomposition of nitrosyl chloride, the equilibrium, Fe Fe ’, I1 - 1’, the polymerization of anthracene and substituted anthracenes. It is not possible to detail these in the scope of this article. One must suffice as an example. , 12 - I’ --Rideal and Williamsz4discovThe Equilibrzum Fe - Fe ered that the rate of reaction in the initial stages of illumination of the thermal equilibrium solution, when the dark reactions compensate each other, provided a means of dissociating the photo-process from the thermal processes. In this way they were able to !how that one molecule of iodine reacted per absorbed light quantum (5790 A). K$akowskyz5 showei that this was true also of the light of wave lengths 5460 A, 4360 A and 3660 A. I n each case the relation was one molecule of iodine reacting per absorbed quantum. This situation is entirely analogous to that discussed in the case of hydrogen iodide. Without quantum theory, ?ne has to explain why 78,500 calories of light energy of wave length 3660 A is only as efficient as 43,850 calories of the longer wave length 5790 A. Immediately the quantum concept is introduced this difficulty entirely disappears, There is an exact relation between the quantum absorbed and the activated molecules, and the method of study in this case eliminates the complicating thermal reactions which would mask the relation. Inhibition The necessity of distinguishing between the absorption act with its immediate chemical consequence and the chemical, purely thermal, reactions which succeed it, is nowhere more evident than in photo-reactions which show the phenomenon of inhibition. The classical example of inhibition in a photoreaction process is the influence of oxygen on the yield of hydrogen chloride in the hydrogen-chlorine combination. We have recorded above that a large number of thermal reactions may succeed the original absorption act in this case and produce yields of the order of 105 - 106 molecules of hydrogen chloride per quantum of absorbed light. Oxygen reduces the yield to an extent roughly proportional to its concentration so that hydrogen-chlorine mixtures
QUASTUM PROCESSES IN PHOTOCHEMISTRY
525
containing one per cent of oxygen are only feebly sensitive to illuminations which would produce explosions in the oxygen-free gas. The role of the oxygen in the process is still under discussion and no completely satisfactory conclusion has yet been reached. From the small concentration of oxygen required it is evident that the effect of the oxygen is exerted either on the atomic chlorine produced by the absorption act or on the thermal reactions subsequently produced. The phenomenon of inhibition is not, however, confined to those photoreactions for the explanation of which the reactivity of atomic species may be invoked. Thus, the photo-decomposition of aqueous hydrogen peroxidez6 and numerous examples of auto-oxidation such as those of benzaldehyde and of aqueous sodium sulphite solutions all show the phenomenon. It has now been shown by Rack~trOm*~ for all the auto-oxidations thus far investigated that the yield of oxidation product is many thousand-fold that to be anticipated on the basis of equivalence between molecules oxidised and quanta absorbed, thus confirming a prediction of Christiansen.** In the case of benzaldehyde the yield is approximately 10,000molecules per quantum. With sodium sulphite the ratio rises to 50~000molecules per quantum, an extremely striking result when it is remembered that this occurs in aqueous solution where there are more than 5 5 molecules of water for every molecule of sulphite (Le., less than I Molar Sulphite solution). It is evident that there must follow the initial absorption act a whole series of thermal reactions producing sulphite oxidation. This agrees with the fact that even the purely thermal oxidation of benzaldehyde and sodium sulphite are also sensitive to inhibitors. The mechanism of the action of the inhibitor is only in the initial stages of study but unpublished results now available in Princeton indicate an intimate relationship between the amount of sulphite oxidised and the amount of inhibitor oxidised. The sulphite oxidised is inversely proportional to the concentration of inhibitor. The inhibitor oxidised is, however, constant over a very wide concentration range and with various alcohols as inhibitors, even though the inhibitor efficiency varies markedly. There is evidence that the amount of inhibitor oxidised is of the order of 1j5000 of the sulphite which would have been oxidised had the inhibitor not been present. This evidence points overwhelmingly to the conclusion that, in the reaction without inhibitor, the initital photo-process is succeeded by a large number of thermal reactions which can be stopped by causing the energy-rich reaction products of the auto-oxidation process to oxidise the inhibitor. The stages of the autooxidation process have been studied in detail by Backstrom. On this basis, the phenomenon of inhibition becomes a special case of the well-known phenomenon of induced oxidation. I n the case of hydrogen peroxide solutions, the mechanism of the thermal processes have not been elucidated in detail. The extent of decomposition yield in the photo-process and the inhibitions of the purely thermal decomposition point, however, to the existence of thermal chain reactions.
526
HUGH S. TAYLOR
Complex Photo-processes We have seen that even in the relatively simple reaction systems already discussed there are still problems which have not yet received their final solution. It has been shown, however, that the method of approach here developed, of distinguishing between, on the one hand, the initial absorption act and its immediate chemical consequences and, on the other hand, the thermal reactions possible in a system thus photochemically produced, leads to an understandable interpretation of the total process. With more complex systems the difficulties increase since the factors to be controlled increase. Nevertheless, it can be shown, with one or two examples from these more complex processes, that the same guiding principles hold. Osidation of Quinine.-The classical researches of Luther and ForbeszQ on the photo-oxidation of quinine in presence of chromic acid are of use in this regard. I n absence of chromic acid the light absorbed by quinine is re-emitted as fluorescence radiation. No photochemical reaction occurs. With small amounts of chromic acid present, a portion of the energyabsorbed by the quinine is consumed in producing fluorescence and the residue in promoting reaction between active quinine molecules and chromic acid. I n such cases it follows that the absorbed quanta are considerably in excess of the reacting molecules. With increase in chromic acid concentration, the probability of a quinine molecule losing an absorbed quantum as fluorescent radiation decreases and, hence, a t a given acid concentration and beyond, a state of affairs obtains in which reaction with chromic acid follows immediately upon the absorption act. I n such case there will be an equivalence between quinine molecules oxidised and quanta absorbed by quinine. There is, however, a complicating factor in that such a relation does not exist between reaction and total light absorbed since chromic acid absorbs but is not thereby converted into a reactive species. One further consequence of this also follows. I n large excess of chromic acid, where the light is practically screened from the quinine, the photochemical yield would fall again from unity to practically zero. Reduction of Fehling's Solution,-Another factor which influences the yield in processes initiated by light may be illustrated by the case of Fehling's solution. Byk30 showed that the photo-decomposition of the solution itself was secured by light of frequencies in the ultraviolet absorbed by the cupricomplex present. S o decomposition occurs if the solution is illuminated by the blue light of the visible absorption band. The system under these conditions is entirely analogous to the quinine system in absence of chromic acid. The absorbed radiations are transformed into radiation of other wave lengths (or otherwise dissipated). If, however, as Leighton3' showed, hydroquinone be introduced into such solutions illuminated by red light, reaction occurs and reduction takes place. The hydroquinone plays the part of the chromic acid in the quinine reaction. With increase in its concentration the photoyield may be expected to rise from zero to unity as in the quinine case also. Other complex systems will doubtless yield to patient and intelligent investigation; without this latter no progress can be expected.
QUANTUM PROCESSES IS PHOTOCHEMISTRY
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Einstein’s Law of Photochemical Equivalence X e are now in a position to examine the law of photochemical equivalence enunciated by Einstein in 1 9 1 2 in the light of the yield of fifteen years of experimental study. The examples discussed in the preceding, together with many others which might have been cited, all suggest that the coupling of the concept of quantised absorption with the idea of net photochemical yield has not been sustained by the experimental test. The idea of photochemical equivalence which the title of the law suggests has not been confirmed by this practical work. Indeed, in the case of some workers, this suggestion of equivalence has given rise to mistrust of the general idea of quantised absorption which is basic to all these modern studies of photochemical processes and is demanded by physical thepry. It would therefore seem desirable to attempt a generalisation of the facts concerning the niechanism of the photochemical process which, while avoiding the difficulties inherent in the acceptance of the Law of Photochemical Equivalence as originally formulated would embody the elements of this law which have found support from its study. For this purpose it seems necessary first to avoid entirely the name which has become usual in reference to this matter, since equivalence has been demonstrated only in exceptional cases rather than as a general rule. The situation may be met by means of two laws of photochemistry. The First Law of Photochemistry would be the Grotthuss-Draper Absorption Law, embodied in the statenlent that: Only light that is absorbed is effective in producing chemical change. This would be followed by the Second L a w of Photochemistry which might thus be expressed: T h e absorption of light i s a quantum process involving one quantum per absorbing molecule (or atom). T h e photochemical yield i s determined by the thermal reactions of the system produced by the light absorption. Of this second law, the quantum concept of absorption is Einstein’s contribution to the progress of photochemistry. The second half is a generalisation from the experiments of numerous workers who, in testing Einstein’s original ideas, have added enormously to the quantitative knowledge of mechanism in photochemical processes and demonstrated the factors which determine the yield from a given illuminated system. References Turner: This Report, 3rd Paper. *Stark: Physik. Z., 9,889, 894 (1908). Einstein: Ann. Ph sik, 37, 832; 38,881 (1912). Cong. App. Chem., 20, 31 (1912). Bancroft: Eighth 6 Baur: Trans. Faraday SOC.,20, 627 (1925). 8 Hinshelwood: “Kinetics of Reactions,” Oxford Univ. Press. ’ Bodenstein and Lind: 2. physik. Chem., 57, 168 (1907). * Christiansen: Danske. Kd. Math. Phys. Medd., 1, 14 (1919). QPolan i 2 Ph sik, 1, 337 (19201. l o Herzzld: Z.Pxysik, 8, 132 (1922). l1 Rideal and Xorrish: Proc. Roy. SOC., 103A,342, 366 (1923). l2 Warburg: Sitaungsher. preuss. Akad., 1918,300. l 3 Bodenstein and Lieneweg: 2. physik. Chem., 119,123 (1926) l4 Lenis: Proc. S a t . .kcad. Sci., 13, 720 (1927). ‘5Tingey and Gerka: J. Am. Chem. Soc., 48.1838 (1926). 1
frit.
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1eBonhoeffer and Steiner: Z. physik. Chem.. 122, 287 (1926). "Bonhoeffer: Z. physik. Chem., 113, 199, 92 (1924). 18Bodenstein and Lutkemeyer: Z. physik. hhem., 114, 208 (1925). 19Cario and Franck: Z. Physik, 11, 162 (1922). 20Sentfleben: Z. Physik, 32, 922 (1925). 21 Cario and Franck and others: See Taylor: J. Am. Chem. SOC.,48, 2840 (1926). 22 That glyoxal is also produced was kindly communicated to me verbally by Dr. W. Frankenburger of the I. G. Laboratories a t Oppau, Germany. 23 Bonhoeffer: Z. Physik, 13, 94 (I 23) 24RidealandWilliams: J. Chem. 8oc.,'127,2j8 (1926). *5 Kistiakowsky: J. Am. Chem. SOC., 49, 976 (1927). 28Anderson and Taylor: J. Am. Chem. SOC., 45, 6jo (1923). 2'Backstrom: J. Am. Chem. SOC., 49, 1460 (1927);hIedd. K. Vet. Akad. Nobel Inst., 6, Nos. 15 and 16 (1927). 28Christiansen: J. Phys. Chem., 28, 1 4 j (1924). 2 9 Luther and Forbes: J. Am. Chem. Soc., 31, 770 (1909). 3OByk: 2. physik. Chem., 49, 6j9 (1904). Leighton: J. Phys. Chem., 17, 205 (1913).
Pnneeton, K.J .
