T H E PHOTOCHEMICAL DECOMPOSITION OF HYDROGEN IODIDE; T H E MODE OF OPTICAL DISSOCIATION1 BY BERNARD
LEWIS^
Introduction
Part I . Kinetzc The problem of determining by direct experiment the true mechanism of the photochemical decomposition of hydrogen-bromide or hydrogen-iodide studied by Warburg is of no little interest and importance since such knowledge would afford information concerning the conditions under which a molecule decomposes after it has become activated as a result of absorption of radiation. It will be recalled that Warburg suggested the following steps in the decomposition after a thermodynamic consideration of the possible occurrence of secondary “dark” reactions: (I) (2)
(3)
+ +
H X hv = H H H X = HP
x + X = XP
+X +X
where X denotes the halogen atom.4 This accounts exactly for the experimentally determined quantum efficiency of two molecules decomposed for each quantum of energy absorbed. Stern and T’olmer5have presented an entirely different point of view which does not violate the observed quantum efficiency. They argue that in this type of reaction a collision is necessary between the active molecule and some other molecule during the mean life of the active state before decomposition can take place and they conclude that the hypothesis that molecular dissociation or decomposition is the primary light process, cannot be entertained. Some of the bases of their arguments have since been found to be untenable. For instance, assuming the heat of dissociation of Clz as 106,000 calories they pointed out the impossibility of a direct splitting of the chlorine molecule into atoms as a result of absorption of radiation since the energy available in the spectral region in which the combination of hydrogen-chloride occurs, namely about X = 4oooA, (Nhv = 71,300 calories) is considerably less than the heat of dissociation of chlorine. A collision therefore seemed necessary to meet the added energy requirement. This must now be ruled out since the heat of dissociation of Clz as more recently determined kinetically6 and spectroscopically7 is 54,000 - 58,000 calories. Thus there is sufficient energy available in the quantum. That Cl? does dissociate in a single and elementary act by absorption of radiation in this region will be evident from spectroscopic considerations presented in Part I1 of this introduction. It is true, however, that experiments on gas fluorescenceb and on the excitation of molecular spectra by electron impactg show that at low pressure a
PHOTOCHEMICAL DECOMPOSITION O F HYDROGEN IODIDE
271
molecule may absorb many times its dissociation energy without dissociating. For example, iodine, whose heat of dissociation is 34,500 calories6 can absorb and emit as a resonance spectrum, an amount of energy five times the work of dissociation, when illuminated by the line X = 1849A.'~ Again a hydrogen molecule excited by electron collision can emit as molecular spectrum several times the amount of the dissociation energy. From the excitation of x-ray spectra, values of the excitation energy can be evaluated which may exceed the work of dissociation a thousand times without leading to dissociation by a primary process. The explanation of this effect seems to be found in the primary employment of the excitation energy in raising the electron system to a higher quantum state, while the oscillation and rotation energies are only altered by the coupling of their periods with those of the electron system.ll Another plausible explanation, more applicable to complex molecules, is that, under certain conditions, there may be a finite interval between the absorption of energy and the decomposition of the molecule, since the former may not be distributed a t once in the particular manner necessary for dissociation. During this interval a collision or some other disturbing factor may cause the emission of the energy as radiation. Conceiving the primary light process as resulting in an energy-rich modification of the molecule, which, on the Bohr conception would be a molecule with an electron in an orbit farther removed from the nucleus, Stern and Volmer prefer to write
where [HX]b indicates the active state. I n order to suit its condition to its surplus energy the active molecule requires a collision with another molecule before it can decompose, [HX]b
+ H X = Hz 4- X2
the steps of which may be several and of the type proposed by Warburg in his thermodynamic treatment.'* According to these authors this will occur provided the active molecule suffers a collision during its term of life, viz., ca. IO-^ sec. which has always been the case in the concentrations hitherto investigated. (Warburg worked at pressures of H I ranging from 80 m.m. to 350 m.m.). Otherwise the energy will be radiated or dissipated as kinetic energy. With inert gases or hydrogen present a small portion of the energy may be transferred to the foreign gas at each collision, the excited HI molecule taking up new and successive rotational quantum states, which, according to Stern and Volmer, seems possible with halogen-hydrides judging from their rather complicated infra-red spectrum. A sufficient number of successive collisions with a foreign gas may finally result in the deactivation of the [HX]b molecule. This would of course presuppose a relatively long life for the active state which is not inconceivable for rotational quantum jumps a3 the calculations of TolmanI3 show. The decomposition of IIBr and HI was studied by Warburg in the presence of S 2and H f . Seither here nor when HI was mixed
272
BERNARD LEWIS
with 0, I or 2 3 4 atmospheres of KP4was the decomposition affected in the least, the quantum efficiency still remaining two. This does not exclude the possibility of the reaction taking the course [HI]b H I = Hn Ia since [HI]b evidently may keep its energy after a collision with certain molecules. It is a fact now coming to light that active molecules show varying susceptibilities to change on collision with different foreign gases.14 B~denstein'~ concluded that the photochemical decomposition of HI is unimolecular and Trautz and Scheifele16 reach the same conclusion for the early stages of the reaction. This does not signify that collisions are unnecessary since the decomposition may still be unimolecular with respect to the active molecules as presented in a mechanism above.I2 From a kinetic point of view probably the most direct method which will enable one to determine whether the excited molecule may decompose without the stimulus of a collision, is to study the quantum efficiency at sufficiently low pressures of pure gas so that a molecule of HI, activated by absorbed radiation, is unable to make a collision with an ordinary H I molecule before its mean free life, vie., IO-' sec.17 has terminated or before it decomposes of its own accord. I n the former case if reversion takes place i.e. if a molecular encounter is a condition which must be fulfilled for decomposition, one would expect the quantum efficiency to be small. I n the latter case if the molecule decomposes in a single act, the quantum efficiency should remain two as at high pressures. The results are markedly different and it should not be difficult to distinguish between these two alternatives. Accordingly, the present paper contains an account of these experiments in full.
+
+
Part II. Spectroscopic Since the writer purposed studying this reaction, spectroscopic data1*of considerable importance have been published on which the results of these experiments have direct bearing. Franck" pointed out that molecules which are bound together by van der Waals' forces such as the halogens can be separated into atoms by an adiabatic process and that illumination with radiation of the proper wave-length may cause dissociation into normal and excited atoms in a single act. The considerations due to Franck leading to this conclusion will be given in brief. Whether a photochemical dissociation is possible in an elementary act is dependent on the magnitude of the changes in oscillation and rotation energies which are coupled with changes in the electron system on absorption of light. Dissociation can take place when this change equals the energy of dissociation for the state considered. I n the absorption spectrum a band convergence limit appears whenever the binding energy of the unexcited molecule in the lowest oscillation quantum state is changed by a suitable amount by transition of the electron system to a new quantum state due to absorption of light Since the electron jump occurs so quickly that the heavy atoms maintain their relative position t o each other during the transition period, potential energy is conveyed to the nucleus due to the electron jump. This
PHOTOCHEhfICAL DECOMPOSITION O F HYDROGEN IODIDE
273
potential energy is dependent on the change in binding. With strong changes in binding, the potential energy is greater than the work of dissociation for the excited state and the molecule dissociates. By exchange of potential to kinetic energy the molecules break up with kinetic energy and instead of a band spectrum a continuous spectrum appears whose maximum is often displaced away from the convergence limit toward shorter wave-lengths. With iodine' the change in binding is so great that the maximum absorption is found in the neighborhood of the convergence limit whereas with chlorine' it is displaced considerably in the continuum toward shorter wave-lengths. I t is concluded that the absorption of light of wave-length equal to or shorter than this limit (A = 4995.4 for I*; X = 4 7 8 j h for C1,) will lead to dissociation of non-polar molecules in an elementary act. The earlier views of Franck have been amplified and clarified re~ent1y.l~ F. Hundz" has shown that the processes involved in the transition of an electron from the system of the anion to that of the cation are indicated from the principles of quantum mechanics. The process of decomposition in an elementary act by absorption of radiation can now be extended to polar molecules. Kondratjew?' has presented what seems to be conclusive experimental evidence that sodium-iodide may so decompose into a normal I atom and an excited K a atom.22 He finds that at IO-^ m.m. Hg the sodium D lines are emitted in a sharply defined beam whose dimensions are given by the boundary of the exciting cone of light. If a collision were necessary to disrupt the NaI molecule the excited molecule could have moved several centimeters (mean free path ca. 5 cms.) outside the zone before suffering a collision and the fluorescence would have been located far removed from the cone of exciting radiation. Furthermore, Kondratjew found that the fluorescence is proportional to the first power of the pressurez3 which seems unequivocally to signify that S a 1 decomposes in an elementary act by absorption of radiation. Tereninz4showed that all wave-lengths below X = 2 5 0 0 give rise to the emission of the sodium D lines in XaI vapor indicating that S a 1 should absorb continuously in this region, which Franck, Kuhn and Rollefson have shown to be the case.'Q It would appear further from the v-ork of Franck, Kuhn and Rollefson and Franck and Kuhnz5that although HI, possessing infra-red rotation-oscillation absorption spectra and a small dipole electric moment, is considered polar in character, its atoms are not bound by the same type of forces as exist in the alkali-halides. The latter are characterized as "ion-linkages" where the forces exist between ions and the H I molecule as an "atom-linkage" where the forces exist between the nuclei, the criterion being the constituent parts into which the molecule decomposes. The view that the halogen-hydrides possess a different type of binding than the alkali-halides is strengthened not only by the criterion set up by these authors for their distinction but also by the work of BelLZ6 From a determination of the constants of the equation of the infra-red absorption bands of the halogen-hydrides, Bell was able to evaluate the variation with distance of the force acting on the hydrogen necleus as it vibrates along the line joining the two nuclei. The results show that the
274
BERNARD LEWIS
hydrogen nucleus is within the halogen electron shell and is buried to the same depth in all. If this is the case, then the union of a hydrogen atom with an iodine atom consists of a transfer of the hydrogen electron to complete the iodine shell and the penetration of the hydrogen necleus within the iodine electron shell 27 One would hardly expect the binding force to be ionic in character but rather as one between the nuclei. I n fact it has been shownz8 that the lattice structure of HC1 is atomic and not ionic. Indeed from these facts as well as both the character of the continuous absorption spectrum and its mode of decomposition one might consider HI as more nearly approximating to a non-polar than to a polar molecule despite its disphy of infra-red spectra and a small dipole electric moment. Both Tingey and GerkeI8 and Bonhoeffer and Steiner18 believe the continuous absorption of hydrogen-iodide to be sufficient evidence to eliminate Stern and T’olmer’s mechanism and to be consistent with Warburg’s mechanism. However, direct experimental proof that a primary decomposition can occur without a collision is desirable, especially since the character of the absorption spectrum of H I is different from that of non-polar and strictly polar molecules such as NaI, no band convergence nor maxima in the cont inuum being observed. Determznation of the Expenmental Pressure of H I to be employed zn the present mvestzgatzon and the probabzlity of colliszon of a n actzve molecule Assuming in agreement with known data, that the mean life of a molecule in its electronic excited state is of the order IO-: sec., one may calculate the gas pressure at which on the average a collision will not occur during this time interval. From the kinetic theory the number of collisons per second between an active H I molecule and any other HI moleculezQis given by Z = 2u2nldzrkTz / m (1) where nl is the number of molecules of H I per C.C. m is the mass of an H I molecule. T f , S f absolute temperature. u ” ” sum of the radii of the participating molecules or the mean distance of approach of their centers. k is the Boltzmann constant.
nl = p X/RT 1333.3 where p is the partial pressure of HI in m.m. Hg. ?i is the no. of molcules per gram mol. From this it follows that
z
=
u2p 5333.2
(2)
’allj
kTnl
where k = R , S The expression T = 1 1 = r/Z, where 1 is the mean free path and u the root mean square velocity, gives the mean time between collisions or thc duration of a mean free path.
PHOTOCHEMICAL DECOMPOSITIOS OF HYDROGEN IODIDE
I/Z where A = 5333.2
T =
T = i/u2pA4
(4)
ysf I
u* X
=
275
IO-'^^
x 3.21
X
-_
-
31.2 X
1021
IO-’
02P
sec.
As first approximation we will allow T = 7 = ~ o - ~ s e the c . mean life of the active state. t h e n p = 31.2/u2 (6) where u is in a%gstrom units. The value zAfor the diameter of theHImolecule given byW,C.Mchwis3a appears to be too small. From considerations above, namely that the hydrogen nucleus is buried within the iodine shell, the diameter of the normal H I molecule should be comparable with that of an I- ion which, having the stable configuration of Xe should approximate the diameter of the latter. A comparison of the diameters may be made by calculating the diameter of HI from van der Waals’ b. Unfortunately only the critical data for HC1 and HBr are available, but by plotting b calculated from the critical data in the Landolt-Bornstein tables, against atomic number we arrive at a valuoe b = 2 162 X IO-^ for HI, whence u = 3.37A. This compare: well with 3.42A for Xe. I n a similar way the valuesfor HBr and Kr are 3.27Aand3.14Arespectively.Since we are concerned with a collision between an excited and a normal molecule of HI some knowledge of the excited state is desirable. Cario3I found in the case of Hg, that the diameter increased from 2 . d for the normal state to 9.1A for the excited state. Assuming in the extreme that the diameter of the i substituting in (6) we obtain as a excited HI molecule increased t o ~ o f and lower pressure limit 31.2
= (5
= 0.693
-t 1 . 6 9 ) ~
m.m. Hg.
I n order to be well within the critical pressure it was decided t o work with pressures of H I of about 0.1 m.m. Hg. At pressures lower than this it becomes very difficult to measure the absorbed radiation. Since T is merely the time between collisions it is not strictly true that a collision will never occur within IO-^ sec. after absorption of radiation. One has to calculate the probability that a molecule will collide while still in the excited state. A method for this calculation has been worked out by L. A. Turne13z and is applied to the present case. If T is the average life of the excited state, then assuming random distribution of lives of excited molecules, ,-t 7 is the chance that a molecule will remain excited in time t. If T is the average time between successive collisions of an excited H I molecule and ordinary H I molecules,
276 ,-t
BERNARD LEWIS T
. the chance that a molecule will not collide in time t is
+
. IS the chance that a molecule will not collide in time t dt The probability that the molecule will collide in the time between t and t d t is the difference between the last two probabilities. Expanding and keeping only the term with d t to the first power, gives: e-t (e-dt - 1) = e - t ’ T d t / T ,-t+dt/T
+
The probability that the molecule will collide in the time between t and dt while still in the excited state is the product of the probabilities for t collision and duration of the excited state. ,-t/Te-t,T d t / T = e -(T+r)t T r dt/T
+
The probability that a molecule will collide while in the excited state for all values of t is
=L m
P
(T+r)t
e - 7 dt/T =
- 7/(T+7)
[e
W
-(TfT)t
=
7/(T+
7)
Thus the probabilityof a collision while the excited state exists is the ratio of the life of excited state and the sum of the time between collisions and the life of the excited state. We may now determine what percentage of the active molecules collide while still excited. Employing equation ( 5 ) and the experimental pressure of 0.1 mm. Hg. of H I , 31.2 X IO-^ ; T = 6.93 X 10-’sec. T= (6.69)* X 0.1 IO-’
p
=
-_-__
= 12.65 (6.93 I ) IO-^ Thus choosing the most unfavorable conditions only 12.670 of the excited molecules collide while still excited while assuming no change in the diameter this reduces to only 3 . ~ 7 ~ .
+
Experimental The method consists in illuminating H I gas at about 0.1 m.m. Hg pressure, measuring the radiation absorbed and the extent of decomposition by freezing out the I? and H I and determining the residual HP. Only the early stage of decomposition was studied to avoid secondary absorption by the iodine liberated. a. Apparatus. The experimental arrangement is shown in Fig. I . I,, the source of radiation, consists of a condensed zinc spark, equipped with a micrometer for fine adjustment, connected in parallel with a large plate glass condenser immersed in transformer oil. This in turn was in parallel with the secondary of a large transformer coil whose primary took 65 amps, a t I IOV. A. C. The latter stepped up to about 40,ooov. The electrodes were vertical and were cooled by a blast of nitrogen to prevent oxidation of the Zn and consequent absorption of the ultra-violet bands by the fog of ZnO. I n this way a steady spark was maintained.
PHOTOCHEMICAL DECOMPOSITIOS O F HYDROGEN IODIDE
277
The optical system 0, consisting of a large quartz prism and suitabl: lenses, served to separate the two strong Zn ultra-violet bands at X = 2080A and X = 2530A'~ which were also employed by Rarburg. The parallel beam w m arranged to just illuminate the entire rear window of the quartz cell. By means of an opening in a zinc sulphide fluorescent screen S, on which the lines were sharply and brilliantly defined, either of the bands could be utilized at will.
FIG.I
The clear quartz reaction cell, C, 30.1 crns. long and 2.; cm. in diameter was provided with optically plane and parallel ends and two quartz-to-Pyrex graded joints, connected on the one side to a liquid-air trap, F, and a magnetic hammer device, A, (shown enlarged) for fracturing a small capillary, G, filled with HI, and on the other to a bifilar quartz fibre manometer, 11, (shown enlarged) for measuring small pressures of Hz, a carefully calibrated McLeod gauge and mercury vapor high vacuum pumps. A cadmium-liquidair trap was used to protect the apparatus, mercury pumps and McLeod. The two quartz fibres in the manometer were sealed together at one end to a small glass indicatoP4 to prevent elliptical motion as suggested by C0olidge.3~The radiation was measured with a sensitive galvanometer protected from stray magnetic fields, and a 31011 thermopile, T, equipped with a fluorite window and receiving funnel 2.60 crns. in diameter. b. Calibration of Moll Thermopile. The thermopile was calibrated against a standard radiation lamp furnished by the Bureau of Standards. Accompanying directions kindly provided by Dr. Coblentz were rigidly adhered to. The table accompanying the lamp is reproduced in Table I.
BERNARD LEWIS
TABLE I Amps
Volts
0.250
79.9 94.8 109.8 125.0
0.300 0.350 0.400
Radiation intensity \Vat t 2 Distance two meters from lamp 4 2 . 6 X IO-^ watts 6 2 . 7 X IO-^ ” 8 6 . 7 X IO-^ ” 1 1 4 . 4 x 10-8 ”
/=
Table I1 shows th calibrat,ion results, t h second column giving the total radiation received by the thermopile (the area of the funnel being 5 3 0 . 7 ~ ’ ) . Experiments indicated that all the radiation entering the funnel reached the thermo-couple. The fourth column allows for absorption of a fluorite window indicated in the directions.
TABLE I1 Amps
Total Radiation Watt/sec.
Deflection of Galvo. in m.m.
2 2 . 6 1 X IO-^ 3 3 . 2 7 X IO+ 4 6 . 0 1 X IO-^
0.250
0.300 0,350
300.0 443.1 618.3
Flux/m.m. deflection allowing 91.6% fluorite transmission; watt. /sec. /m .m. 6 . 9 1 X IO-’ 6 . 8 8 X.IO-’ 6 . 8 2 X IO-?
Average vafue of Flux; ergs/ sec./m.m.
6.87
Calnilation of number of Quanta receiued by the thermopile per second per m. m. deflection.
I. ForX = no8oh. E = hv = hc/X where h is Planck’s constant c i s velocity of light in cm/sec. A is wave-length in cm. 3 X 1olo X 6.55 X IO-^^
E = -_____-
= 9.45
2080 X
x
10-l~
erg (valueof onequantum)
IO-~
6.87
= 9.48
11. For A
x
7.27
x
quanta/sec./m.m.
1 0 ~ ~
10-12
= 2530A.
3 X ioLo X 6.55 X IO-^' _____--
E=---
2530
x
-
7.77 X ro-’*erg.
IO-’
6.87
‘7.77
= 8.84 X IO^'
x
10-12
quanta/sec./m.m.
PHOTOCHEMICAL DECOMPOSITION O F HYDROGES IODIDE
2i9
c. Calzbration of the Quartz-fibre hlanometer. Since hydrogen was to be measured, the manometer was calibrated at low pressures against the hIcLeod using pure dry hydrogen, the time of decay of the amplitude of vibration of the indicator between the glass points p3 and p~ being noted for different pressures. Care was exercised to exclude all Hg vapor. The image of p3 and pl was observed with a traveling micro-telescope with cross-hair. Care must be taken to preserve the distance between the position of the swinging fibres when a t rest and the points p3 and p2. By plotting the pressure of HZ against the reciprocal of the decay time practically a straight line was obtained at low pressures. All conditions were maintained the same as existed in the actual experiments.
d . Preparatzon of capzllarzes containzng H I gas. The volume of the reaction system which included the quartz cell, manometer, liquid-air trap and magnetic hammer, was 232.j C.C. X series of small capillaries about I m.m. in diameter and 4 to 6 cms. long were constricted at the proper length such that their volumes were approximately 1/76oo of the reaction system. Such a capillary filled with H I at standard conditions would when fractured give a pressure of about 0.1 m.m. HI. The exact pressure could be determinedbycalibration of the capillary. H I was drawn from a specially prepared strong solution for the use of which I wish to thank Professor S. C. Lind. The HI gas was frozen out in a liquid air trap, water vapour being removed by a CaCIP PPOstube and most of the 1 2 by a freezing bath. All of the IPwas removed by 3 or 4 low temperature distillations. Further purification was effected just before filling the capillaries by 3 low temperature distillations under high vacuum. The capillaries were thoroughly evacuated and H I allowed to expand into them. The pressure was relieved to that of the atmosphere by gently hesting the end of a small piece of constricted glass and then quickly sealing. Each capillary was removed in turn a t the fine restrictions with a minute jet flame. I n this way a very pure HI gas was obtained. e. Ezperimental procedure. The experimental procedure was as follows : After the reaction system was thoroughly and completely evacuated, the tip of a capillary was broken and the H I frozen out in the liquid air trap. The pumps were again connected to remove the slightest trace of Hzthat may have been present in sealing off the capillary. The amplitude decay time for high vacuum and the galvanometer reading for incident radiation at a given setting of the electrodes were taken before and after each run and were found to check well. The H I was allowed to expand into the reaction system and exposed to the desired ultraviolet band, galvanometer readings being taken every I O seconds, and after a given time of irradiation it was frozen out together with the I1 formed. The decay time for the residual H P was noted and the pressure read from the calibration chart. From this was calculated the number of molecules of H I decomposed. The energy absorbed was the difference between the galvanometer readings for incident radiation and the average reading while the decomposition was in progress. The various parts
2 80
BERNARD LEWIS
of the apparatus and light source were carefully shielded and the reaction system covered with black paper. The experiments were conducted in a dark room free from draughts, the temperature being about 27OC.
Results Before presenting the results certain corrections must be made. The absorption of radiation of one quartz plate must be determined. This was found to :mount to 1 5 . 2 % for the line A = 2080.1 and 8.85% for the line X = 2530A. Therefore the total absorption of the lines must be multiplied by the factors I .18and 1.097 respectively. h correction for difference in areas between that of the cell window and the thermopile necessitates a further multiplication of the absorption measurements by 1.08 (The area of the window was 573 m.m.z as against j30.7 for the thermopile). Table I11 presents the results of a series of runs carried out with each line. A. sample calculation from data in experiment 2 will illustrate the method. Absorption in m.m. 7.83 Time of exposure in seconds 516 3/5 No. of quanta absorbed/sec./m.m. (A = 2080) 7 . 2 7 x 10" Correction for quartz window absorption 1.18 I1 I' area of cell 1.08 Total quanta absorbed = 7 . 2 7 X IO^* X 516 3/5 X 7.83 X 1.18 X 1.08 = 3.76 X 1015 0,OOOi Pressure of HZin m.m. Hg Volume of cell 232. 5 C.C.
Total molecules of HI decomposed =
760 10.5 x 3.76 X
x-x-273 300.2
1 0 ~ ~
= 2.79
232'5 22400
X 6.06 X 1oZ3
moleculeslhv
1015
The difficulty of measuring the absorption of radiation increases greatly at low pressures. It is therefore desirable to determine whether bhe measurements are of the correct order of magnitude. This can be done by calculating the coefficient of absorption from the above data using Beer's law and comparing it with the value found by Warburg a t higher pressures. The average value for p, the absorption c?efficient, was found to be 0 . 0 4 2 7 and 0 . 0 2 5 1 for X = 2080A and A = z53oA respectively whereas Warburg found 0 . 0 2 7 0 and 0.0139. Warburg's values varied for different set-ups of the apparatus. However the coefficients are of the right order of magnitude and the agreement satisfactory.
PHOTOCHEMICAL DECOMPOSITIOS O F HYDROGES IODIDE
0:
N
.
Be
3
i
.
v i v i 0
.
.
.
N
h N
t - h W W N
0
N N
N
mm mgi
. . . .
N
.
h N
gi
ha r n c n h h 0 ~ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0
h
h
h
h
N
N
N
N
g i N
gi
0
wcnw 0 0 0 0
0 0 0 0
0 0 0 0
h
0 0 0 0
m m m m
-2m
:: II
x
o a m m
222 2 0
0
0
0
h m m cn w w w \ o m cno C
-
l
281
BERSARD LEWIS
282
Discussion of Results I t is observed that the number of molecules decomposed for each quantum absorbed is about two as Warburg also found at higher pressures (80 m.m. t o 350 m.m.). This indicates that an activated hydrogen-iodide molecule does not require a collision to suit its condition for decomposition. A question of interest is the fate of the surplus energy namely the difference between the absorbed 137,000 calories (X = 2o8oL&)and 68,000 calories the heat of decomposition of H I from thermochemical data, or 69,000 calories. The first electron level for the hydrogen atom is 10.2 volts or 236,000 calories. Thus excitation of the hydrogen atom is out of the question. From theoretical considerations and spectroscopic data Franck and Kuhna5have pointed out that H I like AgI is a non-ion linkage, the first stage in its dissociation as shown by the long wave-length limit of its continuous absorption spectrum, taking place not into two normal atoms but into a normal and excited atom. ?io maxima were observed in the continuum down to X = 2000A and thus no transition occurs in the mode of decomposition and therefore in the type of binding. From the long wave-length limit of the continuous absorption band, about X = ~ Z O O Awe, obtain 89,000 calories. This value, less the heat of dissociation of H I gives 21,ooo calories which is just the energy required to excite the iodine atom from the stable 2Pa state to the metastable zP1 state. It is noteworthy that absorption does not begin at a point where the energy is just equal to the heat of dissociation of H I ; therefore the latter cannot dissociate into two normal atoms. Similarly in the case of 1 2 the energy corresponding to the band convergence limit where the molecule begins to dis. 21,000 calories, sociate, exceeds the dissociation energy by about 0 . 9 ~ or which represents dissociation into a normal and an excited atom in the 2PI state. There remains 48,000 calories to be accounted for. An attempt was made to observe a visible fluorescence in the iodine liberated but without success. That the iodine atom is not excited further than that represented by the ZP,state is evident from the present results for otherwise a quantum efficiency of 4 would be expected particularly at low pressures where complicating secondary effects prevalent at higher pressures, are absent. R y a collision of the second kind the endothermic reaction (about -28,000 calories) I(aotive)
could proceed and then H
+ HI(ordinary) = H +
+ I-II = H P +
1 2
I(ordinary)
The quantum efficiency at low pressures being slightly in excess of z may be explained on the basis of the occasional occurrence of the above process with an iodine atom in the 2P1 state possessing an exceptional velocity dictated by Maxwellian distribution. It must be concluded that the 48,000 calories are dissipated as kinetic energy.36 In the division of this energy between the hydrogen and iodine atoms, nearly all of it is assumed by the hydrogen on the principle of conservation of momentum. This accounts for the fact that
PHOTOCHEMICAL DECOMPOSITION OF HYDROGEN IODIDE
283
every impact between H and H I (a reaction which is ordinarily exothermic but which nevertheless possesses a critical energy increment for reaction by collision) results in the decomposition of the latter. It, is of interest to note that since collisions are unnecessary for the dissociation of HI, the time bet’ween absorption of radiation and decomposition may be less than 2 x 1 0 - l ~sec., the average time between collisions as cal, culated from Warburg’s highest pressure. From the spectroscopic side it may be concluded that the same interpretation of the continuous spectrum exhibited by hydrogen-iodide may be adopted as was proposed for non-polar molecules; that gaseous hydrogeniodide dissociates in a single and elementary act after absorption of radiation into a normal hydrogen atom and an excited iodine atom.
Summnrv The photochemical dissociation of hydrogen-iodide was studied at low gas pressure where the collision frequency is comparable with the mean life of the excited state. Quantum efficiency of the process was found to be about z which agrees with Warburg’s value for high gas pressures. This indicates that hydrogen-iodide dissociates in an elementary act as a result of absorption of radiation without the necessity of a collision. From the continuous absorption spectrum it is shown that dissociation takes place into a normal hydrogen atom and an excited iodine atom in the metastable 2Plstate, the excess energy if any, being dissipated as kinetic energy. It is pointed out that, the time between absorption and dissociation is shorter than 2 X 10-l~sec. It is a pleasure to acknowledge the kind interest shown by Professor S. C. Lind during the progress of this work.
References Preliminary noticesin Nature, 119,493 (1927) and Roc. Nat. h a d . Sci., 13,720 (1927). National Research Fellow in Chemistry. Sits. Akad. Wiss. Berlin 1916, 314; 1918, 300. This mechanism virtuall; includes the intermediate formation of an active H I molecule. instantanH X hv = H X (active) + H X eously H. Kuhn (Z.Physik, 39,77 (1926))concludes from an analysis of the band and continuous absorption spectrum of the halogens that excitation of the molecule does not enter into the question but rather an immediate dissociation into a normal and excited atom occurs in an elementary act without a collision. 5 Z . wiss. Phot., 19, z (1920). OHenglein: Z. anorg. &%em.,123, 137 (1922). 7 Kuhn,: Z. Physik, 39, 77 (1926). P. Pringsheim: “Fluorescene und Phosphorescene” (1923 For literature see Franck: Physik. Z., (1918),or Footeandkfohler : “Origin of Spectra.” l o 0. Oldenberg: Z. Physik. 18, I (1923). l1 J. Franck: Trans. Faraday Soc., 21, 536 (1926). l2 [HXIb H X = H X HXetc. Anothermechanismsuggestsitself namely theclustering of molecules. However, absor tion by a double molecule to bring about decomposition into a halogen molecule and a Eydrogen molecule cannot exist since temperature or pressure has no effect on the absorption. (Coehn and Stuckardt: Z. physik. Chem., 91, 7 2 2 (1916); Warburg: IC.; andTingeyandGerke: J. Am. Chem. Soc.48, 1838 (1926)). l 3 R. C. Tolman: Phys. Rev., 23, 6 3 (1924). M. Bodenstein: Trans. Faraday so,., 21, 525 (1926). l5 hl. Bodenstein: 2. physik. Chem., 22, 23 (1897). I S Z . wiss. Phot., 24, 177 (1926). 1 2
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1' Since we are dealin with an activation consisting primarily of a chan e in electronic quantum state, the life ofthe active state for emission is of the same genera? order of ma nitude as the mean life of other systems involving roughly similar electron jumps, name& about IO -7 mc. (For literature see Franck and Jordan: "Anregung von Quantumsprtingen Stiisse." durch . . ~ ~ .~~~ . ~(1026). ~ 18 Tin ey'and Gerke: J. Am. Chem. SOC.,48, 1838 (1926). Bon%oeffer and Steiner: Z. Physik. Chem., 122, 287 (1926). 'OFranck, Kuhn and Rollefson: Z. Physik, 43, 155 ( I 27); Birge and Sponer: Phys. Rev., 28, 259 (1926);Leifson: Astrophys. J., 63,73 (1926); E.Witmer: Proc. Nat. Acad. Sci., 12, 238 (1926). zo F. Hund: Z.Ph 'sik, 44,0742(1927). 21 V. Kondratjew: Physlk, 39, 191 (1926). n The possibilit of a dissociation into these component parts is borne out by the appearance of a third maximum in the continuous absorption of XaI (Franck, Kuhn and Rollefson: loc. cit). 98 Both the absorption of radiation and the number of collisions are proportional t o the pressure. 14A. Terenin: Z. Physik, 37, ,98 (1926). z5 Franck and Kuhn: Z. Physlk, 43, 164(1927). Bell: Phil. Mag., 47, 549 (1924). 2' It may be sigmficant that the electron affinity of I - calcuhted from Born's method of grating energy of crystale as 81,000calories (Foote and Mohler: "Origin of Spectra") is quite close to that evaluated from the lon wavelength limit of the continuous absorption of HI. It i s therefore possible that .HI wif only dieaociate when suEc,ient energy has been absorbed to cause I - to part m t h its extra electron. I n the final adjutsment, the energy over and above the thermochemical value goes t o excite the iodine atom as will appear later. 2 8 F. Simon and C. Simon: 2.Physik, 21, I68 (1924). 2 9 The concentration of active H I molecules at any time is negligibly small compared with the total concentration. Therefore impacts between active molecules may be neglected. 80 J. Chem. Soc., 113, 473 (1918). 3' Z.Physik, 10, 188 (1922). 82Phys. Rev. 23, 464 (1924).I am indebted to Dr. George Glockler for bringing this work to my notice. 8s Weighted mean-Eder: Z. wiss. Phot., 13, 38 (1913); Pfliiger: Ann. Physik, 13, 904 (1904). 34 The fibres w e easily joined to lass by inserting them in a capillary opening in the glass indicator and heating the latterqocally until the walls just touch. J. Am. Chem. Soc., 45, 1637 (1923). 86 This conclusion is in agreement with the theory of Franck. A rivate communication from T. R. Hogneas working in Gottingen, Germany, states that wfen gaseous NaI molecules are excited b a frequency reater than that necessary for dissociation, the excess energy is diesipated)in the form o f kinetic energy of the dissociating atoms. The greater the excess e n e r p the greater the kinetic energy o the dmociating atoms, hence the greater the Doppler e ect of the D lines emitted. ~
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School nf Chemistry,
University of Minnesota, September 1, 1987.
