COSTROLLED-ELECTRON REACTIONS
451
C( )STROLLED-ELECTROS REA4CTIOSS' GEORGE GLOCKLER Depnrtmer,f oj'Chc~irisfiI/ arid Chemical E n g i n e e r i n g , State I-nzversLty of Iowa,Iowa Czty, Ioicw
Recezi ed October 25, 1947
-%ctirationof cheniical reactions can he brought about by a variety of external agentb such as catalyst$, invrea-e in temperature of the reacting system, and the presence of various types of iadiation. Radiation n-hich can initiate reaction in a chemically quiescent sy3tem 1: of tivo kinds: (1) light of varying frequency, such as visible or ultraviolet light, x-ray?, and gamma. rays on the one hand; and ( 2 ) particle rays, such a s alpha and beta particles, atoms and radicals, recoil atoms and ions on the other. 'The ultimate agent Tyhich causes chemical reaction to ensue is in many caSes the electron. This particle, if given sufficient speed, can force another electron of a neutral molecule to a higher orbit or even remove it, Le., ionize the molecule. It can produce dissociation accompanying these actions and hence create a nex energy-rich species in a system n-hich then can undergo further chemical change. If an alpha ray or an x-ray passes through a gas there are produced excited molecules, ions, and electrons. These secondary electrons vi11 have varying speed, and they in turn n-ill produce energy-rich molecules, atoms, radicals, and ions. In any form of electrical discharge similar action ivill take place, electrons having been produced in the electric discharge. In these electrical devices electrons of varying speed are created and the n-hole reaction picture is one of great complexity. To arrive at an understanding of the reaction mechanism n-here high-energy particles are the initiators of the reaction sequence, it is of great interest to knonjust what minimum of energy or speed an electron must possess in order to produce a certain kind of activation. Hence it is of interest to study chemical reactions in simple systems and determine the minimum energy of electrons starting the reaction. Furthermore, it is also important t o find out how the reaction varies in amount as the initiating electrons gain greater speed. It would be expected that a molecule would be brought into a reactive state only when the Impacting electron has reached sufficient speed t o lift the molecule from its ground state to some higher energy level in accordance with its energy-level diagram, obtained, for example, from the spectroscopy of the molecule. Were it true that ions of the molecules n-ere the only activating centers for chemical reaction, then it might be expected that reaction should begin only at the ionization potential of the molecule, for the impinging electron would have t o have kinetic energy equal t o the ionization potential of the molecule before ion? could be produced in the system. If ions Jvere the only reactive agents, then resonated states or excited levels of the molecule would not serre as reaction initiators. This proposal is obviously not true, because it is well known that photochemical action is OS1 Presented a t t h e Symposium on Radiation Chemistry and Photochemistry which held at the L-niversity of S o t r e Dame, S o t r e D a m r , Indiana, June 24-27, 1947.
\\
as
452
GEORGE GLOCIiLER
sible in many cases n-here the ahorbed quantum of light doe3 not have sufficient energy t o ionize the molecule. Hence lower energy levels than ionization can lead to reactive statey and it maybe expected that electrons nith energy less than the ionization potential of the molecule can produce active centers in a reaction syst em. The study of chemical reactions started Tvith electrons of kn0n.n speed or “controlled electrons” n-ill give information concerning the resonance states of molecules vhich can start these changes. It uill be ceen that the simple reactions studied so far can be completely understood on the basis of the energy states of the molecules as given by quantum theory and band spectroscopy. The simplest way of obtaining electron streams of knovn speed is t o use the electrons from a hot filament, ae. in a radio tube, aqd to accelerate them by means of an electric field obtained from a battery. I n this ~ a electrons y of any speed may be produced. The potential of the battery is applied hetween a filament and I: grid. This region i, made of .mall dimension (1es.i than 1 mm.) so that very fen- collisions happen b e k e e n filament and grid or hefore the electrons have obtained their full speed corrqonding to the battery potential. Some electrons n-ill not be caught by the grid \vires but n-ill p a v through the meshes of the grid into the region between the grid and plate. IIere they will make impacts with gas molecules, haying attained full speed. The region hetiveen grid and plate is usually field-free and the electron speed is therefore not further altered. Whenever the potential of the battery is changed and reaches qiich a value thst the electrons attain B critical potential of the molecules under investigition, the latter can be placed into an excited or higher energy level. D:lpending on the reaction system, arrangements can be made t o shox that chemiczl reaction hns taken place. The most convenient method is usually to follov- prcasure changes of the reaction system. In this manner the follo~vingreactions have been >rudietl (1): 1. The dissociation of hydrogen into atoms. 2. The dissociation of hydrogen into atoms in the presence of mercury. 3. The dissociation of osygen. 4. The dissociation of nitrogen 5 . The synthesis of ammonia. 6. The reaction behveen nitrogen and oxygen. 7. The decomposition of sodium azide. 8. The reaction between carbon nionoxidr m d hydrogen. 9. The decomposition of sulfur dioxide. The simplest type of chemical reaction that has Iieen studied with electrons of cnontrolled speed is the dissociation of diatomic molecules as, for example, the diimciation of hydrogen molecules into tn-o atoms (3). It might be expected that electrons \\-it11 4.3-1 e.r. of energy could dissociate hydrogen molecules, because this aniount of energy is equivalent t o the heat of dissociation. However, electrons of this $peed cannot transfer this amount of energy to the hydrogen molecule, since thew exists no electronic energy level in the molecule of this
COSTROLLED-ELECTROS REICTIONS
453
amount of energy. -it least the probability of such energy transfer is extremely Ion- and this action has not yet been observed. It x a s found esperimentally that electrons v i t h about 11.0 e.v. can transfer their energy to hydrogen inolecules iind place them into the antisymmetric )":( state:
E- (11.0 e.v.1
+ Hz
+ E- (slow)
(I$)
+ 13%(3Z:)
] % u t the hydrogen molecules in the antisymmetric state will dissociate into t w o :\toms \\-it11kinetic energy:
h i e t i c energy,
I& (33J) 42H
'the r e d t i r i g atoms \\'ere detected hy their reaction \villi a copper oxide surface (kept at room temperature) : 2H
+ CUO -+
CU
+ HzO
The resulting \\-ater vapor \\'as frozen out in a liyuid-air trap and later identified by it4 vapor pressure. In the same reaction vessel :L critical potential of hydrogen was found by the Fmnck-Hertz method (2) at 13.4 e.\-. Hence the demonstration is complete in thi. ca.;e that electrons must bring hydrogen molecule.; into a quantum-mechanical energy level before they i d 1 react chemically. It is then supposed that similar actions n-ill take place in hydrogen gas, whether electron.; are produced b y alpha particles, x-rays, or any other energetic radiation. To this extent then do these experiments v-ith slo\\-electrons also indicate the possible reaction mechanism in electric diqcharges. Electrons of greater speed than 11.0 e.v. will carry on this process n-ith vurying probability, and of course other quantum jump" Tvill become possible as the electrons are given energies greater than 11.0 e.v. h variation of the esperiment (7) on hydrogen dissociation just described involved the admixture of mercury vapor n-ith hydrogen gas. It may then be expected that mercury atoms can be brought into the resonated state (G 3P1) and that they may then transfer their energy (4.9 e.\-.) t o the hydrogen molecules b y :in impact of the hecond kind, cawing their diisociation for ivhich the excitation energy of the mercury atoms is sufficient (4.9 e.v. compared with 4.34 e.v.). Hon-ever, the hrst disappearance of hydrogen molecules \vas noted when the impinging electrons had about 7.7 e.v. of energy. a quantity n-hich is sufficient t o iixnifei~a mercury atom to the 7 ?SI state:
E- (7.7 e.\-.)
+ H:
( G 'SO)
---f
+
H;: (7 jS1) E- (0 e.v.)
It if likely that the excited mercury atonis drop t o the metastable 6 3 P 0 2.;tates \Tit11 e m i 4 o n of radiation:
+
H? (7 'SI) ---f 13: (6 JPp,,,) 1.5461 UL' 4047 -\.) The metastable P *tates have a much longer life and hence much greater opportunity t o collide n-ith hydrogen molecules. Furthermore, this energy is much rnearer to the heat of diqsocintion of hydrogen moleculej, enhancing the possi-
454
GEORGE GLOCKLER
bility of energy transfer, since it is knovn that impacts of the second kind are more likely if the energy t o be dissipated a 5 kinetic energy is a minimum: Hg (6 3P,,,zl
+ H,
(l3;)
-+ 2H (atom?)
+ Hg (6 lSI)
The hydrogen atoms so produced can then react vith the copper oxide surface present in the experimental tube. The ohjerved prescure decrease resultq from the freezing out of the vater formed: 2H (atoms)
+ CuO (solid)
-+
HsO (liquid)
+ Cu (solid)
The mercury-sensitized reaction did not occur at 4.9 e.v. as expected, hut at 7.7 e.v. This fact may be connected vith the relative resonance probability of mercury atoms. Sitrogen molecules are dissociated by electrons of 17.8 volts of energy (12). Again it is seen that electrons cannot transfer the dissociation energy (about 7 or 9 e.v.) to nitrogen molecules with sufficient frequency t o be detected in the experimental set-up used at the time. Lidthe higher voltage mole-ions may be produced, bince the ionization potential of nitrogen molecules is 15.7 e.v. These mole-ions may be the species which is frozen out or adsorbed on the mall5 of the reaction vessel:
E- (17.8 e.v.)
+ S?(lX:)
-+
S$ (22;)
+ E-
(0 e.v.)
The energy of 17.7 e.v. is not high enough t o produce nitrogen atoms and atomic ions (11). Hon-eyer, the act of neutralization of the mole-ion (St) on the plate of the experimental tube mag furnish the energy of dissociation. I n the case of oxygen gas it \vas found that 8.0-volt electrons can cause effective impacts which make oxygen molecules adsorbable on glass and mercury surfaces (8, 9),
E- (8.2 e.\-.)
+ O2 (321)
-+
0 (")
+ 0 (ID)+ G (0 e.v.)
but some slight reaction is also noted even with electrons of lesser energy (3-5 e.v.1, E- (1.62 e.v.) O2 (32;) -+ O2 ('2;) E- (0 e.v.1 or E- (5.1 e.\-.) O2 ("J -+ 2 0 "(p) E- (0 e.v.) Ozone does not seem t o form until the impinging electrons have 25-28 e.v. of energy (9, 17). S o particular change in pressure drop is noted at the ionization potential of the oxygen molecule (12.5 e.v.). Ammonia can be synthesized from a hydrogen-nitrogen gas mixture. The product is formed more readily at 17 e.v. after the ionization potential of the nitrogen molecule (15.7 e.\-.) has been reached. The gas mixtures richer in nitrogen give greater yields, indicating that perhaps nitrogen mole-ions are involved in the reaction mechanism (4). Sitrogen and oxygen react after impact with 19.0-volt electrons with a further increase in reaction rate at 23 e.v. (10, 16). The product is nitrogen dioxide.
+ +
+ +
COSTROLLE D-E LE CTROS RE ACTIOSS
455
Thin films of sodium azide deposited on a plate will decompose n-hen bombarded with electrons of linon-n speed. The first reaction happens a t 11.65 e.v. and further reaction rate changes are noticeable at higher impact potentials of electron;. (14). Carbon monoxide is decomposed by controlled-electron impact at 14 and 19 e.v. (1) I t n-ill react v i t h hydrogen at 14.0, 20.0, and 27.0 e.v. The chief reaction product is formaldehyde. The ionization potential of the carbon monoxide molecule is 14.0 e.v. Excited molecular ions appear at 16.52 and 19.63 e.v. Hence the experiments carried out t o the present ~ ~ o u indicate ld an ion mechanism Hon-ever, the experimental arrangements may not have been sensitire enough t o detect reaction at loiver voltages, xliere either excited neutral states or dissociation alone may he the responiihle reaction mechanism. Sulfur dioxide i5 decomposed at 12.2 and l 5 . i e.v. by electron impact. In the fiist case mole-ion- (SO;) and at the higher voltage SOT ions and oxygen atoms are produced (13). One feature of the usual eqerimental tube uhed in investigations of reactions :~ctiratedby controlled electron;. is the presence of the hot filament. I t can cause I eaction by its high tempeiature, independent of the emitted electrons. I n favornble casea such "zero-volt-rates" may be small and can be readily corrected foi In the case of hydrocubons, pyrolysis on the filament would be beriouc In >uch case3 the filament can be placed in another compartment and the electron- can he allon ed t o enter the reaction chamber proper through a small orifice -%ppropriatepumping can be used t o remore the thermal decomposition products due to the filament. -1cetylenc v-as bhon-n to polymerize (6) in such an apparatus hy using 40-volt elect1 on>. A study on the hynthe3is of ammonia ( 5 ) initiated by alkali ions is of interest t o the present discuqsion of controlled-electron reactions. I n this case ions of lithium, -ium were shot into mixtures of nitrogen and hydrogen of varying pioportions. It i m s found that ammonia \vas formed only after these alkali ions had passed through an accelerating field of sufficient magnitude that the respective positive ions could ionize nitrogen molecules. The latter were considered the important agent in the mechanism, becauqe mixtures rich in nitrogen gave better yields. It is seen that poqitive ions of lo^ velocity do not cause ammonia formation. On the cluster theory (13) reaction might be expected independent of ion velocity. SUI\I.\I;IRT
T h + ~evie\vof reaction- activated by electrons of controlled speed seems t o qhon- thnt the initial ncti of the reaction sequence are to be interpreted on the basi. oi the quantum state. of the interacting molecules, as given by their energylevel diagram. Only very fen- yeactions have been studied. The greatest need is the development of more -ensitive detecting agents, so that reaction on-set can be diwovered even in cases where the probability of energy transfer is very lon- :iiicl \\-here the means used in the past have not been delicate enough t o show the heginning of reaction.
456
,J. .k. HIPPLE:
ISTRODCCTIOS
When a molecule is stiuck by an electron having an energy of 10-20 electron volts or higher, it may qimply lose an electron or, in addition, split into various fragments. This fragmentation is a reproducible phenomenon ivhich is :t fundamental characteristic of the particular type of molecule being studied-the relative probability of the formation of R fragment having a particular mass tieing dependent on the temperature of the molecule.. and the energy of the electruns. The mass ipectrometer ie the most poiyerful tool available for the study of some of the primary proceqSeq n-hich are of basic interest to th0ie attempting t o attain a more complete understanding of the mole complicated reactions in the general field of radiation chemiitry In thiS connection. information is required on the appearance potential\ of the various fragment. as well as their relative abundance. 1
1’1csClltetl at t h e 8\ I ~ ~ ~ ) C I > OII I I I II ~l ~ ~ i l l ~ t i(~hciliistl\ i i .ilitl i ’ h ~ t u ~ h t ~ I\ 11it ~ ~11 ~1, s t ~ ~ I n i r r i s ~ l \of \ ( i t ] ( I l n r n c ~ Y O ( I CD
