Atoms, molecules and radiation

QUARTERLY JOURNAL OF THE

ROYAL METEOROLOGICAL SOCIETY

55’.5.53 ATOMS, MOLECULES AND RADIATION

By Prof. H. DINGLE, A.R.C.S., D.Sc. [The G. J. Symons Memorial Lecture delivered on March 17, 19491

My object in this lecture is to stafe the general problem of meteorology a s it appears to a physicist, and then to explain the statement. I shall make no attempt to solve it, nor do I expect to say much, if anything; with which you are not already familiar. The justification for the lecture, I hope, will reside in the resulting unification of knowledge rather than in additions to it, and if you already know 95 per cent of what I shall say but the other 5 per cent succeeds in imposing order on what was previously an unorganised body of information, I shall consider my object achieved. I shall not, in fact, approach the meteorological problem, but recede from it, in order t o see it in better perspective.

A unidirectional stream of heterogeneous radiation impinges on a shell of gas, which rests on a surface partly solid and partly liquid and rotates once a day so that each element of the shell receives the radiation for periods of n hours duration interrupted by intervals of 24-n hours duration. The gas consists of atoms and molecules situated in a gravitational field directed inwards, and the liquid part of the surface consists of aqueous solutions of inorganic salts. Wha t are the consequences?

To understand this problem we must understand what is meant by I ‘ atoms,” by “ molecules,” and by radiation.” Let us begin with atoms.

The problem of meteorology, then, is this.

ATOMS

The present physical idea of an atom is somewhat abstruse, but fortunately it is legitimate for our purpose to substitute an earlier model. This model breaks down when faced by certain details of laboratory observations, but we may use it with perfect safety in meteorology, and its effectiveness in co-ordinating pheno- mena and suggesting how they may be explained is so vastly superior t o that of the more fundamental but relatively barren conception that I think every working physicist uses it habitually a s a means of visualising his problems. It pictures the atom of an element as a kind of solar system, with a nucleus of positive elec- tricity as the Sun and a number of negative electrons as planets, their total charge being equal but opposite to the charge on the nucleus. \Ye need not consider the nucleus a s anything more than a label bearihg the name of the element. If its charge is one unit the element is hydrogen, and one electron circulates round it. If

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186 H. DINGLE

its charge is two units the element is helium, and two electrons circulate round it. We thus *continue up t o the most massive known element, which is uranium, with a nuclear charge of 92 units and gz satellite electrons.

Each of these travels in a circular or elliptical orbit, and the orbits are arranged in groups. In each group the major axis is the same, but the eccentricity and orientation of the orbit in space may vary. The first group of orbits is called the K group-or K“ shell ” to use the common term; it contains two orbits only. The second group (the L shell), in which the major axis is larger, contains 8 orbits. The third group (the M shell) with a still larger major axis, contains 18 orbits. The succeeding shells contain rapidly increasing numbers of orbits, the rule being that if n represents the order number of the shell (n=1 for K, 2 for L, etc.) the number of orbits.is zn2. The orbits in each shell are further divided into sub-groups in a way which we need not consider.

In an undisturbed atom the electrons revolve in the smallest possible orbits, subject to the condition that no two electrons can occupy the same orbit. Consider, for example, a particular element whose atom has, say, a nuclear charge of 15 and therefore 1 5 electrons; this element is, in fact, phosphorus. The 15 electrons arrange themselves so that iwo are in K orbits, eight in L orbits and the remaining five in M orbits. The K and L shells will thus be fully occupied by electrons, and five out of 18 orbits in the M shell will be occupied. The electrons choose this configuration because it is that in which their energy is least. Each electron has kinetic energy of revolution and potential energy resulting from its position with respect to the attracting nucleus, and its t d a l energy is least when it is a s near as possible to the nucleus. The state of least total energy is therefore that in which the electrons occupy the smallest orbits open to them. (There are, in fact, differences of energy between the orbits in each shell, and the five electrons in the incompletely filled M shell of phosphorus, for instance, choose the five orbits in which their energy is least, but we may ignore this detail for descriptive purposes, and simplify our considerations by thinking of all the electrons in the same shell as having the same energy.)

Now suppose that such an atom is subjected to a source of energy of some kind; it may be energy from an electric current passing through the substance of which the atom is a part, energy of collision with other atoms, energy of radiation falling on it, or energy of any other kind. We find that the atom can absorb such energy in only one way-namely, by one or more of its electrons moving out to larger orbits. Thus, in the phosphorus atom, an electron may move from a K or L to a n M or larger orbit, an M electron may move to an N or larger orbit, and SO on. When this happens the atom is said to be “ excited.” In the ordinary case it is the outer electrons which so move, but movements of the inner ones are possible.

Now the important thing about this movement is that the electron in making *$it takes up precisely the amount of energy necessary for its revolution in the larger orbit-no more and no

Our concern is almost entirely with the electrons.

What will happen?

ATOMS, MOLECULES AND RADIATION 187

less. If the energy available is less than that needed to make the smallest possible change of orbit, the atom will absorb none. If it is slightly larger than that amount, the atom will absorb that amount only, and ignore the remainder. Since every change of orbit corresponds to a definite increment of energy, this means that the atom can absorb energy only in certain standard sizes. These are called quanta, and although an atom may absorb several quanta by simultaneous changes of orbit of more than one electron, it cannot possibly absorb a fraction of a quantum ; its total absorption is the sum of the quanta absorbed simultaneously by its various electrons. Fortunately for physicists, in most acts of absorption only one electron moves, and we will limit our consideration to such cases.

Coqsider now an atom which has absorbed a quantum. Its displaced electron immediately returns inwards, just as a stone thrown into the air never stays there but returns towards the Earth’s centre, only the electron makes the double journey very much more quickly-in about a hundred-millionth of a second. It does not always return direct to the orbit from which it came, If the initial absorption took it out to a remote orbit, it may momen- tarily stop at any or all of the intermediate orbits (with certain exceptions) on its way back. But at each passage to an inner orbit it gives up an amount of energy precisely equal to the difference between the energies required for revolution in the two orbits concerned. Thus, if we number the orbits I , 2 , 3, 4, . . . with corresponding energies E,, E,, E,, E, , . . . , the energy absorbed in moving from orbit I to orbit 4 is E,-E,, and the energy re- emitted on moving back to, say, orbit 2 is E,-E,. But whereas the energy absorbed may be of any kind at al!, the energy re- emitted is always of one kind only-electromagnetic radiation, including light. Its total amount is equal to that absorbed, but its quality may be different. .

An atom may thus be regarded in general terms as a machine for transforming energy into radiation. Energy is transformed, tied up into bundles of standard sizes, and broadcast to space. The collection of quanta which the atom of any particular element can radiate is called the apectrum of that element. The spectra of the various elements differ from one another because, although the kinds of orbits-K, L, M, etc.-are the same for all elements, the energy which an electron possesses in each of those orbits varies with the charge on the nucleus, Consequently, a fall from an M to an L orbit results in the radiation of one quantum of energy in oxyger;, say, and of an unequal one in carbon. I t is on this account that we can analyse substances by examining their spectra. If we find a collection of quanta of certain sizes emitted by a radiating substance, we know that it contains carbon or oxygen, as the case may be.

An atom may absorb so much energy that an electron is removed from it altogether-to an orbit at infinity, as we may express it. When this happens the atom is said to be ionieed. When it is in this state, the energy change associated with a given change of orbit of one of the remaining electrons is different from what it was before; in other words, the spectrum of the ionised element is

There is one thing more to say regarding atoms.

188 H. DINGLE

different from the spectrum of the unionised element, and with regard not only to its spectrum but to its chemical properties also it constitutes in effect a new element. It cannot remain long in this state, however, for it recovers the lost electron (or another to take its place) as quickly as possible, but i f it is subjected con- tinuously to the supply of energy the process may be very rapidly repeated, so that in these circumstances we have effectively a continuous existence of ionised atoms.

An ionised atom may absorb another large amount of energy and lose a second electron, when it is said to be “ doubly ionised.” Treble and multiple ionisation also are possible, and at each stage a new spectrum, and virtually a new element, is brought into existence.

MOLECULES

A molecule is a group of atems, of the same or different elements, in which some at least of the electrons cannot be assigned to one rather than to another of the nuclei contained in the molecule. The number of atoms in a molecule may vary from two up to perhaps a million, but in the molecules of interest in meteorology there are very few-usually two or three.’ For purposes of illustration we will consider for the most part only the simplest-those with only two atoms, or diatomic molecules as they are called.

A group of atoms picked at random may or may not unite to form a molecule. Broadly speaking, the rule is that if the total number of electrons in the atoms is such as to form only completely filled shells, the atoms will unite (granted, of course, favourable physical conditions-temperature, etc.) but otherwise they will not. This is not strictly true, because completion of a sub-shell may be a sufficient condition for a molecule to be possible, but the general principles will be made sufficiently char if we consider only complete shells.

Hydrogen, for example, has one electron in a shell which can contain two, so two hydrogen atoms readily unite to form a molecule since their two electrons then fill the shell. Again, lithium has only one electron in the M shell while fluorine lacks one electron in that shell. Consequently lithium and fluorine form a very stable compound, LiF. The rare gases (e.g. helium, neon) consist of atoms whose shells are already complete ; they therefore combine with no other elements, and their atoms remain single. The number of atoms of hydrogen with which an atom of an element will combine -which measures what chemists call the valency of the element- can be inferred from this rule. Thus, if we take the elements carbon, nitrogen, oxygen and fluorine, which have respectively 4, 5, 6 and 7 electrons in the M shell where 8 can be accommodated, we find that their stable compounds with hydrogen are CH,, NH,, OH, and FH ; the number of hydrogen atoms in the molecule is that which provides just enough elettrons to fill the shell.

In an undisturbed diatomic molecule, the electrons are to be regarded as pursuing orbits of some kind round the system of nuclei, but the nuclei themselves do not remain at rest. They have two. kinds of motion-a vibration of each nucleus to and fro so that the nuclei periodically approach and recede from one another,

ATOMS, MOLECULES AND RADIATION 18B

and a rotation about an axis a t right angles to the line joining them. The periodic time of each motion is constant, so that the molecule has a definite amount of energy of each of three kinds- nuclear rotation, nuclear vibration, and electronic orbital motion.

Now a molecule, like an atom, can act as a machine for transforming energy of all kinds into radiation, but it has a wider range of methods for doing so, for it can make the change through the medium of any of these three forms of energy. Thus, if a molecule is exposed to a source of energy, it may absorb energy by increasing the rate of rotation or vibration of the nuclear system, or by the movement of an electron to a larger orbit; and the energy so absorbed is then re-emitted in quanta of radiation by the return of the molecule towards its former state.

Again, a molecule resembles an atom in its susceptibility to fracture if the energy it absorbs exceeds a certain amount, and here again it has a richer store of possibilities. I t may, like an atom, become ionised by the loss of an electron, but it may also be dissociated (i.e. broken up into separate atoms) if the motion of rotation and vibration is vigorous enough. Each of these atoms may carry off its normal number of electrons with it, or one or both of them may leave the molecule as an ionised atom, and free electrons may be left over. The possibilities, as may easily be. imagined, are numerous in the general case, and much study, both theoretical and experimental, is necessary to determine what is likely to happen in given circumstances.

RADIATION

Radiation, which includes light, infra-red and ultra-violet radiation, is something which is emitted in one form or another by all bodies, and travels through empty space a t a constant velocity of 300,000 km./sec. That is all we know about it, but just as with atoms and molecules, we form for working purposes a model of what it might be. Here, however, a single model does not meet all our needs, so we make two independent ones and use whichever is the more convenient in our particular problem. According to one model, radiation consists of waves; according to the other it consists of particles called photons. The former is probably the more familiar, but the latter is the better applicable to our present problem. We must not try to " reconcile " the two models ; they are alternatives, not parts of a larger whole. What we must try to do is to establish a correspondence between their separate details.

According to the wave theory, the two chief characteristics of radiation-its qua2ity (i.e. its colour when it is visible light) and its intensity-are represented by the frequency or wave-length and the amplitude of the waves, respectively. Since y e shall be dealing throughout with almost empty space-for the density of the atmosphere is too small to affect the properties of radiation appreciably -the frequency is always inversely proportional to the wave-length and so does not call for separate consideration; we may therefore say that the differences in the quality of radiation are represented only by differonces in the wave-length, and that waves of all lengths travel through empty space with the same velocity. Waves

I will describe them briefly in turn.

1 so H. DINGLE.

of red light are longer than those of blue, while infra-red waves are longer, and ultra-violet waves shorter, than waves of any visible light. The intensity of the light depends on the energy of the waves, and this is measured by the square of their amplitude.

On the particle theory the quality of radiation is represented by the size of the individual photons, and the intensity by the total size of all of them together. Thus a beam of red light consists of ;I number of photons of a particular size, and a stronger beam of red light consists of more photons of the same size. A beam of blue light consists of larger photons, and so, in order to have the same intensity as a beam of red light, it will have fewer photons.

A photon is to be regarded as a quantity of energy, not as a particle carrying energy, and what I have called its “ size ” represents the quantity of energy of which it consists. I ts energy is n-ot the kinetic energy of its motion. I ts velocity in empty space is unalterable and is independent of its size; we cannot think of it without that velocity. Since a red and a blue photon have the same velocity but have different energies, we should have to say that they have different rest-masses if their energy were kinetic, but actually neither of them has any rest-mass at all; if it had, its energy when moving with its actual velocity would be infinite. When a photon gives up its energy in some other form-as when the Sun’s rays warm our bodies-it does not remain a s a particle without energy ; it vanishes altogether. Similarly, when energy of some other kind is converted into radiation, photons are created.

There is one important difference between the two models of radiation. On the wave theory the energy of a beam of light may be indefinitely small; there is no limit to the reduction we can imagine in the amplitude of a wave. On the particle theory, however, there is a definite limit to the energy of a beam of a particular colour-namely, the energy of a single photon. This is a quantum of energy for that colour. True, we may have smaller photons, but then the light has a different colour. We can have an indefinitely small quantity of radiation on either model, but we can have an indefinitely small quantity of radiation of a particular colour only on the wave model. I t is believed that the photon model is the truer in this respect, for, as we have seen, whenever an atom or a molecule emits radiation, it is always as a quantum, but as we shall never need to consider smaller amounts of radiation than single quanta, the difference is not important for our purpose.

Let us now return to our picture of the atom (or molecule) emitting radiation. We have seen that when an electron falls from orbit 4 to orbit 2, it emits energy E,- E , as a quantum (i.e. a single photon) which will accordingly have a particular quality or colour. A fall from orbit 4 to orbit 3 corresponds to the emission of energy E , - E , , and this again is a single quantum, but of a different colour. On the wave theory we would say that the two transitions gave waves of different lengths. Each atom or molecule, then, can radiate certain definite photons, or wave-lengths, and these constitute its spectrum. In a piece of luminous matter of ordinary size there are billions of atoms or molecules, and at any instant some will be radiating one photon and others another, so that the whole body radiates the whole spectrum a t each instant.


When the atoms of a body are so close together that the electrons cannot move out to larger orbits without encountering other atoms (as in a solid or liquid), or when (as in the interior of the Sun) other disturbing influences interfere with the process of radiation, the definite set of photons characteristic of each free atom can no longer be produced, but instead photons of all sizes between certain indefinite and widely separated limits are emitted. We then have what is called a continuous spectrum-consisting, on the wave theory, of radiation of all wave-lengths between the extremes. The photons of different sizes are not all produced in equal numbers, however. The number is a maximum for photons of a particular size, and falls off on either side of it. The higher the temperature of the radiating body, the larger is the photon whose number is a maximum, and this, in fact, gives us a good method of measuring the temperature of a very hot body.

T H E METEOROLOGICAL PROBLEM

Let us now look a t our meteorological problem in the light of these considerations. We have a mass of air, consisting of atoms and molecules of different substances, gradually decreasing in density and changing in composition upwards owing to the Earth’s gravitational field. On this mixture a stream of photons of various kinds is projected intermittently from the Sun. The atoms and molecules are in motion, their kinetic energy depending on the temperature; and, moving a t random in all directions, they fre- quently collide with one another. The photons, moving with the velocity of light, also collide with them, and as a result of all these collisions there may be interchanges and transformations of energy, ionisation of atoms, formation and dissociation of molecules, and destruction and creation of pho top . Our problem is to determine the net result of all these processes.

In our present state of knowledge we must ask the experi- menter to furnish us with certain information, e.g. the elements composing the atmosphere and the effect on the atmospheric composition of activities, natural and artificial, which occur on the Earth’s solid and liquid surface ; the temperature, density and pressure a t various heights; the energies of the atoms and molecules in their various possible states of excitation ; and the number of photons of various sizes entering the atmosphere from the Sun. Provided with these data we should then be able to determine theoretically what kind of atmosphere we shall have. The problem is, of course, exceedingly complicated, but it is in principle soluble. We approach it by considering the possible effects of the collisions in more detail.

Let us first ignore the incoming stream of radiation, and consider only the collisions of the various material particles with one another, as though we experienced a perpetual midnight. The particles include various atoms and molecules, neutral and ionised, together with a number of electrons set free in processes of ionisation. Our general guiding principle is that when an atom or molecule receives a collision from another particle there is a redistribution of energy, the total quantity of energy remaining

192 H. DINGLE

constant. There are various ways in which this redistribution can occur.

First, an atom or molecule can behave like a mass-point in ordinary mechanics, and suffer an increase (or a decrease, if it gives some of its energy to the colliding particle) of its kinetic energy. In this case no outward or visible sign of the collision appears; kinetic energy is redistributed among the particles, but its total and its average amount remains the same and there is nothing t o be observed. This happens whenever the energy of collision is less than that necessary to make the smallest quantum change of which the atom or molecule is capable, and even when it is larger than this amount some of the energy may be redistributed in this way.

Secondly, an atom or molecule can absorb a quantum of energy by the outward movement of one of its orbital electrons or by a definite increase in the rotational or vibrational energy of its system of nuclei. When this happens the absorbed energy is immediately converted into a beam of radiation in the way already explained, and this travels off into space until it encounters another atom or molecule. This process therefore results on the whole in some definite change-the transformation of kinetic energy into radiation. Obviously, the higher the temperature of the gas the greater will be the number of collisions by which this can happen, and so the faster will the heat energy of the gas be radiated away ; but it will always occur to some extent, however low the temperature, because there will always be some molecules moving sufficiently faster than the average to bring it about. That is why any material body, prevented from receiving energy from the outside, cools by the radiation of its energy into space.

Thirdly, the energy of collision may be used up in detaching an electron from an atom or molecule and setting it free to collide with other particles. A certain minimum amount of energy is needed for this, which depends on the element or compound concerned, for the various nuclei hold their electrons with different degrees of tenacity. If the energy of collision exceeds this minimum amount the released electron carries off the excess a s kinetic energy. This process is not detectable unless special experimental arrangements are made, but its consequences may be observed, for a free electron is very prone to re-capture by any ionised atom or molecule, and when it thus loses its freedom it gives up the energy it no longer requires a s a quantum of radiation.

Fourthly, when atoms or molecules collide they may unite t o form a single molecule; or, conversely, they may be broken up into simpler molecules or single atoms, either ionised or not. This process also is often accompanied by radiation, for the energy which an electron may require to revolve round its new system of nuclei may be less than that needed for the old, and the balance may then be radiated. Here again what will happen depends on the relative energy values of the system in its possible states. There is always a tendency for a system t o take up the state in which its total energy is least, and some molecules may find that state in dissociation and others in combination. The chemist has long known that the application of heat dissociates some molecules


into atoms, while with other substances it makes possible the union of atoms into molecules.

There are other consequences of collisions into which we need not enter in detail. Thus it may happen that three particles meet at the same time, thus opening up a wider field of possibility; or an atom or molecule, after being excited to a higher energy state by one collision, may receive another before it has had time to return to its normal condition. These and other occurrences, though comparatively rare, are yet frequent enough in an atmosphere containing billions of particles to have important effects ; but since our purpose now is merely to get a general view of the character of the problem, we need not pursue their consequences.

The net result of all these processes, in course of time, would thus be as follows. There would be a continuous production of radiation, some of which would escape into space, and consequently a continuous decline in the kinetic energy of the particles which determines the temperature of the gas. This would go on until the temperature had fallen almost to the absolute zero-that is, to a value such that no atom or molecule could collide with another with sufficient energy to change its rotational, vibrational, or electronic state by the smallest possible quantum. Ionisation would be reduced to vanishing point, for recombinations of electrons with ionised particles would go on after the collisions had become too feeble to produce more ions, and the various combinations of atoms into molecules and dissociations of molecules into atoms or simpler molecules would result finally in an atmosphere of fixed composition in which no further change could take place without the introduction of energy from outside. From our knowledge of the elements in the atmosphere and laboratory data of the relative energies of the particles in their various possible states, the ultimate temperature and composition would be calculable, and although the problem would not be a simple one, it would not be beyond solution.

Superposed on all this, however, we have a diurnal stream of photons of various kinds pouring into the atmosphere from the Sun. Now the collision of a photon with an atom or molecule (a collision between a photon and an electron has no effect relevant to our considerations) resembles that of a material particle except that if a transfer of energy occurs it is always from the photon to the particle and never from the particle to the photon. I t is possible in very special circumstances for a photon to give up part of its energy on collision, and to change into a smaller photon, but this is a very exceptional process and we will not consider it further. Normally, a photon either gives up the whole of its energy and goes out of existence, or it gives up none, but travels through the gas with no change except a very slight decrease of velocity which we may ignore.

Simply the composition of the gas. A.photon is a particle of energy of definite amount. If an atom is to absorb it, it must be able to absorb just that amount, and this means (considering a n atom only for simplicity, though the same principles apply to the processes of molecular absorption also) that unless an atom has a possible orbit, for a transition to which it would require energy exactly equal to that

What determines which it will do?

194 H. DINGLE

of the photon, it will not absorb the photon. Now the orbital energies vary from element to element. Consequently, any particular photon will be liable to absorption only by that atom which has an orbital difference of energy equal to it. If the gas does not contain such an atom, it will be transparent to that photon, which will travel through it, almost as though it were not there.

It follows from this, of course, that if the photon has been produced by the radiation of an atom of a particular element-say, calcium-in the Sun, it will be absorbable only by an atom of calcium in our atmosphere, and if there is no such atom there, the photon will not be absorbed at all. This follows because the energy of the photon emitted in the Sun was, as we have seen, the difference between the energies of an electron in two calcium orbits, and therefore a calcium atom must necessarily have orbits with the requisite difference of energy. Thus we have the general rule that a gas is able to absorb the same kinds of photon as it can emit. It does not follow, however, that such photons, if they reach us from the Sun, have necessarily been emitted by calcium atoms there, because, as we have seen, in the special conditions existing inside the Sun photons of all sizes are emitted, irrespective of the chemical composition of the Sun, owing to the high degree of interference of atoms with one another. I t nevertheless remains true that if photons of the size in question enter our atmosphere, they will pass through it without absorption unless they encounter atoms of calcium, in which case they will be absorbed.

They are immediately radiated again, either in the same form as before or as two or more smaller photons if the calcium electron returns to its normal place by easy stages. Very little, therefore, appears to have been done, but there is this change, that whereas all the incident photons came from the same direction-that of the Sun- those reradiated go out in all directions, so that the fraction of them reaching an observer on the Earth's surface who looks towards the Sun is very small, and to him it is as though the absorption were nearly complete.

The final effect, then, of the interaction between the atmosphere and solar radiation, so far as this process is concerned, is simply that some of the radiation coming towards the Earth is scattered in all directions. The particles of the atmosphere which produce the scattering return to their original state and are left as though nothing had happened to them, and if the solar radiation consisted only of photons of such a size that they could be absorbed by the gases of the atmosphere in this way, it would all be scattered while the atmosphere would remain unchanged except for a small secondary effect which is just worth mentioning before we proceed. When an atom has absorbed a photon it might, before it has had time to return to its normal state, receive a collision from another particle sufficiently violent to ionise it, though the same energy would have been insufficient to do so if the atom had not been excited. The atmosphere therefore becomes rather more strongly ionised as the result of scattering the solar radiation, and this, being in effect a conversion of photon energy into kinetic energy

Suppose they are absorbed; what happens then?


of the atmospheric particles, slightly heats the atmosphere and SO

opposes the process of cooling which we visualised just now. A much stronger tendency in the same direction, however,

results from the presence of photons of such energy that atoms become ionised in the act of absorbing them. When this happens absorption is no longer restricted to those atoms which have orbital energies of just the right values; any atom can absorb any amount of energy exceeding that necessary for ionisation, for the electron set free can carry off the excess of energy as its own kinetic energy. Such high energy photons are not necessarily absorbed by any atom they meet, but many of them are. Hence, when they are present, the degree of ionisation of the gas, and also its temperature, are kept up, and if the incidence of such photons is continuous, the final cold, inert state of the gas is never reached, but instead a much higher degree of activity is given to the gas.

It is easy to see that the density of the gas is an important factor in determining how highly it will become ionised as a result of this process. When an electron is released from an atom or molecule it remains free until it encounters a n ionised atom or molecule (not necessarily of the same kind) which can recapture it, and its chance of doing this is less the more rarefied the gas may be. Consequently the upper atmosphere would be expected to be more highly ionised than the lower, both because it is less dense and also because it is the first region to suffer bombardment by solar photons and so protects the lower regions from the photons of greatest energy. It is well known from wireless observations that the upper atmosphere is in a comparatively highly ionised state, and we can understand from these considerations why its temperature is so much higher than was once thought possible.

Now, clearly, in order to determine exactly what will happen in the atmosphere, we must know precisely what photons the S u n emits. This is not a simple matter, because we are situated at the bottom of the atmosphere, and can directly observe only what gets through to us, not what falls on the upper atmosphere and fails to get through. We can, however, deduce what we want to know partly by observation and partly by theory.

Let us look first of all at the spectrum of the sunlight which reaches the Earth’s solid surface. This consists in the main of continuous radiation-i.e. of photons of all sizes from far into the infra-red to a considerable distance into the ultra-violet-but we notice that there are numerous gaps in the sequence, indicating a scarcity of photons of certain particular sizes. These we know, from laboratory observations, are photons which can be absorbed by certain known elements and compounds, and we conclude accordingly that they have been so absorbed. But now many of them-most, in fact-are absorbable only by elements (e.g. iron and many other metals) which we have no reason to think are in our atmosphere at all, and the inference is that the Sun also has an atmosphere which has absorbed them. (The absorption is in no case quite complete, nor would we expect it to be so since, as we have seen, absorption is always followed by re-emission, and a small fraction of the re-emitted photons travel in the originat direction.) W e picture the interior of the Sun as a source of

196 H. DINGLE

photons of all sizes, certain particular ones of which are largely absorbed by the solar atmosphere while the remainder travel through space to enter our atmosphere.

What we receive, then, is the original continuous radiation reduced by two distinct processes of absorption-one in the solar and the other in the terrestrial atmosphere. We must distinguish the results of these processes before we can know exactly what radiation falls on the upper regions of the Earth’s atmosphere.

There are two or three observational methods of doing this. We can observe the solar spectrum at different altitudes. Terrestrial absorption would decrease with altitude while solar absorption, of course, would be unaffected. Better still, we can observe the Sun on the horizon and at its greatest altitude in the sky. In the former case the light will have travelled through a greater thickness of the Earth’s atmosphere, and terrestrial absorption will therefore be relatively stronger. Perhaps the best method of all depends on a comparison of the light from the east and west limbs of the rotating’sun, but that we cannot enter into now. Suffice it to say that in one way or another we now know fairly well, though not so well as we should wish, what photons fall on the Earth’s atmosphere and what are destroyed on their passage through it.

The theoretical physicist is now in a position to tackle the general problem. He knows something of the quality and intensity of the incident photons ; he knows the chief elements present in the Earth’s atmosphere, and he has a good idea of the temperature and density at various heights. He has also much data concerning the possible energy states oi the atoms and molecules concerned. I t remains for him to apply the principles previously outlined, and tell us, so far as the incomplete data allow, everything else that we want to know about meteorology. Here I should leave the matter because, as I have said, my object has been to state the problem and not describe the steps which have been taken to solve it, but it may be helpful to give in conclusion one example of the analysis of atmospheric processes from this point of view. I choose the problem of the behaviour of oxygen in the atmosphere -a problem which has been considered especially by Professor Chapman.

Oxygen can exist in the form of single atoms or as diatomic or triatomic molecules. In a jar of undisturbed oxygen at ordinary temperatures practically all the gas is in the form of diatomic molecules, and if by laboratory processes we dissociate the molecule into atoms or build up triatomic molecules (ozone), these abnormal forms quickly disappear and diatomic oxygen is formed again.

In the atmosphere at ground level also oxygen is mainly diatomic, but in the upper levels this is not so. The reason is that here the oxygen is exposed to high energy photons from the Sun which in the first place break up the molecules into atoms, thus-

0, + photon = z 0 The photons required to do this correspond to ultra-violet radiation of about z53oA. or less in wave-length, and such photons are all destroyed in performing this and other operations and so never reach us at ground level. The oxygen atoms thus formed unite


again when they meet one another under favourable conditions, and so there is a double process of destruction and re-formation of molecules continually at work, with the result that at any instant there are always more or less definite proportions of 0 atoms and 0, molecules in the upper air; the relative numbers could be calculated exactly if we knew sufficiently well the number of effective photons and other data.

But now other processes occur besides occasional collisions between 0 atoms. An 0 atom may meet an 0, molecule, it may strike a molecule of another gas-say, nitrogen-and so on ; and we have to consider the relative frequency of such encounters and what happens in them. We find, for instance, that when an 0 atom meets an N, molecule, in the great majority of collisions nothing iiappens except a transfer of kinetic energy. A molecule N,O is possible, and is well known as nitrous oxide, or laughing gas, but the conditions are not favourable for the formation of this substance. The particles meet with a certain amount of kinetic energy, and something has to happen to this energy if they unite. If it were needed to give the electrons the energy they require in their new orbits or to supply energy of rotation or vibration .to the resulting molecules, all would be well; union would take place and a small excess or defect of energy could always be given to or taken from the kinetic energy of the more complex molecule. But if the new molecule does not require such energy for its formation -and especially if it requires less than the sum of the energies of the component parts when they are at rest-then the overplus of energy may simply break up the molecule as fast as it is formed. This appears to be what happens when 0 atoms meet N, molecules in the upper air.

Things are different, however, if a third particle is present in the collision. Suppose, for instance, that an 0 atom, an 0, molecule and an N, molecule all meet at the same time. I t can be calculated that in such a case the 0 atom unites with the 0, molecule and forms O,-ozone-while the balance of energy is carried off as kinetic energy by the unchanged N, molecule. This appears to be the chief process by which 0, is formed in the atmosphere.

Any particular 0, molecule has probably only a short life. When it meets an 0 atom it reverts to the 0, state, giving up its third atom to form another 0, molecule with the colliding atom, thus-

0 , + 0 = 2 0, But new 0, molecules are continually being formed, so that there is always a certain proportion of the oxygen in the form of 0,.

Now this 0, when once formed is capable of absorbing less energetic photons than those which broke up the original 0, molecule. It can absorb those corresponding to a wave-length of about 2950A or less, and, as a matter of fact, we never, a t the surface of the Earth, receive photons of higher energy than this because they are absorbed by the ozone in the upper regions of the atmosphere.

This brief example will be sufficient to show the very complex possibilities of exchange of energy and formation and dissociation

198 CORRESPONDENCE AND NOTES

of molecules which a r e opened up by t h e incidence of solar photons on t h e Ear th’s atmosphere. I have somewhat simplified the problem by omit t ing all consideration of cosmic rays, turbulence and wind currents in t h e atmosphere, a n d other complicating factors, bu t I hope I h a v e been able to show t h a t modern physics enables us to view some of t h e problems of meteorology in a very promising light.

Buchan Prize, 1943 The Secretary, Royal Meteorological Society. DBAR SIR,-

I wish to acknowledge with grateful thanks receipt of the Buchan Prize Certificate together with the accompanying cheque.

For obvious reasons, research work was carried out during the last few years a t the Pretoria Meteorological Office under rather adverse circumstances, and for that reason we appreciate all the more that our modest efforts should have received recognition in a very tangible form from the Royal Meteorological Society. I a m sure that the members of my staff were as pleased as myself that the Buchan Prize should have found its way to South Africa; and I wish to express the hope that it will not be the last time for a South African to receive this honour.

Yours very sincerely, T. SCHUYANN.

73, Anderson Street,

March 16, 1943. Pretoria.

Earth temperatures at Camden Square, London 55’ .525 Records of earth thermometers a t depths of I ft., 4 ft., and 10 ft.

are available a t Camden Square back to 1874, 1895, and 1908 respectively. The instrument used for the temperature a t I ft. is one of the original Symons earth thermometers (for description see Q.J., 1934, p. 535, and the report of the Glasgow meeting of the British Association in 1876). In 1936 a Symons earth thermometer of modern type (see description in Observer’s Handbook, 1939, pp. 24 and 33) was installed in addition to the old instrument. The suggestion a t that time was to withdraw the old instrument from service, after a sufficient time had elapsed for comparison with the new thermometer, and to place it on exhibition provided there was no substantial difference in the readings owing to lag, which might spoil the continuity of the long record.

Miss L. F. LEWIS, of the Meteorological Office, and a Fellow of the Society, has made a comparison of the readings for the six years 1937 to 1942, and has drawn up a table of the monthly means for each year obtained from the two instruments. The comparison shows the high efficiency of the old thermometer in as much as none of the differences in the monthly means exceeds 0. r°F. Individual readings, however, occasionally differ by upwards of o.g°F. and there is a tendency for these larger differences to occur mainly in winter. The data have been filed by the Society for reference by investigators interested in this long record of London earth temperatures, and the Society is grateful to Miss Lewis for making the comparisons available.

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Atoms, molecules and radiation