[The invitation for this piece was prompted by the appearance of an article entitled “Huge electric field found in ice-cold laughing gas” in Science Alert]
Is laughing gas laughing at us?
How do scientists discover new phenomena, and, just as important, how do they persuade other scientists that they have discovered something new? First, they must persuade themselves; this can be a long and tortuous process. During its course, they do their very best to prove that their discovery is wrong, perhaps because it contradicts some well-established law. They set out to show that their new phenomenon may, in the polite phraseology of science, be an artifact, or in the more colloquial form, a complete cock-up. They think up every reasonable test, using as many different techniques as possible, throwing at their new phenomenon every tool that they can lay hands on — to make sure that they really have gotten something new. Woe betide them if they do not follow this course of action!
Let us restrict ourselves here to the quite serendipitous, experimental discoveries, those that take place quite unexpectedly. A few examples will clarify what I mean. First, take Fleming’s chance observation of destruction of bacteria by penicillin, which apparently must have flown in through a nearby open window in his untidy laboratory. Or consider Bequerel’s discovery of radioactivity in which the chance juxtaposition, over a weekend, of a photographic plate, a key and pitchblende (uranium ore) created an image of the key, when the photographic plate was developed. Even better perhaps, the “experiment,” that many people would have wished to have been present to see, when Roentgen put his hand between his X-ray tube and the detection screen and saw, to his astonishment, the image of the bones in his hand. It is revealing of the character of discovery that subsequently Roentgen conducted further experiments in private, rather than expose himself to the ridicule of the scientific community if he turned out somehow to be imagining, rather than imaging, things. Fear of failure and doubt of their results are not confined to experimentalists. Both Schrödinger and Dirac have both recorded the same sensations in their respective paths to the discovery of quantum mechanics and anti-matter.
We have obviously no choice but to admit that chance plays an important role in scientific discovery. But there is much more to it than that. How many researchers, prior to Fleming, had glanced at the destruction of bacteria and washed the stuff down the sink, without giving it another thought? Again, the fogging of photographic plates by pitchblende had been seen before. The conclusion that had been drawn was that one should not leave pitchblende near photographic plates; it ruins them. Or take the annoying radio-astronomy hiss that seemed to come from every direction in the universe and proved impossible to suppress. This was certainly observed before Penzias and Wilson took it seriously — having first cleaned the pigeon mess off their radio dish, a good example of removing a possible artifact. So it was that observational cosmology was born, through recording of the cosmic microwave background.
The case study which I seek to illustrate here cannot be classed with these great discoveries of the past, but it is “a small thing but mine own” and, I believe, illustrates features of the nature of discovery. If you take a gas, such as laughing gas, that is, nitrous oxide, and expose it to a cold surface at (say) -40 oC, it then condenses to form a solid film. What my colleagues and I found was that the surface of this film apparently had a positive voltage on it, just as if the positive end of a battery was connected to it. This was indeed serendipitous. We did not set out to find this effect: after all, we did not know that it existed. Rather we were studying the interaction of surfaces with low energy electrons. So our discovery was by chance: but more than that, we thought that it was probably wrong, or so we had to assume. Without going into detail, we observed the passage of a current through a system, as if the surface of the nitrous oxide had a positive voltage on it. We did not expect to see this current and it should not have been present. That is, the machine was electronically incontinent and, unless we could put a nappy on the damn thing, it was useless for any meaningful experiments. This, if you like, was the “eureka” moment, but at the time, despite an inkling of something else, we schooled ourselves to believe that the machine had broken down.
Discovery — or not: rip it apart or press on?
Now we had a choice. We could succumb to our damning observations, rip apart the machine, clean it and pamper it and hope that it would work “correctly” when it was back together again. One might term this the “pitchblende fogs photographic plates: keep them away from each other” or “don’t leave the window open near bacterial cultures, it destroys them” approach. Or we could press on making the provisional assumption that, just maybe, there was something in our rather crazy observation that the surface of a thin film of nitrous oxide was spontaneously at a positive voltage of several volts. Remember that, since the film was very thin, just a few per cent of one millionth of a meter, then several volts suggested enormous electric fields in the film, expressed as volts per meter. Our observations implied that there were electric fields in the film of one hundred million volts per meter. This was about as big as it could be, since it was close to the field that would cause electrical breakdown, like a spark in damp air.
So it was that there was a knock on my door in the Department of Physics and Astronomy at the University of Aarhus, where this saga took place, and Richard Balog and Peter Cicman, my two postdocs working on the project, entered to tell me that the apparatus was misbehaving. I should mention that the apparatus itself was attached to the ASTRID storage ring synchrotron source and I must acknowledge here the technically outstanding people who built this source and maintain it. Without this resource and these scientists, none of what I describe would have been possible. These same people have recently built another source in the basement of our department. Previously Aarhus Physics and Astronomy was probably the only department in the world to have a synchrotron storage ring in its basement: now we are certainly the only department to have two such sources.
At any rate, Richard and Peter took the brave step of following their instincts and decided to make the assumption that, just maybe, we were on to something. First though, as I said, we needed to persuade each other that we had really discovered a new phenomenon and were not just fooling ourselves. I want to take you through this process — it shows how scientists work, setting ourselves up, perhaps immodestly, as model scientists! Richard and Peter started by showing that the measured voltage on the surface increased exactly in proportion to the thickness of the film. This agreed with the two hundred year old Poisson equation, which is fundamental to the subject known as electrostatics. We had not violated anything basic at this point. So we were on our way.
We then varied the temperature at which the film was condensed. We found that the higher the temperature of condensation of nitrous oxide, the lower the potential measured on the surface, for the same film thickness. For example, for condensation at -213 oC compared to -233 oC, the voltage on the surface was more than two and a half times smaller. This seemed sensible. Higher temperature means that the molecules of nitrous oxide push and shove each other more. You would expect this to create a more disorderly system. A more disorderly system would somehow seem likely to produce a lower voltage on the surface. But now we were confronted with this “somehow.” It is not enough in science just to proclaim the facts of observation, you need also to offer some sort of rationale.
Some dipolar moments
We needed first to address the question: how could there be a spontaneous voltage on the top of films of nitrous oxide? Age old electrostatics tells you that, since you have measured a constant field, there are no free charges, for example electrons, on or in the film. This is not obvious, but I beg you to accept it. Now, while voltages are generally due to the presence of free charges, which we had excluded, the charge does not in fact have to be free to create a voltage, it can be contained within, that is, be intrinsically part of a molecule at the surface. Molecules are overall neutral but can have one end positive and one end negative. Such molecules, and they are the great majority, are said to possess “dipole moments.”
If the positive end of the molecule sticks out of the top of the film, the surface will appear positively charged. Could this be what is happening here, we thought: the positive nitrogen end of nitrous oxide sticks out the surface whilst the negative oxygen end remains buried? This would also explain why higher temperature leads to lower voltages. The amount of voltage on the surface depends on how parallel the molecules are to one another, that is, their degree of orientation. At higher temperatures, they push and shove more and therefore they are less well oriented. Hence, there would be less tendency for positive ends to stick out of the surface.
There was something rather strange here, however. This model may have explained our observations, but it carried with it some rather unfortunate baggage. The model required that the plus end of one molecule tends to associate with the plus end of another, and the same with the minus ends. But plus repels plus and minus, minus, so why should the system configure itself in this way spontaneously? It should find its most favorable state, with plus to minus, just as you sink most comfortably into an armchair. But let us sweep this under the carpet for the present.
With our experimental evidence and despite our reservations about our understanding of the cause of what we had observed, we felt prepared to publish our findings . Yet, there were a lot more experimental questions to answer, quite apart from the theoretical one which we have just swept out of sight. Does it make any difference on what surface you condense the nitrous oxide? Is nitrous oxide the only molecule to show this effect? Have we observed a truly general effect or is it just special to one system? If it were special, it would still be interesting, but it would be much more interesting if it were a general phenomenon. And if you heat a film, would the effect go away?
Physics or stamp collecting?
Let us answer these questions one by one, without going into too much detail. The nature of the surface, upon which you condense the nitrous oxide, makes no difference to our observations. For example, you can condense nitrous oxide on films of condensed atoms of xenon and you see no change in the surface voltage. In answer to the second question, nitrous oxide is by no means the only molecule to show this effect. Taking note of Rutherford’s famous injunction against stamp collecting, we tried nine chemically diverse materials, but all with dipole moments, of which eight showed the same effect as nitrous oxide. Some, however, had a negative voltage on their surface: presumably they had the negative end of the molecule sticking out of the surface. The effect is general. If you heat a film, then, yes, the effect does disappear and it does so rather abruptly over a small range of temperature. For example, a film of isoprene composed of 300 layers of molecules and condensed at -233 oC has a surface potential of about nine volts. Warming this layer to -201 oC causes the potential to disappear, reaching zero at -197 oC. This was one more clear step towards showing that we were beginning to understand the physical basis for the phenomena that we were observing. Also, since we now felt that the phenomenon was general, we gave it a name: the “spontelectric” effect.
However, no physicist can sleep at night unless he or she has some mathematical model to describe quantitatively what they observe. So armed with all these experiments, I began, with help from Hans Fogeby and Axel Svane in our Department, to write down a couple of equations based upon the model of oriented dipoles mentioned above. This was found to fit the observations of electric field versus deposition temperature for nitrous oxide very well. The theoretical model provided one more piece of evidence that we were on the right track. At this stage, chance intervened once again. I had an astronomer working with me, Cécile Favre, interested in the radio-astronomy of methyl formate in Orion. She wanted to know the temperature at which methyl formate sublimes and we decided to measure this on our machine. This was nothing to do with spontelectrics, at first. But since we had methyl formate in the system, we decided to look and see if this was spontelectric too.
The International Brigade: Fate intervenes
At this point, enter Oksana, Ukrainian by birth, with a Russian father, Ukrainian mother, educated in Armenia, working previously at the Synchrotron Laboratory in Trieste in Italy. Enter also Andrew Cassidy, from Ireland who speaks some Irish Gaelic, if pushed, but probably more importantly, he is a chemist with a PhD from Cambridge (England). By the way, Peter Cicman, with two PhDs, one from Japan and one from Austria, and Richard Balog, with a PhD from Berlin, were both Slovak by birth. I mention the lineage of my co-workers to emphasize how international European science has become in the last decade or so. Scientific research makes for good international relations.
As well as performing more experiments on nitrous oxide, Oksana showed that methyl formate was indeed spontelectric. This became quite an epic adventure, as Oksana tried higher and higher temperatures of deposition, sixteen in all. She found that at above -193 oC the electric field in films of methyl formate started sharply to increase, instead of decreasing. Dismay! Everything that we thought that we had understood was wrong — or was it? Oksana found that at -188 oC, the field was the same as it had been at -233 oC and then went more than 50% higher again before collapsing at -183 oC. In fact the spontelectric field at -233 oC deposition temperature was twenty-eight million volts per meter, whereas at -185 oC, it was forty million volts per meter. This was an odd intervention of fate, more than odd, in fact. It appeared to cut through all our understanding of spontelectrics which we had so carefully built up around data for nitrous oxide and other molecules. Surely more pushing and shoving causes less dipole orientation and less field, not more!
And what about the lovely theory which worked so well for nitrous oxide? Was it flawed? I had surely put nothing in the equations that could show the behavior of methyl formate. What was missing? Nothing, as it turned out, to my continuing surprise. Using my two equations, the rate of variation of electric field with temperature of film condensation can be written as a fraction, that is, something divided by something. If the second something is zero, then this expression becomes infinity. On one side of the infinity, that is, at lower temperature, the rate of variation of electric field with temperature is negative, as we expected and observed in nitrous oxide and in methyl formate, below -193 oC. On the other side, it is positive. This latter is the anomalous behavior which we observe in methyl formate above -193 oC, and indeed what one predicts, if, in my equations, one uses the parameters for methyl formate derived from fitting lower temperature spontelectric data for this molecule. Spontelectrics just became curiouser and curiouser. We could reproduce our observations, but we could not be said to understand them physically.
At this stage we needed to sit back and take a deep breath. Spontelectrics had turned out to be something of a rollercoaster. What did we therefore do? We wrote a review of the whole topic as we understood it in early 2013 . But on a rollercoaster, one moment you think that you are safe and the next you are hurtling down some endless slope, having left your stomach somewhere behind you. For there is still more curious behavior to consider — which we knew about already but had quietly ignored in order to preserve our sanity.
If you lay down a film of toluene at -198 oC, it is not spontelectric for the first one hundred monolayers, where a monolayer is a single layer of molecules. Put down a bit more, and the surface potential takes off and the film becomes spontelectric. The same thing happens with isoprene at -203 oC, except that the spontelectric effect comes in after 50 to 75 monolayers have been laid down. Apparently, the molecules like to get head-to-head and tail-to-tail, plus to plus, minus to minus, only when the film has achieved a certain thickness. Somehow, toluene layer number 101 knows that it is number 101 and decides to “go spontelectric.” In other words, the molecules know about each other’s presence: they communicate with one another. Apparently there has to be a certain number of molecules of toluene or isoprene before the films switches into a spontelectric state.
What we observed showed that every part of the system depends on every other part. Communication extends right across the film, over macroscopic distances for which pair-wise interaction between molecules is completely negligible. This leads one to appreciate that spontelectrics have properties which are much more than just the sum of pairwise interactions between molecules, or even three-, four- or five-wise, but many thousand-wise in the case of toluene laid down at -198 oC. Such systems are called “non-local.”
So spontelectrics have two crazy properties: the effect can get greater at higher temperature of deposition, as in methyl formate, and the effect seems to require that all the molecules in the film talk, or feed back, to one another. If we are going to claim to understand spontelectrics, we are going to need to understand these two fundamental aspects. This is where I make my escape: no nice explanation is forthcoming. Simple reasoning based on cause and effect, in a system dominated by feedback, is difficult because the first system, one molecule of toluene, say, influences the second, and the third, the fourth, the fifth etc. and in turn each of these influences each of the others. If you could model a film as a repeating unit of a cube of ten by ten by ten molecules, you would have to consider 499,500 pairwise interactions, some very weak and every one dependent on every other. This might allow you to show that a film would settle into some stable spontelectric state, through mutual agreement between the molecules.
Not so grand Finale
From that last example of 499,500 interactions, you can see why I need to quit the footlights and exit by the stage door. There are other aspects of spontelectrics too: for example, how you can build different spontelectrics on top of each other and make any geometrical form of electric field you wish, work which we did in collaboration with Jack Dunger, a very talented young post-graduate from Cambridge (England) . There are also some lovely experiments carried out by Andrew Cassidy which show what happens when nitrous oxide is diluted in xenon , giving us additional insight into the non-local, non-linear nature of spontelectrics. Furthermore, in the spirit of throwing as many techniques as possible at the subject, Jerome Lasne, Alexander Rosu-Finsen and Martin McCoustra at Heriot Watt University, Edinburgh, have recently been performing some experiments on how spontelectrics absorb light. Their independent data also reveal the presence of the spontelectric field. This helped to allay some of the last traces of skepticism which remained in a corner of my mind .
To return to a possible explanation of how spontelectrics may form: nature provides you with an accomplished fact, the spontelectric film is there in front of you. I am hesitating by the stage door and I am about to make a run for it because I have no explanation, save conjecture, of how a film gets itself into the spontelectric structure. How do the molecules move about as they make the film, condensing from the gas phase, and why do they spontaneously choose an apparently unfavorable structure, with plus to plus and minus to minus? This is the problem which we swept under the carpet above, but that we need to face. Unfortunately, we do not know how to do this. However, I will allow myself the luxury of speculation, without thinking about 499,500 interactions, before I finally do make my exit.
Our current speculation goes something like this . Fluctuating movements of the molecules at the surface locally create, by chance, some fleeting orientation of the molecules, with plus to plus and minus to minus. This in turn creates an electric field opposing this orientation. The electric field will also be found in a region outside the fluctuation that caused it. There, the field creates a dipole orientation in the opposite sense to that of the fluctuation and this propagates throughout the film, locking the dipole orientation into position and creating the spontelectric state. We cannot say how much truth there is in this hand-waiving — maybe not too much. There is something called chemical dynamics which may, using mighty computers, give us the answer. The doors of the Department of Physics and Astronomy at Aarhus University are open for anyone who would like to join us in this search for greater understanding.
At all events, at the very start I asked how scientists discover things, and how they convince first themselves and then other scientists that they have discovered something new. In this case study, I have described how a quite unexpected discovery was made in solid state physics, where counter-intuitive stacking of molecules leads to properties never before observed in solids. I have tried to sketch the thought processes which accompanied the verification of this discovery: first denial and doubt, then testing to destruction, and ultimately some degree of confidence. My hope is that this short account has provided some insight into how discoveries in general are made and eventually validated.
David Field is Professor Emeritus in the Department of Physics and Astronomy at Aarhus University in Denmark. He has published over 175 papers, is on the editorial board of Astrobiology, and has played a key role in the discovery of spontelectrics.
 Balog R., Cicman P., Jones N.C., and Field D. (2009) Spontaneous Dipole Alignment in Films of N2O. Phys. Rev. Lett., 102, 073003.
 Field D., Plekan O., Cassidy A., Balog R., Jones N.C., and Dunger J. (2013) Spontaneous Electric Fields in solid Films: Spontelectrics. Int. Rev. Phys. Chem., 32, 345.
 Cassidy A., Plekan O., Balog R., Dunger J. and Field D. (2014) Electric Field Structures in thin Films: Formation and Properties. J. Phys. Chem. A 118, 6615.
 Cassidy A., Plekan O, Dunger J., Balog R., and Jones N.C. (2014) Investigations into the Nature of Spontelectrics: Nitrous Oxide diluted in Xenon. Phys. Chem. Chem. Phys. 16, 23843.
 Lasne J., Rosu-Finsen A., Cassidy A., McCoustra M. R. S., and Field D. (2015) Spontaneously electrical solids in a new light. Submitted for publication.