Positronium – Hydrogen Like and Unlike*

CROATICA CHEMICA ACTA CCACAA, ISSN 0011-1643, e-ISSN 1334-417X Croat. Chem. Acta 82 (4) (2009) 791–800. CCA-3373 Author’s review Positronium – Hydrogen Like and Unlike* Milan Randić** National Institute of Chemistry, Ljubljana, Slovenia (E-mail: [email protected]) RECEIVED SEPTEMBER 2, 2008; REVISED JANUARY 13, 2009; ACCEPTED MARCH 30, 2009 On the occasion of the 2008 Brijuni Conference on Hydrogen – the most abundant atomic species in the Universe, it seems fitting to draw attention of the participants of this conference, as well as chemists at large, to Positronium – one of the least abundant atom-like species in the Universe, if for no other reasons then because it was theoretically predicted by a Croatian scientist, Stjepan Mohorovičić some 75 years ago, about 100 miles away, in the city of Zagreb, the capitol of the Republic of Croatia. Abstract. A brief review on positronium, Ps, hydrogen-like system built from positron and electron, is out- lined from its beginning in 1935, the first theoretical study on this relatively stable matter-antimatter system by Stjepan Mohorovičić, to the most recent works on positronim hydride PsH, and positronium molecule Ps2, analogue of hydrogen molecule. Mohorovičić calculated spectra of Ps and was even looking for it in the sky searching for its spectrum, but experimental observations of positronium Lyman-α radiation Lyα λ2430 line waited for another 40 years before being successful identified in a laboratory in 1975 by Canter and collaborators. It took another ten years for astronomical observation of the presence of positronium in outer space in 1984 by McClintock, who observed Lyα λ2430 line in spectra of Crab Nebula, 50 years after the attempts of S. Mohorovičić to detect positronium lines. The work of Mohorovičić was mostly ignored in his native Croatia, until the most recent time, an illustration of “historical blunder” of local physics community – phenomenon not so unheard of in science in general, as has been recently worldwide illustrated with hesitation of acceptance of the notion of nonlinear dose response (hormesis); the density functional theory; and chemical graph theory. Keywords: positronium, positronium hydride, molecular positronium INTRODUCTION also first to measure fluctuations between tide and ebb of the Adriatic Sea, and initiated measuring of varia- “The father of positronium,” Stjepan Mohorovičić, was tions in ozone in the atmosphere in Zagreb, about 80 born in 1890 in Bakar, a small coastal city of Croatia years ago, when few even heard of the problems of some 63 miles (100 km) from Brijuni and died at the ozone. age of 90 in Zagreb. At best he had a miserable acceptance by local scientific and local political authorities, Recognitions of accomplishments of a scientist by and was prevented by different and opposing political their contemporary colleagues are uncommon even regimes admittance to be a faculty at Zagreb University, when the work is in commonly recognized areas of despite the prestigious position of his father at the Uni- science, and scientists are in the leading institutions and versity. Stjepan Mohorovičić is son of the famous Croa- the leading scientific countries of the world. But for a tian geophysicist, Andrija Mohorovičić (1857−1936), scientist from an “unheard” of country, and an “un- born in Volosko, another small coastal town of Croatia, heard” of university, or no university at all, as has been less than 30 miles from Brijuni, who predicted the MO- the case with Stjepan Mohorovičić, the “father of posi- HO layer (a discontinuity at the boundary between the tronium,” a proper recognition of their scientific accom- Earth crust and the mantle). Andrija Mohorovičić was plishment is beyond reality. On the top of this, Stjepan * This contribution is dedicated to the memory of my physics teachers, Marija Lukšić (high school teacher), Mladen Paić (1905- 1997), Branimir Marković (1917-1973), Vatroslav Lopašić (1911-2003), Milena Varićak (1903-1971), Ivan Supek (1915-2008), Zlatko Janković (1916-1987), Ivan Babić-Gjalski (1925-1953), Vladimir Glaser (1924-1984), Borivoj Jakšić (1925), Krunoslav Ljolje (1928-1994) and Aleksandar Grossman (1930) at the University of Zagreb. ** Visitor; Permanent address: Emeritus, Department of Mathematics and Computer Science, Drake University, Des Moines, IA 50311, USA. 792 M. Randić, Positronium: Hidrogen-like and unlike Mohorovičić has additional burden to face, living in the vered” at home, sooner or later. From my correspon- shadow of his father, a famous and recognized scientist. dence with Academician Paar I learned that it was soon- However, there is no doubt that Stjepan Mohorovičić er thanks to an “accidental” contact between Academi- was outstanding scientist and that his work was one of cian V. Paar (theoretical physicist) and Professor Kre- the most outstanding accomplishments of Theoretical šimir Balenović, the head of organic chemistry at the Physics of the early Quantum Theory period. How little University of Zagreb. Professor Balenović draw atten- was known of his work, although it has been published tion of V. Paar to Stjepan Mohorovičić, who had been in respectable scientific journal, is seen from the fact teaching physics in a high school attended by Balenović that on two occasions his calculations were re-produced. many years back. Professor Balenović was somewhat The title of the paper of S. Mohorovičić:1 “Möglicheit irritated by complete disregard among contemporary neuer Elemente und ihre Bedeauting in Astrophysik” Croatian physicists for his former physics teacher, Stje- (Possibility of novel elements and their meaning in pan Mohorovičić, as if he did not exist. This disregard astrophycics) is indicative that S. Mohorovičić was fully went so far that around 1950 none of the physicists of aware of the significance of his work. For an introduc- the University of Zagreb with a scientific reputation was tion to the discovery of positronium see a brief review2 willing to testify that Stjepan Mohorovičić is bona fide by Helge Kragh entitled: “From “electrum” to positro- scientist (who had about dozen publications in reputed nium.” In addition there are two similar “popular” ar- German astrophysics journals). This “don’t see, don’t ticles in Croatian language by H. Galić3 and V. Paar4 hear, don’t speak” attitude towards Mohorovičić de- about Stjepan Mohorovičić and positronium. I may add prived Mohorovičić to qualify for a better retirement that during the four years of being student of Theoreti- pension, to which scientist have right to apply. Today’s cal Physics at the University of Zagreb, the name and interpretation of the three wise monkeys: “See no evil, the work of Stjepan Mohorovičić was never mentioned, hear no evil, speak no evil,” according to Wikipedia, is even though his work has been known to few abroad. commonly used to describe persons who do not want to An exception in Croatia was Professor Vladimir Glaser, be involved in a situation, persons turning blind eye to who in his book on quantum electrodynamics, published the immorality of an act in which they are involved – in 1955 in Croatian language,5 has mentioned the work and for reasons that remain unknown that appears to be of Stjepan Mohorovičić on positronium. In passing, the the position of then the scientific (physics) “Establish- book of V. Glaser was one of the first monographs on ment” in Croatia. It also appears that Vladimir Glaser, quantum electrodynamics ever published. It was pub- who has been the best among theoretical physics in lished three years before the book edited by J. Schwing- Croatia at that time and the time that followed, was the er “Selected Papers on Quantum Electrodynamics”,6 and only person who knew better. 30 years ahead of the “classic” of R. P. Feynman: The paper on Positronium of Stjepan Mohoro- “Quantum Electrodynamics: The Strange Theory of vičić,1 particularly when viewed from the present time, Light and Matter”.7 could have been worthy of the Nobel Prize. At first At this point it is worthwhile to recall a comment sight this may appear as an exaggeration if not a prepos- of Vannevar Bush (1890−1974), an engineer who de- terous speculation, but enough time has passed that in veloped the differential analyzer (analog computing) in retrospect one can better appreciate the importance of 1928 at MIT, and was influential government advisor the “discovery” than would have been possible 70 years and science administrator. He refereed to Mendel in the ago. As we know today the postulate on positronium has following way: lead to novel insights in physic, goes beyond positronium not only as a component in search for elusive anti- “...Mendel's concept of the law of genetics was lost proton, detected in cosmic rays about 30 years ago,8 to the world for a generation because his publication which is the core of antihydrogen, a counterpart of hy- did not reach the few who were capable of grasping and drogen,9−11 for arrival at “hybrid” molecules, parts con- extending it; and this sort of catastrophe is undoubtedly sisting of atoms and parts of positronium (Ps), like posi- being repeated all around us, as truly significant at- tronium hydride PsH,12 lithium positronium PsLi,13 and tainments become lost in the mass of inconsequential.” of recently opened the field of molecular positroniums 14 The case of Stjepan Mohorovičić is different in the Ps2. One can view this in parallel with the discovery of sense that his work did reach the few who were capable buckminsterfullerene, C60, by Curl, Kroto and Smalley of grasping and extending it but the importance of his in 1985,15 for which they shared Nobel Prize in 1996. significant attainment became almost lost for his imme- Few have thought in the mid 1980s that this novel allo- diate contemporaries in Croatia and the innocent ge- trope of carbon, despite that it revolutionized our know- neration that followed, even though eventually his major ledge on carbon, would be more than a interesting cu- accomplishment, the postulate on existence of positro- riosity of chemistry, such as is, for instance bullvalene, 16 nium, being known abroad, could not but be “redisco- C10H10 molecule, conceived by von Doering and Roth Croat.

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