E. A. Ogryzlo
University of British Columbia Vancouver, Canada
Why Liquid Oxygen Is Blue
A
substance appears colored to the human eye when it absorbs a portion of the visible spectrum (4000 to 7000 A). Since the energy of a quantum of visible light is very much greater than that required to excite vibrations and rotations in molecules, absorption of visible light can normally be traced exclusively to the excitation of an electron from one energy level (state) to another. Studies of such absorption spectra have led to the identification of a considerable number of low-lying excited states of atoms and small molecules. I n the case of the oxygen molecule all the low-lying states have been observed or their positions have been quite accurately fixed by theoretical calculations. The six lowest energy states of oxygen are shown in Figure 1. The first excited state (labeled 'A,) lies 0.98 ev above the ground state (labeled %2,-). A transition between these states gives rise to a weak absorption at 12,690 A which is in the infrared region. The next excited state (lcbeled I&+) gives rise to an absorption band a t 7619 A (also in the infrared region) and a much wcaker band, on the edge of the visible region, at 6990 A, which is due to a transition to the first excited vibrational level of the I&+ state. The '2,-, %Au,and 3 2 . + states lie at much higher energies and give rise to the very weak "Hewberg bands" in the ultraviolet. There are no other states of 0% which can give rise to absorption bands in the visible region. However, when oxygen is condensed (at -183'C) the liquid is blue. The absorption bands which are responsible for this color are shown in Figure 2. There is only one
band in this spectrum which can be attributed to isolated O2moleculesthe weak band at 6990 A whose origin was described above. The remaining strong bands require quite a diierent explanation which was first suggested by Ellis and Knesser in 1933 (1). R e cent work in the Soviet Union (Z), Holland ($), and Canada (4, 6) has supported the original assignment, and there is now little doubt as to the origin of these bands. They arise when a single photon simultaneously elevates two electrons on two different molecules t o excited states. Thus twice the energy required to excite a molecule t o the 'Ag state is possessed by a photon at 6340 A. The absorption of these photons gives rise to peak a in Figure 2. Peaks b, c, and d result from the same simultaneous electronic transition when it is accompanied by the additional excitation of 1, 2, and 3 vibrational quanta respectively. Peaks a', b', and c' form a similar series when a simultaneous electronic transition occurs to the 'A, state in one molecule and the '2,+ state in the other. Because these peaks are lower than those in the unprimed series, most of the absorption occurs in the red, yellow, and green region giving liquid oxygen its characteristic blue color. For many years it was thought that the simultaneous electronic transitions described above were unique to oxygen because of the possible formation of an Oaspecies. It is now clear from both the absorption (6) and emission (7) studies that the pair of molecules taking part in this process are not bound to each other but are simply a colliding pair. Furthermore, there appears no reason why this could not be a fairly common phenomenon.
1
INTERNUCLEAR DISTANCE (ANGSTROMS) Figure 1.
Potential energy curves for the six low lying states of
0%
W A V E L E N G T H (ANGSTROMS) Figure 2.
Absorption spectrum of oxygen in the visible region.
Volume 42, Number 12, December 1965 / 647
The reason why it is seldom observed is that twice the energy of a given electronic transition is almost always in a region where another strong transition dominates the spectrum. However, there are at least two other of transitions in the literature. In 1961 (8)a simultaneous electronic transition was reported for two pra+ions in a solid crystal of
systems, undoubtedly, additional simultaneous electronic trausii.ions will be discovered.
that the two Pr3+ions that are excited by a single photon are se~aratedfrom one another bv chloride ions. The other system for which a simultaneous electronic transition has been reported is a solution of naphthalene and oxygen in chloroform (9). In this system a 3500 A photon was found to simul~aneouslyexcite o2 to the 'A, state and naphthalene to the lowest triplet state (3B2.). AS more careful studies are made of other
A&, 18,1(1962). (5) BADER,L. W., AND OGRYZLO, E. A,, D i s m s i o n ~F a d a y
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Journol o f Chemicd Education
Literature Cited (1) ELLIS,J. W., AND KNESSER, H. O., 2.Physik, 86,583 (1933). (2) DIANOY-KLOKOV, V. I., o p t . i Speet~oskopiya,6,457 (1959). (3) FAERENFORT. J.. Thesis. Universitv of Amsterdam. 1955.
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DOC., 0,s 40 ( 1 J D Y I .
(6) SALOW, H., AND STEINER, W., Z. Pkysik, 99,137 (1936). (7) ARNOLD,S. J., BROWNE, R. J., AND OGRYZLO, E. A., J. P k o t o e h i s t ~ yand Photobwlogy, December, 1965. (8) VAN~ANYI, F.2 AND DIEKE,G. Hv P ~ Y sRev. . Letlers, 7,442 (1961). (9) D~KGRAAF, c., SI-RS, R., AND HOIJTINK,G. J., MOI. Phys., 5,643 (1962).
