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 ex- clusively to the excitation of an electron from one en- ergy level (state) to another. Studies of such absorp- tion spectra have led to the identification of a considera- ble number of low-lying excited states of atoms and small molecules. In the case of the oxygen molecule all the low-lying states have been observed or their posi- tions 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 32 .+ states lie at much higher energies and give rise to the very weak "Hewberg bands" in the ultra- violet. There are no other states of 0% which can give rise to absorption bands in the visible region. How- ever, 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

E. A. Ogryzlo University of British Columbia

Vancouver, Canada

INTERNUCLEAR D ISTANCE ( A N G S T R O M S )

Figure 1. Potential energy curves for the six low lying states of 0%

Why Liquid Oxygen Is Blue

band in this spectrum which can be attributed to iso- lated O2 moleculesthe 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 simultane- ously elevates two electrons on two different molecules to excited states. Thus twice the energy required to excite a molecule to 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 un- primed series, most of the absorption occurs in the red, yellow, and green region giving liquid oxygen its char- acteristic 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 Oa species. It is now clear from both the absorption (6) and emis- sion (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

W A V E L E N G T H ( A N G S T R O M S )

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 systems, undoubtedly, additional simultaneous elec- energy of a given electronic transition is almost always tronic trausii.ions will be discovered. in a region where another strong transition dominates the spectrum. However, there are at least two other Literature Cited

of transitions in the (1) ELLIS, J. W., AND KNESSER, H. O., 2. Physik, 86,583 (1933). literature. In 1961 (8) a simultaneous electronic transi- (2) DIANOY-KLOKOV, V. I., o p t . i Speet~oskopiya, 6,457 (1959). tion was reported for two pra+ ions in a solid crystal of (3) FAERENFORT. J.. Thesis. Universitv of Amsterdam. 1955.

that the two Pr3+ ions that are excited by a single pho- A&, 18,1(1962).

ton are se~arated from one another bv chloride ions. (5) BADER, L. W., AND OGRYZLO, E. A,, D i s m s i o n ~ F a d a y 0.. ." DOC., 0,s 40 (1JDYI.

The other system for which a simultaneous electronic (6) SALOW, H., AND STEINER, W., Z. Pkysik, 99,137 (1936). transition has been reported is a solution of naphthalene (7) ARNOLD, S. J., BROWNE, R. J., AND OGRYZLO, E. A., J. and oxygen in chloroform (9). In this system a 3500 A P k o t o e h i s t ~ y and Photobwlogy, December, 1965.

photon was found to simul~aneously excite o2 to the (8) VAN~ANYI, F.2 AND DIEKE, G. Hv P ~ Y s . Rev. Letlers, 7,442 (1961).

'A, state and naphthalene to the lowest triplet state (9) D~KGRAAF, c., SI-RS, R., AND HOIJTINK, G. J., MOI. (3B2.). AS more careful studies are made of other Phys., 5,643 (1962).

648 / Journol of Chemicd Education