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E. A. Ogryzlo
of
British
Columbia
University
Vancouver, Canada
Why Liquid Oxygen Is Blue
M 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. In 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
lies
0.98
ev
above
the
1Aa)
ground state (labeled 32„”).
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 (labeled 1Sff+) gives rise to an
absorption band at 7619 A (also in the infrared region)
and a much weaker 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 12ff+ state. The 1 —,
+
3A„, and 32u states lie at much higher energies and give
rise to the very weak “Herzberg bands” in the ultraviolet. There are no other states of 02 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 02 molecules—the weak band at 6990 A whose
origin was described above. The remaining strong
bands require quite a different explanation which was
first suggested by F41is and Ivnesser in 1933 (1). Recent work in the Soviet Union {%), Holland (5), and
Canada (4-, 5) 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
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 1Af
state in one molecule and the l2ff+ 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 0.i species.
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.
WAVELENGTH
Figure 1.
Potential energy
curves
for the six low lying states of O2.
Figure 2.
(ANGSTROMS)
Absorption spectrum of oxygen
in the visible region.
Volume 42, Number 12, December 1965
/
647
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
examples of simultaneous electronic transitions in the
literature. In 1961 (8) a simultaneous electronic transition was reported for two Pr3+ ions in a solid crystal of
PrCl;;. The striking thing about this observation is
that the two Pr3+ ions that are excited by a single photon are separated from one another by 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 simultaneously excite 02 to the
1Ag state and naphthalene to the lowest triplet state
careful studies are made of other
(3£>2„). As more
The
648
/
Journal of Chemical Education
systems, undoubtedly, additional simultaneous electronic transitions will be discovered.
Literature Cited
(1) Ellis, J. W., and Knesser, H. O., Z. Physik, 86, 583 (1933).
(2) Dianov-Klokov, V. I., Opt. i Spectroskopiya, 6, 457 (1959).
(3) Fahrenfort, J., Thesis, University of Amsterdam, 1955.
(4) Landau, A., Allin, E. J., and Welsh, H. L., Spectrochim.
Acta, 18, 1 (1962).
(5) Bader, L. W., and Ogryzlo, E. A., Discussions Faraday
Soc.,37,46(1964).
(6) Salow, H., and Steiner, W., Z. Physik, 99, 137 (1936).
(7) Arnold, S. J., Browne, It. J., and Ogryzlo, E. A., J.
Photochemistry and Photobiology, December, 1965.
(8) Vansanyi, F., and Dieke, G. H., Phys. Rev. Letters, 7, 442
(1961).
(9) Dijkgraaf, C., Sitters, R.,
Phys., 5,643(1962).
and
Hoijtink,
G. J., Mol.