The Balmer spectrum is the portion of the spectrum of
atomic hydrogen that has emission and absorption lines visible
to the eye. The Bohr theory depicts a nucleus of mass M and
electron of mass m in motion about their common center of
mass, which is always much closer to the nucleus than the
electron. In the center-of-mass frame, a nucleus of mass M
executes a small orbital motion, with a radius of (m/M ) times
the atomic radius, and the kinetic energy of that minor orbital
motion adds directly to the energy of the atom. For hydrogen,
m/M = 1/1,836, so this is quite a small effect, even less for possible heavier isotopes of hydrogen.
Low-temperature physics joins
atomic spectroscopy
The energy of nuclear motion introduces a shift of about 0.1 nm
between the wavelengths of the Balmer lines of hydrogen and
a hypothetical isotope of mass 2, an effect that was well within
the resolving power of the diffraction grating spectrometer
that Urey had built at Columbia. He and his associate George
M. Murphy evidently saw the lines of the mass 2 isotope
soon after they started looking for them. However, due to the
low natural abundance of the heavy isotope, these lines were
weak features that were comparable to some known artifacts
in the spectrum.
Urey decided to try to make samples of hydrogen in which
the concentration of heavy isotopes would be enriched. He
settled on the idea of creating liquid hydrogen and evaporating
it near its triple point. This distillation should leave behind a
liquid in which heavier isotopes are concentrated, since heavier
molecules have lower speeds of thermal motion, and should
be less likely to evaporate from the liquid. To pursue this
approach, Urey enlisted the help of Ferdinand G. Brickwedde,
chief of the Low Temperature Laboratory of the National
Bureau of Standards (NBS) in in the autumn of 1931 in Washington, D.C. NBS is now the National Institute of Standards
and Technology, or NIST.
Then only 28 years old, Brickwedde had already developed
a reputation in low-temperature physics. Earlier that year, he
led an NBS team that produced the first liquid helium made in
the United States (which, by the way, made NBS only the fourth
laboratory in the world to liquefy helium in the 23 years since its
first production by Heike Kamerlingh Onnes in Leiden, Holland). His laboratory was then one of only two in the United
States that could produce liquid hydrogen on a regular basis,
the other being that at the University of California, Berkeley,
directed by William F. Giauque (subsequently Nobel Laureate
in chemistry, 1949).
In the hunt for a heavy isotope of hydrogen, Brickwedde
prepared several samples of liquid hydrogen at different levels
of distillation. The most enriched was a sample that started
with 4,000 cm3 of liquid hydrogen and had all but 1 cm3
evaporated off. The samples were sent by railway express from
NBS to Columbia University, where their emission spectra
were recorded with Urey’s high-resolution spectrometer. The
Ferdinand G. Brickwedde with his wife, physicist Marion Langhorne Howard Brickwedde (1909-1997). Between them is the apparatus for making water enriched with deuterium.
Smithsonian Institution Archives/ Wikimedia
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Ferdinand Brickwedde prepared three samples to increas-
ingly concentrate any heavy isotopes of hydrogen. H1b is
the ordinary Balmer line of atomic hydrogen at 486.1 nm,
in the blue-green. H2b and H3b mark the lines predicted by
the Bohr theory for emissions by isotopes of mass 2 and 3,
respectively. The “ghost” and “halation” features are instru-
mental artifacts; their prominence with respect to the obvious
H2b feature motivated Urey to show that that feature was
enhanced by distilling liquid hydrogen, resulting in a relative
increase of the mass 2 isotope (deuterium). No evidence for
mass 3 (tritium) is seen.
