Optics & Photonics News

Chemists noticed that the atomic weight of hydrogen as measured by chemical methods differed slightly from the physical value found in Aston’s mass spectrograph. This discrepancy could be explained by the existence of a heavy isotope of hydrogen.

Few substances have had such rapid development from basic discovery to transformational applications. Industrial-scale production began in 1934. The 21st birthday of the discovery was marked by the ignition of the first nuclear fusion bomb, which was fueled by liquid deuterium.

Deuterium is now widely used in a variety of scientific and industrial tasks. The atomic spectroscopy used to find it is now a standard experiment in undergraduate physics laboratory courses. And new applications keep cropping up. For example, scientists recently conducted a more elaborate version of the same experiment to confirm the validity of the standard cosmological model of nucleosynthesis at the dawn of the universe.

As of Thanksgiving 1931, researchers had accumulated
much empirical knowledge on the nature of isotopes. The
masses of isotopes had all been found to be close to integer
multiples of the mass of atomic hydro-
gen, and the charges of all ions had been
revealed to be multiples of the elemen-
tary charge of the hydrogen nucleus.

Spectrum of the Sun showing the absorption lines of Ha at 656 nm and Hb at 486 nm, as well as the famous D lines of sodium, D1 at 589 nm and D2 at 596 nm. This image was created from a Fourier transform spectrum at Kitt Peak National Observatory. The spectrum was chopped into 50 slices, each covering 6 nm and pasted together to simulate an echelle spectrogram, typical of what is often used with astronomical telescopes. Above the Sun’s absorption spectrum is an emission spectrum showing Ha and Hb as well Hg at 434 nm and Hd at 410 nm.

The hunt for heavy hydrogen

In early 1931, chemists noticed that the atomic weight of hydrogen as measured by chemical methods differed slightly from the physical value found in Aston’s mass spectrograph. This discrepancy could be explained by the existence of a heavy isotope of hydrogen in nature.

Harold C. Urey, then an associate professor of chemistry at Columbia University, spearheaded the search. Fixed to the wall of his laboratory was a chart similar to the one on the left side of the figure on p. 37, which served as a constant reminder to Urey of the likely existence of a heavy isotope of hydrogen. At some point Urey realized, as he wrote in his Nobel Prize lecture in 1934, that “Bohr’s theory, given some 20 years ago, permits the calculation of the Balmer spectrum of the heavier isotopes of hydrogen from [the] spectrum of hydrogen.”