Number 110 (Story #1), January 15, 1993 by Phillip F. Schewe and Ben Stein ATOMIC HYDROGEN IN A MAGNETIC TRAP has been studied directly using optical spectroscopy for the first time. Hydrogen usually exists on Earth as a diatomic molecule, but atomic hydrogen (H) can be created and maintained at low temperatures by spin polarizing the atoms in a magnetic field; the Pauli exclusion principle then keeps the atoms from pairing up. This effort is worthwhile since atomic hydrogen is the one substance in nature that remains a gas even at a temperature of absolute zero, making it an interesting test ground for quantum mechanics. Previous studies of trapped H had observed the atoms only after they had left the trap. Now, Jook Walraven and his colleagues at the University of Amsterdam have studied the transmission spectroscopy of trapped H in situ using pulsed (third-harmonic generated) laser light at the so-called Lyman-alpha wavelength (121.6 nm, corresponding to the 1S-2P transition). The spectrum can be used to monitor the temperature and density of the hydrogen sample as a function of time, which will be useful for later studies of H at even lower temperatures. (O.J. Luiten et al., 1 Feb. issue of Physical Review Letters.)