  Supersonic beams of hydrogen atoms, prepared selectively in Rydberg-Stark
states of principal quantum number $n$ in the range between 25 and 35, have
been deflected by 90$^\circ$, decelerated and loaded into off-axis electric
traps at initial densities of $\approx 10^6$ atoms/cm$^{-3}$ and translational
temperatures of 150 mK. The ability to confine the atoms spatially was
exploited to study their decay by radiative and collisional processes. The
evolution of the population of trapped atoms was measured for several
milliseconds in dependence of the principal quantum number of the initially
prepared states, the initial Rydberg-atom density in the trap, and the
temperature of the environment of the trap, which could be varied between 7.5 K
and 300 K using a cryorefrigerator. At room temperature, the population of
trapped Rydberg atoms was found to decay faster than expected on the basis of
their natural lifetimes, primarily because of absorption and emission
stimulated by the thermal radiation field. At the lowest temperatures
investigated experimentally, the decay was found to be multiexponential, with
an initial rate scaling as $n^{-4}$ and corresponding closely to the natural
lifetimes of the initially prepared Rydberg-Stark states. The decay rate was
found to continually decrease over time and to reach an almost $n$-independent
rate of more than (1 ms)$^{-1}$ after 3 ms. To analyze the experimentally
observed decay of the populations of trapped atoms, numerical simulations were
performed which included all radiative processes, i.e., spontaneous emission as
well as absorption and emission stimulated by the thermal radiation. These
simulations, however, systematically underestimated the population of trapped
atoms observed after several milliseconds by almost two orders of magnitude,
although they reliably predicted the decay rates of the remaining atoms in the
trap. The
