  The time evolution of thermal and thermo-reactive instabilities of primordial
gas in the presence of ionizing UV radiation is studied. We obtain conditions
(depending on density, temperature, and UV radiation intensity) favorable for
the formation of a multi-component medium. Nonlinear effects, especially those
attributable to opacity, can play an important role. In certain parameter
regimes, dramatic, rapid evolution of ionization states away from ionization
equilibrium may occur even if system control parameters evolve slowly and the
system starts in or near ionization equilibrium. Mathematically, these rapid
changes may best be understood as manifestations of bifurcations in the
solution surface corresponding to ionization equilibrium. The astrophysical
significance of these instabilities is that they constitute a nongravitational
mechanism (thermo-reactive instability) for decoupling isolated spatial regions
with moderate initial fluctuations from their surroundings, even under the
influence of heating processes due to ionizing UV radiation. In particular,
isolated, decoupled regions with relatively high neutral hydrogen density may
result, and these could be associated with Lyman alpha clouds of high column
density or with Lyman limit systems.
