Nontransient phase-coherent effects in multilevel systems

We examine the effect of including phase-coherent coupling between a driving laser field and a gas of multilevel molecules ($N=20,40\mathrm{}$) on a nontransient time scale. Although multiphoton enhancement of upper energy-level populations is known to be negligible for systems containing a small number of levels ($N=3,4\mathrm{}$), such coherent enhancements increase strongly as a function of the number of consecutive radiatively coupled levels, and may play a significant role in recently observed laser-induced chemical reactions.