Theory of electronic-state coherences produced in combined laser-field—collisional reactions

A method for producing electronic-state coherences using either collisionally-aided radiative excitation (or "optical collision") or laser-induced collisional energy transfer (or "radiative collision") is proposed. Two atoms, $A$ and $A^{\prime }$, collide in the presence of two pulsed-laser fields having frequencies $\Omega$ and ${\Omega }_1$. It is shown that by choosing ${\Omega }_1\pm \Omega$ such that an energy-conserving transition can occur in the composite $A{A}^{\prime }$ system, one can create an electronic-state coherence in the $A$ or $A^{\prime }$ atoms. The coherence can be produced between states of the same or of opposite parity; if it is between states of opposite parity, coherent emission at frequency ${\Omega }_1\pm \Omega$ may be generated. The relationship of this work to theories of "pressure-induced extra resonances" in four-wave mixing is discussed.