Determination of population, alignment, and orientation using laser induced fluorescence with unresolved emission

Abstract

A method is presented for determining the population, A^{0}₀₊, the alignment moments, A^{2}_q± and A^{4}_q±, and the orientaion moments, A^{1}_q± and A^{3}_q±, for a ground state distribution of diatomic molecules probed by 1+1 laser induced fluorescence. General expresssions are developed for all rotational branches as a function of the rotational quantum number for excitation with linearly, circularly, or elliptically polarized light. Specific expressions are evaluated for the case in which the emission is unresolved and collected independent of its polarization and for the case in which the emission is unresolved but is analyzed with a polarizer. When the emission is collected independent of its polarization, the real polarization moments, A^{k}_q±, cannot be independently determined and only the apparent moments, A^{k}_q± (app), can be measured, explicit expressions for the apparent moments in terms of the real moments are presented. However, for the case in which the excitation light is created by passing linearly polarized light through a quarter‐wave plate and the emitted light is analyzed with a quarter‐wave plate and a linear polarizer, the real alignment and orientation moments can be independently determined.