Determination of molecular orientation and alignment from polarized laser photofragmentation measurements. Oriented CH3I molecular beams

Abstract

It is important to determine the degree of orientation and alignment of a given ensemble of molecules, whether they be reagents (prepared via hexapole focusing or laser excitation), or products from crossed-beam reactions or scattering from surfaces. In general, the orientational probability density function P(cos θ), i.e. the spatial distribution function of the molecular framework, can be expressed as a Legendre expansion in cos θ(ρ), where the angle θ is defined with respect to the axis of a reference electric field. The first moment of P(ρ), i.e.〈P₁(ρ)〉P₁, is the average orientation, while the second moment, P₂, is the average alignment. Unfortunately, because of hyperfine coupling effects (dependent upon the electric field strength and upon the time-dependence of the passage of the oriented molecules from strong to weak field, e.g. from the hexapole through guiding and orientation fields), it is not always possible to ascertain the actual P(ρ). The present paper provides the methodology for inversion of experimental measurements of the asymmetry parameters, κ_∥, κ⊥, from the polarized laser-induced photofragmentation of oriented symmetric-top molecules, to yield the first few Legendre moments of P(ρ) and thus characterize a given ensemble of molecules in terms of its orientation and alignment. As an example, this method is applied to available experimental measurements on CH₃I in weak electric fields, for which computations dealing with certain limiting (adiabatic, sudden) hyperfine coupling cases have been carried out.