Orientation Dependence of an Unresolved Mössbauer Quadrupole Doublet in an Aligned Liquid Crystalline Glass

Abstract

In an aligned system, an unresolved Mössbauer quadrupole doublet will appear to be a broadened single line absorption with an orientation dependent line position (1.p.). Previously, we have reported an orientation dependent 1.p. for the ¹¹⁹Sn absorption for the solute, 4-trimethyltin-benzylidene-4′-n-butylaniline (Sn-BBA), in the smectic B glass of 4-n-butoxybenzylidene-4′-n-octylaniline (BBOA). The 1.p. depended on the angle (θ) between the preferred moleuclar direction as determined by a 9 kG magnetic field and the direction of the γ-ray beam. In this report, we present a method of obtaining the isomer shift (I.S.), the quadrupole splitting (δE _Q) of the unresolved doublet, and the orientational order parameter [S = <3/2 cos² δ - ½>] for the Sn-bearing molecules. The angle δ is the angle which a given molecule makes with the field direction and for the glass phase the brackets indicate a spatial average. The method consists of constructing a doublet spectrum for each θ based on initial guesses of I.S., δE _Q, S, and the single line linewidths. The constructed doublet spectrum is then computer fit with a single Lorentzian lineshape which shows a θ-dependent position and linewidth. The Mössbauer parameters are then adjusted to reproduce the θ variation of the line position and linewidth of the apparent singlet experimental spectrum. This method yields δE _Q = -0.33 mm/sec and S = 0.48 for Sn-BBA in the BBOA glass at 77°K. This S-value is compared to other determinations of S for ⁵⁷Fe and ¹¹⁹Sn bearing solutes which show resolved doublet spectra in liquid crystalline glases.