Recent Attempts to Determine the Thickness and Refractive Index of Unimolecular Films

The Hartman method for measuring the thickness and refractive index of nonabsorbing unimolecular films is investigated. Both theoretical and experimental evidence lead to unfavorable conclusions. A test experiment is reported in which the Hartman method is used to measure the optical constants of barium stearate multilayers from 50 A to 250 A in thickness. The results obtained disagree with the relatively well-known properties of barium stearate films. To determine the cause of the disagreement, the method is investigated theoretically. It is shown by numerical calculation and algebraic analysis that, in terms of the present accuracy of the optical measurements involved, the Hartman method is too insensitive to be of practical use for films of molecular thickness.