The two images produced by each feature in a Nomarski polarization interferogram are in general not equivalent. In the case of an etch pit or other isolated, sloped feature, the angle ϑ between the feature and the flat background surface can be computed from the fringe spacing l in the feature by the relationship sin[ϑapp−(ε/2) cosα] =λ/2l. The angle ε/2 is calculated from the background fringe spacing l0 and the relation sin(ε/2) =λ/2l0. The angle α is the orientation of the feature with respect to the background interference fringes on the flat surface. An additional correction is necessary to calculate ϑ from ϑapp because of the distortion of the fringe spacings which occurs in microinterferometers when using microscope objectives of large numerical aperture. Empirical observation shows that this obliquity correction can be calculated accurately by a formula first derived by Ingelstam. The method of determining slopes from fringe spacings in Nomarski polarization interferometry is described in detail. This description may be of interest to investigators who study crystal surfaces or changes in their topography during chemical reactions, because the Nomarski microinterferometer is suitable for continuous observation of surface changes which accompany reactions in a controlled-atmosphere heating stage.

Recent observations indicate that the rate of evaporation of elemental arsenic into a vacuum is determined by the rate of formation, at screw dislocations, of kinks in ledges of molecular height. Once formed, a kink advances along the ledge, which makes a continuous spiral ramp outward from the dislocation, until the ledge terminates. A kink advances by releasing As(4) molecules from the ledge. A given kink releases almost 10(6) As(4) molecules before the next kink is initiated.

The initial vaporization of freshly cleaved As(111) surfaces has been studied by a combination of microbalance weight-loss measurements and continuous microinterferometry. Vaporization is accompanied by formation and growth of triangular pits on the originally flat surface. The slopes (5°–9°) and other characteristics of the pits show that they have a terrace–ledge structure and form at dislocations. Pit sides have constant slope indicative of an average, constant terrace width between ledges. Isolated pits grow outwards at a constant rate while mass loss from a freshly cleaved increases quadratically with time. The mass-loss rate attains steady state, independent of the pit density, when the surface is covered by intersecting pits. The slopes of pits formed on surfaces sublimed into a vacuum are greater than those on surfaces vaporized under saturations nearer equilibrium. The pit growth rates and characteristics, as well as the time-dependent mass-loss rates, are quantitatively consistent with a vaporization mechanism in which the vaporization rate is proportional to the total ledge length on the surface. The results imply that formation of As4 vapor species occurs with equal probability, per unit length of ledge, along any ledge segment.