Figure 5

Plotted is the deviation from the PAR condition represented by the equality in eqn. (5). The black (dashed) and white lines denote values for which the frequency and incident angle satisfy the PAR conditions of eqns (4) and (5) respectively. Crossing points of the white and black dashed lines which satisfy the conditions for PAR agree well with the narrow dip in reflectivity of Fig. 2 between 1.55–1.57 rad/fs.