n(k) Structure and Matrix Elements in Photoemission: Results from   Insulating Ca_2CuO_2Cl_2
  The photon energy dependence of angle-resolved photoemission data on Ca_2CuO_2Cl_2 is presented, and the role of the matrix element is investigated. It is shown that despite strong variations in the matrix element, the loss in intensity as one crosses the antiferromagnetic zone boundary is a robust feature of the Mott insulator. As expected for a two dimensional system, the dispersion remains independent of the photon energy even when the matrix element does not. 
F. Ronning, C. Kim, K.M. Shen, N.P. Armitage, A. Damascelli, D.H. Lu,   and Z.-X. Shen (Stanford University), L.L. Miller (I...