  The separation $\del$ between two peaks in the gamma-ray pulse profile is
calculated as a function of energy for several polar cap models with
curvature-radiation-induced cascades. The Monte Carlo results are interpreted
with the help of analytical approximations and discussed in view of the recent
data analysis for the Vela pulsar (Kanbach 1999). We find that the behaviour of
$\del$ as a function of photon energy $\epsilon$ depends primarily on local
values of the magnetic field, $B_{\rm local}$, in the region where
electromagnetic cascades develop. For low values of $B_{\rm local}$ ($<
10^{12}$ G), $\del(\epsilon)$ is kept constant. However, for stronger magnetic
fields ($\ga 10^{12}$ G) in the hollow-column model $\del$ decreases with
increasing photon energy at a rate dependent on maximum energy of beam
particles as well as on viewing geometry. There exists a critical photon energy
$\et$ above which the relation $\del(\epsilon)$ changes drastically: for
$\epsilon > \et$, in hollow-column models the separation $\del$ increases
(whereas in filled-column model it decreases) rapidly with increasing
$\epsilon$, at a rate of $\sim 0.28$ of the total phase per decade of photon
energy. The existence of critical energy $\et$ is a direct consequence of
one-photon magnetic absorption effects. In general, $\et$ is located close to
the high-energy cutoff of the spectrum, thus photon statistics at $\et$ should
be very low. That will make difficult to verify the existence of $\et$ in real
gamma-ray pulsars. Spectral properties of the Vela pulsar would favour those
models which use low values of magnetic field in the emission region ($B_{\rm
local} \simless 10^{11}$ G) which in turn implies a constant value of the
predicted $\del$ within EGRET range.
