  The excitation of wakefields by a triangular temporal profile laser pulse
with an abrupt fall in a step density profile plasma channel is investigated
analytically and an exact solution is obtained. The excitation is more
efficient for t~tp, where t is the rise time of the pulse and tp is the plasma
period, though it is still significant for t~20 tp. The wake potential in the
former case is 1.7 times the ponderomotive potential, whereas it falls to 1.4
times the ponderomotive potential for the latter.
