Time-dependent particle number densities of the excited components (Xe/sup +*/, He/sup */) in a longitudinal-discharge XeCl excimer laser have been measured by laser absorption probing with a CW dye laser. The densities are compared with those in transversal-discharge XeCl excimer lasers. The formation dynamics of the XeCl excimer molecules, excited atoms, and excited ions are discussed. The dependence of the particle number densities of the Xe/sup +*/ ions on the buffer gas pressure, the Xe gas pressure, and the HCl gas pressure is investigated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>