Line shapes in high-resolution triple-resonance ionization spectroscopy have been calculated and compared with experimental measurements on the 4s~(2) ~(1)S_(0)→4s4p ~(1)P_(1)→4s4d ~(1)D_(2)→4snf~(1)F_(3)→Ca~(+) system of calcium. Calculations based on the density matrix formalism integrated the fundamental equations over experimental atomic angular and velocity distributions and laser intensity profiles. The measurements reveal and confirm all predicted structures arising from the complex coupling of four atomic states with three laser fields and the Doppler distribution of the atomic ensemble. Effects of different laser beam geometries on the line shapes have been investigated. The agreement between calculated and experimental spectra is generally good over a dynamic range of 10 orders of magnitude. Thus these calculations can accurately predict optical isotopic selectivity in multistep resonance ionization, with a value of S_(opt)～10~(10) expected for detection of the ultratrace isotope ~(41)Ca.
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