Photoelectron-current measurement by low-frequency electromodulation probe (PMMP) is proposed as a sensitive method to determine the rate G, of neutral free radical production by the photodeionization of negative ion beams (PDINIB). The PMMP method was employed to evaluate the production rate in a trial surface-processing apparatus developed in the author's laboratory utilizing a steady-flux refined beam of neutral free radicals produced by the PDINIB procedure. A Cu-63(-) negative ion beam of kinetic energy E-i varied up to 15 keV was irradiated with a 514.5 nm visible light beam from a 25 W cw Ar+ ion laser. The detection limit of the production rate by the measurement setup was 6 x 10(9)/s under the conditions that E-i = 15 keV, the negative ion beam current I-i = 4 muA, and the laser power P = 6 W. Based on the results of these basic experiments, furthermore, the photodeionization efficiency defined by eG(r)/I-i where e is the elementary electric charge was estimated to be 27 +/- 6% under the conditions that E-i = 15 keV, I-i = 40 AA, and P = 18 W.