7 Figure captions Fig.1 Schematic diagram of the three-center model as a triatomic negative ion Fig.2 The total photodetachment cross section for different values of d using Eqs. (5)-(7). From (a) to (d) show that with the increase in d the oscillation amplitude decreases and oscillation frequency increases Fig.3 Fourier transformation of the total photodetachment cross section (a) calculation, (b) com- parison: the dashed line is the result with two-center system and solid line is the result with three-center system. The calculations are carried out using d = 250 a.u.