4 dσ ( q ) ds = 2 πE$_{ph}$ c ⃗ j$_{r}$ · ˆ n (4) where q is the coordinate on the surface Γ, ˆ n is the exterior norm vector at q, c is the speed of light in a.u. and ds = r $^{2}$sin θdθdφ is the differential area on the spherical surface. The total cross section of the triatomic negative ion can be derived by the integration of the differential cross section over the entire surface, σ ( q ) = ∫ Γ dσ ( q ) ds ds. Substituting the value of ⃗ j$_{r}$ from Eq. (3) in Eq. (4) and integrating: After solving the integration, the following simple result is obtained: where σ$_{0}$ ( E ) is the smooth total photodeatchment cross section of H − [28] and A$_{|}$$_{|}$ ( kd ) is the modula- tion function due to the triatomic negative ion. The total photodetachment cross-section expressed in Eq. (5) is plotted in Fig. 2 as a function of the incident laser photon enenrgy (eV) for several d values. The figure shows strong oscillation for the laser polarization parallel to the axis of the linear triatomic molecule, in contrast to the recently reported [1] laser polarization perpendicular to the triatomic molecule. Closed orbit theory [19, 24, 25] is used to explain the origin of these oscillations. The detached-electron wave produced at center 1 while propagates out of the source reaches to center 3 and center 2 on its way out. The overlap of the detached-electron wave from center 1 with the source at center 3 and the source at the center 2 produces oscillations in the total cross section of the triatomic negative ion. As a result we encounter two special orbits of the detached-electron; one is generated from center 1 to the nearby center 3 and the other one is from center 1 to center 2. These two special orbits of the detached-electron are responsible for the oscillations. Similar oscillations are also reported in the diatomic negative ion (two-center system) [2]. A comparison of our results with the two-center model is shown in Fig. 3. Fourier transformation in Fig. 3 is used to investigate special orbits of the detached-electron. A$_{|}$$_{|}$ ( kd ) = 1 + 4 [ sin( kd ) ( kd ) + 2 cos( kd ) ( kd ) 2 − 2 sin( kd ) ( kd ) 3 ] + 2 [ sin(2 kd ) (2 kd ) + 2 cos(2 kd ) (2 kd ) 2 − 2 sin(2 kd ) (2 kd ) 3 ] (7) σ$_{0}$ ( E ) = 8 π $^{2}$k | C | $^{2}$E$_{ph}$ 3 c with C = 4 kBi ( k 2 b + k $^{2}$)2 (6) σ ( E ) = σ$_{0}$ ( E ) A$_{|}$$_{|}$ ( kd ) (5) σ ( E ) = 2 πkC $^{2}$E$_{ph}$ 3 c ∫ π 0 ∫ 2 π 0 cos $^{2}$θ sin θ [1 + 4 cos( kd cos θ ) + 4 cos $^{2}$( kd cos θ )] dθdφ