laser beam while it approaches to one-center model for a low energy laser. In the present article we have investigated the photodetachment cross section for a linear triatomic negative ion with the laser beam polarized parallel to the axis of the triatomic ion. A procedure similar to the one reported by Afaq et al [1] is used in the present calculation. 

Three centers of the linear triatomic negative ion is supposed as a coherent source of electrons similar to H − and the detached-electron wave function is obtained as a superposition of the three coherent waves originating from each center of the system. The detached-electron flux, due to the superposition of these three waves, on a screen is calculated. For the total photodetachment cross section, of a linear triatomic negative ion, an analytical formula is derived by the integration of the electron flux in all directions. Our present article will be helpful in the understanding of the structural information of a linear triatomic negative ions like BeCl − 2 , HCN $^{−}$, CS − 2 , CO − 2 etc. [26]. Atomic units are used throughout the article or mentioned otherwise. 

## II. DETACHED ELECTRON WAVE FUNCTION 

The schematic diagram of the linear triatomic negative ion as a three-center model is shown in Fig.1. Numbers 1, 2 and 3 on the z axis represent the three centers of the system for the negative molecular ion, such that number 3 is set as the origin of the coordinates system. The ± symbols indicate the sign of the two lobes of the p-orbital wave function. The screen is placed at a distance L from the three-center system and perpendicular to the z axis. Assume d is the distance between the two adjacent centers and is of the order of a few atomic units, while L is much larger than d and is equal to several thousands atomic units [22, 23]. These two variables can be changed in our model. 

Let there is only one active electron like the H − model [18]. The normalized wave function for the active electron in the three-center system is the linear superposition of the H − like bound state at the three centers, φ$_{T}$ = ( φ$_{1}$ + φ$_{2}$ + φ$_{3}$ ) $_{/}$√$_{3.}$ Where φ$_{1}$ , φ$_{2}$ and φ$_{3}$ are the normalized wave functions for H − but centered at 1, 2 and 3 in Fig. 1. Photodetachment process is a two step process [27]. In the first step, a negative ion absorbs one photon energy E$_{ph}$ and generates an outgoing electron wave, while in the second step, this outgoing wave propagates to a large distance. 

A z-polarized laser light similar to Ref. [1] is used. The detached-electron wave function can be obtained by the linear superposition of the three coherent waves generated from the each center. These coherent waves can be achieved from the results of the H − photodetachment in the absence of an electric field [20]. Assume Ψ + 1 , Ψ + 2 and Ψ + 3 are the waves emitted from centers 1, 2 and 3. 

