energy transfer from a projectile to a target molecule. This process depends on the concept of kinematics and mass perceptibility. Probability of occurrence of collision provides information about scattering cross section. Average loss of energy of an atom moving through a dense medium gives idea on stopping cross section and capability of depth perception. Statistical fluctuations caused by the energy loss of an atom while moving through a dense medium. This process leads to the concept of energy straggling and a limitation to the ultimate depth and mass resolution in back scattering spectroscopy. Physical concepts that are highly important in interpretation of forward recoil spectrum are depth profile, energy straggling, and multiple scattering. These concepts are described in detail in the following sections : Depth profile and Resolution analysis A key parameter that characterizes recoil spectrometry is the depth resolution. This parameter is defined as the ability of an analytical technique to measure a variation in atomic distribution as a function of depth in a sample layer. In terms of low energy forward recoil spectrometry, hydrogen and deuterium depth profiling can be expressed in a mathematical notation. Δx = ΔEtotal/(dEdet/dx) where δEdet defines as the energy width of a channel in a multichannel analyzer, and dEdet/dx is the effective stopping power of the recoiled particles. Consider an Incoming and outgoing ion beams that are calculated as a function of collisional depth, by considering two trajectories are in a plane perpendicular to target surface, and incoming and outgoing paths are the shortest possible ones for a given collision depth and given scattering and recoil angles . Impinging ions reach the surface, making an angle θ1, with the inward-pointing normal to the surface. After collision their velocity makes an angle θ1, with the outward surface normal; and the atom initially at rest recoils, making an angle θ1, with this normal. Detection is possible at one of these angles as such that the particle crosses the target surface. Paths of particles are related to collisional depth x, measured along a normal to the surface. This Figure is plane representation of a scattered projectile on the target surface, when both incoming and outgoing paths are in perpendicular to target surface For the impinging ion, length of the incoming path L1 is given by : The outgoing path length L2 of the scattered projectile is : And finally the outgoing path L3 of the recoil