In recent years, the collisional redistribution of radiation and collision-induced broadening of Rydberg atomic spectral lines by buffer gas perturbation have aroused the renewed interest. Rydberg atoms having a large dipole moment and long lifetime can interact with each other coherently for relatively long time, which makes them a potential candidate for quantum information processing. Besides, collisional redistribution has an important potential application in laser cooling and trapping. n Based on previous experimental data, in this paper, two-nondegenerate four-wave mixing (NFWM) for studying atom collision, composed of two-photon resonant NFWM and collisional redistribution NFWM, is reported. The spectrum variation of the two-NFWM affected by the pressure, temperature, detuning and collision-broadening rate coefficient is analyzed. n The principle of two-NFWM involving three incident beams is explained as follows. Consider two-NFWM in a|0⟩-|1⟩-|2⟩cascade three-level system, where states between|0⟩and|1⟩and between|1⟩and|2⟩are coupled by resonant frequenciesΩ1 andΩ2, respectively. Beam 1 with frequencyω1 propagates along the direction opposite to the direction of beam 2, beams 2 and 2′have the same frequencyω2, and between their directions there exists a small angle. Assuming thatω1≈Ω1 andω2≈Ω2 so thatω1 drives the transition from|0⟩to|1⟩whileω2 drives the transition from|1⟩to|2⟩, the simultaneous interactions of atoms with beams 1 and 2 will induce atomic coherence between |0⟩ and |2⟩ through two-photon excitation. This coherence is probed by beam 2′, and as a result a two-photon resonant NFWM signal of frequency ω1 is generated in the direction almost opposite to the direction of beam 2′. To avoid strong absorption at the resonant frequency of transition from|0⟩to|1⟩, here the wavelength of beam1 is detuned from the exact resonance. An atom population of level |1⟩ caused by collisional redistribution can be induced when a certain buffer gas pressure is imposed. The collisional redistribution NFWM process also exists in this case. Beam 2 drives the transition from|1⟩to |2⟩ to induce an atomic coherence which is probed by beam 2′ for giving rise to an atomic population grating. A collisional redistribution NFWM signal propagating along the same direction as the two-photon resonant NFWM signal is generated when beam 1 is scattered by the grating. Much information about atomic collisions can be obtained by analyzing the two NFWM signals. n In a cascade three-level system composed of ground state, intermediate state and Rydberg state, and the two-NFWM can be used to investigate not only the broadening and shifting of the Rydberg level but also the collisional redistribution of the intermediate state. Unlike other experiments studying the pressure dependence of the longitudinal relaxation rate of atom states, this technique is a purely optical coherent means, and can measure the transverse relaxation rate Γ20 between Rydberg state and ground state as well as the pressure dependence of the transverse relaxation rateΓ21 between Rydberg state and intermediate state.%在已有实验的基础上，提出了由双光子共振非简并四波混频和碰撞再构非简并四波混频组成用于研究原子碰撞效应的双非简并四波混频理论。分析了缓冲气压、温度、共振失谐和碰撞展宽系数对双非简并四波混频谱线的影响。在由基态、中间态和激发态组成的级联三能级系统中，双非简并四波混频可同时研究碰撞引起的激发态谱线展宽和碰撞引起的中间态能级再分布现象。与测量纵向弛豫碰撞展宽的传统实验方法不同，本文方法是一种纯光学的相干测量技术，可以同时测量激发态与基态间的横向弛豫Γ20和中间态与基态间的横向弛豫Γ21的碰撞展宽。
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