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Abstract: A newly developed molecular beam scattering apparatus, featuring a flat liquid jet, has been designed to investigate molecular interactions at the gas-liquid interface. The flat liquid surface, coupled with a rotatable mass spectrometer that offers mass, energy, and angular selectivity, provides unique insights into this crucial chemical boundary. The instrument's feasibility is demonstrated through studies on neon gas evaporation and scattering from liquid dodecane. To explore more complex interactions, polyatomic gases, including deuterated methane and water, are also investigated, offering a deeper understanding of energy transfer at the interface, with the potential for internal excitation of the scatterer. The energetic distributions observed in the scattering experiments are modeled using a bimodal system consisting of an impulsive scattering (IS) component, which peaks at the specular angle, and a thermal desorption (TD) component, which follows a Maxwellian distribution. Energy transfer between the gas and liquid surface is described using a "soft-sphere" kinematic model, revealing that molecules experience greater internal energy excitation through rotational modes compared to atoms. These experiments have provided fundamental insights into the gas-liquid interface through a collision-based model. In the near future, we plan to use this apparatus to investigate both nonreactive and reactive scattering from a water flat jet, which could serve as a useful analogue to atmospheric environments.