Tbe thereotical work of Matsnzawa based on the essentially free electron model suggests that the rate constant for collisional ionization of a high rydberg atom is essentially the same as that for attachment of free electron haying the same momentum distribution. In the present work we test the essentially free electron model by comparing rate constants measured for the following reaction with those expected on the basis of free electron studies. The data are in good agreement indicating that studies of Rydberg atom collisions can provide information on very-low-energy electron interaction.