We demonstrate a possibility that helium surface electrons at cryogenic temperatures can be used as a new source of very low energy electrons. Since both electrons (<$E roman e sup ->) and hydrogen atoms (H) are bound on liquid helium surface, two-dimensional mixture gas of these two species is available on the surface. We found that low energy collision of <$E roman e sup -> and H drives electron attachment to form a negative hydrogen ion (<$E roman H sup ->) in the mixture. From our temperature dependence measurement of the reaction rate, it was found that another H atom participate in the reaction. Namely, the reaction is expressed as <$E roman {H~+~H~+~e sup - ~symbol О~H sup -~+~H}>. Possible reaction mechanisms are discussed in terms of direct three-body process and dissociative attachment process. Measurements in applied magnetic field (B) show that the reaction rate coefficient is suppressed as ~ <$E B sup -2>. This implies that electron spin singlet collision is relevant for electron attachment.