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A new method for measuring absolute electron-impact differential cross sections (DCS) for elastic scattering in gases is described. The technique uses a well-defined crossed-beam scattering geometry in which the relative target densities of an unknown and a secondary standard gas are accurately determined by a calibrated flowmeter and a pressure gauge. The method is applied to the measurement of elastic scattering from H2 (which, in the present work, also includes rotational excitation) relative to elastic scattering from He, the secondary standard. The relative DCS for H2 are then placed on the absolute scale by using recently measured absolute elastic DCS for He. Measurements for H2 are reported at incident electron energies of 3–75 eV, and in the angular range 20°–135°. Differential and integral cross sections at each energy are compared with previous measurements and recent theoretical calculations.

Using an electron-beammolecular-beam apparatus and employing the relative flow technique, ratios of the differential elastic scattering cross sections (DCS s) of H2 to He were measured at incident electron energies of 15 -100 eV and angular range of 10'-125'. From these ratios, the absolute elastic DCS's for H2 were determined by normalization to accurate, available elastic DCS's of He. Since pure rotational structure was not resolved in this work, the DCS's reported are the sum of elastic and rotational excitations of H& at room temperature. The reliability of the relative flow normalization to He was checked at each energy and angle by performing similar elastic DCS measurements on Ne |'for which the cross sections are known). The resulting absolute Ne DCS's were found io be in good agreement (within 10%) with the Ne elastic DCS s measured previously [D. F. Register and S. Trajmar, Phys. Rev. A 29, 1785 {1984)]. From the DCS's, integral and momentumtransfer cross sections were calculated. The present results are compared with other recent measurements.

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