Energy loss of 18 keV electrons in gaseous T2 and quench condensed D2 films

Measurements of the energy loss of East electrons at an energy of 18 keV have been performed on molecules of hydrogen isotopes, gaseous T-2 and frozen Da. Whereas in the case of gaseous T-2 the values of total inelastic cross-section (sigma(tot,) (gaseous) = (3.40 +/- 0.07) x 10(-18) cm(2) for E = 18.6 keV), average energy loss (<(epsilon)over bar>(gaseous) = (29.9 +/- 1.0) eV) and peak position of the energy loss spectra (epsilon(1,gaseous) = 12.6 eV) agree well with the expectations, the corresponding values for quench condensed D-2 differ significantly from the ones for gaseous T-2. We observe a significant lower total inelastic cross-section (sigma(tot,) (solid) = (2.98+/-0.16) x 10(-18) cm(2), for E = 18.6 keV) larger average energy loss (<(epsilon)over bar>(solid) = (34.4+/-3.0) eV) and higher peak position (epsilon(1,) (solid) = (14.1(-0.6)(+0.7)) eV). These differences may be interpreted in terms of changes of the final state spectrum. A CI calculation for a D-2 cluster shows indeed a clear shift of the excited states in agreement with the observation.