Energy loss of 18 keV electrons in gaseous T-2 and quench condensed D-2 films

Measurements of the energy loss of East electrons at an energy of 18 keV ha ve 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 = 1 8.6 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 quenc h condensed D-2 differ significantly from the ones for gaseous T-2. We obse rve 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 lo ss (<(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 int erpreted 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 agree ment with the observation.