Inelastische Streuung langsamer Neutronen an festem und fl�ssigem Wasserstoff

Abstract

Para-Ortho transitions in solid hydrogen above the 2-point have been measured by inelastic neutron scattering. From the scattered spectra the widths of the rotational lines caused by the quadrupole-quadrupole interaction between the ortho-molecules are determined. The full width at half intensity is found to be 1.22_+ 0.07 and 0.80+ 0.07 meV for 68 and 27 percent ortho-concentration respectively in good agreement with theoretical values. The phononspectrum of solid hydrogen could be extracted from the neutron scattering data. The specific heat and the Debye-temperature which were calculated from the phononspectrum are in fairly good agreement with experimental values. Moreover the double differential scattering cross section of liquid hydrogen (T= 19.8 ~ was measured at 87 and 21.8 meV incident neutron energy. In the case of 87 meV energy there is a relatively good agreement with a simple free rotator model. At 21.8 meV the binding of the molecules in the liquid influence the spectrum. Therefore a description of the measured spectrum by a convolution of the phononspectrum of the solid and a simple diffusion model was tried. This model is superior to the gas model but discrepancies between model and experiment still remain.

Collective excitations in liquid para-H2: A neutron polarization-analysis study

“…In agreement with previous observations in liquid normal D 2 , 9,10,23 the wave vector dependence near Q P seems to be well accounted for by a parabolic dependence of the type proposed by Landau in the context of roton excitations in 4 He, with an excitation gap, ⌬ H 2 ϭ1.65 meVХ⌬ D 2 /ͱ2, where ⌬ D 2 ϭ2.26 meV is the value recently reported 23 for liquid normal D 2 near its triple point. The comparison of the results for the two liquids can be understood if we assume the existence of a diffuse Brillouin zone limit near Q P /2, within the time scales probed by neutrons in our experiment, where the dispersive behavior shown in the low-Q limit is reflected.…”

“…3 These latter results are obtained by threeaxis spectrometry with unpolarized neutron beams and follow earlier efforts to characterize the dispersive behavior and the range of existence of collective excitations in this liquid. 4 Even though the liquid hydrogens are relatively close to the quantum limit in their translational degrees of freedom, assuming that the individual molecules behave as distinguishable particles that obey Boltzmann statistics in the liquid phase allows the partition of the coherent and incoherent dynamical scattering functions into self, S s (Q,E), and distinct, S d (Q,E), molecule components. The uncertainty placed on our previous results by the lack of direct experimental access to S d (Q,E) can be alleviated by the use of neutron classical polarization analysis, as was early recognized.…”

Velocity autocorrelation in liquid parahydrogen by quantum simulations for direct parameter-free computations of neutron cross sections

“…E) alone is generally sufficient to account for the scattering of thermal and hot neutrons from this liquid. In this respect, the results on liquid para-H 2 provide a fundamental test of the general quality of possible representations for the single-molecule dynamics.The simplest algorithms for the evaluation of the single-molecule part of the DDCS3,5,16,17,22,23 originate from direct or empirically modified use of, basically, two possible analytical models for S CM,self (Q, E): either the ideal gas (IG) law (see e.g [1]25 …”

“…/sr/meV/molecule] Scattered neutron energy E 1 [meV] (Color online) Experimental DDCS of H 2 at 19.8 K with a 41% ortho concentration, (black squares)22 and CMD+GA results (red dots with thin line). The blue empty circles are the DDCS values calculated by Morishima and Nishikawa 18.…”

Measured Total Cross Sections of Slow Neutrons Scattered by Gaseous and LiquidH22

“…A specific interest in the scattering of neutrons by hydrogen comes from work to develop cold neutron sources; experimental measurements for liquid and solid 1 H 2 and 2 H 2 [8,9] exposed differences between the values for cross sections measured and those calculated using the YK model, particularly for neutrons below 4 meV interacting with 2 H 2 liquid. Several explanations and models [10 -15] have been put forward to explain these discrepancies.…”