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Single rotational states were populated in vibrationally excited hydrogen by stimulated Raman pumping. The population in H2 X 1∑+g(v″=0,1) and D2 X 1∑+g(v″=0,1) was probed state selectively by tunable vacuum ultraviolet (VUV) laser radiation around λ=110 nm, and the fluorescence induced when exciting the hydrogen molecules in the (0–0), (1–0), (2–0), (3–1), and (4–1) Lyman bands of the (B 1∑+u←X 1∑+g) transition monitored. From a comparison of line heights, the stimulated Raman pumping efficiency is estimated to be 30%–50% in the focal volume. Rotational transitions in X 1∑+g(v″=1) were induced in collisions with H2, D2, and He. State-to-state rotational relaxation rates were measured for the (J″=1→J″=3) transition in H2(v″=1) and for the (J″=2→J″=0,4) transitions in D2(v″=1). These rates were found to be generally higher than the corresponding previously determined ones in ground state hydrogen, in qualitative accord with recent theoretical calculations. A comparison with available theoretical state-to-state cross sections shows that the rates obtained with these cross sections are generally lower than the relaxation rates directly measured in this work. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of A state ammonia molecules (both NH3 and ND3) has been further investigated using the technique of H(D) atom photofragment translational spectroscopy. The resulting NH2(ND2) fragments are observed to carry high levels of internal excitation, the precise disposition of which is sensitively dependent upon the parent v’2 level excited. Dissociation from the v2=0 level of the A state yields ground state NH2(ND2) fragments, primarily in their zero-point level, but with high levels of rotational excitation specifically concentrated about the a-inertial axis; the population distribution over the energetically accessible product rotational levels with N[bar_over_tilde:_approx._equal_to]Ka appears near to statistical. In contrast, dissociation from the parent v’2=1 level yields a markedly inverted fragment internal energy distribution. These different energy disposals have been rationalized via classical trajectory calculations employing the best available ab initio potential energy surfaces for the A and X states of the ammonia molecule. The energy disposal following excitation to the parent v2=2 and 3 levels is found to mimic that observed for, respectively, the v’2=0 and 1 levels. These results provide clear evidence for the importance of anharmonic coupling (whereby an even number of bending quanta are redistributed into stretching motions) in promoting the fragmentation of parent levels with v2≥2. The threshold energy for producing electronically excited NH2(A2A1) fragments is 6.02 eV [∼6.16 eV for ND2(A)]. The present studies of NH3 photolysis suggest that this fragmentation channel opens at threshold and clearly indicate that branching into this channel occurs with much higher quantum yield than hitherto believed. [ABSTRACT FROM AUTHOR]

The photofragmentation dynamics of ammonia molecules following pulsed laser excitation to the two lowest levels (v’2 =0 and 1) of their A 1A‘2 excited state has been investigated by monitoring the time-of-flight spectra of the nascent H-atom products. These spectra show well resolved structure. Analysis of this structure confirms recent revised estimates of the quantity D00 (H–NH2) (4.645±0.01 eV) and reveals that the majority of the accompanying NH2 (X 2B1) fragments are formed vibrationally unexcited, but with high levels of rotational excitation specifically concentrated about the a-inertial axis. The detailed energy disposal is sensitive to the initially excited parent vibronic (and even rovibronic) level: the NH2 (X) fragments resulting from photodissociation via the v’2 =1 level of NH3 (A) carry a higher level of excitation of the N=Ka rotational levels, which show an inverted population distribution. We also describe the results of trajectory calculations employing the recently reported [M. I. McCarthy et al., J. Chem. Phys. 86, 6693 (1987)] ab initio potential energy surfaces for the A and X states of ammonia. These provide a detailed rationale for the experimentally observed energy disposal and highlight the massive influence on the eventual fragmentation dynamics of the conical intersection between these surfaces along the H–NH2 dissociation coordinate. [ABSTRACT FROM AUTHOR]

This is a review of those experiments in the area of Fundamental Physics that are either approved by ESA and NASA, or are currently under development, which are to be performed in the microgravity environment of the International Space Station. These experiments cover the physics of liquid Helium (SUE, BEST, MISTE, DYNAMX, and EXACT), ultrastable atomic clocks (PHARAO, PARCS, RACE), ultrastable microwave resonators (SUMO), and particle detectors (AMS and EUSO). The scientific goals are to study more precisely the universality properties of liquid Helium under microgravity conditions, to establish better time standards and to test the universality of the gravitational red shift, to make more precise tests of the constancy of the speed of light, and to measure the particle content in space directly without disturbances from the Earth's inner atmosphere. [ABSTRACT FROM AUTHOR]

This paper reviews the opportunities for microgravity and Earth-based measurements of the thermal conductivity λ( t, L) of4He confined in cylindrical geometries of radius L with axial heat flow at temperatures near the bulk superfluid-transition line T λ ( P) ( t is the reduced temperature T/ T λ −1). It provides an evaluation of existing data for L=1 µm near T λ at saturated vapor pressure (SVP), and uses these to derive a scaling function for the resistivity R( t, L)=1/ λ( t, L). The purpose of future measurements over a wide range of L and of the pressure P will be to test the applicability of this function. In the present paper the scaling function is used to assess quantitatively the effect of gravity on potential Earth-based measurements. It is found that the gravity effect for R is particularly severe below T λ . For typical three-mm-high samples at SVP, values of L significantly larger than 8 µm can only be investigated fully in micro-gravity. At higher pressures the gravity effect is larger. At 30 bar, three-mm samples with L≳4 µm require microgravity for measurements below T λ ( P). Modern thermometry has sufficient resolution to permit quantitative measurements in micro-gravity of the anticipated finite-size effect for values of L as large as 50 µm. [ABSTRACT FROM AUTHOR]