Atomic and Molecular Hydrogen

FIGURE 18 Illustration of the formation of a P Cygni profile (emission line near the rest wavelength, with blue-shifted absorption) in a spectral feature, resulting from high-velocity outflow of gas from a stellar atmosphere. 

The Copernicus satellite also made the first direct observations of deuterium ( heavy hydrogen ) in the interstellar medium, both in atomic form and in the molecule HD (Fig. 19). Accurate knowledge of the ratio of D to H in the interstellar gas is of relevance to theories of the origin and evolution of the universe. 

Currently, observations of interstellar molecular hydrogen, as well as of atomic hydrogen, deuterium, and HD, 

INTERSTELLAR MOLECULAR HYDROGEN ABSORPTION LINES TOWARD ZETA OPHIUCHI 

FIGURE 19 Copernicus high-resolution (0.005 nm) spectrum of the star Ophiuchi showing absorptions due to interstellar molecular hydrogen (H2) and HD. [Spitzer, L., and Jenkins, E. B. (1975). Annu. Rev. Astron. Astrophys. 13, 133.] 

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The name hydrogen refers to both atoms and molecules. To minimize confusion, we refer to atomic hydrogen when we mean hydrogen atoms and molecular hydrogen when we mean hydrogen molecules. 

Shocks were first invoked to explain the unusually high abundance of the radical CH+in diffuse clouds (n 100 cm-3 T = 100 K), where standard chemical models predict far too little of this substance which is reactive both with electrons and with atomic and molecular hydrogen.128 In a high temperature medium, however, the reaction,... 

These may include atomic and molecular hydrogen, as well as hydrogen bonded at oxygen-related clusters. Passivation of the 450°C thermal donors has also been investigated as a function of cluster size by Johnson and Hahn (1986) and Johnson et al. (1986). 

Fig. 8.14. Surface densities of atomic and molecular hydrogen in the Galaxy as a function of Galactocentric distance the Sun is at 8.5 kpc. Beyond that distance, the deduced surface density depends on the assumed law of Galactic rotation KBH refers to Kulkarni, Blitz and Heiles (1982). Assuming their rotation curve, the total gas surface density falls by about a factor of 2 between 4.5 and 13 kpc, corresponding to an exponential fall-off with a scale length a l of about 12 kpc. After Dame (1993). Courtesy T.M. Dame.

The very light gases, atomic and molecular hydrogen, which have their origin in the water vapor in the mesosphere, are subject to vertical diffusion and are transported to the upper levels of the atmosphere, where they form the terrestrial hydrogen corona, and from where finally they escape continually into interplanetary space. 

Thermodesorption mass-spectra of water, atomic and molecular hydrogen were taken on a MI 1201 mass-spectrometer in the temperature range from 20 to 600 °C [2],... 

On the Fig. 2 we present kinetic curves of hydrogen accumulation at irradiation of the water-ethanol mixtures, containing powdered mesoporous Ti02, preliminary exposed to HTT and calcination at 350 - 500 °C, as well as commercial Ti02 Degussa P25, used without any pre-treatment. It could be seen from the Fig. 2, that the photoprocess proceeds with some acceleration at the initial stage of irradiation ( 0.5 h). This phenomenon may be explained by the formation of nickel nanoparticles and their saturation with atomic and molecular hydrogen (reactions (4)-(8)). 

The reaction chemistry of smaller IS negative ions is receiving continued attention. Recently, Bierbaum and co-workers have used the Boulder FA-SIFT to monitor the reactivity of various C and C H ions with atomic and molecular hydrogen [245]. The absence of any reaction with H2 is important given the predominance of hydrogen as an IS molecule, while for C7, CR, Cy and C,r, associa-... 

Fig. 4. Fraction of protons and neutral hydrogen projectiles in a hydrogen beam when colliding with atomic and molecular hydrogen targets as a function of the projectile energy. The experimental points are from Allison and Garcia-Munoz [43].

The quantum efficiencies for these different channels vary with wavelength. The values quoted in Chapter 4 (see also Brownsword et al., 1997) for Lyman a excitation need to be confirmed. CH2 produced by two of these channels reacts rapidly with 02 to form CH20 + O, HCO + OH, CO + H20 and C02 + H2 (or H + H). The photolysis of methane produces atomic and molecular hydrogen. However, since the amount of methane in the upper part of the middle atmosphere is small, CH4 photolysis can generally be neglected in the global atmospheric methane budget. 

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