Lyman-a radiation

When A and B are identical atomic species (e.g., H + -H259 or He+-He260), radiative emissions may result from neutral target excitation as well as from electron capture by the ionic projectile into the same excited state. Both processes yield Lyman a radiation in the case of H+-H collisions,259... 

Fig. 3. The absorption spectrum for solar radiation in the Earth s atmosphere 19X On the ordinate is plotted the altitude at which the radiation intensity is reduced by a factor e 1 from its unattenuated value. The species predominantly responsible for the absorption in the various wavelength ranges are as indicated. The wavelength of the H Lyman-a radiation closely coincides with a window in the O2 absorption spectrum...

Continuous wave coherent Lyman-a radiation has recently become available [85] so that laser cooling or sensitive shelving spectroscopy of magnetically trapped hydrogen atoms is coming within reach. The ability to work with a small number of atoms is of particular interest for laser spectroscopy of antihydrogen, a goal pursued by the ATRAP and ATHENA collaborations at CERN [8]. 

In the case of the 1S-3S transition in hydrogen and for an estimated velocity of v=3km/s, the shift is Av =l4A kHz. We can t measure the velocity distribution by observing the Doppler broadened 1S-2P transition at 121 nm with a colinear laser beam, because the production of Lyman-a radiation is very difficult. In 1991 a method to compensate or at least to measure this effect was proposed by F. Biraben [7]. The basic idea is to apply a transverse magnetic field B in the atom-laser interaction region. This field has two effects ... 

Figure 5. H atom kinetic energy distribution from the photolysis of PH3 at 193.3 nm as deduced from Doppler profiles (using Lyman-a radiation). The two curves bound the range of acceptable fit to the measured profiles. [Adapted from Z. Xu, B. Koplitz, S. Buelow, D. Baugh, and C. Wittig, Chem. Phys. Lett. 127, 534(1986).]...

The D-region is located between about 60 km and 90-95 km. In this layer, ionization results mostly from photoionization of NO by Lyman a radiation (121.6 nm). High energy cosmic rays also contribute to the ionization of O2 and N2 below 70 km. Radio waves are readily absorbed in the D-region, as will be discussed in Section 7.6. 

In the absence of solar radiation, the ionization is due primarily to the precipitation of high energy particles (e.g., see Vampola and Gorney, 1983), diffuse Lyman a radiation and cosmic rays. The effect of X-rays emitted by certain stars (Sco XR1, GX333-2.5) plays only a minor role. 

If the microwave discharge is pulsed the Lyman a-radiation emitted from the excited H-atoms can be detected behind a time gate as a function of the time de-... 

In interstellar regions subjected to fast shocks, the radiation field may have a very high intensity at 1215.6 A, because of recombination and collisional excitation of atomic hydrogen leading to Lyman a radiation (Hollenbach and McKee 1979 Neufeld and Dalgarno 1988). It is thus important to know whether a molecule has a photodissociation or photoionization channel at this wavelength. Molecules such as Hj, CO and Nj cannot be destroyed by Lyman a radiation, whereas other species like OH and H2O have cross sections of a few times 10 cm at 1215.6 A and are thus easily dissociated. Another strong peak in the radiation field in shocks is provided by the C III resonance line at 977 A. 

In the absence of external radiation, however, the theory predicts that if the atom is prepared in this state at time t=0 then it will remain in that state indefinitely. This is clearly incorrect, since hydrogen atoms in the 2p state are observed to decay rapidly to the ground state Is with the spontaneous emission of Lyman a radiation, A = 1216 8. 

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