Lyman radiation

Cross sections for ionization excitation of He for impact of (6.1-12.1)i o e , p, and projectiles have been obtained recently by Pedersen and Folkmann [6.22]. They detected the emission of Lyman radiation (np- ls) from the He ions created in the collisions. An analysis using eq. (6.4) gave experimental values of double ionization. Pedersen and Folkmann calculated Oj and o-n, using the Weizsacker-Williams method, and found good agreement with the experimental values. Furthermore, the calculations showed that, as for double ionization, close collisions contribute substantially to the IE process, and that the experimentally obtained Oj , amounted to (50-75)% of the theoretically estimated maximum possible value [(o-j -Od)tr,i] l The interference gave rise to a difference between the electron- and proton-impact cross section so that [Pg.182]

It is possible to change the conditions in the helium discharge lamp so that the helium is ionized predominantly to He (He II). The radiation is due mainly to the n = 2 — n = transition of He II (analogous to the first member of the Lyman series of the hydrogen atom in Figure 1.1) at 30.4 nm with an energy of 40.81 cY A thin aluminium foil filter can be used to remove any He I radiation. 

For ion TOF measurement a probe laser was used to ionize reaction products in the reaction zone. The (1 + F) resonance-enhanced multiphoton ionization (REMPI) method was adapted for H-atom detection. The necessary vacuum ultraviolet (VUV) radiation near 121.6 nm (for Lyman-a transition) can readily be generated by a frequency-tripling technique in a Kr cell.37 The sensitivity of this (1 +1 ) REMPI detection scheme is extremely high owing to the large absorption cross-section of Lyman-a transition,... 

The primordial Li abundance was sought primarily because of its ability to constrain the baryon to photon ratio in the Universe, or equivalently the baryon contribution to the critical density. In this way, Li was able to complement estimates from 4He, the primordial abundance of which varied only slightly with baryon density. Li also made up for the fact that the other primordial isotopes, 2H (i.e. D) and 3He, were at that time difficult to observe and/or interpret. During the late 1990 s, however, measurements of D in damped Lyman alpha systems (high column-density gas believed to be related to galaxy discs) provided more reliable constraints on the baryon density than Li could do (e.g. [19]). Even more recently, the baryon density has been inferred from the angular power spectrum of the cosmic microwave background radiation, for example from the WMAP measurements [26]. We consider the role of Li plateau observations post WMAP. 

In gas clouds containing one or more hot stars (7 cn > 30 000 K), hydrogen atoms are ionized by the stellar UV radiation in the Lyman continuum and recombine to excited levels their decay gives rise to observable emission lines such as the Balmer series (see, for example, Fig. 3.22). Examples are planetary nebulae (PN), which are envelopes of evolved intermediate-mass stars in process of ejection and... 

Lyman-alpha radiation spect Radiation emitted by hydrogen associated with the spectral line in the Lyman series whose wavelength is 121.5 nanometers. iT-man al-fo, rad-e a-sh3n ... 

It is seen from the above that the energy of the molecule in the excited state must be enough to provide for the energy of dissociation, the kinetic energy of the atoms relative to each other and for the energy which is radiated as continuous spectrum. That a similar process occurs in absorption has been shown by Dieke and Hopfield. The work of Witmer on the analysis of bands in the Lyman region gives as the heat of dissociation a value of 4.34 volts and that found by Dieke and Hopfield was 4.38 volts. 

Emami B, Lyman J, Brown A, et al. Tolerance of normal tissue to therapeutic irradiation. Int J Radiat Oncol Biol Phys 1991 21(1) 109-122. 

Other spectral regions are also important because the detection and quantification of small concentrations of labile molecular, free radical, and atomic species of tropospheric interest both in laboratory studies and in ambient air are based on a variety of spectroscopic techniques that cover a wide range of the electromagnetic spectrum. For example, the relevant region for infrared spectroscopy of stable molecules is generally from 500 to 4000 cm-1 (20-2.5 /Am), whereas the detection of atoms and free radicals by resonance fluorescence employs radiation down to 121.6 nm, the Lyman a line of the H atom. 

The ionization of NO by the Lyman-a line is the main source of ions in the D region. The photodissociation of NO in the upper atmosphere occurs from the /t2Z + (F > 4), B2n (c > 7), and C2n (F > 0). The dissociation rate of NO by the solar radiation is proportional to the integrated absorption coefficient of various bands (that is, the oscillator strength). From Table V 4 it can be seen that absorption by the /if (12,0) and 6 bands is most important in leading to photodissociation. 

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... 

In order to study the photochemical action of solar radiation on tropospheric, stratospheric, and mesospheric constituents, the solar spectrum must be divided in various ranges.1 The radiation at wavelengths less than 100 nm, which is absorbed by nitrogen and oxygen in the thermosphere above 100 km, leads essentially to ionization processes and is, therefore, not considered there. Only X-rays of wavelengths less than 1 nm can penetrate into the atmosphere below 100 km, and lead indirectly to the dissociation of molecular constituents. Nevertheless, their principal role is the photoionization in the D region of the ionosphere below 100 km where the solar line Lyman-a at 121.6 nm ionizes the nitric oxide molecule, NO. 

The first crude spectroscopic measurement performed on excited state positronium was the identification of the 243 nm Lyman-o radiation emitted in the 2P-1S transition. The first observation of this line was... 

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 ... 

Fig. 1. Four-wave mixing scheme to produce radiation at Lyman-a in mercury vapor...

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