Lyman-a line

Fig. 12.5. Parts of the spectra of two QSOs (with emission redshifts 2.9 and 3.3 respectively) taken with the 4-m William Herschel Telescope on La Palma with a resolution of about 50kms 1 showing Lyman-a lines with damping wings (column densities N(H i) 2 x 1021 cm 2 or 6M0pc-2 and 8 x 1020 cm-2 or 2.4 Mq pc-2 respectively). After Pettini et al. (1997).

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. 

FIGURE 11. LIF Measurements of the Doppler width of the Lyman a line. The H atoms are produced in the photolysis of HI. 

The use of a Johann-type curved crystal spectrometer and an electron beam ion trap allow us to obtain X-ray specta of the Lyman-a lines of medium Z... 

OGO-5, a few other emissions could be identified (Fig. 4). The OH-emissions at 3090 A which are just barely visible also from the ground turned out to be the strongest lines besides the Lyman a line, and for the first time production rates for this radical could be derived and compared with those for neutral hydrogen. A description of the method by which production rates can be derived from the observed line intensities is e.g. given by KeUer These calculations require the assumption of a certain coma model. The results from several recent comets favor the interpretation that the observed hydrogen and hydroxyl have been formed by dissociation of water. This is roughly demonstrated by the abundances of H and OH given in Table 2. In the case of HjO dissociation one should expect about twice as many H atoms ls OH... 

Figure 4-33. Spectral distribution of the absorption cross section of oxygen from 105 to 135 nm. The cross section exhibits a minimum for the wavelength corresponding to the solar Lyman a line. After Watanabe et ah, (1953).

The absorption spectrum of water vapor consists of a continuum from 145 to 186 nm, diffuse bands from 69 to 145 nm, and a continuum below 69 nm. A maximum in the intensity of a diffuse band (121.8 nm) is located in the vicinity of the Lyman-a line (121.6 nm). Figure 4.39 shows the shape of the absorption cross section of water vapor above 120 nm. Measurements of this cross section were reported by Watanabe and Zelikoff, 1953, Thompson et al. (1963), Laufer and McNesby (1965), Schurgers and Welge (1968), Yoshino et al. (1996b), and Cantrell et al. (1997). In particular, the value of cross section longward of 175 nm. Beyond this wavelength, the penetration of solar radiation is determined by the 02... 

Figure 4.40 shows the contributions of the Lyman a line and the Schumann-Runge bands to the vertical distribution of Jh2o-Photodissociation in the middle atmosphere occurs at wavelengths above about 100 nm, while photoionization occurs for wavelengths less than 98 nm. In the mesosphere and thermosphere, water vapor is photodissociated mainly by the solar Lyman a line, leading to a substantial fraction of the hydrogen atom production at these altitudes. 

Methane only photodissociates in the upper part of the middle atmosphere because its absorption cross section becomes very weak at wavelengths longer than 145 nm (Figure 4.39). The most intense part of the spectrum is located below 130 nm, where the cross section is about 1.9 xl0 17cm2. The Lyman a line dominates the photolysis rate with... 

In the spectral intervals close to the solar Lyman-a line, the solar flux as well as the O2 absorption cross section vary relatively rapidly with wavelength, and parameterizations similar to those used for the Schumann-Runge intervals have been used (Chabrillat and Kockarts, 1997). 

In the D-region, under typical conditions, most of the ionization is due to the effect of the solar Lyman a ray on nitric oxide (Nicolet, 1945). The ionization potential of the NO molecule is only 9.25 eV, which corresponds to a wavelength of 134 nm. In the spectral region of the Lyman a line an atmospheric window exists due to the low absorption cross section of O2 in this interval, and thus the ionizing radiation can... 

For low levels of solar activity, this source of ions is small compared to that produced by the Lyman a line. On the other hand, its contribution becomes more important during disturbed conditions from 0.1 to 0.8 nm, for example, the solar irradiance is about 1000 times more intense for active conditions, and may increase by an additional factor of 100 during solar flares. 

While the reduced mass effect is easily detectable spectroscopically, the small extra shift is well beyond the limits of accuracy of any spectroscopic work which has yet been done. (It might just be detectable in an absolute measurement of the wavelength of the Lyman a line of LrfL) We shall consider in the next chapter the experimental test of the Dirac theory. 

Fig. 9. Fin structure of the Lyman a-line (deuterium) showing the calculated Lamb shift in the ground level. The Doppler width corresponds to a temperature of 80°K. Intervals in cm 1.

H atoms absorbing the Lyman-a line at 121 "5 nm, sensitivity to [H] values in the 10 mole cm range is achieved with a shock tube setup. These methods have not been applied to near-stoichiometric H2-O2 systems, but the rate of formation of H atoms from the H2-O2 chain reaction in Hj-Ar mixtures containing traces (0 1 % and 0-01 %) of O2 has been measured and interpreted successfully. ... 

The Lyman-a line also dissociates with H2O at altitudes between 80 and 85 km, where the supply of water vapor is maintained by methane oxidation even for very dry conditions at the tropospheric-stratospheric exchange region. An increase in HO c follows and thereby a sharp drop in the ozone concentration nears the meso-pause (Chapter 5.3.7 concerns stratospheric O3 cycles). The ozone concentration... 

Another PHg source is benzylphosphane CgHsCHgPHg. Its thermal decomposition starts at 650 C [49] or 600°C [50] and was approximately complete at SSO C [49] or 800 C [50]. The PHg fragment was mass-spectrometrically detected following either electron impact [49] or photoionization with the Lyman-a line [50]. The formation of PHg was confirmed by its reaction with CH3 (from the simultaneous pyrolysis of ethyl nitrite) to CH3PH2 which was mass-spectrometrically detected [49]. 

Note that hwD increases linearly with the frequency coq and is proportional to The largest Doppler width is thus expected for hydrogen (M= 1) at high temperatures and a large frequency o> for the Lyman a line. 

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