Photodissociation Schumann-Runge continuum

The f1/)) state of atomic oxygen yields the forbidden red-line radiations (6300 and 6364 A) that are prominent in the aurora and dayglow, twilight, and nightglow. In the dayglow, the major source of Of1D) is photodissociation in the Schumann-Runge continuum [180] ... 

Figure 4.14 shows the photodissociation rate coefficient for 02, namely, j0l, as a function of altitude above 30 km. From 30 to 60 km, the Herzberg continuum provides the dominant contribution to j0l At about 60 km, the contribution from the Schumann-Runge bands equals that from the Herzberg continuum above 60 km, the Schumann-Runge bands predominate until about 80 km, where the Schumann-Runge continuum takes over. In the mid-to upper stratosphere, at solar zenith angle = 0°, an approximate value of jo2 is jo2 — 10 9 s. ... 

Reaction (5.16) refers to the Herzberg continuum (230-280 nm), which consists of three bands. It is clear that O3 formation (reaction 5.14) as well as O3 photolysis occurs. Hence, at 30 km (above the peak in the ozone layer), we observe substantial reductions in the amount of radiation received between 225 and 275 nm. The O2 photodissociation in the Schumann-Runge continuum (125-175 nm) directly produces O ( D) and opens many radical reactions (but almost all molecules photodisso-ciates at such hard radiation) this is in altitude of 100-200 km. The solar H Ly-man-a line (121.6 nm) is, through O2 photodissociation, an important source of O ( D) production throughout the mesosphere and lower thermosphere. 

B. Atomspheric photochemistry. The photodissociation of oxygen in sunlight is the major photochemical process occurring in earth s atmosphere. The first intense allowed transition in O., is B - X 32 which occurs at 202.6 nm and is called the Schumann-Runge band system (Section 2.8). It merges intoa continuum beyond 175.9 nm and correlates with one oxygen atom 0(23P) in the ground state and one in the excited state O (2 D)... 

As shown by Johnston and Selwyn (1975), the cross section of N2O varies strongly with temperature. The quantum yield for photodissociation is unity, and the products are N2 and 0(1D). The atmospheric photolysis rate comes predominantly from the absorption of solar radiation in the 02 Herzberg continuum and Schumann Runge bands. Jn2o 9 x 10-7s-1 for A > 175 nm. 

The absorption cross sections (Figure 4.50) for hydrochloric acid (or hydrogen chloride) HC1 were measured by Vodar (1948), Romand and Vodar (1948), and Romand (1949), and later by Inn (1975). The latter measurements extend from 140 to 220 nm, leading to a photodissociation frequency of about 3 x 10-6s-1 at zero optical depth. To calculate Jhci in the atmosphere, the Schumann Runge bands and continuum must be considered, as well as the Herzberg continuum. 

For compounds X other than O2 (or NO) for which the absorption cross section varies slowly with wavelength (continuum) over the Schumann-Runge region (e.g., H20, CH4, CO2), the photodissociation frequency over interval AAj can be derived from... 

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