Photon-dominated regions

Photon-dominated regions around stars where the intense UV and for-UV radiation fields are responsible for photolysis of all molecules, leaving atoms, some of which are ionised. 

Photon-dominated region (PDR) The region around a bright star where photochemical processes dominate the ion-electron, plasma-like state of matter. 

Teyssier D., Fossd D., Gerin, M., et al. (2004). Carbon budget and carbon chemistry in photon dominated regions. Astronomy Astrophysics 417 135. 

Figure 3.9. The Fourier transform of spectra of Fig. 3.8 Projection PK(a>) of the pure excitonic state on the eigenstates of the coupled system of an exciton K and the effective photon continuum in a 2D lattice, for various values of the wave vector K (KXd). The vertical peak represents a discrete state, whose weight is represented by a rectangle (a-d). For an exciton with K < K0, the continuum band (matter-contaminated photons) dominates the spectrum, with a quasi-lorentzian resonance (a,/). For K > K0, the discrete state dominates (d,e). In the intermediate region (b, c) the spectrum reflects the complicated behavior of its Fourier transform cf. Fig. 3.8. 

Cosmic abundances in the interstellar medium are derived by measuring elemental abundances in stellar photospheres, the atmospheric layer just above the stellar surface. Such measurements indicate the amount of elements available for the formation of molecules and particles. Cosmic dust models indicate that up to 80% of the carbon in the photon-dominated diffuse interstellar medium is incorporated into solid aromatic macromolecules and gaseous polycylic aromatic hydrocarbons (41,30). CO gas and C-based ice species (such as CO, CO2, CH3OH and others) may be responsible for up to -25 % of the carbon in cold dense interstellar regions. 

The rectifier, or diode, is an electronic device that allows current to flow in only one direction. There is low resistance to current flow in one direction, called the forward bias, and a high resistance to current flow in the opposite direction, known as the reverse bias. The operation of a pn rectifying junction is shown in Figure 6.17. If initially there is no electric field across the junction, no net current flows across the junction under thermal equilibrium conditions (Figure 6.17a). Holes are the dominant carriers on the / -side, and electrons predominate on the n-side. This is a dynamic equilibrium Holes and conduction electrons are being formed due to thermal agitation. When a hole and an electron meet at the interface, they recombine with the simultaneous emission of radiation photons. This causes a small flow of holes from the jp-region... 

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