Generation, photonics

Light is generated in semiconductors in the process of radiative recombination. In a direct semiconductor, minority carrier population created by injection in a forward biased p-n junction can recombine radiatively, generating photons with energy about equal to E. The recombination process is spontaneous, individual electron-hole recombination events are random and not related to each other. This process is the basis of LEDs [36]. 

Modem instrumentation has improved substantially in recent years, which has enabled the measurement of XPS spectra of superior resolution necessary to reveal the small BE shifts present in highly covalent compounds such as those studied here. In a laboratory-based photoelectron spectrometer, a radiation source generates photons that bombard the sample, ejecting photoelectrons from the surface that are transported within a vacuum chamber to a detector (Fig. 2). The vacuum chamber is required to minimize the loss of electrons by absorption in air and, if a very high quality vacuum environment is provided (as is the case with modem instruments), the surface contamination is minimized so that the properties of the bulk material are more readily determined. 

Chemiluminescence sensors utilize chemicals that generate photons, e.g. 

Some electrochemically active substances that can generate photons on an electrode surface are suitable labels for homogeneous immunoassays. A labelled antigen exhibits an electrochemical reactivity and produces luminescence, but when it is immunochemically complexed, the labelled antigen loses its electrochemiluminescent properties. One optical immunosensor for homogeneous immunoassays was assembled by spattering platinum on the end surface of an optical fibre. Spattered platinum maintains optical transparency and functions as an electrode. An optical electrode efficiently... 

The use of lanthanides are common for optical purposes because of their narrow and sharp bands, and distinguishable long lifetimes, accomparied by low transition probabilities due to the forbidden nature of the transitions [10-13]. Thus chromophoric sensitization of ligand to metal has been subjected to numerous theoretical and experimental investigations [14—16]. However, only limited classes of organic-lanthanide complexes have been developed and shown to display nonlinear processes [17-19]. Common nonlinear processes from lanthanide complexes include harmonic generation, photon up-conversion and multiphoton absorption induced emission. 

All of these ionization sources are classical sources used also in optical spectroscopy. The only fundamental difference is that these sources are not used for atomization/excitation processes to generate photons but to generate ions. 

A source generating photons, most commonly a lamp with hollow cathode... 

Chemical reactions provide the energy to generate photons in chemiluminescence. These chemical reactions often involve oxygen. 

The rate of detected pulses equals the rate of photons striking the photocathode times Q. In other words, Q is the fraction of photons that ejects photoelectrons and results in an output pulse. The dark signal results from thermal ejection of electrons from the photocathode. Since Raman spectrometers often operate in the red and near infrared wavelength regions, the work function of the photocathode surface must be quite low. This small work function makes it difficult to prevent dark electron generation. Photon counting... 

The x-ray tube assembly is a simple and maintenance-free device. However, the overall efficiency of an x-ray tube is very low - approximately 1% or less. Most of the energy supplied to the tube is converted into heat, and therefore, the anode must be continuously cooled with chilled water to avoid target meltdown. The input power to the sealed x-ray tube ( 0.5 to 3 kW) is therefore, limited by the tube s ability to dissipate heat, but the resultant energy of the usable x-ray beam is much lower than 1% of the input power because only a small fraction of the generated photons exits through each window. Additional losses occur during the monochromatization and collimation of the beam (see section 2.3). 

The conversion dynode of a photomultiplier detector generates electrons that impinge on a phosphor, which subsequendy generates photons that are detected... 

Figure 2. Second-harmonic generation involving two incident photons of frequency u) and a sum-frequency generated photon of frequency 2w...

The next survey of Phosphor Devices for Electronic Display will cover those in which generated photons are directly involved in phosphor excitation. 

As shown in Section 11in TOP instruments, positive ions are produced periodically by bombardment of the sample with brief pulses of electrons, secondary ions, or laser-generated photons. The ions produced in this wav are then accelerated into a lield-free drift tube by an electric field pulse of 111 to 10 V (see Figure 20-15). Separation ol ions by mass occurs during the transit of the ions to the detector located at the end of the tube. Because all ions entering the tube hare the same kinetic eneres, their velocities in the tube varv in-... 

Despite its successes, Bohr s theory did not resolve the fundamental issues that lie at the heart of the quantum theory of Planck and Einstein What is behind the quantization of oscillators and atoms What happens when a system changes its state, resulting in the creation of a photon How can the generated photon behave like a particle under certain conditions and like a wave under other conditions ... 

The shape of the IRF is remarkably clean, without any secondary peaks. There is, however, a slow tail in the response measured at 785 nm. This diffusion tail" is typical for APDs operated at long wavelengths. It is caused by photons which penetrate the depletion layer and generate photons in the neutral regions nearby. The tail has negligible amplitude at 468 nm see Fig. 6.57, right. 

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