Radiation incoherent

Finally, if one has a condition with incoherent radiation of a small band width Av exciting a broad absorption band with a(v + A ) one finds ... 

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

The system is prepared at t=0 in the quantum state Pik> and the question is how to calculate the probability that at a later time t the system is in the state Fjn>. By construction, these quantum states are solutions of molecular Hamiltonian in absence of the radiation field, Hc->Ho Ho ik> = e k Fik> and H0 Pjn> = Sjn xPJn>. The states are orthogonal. The perturbation driving the jumps between these two states is taken to be H2(p,A)= D exp(icot), where co is the frequency of the incoherent radiation field and D will be a time independent operator. From standard quantum mechanics, the time dependent quantum state is given by ... 

The computation of far-field radiation from a collection of incoherently radiating dipoles is in general quite a complicated problem. To calculate the angular dependence of the far-field intensity, the volume distribution of excited states must first be obtained, which, as we have seen, depends on the volume distribution of the absorbers and the electromagnetic field which stimulates them. The fields in turn depend on the frequency and linewidth of the exciting light source. Then the emission problem for the excited-state distribution (both spatial and frequency) must be solved including reorientation and depolarization effects. 

Compton Normalization (CN) is an internal standard, in which spectra are normalized to the Compton peak, which is produced by incoherent backscattering of the source radiation and is present in every sample. The intensity of the incoherent radiation backscatter reflects... 

This assumes that each photon placement is independent of all others, as would be true for an incoherently radiating source. Stated as an equality,... 

The most important difference between excimer and standard UV lamps is that the former are incoherent radiation sources and can therefore be used for large area applications. Different wavelengths can be produced by choosing different gas fills in the gap between the quartz tubes. 

Artificial sources of incoherent radiation functioning continuously involve ... 

Sources of pulsed incoherent radiation are so-called flash lamps, which produce short light pulses of broadband continuous-wave characteristic. Commonly, an intense pulse of short duration is used to generate sufficient concentration of a transient species suitable for spectroscopic observation. 

Incoherent radiation Not having the properties of the coherent radiation. 

Luminescence processes usually produce incoherent radiation. The population of the energy states 2 and 1, according to Boltzmann s law, may be described by a temperature. These states are not usually in thermal equilibrium with the other energy states of the ensemble, which normally correspond to a low temperature. 

The subjects dealt with in these papers have lost none of their interest or attraction for investigators since our conference was held. The real cause of superconduction still remains a mystery. On the other hand, Heisenberg has quite recently found a method, based on the Thomas-Fermi distribution, for calculating the atom form factor for the incoherent radiation, a method which is just as simple as that previously known for the coherent part of the radiation. Many other problems will suggest themselves to the readers of this book. That, indeed, is the object of its publication, and I accordingly welcome the extension of its influence by the present English translation. 

Radiation so scattered is called Compton modified radiation, and, besides having its wavelength increased, it has the important characteristic that its phase has no fixed relation to the phase of the incident beam. For this reason it is also known as incoherent radiation. It cannot take part in diffraction because its phase is only randomly related to that of the incident beam and cannot therefore produce any interference effects. Compton modified scattering cannot be prevented, however, and it has the undesirable effect of darkening the background of diffraction patterns. 

In the general case, a beam of partially polarized incoherent radiation can be considered a mixture, in unequal amounts, of two (incoherent) beams polarized in orthogonal directions. 

In the case of incoherent radiation, which would then indicate inelastic scattering or also Compton scattering, the wavelengths of the radiations emitted by the various electrons are different, depending on what orbital is considered. Therefore, there is no interference. The intensity scattered by the entire atom is the sum of the intensities scattered by each of the electrons and it is written ... 

Excimer lamps were selected to study the low fluence irradiation region, where linear (no ablation) photochemistry is taking place. This is the fluence range (e.g., insert in Fig. 47 of the previous chapter), where a linear relation between reaction products and laser fluence is observed. This may correspond to the range of linear photochemistry, i.e., below the threshold of ablation (see, e.g., Figs. 25 and 26), or the so-called Arrhenius tail. The excimer lamps emit at the same wavelengths as the excimer lasers, but with incoherent radiation, and in quasi-CW mode. The peak photon fluxes of the lamps are low compared to the excimer laser, suggesting that multiphoton processes are not important. Thin films of the triazene polymer on quartz substrates were irradiated with the excimer lamps under different conditions, i.e., in Ar, air, and 02. 

Lasers with short pulses are not used in Raman spectrometers, mainly because the detectors in Raman spectrometers are tuned to high sensitivity. Such detectors are very easy to saturate and this is a case where short and intense laser pulses are employed for excitation of Raman scattering. It must be noted, that gas lasers are not perfect sources of monochromatic radiation. Together with intense coherent radiation such lasers produce weak incoherent radiation, caused by a different transition between electronic energy levels of the gas. The intensity of this incoherent and noncollimated radiation can be suppressed by increasing the distance between the laser and the sample, by placing a spatial filter (consisting of two lenses and a pinhole) or a narrow-band filter (usually an interference filter) into the laser beam. 

In order to illustrate the advantages of absorption spectroscopy with tunable lasers, we first compare it with conventional absorption spectroscopy, which uses incoherent radiation sources. Figme 1.1 presents schematic diagrams for both methods. 

An electron beam that traverses an undulatory magnetic field emits incoherent radiation. Indeed, this is the mech-... 

For incoherent radiation one wouid have to introduce a phase that depends on the frequency of the radiation. 

Fig. 9.3 Evolution of the population of the upper level in a two-level system, driven by a coherent radiation field (thin fine), by an incoherent radiation field (heavy line), and by an adiabatic-passage process (dashed line). The time scale is in units of the Rabi oscillation period.

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