Electromagnet absorption spectrum

As described in earlier sections, any two material bodies will interact across an intermediate substance or space. This interaction is rooted in the electromagnetic fluctuations— spontaneous, transient electric and magnetic fields—that occur in material bodies as well as in vacuum cavities. The frequency spectrum of these fluctuations is uniquely related to the electromagnetic absorption spectrum, the natural resonance frequencies of the particular material. In principle, electrodynamic forces can be calculated from absorption spectra. 

For polarized measurements the relationship between refractive index and absorbance is critical. Directional absorbance changes lead to birefringence, which in turn will affect strongly the nature of polarized light. The Kramers-Rronig relations come into play for cases like this one (15). They relate the complete electromagnetic absorption spectrum of a material to its refractive index at all frequencies and vice versa. A form of the relation is... 

Microwave spectroscopy began in 1934 with the observation of the -20 GHz absorption spectrum of ammonia by Cleeton and Williams. Here we will consider the microwave region of the electromagnetic... 

Absorption spectrum (Section 12.5) A plot of wavelength of incident light versus amount of light absorbed. Organic molecules show absorption spectra in both the infrared and the ultraviolet regions of the electromagnetic spectrum. 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

When electromagnetic radiation passes through transparent matter, some of it is absorbed. Strong absorption will occur if there is a close match between the frequency of the radiation and the energy of one of the possible electronic or molecular absorption processes characteristic of the medium. A plot of absorbance (A) against wavelength (X) or frequency (v) for a particular material is termed an absorption spectrum. The complexity of the absorption spectrum depends on whether atomic (simple, with a few sharp absorption bands) or molecular (complex, with many broad bands) processes are responsible. 

Since the photophoretic force depends on the electromagnetic absorption efficiency Q y , which is sensitive to wavelength, photophoretic force measurements can be used as a tool to study absorption spectroscopy. This was first recognized by Pope et al. (1979), who showed that the spectrum of the photophoretic force on a 10 foa diameter perylene crystallite agrees with the optical spectrum. This was accomplished by suspending a perylene particle in a Millikan chamber with electro-optic feedback control and measuring the photophoretic force as a function of the wavelength of the laser illumination. Improvements on the technique and additional data were obtained by Arnold and Amani (1980), and Arnold et al. (1980) provided further details of their photophoretic spectrometer. A photophoretic spectrum of a crystallite of cadmium sulfide reported by Arnold and Amani is presented in Fig. 11. 

When a photon passes close to a molecule, there is an interaction between the electromagnetic field associated with the molecule and that associated with the radiation. If, and only if, the radiation is absorbed by the molecule as a result of this interaction, can the radiation be effective in producing photochemical changes (Grotthus-Draper law, see, e.g., Finlay son-Pitts and Pitts, 1986). Therefore, the first thing we need to be concerned about is the probability with which a given compound absorbs uv and visible light. This information is contained in the compounds uv/vis absorption spectrum, which is often readily available or can be easily measured with a spectrophotometer. 

A high-resolution spectrum of the clock transition is shown in Fig. 2. The clock-laser power was reduced to 30 nW to avoid saturation broadening. The fit with a lorentzian curve results in a linewidth of 170 Hz (FWHM), corresponding to a fractional resolution bv/v of 1.3 10-13. A spectral window of 200 Hz width contains 50% of all excitations. According to our present experimental control of the ion temperature, electromagnetic fields and vacuum conditions, no significant Doppler, Zeeman, Stark or collisional broadening of the absorption spectrum of the ion is expected beyond the level of 1 Hz. The linewidth is determined by the frequency instability of the laser and the lineshape is not exactly lorentzian... 

Why Because the frequencies at which charges spontaneously fluctuate are the same as those at which they naturally move, or resonate, to absorb external electromagnetic waves. This is the essence of the "fluctuation-dissipation theorem." It states that the spectrum (frequency distribution) over which charges in a material spontaneously fluctuate directly connects with the spectrum of their ability to dissipate (absorb) electromagnetic waves imposed on them. Computation of charge-fluctuation forces is essentially a conversion of observed absorption spectra. By its very nature, the measured absorption spectrum of a liquid or solid automatically includes all the interactions and couplings among constituent atoms or molecules. 

In an absorbing medium, /cabs / 0, this wave will be attenuated as e" abs rrf. The imaginary part k of the complex index of refraction, nKf + Kabs, is a measure of the attenuation of an electromagnetic wave as seen, for example, in a spectrophotometer. We speak of k k(o>) as the absorption spectrum of light. In the limit of high absorption (k3bS... 

The spectrometer to be used need cover only the visible portion of the electromagnetic spectrum. Preferably it should be a double-beam instrument, to allow for compensation for absorption by the cell windows. However, a single-beam instrument may be used if the absorption spectrum of an equivalent set of cell windows is obtained separately, so that the absorbances of the windows can be subtracted from those of the iodine-containing cell. The spectrophotometer must have a cell compartment large enough to contain the... 

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