Delayed electromagnetic radiation

Figure 24-6 Photoluminescence methods (fluorescence and phosphorescence). Fluorescence and phosphorescence result from absorption of electromagnetic radiation and then dissipation of the energy by emission of radiation (a). In (b), the absorption can cause excitation of the analyte to state 1 or state 2. Once excited, the excess energy can be lost by emission of a photon (luminescence, shown as solid line) or by nonradiative processes (dashed lines). The emission occurs over all angles, and the wavelengths emitted (c) correspond to energy differences between levels. The major distinction between fluorescence and phosphorescence is the time scale of emission, with fluorescence being prompt and phosphorescence being delayed.

There are two ways in which molecular vibrations affect non-linear optical properties. The first, which is well understood, is zero-point-vibrational averaging of the calculated electronic properties. This need not delay us long. The second comes about from the effect that the electromagnetic radiation has on the vibrational motions themselves and this leads to the vibrational polarizabilities and hyperpolarizabilities which are the exact counterparts of the electronic ones which stem from the effect that the radiation has on the electronic motions. This phenomenon is now receiving long overdue attention and will be the main subject of this section. A more extensive review is available elsewhere [2]. 

The first successful experimental observation of the dynamics of the chemical bond was accomplished in the 1980s [2]. The clocking of such an ultrafast event requires an almost instantaneous initiation of the dynamics which can be accomplished by a femtosecond laser pulse. After this flash of electromagnetic radiation -the pump pulse - the dynamics of the induced photochemical reaction is probed via another ultrashort interaction with a probe pulse. This is repeated at various time delays between the pump and probe pulse and a series of snapshots are obtained which together constitute a molecular movie of the dynamics. 

The modern student, to whom the Bohr frequency rule has become commonplace, might consider that this rule is clearly evident in the work of Planck and Einstein. This is not so, however the confusing identity of the mechanical frequencies of the harmonic oscillator (the only system discussed) and the frequency of the radiation absorbed and emitted by this quantized system delayed recognition of the fact that a fundamental violation of electromagnetic theory was imperative. 

From Maxwell s equations, the fundamental difference between static near fields (electromagnetic fields) and radiation (electromagnetic waves) is demonstrated. In Section 6.2, we treated the dipole. Consider that the dipole moment varies as a sine function with time. Now if the frequency is very high, the time delay will be noticeable if we are at a distance much longer than the wavelength X = c/f from the dipole. 

Electrical performance failures can be caused when individual components have incorrect resistance, impedance, voltage, current, capacitance, or dielectric properties or by inadequate shielding from electromagnetic interference (EMI) or particle radiation. The failure modes can be manifested as reversible drifts in electrical transient and steady-state responses such as delay time, rise time, attenuation, signal-to-noise... 

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