A molecule in an oscillating electric field

5 A MOLECULE IN AN OSCILUTING ELECTRIC FIELD Constant and oscillating components 

A non-zero hyperpolarizabihty indicates a non-linear response (contributions to the dipole moment proportional to the second and higher powers of the field intensity). This may mean an inflated reaction to the applied field, a highly desired feature for contemporary optoelectronic materials. One such reaction is the second- and third-harmonic generation (SHG and THG, respectively), where light of frequentty w generates in a material light with frequencies 2w and 3w, respectively. A simple statement about why this may happen is shown below.  

Let us imagine a molecule immobilized in a laboratoiy coordinate system (like in an oriented crystal). Let us switch on a homogeneous electric field S, which has two components, a static component and an oscillating one with frequency (o  

We may imagine various experiments here the steady field along x,y or z and a light beam polarized along x, y or z, we may also vary w for each beam, etc. Such choices lead to a rich set of non-linear optical phenomena. What will the reaction of the molecule be in such an experiment Let us see.  

The total dipole moment of the molecule (i.e. the permanent moment /xq plus the induced moment lUin ) will depend on time, because does  

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Forster energy transfer or energy transfer at a distance occurs between two molecules, a donor (the excited fluorophore), and an acceptor (a chromophore or fluorophore). Energy is transferred by resonance, i.e., the electron of the excited molecule induces an oscillating electric field that excites the acceptor electrons. As a result of this energy transfer, the fluorescence intensity and quantum yield of the emitter will decrease. Energy transfer is described in Chapter 14. 

Dipole-dipole interaction between molecules placed in an oscillating electric field results in energy being transferred from the oscillating field to the sample. This phenomenon manifests itself as a frequency dependent dielectric constant and is a property common to all materials. The physical basis of the measurement is expressed by Equation 3. 

The working principle of a third type of mass filter, the ion trap detector (ITD), is illustrated in Fig. 6. It operates like the quadrupole based on the concept that the trajectories of ions, traveling in an oscillating electric field, become influenced by the wave frequency. Dissimilar to the quadrupole filter, however, the ion separation of the ITD occurs in a closed cavity, where the dc and ac currents applied to a ring electrode and an end cap of the cell set up the electric field. The molecules enter the cell in which they are ionized by an electron beam, and under the control of the given electric field the ions are forced to move in an orbit within the space of the cell. When the ac or dc potential is changed, the motion of some ions becomes unstable. 

Rotational energy and transitions If a molecule has a permanent dipole moment, its rotation in space produces an oscillating electric field this can also interact with electromagnetic radiation, resulting in light absorption. 

Molecules that have a permanent dipole moment (e.g., water) can rotate in a fast changing electric field of microwave radiation. Additionally, in substances where free ions or ionic species are present the energy is also transferred by the ionic motion in an oscillating microwave field. Owing to both these mechanisms the substance is heated directly and almost evenly. Heating with microwaves is therefore fundamentally different from conventional heating by conduction. The magnitude of this effect depends on dielectric properties of the substance to be heated. 

Ion trap, by which ions of a given mass are trapped within a set of three hyperbolic electrodes with a DC electric held on each of two end-cap electrodes and an oscillating electric field on the ring electrode. Ions are injected in a pulsed manner, stored (or trapped) for a short amount of time, then extracted one mass at a time by the pulsed extraction grid. These instruments can detect molecules up to molecular 70,000 Da with very high sensitivity (Figure 1.8). 

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