Aligning molecules with polarized radiation

Preparing aligned molecules with polarized radiation... 

Aromatic plants are usually constituted from cellulose, essential oil, and water. If these three compounds are heated by microwaves at a fixed radiation power and for a set time, the heating rate will be the highest for water, followed by essential oil and cellulose, respectively. One of the interactions of the microwave energy with the matrix is called the dipolar polarization mechanism. A substance can generate heat when irradiated with microwaves if it has a dipole moment, for example that of the water molecule. A dipole is sensitive to external electric fields and will attempt to align itself with the field by rotation. 

Molecules orient themselves when exposed to microwave radiation, which results in polarization of the molecules as they align themselves with the beam. As the beam reverses its direction, at a rate more than two million times per second, the continuous reorientation and reversal of the molecules produces molecular friction, generating heat. The heating results from the release of kinetic energy from the molecular motion the heating effect is greatest in the interior of the material exposed to the radiation. 

Figure B3.6.12 Depolarization of fluorescence indicates rotation of the chromophore. Monochromatic radiation from the source (S) has all but the vertically polarized electric vector removed by the polarizer (P). This is absorbed only by those molecules (see Fig. B3.6.5) in which the transition dipole of the chromophore is aligned vertically. In the case where these molecules do not rotate appreciably before they fluoresce ( no rotation"), the same molecules will fluoresce (indicated by shading) and their emitted radiation will be polarized parallel to the incident radiation. The intensity of radiation falling on the detector (D) will be zero when the analyzer (A) is oriented perpendicular to the polarizer. In the case where the molecules rotate significantly before fluorescence takes place, some of the excited chromophores will emit radiation with a horizontal polarization ( rotation ) and some with a vertical polarization. Finite intensities will be measured with both parallel and perpendicular orientations of the analyzer. The fluorescence from the remainder of the excited molecules will not be detected. The heavy arrows on the left of the diagram illustrate the case where there is rotation.

Since synchrotron radiation is polarized, an anisotropic orientational distribution of molecules can be produced by core electron excitation. This distribution is determined by the orientation of the transition dipole moment with respect to the electric vector of the radiation. Information about this alignment and subsequent time evolution is eontained in the angular distributions of fragmentation products resulting from the excitation. Details have been worked out and described in several references for the case of valence electron excitation and ionization (Zare 1972 Yang and Bersohn... 

Microwaves work by using a process called dielectic heating. The microwave surrounds your food in a field of electromagnetic radiation that is constantly changing directions. The polar molecules in your food, particularly water, align their dipoles with the direction of this applied field. The constant shifting of the direction of this field causes the polar molecules to tumble around, and this molecular motion warms your food. 

The cooking action in a microwave oven results from the interaction between the electric field component of the radiation with the polar molecules— mostly water—in food. AH molecules rotate at room temperature. If the frequency of the radiation and that of the molecular rotation are equal, energy can be transferred from the microwave to the polar molecule. As a result, the molecule will rotate faster. This is what happens in a gas. In the condensed state (for example, in food), a molecule cannot execute the free rotation. Nevertheless, it still experiences a torque (a force that causes rotation) that tends to align its dipole moment with the oscillating field of the microwave. Consequently, there is friction between the molecules, which appears as heat in the food. 

Fourier transform techniques have become very powerful, particularly when used with pulsed sources of the sample. They are ubiquitous in studies of van der Waals molecules, in which two or more entities are very weakly bound together in the gas phase, such as HCT Ar, but are also used to determine structures of many other compounds, of which the mixed alkali halide dimer LiNap2 is an example. In a typical experiment, the sample is introduced into the cell in a pulsed supersonic jet expansion, the rotational temperature being reduced to a few Kelvin. A pulse of microwave radiation then aligns the dipole moments of the molecules, so that the sample is polarized on a macroscopic scale. The subsequent decay of this polarization is recorded, and Fourier transformation of the free induction decay yields the spectrum [9]. 

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