Circularly polarized radiation absorbance

The quantities /4 and Ajf are the decadic absorbances for left and right circularly polarized radiation, respectively. The ordinary IR absorption is given by the average of /4 and A, namely,... 

In this equation, Al(v,) and An(v,) indicate, respectively, absorbances for right- and left-circularly polarized radiation at wavenumber v,. To determine Al(v,) - Ar(v ), the value of 2 /i(5q)G(v,) should be known in advance of the VCD measurement by the method to be described later. 

As described above, there are two forms of vibrational optical activity, one derived from infrared absorption and the other from Raman scattering. Both forms involve the differential response of a molecule to the modulation of polarization of the interacting radiation between right and left circularly polarized states. In the case of infrared absorption, VCD is defined as die differential absorbance for left minus that for right circularly polarized infrared radiation. This is expressed by the relation ... 

The difference in absorbance of left and right circularly polarized incident radiation (MCD), or the extent of the rotation of a beam of plane polarized light (MORD), with the sample located in a magnetic field aligned parallel to the axis of light propagation is observed. 

CD spectroscopy measures the difference in absorbance between left- and right-circularly polarized incident radiation, in the near ultraviolet to visible and infrared regions of the spectrum. 

The two circular-polarized beams of the incident radiation (I, levorotatory r, dextrorotatory) not only can be influenced with respect to their direction of rotation, but also, in the region of an absorption band, can be absorbed to a different extent by the sample, so that in addition to so-called optical rotatory dispersion (ORD), circular dichroism (CD) or the Cotton effect is observed [35] - (38J. Along with these classical methods of analysis, modem methods for the analytical investigation of surfaces and boundary layers have become very important, including the use of polarized light in ellipsometry ( Surface Analysis) and surface plasmon resonance. Under the influence of external forces (e.g., a magnetic field), even optically inactive substances can be caused to produce magnetooptic rotation. 

The absorbance for circularly polarized infrared radiation is proportional to a quantity called the transition strength B expressed by the following equation, where superscripts -I- and - on B, shown as +/-, indicate, respectively, L and R in Equation (22.1), and the corresponding symbol of the same sign (-1- or -) is used on the right-hand side at of the equation. 

Figure 1 Energy-level diagram illustrating the definition of VCD as the difference in the absorbance of a molecule for left versus right circularly polarized IR radiation in a vibrational transition between states gO and g in the ground electronic state.

In its absorption spectral region, an optically active sample may absorb the two circularly polarized components of a linearly polarized radiation to different extents. In such a case the species exhibits different absorption coefficients for left- and right-handed light, that is, Sl(A) r( )- This phenomenon is called circular dichroism CD) [3,4], When circular dichroism occurs, the two circularly... 

It is important to remember that both enantiomers are absorbing the radiation and being destroyed, one is just being photo-chemically destroyed more rapidly than the other. The longer the exposure to circularly polarized light, the more chiral the sample becomes, but, of course, too long an exposure will destroy all of the molecules. A rough estimate is that a chiral excess of 10% is probably the maximum one could expect from this process. 

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