Magnetic Circular Dichroism - Measurement

Measurement

The MCD signal ΔA is derived via the absorption of the LCP and RCP light as

This signal is often presented as a function of wavelength λ, temperature T or magnetic field H. MCD spectrometers can simultaneously measure absorbance and ΔA along the same light path. This eliminates error introduced through multiple measurements or different instruments that previously occurred before this advent. An MCD spectrometer begins with a light source that emits a monochromatic wave of light. This wave is passed through a Rochon prism linear polarizer, which separates the incident wave into two beams that are linearly polarized by 90 degrees. The two beams split into different paths- one beam (the extraordinary beam) traveling directly to a photomultiplier (PMT), and the other beam (the ordinary beam) passing through a photoelastic modulator (PEM). The PMT for the extraordinary beam detects the light intensity of the input beam, while the PEM causes a 1/4 wavelength shift that converts linearly polarized light into circularly polarized light. Linearly polarized light has two circular components with intensity represented as

The PEM will delay one component of linearly polarized light with a time dependence that advances the other component by 1/4 λ (hence, quarter-wave shift). The departing circularly polarized light oscillates between RCP and LCP in a sinusoidal time-dependence as depicted below:

The light finally travels through a magnet containing the sample, and the transmittance is recorded by another PMT. The schematic is given below:

The intensity of light from the ordinary wave that reaches the PMT is governed by the equation:

Here A_– and A₊ are the absorbances of LCP or RCP, respectively; ω is the modulator frequency – usually a high acoustic frequency such as 50 kHz; t is time; and δ₀ is the time-dependent wavelength shift.

This intensity of light passing through the sample is converted into a two-component voltage via a current/voltage amplifier. A DC voltage will emerge corresponding to the intensity of light passed through the sample. If there is a ΔA, then a small AC voltage will be present that corresponds to the modulation frequency, ω. From such voltage, ΔA and A can be derived using the following relations:

where V_ex is the (DC) voltage measured by the PMT from the extraordinary wave, and V_dc is the DC component of the voltage measured by the PMT for the ordinary wave (measurement path not shown in the diagram).

Some superconducting magnets have a small sample chamber, far too small to contain the entire optical system. Instead, the magnet sample chamber has windows on two opposite sides. Light from the source enters one side, interacts with the sample (usually also temperature controlled) in the magnetic field, and exits through the opposite window to the detector. Optical relay systems that allow the source and detector each to be about a meter from the sample are typically employed. This arrangement avoids many of the difficulties that would be encountered if the optical apparatus had to operate in the high magnetic field, and also allows for a much less expensive magnet.