The design, construction, and evaluation of a combination spectrometer for measuring electronic absorption (EA), natural circular dichroism (CD), and magnetic circular dichroism (MCD) are described. Around the optical components of a JASCO ORD/UV-5 spectropolarimeter, a new EA/CD/MCD instrument was built with the realized intentions of increasing sensitivity and upgrading the analog tube type circuitry to a solid-state digitally, computer-controlled spectrophotometer. It is a flexible, dynamic, and user-controllable system, interfaced to an Apple II Plus computer, for studying instrument and signal parameters. The monochromator (M), photoelastic modulator (PEM), photomultiplier tube applied voltage (PMHV), and photomultiplier tube dc output current (PMdc) are under complete and independent software control. Our system has two unique aspects for obtaining the circular dichroism. First, the ac signal is measured with a voltage-to-frequency (V/f) converter; and, second, both the ac and the dc are independently recorded and their ratio is digitally calculated. This design has several advantages which include the elimination of voltage divider integrated circuits or division electronics, a wide dynamic range, a greater precision of ac values at low percentages of full scale, and the capability of continuous integration over long time periods. Also, both types of spectra, EA and CD or MCD, are obtained from the current output of the PM. This paper not only describes the design of the instrument for obtaining the two types of spectra but also compares four methods of obtaining the circular dichroism. Sensitivities of ∼1×10−7ΔA units are achievable as determined by measuring CD spectra of the well-known enantiomer (+)−[Co(en)3]3+.

A stable high-current transistor-based regulator has been designed to stabilize 450-W dc xenon arc lamps. The circuit employs commercially available components. The regulator provides stabilization beyond that found in commercial optical instruments. The improvements using this circuit are a 0.20% ac-to-dc regulation representing a 44% decrease in the peak-to-peak ac ripple, achievement of nondrifting dc level, and being able to use the spectrometer immediately following lamp ignition.