A new device, a selective digital integrator ͑SDI͒, for the acquisition of modulated polarization spectroscopy ͑MPS͒ signals is described. Special attention is given to the accurate measurement of very small signals ͑ac component of interest Ͻ10 Ϫ3 ϫdc component͒ buried in noise. The polarization of light is altered with fast modulators ͑modulation frequency ϳ50 kHz͒. A SDI connected to a single detector is used to separately and selectively measure the polarization-dependent intensities of electronic transitions. The SDI is a major improvement over the lock-in approach: it has on-the-fly time-resolved data acquisition and integration capability, and a wide gain-switching-free dynamic range ͑10 orders of magnitude͒. This design permits a variety of applications that require detection of modulated or pulsed signals. The advantages of the MPS-SDI method are demonstrated on the first Rydberg electronic transitions of ͑ϩ͒-3-methylcyclopentanone. The selectively measured MPS intensities are provided in 3D form ͑intensity versus both wavelength and light polarization͒. A discussion of the scattering problems that bedevil measurements at higher optical densities is included, and ways to correct for it are outlined.

The absorption of a modulated light beam in which the state of the polarization is continuously variable is discussed. It is now possible to measure the average time-dependent profile for the polarization cycle of such a beam, and corresponding theoretical descriptions are needed in order to extract the information contained in it. This work generates such a description. Based on a semiclassical treatment of light/matter interactions, a three-parameter fit function is derived that permits the simultaneous extraction of absorbance, linear dichroism, and circular dischroism from the profile for various types of uniformly oriented, partially oriented, and isotropic materials and combinations of them. The theoretical expressions are used to analyze data for (+)-3-methylcyclopentanone, the results being (1) an increase of the S/N ratio, (2) the simultaneous generation of absorption and dichroism spectra of identical quality, and (3) a description of window adsorbates of the cyclopentanone material. The Jones parameters for the macroscopic description of polarized light interactions with matter are reformulated at the microscopic (i.e., transition moment) level, and an alternative derivation of the Jones matrix is given.