Selective digital integrator for modulated-polarization spectroscopy: An evaluation using (+)-3-methylcyclopentanone

A new device, a selective digital integrator (SDI), for the acquisition of modulated polarization spectroscopy (MPS) signals is described. Special attention is given to the accurate measurement of very small signals (ac component of interest <10−3×dc component) buried in noise. The polarization of light is altered with fast modulators (modulation frequency ∼50 kHz). 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 magnitude). 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 (+)-3-methylcyclopentanone. The selectively measured MPS intensities are provided in 3D form (...