SELECTIVE DIGITAL INTEGRATOR FOR MODULATED-POLARIZATION SPECTROSCOPY - AN EVALUATION USING (-3-METHYLCYCLOPENTANONE())

A new device, a selective digital integrator (SDI), for the acquisitio n of modulated polarization spectroscopy (MPS) signals is described. S pecial attention is given to the accurate measurement of very small si gnals (ac component of interest < 10(-3) xdc component) buried in nois e. The polarization of light is altered with fast modulators (modulati on frequency similar to 50 kHz). A SDI connected to a single detector is used to separately and selectively measure the polarization-depende nt intensities of electronic transitions. The SDI is a major improveme nt over the lock-in approach: it has on-the-fly time-resolved data acq uisition and integration capability, and a wide gain-switching-free dy namic range (10 orders of magnitude). This design permits a variety of applications that require detection of modulated or pulsed signals. T he advantages of the MPS-SDI method are demonstrated on the first Rydb erg electronic transitions of (+)-3-methylcyclopentanone. The selectiv ely measured MPS intensities are provided in 3D form !intensity versus both wavelength and light polarization). A discussion of the scatteri ng problems that bedevil measurements at higher optical densities is i ncluded, and ways to correct for it are outlined. (C) 1998 American In stitute of Physics.