Signal-to-noise improvements with a new far-IR rapid scan Michelson interferometer

Abstract

In this paper signal-to-noise issues in the infrared spectral region are discussed, presenting an update on instrumentation developments that have focused on this topic. Reproducibilities in the 0.01% range for spectra measured in around 1 min on samples with an area of 1 mm2 illuminated with an f/10 beam were achieved. It is shown how this result is consistent with the synchrotron source intensity and detector noise and a comparison with a conventional globar source is also shown. For these new studies, a NicoletTM Impact 400 rapid scan Michelson interferometer was modified by Pike Technologies and installed in vacuum at the U4IR infrared beamline at the NSLS. The instrument is capable of scanning at an optical retardation rate of 3.2 cm/s, and of a data-collection frequency of 50 kHz triggered by the colinear reference beam of a HeNe laser. A proprietary NicoletTM solid-state beam splitter was used to cover the range from 10 to 2500 cm−1. Spectra were taken in reflection at grazing incidence off a single-crystal Cu surface in ultrahigh vacuum using liquid helium cooled detectors of the photoconductive type (Cu/Ge) or bolometric type (B/Si). The sample throughput for this system was 0.05 mm2 sr.