Mei-Wei Tsao

Design and Performance of a Planar Array Infrared Spectrograph that Operates in the 3400 to 2000 cm -1 Region

A prototype, no-moving-parts, plane array infrared spectrograph (PA-IR) capable of routine spectral acquisition in the 3400 to 2000 cm−1 region has been constructed. The instrument includes a continuous source, Czerney–Turner type monochromator system and an infrared camera that incorporates a 320 × 256 pixel InSb focal plane array detector cooled with liquid nitrogen. PA-IR spectra (∼3400 to 2550 cm−1) of polystyrene (PS) and poly(ethylene naphthalate) (PEN) films have been obtained at a resolution of ∼8 cm−1 with excellent signal-to-noise ratios (SNR). Peak-to-peak noise levels of ∼1 × 10−3 absorbance units are observed for single acquisition spectra with 1.5 ms integration times and 17 ms total acquisition times. Integration times as low as 10 μs are possible (with good SNR); however, data acquisition is limited by the frame rate (60 frames/s) of the software acquisition package currently used. In this work, an apertured image of the source is displayed over ∼20 rows of the array and the expected square root improvement in SNR is observed when multiple rows and/or frames are averaged. We have also shown that a square root improvement in SNR continues to occur with further signal averaging, providing noise levels as low as 1.5 × 10−5 absorbance units. Several additional advantages and options associated with the PA-IR method are discussed, including time-resolved spectroscopy, real-time monitoring, and spectroscopic mapping.

COMPARISON OF THE ANCHORING OF NEMATIC LIQUID CRYSTALS ON SELF-ASSEMBLED MONOLAYERS FORMED FROM SEMIFLUORINATED THIOLS AND ALKANETHIOLS

The anchoring of nematic liquid crystals on self-assembled monolayers (SAMs) formed by the chemisorption of semifluorinated thiols or alkanethiols on gold is compared and contrasted. The planar anchoring of 4-n-pentyl-4-cyanobiphenyl (5CB) observed in the past on SAMs formed from alkanethiols is also observed on SAMs formed from semifluorinated thiols. The azimuthal anchoring of 5CB, however, differs on these two types of surfaces: nematic 5CB anchored on SAMs formed from alkanethiols has a grainy appearance due to the formation of domains with sizes 10 mum whereas 5CB forms large domains (100 mum) with diffuse branches emerging from defects of strength 1/2 when anchored on SAMs formed from semifluorinated thiols. Mixed (two-component) SAMs formed from either short and long semifluorinated thiols or short and long alkanethiols cause homeotropic anchoring of 5CB. We discuss these results in light of the known differences in the structure of SAMs formed from alkanethiols and semifluorinated thiols, i.e. the...