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Evaluation of Polynomial Fitting Functions for Use with CCD Arrays in Raman Spectroscopy

Volume 47, Number 1 (Jan. 1993) Page 125-126

Wollman, S.T.; Bohn, P.W.


Calibration of wavelength is vital to the accurate assessment of Raman shifts using multichannel detectors, such as a charge-coupled device (CCD) detector in combination with multistage spectrometers. Although in zeroth order, the position in the image plane (i.e., CCD) may be approximated as linearly related to wavenumber, this approximation fails badly for accurate determinations of band position at even moderate spectral coverage (>50 cm−1). A Gaussian development of the imaging properties of a Czerny-Turner spectrometer leads to a quadratic relationship. However, the use of flat-field optics in modern triple monochromators results in position-wavenumber relationships that are not described well by a quadratic expression and are best fit by empirical relationships. Establishing the goodness of fit of several trial functions is the purpose of this current work. One way to achieve the relationship between position, or pixel number, and wavelength, as dispersed by a grating spectrometer, is to observe a standard sample that has several Raman lines of accurately known wavenumber shift. Indene, C9H8, has been used as such a standard for many years due to its well-calibrated and evenly spaced spectral features. This paper will explore the use of indene as a Raman shift calibration standard for CCD based spectrometers in the diagnostically important range of 700 to 1600 cm−1.