Determining the Spectral Resolution of a Charge-Coupled Device (CCD) Raman Instrument

A new method based on dispersion equations is described to express the spectral resolution of an applied charge-coupled device (CCD) Czerny-Turner Raman instrument entirely by means of one equation and principal factors determined by the actual setup. The factors involved are usual quantities such as wavenumber values for the laser and the Raman band, the diffraction grating groove density, the second focal length, the angle between the incident and the diffracted light, and the full width at half-maximum (FWHM) value of the signal on the detector.

A basic formula is derived to estimate the spectral resolution of the Raman instrument. An essential feature of the new method is a proposed way to compensate for non-ideality (diffractions, aberrations, etc.) by use of a hyperbola model function to describe the relationship between the width of the entrance slit and the image signal width on the CCD.

The model depends on the spectrometer magnification and a diffraction and aberration compensation factor denoted as A. A could be approximated as a constant that can be determined by the experimental method. The validity of the new expression has been examined by measuring the band width of the 1332.4 cm(-1) diamond Raman fundamental band, excited with two quite different wavelengths (a deep ultraviolet 257.3 nm laser line and a visible green 514.5 nm line).

A low pressure mercury line at 265.2042 nm also was applied to give further verification of the given expression. A useful method to find true Raman band widths is also provided. A final finding was that the known significant changes in spectral resolution along the Raman shift axis make static recording and synchronous (extended) scanning modes differ significantly with respect to their resolution properties; this feature has been often overlooked in many contemporary works reporting Raman spectra.

A reason for this is that many Raman bands are too wide to show the effect.