Subwavenumber charge-coupled device spectrometer calibration using molecular iodine laser-induced fluorescence

Spectrometers configured with charge-coupled devices (CCD) or other array-based detectors require calibration to convert from the pixel coordinate to a spectral coordinate. A CCD calibration method well suited for Raman spectroscopy has been developed based on the 514.5 nm Ar(+) laser-induced fluorescence (LIF) spectrum of room-temperature molecular iodine vapor. Over 360 primary and secondary I(2) LIF calibration lines spanning 510-645 nm were identified as calibrant peaks using an instrumental resolution of 1 cm(-1). Two instrument calibration functions were evaluated with these peaks: a second-order polynomial and a function derived from simple optomechanical considerations. The latter function provided better fitting characteristics. Calibration using I(2) LIF was tested with measurements of both laser light scattering and Raman spectra. The I(2) LIF reference spectra and the signal spectra were recorded simultaneously, with no cross talk, by separating the two signals spatially along the vertical axis of the CCD imager. In this way, every CCD image could be independently calibrated. An accuracy and a precision of +/-0.05 cm(-1) were achieved with this calibration technique.