Plane-grating Monochromator Research Articles

A New Plane Grating Monochromator with Off-Axis Paraboloids and Curved Slits

A design for a new plane grating monochromator is described. The system employs a symmetrical configuration of off-axis paraboloids for both the collimator and camera mirrors. The entrance and exit slits are mounted on the same side of the grating and are curved in order to eliminate wavelength errors due to spectral line curvature. There is, in fact, a remarkable coincidence between the curvature which is required to avoid wavelength errors and the curvature which is required to obtain equal curvatures in the object and image planes. Ray tracing calculations for a symmetrical configuration of off-axis paraboloids show that the image of a curved slit with a length equal to 0.05 times the focal length of an f/5 mirror is diffraction limited for wavelengths lambda > 0.2 . micro.

Optical Properties of Plane-Grating Monochromator

The optical properties of plane-grating monochromators such as the Ebert and Czerny–Turner types are examined by geometrical optics. The grating equation and the expression for the focal surface are given; by using them, expressions are found for the image form; the optical system that minimizes wavelength impurity is discussed; and such residual aberrations as coma and astigmatism are examined collectively. In any monochromator, the focal surface depends upon the grating position and the off-axial properties of the collimator. In an Ebert-type monochromator with a circular-arc-shaped entrance slit the image form is a circular arc only in a symmetric arrangement; it is elliptic in other arrangements; the light through a circular-arc-shaped exit slit is free of wavelength error; and even if the slit is long, the image will not be much blurred by residual aberrations. Therefore, it is advisable to use long circular-arc slits if the system is of the Ebert type. In the Ebert- or Czerny–Turner-type monochromator with a straight entrance slit the image is parabolic. In the Czerny–Turner type both the image form and focal surface are affected by its nonconcentric character.

Photoelectric Detection and Intensity Measurement in Raman Spectra

Preliminary experiments on the photoelectric detection of Raman spectra are described making use of commonly available spectroscopic equipment in conjunction with the new RCA-931 photomultiplier tube. A more eleborate experiment is described using a large planegrating monochromator as the dispersing device. The photoelectric current was suitably amplified and continuously recorded making use of a galvanometer and photographic galvanometer deflection recorder. The limit of detection set by the thermionic emission at room temperature of the photomultiplier has been reached. It is shown that the minimum intensity observable photoelectrically compares favorably with the minimum commonly observed photographically in Raman spectra. A rough order of magnitude experiment was performed with the photomultiplier to ascertain the feasibility of the photoelectric method for measuring intensities in arc and spark spectra under high dispersion. An arrangement of apparatus for quantitative spectroscopic analysis photoelectrically is suggested.