Shimadzu ICPE-9000, Инструкция по эксплуатации

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4 Basic Principles
4-6
4.3.2 Mounting of Spectrometer
Entrance slit
Collimator
Prism
Telemeter
CCD
Schmidt mirror
Echelle diffraction grating
Fig. 4-2 Spectrometer Incorporating Echelle Diffraction Grating
The arrangement of a spectrometer's optical elements is called a "mounting". "Fig. 4-2" shows the
mounting for an echelle spectrometer incorporating an echelle diffraction grating.
The divergent light through the entrance slit is converted to parallel light by the collimator. It is then
diffracted and divided by the diffraction grating and the prism, it undergoes aberration correction at the
Schmidt mirror, and an image is formed by the telemeter mirror at the exit slit. With the ICPE-9000, a
two-dimensional semiconductor detector is used instead of an exit slit.
The echelle spectrometer is designed to improve resolution by increasing the order of diffraction , as
shown in equation (5) in "4.3.1 Basic Equations for Diffraction Gratings", and to improve the angular
dispersion by increasing the diffraction angle,
β (i.e., reducing the value of cos β ), as shown in equation
(2). In general, the order is in the range 30 to 130 and the diffraction angle is in the range 60
°
to 70
°
.
When light is divided with an echelle spectrometer, all light for which the product of the order and
wavelength is the same will be radiated in the same direction and so an order-dispersive element is
required in order to isolate light of the target wavelength. In light divided with an echelle spectrometer, if
λ and λ ' are wavelengths of two consecutive orders that appear at the same position in the spectrum, then
the difference in the wavelength, ∆λ , is called the "free spectral range", and represents the wavelength
range measured for each order.
Applying
β α
≅
to equation (1) gives the following:
( )
β λ λ
sin 2 1 d n n
=
′
+ =
····························· (6)
n /
λ λ λ λ ′
=
′
− = ∆
······································ (7)
In general, the range is from the 30th to the 130th order and from 1 to 10 nm.