Phys 4910 - Spectroscopy
The monochromator in our laboratory is a SpectraPro
750i 75 cm Czerny Turner monochomator from Roper
Scientific (now part of Princeton
The following are some of the specifications
are taken from the Roper Scientific web site. (Note: they are specific
to an installed 1200g/mm grating). The indented notes are the implication
of these specifications to our laboratory.
Focal length: 750 mm, Aperture Ratio: f/9.7
Focal length refers to the mirrors which form part of the Czerny Turner
Aperture ratio is a measure of the light collecting ability. The larger
the diameter of the mirrors the more light is collected by them, and the
the f number. With an f number of 9.7 the diameter of the mirrors is 750
mm / 9.7 = 77 mm
Optical Design: Imaging Czerny-Turner with original polished aspheric mirrors
Optical Paths: 90° standard, 180° and multi-port optional
The SpectraPro-750 can be equipped with up to four ports (2 entrance/2
exit) for maximum versatility. Multi-port configurations permit using the
SpectraPro-750 with multiple detectors. Our monochromator is equipped with
one entrance port and two exit ports
In the straightforward configuration the exit port is equipped with a diode
In the sideways configuration the exit port is equipped with an exit slit
and a photomultiplier.
Exit port selection is computer controlled for automated selection of detector.
Operating Range: 185 nm to the far infrared with available gratings and
A wavelength of 185 nm is well into the ultraviolet part of the spectrum.
It is the minimum wavelength that will pass through dry air without being
absorbed. Monochromators operating below this wavelength need to be evacuated
(and hence the region is known as the vacuum ultra violet (VUV)) or filled
with a non absorbing gas such a helium. With helium operation is possible
down to 50.4 nm, the wavelength of the 1s-2p resonance transition in helium.
Between 185 nm and about 300 nm although the radiation will pass through
air it will not pass though glass. This is not a problem for the monochromator,
but it a problem for the detectors. Below 300 nm detectors have to be fitted
with quartz windows which will let short wavelength radiation through.
Our detectors have glass windows, and so are limited to operation above
about 300 nm.
Resolution: 0.023nm at 435.8nm, 10µm slits,
Dispersion is a measure of how well the light is split into its different
wavelengths. It is quoted as the difference in wavelength corresponding
to a physical separation of 1 mm in the spectrum. A small number is better,
since for maximum resolution you would like the exit slit of the monochromator
to let through light of wavelength, but to block light of a slightly different
Using the 1800 l/mm grating; 0.75 nm/mm (nominal)
Using the 600 l/mm grating; 2.2 nm/mm (nominal)
Repeatability: ±0.05nm, Drive Step Size: 0.0025nm (nom.)
Detector Coverage: ~28nm across a 1.0” wide focal plane (~56nm: 600g/mm
Using the diode array, this the width (in nanometers) of the portion of
the spectrum which viewed at one time. Wider spectra are obtained by recording
multiple portions and the 'gluing' them together using the software.
When using the 1800 l/mm grating (see below) the detector coverage should
be ~ 19 nm.
The diode array width is 1024 pixels. The maximum possible resolving power
is therefore 19 nm / 1024 = 0.019 nm.
This specification has no relevance when using the photomultiplier.
Standard Slits: Micrometer adjustable from 10µm to 3mm wide. Slit
heights: 4 or 14mm
Grating Mount: Triple Indexable grating turret for 1, 2, or 3 gratings.
We have two different gratings installed
600 lines/mm, blazed
for 500 nm
1800 lines/mm, blazed
for 500 nm
For all experiments we shall us the 1800 l/mm grating, since it gives the
higher resolution. The other grating is normally only used for applications
in which a wider portion of the spectrum is to be viewed at one time, but
with lower resolution.
Scan Linearity: The SpectraPro-750i scans linear with respect to wavelength