![]() With the 300 gr/mm the spectral range goes to ~7400 cm -1, whilst the 1800 gr/mm grating only reaches approx. The spectra above clearly show the decrease in spectral range as the groove density is increased. Insert shows the high wavenumber region of the sample with 300 gr/mm and 1800 gr/mm grating. The spectral range of the spectrometer is inversely proportional to the groove density of the grating.įigure 5: Raman spectra of cyclohexane with different gratings and extended scan with an 1800 gr/mm grating (magenta). The spectra again show how the resolution increases with increasing groove density, but now also demonstrate the downside of high groove density reduced spectral range. A sample of cyclohexane was placed in a cuvette holder for analysis with 638 nm excitation using all five gratings (Figure 5). This value decreases to 4.8 cm -1 when using an 1800 gr/mm grating, highlighting the improvement in spectral resolution with increasing groove density.Ĭhanging the groove density of the grating affects the spectral resolution as discussed, but it will also impact the spectral range. Starting with the 300 gr/mm grating, we observe a FWHM of 22.9 cm -1. Ramacle ® can provide FWHM values (the width of the peak at half its maximum intensity) and display these on the spectra. This measurement exemplifies the importance of spectral resolution.įigure 4: Raman spectrum of MoS 2 acquired using five different gratings.įigure 4 also shows the full width half maximum (FWHM) values of the silicon peak from the sample. The better the resolution of these two peaks the more accurate the information on peak position and number of layers. As the gr/mm increases we see the improved resolution of the two individual peaks. Using a 300 gr/mm grating, the spectral resolution is insufficient to resolve the two individual peaks and a single broad feature is seen. These peaks are critical for detecting the number of layers of MoS 2 present. The analysis of MoS 2 focuses on the two peaks between 350-450 cm -1. To illustrate the effect of groove density on spectral resolution, the spectrum of MoS 2 on a silicon substrate was measured (Figure 4) using five diffraction gratings, and a 532 nm excitation wavelength. The simple rule of thumb is when the number of grooves is doubled, the resolution is roughly doubled.įigure 3: Dispersion of light by a low and high groove density grating. The higher groove density grating spreads the light over a larger area of the CCD increasing the spectral resolution. The dispersion of light by a low and high groove density grating is illustrated in Figure 3. The higher the groove density, the better the spectral resolution, for example, a 1200 gr/mm grating will provide higher spectral resolution than a 150 gr/mm grating. Gratings have a defined groove density (measured in grooves per millimetre, gr/mm), which controls the dispersion of light. When selecting a diffraction grating for a Raman spectrometer there are four main considerations: spectral resolution, spectral range, blaze wavelength, and excitation wavelength.įigure 2: Ramacle ® software in measurement set up, highlighting ease of grating selection. Ramacle ® will then display the spectral range achievable with this grating and excitation laser both in wavenumber and wavelength. After selection, the grating turret will automatically move to the selected grating meaning there is no need for the user to manually exchange gratings. The gratings can be selected by the user from a drop-down menu in the Ramacle ® software, Figure 2, on the RM5 and RMS1000. ![]() To operate with multiple laser lines the spectrographs of the RM5 and RMS1000 can host up to five diffraction gratings so that a grating best suited for the laser’s wavelength and the users’ requirements. The RM5 Raman Microscope can host up to three lasers, and the RMS1000 Raman Microscope up to five with the option of additional external lasers. When analysing samples by Raman spectroscopy multiple excitation sources may be required to cover the users range of samples, for example, lasers in the UV, visible, or NIR regions. All Raman spectrometers will require at least one diffraction grating and will frequently be configured to contain more than one to allow the user optimum grating selection for their samples and excitation wavelength(s).įigure 1: Edinburgh Instruments RMS1000 (left) and RM5 (right) Raman Microscopes. In a Raman spectrometer a diffraction grating is used to separate the constituent wavelengths of the collected Raman scatter onto different pixels of the CCD camera for detection. A diffraction grating is used to separate polychromatic light into its constituent wavelengths. ![]()
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