PhysicsOpenLab Modern DIY Physics Laboratory for Science Enthusiasts
Abstract: in this article we describe the construction of a DIY Raman system, based on a 532 nm DPSS laser and a B&W Tek surplus spectrometer. A 90° configuration was chosen for the collection of scattered light, particularly suitable for the analysis of liquid samples. The system is equipped with optical fiber and sharp-edge filters for blocking the radiation from excitation laser. The system has been used for acquisition of the Raman spectra of numerous organic and inorganic substances.
Introduction
In Raman spectroscopy a laser light is typically used in the visible range, or in near infrared or near ultraviolet range in order to excite the vibrational energy levels of the molecules. The radiation scattered by the molecules has a predominant component that has the same wavelength as the excitation radiation, this component is called Rayleigh scattering. The other component, of much weaker intensity than the first, has a longer wavelength because part of the incident energy was absorbed by the molecule and served to excite vibrational energy levels, this component is called Raman scattering. Fig. 1 shows the basic scheme of the interaction. The intensity of Raman scattering is much lower than Rayleigh scattering: it is calculated that only one photon out of 106 incident photons undergoes Raman scattering: from this we understand the difficulty of studying this radiation which requires powerful excitation sources and very sensitive instruments.
Fig 1- Diagram of Rayleigh and Raman scattering (from Semrock)
Raman spectroscopy is therefore a scattering spectroscopy technique where the monochromatic electromagnetic radiation of known intensity and frequency hits the sample and the scattered radiation is measured by means of a detector placed at 90º or 180º with respect to the optical path along the sample. The radiation can be scattered in three ways: Stokes, anti-Stokes and Rayleigh (which corresponds to elastic scattering). Stokes radiation has less energy than the original incident radiation, because part of its energy is used to promote a roto-vibrational transition to a higher energy level. The anti-Stokes radiation (normally of much weaker intensity) instead receives an energetic contribution from the excited state when the molecule passes to a lower energy level, for which it is characterized by greater energy. Rayleigh radiation, on the other hand, results from elastic scattering and has the same energy and therefore the same wavelength as the incident radiation. Fig. 2 shows the diagram of the energy levels corresponding to the different transitions. Note that there is a “virtual excited state” of the molecule which does not correspond to a real state but only to a transient situation corresponding to the interaction of the incident photon with the molecule.
Fig 2 – Diagram of the energy levels involved in scattering
The basic scheme of a Raman spectrometer with collection of scattered light at 90° with respect to the direction of the incident beam is shown in Fig. 3. The apparatus is composed of an excitation laser, an excitation filter that allows the transmission of only the wavelength of the laser, and the sample being irradiated. The scattered radiation is collected through a “sharp-edge” filter which can be “long pass” or “notch-filter“, which has the purpose of rejecting the Rayleigh radiation scattered by the sample (having a wavelength equal to that of the laser) and transmit the Stokes components (and possibly also anti-Stokes component). Finally, there is the spectrometer for the analysis of scattered light.
Fig 3 – Basic scheme of a Raman spectrometer (from Horiba)
To obtain good quality Raman spectra it is important that the excitation laser has a narrow emission line (<1 nm) and good operating stability. Raman scattering has very low intensity, in general it will therefore be necessary to use excitation lasers with high emission power (at least 50-100 mW) and to acquire the Raman diffusion with the spectrometer for several seconds (we used integration times 10 seconds). The blocking filter is also very important for removing the Rayleigh component and any reflections that could easily cover the weak Raman signal. The sensitivity and resolution of the spectrometer must be adequate for the acquisition of low intensity light, it is important that the sensor has low noise and can be used for acquisition with long integration times. In the image of Fig. 4 we show how the scattering radiation from methanol appears, without filter and with filter. In the image taken with the filter you can see the orange color trace due to Raman scattering appears, while without filter that trace is covered by the Rayleigh radiation.
Fig 4 – Raman scattering demonstration
A bit of Theory
As I said earlier, Raman scattering takes place when light interacts with the atoms of molecules, causing them to vibrate. This phenomenon is related to the infrared spectroscopy, but follows different rules. To obtain the Raman effect, a change in the polarizability of the molecule during vibration is required. Given the difference in selection rules, the Raman spectrum presents vibrations that could be not allowed in the infrared absorption spectrum and vice versa. For example, unlike the absorption of infrared rays, Raman spectroscopy is very useful in the study of the carbon atoms that form the structure of the diamond, for example. The results are graphically reproduced in the form of Raman spectra. The graph shows the intensity of the scattered light (on the Y axis) as a function of the emission energy (frequency) (on the X axis). Typically, frequency is measured in units called wave number (number of waves per cm, cm-1). The formula that links the molecule’s vibration frequency to the wavelengths of the incident radiation λ0 and of the scattered radiation λ1 is the following:
We know λ0 and with the spectrometer we measure λ1 and from the above relation we obtain Δν. From the equation it is evident, in order to make precise measurements, the importance of having a monochromatic excitation source and a high resolution spectrometer.
Molecular vibrations can be of two types: stretching of the chemical bond and deformation of the bond angle (bending). Stretching consists of a periodic variation of the interatomic distance and can be symmetrical if the two atoms approach or move away simultaneously (indicated with νs) or asymmetrical in the opposite case (indicated with νa). The deformation can also be symmetrical or asymmetrical and can occur along the plane on which the bond angle lies or outside that plane. The symmetrical deformation in the plane is called scissoring (opening and closing of a scissor, indicated with δ), while the asymmetrical one is called rocking (oscillation, indicated with ρ); the asymmetric out-of-plane deformation is called twisting (torsion, denoted by τ) while the symmetrical out-of-plane deformation is called wagging (agitation, denoted by the letter w).
Considering a linear molecule formed by N atoms, based on the orientation along the three Cartesian axes (x, y, z) 3N-5 different vibrational modes are possible; for a non-linear molecule these modes are 3N-6. This is because we have 3N degrees of freedom minus 3 of translation and 3 of rotation (therefore 3N-6) for nonlinear molecules and 3 of translation and 2 of rotation (therefore 3N-5) for linear molecules. For example, for the water molecule H2O we have three atoms arranged in a non-linear way, the modes will therefore be 9-6=3. In fact, the water molecule has two vibrational modes of stretching the O-H bond, one symmetrical and the other asymmetrical, and a vibrational mode of scissoring for the H-O-H bond.
The frequencies of vibration depend on the masses of the atoms and the strength of the bonds. Heavy atoms with weak bonds vibrate at low frequencies and therefore show a small Raman shift. Light atoms with strong bonds vibrate at higher frequencies and therefore produce a larger Raman shift. This intuitive observations are also demonstrated by the classical theory of the harmonic oscillator which allows to calculate the vibration frequency with the relation:
ω = (k/m)½
Where ω is the pulsation of the vibration (ω = 2πf), k is the binding force and m is the reduced mass of the system. Stronger chemical bonds cause vibrations at higher frequencies while larger masses cause vibrations at lower frequencies.
The Raman spectrum is unique to a specific material and therefore Raman spectroscopy is an excellent technique for identifying compounds. A Raman spectrum is made up of a series of maxima, each associated with a vibrational mode. Each type of chemical bond (eg C-H, C-C, C=C) has unique modes of vibration, which lead to specific interactions with light radiation and thus produce distinct Raman Shifts (see Table below for some examples). This information can be used to identify molecules and study chemical bonds.
| Functional Group | Vibration Band | Intensity |
| C-C | ∼ 600-1300 cm-1 | Variable |
| C=C | ∼ 1600 cm-1 | Variable |
| C≡C | ∼ 2100-2300 cm-1 | Variable |
| C-H | ∼ 2700-3100 cm-1 | Strong |
| O-H | ∼ 3200-3400 cm-1 | Variable |
| C=O | ∼ 1700-1750 cm-1 | Strong |
| C≡N | ∼ 2200-2300 cm-1 | Variable |
| N-H | ∼ 3300-3500 cm-1 | Medium |
Tab 1 – Some examples of functional groups with the relative band of stretching vibrational frequencies
Apparatus
Our apparatus, shown in Fig. 5, is essentially composed of a DPSS laser source emitting at 532 nm (green laser) with a power of about 100 mW, equipped with TEC cooling. The laser beam is directed towards the cuvette holder entrance window. On the entrance window there is a band-pass filter with a bandwidth of about 15 nm centered on 532 nm in order to allow to pass only the excitation beam at 532 nm. The liquid sample is placed in a quartz cuvette. The light scattered by the sample is collected, in a 90° geometry, by a lens that focuses it onto an optical fiber, which leads it to the spectrometer. The light collected by the fiber is filtered with a sharp-edge long-pass filter that blocks wavelengths below 540 nm and transmits the higher wavelengths. In this way we avoid collecting the Rayleigh scattering, at the same wavelength as the excitation, and also any spurious reflections that would make the spectrum acquired by the instrument more blurred.
Fig 5 – the Raman Apparatus
The image in Fig. 6 shows the laser we used as a Raman excitation light source. It is a DPSS laser that emits at 532 nm. It is the 100 mW PGL-H-532nm model from the manufacturer CNI Laser. It is a source with good monochromaticity with a line width <0.2 nm and a divergence <1.5 mrad. The laser is powered by a voltage of 3 V and absorbs about 0.3 A. The laser is equipped with a cylindrical aluminum heat exchanger which is kept cool by a TEC device. The laser must be “warmed up” for a couple of minutes before being used as a Raman excitation source.
Fig 6 – Detail of the Laser and cuvette holder
In Fig. 7 we show the emission spectrum of the source, also in high resolution, centered at 532 nm.
Fig 7 – Emission line of the DPSS laser
Cuvette holder, collimator and sharp-edge Raman filter come from manufacturer Thunder Optics, which also supplies complete Raman spectrometers and systems. In Fig. 8 we report the transmission spectrum of the Raman filter which is centered on 540 nm: below this wavelength the transmission is practically zero, while above the transmission it is close to 100%. The filter is placed at the collimator input of the optical fiber which carries the signal towards the spectrometer. As already mentioned above, the purpose of the filter is to make sure that the radiation of the Rayleigh scattering – much more intense than the Raman scattering – does not reach the spectrometer where it could cause problems in detecting the Raman bands with lower Raman shift, i.e. more close to the excitation wavelength.
Fig 8 – “Sharp-Edge” filter transmission diagram
Spectrometer
The spectrometer is a B&W Tek model that can be found in the surplus market at a reasonable price (eBay link). It is a compact CCD spectrometer coupled to optical fiber with SMA-905 connector and powered at 5 Vdc. It is based on a crossed Czerny-Turner design, as the detector uses a Sony ILX511 linear CCD with a 100 µm slit. The diffraction grating is 1800 l/mm, the wavelength range covered is 250 nm and is normally between 400 nm and 650 nm. In Fig. 9 we show the instrument inserted in a metal box. A valid alternative is also the R-Spectrometer instrument also from Thunder Optics.
Fig 9 – B&W Tek spectrometer
The B&W Tek instrument optics should be checked as described in the documents found on the Science-Surplus website. In particular, the presence or absence of a filter on the entrance slit must be checked. The instrument was originally conceived as a Raman spectrometer with an excitation wavelength of 473 nm, for this reason there may be a 480 nm long-pass filter on the entrance slit. When using the instrument with an excitation of 532 nm the filter must be removed. The diffraction grating must also be rotated so as to center the range of wavelengths of interest on the CCD sensor. The mirrors should already be aligned, otherwise you need to follow the procedure described on the site Science-Surplus. In Fig. 10 and Fig. 11 we show the inside of the instrument.
Fig 10 – Crossed Czerny-Turner setup of the spectrometer
Fig 11 – Detail of the optical bench of the spectrometer
Some Raman Spectra
Spectra can be acquired for example with the Science-Surplus software downloadable from the Science-Surplus website. The spectra can be saved in csv format and subsequently processed with excel for the calculation of the Raman Shift using the formula described in the paragraph relating to the theory. It is necessary to accurately measure the wavelength of the excitation radiation, it should be 532 nm but the spectrometer, depending on the accuracy of its calibration, could give a slightly different measurement. The Raman scattering radiation is very weak so it is necessary to make acquisitions with integration times of at least 10 seconds. With these long times the spectrum obtained is very disturbed, so it is necessary to acquire a background spectrum in advance and subsequently, during the processing phase, subtract the background from the Raman spectrum. After these processing steps, which can be automated, rather good Raman spectra are obtained with main maxima that correspond to the vibrational energy levels of the molecule. In the following graphs we show the spectra of some molecules.
Fig 12 – Raman spectrum of dimethyl sulfoxide
Fig 13 – Raman spectrum of water
Fig 14 – Raman spectrum of methanol
Fig 15 – Raman spectrum of Nitric Acid
Conclusions
In this project we have shown how it is possible to make a Raman spectrometer without spending astronomical amounts. The “key” of the project is the “second hand” B&W Tek spectrometer: it is not a brand new instrument but it does its job very well. A good laser source and a good quality sharp-edge filter makes the trick. Both the laser and the filter can be found on the online market for affordable price. In the next articles we will show in more detail the applications of our Raman DIY apparatus.
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