18.2: Instrumentation

The basic instrumentation for Raman spectroscopy is similar to that for other spectroscopic techniques: a source or radiation, an optical bench for bringing the source to the sample, and a suitable detector.

Sources

One of the notable features of the Raman spectrum for CCl₄ (see Figure 18.1.1) is the low intensity of the Stokes lines and the anti-Stokes lines relative to the line for Rayleigh scattering. The low intensity of these lines requires that we use a high intensity source so that there are a sufficient number of scattered photons to collect. For this reason, a laser is the most common source for Raman spectroscopy, providing a high intensity, monochromatic source. Table \(\PageIndex{1}\) summarizes some of the more common lasers.

Table \(\PageIndex{1}\). Examples of Laser Sources for Raman Spectroscopy

type of laser	wavelengths (nm)
Ar ion	488.0 or 514.5
Kr ion	530.9 or 647.1
He/Ne	632.8
Near Infrared (NIR) Diode Laser	785 or 830
Nd/YAG	532 or 1064

The intensity of Raman scattering is proportional to \(\frac{1}{\lambda^4}\), where \(\lambda\) is the wavelength of the source radiation; thus, the smaller the wavelength, the more intense the intensity of scattered light. For example, the intensity of scattering using an Ar ion laser at 488.0 nm is almost \(23 \times\) greater than the intensity of scattering using a Nd/YAG laser at 1064 nm

\[\frac{(1/480)^4}{(1/1064)^4} = 22.6 \nonumber \]

The increased scattering when using a smaller wavelength laser comes at a cost, however, of an interference from fluorescence from species that are promoted into excited electronic states by the source. The NIR diode laser and Nd/YAG laser, when operated at 1064 cm^–1, discriminate against fluorescence and are useful, therefore, for samples where fluorescence is a problem.

Samples

Raman spectroscopy has several advantages over infrared spectroscopy. Because water does not exhibit much Raman scattering it is possible to analyze aqueous samples; this is a serious limitation for IR spectroscopy where water absorbs strongly. The ability to focus a laser onto a small area makes it possible to analyze very small samples. A liquid sample, for example, can be held in the tip of a 1-mm inner diameter capillary tube, such as that used for measuring melting points. Solid samples and gaseous samples can be sampled using the same types of cells used in IR and FT-IR (see Chapter 17). Fiber optic probes make it possible to collect samples remotely. Figure \(\PageIndex{1}\) shows the basic set-up. A small bundle of fibers (shown in blue) brings light from the source to the sample where a second bundle of fibers (shown in green) brings the scattered light to the slit that passes light onto the detector.

Figure \(\PageIndex{1}\): Illustration of a fiber optic probe for Raman spectroscopy. The darker blue circles are cross-sections of individual fibers that bring light from the source to the sample, and the lighter blue circles are cross-sections of individual fibers that collect scattered light from the sample and bring it to the detector. The circular arrangement of the collection fibers are gathered into a linear array and focused onto the entrance slit of the instrument's detector.

Optical Bench

Raman spectrometers use optical benches similar to those for UV/Vis or IR spectroscopy, which were covered in Chapter 7. Dispersive instruments place the laser source and the detector at 90° to each other so that any unscattered high intensity emission from the laser source is not collected by the detector. A filter is used to remove the Rayleigh scattering. To record a spectrum one either uses a scanning monochromator or a multichannel detector. Fourier transform instruments are similar to those used in FT-IR and include a filter to isolate the Stokes lines.