15 Raman Spectroscopy

Raman spectra are plots of scattering intensity versus Raman shift, where the latter is calculated as Eqn. 15.1, where [latex]\bar{ν}[/latex] is a wavenumber (i.e. 1/λ). The shifts of Raman bands correspond to vibrational transitions in a sample and Raman spectra thus report on chemical composition. While some Raman spectral bands align with bands in IR absorption spectra, other bands are unique to Raman spectra (and vice versa). Raman spectra tend to have fewer bands than IR absorption spectra, making them more amenable to compound identification.

        (Eqn. 15.1)        [latex]Raman \:Shift = \bar{ν}_{incident} – \bar{ν}_{Raman}[/latex]

Instrument Design

Raman scattering spectra are measured using lasers. Lasers are the best light sources for Raman spectroscopy because they provide a high intensity of incident photons (compensating for the low efficiency of Raman scattering) and are monochromatic (making it possible to resolve the small frequency shift of Raman scattered light). Notch filters are often used to block (or greatly reduce) Rayleigh scattered laser light, and monochromators are used to resolve the Raman scattering bands.

Some tradeoffs are necessary in Raman spectroscopy. Stokes Raman is much more intense than anti-Stokes Raman, but, unlike Anti-Stokes Raman, Stokes Raman often suffers from background from sample fluorescence. Raman scattering efficiency increases significantly as excitation wavelength decreases (proportional to λ^–4), but so does sample fluorescence in many cases. The use of red and NIR lasers for Raman spectroscopy is thus common, albeit that shorter wavelengths (including UV) are also used, with various advantages and disadvantages for each approach.

In addition to one or more laser sources and the corresponding notch filters, modern Raman spectrometers typically have a grating-based polychromator with a CCD detector or sometimes an EM-CCD for even better sensitivity to weak Raman signals. The CCD also enables fast acquisition of full spectra without scanning through grating angles. Raman spectrometers can be sized for laboratory bench tops and also made handheld. Handheld instruments often use optical fiber-bundle probes for illuminating samples and collecting scattered light.

Raman microscopy, where a Raman spectrum is recorded at each image pixel, is also possible. A focused laser beam is raster scanned across a sample and records a new spectrum at each pixel (i.e. location) in the field of view.

Fluorescence Background

Raman scattering spectra are measured using lasers, which have the potential to excite fluorescence from a sample. The fluorescence emission will often generate high background in Raman spectra. Many strategies are used to minimize this background. Some of the simplest include using a longer laser wavelength and measuring the anti-Stokes spectrum. (Raman peaks for water and other solvents also appear in fluorescence emission spectra, but the narrow peaks tend not to cause significant difficulties.)

Connections

• Raman spectrometers most closely resemble fluorescence spectrometers (Ch. 9) and, despite being a vibrational spectroscopy, utilize UV-visible light and thus incorporate similar lasers (Ch. 3), filters and gratings (Ch. 4), and photodetectors (Ch. 5).
• Like fluorescence spectra (Ch. 9), Raman spectra plot the detected light intensity on the vertical axis, but, like IR absorption spectra (Ch. 13), plot wavenumber on the horizontal axis.
• The use of a polychromator based on a grating and CCD detector for Raman has the same benefits as the use of array detectors for UV-visible spectrophotometry (Ch. 7), AES (Ch. 11), or fluorescence (Ch. 9).

Post-Reading Questions

1. A sample is illuminated with a 532 nm laser and the spectrum of scattered light is measured. A peak is observed at a Raman shift of 400 cm–1. At what wavelength is this peak observed?
2. The sample in Question 2 is illuminated with a 785 nm laser. What will be the Raman shift of the peak?
3. Is Stokes or anti-Stokes Raman scatter affected by background fluorescence?
4. Why are lasers used as sources for Raman spectroscopy?
5. What type of filter is used to minimize the Rayleigh scatter peak in Raman spectra?

Topic Learning Objectives

The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:

• Draw and label block diagrams of a Raman spectrometer.
• Calculate Raman shifts.
• Sketch and label Raman spectra.
• Use peaks in Raman spectra to identify functional groups.
• Recognize fluorescence background in Raman spectra and recognize Raman peaks in fluorescence spectra.