Engineering Physics 3(2+1)

LESSON 14. Practical view of Raman Spectroscopy

• Excitation source (Laser).

• Sample illumination system and light collection optics.

• Wavelength selector (Filter or Spectrophotometer).

• Detector (Photodiode array, CCD or PMT).

A sample is normally illuminated with a laser beam in the ultraviolet (UV), visible (Vis) or near infrared (NIR) range. Scattered light is collected with a lens and is sent through interference filter or spectrophotometer to obtain Raman spectrum of a sample. Since spontaneous Raman scattering is very weak the main difficulty of Raman spectroscopy is separating it from the intense Rayleigh scattering. More precisely, the major problem here is not the Rayleigh scattering itself, but the fact that the intensity of stray light from the Rayleigh scattering may greatly exceed the intensity of the useful Raman signal in the close proximity to the laser wavelength. In many cases the problem is resolved by simply cutting off the spectral range close to the laser line where the stray light has the most prominent effect. People use commercially available interference (notch) filters which cut-off spectral range of ± 80-120 cm-1 from the laser line. This method is efficient in stray light elimination but it does not allow detection of low-frequency Raman modes in the range below 100 cm-1. Stray light is generated in the spectrometer mainly upon light dispersion on gratings and strongly depends on grating quality.

Raman spectrometers typically use holographic gratings which normally have much less manufacturing defects in their structure then the ruled once. Stray light produced by holographic gratings is about an order of magnitude less intense then from ruled gratings of the same groove density. Using multiple dispersion stages is another way of stray light reduction. Double and triple spectrometers allow taking Raman spectra without use of notch filters. In such systems Raman-active modes with frequencies as low as 3-5 cm-1 can be efficiently detected.

In earlier times people primarily used single-point detectors such as photon-counting Photomultiplier Tubes (PMT). However, a single Raman spectrum obtained with a PMT detector in wave number scanning mode was taking substantial period of time, slowing down any research or industrial activity based on Raman analytical technique. Nowadays, more and more often researchers use multi-channel detectors like Photodiode Arrays (PDA) or, more commonly, a Charge-Coupled Devices (CCD) to detect the Raman scattered light. Sensitivity and performance of modern CCD detectors are rapidly improving. In many cases CCD is becoming the detector of choice for Raman spectroscopy.

Ways to improve Raman signal intensity

Raman signal is normally quite weak and people are constantly improving Raman spectroscopy techniques. Many different ways of sample preparation, sample illumination or scattered light detection were invented to enhance intensity of Raman signal. Here we will examine some of them.

Stimulated Raman

It was found that if the sample was irradiated with a very strong laser pulse, new “non-linear” phenomena were observed in Raman signal. In comparison with continuous wave (CW) lasers with electric field of about only 104 V·cm-1 pulsed lasers with electric field of about 109 V·cm-1 transform a much larger portion of incident light into useful Raman scattering and substantially improve signal-to-noise ratio.

Stimulated Raman scattering is an example of “non-linear” Raman spectroscopy. Very strong laser pulse with electric field strength > 109 V·cm-1 transforms up to 50% of all laser pulse energy into coherent beam at Stokes frequency υ0 – υm.

The Stokes beam is unidirectional with the incident laser beam. Only the mode um which is the strongest in the regular Raman spectrum is greatly amplified. All other, weaker Raman active modes are not present. The Stokes frequency is so strong it acts as a secondary excitation source and generates the second Stokes line with frequency υ0 - 2υm. The second Stokes line generates the third one with the frequency υ0 - 3υm etc. Stimulated Raman technique enjoys 4-5 orders of magnitude enhancement of Raman signal as compared to the spontaneous Raman scattering.

CARS

Coherent Anti-Stokes Raman, CARS, is another type of “non-linear” Raman spectroscopy. It stands for Coherent Anti-Stokes Raman Spectroscopy. Instead of the traditional one laser, two very strong collinear lasers irradiate a sample. Frequency of the first laser is usually constant, while the frequency of the second one can be tuned in a way that the frequency difference between the two lasers equals exactly the frequency of some Raman-active mode of interest. This particular mode will be the only extremely strong mode in the Raman signal.

With CARS we can obtain only one strong Raman peak of interest. In this case a monochromator is not really required. A wideband interference filter and a detector behind the filter would do the job. Below is more specific description with a little bit of math to understand CARS more in depth.

Two laser beams with frequencies υ1 and υ2 (υ1 > υ2) interact coherently, and because of the wave mixing, produce strong scattered light of frequency 2υ1 - υ2

If the frequency difference between two lasers υ1 - υ2 is equal to the frequency um of a Raman-active rotational, vibrational or any other mode then a strong light of frequency υ1 + υm is emitted. In other words, to obtain strong Raman signal the second laser frequency should be tuned in a way that υ2 = υ1 - υm . Then the frequency of strong scattered light will be 2υ1 - υ2 = 2υ1- (υ1 - υm) = υ1 + υm, which is higher than the excitation frequency u1 and therefore considered to be Anti-Stokes frequency. Coherent Anti-Stokes Raman Spectroscopy derives its name from the fact that it uses two Coherent laser beams and the resulting signal has Anti-Stokes frequency.

Resonance Raman (RR)

Many substances, especially colored ones, may absorb laser beam energy and generate strong fluorescence which contaminates Raman spectrum. This is one of the central problems in Raman spectroscopy especially when UV lasers are used. However, it was found that under certain conditions some types of colored molecules can produce strong Raman scattering instead of fluorescence. This effect was called Resonance Raman. The Resonance Raman effect takes place when the excitation laser frequency is chosen in a way that it crosses frequencies of electronic excited states and resonates with them. Intensity of Raman bands which originate from electronic transitions between those states are enhanced 3-5 orders of magnitude. Not all the bands of spontaneous Raman spectrum are enhanced. The so-called chromophoric group, which is responsible for the molecule’s coloration, experiences the highest level of enhancement. The reason is the chromophoric group normally has the highest level of light absorption.

The highest intensity of Resonance Raman signal is obtained when laser frequency equals to the first or the second electronic excited state

Therefore, tunable lasers are the most appropriate choice for the RR technique. Although,

Even when the frequency of the laser does not exactly match the desired electronic excited states an impressive enhancement of Raman signal occurs.

SERS and SERRS

Surface-Enhanced Raman Spectroscopy utilizes the following effect. Raman signal from molecules adsorbed on certain metal surfaces can be 5-6 orders of magnitude stronger then the Raman signal from the same molecules in bulk volume. The exact reason for such dramatic improvement is still under discussion. However, since intensity of Raman signal is proportional to the square of electric dipole moment P = αE, there are two possible reasons - the enhancement of polarizability α, and the enhancement of electrical field E. The first enhancement of polarizability a may occur because of a charge-transfer effect or chemical bond formation between metal surface and molecules under observation. This is a so-called chemical enhancement. The second one takes into account interaction of the laser beam with irregularities on the metal surface such as metal micro-particles or roughness profile. It is believed that laser light excites conduction electrons at the metal surface leading to a surface plasma resonance and strong enhancement of electric field E. It is also called electromagnetic enhancement. In all cases choice of appropriate surface substrate is very important. The most popular and universal substrates used for

SERS are electrochemically etched silver electrodes as well as silver and gold colloids with average particle size below 20 nm.

One disadvantage of SERS is the difficulty of spectra interpretation. The signal enhancement is so dramatic that Raman bands that are very weak and unnoticeable in spontaneous Raman spectra can appear in SERS. Some trace contaminants can also contribute additional peaks. On the other hand, because of chemical interactions with the metal surface, certain peaks which are strong in conventional Raman might not be present in SERS at all. Non-linear character of signal intensity as a function of concentration complicates things even further. Very careful consideration of all physical and chemical factors should be done while interpreting SERS spectra which makes it extremely difficult for practical use. The SERRS technique, Surface-Enhanced Resonance Spectroscopy, was developed because of such complications. It utilizes both Surface-Enhancement effect and Raman Resonance effect so the resulting enhancement in Raman signal intensity can be as high as 1014. The main advantage of SERRS is its spectra resemble very much regular Resonance Raman spectra, which makes it much easier to interpret.