29 LASER SPECTROSCOPY

Devendra Mohan

Contents:

1. Absorption Spectroscopy
2. Photo acoustics Spectroscopy
3. Emission or Scattering of Light
4. Rayleigh and Raman Scattering
5. Advantages and Disadvantages of using UV laser source for Raman Spectroscopy
6. Remarks
7. Summary

Learning Outcomes

• From this module students may get to know about the following:
• The role of lasers in absorption and Photo-acoustics Spectroscopy, Laser induced fluorescence, Rayleigh and Raman scattering.

As a matter of fact, when the light interacts with matter, the strength of interaction is measured as a function of the wavelength or optical frequency. The spectrum so obtained plays an important role. Before the advent of Lasers the radiation sources with broad emission continuum were being used, for example, Xenon flash lamps, Mercury lamps, etc. With the development of tunable laser source of dye laser, the conventional sources were replaced by laser due to its properties: temporal and spatial coherence, narrow line width and wavelength tenability and peak power. Ultra short pulse generation is the added advantage for laser spectroscopy. It is to note that the earlier fixed frequency lasers have shown poor response as were not suited for such type of studies.

1.  Absorption Spectroscopy:

The most commonly used interaction between light and matter is the absorption of light that leads to absorption spectroscopy. As the atoms and molecules exhibit distinct absorption features, the different atoms or molecules can easily be distinguished if the absorption versus wavelength (absorption spectrum) is measured. Particularly in the mid-infrared spectral region, molecules have relatively strong and narrow absorption lines related to their vibrational and rotational modes. Thus a wide range of substances can be detected with a very high sensitivity.

As molecules can have many different absorption lines, some of which may overlap for different species, detection of a single line is often not sufficient for discriminating between different molecules. Recording the absorption spectrum for some sufficiently wide wavelength range gives a clear spectroscopic view. The mid-infrared spectral region is quite ideal for sensitive spectroscopy, for example, of many gases and air pollutants. However, mid-infrared lasers are less powerful and expensive than near-infrared laser sources.

Light transmitted through the medium, solid, liquid or gas, experiences attenuation due to scattering and absorption; the latter being the dominant loss contribution. Loss measurements can be used to deduct the concentration of a gas or liquid of interest with known absorption strength. The light attenuation for sufficiently small absorption is described by the Beer-Lambert Law:

Here I is the intensity measured by the detector, I₀ is the intensity entering the gas/liquid cell, σ is the absorption cross section of the absorber (in cm²/molecule) and N is the number density of absorbers (in molecules/cm³). The absorption coefficient (α) is then

α   = σ ∙ N, in 1 / cm and L is the path length of the light-matter interaction

Fig.1 Simple absorption spectroscopy setup

When employing absorption spectroscopy it is important to ensure specificity and sensitivity. The optical cavities with high reflective mirrors are used for increasing sensitivity while maintaining a small resonator size as the mirrors will enlarge the path length of the light, increasing the possibility of light-matter interaction and hence to get the measured signal from the constituents of the sample under investigation.

Fig.2

There are also resonant techniques that use absorption inside a passive optical resonator or within the laser resonator. Now a day, other tunable laser sources like Ti:Sapphire and broad-band fiber laser are being used.

The use of absorption spectroscopy is in Identification of tiny particles in air pollutants and one may also distinguish different isotopes.

However, there is a drawback of simple absorption spectroscopy as one finds the reduction in optical power due to the absorption in the material. To overcome the problem, there are other possibilities, for example, photo- acoustic spectroscopy wherein one exploits the generation of sound waves by absorption of light .The absorbed light leads to some heating that causes expansion of the gas and thus generates a sound/mechanical waves that can be detected with a sensitive microphone/suitable transducer.

2. Photo- acoustic Spectroscopy:

Photo- acoustic spectroscopy (PAS) involves irradiating the light onto a sample and then detecting the periodic temperature fluctuations in the sample as pressure fluctuations. It permits measurements without pretreatment of the sample, regardless of sample form and is contactless technique. It can be used for the

samples that do not transmit or reflect incident light. The technique is sensitive that increases as the light source intensity increases. This method is useful for depth profiling as it obtains information at different depths by changing the velocity of the movable mirror in the interferometer. The PA spectroscopy is beneficial for analyzing irregularly shaped samples; perform measurement of the surface treatments on fabrics, films, and powders.

Fig.3 A general layout of the Photo-acoustic set up

The measurement is based on the photo acoustic effect. The photo acoustic effect was discovered by Alexander Graham Bell in 1880. When intermittent light is irradiated onto a substance, the substance emits acoustic waves of the same frequency as that of the light pulse frequency. With the development of highly-sensitive microphones/ transducers and other advances in electronics with the passage of time, research progressed into the measurement of pressure waves generated from the gas samples, in particular and then slowly in solid samples. The application of the photo acoustic effect in the infrared region is done using highly-sensitive Fourier Transform Infrared (FTIR) instruments which are being widely used for the analysis of solid samples.

The solid sample to be measured is placed in a sealed vessel to which small microphone/transducer is attached. In case of FTIR photo acoustic spectroscopy,

when a modulated infrared light beam is absorbed by the sample, heat is generated due to the incident light. This heat causes pressure changes in the surrounding gaseous layer that is detected by the highly sensitive microphone. The signals from the microphone are acoustic interference waves and Applying Fourier transformation to these signals produces a spectrum similar to an absorption spectrum.

However, the theory of photo acoustic spectroscopy differs according to the form of and design of photo acoustic cell.

Rosencwaig-Gersho (RG) theory is sufficient to describe the photo acoustic signal generation in condensed media. It is a one-dimensional analysis for the generation and flow of heat from a sample on irradiation. RG theory based on thermal diffusion equations is a well-known theory for solid samples. The three important parameters involved are;

• Sample thickness l,
• Light absorption length lß (where, lß = 1/ß and ß is the absorption co-efficient), And
• The thermal diffusion length µs (where, µs = 1/as and as is the thermal diffusion coefficient).

It is classified according to whether the sample is optically transparent (lb > l) or opaque (lb < l), thermally thick (µs < l) or thermally thin (µs > l). The photo acoustic signal reflects the sample optical properties, for an optically transparent sample, regardless of the sample thermal properties. It is obvious that the photo acoustic signal is proportional to the sample absorption coefficient and the thermal diffusion length only when the sample is optically opaque and thermally thick (µs < lb). Though the sample is optically opaque, it is considered as photo- acoustically transparent because the thermal diffusion length is a function of the modulation frequency of the incident light. Increasing the modulation frequency and reducing the thermal diffusion length, such that (µs < lß), makes an optically opaque sample photo acoustically transparent. Further, the photo acoustic signal is proportional to the thermal diffusion length for a thermally thick sample; the depth from which the photo- acoustic signal is emitted can be changed by changing the modulation frequency. It infers that increasing the modulation frequency reduces the depth and hence the signal intensity.

The photo acoustic cell comprises a small, well-sealed vessel fitted with a highly sensitive microphone/transducer. The volume of the cell is made as small as possible to increase the sensitivity. The cell is mounted on a vibration-isolation stand to eliminate vibrations from the floor in order to increase signal to noise ratio.

3. Emission or Scattering of Light:

When the light interacts with sample, the interaction results in the excitation of sample species such as atoms, ions, or molecules to excited quantum levels. This is also possible with simple heating or by illumination with sun light even. A spectrometer on a satellite is able to record spectra of scattered light from the earth surface in order to distinguish plants, lake surfaces and rocks etc. The absorbed light may not only lead to heating, but also excite atoms or molecules, which then subsequently emit florescence light, that depends upon material property. The spectrum of the emitted light as a function of the wavelength of the pump light (laser) can be recorded.

In general, a single photon induces the molecular transition, but where nonlinear interactions are involved, two or more laser photons are required to excite the molecules simultaneously.

The laser photons need to have sufficient energy for the desired transition. However, there are selection rules to be followed in terms of both molecular symmetry and photon polarization.

Initially for absorption of laser photon, the photon energy hν must match the difference between two quantum states of the molecule, E2-E1.. Now, the energy conservation principle can be extended to the multi- photon processes as

n hν= E2-E1

The figure depicts that the two photons have been absorbed simultaneously.

In an insulator or semiconductor, two-photon absorption (TPA) can occur only if the photon energy is at least half the band gap energy. The phenomenon of two-photon absorption finds applications in various technical areas.

4. Rayleigh and Raman Scattering

Raman spectroscopy, named after Sir C.V. Raman, is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a molecular system.

The Raman Effect occurs when electromagnetic radiation impinges on a molecule and interacts with the polarizable electron density and the bonds of the molecule in the solid, liquid or gaseous media and the environment of the molecule. The spontaneous Raman effect is a form of inelastic light scattering wherein a photon excites the molecule in either the ground or vibronic state (lowest rotational and vibrational energy level of the ground electronic state) or an excited vibronic state. This results in the molecule being in a virtual energy state for a short period of time before an inelastically scattered photon results. The resulting inelastically scattered photon that is “emitted”/”scattered” can be of eitherlower (Stokes) or higher (anti-Stokes) energy than that of the incoming photon. In Raman scattering, the resulting rovibronic state (rotational sublevel of a vibrational level of an electronic state) of the molecule is a different rotational or vibrational state than the one in which the molecule was originally, before interacting with the incoming photon (electromagnetic radiation). The difference in energy between the original rovibronic state and this resulting rovibronic state describes a shift in the emitted photon’s frequency away from the excitation wavelength, the so-called Rayleigh line. The Raman effect is due to inelastic scattering and is different from emission (fluorescence or phosphorescence) in which, a molecule in an excited electronic state emits a photon of energy and returns to the ground electronic state.

Case 1.If the final vibrational state of the molecule is more energetic than the initial state, the inelastically

scattered photon is shifted to a lower frequency for the total energy of the system to remain balanced. This

shift in frequency is designated as a Stokes shift.

Case 2.If the final vibrational state is less energetic than the initial state, then the inelastically scattered

photon will be shifted to a higher frequency, and this is designated as an anti-Stokes shift.

Raman scattering is a good example of inelastic scattering because the energy and momentum transfer between the photons and the molecules during the interaction results in a difference in energy between the incident and scattered photons. In particular, the difference is equal to the difference in energy between the initial and final (rovibronic) states. This contrasts with infrared absorption, where the energy of the single absorbed photon matches this difference in energy (between those same two states).

Rayleigh scattering is an example of elastic scattering, where the frequency of the Rayleigh scattered light is the same as that of the incoming electromagnetic radiation.

A change in the molecular electric dipole-electric dipole polarizability with respect to the vibrational coordinate corresponding to the rovibronic state is required for a molecule to exhibit a Raman effect. The intensity of the Raman scattering is proportional to the change in dipole-dipole polarizability. The Raman spectra describing Raman scattering intensity as a function of the Stokes and anti-Stokes frequency shifts, is dependent on the rovibronic states of the sample. This dependence on the dipole- dipole polarizability derivative differs from infrared spectroscopy where the interaction between the molecule and light is determined by the electric dipole moment derivative.

The contrasting feature allows one to analyze transitions using Raman spectroscopy that might not be IR active, as in case of cetntro-symmetric molecules. Bands which have large Raman intensities in many cases have weak infrared intensities and vice versa. For very symmetric molecules, certain vibrations may be both infrared and Raman inactive so,one can use a technique called inelastic incoherent neutron scattering to determine the vibrational frequencies. The selection rules for inelastic incoherent neutron scattering (IINS) are different from those of both infrared and Raman scattering. Hence, the three types of vibrational spectroscopy are complementary.

In general, a sample is illuminated with a laser beam. Electromagnetic radiation from the illuminated spot is collected with a lens and sent through a Monochromator onto a detector. Elastic scattered radiation at the wavelength corresponding to the laser line (Rayleigh scattering)) is filtered out by using either a notch filter, edge pass filter, or a band pass filter. It is noteworthy thatSpontaneous Raman Scattering is typically very weak, and as a result the main difficulty of Raman spectroscopy is separating the weak in-elastically scattered
light from the intense Rayleigh scattered laser light

When a monochromatic radiation of frequency v₀ is passed through a nonabsorbing medium, it is found that most of it is transmitted without any change, and some of it is scattered. If the scattered energy is analyzed by means of a spectrometer, the bulk of the energy is found at the frequency of the incident beam v₀, but a small portion of the scattered energy will be found at frequencies v’=v₀+v_m The displaced frequencies are associated with transitions between rotational, vibrational and electronic levels of the molecular systems.

The scattering of radiation without change of frequency arising from scattering centers (that are much smaller than the wavelength of the incident radiation) is called Rayleigh scattering, named after Lord Rayleigh who studied its various features.

The scattering of radiation with change of frequency is called Raman scattering and is always accompanied by Rayleigh scattering. The spectrum of the scattered radiation, thus, consists of the original line and Raman lines. Raman lines at wave-numbers less than the incident wave-numbers, v₀-v_mare known v₀,(v_o+v_m)as Stokes lines and those with wave numbers greater than as antistokes lines.

The frequency shifts of the Raman lines, their intensity and polarization are characteristic of the scattering substance. According to the classical theory, Stokes and anti-Stokes lines should appear with equal intense. The figure shows the levels involved in the Raman spectra. The level w′ shown in the figure does not, in general, correspond to a stationary state. It is a virtual level. Since energy is not preserved in a transition to a virtual level, the molecule will immediately return to a stationary state under emission of a photon. The principal difference between fluorescence and Raman effect is that in fluorescence there is always a transition to a stationary upper level, whereas in Raman effect there is no such state.

The high intensity and narrow line width of the laser radiation have made it possible to measure the Raman scattering properties of materials under high resolution. The development of tunable dye lasers and the frequency doubling techniques which give an access to the ultraviolet region, have added a new dimension to the studies in Raman effect.

There are many variants of the Raman effect which have become popular with the use of lasers (i) nonlinear stimulated Raman scattering (ii) the hyperRaman effect, and (iii) coherent anti-Stokes Raman scattering (CARS). The giant-pulse or Q-switched laser systems are commonly used for such investigations such as ruby laser emitting at l = A o 6943.3 and neodymium-glass laser with an output at l =10600A. o The outputs of these lasers are of short duration (about 10 to 100 nanoseconds) and of very high powers.

There are also a number of advanced types of Raman spectroscopy: surface enhanced Raman, resonance Raman, polarized Raman, stimulated Raman (analogous to stimulated emission), and transmission Raman, and hyper Raman.

5. Advantages and disadvantages of ultra-violet (UV) lasers for Raman Spectroscopy

Ultra-violet (UV) lasers for Raman spectroscopy typically include laser wavelengths ranging from ~244 nm through to ~364 nm.

Though UV Raman spectroscopy is no different from standard analysis using visible laser wavelengths, but there are a number of practical difficulties and disadvantages which must be considered.

Advantages

(i) With certain samples, visible laser sources are not possible for excitation laser excitation while UV

sources can interact.

(ii) In semiconductor materials the penetration depth of UV light is typically in the order of a few

nanometers, and thus UV Raman can be used to selectively analyze from a thin top surface layer.

(iii) UV excitation can give rise to specific resonance enhancement with biological moieties, particularly

protein, DNA and RNA structures. Specific analysis of these materials within tissue is difficult using visible

laser wavelengths.

(iv) Fluorescence suppression can often be assisted using UV lasers, by spectrally separating the Raman and

fluorescence signatures. With visible lasers it is common that Raman and fluorescence are superimposed,

and the incomparable strength of the fluorescence is what can perturb (or completely mask) the Raman

spectrum. With UV excitation the Raman spectrum lies close to the laser line, whereas the fluorescence is

often slightly shifted to higher wavelengths. Thus, they no longer overlap, and the fluorescence is no longer

an issue.

Increased sensitivity can result from UV excitation as Ramanscattering efficiency is proportional to λ^-4, where λ is the laser wavelength. Thus, Raman scattering at 325 nm is a factor of 14 more efficient than that at 632.8nm.

Disadvantages

(i) UV Raman still remains a more sophisticated technique that requires greater expertise to handle. Reasons

for this include the fact that the laser beam is now invisible, and that the lasers are larger, more complex,

and considerably more expensive.

(ii) Samples are more prone to burning and degradation from the laser beam since the energy per photon is

increased.

(iii) Many Raman systems designed for visible and near infra-red analysis are not suitable for UV Raman. UV

Raman requires specific mirror coatings, microscope objectives, diffraction gratings, and CCD detector.

5. Remarks

(1)Although the inelastic scattering of light was predicted by Adolf Smekel in 1923, it was not until 1928 that it

was observed in practice. The Raman effect was named after one of its discoverers, the Indian scientist Sir

C.V. Raman who observed the effect by means of sunlight (1928, together with K.S. Krishnan and

independently by Grigory Landsberg). Raman won the Nobel Prize in Physics in 1930 for this discovery

accomplished using sunlight, a narrow band photographic filter to create monochromatic light, and a

“crossed filter” to block this monochromatic light. He found that a small amount of light had changed

frequency and passed through the “crossed” filter.

(2) Raman shifts are typically reported in wavenumbers, that have units of inverse length, as this value is

directly related to energy. In order to convert between spectral wavelength and wavenumbers of shift in the

Raman spectrum, the following formula can be used:

Fig.5 Rayleigh and Raman Scattering

Summary

The advantages of Photo acoustics spectroscopy in comparison with absorption spectroscopy have been

discussed. Rayleigh and Raman scattering alongwith advantages and disadvantages of using UV laser source

for Raman Spectroscopy have also been understood.