Week 12 (Raman spectroscopy) Lecture Notes PDF

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Monash University CHM2922: Molecular Spectroscopy Lecture Notes (Week 9 to 12) by Dr. Toby Bell...

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CHM2922 – Raman spectroscopy

2021

Objectives

1. Discuss the theory of Raman spectroscopy 2. Distinguish between Rayleigh scattering and Raman scattering 3. Investigate the importance of lasers, array detectors & FT techniques for Raman spectroscopy 4. Understand what is meant by polarisability of a molecule

5. Explore instrument components in Raman spectrometers 6. Look at Resonance Raman spectroscopy

What is Raman Spectroscopy?

P.of.I.A 18A

• Raman spectroscopy is a kind of vibrational spectroscopy. • That is, it yields information about the vibrational levels of a molecule. • Raman spectroscopy is often referred to as a complementary technique to infrared spectroscopy.

• BUT! … it uses visible or near IR light.

Δcm-1 or cm-1

What is Raman Spectroscopy?

P.of.I.A 18A

• Raman spectra are typically acquired by irradiating the sample with a visible or near-infrared laser source.

• The sample may either absorb, transmit or scatter the radiation - often all three processes occur.

• The scattered radiation may be measured at some angle (often 90o)

What is Raman Spectroscopy?

P.of.I.A 18A

• C.V. Raman - Nobel Prize in Physics 1930 • Observed that when visible light was scattered by certain molecules, a small amount had either higher or lower, yet discrete wavelengths.

➢ A quantum effect! • This “Raman” scattering is a only a weak phenomenon, and it has taken until the last few decades with the advent of lasers, array detectors and cheaper modern instruments before it has come into regular analytical use.

What is Raman Spectroscopy?

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Key concept: We are irradiating the sample with visible (or near IR) radiation. How can this give us information about fundamental vibrations? Energies of these correspond to photons from the mid-IR!

Differences in energies of incident and scattered photons contain information about vibrational energy levels

What is Raman Spectroscopy?

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• Most photons are scattered elastically and their energy (i.e. frequency/wavelength) is unchanged. • This is called “Rayleigh” scattering.

Rayleigh

• Stokes and anti-Stokes scattering occur when the interaction is inelastic and a quantum of vibrational energy is lost or gained by the scattered photon • The molecule also gains or loses a quantum ensuring momentum is conserved.

Stokes & Anti-Stokes Scattering

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• One way to think about scattering is by invoking a “virtual state”.

• The molecule is excited very briefly to a virtual state before spontaneously scattering a photon.

• Inelastic scattering provides the useful information, however only as little as 1 in a 1,000,000 photons are Stokes or anti-Stokes Raman scattered. Very Weak!

The Role of Lasers

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• Solution? Use lots of photons! • Until the development of lasers, Raman spectroscopy was not generally used for analytical applications, however the photon densities provided by lasers allow a more detectable yield of Raman scattered photons.

Lasers – CHM3952!!!!

So how did Raman do the experiment?

Key point in Raman Spectroscopy Rayleigh

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• Raman spectrum plots the difference between rayleigh (excitation) and raman scatter • Difference is caused by loss (or gain) of a quantum of vibrational energy

ν or Δν / cm-1

Intensity of Normal Raman Bands.

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• In infrared spectroscopy, we saw that the intensity of a band was proportional to the square of the transition dipole moment. • Band intensities in Raman are dictated by a complex interaction of factors including the polarisability of the molecule, the intensity of excitation source, the frequency of the excitation source and the concentration of the sample. • Polarisability is the ratio between the induced dipole moment  by an electric field, E (in this case from the electromagnetic radiation) and the electric field.

=

 E

Polarisability of CO2 vibrational modes

Intensity of Normal Raman Bands. • This distinction between IR and Raman intensities can produce markedly different spectra

• Strong bands in the IR are often weak in Raman and vice versa.

P.of.I.A 18A-4

Can do dispersive and FT Raman • Dispersive instruments simultaneously measure all wavelengths by separating them onto a 2D element such as a CCD detector.

grating

CCD detector Raman source

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• FT instruments simultaneously measure all wavelengths via a Michelson interferometer.

FT Raman Spectrometers

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Gases, Liquids & Solids • Gases may be analysed by passing the excitation source directly through a gas cell. • External mirrors allow multiple passes to maximise Raman scattering.

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Gases, Liquids & Solids • Liquids can be illuminated directly onto (b) capillaries, (c) cylindrical cells, or small ampoules. • Via capillaries volumes as small as a nanolitre may be measured. • Unlike in the infrared, glass sample holders can be used since visible and near-IR radiation is transmitted • Sample handling is significantly less complicated!

P.of.I.A 18B-2

Gases, Liquids & Solids • Solids may be measured as (d) pellets (bottom right) or even ‘as-is’ without additional preparation.

• Since near-IR transmits efficiently via optic fibre, instruments are often able to utilise several metres of emission fibre into the monochromator.

P.of.I.A 18B-2

Resonance Raman Spectroscopy • In RRS the incident photons are deliberately tuned to an electronic transition of the molecule

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S1

• Notice how this transition mechanism is different to fluorescence. • In RRS emission occurs directly from the excitation level. • Intensity increase can be orders of magnitude in the right conditions!

S0 10-14 s

10-10-10-6 s

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CHM2922 – Gas analysis

2021

Objectives

1. Examine how detection of gases by vibrational and UV-vis spectroscopy can be achieved 2. Examine some applications of various spectroscopic techniques in gas analysis 3. Justify choice of instrumental method(s) to monitor gases in exhaust plumes, workplace environments and in the atmosphere

Atmospheric CO2 • CO2 is present at 100s of ppm level: It just reached 410 ppm (Up from ~280 ppm in mid 19th Century) • Careful and accurate measurement is vital!

Methane is 23 times more potent than CO2; N2O is 310 times more potent! https://www.co2.earth/

Gas Analysis

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• Atmospheric monitoring • Remote Sensing - performing measurements on the environment at a distance • Sampling

• Measurements can be made using emission or absorption detect photons emitted by excited molecules

detect reduction in incident photons caused by absorption

M*

M*

M

M

IR Gas Analysis - Absorption • Sample collection for lab analysis • Collection from site or via plane, balloon etc… • Spectrum of gas is obtained at leisure

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long path cells - in lab or on site

IR Gas Analysis - Absorption

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“ALIAS” has flown over 300 times in 7 major NASA missions!

• High resolution, 4-channel scanning tunable laser spectrometer (3.4 to 8 µm) • Direct, simultaneous measurements of HCl, NO2, CH4, N2O, and CO in the strato- and troposphere at sub-ppb! • 80-m pathlength in a 1-m multipass Herriott cell • ‘Spatially resolved’ atmospheric data!

IR Gas Analysis - Absorption

P.of.I.A 17A

• Solar Occultation: • Uses the sun as UV-vis & IR source for absorption spectrum

• Ground base - long path, long observation time, pressure broadening, water E.g. The Wollongong centre for atmospheric chemistry monitors for: O3, ClONO2, HNO3, HCl, CFC-11, CFC-12, CFC-22, NO2, N2O, NO, HF, C2H2, C2H4, C2H6, CH4, CO, COF2, H2O, HCN, HO2, NO2, NH3, N2, OCS. https://www.uow.edu.au/science-medicinehealth/research/centre-for-atmospheric-chemistry/

IR Gas Analysis - Absorption

P.of.I.A 17A

• Solar Occultation: • From a plane or a balloon – or a space shuttle!

• VERY long path lengths – high sensitivity

Space shuttle 200 km profile in a 30 sec sunrise/sunset! - requires very fast spectrometer (0.01 cm-1 spectra in...