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Time Resolved Raman Spectroscopy for Depth Measurements Through VU Research Portal Time-Resolved Raman Spectroscopy for depth analysis in scattering samples Petterson, I.E.I. 2013 document version Publisher's PDF, also known as Version of record Link to publication in VU Research Portal citation for published version (APA) Petterson, I. E. I. (2013). Time-Resolved Raman Spectroscopy for depth analysis in scattering samples. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. E-mail address: [email protected] Download date: 30. Sep. 2021 Chapter 1. Introduction Concepts of Raman spectroscopy 11 1.1 Light scattering and the Raman effect Types of electromagnetic scattering When photons from a monochromatic light source such as a laser illuminate a material, the light can be transmitted, absorbed or scattered. In the case of scattering, the majority of this light is elastically scattered by the material with the same photon energy as the incident beam. This elastic scattering is called Rayleigh scattering for small particles and molecules, Debye scattering for particles with a size on the order of the incident wavelength, and Mie scattering in the case of even larger particles.1 The angular distribution of the scattering by these larger particles is not uniform in all directions and generally forward scattering is favored.1 In addition to elastic Rayleigh scattering, illuminating molecules also results in light scattering of a different color than the incident excitation beam. A small fraction of photons is scattered inelastically after a transfer of vibrational or rotational energy to the scattering molecules. Possibly the first mention of this phenomenon was in 1878, when Lommel2 described characteristics of fluorescence which were dependent on the sample and excitation energy. The inelastic scattering was predicted as a distinct phenomenon by Smekal3 as an analogue to the Compton effect discovered just previously in 1923, and thereafter predictions on the basis of classical electromagnetic theory were made by Kramers and Heisenberg4, Schrödinger5, and Dirac6 in subsequent years.7,8 The scattered light of a different color than the excitation beam was first observed and interpreted as an exchange of vibrational energy by C.V. Raman and K.S. Krishnan in 1928,9,10 and was virtually simultaneously reported by Landsberg and Mandelstam.11 The importance of the finding was recognized with the Nobel prize in Physics awarded to Raman in 1930, and henceforth the effect was named after him.7 12 Resonance Fluorescence Raman Vibration S1 S1 Rayleigh Raman Scattering Scattering Stokes Virtual state = IR absorption Vibration S S0 Elastic Inelastic 0 Figure 1.1. A simplified Jablonski energy level diagram depicting the different energetic processes of elastic and inelastic light scattering, infrared (IR) absorption, Resonance Raman scattering (RRS), and electronic absorption, followed by fluorescence emission. S0 and S1 are the electronic ground state and the first excited state, respectively. Stokes and anti-Stokes Raman scattering Under ambient conditions, by far the most common mode of Raman scattering is Stokes shifted; incident photons excite a molecular vibration of the material from the ground electronic (S0) and vibrational state, and photon energy is lost in the vibrational motion of this molecular bond. Hence these scattered Raman photons will have lower photon energy than the incident beam, and as a result the wavelength of the scattered light is longer. Anti-Stokes Raman scattering (not depicted in Figure 1.1), produced by inelastic scatter with a molecule that already existed in a higher vibration of the S0 level, will result in scattered photons with a gain in energy compared to the incident beam. Anti-Stokes Raman scattering is especially rare under ambient conditions, due to the decreased population of these higher excited vibrational occupancies, as characterized by the Boltzmann distribution.12 The difference in photon energy between the incident excitation and the Raman scattering is equivalent to the energy of a molecular vibration, as will be obvious from Figure 1.1 (wherein 13 the various processes relevant in vibrational spectroscopy are schematically depicted). This energy difference is distinct for each vibration in a molecule. Molecules with N atoms can in theory produce Raman scattering of 3N-6 different energies (3N-5 for linear molecules). While electromagnetic radiation is often described by wavelength (λ), this energy difference (Raman shift) in Raman spectroscopy is usually expressed in wavenumber (σ), or reciprocal wavelength (cm-1). The relationship between wavelength and wavenumber is σ = 1/ 8,12 λ0. Raman shifts will always have the same value when measured on a wavenumber scale, but not on a wavelength scale. Selection rules It is important to note that not all molecular vibrations produce Raman scattering of equal intensity. In general there exists the “rule of mutual exclusion” for infrared (IR) and Raman spectroscopies, namely that in a molecule with a center of symmetry, a vibration that is Raman active is generally not pronounced in IR spectroscopy, and vice versa.7 The molecular center of symmetry determines the electrical dipole moment and polarizability of the molecule; the latter is a measure of the change in electron cloud distribution of a molecule in response to an applied electric field. IR absorption by a molecule results from a direct resonance between the incident light and a vibrational mode of the molecule; the resonant interaction is created by the change in dipole moment with respect to a vibrational motion. In the case of polarizable molecules that may or may not have a permanent dipole, a temporary distortion of the electron cloud around a certain molecular bond is created by interaction with incoming electromagnetic radiation from the excitation beam, creating an induced dipole moment in the molecule, which subsequently allows for radiation emitted in the form of Rayleigh and Raman scattering.8 Thus, in a Raman spectrum only those vibrations are observed that result in a change in polarizability. The following equation expresses the relationship of polarizability to the induced molecular electrical dipole moment, μ, where E denotes the electric field vector and α is the polarizability tensor with units of C m2 V-1.12 μ = α E Additionally the Raman scattering efficiency of a molecule can be expressed by the following relationship, where I0 is the intensity of the incident light, ν0 is its frequency, and Δα is the change in polarizability during the particular vibration: 14 4 2 Raman Intensity, I ~ I0ν0 Δα From this equation it is clear that using excitation sources of higher frequency increases the Raman intensity non-linearly, in theory by a 4th power. In practice one must also consider the effects of this higher frequency excitation on sample photodegradation, as well as towards the creation of fluorescence interference. Thus a molecule will not only produce Raman scattering of different energies, but because of the vibrational dependence on Δα, the different energies will also have different ratios of intensity. By measuring these various energy differences a spectrum is obtained that provides molecular vibrational information unique to each type of molecule. Raman spectroscopy, the observation and measurement of Raman scattering, is a powerful analytical tool in that it provides molecular information, and in most Raman modes is non-invasive. Additionally of note is that due to their low polarizability, water molecules are not very Raman active, and do not generally interfere strongly in Raman measurements as they do in IR spectroscopy. This is particularly helpful in applications of Raman spectroscopy for the study of biomedical specimens and biological samples, which can often contain large amounts of water. 1.2 Fluorescence interference A process that cannot go unmentioned in discussions of Raman scattering is interference from fluorescence emission, also shown in Figure 1.1. When a molecule is excited by an incident beam to the S1 state or higher a photon is absorbed. The excited molecule then rapidly loses energy until it reaches the lowest vibrational state of the S1 level, generally with an efficiency of 100%. Fluorescence as a process occurs in the subsequent transition from the S1 to one of the vibrational states of the electronic ground state S0, via the emission of a photon. As a rule, this emitted photon is of a lower energy than the incident excitation photon, and its energy is otherwise also generally independent of the excitation energy.13 While Raman scattering is considered to be an instantaneous process (occurring in the time it takes for a photon to travel one wavelength, typically a few femtoseconds), fluorescence emission rates are typically 108 s-1 with lifetimes generally on the order of a few nanoseconds.13 15 The time scale difference between these processes is an important consideration for Time- Resolved measurements. Not all molecules fluoresce, but for compounds that do show fluorescence it can be 106 times more probable than Raman scattering. Therefore, fluorescence emission and Raman scattering from a sample may have a great overall difference in intensity. Most samples include at least a small amount of some fluorescent species, and even minute fluorescent contaminants can greatly interfere with standard Raman spectroscopy, as fluorescence photons result in a strong background and increasing overall shot noise, masking the much weaker Raman signal.
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