Optical Absorption PDF

OPTICAL ABSORPTION OPTICAL ABSORPTION ◼Biological tissue consists of molecules. (water, proteins, lipids, DNA, etc). ◼ When light is absorbed by tissue, it is absorbed by specific molecules that are part of,or within, the tissue. ◼ Absorbing molecules are called chromophore...

OPTICAL ABSORPTION OPTICAL ABSORPTION ◼Biological tissue consists of molecules. (water, proteins, lipids, DNA, etc). ◼ When light is absorbed by tissue, it is absorbed by specific molecules that are part of,or within, the tissue. ◼ Absorbing molecules are called chromophores. OPTICAL ABSORPTION ◼ Different chromophores havedifferent absorption spectra, in other words, they absorb different wavelengths of l i g h t more or less strongly. ◼ The different absorption spectra arise due to the differences in the molecular, or atomic, structure of the chromophores. OPTICAL ABSORPTION TABLE OF CONTENT ◼Molecular Energy Levels ◼Light Absorption ◼Absorption Spectra Absorption Coefficient ◼Nonlinear effects O ▪ Molecular Energy Levels ▪ What is Energy? ▪ Internal Energy ▪ Energy Quantization ▪ Thermal Equilibrium: Boltzmann’s Distribution O ▪. Energy comes in many forms. ▪ Two broad classes : ▪ Kinetic energy, which is energy that an object or system has by virtue of its motion (or of the motion of its internal components), ▪ Potential energy, which is the energy an object or system has because of its position.  OPTICAL ABSORPTION MOLECULAR ENERGY LEVELS WHAT IS ENERGY ◼ In therapeutic applications of tissue optics the: ◼ the light is absorbed and the kinetic energy becomes internal energy of the molecule, eg. ◼This energy can, in turn, cause thermal or chemical effects, ◼Molecules can only store certain fixed amounts of energy, not any arbitrary amount. ◼These are called the ‘energy levels’ of the molecule OPTICAL ABSORPTION MOLECULAR ENERGY LEVELS INTERNAL ENERGY ◼ A molecule can store energy in a number of different forms, for example: ◼ Rotational kinetic energy, ◼ Vibrational energy (with potential and kinetic parts), ◼ Translational kinetic energy (if the molecules are free to move as theyare in a fluid), ◼ Electronic potential energy. OPTICAL ABSORPTION MOLECULAR ENERGY LEVELS INTERNAL ENERGY It is the structure of these energy levels that gives rise to a molecule’s absorption spectrum. The sum of all these types of molecular energy is called the internal energy. OPTICAL ABSORPTION MOLECULAR ENERGY LEVELS INTERNAL ENERGY ◼The kinetic part of the internal energy is associated with thermal motion, and so is related to temperature when viewed from a macroscopic perspective. OPTICAL ABSORPTION :MOLECULAR ENERGY LEVELS INTERNAL ENERGY ◼ Rotational Kinetic Energy ◼ A molecule that is not bound to another molecule is free to rotate as a solid body, so it can store energy in the form of rotational motion. ◼ Similarly, two parts of a molecule held together by a bond that allows them to rotate with respect to each other can also store rotational energy. ◼ Rotational modes such as this are very easily excited - very little energy, so these usually correspond to absorption in the microwave region of the EM spectrum (longer wavelengths than visible light; lower energy photons) OPTICAL ABSORPTION MOLECULAR ENERGY LEVELS INTERNAL ENERGY ◼Vibrational energy ◼Different parts of molecules can also vibrate with respect to other parts as the bonds holding the constituent atoms together are not completely rigid but a little elastic. ◼ OPTICAL ABSORPTION MOLECULAR ENERGY LEVELS INTERNAL ENERGY ◼Vibrational modes typically require much more energy than rotational modes to excite them, so a vibrational transition requires photons in the near- infrared or red end of the visible spectrum, which carry more energy OPTICAL ABSORPTION MOLECULAR ENERGY LEVELS INTERNAL ENERGY ◼ Electronic energy levels refer to the potential energy of electrons with respect to their distance from the nucleus. ◼ To movean electron to a more energetic orbit (on average further from the nucleus) typically requires much more energy than is required to excite vibrational modes, so blue or ultraviolet (UV) light is necessary to excite valence (outer) electrons, and the more tightly bound inner electrons require X-ray photons. In general the energy of electronic transitions, !"!#, are much greater than those of vibrational modes which themselvesare muchgreater than those of rotational modes: ◼. When electrons have been excited by photon absorption, the molecule is more likely to react with neighbouring molecules, and we enter the realm of photochemistry OPTICAL ABSORPTION MOLECULAR ENERGY LEVELS INTERNAL ENERGY LIGHT ABSORPTION RESONANCE ABSORPTION ◼Light Absorption ◼Resonance Absorption LIGHT ABSORPTION RESONANCE ABSORPTION ◼. It is typical of vibrating systems that it is easiest to couple energy into them at their natural frequencies. ◼ When the frequency of the driving force and the natural frequency match it is called a resonant condition or resonance. ◼ In our application, light is the driving force with frequency and the molecule it is potentially interacting with is the system with a natural frequency. ◼ In fact, a system with many natural frequencies, one corresponding to each ofthe energy levels described above. LIGHT ABSORPTION RESONANCE ABSORPTION ◼ So, for the energy of the light to be coupled into the molecule the frequency ofthe light must match a natural frequency of the molecule. ◼ Absorption is highly likely when the photon energy equals the energy difference between the current state (the current energy level) and an excited state (When absorption does occur, it occurs very quickly, on the timescale of a period, (femtosecond). ◼Absorption Spectra ◼ Spectral Lines & Line Broadening. ◼ Chromophores of Clinical Interest ◼ Near Infrared Window SPECTRA ABSORPTION LINES SPECTRAL ◼ The emission spectrum of a gas, contains very narrow peaks of strong emission at frequencies – or equivalently wavelengths, or photon energies ◼. It is also only at those frequencies that light can be absorbed, and gasses therefore often have strong and distinct absorption lines. ◼ Actually, that is not quite accurate, as these spectra lines are not infinitely thin: ◼ In other words, absorption will still occur with a finite probability at wavelengths slightly different from the resonance frequency. SPECTRA ABSORPTION LINES SPECTRAL ◼ The emission spectrum of a gas, contains very narrow peaks of strong emission at frequencies – or equivalently wavelengths, or photon energies ◼. It is also only at those frequencies that light can be absorbed, and gasses therefore often have strong and distinct absorption lines. ◼ Actually, that is not quite accurate, as these spectra lines are not infinitely thin: ◼ In other words, absorption will still occur with a finite probability at wavelengths slightly different from the resonance frequency. ◼ The line shape function, such as the function describes the shape of the absorption spectrum close to the resonance frequency !. It peaks at frequency but is also non-zero at nearby frequencies, indicating that the absorption bandof an allowed transition is not a single precise line at the resonance frequency but covers a small range of wavelengths. ◼ This is called spectral broadening. SPECTRA ABSORPTION LINES SPECTRAL ◼ The line shape function, such as the function describes the shape of the absorption spectrumclose to the resonance frequency ! ◼ It peaks at frequency but is also non- zero at nearby frequencies, indicating that the absorption band of an allowed transition is not a single precise line at the resonance frequency but covers a small range of wavelengths. ◼ This is called spectral broadening.  INTERESTCLINICAL OF SPECTRA CHROMOPHORES ABSORPTION ◼In complex condensed matter such as biologicaltissue, however, the absorption spectra are much more smoothly varying. ◼ Many biomoleculesare quite large and complex with very many vibrational energy levels between the electronic peaks. ◼ Combined with the broadening mechanisms described above, this can give rise to a continuously varying absorption spectrum. INTEREST CLINICALOF SPECTRA CHROMOPHORES ABSORPTION ◼ further reason is that biological tissue is rarely, if ever, homogeneous but consists of many different types of molecules. ◼Measured absorption spectra, therefore, will be a combination of the spectra of the component molecules present. INTEREST CLINICALOF SPECTRA CHROMOPHORES ABSORPTION INTEREST CLINICALOF SPECTRA CHROMOPHORES ABSORPTION ◼ the absorption spectra for various important endogenous chromophores. ◼Endogenous absorbers are those naturally occurring in the body, as opposed to exogeneous absorbers, which are those introduced, such as contrast agents or tattoos. INFRARED ABSOPRTION SPECTRA NEAR WINDOW ◼The wavelength range between about 650-1300nm is known as the near-infrared window, ◼those are the optical wavelengths that can penetrate deepest into tissue. WINDOWINFRARED ABSOPRTION SPECTRA NEAR ◼The reason is that the absorption of endogenous chromophores, water, hemoglobin, etc are all low at these wavelengths, and optical scattering, which tends to increase at shorter wavelengths, still allows the light to penetrate to some extent. ◼Absorption Coefficient ◼Absorption Cross-Section ◼Molar Absorption Coefficient ◼Absorption Coefficient ◼Absorption Coefficient as a Probability COEFFICIENT ABSORPTION CROSS-SECTION ABSORPTION ◼For a photon of frequency , the likelihood of a transition from state a to state b is often given in terms of the absorption cross-section, ◼The absorption cross-section is the ratio of the power absorbed by the molecule to the incident power per unit area, so has units of area. COEFFICIENT ABSORPTION CROSS-SECTION ABSORPTION COEFFICIENT ABSORPTION COEFFICIENT ABSORPTION MOLAR ◼The description of the absorption of a single molecule is not muchuse on its own in tissue optics, and it is necessary to moveto a description of the absorption of bulk matter consisting of very many molecules. ◼ The molar absorption coefficient, , is obtained by summing the absorption cross-sections of all the molecules in one mole of a substance. COEFFICIENT ABSORPTION COEFFICIENT ABSORPTION MOLAR ◼One mole is just NA molecules, where Avogadro’s constant NA= 6.022×10-23, so multiplying by NA gives: COEFFICIENT ABSORPTION ◼As most biological media contain many types of absorbers, eg. blood, lipids, water, melanin, the total absorption coefficient of a material consisting of K chromophores is the sum of the molar absorption coefficients weighted by their concentrations: COEFFICIENT ABSORPTION CROSS-SECTION ABSORPTION Nonlinear ◼ effects............................................................................................................... 18 Multiphoton 2.6.1 ◼ Absorption............................................................................................ 18 Depletion, Ground-State 2.6.2 ◼ Saturation.......................................................................... 19 and Bleaching 2.6.3 ◼ Photostability..................................................................................... 19