Infrared spectroscopy involves the interaction of infrared radiation with matter. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemical substances. Samples may be solid, liquid, or gas. The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrometer to produce an infrared spectrum. An IR spectrum can be visualized in a graph of infrared light absorbance on the vertical axis vs. frequency or wavelength on the horizontal axis. Typical units of frequency used in IR spectra are reciprocal centimeters, with the symbol cm−1. Units of IR wavelength are commonly given in micrometers, symbol μm, which are related to wave numbers in a reciprocal way. A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer. Two-dimensional IR is also possible as discussed below.
Spectroscopy is the study of the interaction between matter and electromagnetic radiation. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, by a prism. Later the concept was expanded greatly to include any interaction with radiative energy as a function of its wavelength or frequency, predominantly in the electromagnetic spectrum, although matter waves and acoustic waves can also be considered forms of radiative energy; recently, with tremendous difficulty, even gravitational waves have been associated with a spectral signature in the context of the Laser Interferometer Gravitational-Wave Observatory (LIGO) and laser interferometry. Spectroscopic data are often represented by an emission spectrum, a plot of the response of interest, as a function of wavelength or frequency.
Raman spectroscopy ; is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman spectroscopy is commonly used in chemistry to provide a structural fingerprint by which molecules can be identified.
Fluorescence spectroscopy is a type of electromagnetic spectroscopy that analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electrons in molecules of certain compounds and causes them to emit light; typically, but not necessarily, visible light. A complementary technique is absorption spectroscopy. In the special case of single molecule fluorescence spectroscopy, intensity fluctuations from the emitted light are measured from either single fluorophores, or pairs of fluorophores.
Brillouin scattering, named after Léon Brillouin, refers to the interaction of light with the material waves in a medium. It is mediated by the refractive index dependence on the material properties of the medium; as described in optics, the index of refraction of a transparent material changes under deformation.
Raman scattering or the Raman effect is the inelastic scattering of photons by matter, meaning that there is an exchange of energy and a change in the light's direction. Typically this involves vibrational energy being gained by a molecule as incident photons from a visible laser are shifted to lower energy. This is called normal Stokes Raman scattering. The effect is exploited by chemists and physicists to gain information about materials for a variety of purposes by performing various forms of Raman spectroscopy. Many other variants of Raman spectroscopy allow rotational energy to be examined and electronic energy levels may be examined if an X-ray source is used in addition to other possibilities. More complex techniques involving pulsed lasers, multiple laser beams and so on are known.
Resonance Raman spectroscopy is a Raman spectroscopy technique in which the incident photon energy is close in energy to an electronic transition of a compound or material under examination. The frequency coincidence can lead to greatly enhanced intensity of the Raman scattering, which facilitates the study of chemical compounds present at low concentrations.
Functional near-infrared spectroscopy (fNIRS), or Optical Topography as it is called in Japan exclusively, is the use of near-infrared spectroscopy (NIRS) for functional neuroimaging. Using fNIRS, cerebral hemodynamic responses are measured by near-infrared light, which go in line with cerebral activation or deactivation. In particular, this technology is capable of visualizing changes both in oxy- and deoxyhemoglobin concentration.
Surface-enhanced Raman spectroscopy or surface-enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on rough metal surfaces or by nanostructures such as plasmonic-magnetic silica nanotubes. The enhancement factor can be as much as 1010 to 1011, which means the technique may detect single molecules.
X-ray absorption near edge structure (XANES), also known as near edge X-ray absorption fine structure (NEXAFS), is a type of absorption spectroscopy that indicates the features in the X-ray absorption spectra (XAS) of condensed matter due to the photoabsorption cross section for electronic transitions from an atomic core level to final states in the energy region of 50–100 eV above the selected atomic core level ionization energy, where the wavelength of the photoelectron is larger than the interatomic distance between the absorbing atom and its first neighbour atoms.
Chemical imaging is the analytical capability to create a visual image of components distribution from simultaneous measurement of spectra and spatial, time information. Hyperspectral imaging measures contiguous spectral bands, as opposed to multispectral imaging which measures spaced spectral bands.
Photothermal microspectroscopy (PTMS), alternatively known as photothermal temperature fluctuation (PTTF), is derived from two parent instrumental techniques: infrared spectroscopy and atomic force microscopy (AFM). In one particular type of AFM, known as scanning thermal microscopy (SThM), the imaging probe is a sub-miniature temperature sensor, which may be a thermocouple or a resistance thermometer. This same type of detector is employed in a PTMS instrument, enabling it to provide AFM/SThM images: However, the chief additional use of PTMS is to yield infrared spectra from sample regions below a micrometer, as outlined below.
Fellgett's advantage or the multiplex advantage is an improvement in signal to noise ratio that is gained when taking multiplexed measurements rather than direct measurements. The name is derived from P. B. Fellgett, who first made the observation as part of his PhD. When measuring a signal whose noise is dominated by detector noise, a multiplexed measurement such as the signal generated by a Fourier transform spectrometer can produce a relative improvement in signal-to-noise ratio (SNR), compared to an equivalent scanning monochromator, of the order of the square root of m, where m is the number of sample points comprising the spectrum.
Transmission Raman spectroscopy (TRS) is a variant of Raman spectroscopy beneficial in probing bulk content of diffusely scattering samples. Although it was demonstrated in the early days of Raman spectroscopy it was never exploited in practical settings, probably due to limitations of technology at the time. It was rediscovered in 2006, where the authors showed it was capable of allowing Raman spectroscopy through many millimetres of tabletted or powdered samples. In addition, this research has also identified several highly beneficial analytical properties of this approach, including the ability to probe bulk content of powders and tissue in the absence of subsampling and to reject Raman and fluorescence components originating from the surface of the sample.
The Raman microscope is a laser-based microscopic device used to perform Raman spectroscopy. The term MOLE is used to refer to the Raman-based microprobe. The technique used is named after C. V. Raman who discovered the scattering properties in liquids.
Catherine Margaret Shachaf is an Indian cell biologist. She previously held an instructor position at Stanford University School of Medicine and has made ground-breaking discoveries in cancer research. Shachaf has spoken at scientific conferences and has published more than 17 journal articles. Her leading work was published in Nature (2004), "MYC Inactivation Uncovers Pluripotent Differentiation and Tumor Dormancy in Hepatocellular Cancer." Shachaf is on the Editorial Board of the International Journal of Green Nanotechnology.
The technique of vibrational analysis with scanning probe microscopy allows probing vibrational properties of materials at the submicrometer scale, and even of individual molecules. This is accomplished by integrating scanning probe microscopy (SPM) and vibrational spectroscopy. This combination allows for much higher spatial resolution than can be achieved with conventional Raman/FTIR instrumentation. The technique is also nondestructive, requires non-extensive sample preparation, and provides more contrast such as intensity contrast, polarization contrast and wavelength contrast, as well as providing specific chemical information and topography images simultaneously.
AFM-IR is one of a family of techniques that are derived from a combination of two parent instrumental techniques; infrared spectroscopy and scanning probe microscopy (SPM). The term was first used to denote a method that combined a tuneable free electron laser with an atomic force microscope equipped with a sharp probe that measured the local absorption of infrared light by a sample; it required that the sample be coupled to an infrared-transparent prism and be less than 1μm thick. It improved the spatial resolution of photothermal AFM-based techniques from microns to circa 100 nm.
Stimulated Raman spectroscopy, also referred to as stimulated raman scattering (SRS) is a form of spectroscopy employed in physics, chemistry, biology, and other fields. The basic mechanism resembles that of spontaneous Raman spectroscopy: a pump photon, of the angular frequency , which is absorbed by a molecule has some small probability of inducing some vibrational transition, as opposed to inducing a simple Rayleigh transition. This makes the molecule emit a photon at a shifted frequency. However, SRS, as opposed to spontaneous Raman spectroscopy, is a third-order non-linear phenomenon involving a second photon—the Stokes photon of angular frequency —which stimulates a specific transition. When the difference in frequency between both photons resembles that of a specific vibrational transition the occurrence of this transition is resonantly enhanced. In SRS, the signal is equivalent to changes in the intensity of the pump and Stokes beams. Employing a pump laser beam of a constant frequency and a Stokes laser beam of a scanned frequency allows for the unraveling of the spectral fingerprint of the molecule. This spectral fingerprint differs from those obtained by other spectroscopy methods such as Rayleigh scattering as the Raman transitions confer to different exclusion rules than those that apply for Rayleigh transitions.
Micro-spatially offset Raman spectroscopy (micro-SORS) is an analytical technique developed in 2014 that combines SORS with microscopy. The technique derives its sublayer‐resolving properties from its parent technique SORS. The main difference between SORS and micro-SORS is the spatial resolution: while SORS is suited to the analysis of millimetric layers, micro-SORS is able to resolve thin, micrometric-scale layers. Similarly to SORS technique, micro-SORS is able to preferentially collect the Raman photons generated under the surface in turbid media. In this way, it is possible to reconstruct the chemical makeup of micrometric multi-layered turbid system in a non destructive way. Micro-SORS is particularly useful when dealing with precious or unique objects as for Cultural Heritage field and Forensic Science or in biomedical applications, where a non-destructive molecular characterization constitute a great advantage.
