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Vibrational spectroscopy: infra-red and Raman spectroscopy
Published in D. Campbell, R.A. Pethrick, J.R. White, Polymer Characterization, 2017
D. Campbell, R.A. Pethrick, J.R. White
The directional properties of Raman spectra are determined both by the symmetry of the vibrations and by the orientation of the molecules relative to the plane of polarization. Even for randomly orientated molecules, such as in liquids, the intensities of Raman lines are dependent on the polarization of the incident beam and the symmetry of the vibrational modes. For oriented specimens, the intensity of the scattered light is dependent also on the molecular orientation relative to the plane of polarization of the incident radiation as well as on the direction and plane of polarization of the scattered radiation. Polarization effects are expressed quantitatively by the depolarization ratio (ρ) defined as ρ =I///I⊥, where I// is the intensity of the light scattered with its plane of polarization in the same direction as that of the incident light, and I⊥ is the intensity of light having its plane of polarization at right angles to that of the incident light. A band is considered as being depolarized when ρ = 0.75 and polarized for ρ < 0.75. It is fortuitous that the light emitted by gas lasers is highly plane polarized.
Raman spectroscopy in biomineralization
Published in Elaine DiMasi, Laurie B. Gower, Biomineralization Sourcebook, 2014
Karen Esmonde-White, Francis Esmonde-White
e depolarization ratio is calculating by taking the Raman spectral intensity recorded with opposite polarization states set on the two paths and dividing by the Raman spectral intensity recorded with the same polarization state set. Raghavan et al. have demonstrated that orientation of particular molecular species can be quantitatively assessed using this method (Raghavan et al. 2010). A common question by those familiar with microscopy, but unfamiliar with Raman spectroscopy, is "What is the spatial resolution of this Raman microscope?" Nominally, it should be possible to measure the Raman spectrum from a region approximately half the size of excitation light wavelength. Unfortunately, in practice, there is no precise answer in most cases where Raman microscopy is applied to biomedical tissues. Light scattering causes the excitation light to be di used, and multiple scattering can greatly broaden the region from which Raman-scattered light is collected. Together these e ects typically increase the size of the sampled region. This is also important because light scattering is not uniform through tissue. As a result, as a sample is imaged, the interrogated volume will change. Finally, microscopes typically have much ner resolution in the plane of the stage than along the optical imaging axis. As a result of these many confounding issues, the sampling volume is not necessarily symmetric (Everall 2000; Everall et al. 2007). Optimal spatial resolution is obtained using a confocal Raman microscope with a well-focused single-mode laser beam, a very high-magni cation objective, and a pinhole or other small aperture to limit collection of out-of-focus light. The speed of acquiring a map (or image) must be balanced with the spatial resolution of the system. In practice, fast imaging is not possible with low excitation power in confocal imaging con gurations. In many applications, high spatial resolution is not required. For experiments that require either rapid signal collection or scanning over a large region, semiconfocal systems are a compromise. These systems are equipped with a line-shaped laser and a slit, rather than a pinhole. We note that the spatial resolution along the slit is degraded substantially, especially when compared to a pinhole. While the results of most Raman microscopy is not compromised by spatial resolution degradation, results of polarized Raman spectroscopy can be strongly a ected by multiple optical scattering. To avoid these e ects in polarized Raman spectroscopy, a high-magni cation objective and small pinhole or slit are typically used.
MRS-STFF: Evaluation of biomass energy combustion and associated pollutants
Published in Human and Ecological Risk Assessment: An International Journal, 2022
Desheng Liu, Liuyan Wang, Yue Sun, Shuaishuai Lian
Figure 4 shows the results of lidar impact processing at three time nodes in the selected research area in the spring plowing season of 2017. The backscattering coefficient is an important parameter in CALIPSO aerosol products, and the larger backscattering coefficient indicates the stronger the scattering power of particles in the atmosphere and the weaker vice versa. The color ratio images were used to determine the size of aerosol particles, and the aerosol layer ≤1, Cloud color ratio >1. For the biomass combustion aerosol, the decay color ratio increases significantly, the depolarization ratio image can determine whether the particle is a spherical aerosol based on whether the different shape can change the polarization state of the dorsal scattered light. And the smaller the depolarization ratio, the more regular the particulate matter.
A polarization-resolved light scattering method for eliminating the interference of water aerosol in industrial stack PM measurement
Published in Aerosol Science and Technology, 2020
Vipul Dogra, Satyanarayanan Seshadri
The elements of the scattering matrix can be derived from the measured intensity, once the details of incident and detected Stokes vectors are known, including the characteristics of the optical components used. Analysis of the elements of the scattering matrix further reveals the nature of the scatterer in response to incident light. Typically a detector is setup to measure the total intensity of light scattered from the illuminated sample. These intensity measurements are used to estimate the matrix elements as there exists specific relationship between the two. However, in most cases, the intensity values are directly used to study the behavior of the scatterer. Iannone et al. (2011), used the ratio of scattered intensities, called the polarization ratio, for studying the effect of polarized light on scattering by absorbing and non-absorbing particles. They had also defined the depolarization ratio as In these representations, the first and second subscripts denote the polarization planes of incident light and detected signals, respectively.
In vivo spectroscopy: optical fiber probes for clinical applications
Published in Expert Review of Medical Devices, 2022
Ajaya Kumar Barik, Sanoop Pavithran M, Jijo Lukose, Rekha Upadhya, Muralidhar V Pai, V.B. Kartha, Santhosh Chidangil
Another disadvantage of using optical fibers is that the polarization properties of radiation in both light delivery and collection fibers will be scrambled, thus making them unsuitable to measure the depolarization ratio in Raman spectra (which depends upon the symmetry of the molecule and the normal vibrational modes) for characterization of the band types in different samples of same composition, differing in surroundings, complex formation etc.