Dispersion analysis of arbitrarily cut uniaxial crystals

Höfer, Sonja; Uecker, R.; Kwasniewski, Albert; Popp, Jürgen; Mayerhöfer, Thomas G.

doi:10.1016/j.vibspec.2015.03.004

Cited by 10 publications

(7 citation statements)

References 18 publications

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“…This process could potentially also occur for quartz, a uniaxial crystal, but because the differences arising from crystal orientation are less significant for quartz (compare Figures 8 and 9 in Höfer et al . []), it is less noticeable in the polished microcrystalline quartz spectrum. The polished microcrystalline quartz sample is spectrally similar to macrocrystalline quartz (Figure b).…”

Section: Discussionmentioning

confidence: 97%

“…As shown in Höfer et al . [], the fundamental calcite spectral features change significantly when viewed along directions that are not perpendicular to the major crystallographic axes. Thus, we propose that the process of polishing disrupts a surface composed of crystal cleavage planes, resulting in a spectrum that differs from a cleavage face of Iceland spar.…”

Section: Discussionmentioning

confidence: 99%

“…Due to its orientation relative to the crystallographic axes, the spectrum of a calcite cleavage face cannot be modeled with just one of the principle indices of refraction (ñ e , ñ o ), nor with a simple 1/3 + 2/3 mixture of the two (a combination that is conventionally used to approximate the contribution from randomly oriented grains [e.g., Chandrasekhar, 1951;Draine, 1988;Draine and Malhotra, 1993;Pitman et al, 2005]). As shown in Höfer et al [2015], the fundamental calcite spectral features change significantly when viewed along directions that are not perpendicular to the major crystallographic axes. Thus, we propose that the process of polishing disrupts a surface composed of crystal cleavage planes, resulting in a spectrum that differs from a cleavage face of Iceland spar.…”

Section: Spectral Differences Between Macrocrystalline and Polished Mmentioning

confidence: 99%

“…Indeed, the broadened~1450 cm À1 feature of the polished sample appears similar to a theoretical emissivity spectrum of particulate calcite with random crystal orientations, calculated using the Fresnel equation under normal incidence [e.g., Glotch et al, 2006], with calcite optical constants from Lane [1999] in the 2:1 ñ o :ñ e proportions described above as inputs ( Figure 13). This process could potentially also occur for quartz, a uniaxial crystal, but because the differences arising from crystal orientation are less significant for quartz (compare Figures 8 and 9 in Höfer et al [2015]), it is less noticeable in the polished microcrystalline quartz spectrum. The polished microcrystalline quartz sample is spectrally similar to macrocrystalline quartz (Figure 7b).…”

Section: Spectral Differences Between Macrocrystalline and Polished Mmentioning

confidence: 99%

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Thermal emission spectroscopy of microcrystalline sedimentary phases: Effects of natural surface roughness on spectral feature shape

et al. 2016

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Distinguishing between microcrystalline and macrocrystalline mineral phases can help constrain the conditions under which those minerals formed or the degree of postdepositional alteration. This study demonstrates the effects of crystal size and surface roughness on thermal infrared emission spectra of micro and macrocrystalline phases of the two most common minerals on Earth, quartz and calcite. Given the characteristic depositional and environmental conditions under which microcrystalline minerals form, and the recent observations of high-silica deposits on Mars, it is important to understand how these unique materials can be identified using remote infrared spectroscopy techniques. We find that (a) microcrystalline minerals exhibit naturally rough surfaces compared to their macrocrystalline counterparts at the 10 μm scale; and that (b) this roughness causes distinct spectral differences within the Reststrahlen bands of each mineral. These spectral differences occur for surfaces that are rough on the wavelength scale, where the absorption coefficient (k) is large. Specifically, the wavelength positions of the Reststrahlen features for microcrystalline phases are narrowed and shifted compared to macrocrystalline counterparts. The spectral shape differences are small enough that the composition of the material is still recognizable, but large enough such that a roughness effect could be detected. Petrographic and topographic analyses of microcrystalline samples suggest a relationship between crystal size and surface roughness. Together, these observations suggest it may be possible to make general inferences about microcrystallinity from the thermal infrared spectral character of samples, which could aid in reconstructions of sedimentary rock diagenesis where corresponding petrographic or microimaging is not available.Acquisition of laboratory spectra of a wide variety of reference sedimentary materials is becoming increasingly important for Mars exploration. From both landed and orbital missions, rocks have been identified on Mars that have formed or been modified in the presence of ground water or surface water [Squyres et al., 2004;Ming et al., 2006;Mustard et al., 2008;McLennan et al., 2005;Ehlmann et al., 2009;Grotzinger et al., 2013]. In the infrared spectral range (~1.5-100 μm), spectral features arise from molecular vibrations within HARDGROVE ET AL.IR SPECTRA OF MICROCRYSTALLINE PHASES 542 PUBLICATIONS

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Section: Discussionmentioning

confidence: 97%

Section: Discussionmentioning

confidence: 99%

Section: Spectral Differences Between Macrocrystalline and Polished Mmentioning

confidence: 99%

Section: Spectral Differences Between Macrocrystalline and Polished Mmentioning

confidence: 99%

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Thermal emission spectroscopy of microcrystalline sedimentary phases: Effects of natural surface roughness on spectral feature shape

et al. 2016

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“…The later immediately followed the first quantitative analysis of triclinic crystals in 2013 . Recently, we pushed this approach further to allow a quantitative analysis of single crystals with a priori unknown crystal symmetry and orientation …”

Section: Introductionmentioning

confidence: 99%

Employing Theories Far beyond Their Limits – Linear Dichroism Theory

Mayerhöfer

2018

ChemPhysChem

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Using linear polarized light, it is possible in case of ordered structures, such as stretched polymers or single crystals, to determine the orientation of the transition moments of electronic and vibrational transitions. This not only helps to resolve overlapping bands, but also assigning the symmetry species of the transitions and to elucidate the structure. To perform spectral evaluation quantitatively, a sometimes "Linear Dichroism Theory" called approach is very often used. This approach links the relative orientation of the transition moment and polarization direction to the quantity absorbance. This linkage is highly questionable for several reasons. First of all, absorbance is a quantity that is by its definition not compatible with Maxwell's equations. Furthermore, absorbance seems not to be the quantity which is generally compatible with linear dichroism theory. In addition, linear dichroism theory disregards that it is not only the angle between transition moment and polarization direction, but also the angle between sample surface and transition moment, that influences band shape and intensity. Accordingly, the often invoked "magic angle" has never existed and the orientation distribution influences spectra to a much higher degree than if linear dichroism theory would hold strictly. A last point that is completely ignored by linear dichroism theory is the fact that partially oriented or randomly-oriented samples usually consist of ordered domains. It is their size relative to the wavelength of light that can also greatly influence a spectrum. All these findings can help to elucidate orientation to a much higher degree by optical methods than currently thought possible by the users of linear dichroism theory. Hence, it is the goal of this contribution to point out these shortcomings of linear dichroism theory to its users to stimulate efforts to overcome the long-lasting stagnation of this important field.

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Dispersion analysis of anisotropic crystals—Examples

Mayerhöfer

2024

Wave Optics in Infrared Spectroscopy

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Dispersion analysis of arbitrarily cut uniaxial crystals

Cited by 10 publications

References 18 publications

Thermal emission spectroscopy of microcrystalline sedimentary phases: Effects of natural surface roughness on spectral feature shape

Thermal emission spectroscopy of microcrystalline sedimentary phases: Effects of natural surface roughness on spectral feature shape

Employing Theories Far beyond Their Limits – Linear Dichroism Theory

Dispersion analysis of anisotropic crystals—Examples

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