Absorption polarisation spectroscopy

An important development in microwave/optical double resonance, called microwave/optical polarisation spectroscopy, was described by Ernst and Torring [42], The principles of this technique are illustrated in figure 11.22. A linearly polarised probe beam from a tunable laser is sent through the gas sample and a nearly crossed linear polariser, before its final detection. Polarised microwave radiation resonant with a rotational transition in the gas sample is introduced via a microwave horn as shown, and resonant absorption results in a partial change in polarisation of... 

The saturated absorption method has been applied by a number of authors to the study of the Ha transition. The technique is often preferred over its rival polarisation spectroscopy because the latter gives in general a mixture of symmetrical and asymmetrical lineshapes... 

Polarisation modulation infrared rejiection-absorption spectroscopy (PM-IRRAS or JRRAS). Potential modulation IR studies rely on switching the potential at a reflective electrode between rest and active states, generating difference spectra. However, the EMIRS technique has several drawbacks the relatively fast potential modulation requires that only fast and reversible electrochemical process are investigated the absorption due to irreversibly chemisorbed species would be gradually eliminated by the rapid perturbation. Secondly, there is some concern that rapid modulation between two potentials may, to some extent, in itself induce reactions to occur. 

The selection rules for the Raman effect are quite different from those for IR spectroscopy. The mechanism involves interaction between the incident radiation and the fluctuating polarisability of the molecule, in contrast to the fluctuating dipole moment in IR absorption. The dipole moment is a vector quantity, and can be resolved into components along three Cartesian axes. The polarisability is a tensor quantity, whose components can be written as products of Cartesian axes. For a molecule having no symmetry at all, or having only a plane of symmetry, all... 

Both these concerns were addressed by the development of modified IR techniques. In the technique of Subtractively Normalised Fourier Transform IR Spectroscopy (SNIFTIRS) or Potential Difference IR (SPAIRS or PDIR) [37], the increased stability and sensitivity of Fourier Transform IR is exploited, allowing usable spectra to be obtained by simple subtraction and ratioing of spectra obtained at two potentials without the need for potential modulation or repeated stepping. A second technique which does not call for potential modulation, but actually modulates the polarisation direction of the incoming IR beam is termed Photo-elastically Modulated Infra-Red Reflectance Absorption Spectroscopy (PM-IRRAS) this was applied to the methanol chemisorption problem by Russell and co-workers [44], and Beden s assignments verified, including the potential-induced shift model for COads. 

Reflection spectroscopy is used for more accurate quantification of the radiation reflected by a sample the intensity, spectral composition, angular distribution and polarisation can be analysed. This method is particularly apt for measuring samples that are impervious to light, that is to say, wherever absorption spectroscopy cannot be used. 

In Sect. 2.5,1 pointed out that the auyu and 7i yu configurations differ mainly in the density of their excited states. The former should create 16 states, the latter 32. Polarised absorption spectroscopy only detects 12 excited states, so both configurations remain possible. The cynic can claim that many more states are present but are undetected. This argument can never be completely overturned, but can be undermined by increasing the number of observables characterising the states, and by reducing the probability that some states are not detected. Two-photon absorption (TPA) is very helpful in this regard. 

Pure MCD with no rotation occurs if the sample is optically thick and completely absorbing in one circular polarisation, but not in the other. Pure MOR will occur when both circular polarisations are equally absorbed, i.e. the absorption coefficients ot+ v) and ot-(v) are equal, but the refractive indices n+(v) and n-(v) are not equal. The latter condition is satisfied at the centre of symmetry of the rotation pattern, viz. the field-free resonance frequency vq.6 In principle, the situation seems simpler when either pure MOR or pure MCD occurs, which is why most of the effort has traditionally been expended in separating one from the other, leading to MOR and MCD spectroscopies. 

This is called the Kramers-Kronig (KK) relationship, from which the dielectric function e = ej + e2 can be derived [3.25]. Since e is also a linear response function, ej and 2 are again related by the KK relationship, thus the information contained in the dielectric function can be examined by concentrating on one of the two components of the dielectric function. We choose to work with 2(m) because it is what optical (X-ray) absorption spectroscopy measures and can be directly related to the atomic polarisability Im[a(o )] that appeared in (3.5). 

The expression above must be summed over all possible transitions between states m and n to obtain a total absorption coefficient. In rotational spectroscopy, every transition originating in state J consists of 27 + 1 overlapping transitions whose magnetic quantum number M ranges from -7 to +7, but whose frequencies are identical. For plane polarised MMW interacting with all molecules, the relative strength of each M component works out as — and, after... 

PMFTIRRAS Polarisation modulated Fourier transform infrared reflection absorption spectroscopy... 

The usual approach used to characterise the molecular orientation in biaxially oriented samples by IR spectroscopy is used to measure spectra with polarisation in all three directions - machine, transverse and normal (or thickness). However, the latter measurement is rather difficult to make experimentally. A new approach to characterising the molecular orientation in both uniaxially and biaxially oriented samples of PETP is employed. It makes use of the ratio of the absorption bands near 1250 and 1725 cm-1, the first of which shows parallel dichroism and the second perpendicular dichroism. An equation is developed that relates this ratio to the molecular orientation with respect to the direction of measurement. Thus it is possible to determine individually the orientation functions with respect to the machine and transverse directions. The validity of functions determined in this way is confirmed by comparison with birefringence results. 9 refs. 

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