Spectral intensities, fluctuations

The formal approach of 2D correlation analysis to time-dependent spectral intensity fluctuations has been extended to UV, Raman [1010], and near-IR spectroscopy [1011-1014] 2D fluorescence is upcoming. 

The basic concept of 2D IR spectroscopy based on the correlation analysis of perturbation-induced time-dependent fluctuations of IR intensities could be readily extended to other areas of polymers spectroscopy. The 2D correlation analysis has been successfully applied to the time-dependent variations of small angle X-ray scattering intensity measurement [4], In this study, a small amplitude dynamic strain is applied a sheet of microphase-separated styrene-butadiene-styrene triblock copolymer sample. Intensity variation of scattered X-ray beam due to the strain-induced changes in the interdomain Bragg distances coupled with the reorientation of microdomain structures is analyzed by using the 2D correlation map. Similarly, the formal approach of 2D correlation analysis to time-dependent spectral intensity fluctuations has been extended to UV, Raman [63], and near-IR spectroscopy [64], There seems no intrinsic limitation to the application of this versatile technique in polymer spectroscopy. 

As the next step, one must Fourier transform the dynamic spectra measured in the time-domain into the frequency domain (18). The forward Fourier transform Ti([Pg.67]

Levine R D 1987 Fluctuations in spectral intensities and transition rates Adv. Chem. Phys. 70 53-95... 

Source compensation Pulse-to-pulse intensity variations and intensity fluctuations in the spectrometric excitation source are often the dominant noise source affecting the performance of the detection system. However, since OIDs are parallel multichannel detectors, these intensityvariations do equally and simultaneously affect the entire spectral distribution as a whole. Thus, with the aid of a single-channel reference detector, monitoring a portion of the source s light flux, it is possible to accurately normalize for spectrum-to-spectrum variations and practically eliminate these and any other source flicker noise related effects. 

Another factor which can cause spectral distortions is the fluctuation of the output energy of the radiant source. The OMA 2 system incorporates a capability to correct spectral data for variations in source output energy (17). The source compensation mode continuously monitors the output energy of the lamp and automatically corrects incoming spectral data for source intensity fluctuations prior to storing that data. 

One of the shortcomings of LIBS, particularly in relation to quantitative elemental analysis, arises from the instability of the laser-induced plasma emission resulting from laser intensity fluctuations (1-5%) the amount of scattered light present depends on local matrix effects and on physical and chemical properties of the target material. The most common way of compensating for signal fluctuations in LIBS is by calculating the ratio of the spectral peak intensity to that of a reference intensity. However, this internal calibration method provides relative rather than absolute concentrations. 

FLUCTUATIONS IN SPECTRAL INTENSITIES AND TRANSITION RATES 79 Here p0 has been defined by... 

Another nonlinear technique that is potentially applicable to thermometric measurements is DPWM [7,9]. Por instance, a Boltzmann plot constructed out of the relative line intensities of a DPWM spectrum can lead to temperature predictions that can be as accurate as CARS in some cases. An alternative method is to fit theoretical simulations to the experimental spectrum. Nonetheless, the versatility of CARS is not equaled by DPWM. In effect, single pulse measurements seems to be limited to some radical species and mode fluctuations of conventional lasers perturb the data severely. To avoid troubles with such laser intensity fluctuations, saturated DPWM is often employed, but the difficulties of spectral S5mthe-sis remain a serious hindrance to a major role of DPWM thermometry. 

Because of relaxation of surface elevations, the scattered light has a broadened spectral distribution compared with the incident light. The broadening is too small to be analyzed by the conventional Fabry-Perot interferometry," however, so the more recent technique of light beating" must be used. We call this technique intensity fluctuation spectroscopy (IFS). 

In this section we present a point of view that provides a convenient parametrization of the spectrum and apply it to discuss three aspects (i) the different frequency scales in the spectra and the interpretation in terms of the sequential exploration of phase space, (ii) the fluctuation of spectral intensities, and (iii) the extraction of the time crosscorrelation function from observed Raman excitation profiles. For the convenience of the reader we first summarize the practical results so that one can skip Sec. IV.A.1-3 and proceed directly to the applications in Sec. IV.B. The derivation of the parametrization, in Sec. IV.A.2, uses the maximal entropy formalism in the manner of (101) and further details can be found in (102,103). Two extensive reviews are (104,105). Section IV.A.l is a brief discussion of why we use the maximum entropy formalism. The discussion is brief and cannot replace the more detailed presentations available (e.g., (55,101,107,108)) in the literature. 

The RRK rate monotonically increases with increasing energy of the molecule. On a state-by-state basis the rate of dissociation shows much more irregularity (123,124), in a manner similar to that discussed in Sec. IV for the spectral intensities (which are the rates of transition induced by the light). What the RRK rate provides is the mean about which the fluctuations occur. Hence, as in Eq. (86) one can write... 

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