Nonresonant background

As a result, the CRS lineshape is asyimnetric and more complicated due to this nonresonant background interference. 

The nomesonant background prevalent in CARS experiments (discussed above), although much weaker than the signals due to strong Raman modes, can often obscure weaker modes. Another teclmique which can suppress the nonresonant background signal is Raman induced Kerr-efifect spectroscopy or RIKES [96, 97]. 

For adsorbates on a metal surface, an SFG spectmm is a combination of resonant molecular transitions plus a nonresonant background from the metal. (There may also be a contribution from the water-CaF2 interface that can be factored out by following electrode potential effects see below.) The SFG signal intensities are proportional to the square of the second-order nonlinear susceptibility [Shen, 1984] ... 

Lagutchev A, Hambir SA, Dlott DD. 2007. Nonresonant background suppression in broadband vibrational sum-frequency generation spectroscopy. J Phys Chem C 111 ... 

So far we have considered only the recoil-free fraction of photons emitted by the source. The other fraction (1 —/s), emitted with energy loss due to recoil, cannot be resonantly absorbed and contributes only as a nonresonant background to the transmitted radiation, which is attenuated by mass absorption in the absorber... 

This expression holds only for an ideal detection system, which records only Mossbauer radiation. Practical problems with additional nonresonant background contributions from y-ray scattering and X-ray fluorescence are treated in detail in Sects. 3.1 and 3.2. 

The complexity of the Co emission spectrum and the low fraction of the desired 14.4 keV radiation require an efficient Mossbauer counting system that is able to discriminate photons of different energies and reject the unwanted events. Otherwise a huge nonresonant background would add to the counting statistics of the spectra and fatally increase the noise of the spectrometer. 

In summary, pulse-height analysis (PHA) prior to a Mossbauer measurement is an essential step in tuning a Mossbauer spectrometer. PHA allows the adjustment of the y-detection system to the Mossbauer photons and the reduction of noise by rejecting nonresonant background radiation. 

Scattered radiation. In a transmission experiment, the Mossbauer sample emits a substantial amount of scattered radiation, originating from XRF and Compton scattering, but also y-radiation emitted by the Mossbauer nuclei upon de-excitation of the excited state after resonant absorption. Since scattering occurs in 4ti solid angle, the y-detector should not be positioned too close to the absorber so as not to collect too much of this unwanted scattered radiation. The corresponding pulses may not only uimecessarily overload the detector and increase the counting dead time, but they may also affect the y-discrimination in the SCA and increase the nonresonant background noise. 

Fig. 3.13 Signal-to-noise ratio of Mossbauer spectra as a function of the area density l of the sample for thin absorbers ( < 1) and negligible nonresonant background, A b A oo...

The derivation of the expressions (3.3)-(3.6) is found in Appendix A (cf. CD-ROM). Since for most Fe-spectra the level of nonresonant background counts, Ab, may be in the range of 10-30% of the total counts, the absorber thickness is usually best adjusted to a value between the limits given above. The maximum of SNR( ) is naturally rather broad, such that deviations from / opt of even dz50% are fairly immaterial. 

Fig. A.l Contributions to a Mossbauer transmission spectrum. N, is the nonresonant background from scattered high-energy y-radiation and X-ray fluorescence in the source and the absorber...

The nonresonant background in CARS spectroscopy originates from instantaneous four-mixing processes, while the resonant contribution involves real vibrational states. This provides a basis for possible discrimination against the nonresonant background. To do so, one has to come up with a pair of pulses that excite the vibrational state, and the third, time-delayed pulse will only contribute to the resonant part of the CARS signal. However, to make this scheme work efficiently, one has to overcome certain obstacles. To achieve high spectral resolution, the bandwidth of the third pulse should... 

By making the excitation pulses overlap with the minimum of the probe pulse preceding its main maximum, the nonresonant background is further suppressed (Pestov et al. 2007). The same idea can be exploited with a single pulse excitation (Dudovich et al. 2003), when both pump pulses at frequencies i, 2 are derived from a single ultra-broadband pulse. 

The spectral line shape in CARS spectroscopy is described by Equation (6.14). In order to investigate an unknown sample, one needs to extract the imaginary part of to be able to compare it with the known spontaneous Raman spectrum. To do so, one has to determine the phase of the resonant contribution with respect to the nonreso-nant one. This is a well-known problem of phase retrieval, which has been discussed in detail elsewhere (Lucarini et al. 2005). The basic idea is to use the whole CARS spectrum and the fact that the nonresonant background is approximately constant. The latter assumption is justihed if there are no two-photon resonances in the molecular system (Akhmanov and Koroteev 1981). There are several approaches to retrieve the unknown phase (Lucarini et al. 2005), but the majority of those techniques are based on an iterative procedure, which often converges only for simple spectra and negligible noise. When dealing with real experimental data, such iterative procedures often fail to reproduce the spectroscopic data obtained by some other means. 

Kamga, F. M., and Sceats, M. G. 1980. Pulse-sequenced coherent anti-Stokes Raman scattering spectroscopy A method for suppression of the nonresonant background. Opt. Lett. 5(3) 126-28. 

The main source of contrast in FE-CARS is based on differences in the amplitude and phase of The spectral phase plays an important role in FE-CARS. While the phase of the nonresonant CARS signal is independent of co, the resonant part of exhibits a characteristic r-jump in the vicinity of a vibrational resonance Or. In the presence of a spatial r-step in focus, the nonresonant background destructively... 

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