Raman scattering efficiency

Higher laser power is preferred since it proportionally reduces acquisition time (signal strength). The maximum sampling rate in a process application is limited by acquisition time, which depends on the molecule s Raman scattering efficiency, though adjustments to the standard data transfer hardware and software... 

In summary, resonance Raman enhancement occurred in reduced molybdenum oxides when the exciting frequency corresponded to the Mo5+ Mo6+ IVCT transition at about 2 eV. This resonance enhancement was found to be crucial for catalyst characterization during operation. Excitation of the Raman spectra of such reduced oxides is of course also possible with other laser frequencies (vide supra). However, then the overall Raman scattering efficiency is much smaller, and small concentrations (e.g., in a catalytic reaction experiment) may not be detectable. 

The probability of a molecule producing Raman scattering is much less than the probability of IR absorption. However, advances in Raman instrumentation and lasers have narrowed the gap considerably, in some optimal cases, the sensitivity of normal Raman scattering compares favorably with IR absorption. In addition, the Raman scattering efficiency can be greatly increased by use of the surface enhanced and/or RR effects. 

The Raman scattering efficiency for sp bonds is more than fifty times the efficiency for sp bonds for graphitic domains smaller than 10 nm. As a result, the technique is capable of detecting minute amounts of graphite bonds (such as may present in some diamond-like carbon). However, it must be recognized that the techniques cannot readily define the state of aggregation of the constituents. 

Because of the momentum conservation rule during the light scattering process, first-order Raman scattering is caused by phonons at the Brillouin zone center. The Raman scattering efficiency S with polarization detection is given by ... 

Though normal Raman spectroscopy is a very selective technique for chemical analysis, there are some serious experimental disadvantages related to the sensitivity, large fluorescence interference, and lack of time resolution of the technique. These weaknesses have been addressed in the creation of new Raman-based techniques. The weak Raman signals due to inherently small Raman scattering efficiencies has been addressed by resonance Raman, surface-enhanced Raman and SPP-Raman techniques. Fourier transform-Raman spectroscopy and con-focal Raman microscopy address the disadvantage of... 

SERS offers considerable promise for the study of polymers for several reasons. The enhancement effect can increase Raman scattering by a factor of 10 -10 . Adsorption of molecules on the SERS-active metal surface causes fluorescence quenching in highly fluorescent compounds. In addition, surface-enhanced resonance Raman scattering can further enhance the Raman scattering efficiency by a factor of 10 -10 above that observed under resonance or surface-enhanced conditions alone. 

The first temi results in Rayleigh scattering which is at the same frequency as the exciting radiation. The second temi describes Raman scattering. There will be scattered light at (Vq - and (Vq -i- v ), that is at sum and difference frequencies of the excitation field and the vibrational frequency. Since a. x is about a factor of 10 smaller than a, it is necessary to have a very efficient method for dispersing the scattered light. 

The scattered radiation V3 is to high wavenumber of Vj (i.e. on the anti-Stokes side) and is coherent, unlike spontaneous Raman scattering hence the name CARS. As a consequence of the coherence of the scattering and the very high conversion efficiency to V3, the CARS radiation forms a collimated, laser-like beam. 

Another method, called photobleaching, works on robust soHds but may cause photodecomposition in many materials. The simplest solution to the fluorescence problem is excitation in the near infrared (750 nm—1.06 pm), where the energy of the incident photons is lower than the electronic transitions of most organic materials, so fluorescence caimot occur. The Raman signal can then be observed more easily. The elimination of fluorescence background more than compensates for the reduction in scattering efficiency in the near infrared. Only in the case of transition-metal compounds, which can fluoresce in the near infrared, is excitation in the midvisible likely to produce superior results in practical samples (17). 

The present study demonstrates that the analytic calculation of hyperpolarizability dispersion coefficients provides an efficient alternative to the pointwise calculation of dispersion curves. The dispersion coefficients provide additional insight into non-linear optical properties and are transferable between the various optical processes, also to processes not investigated here as for example the ac-Kerr effect or coherent anti-Stokes Raman scattering (CARS), which depend on two independent laser frequencies and would be expensive to study with calculations ex-plictly frequency-dependent calculations. 

Since the Raman scattering is not very efficient (only one photon in 107 gives rise to the Raman effect), a high power excitation source such as a laser is needed. Also, since we are interested in the energy (wavenumber) difference between the excitation and the Stokes lines, the excitation source should be monochromatic, which is another property of many laser systems. 

In resonant Raman spectroscopy, the frequency of the incident beam is resonant with the energy difference between two real electronic levels and so the efficiency can be enhanced by a factor of 10 . However, to observe resonant Raman scattering it is necessary to prevent the possible overlap with the more efficient emission spectra. Thus, Raman experiments are usually realized under nonresonant illumination, so that the Raman spectrum cannot be masked by fluorescence. 

---