Multiphoton advantage

In the calorimeter depicted in figure 13.6 there are two independent systems to measure the sample transmittance. Both have advantages and disadvantages, and both have been adopted in other photoacoustic calorimeters described in the literature. When used simultaneously, they provide a straightforward way to test whether the assumptions of equation 13.1 are being met (e.g., testing for multiphoton effects) and generally allow for more confidence in the experimental results. 

The procedure described, involving the variation of the laser energy, has some advantages relative to the alternative method of using several solutions with different transmittances. First, it provides a check for multiphoton effects simply by analyzing the quality of the linear correlations obtained. It should be stressed that the excellent correlations in figure 13.7 are typical, that is, correlation factors are usually better than 0.9995. Second, the method requires considerably less sample (only one solution is needed). Third, the analysis of experimental data is also conceptually simpler, because no normalization is required. 

Not illustrated is the use of multiphoton excitation of fluorescence (12.15). thus far demonstrated in flame systems only for excitation of atoms. It affords the means to excite otherwise inaccessible states and offers other potential advantages in spatial resolution and for optically thick flames, in spite of inherently low signal levels. 

Resonance-enhanced multiphoton ionization (REMPI) has proved to be a versatile ionization technique for MS offering a number of advantageous features in the field of chemical analysis. Since it makes use of substance-specific excited states for the ionization process, it involves UV spectroscopy of the molecule to be ionized. Thus, it enables ionization of preselected compounds, control of the degree of fragmentation and, for a large number of substances, a high ionization efficiency. These features require that the excited molecular state(s) involved in the REMPI process not be significantly depleted... 

The small cross sections in multiphoton processes are of course a weakness of nonlinear spectroscopy. Especially in microscopy, this problem becomes serious because of the small volume of a sample. By the use of the signal enhancement techniques, however, the disadvantage can be turned into an advantage of back-ground-free selective measurements. For example, the combined use of HRS with the plasmonic enhancement provides us a chemical imaging with nanoscale spatial resolution when a laser-illuminated metal tip is located adjacent to a sample surface, signal enhancement is locally induced near the tip. This spatial resolution is expected to overcome optical diffraction limit. Such tip-enhanced spectroscopy has already been reported in conventional CARS [15]. 

Time-resolved measurements are also one of the advantages in multiphoton spectroscopy. In the case of nanomaterials, physical and chemical properties are largely affected by interaction with surrounding environments. Therefore, the dynamical study should be useful for characterizing nanomaterials from the viewpoints of nanodevice applications. 

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