ABSTRACT
In this final chapter, two less-widely known imaging techniques are covered: the first one employing absorption spectroscopy, and the second one utilizing ion detection methods. Two specific approaches utilizing molecular absorption are discussed, which have alleviated the problems of attenuation when light propagates through dense media. In the first, terahertz radiation is exploited, which is both absorber-molecule specific and little affected otherwise. By and large, terahertz imaging is used to detect “hidden structures” (like explosives in homeland security applications). The second methodology is known as photoacoustic microscopy—with acoustic or optical resolution (AR-PAM/OR-PAM), which records localized absorption via thermal “shockwaves.” A range of examples is described, which reveal spectrochemical information with high spatial resolution, and even superresolution becomes possible using the “absorber-bleaching” modality of Pi-PAM. The final topic covers charged-particle imaging, a methodology primarily reserved for fundamental research applications in molecular reaction dynamics. First, the concepts of charged-particle imaging, distinctly different from photon-based imaging, are outlined. Theoretical aspects relevant to reaction dynamics (e.g., the Newton sphere) are described in some depth; and key aspects of spatial resolution are outlined, for example, how it can be increased implementing approaches like velocity-map imaging (VMI) or slice imaging, and how direct 3D images can be generated. Representative examples of uni- and bimolecular reactions are discussed, ranging from photodissociation of oriented molecules to product pair correlation in reactive scattering, in which the underlying reaction mechanism can be “seen” looking at the respective VMI.
