Photon-based methods spectroscopy

In sharp contrast to conventional spectroscopic methods based on direct mie-photon absorption, IRMPD spectroscopy relies on the sequential absorption of a large number of IR photons. This excitation mechanism leaves an imprint on the observed IR spectrum in the sense that vibrational bands are typically broadened, red-shifted and affected in relative intensity to some extent. While the intramolecular processes underlying these spectral modifications have been addressed and qualitatively modelled in a large number of studies [166-172], it is often hard to predict quantitatively an IRMPD spectrum because the required molecular parameters, in particular the anharmonic couplings between vibrational normal modes at high internal energies, are usually unknown and cannot be calculated accurately using current quantum-chemical methods, fri practice, most experimental IRMPD spectra are therefore analysed oti the basis of computed linear absorption spectra, which usually provide a reasonable approximation to the IRMPD spectrum. 

Abstract In this chapter we review recent advances in theoretical methods to understand and rationalize anharmonic vibrational spectroscopy (IR-MPD and IR-PD) and collision induced dissociations (CID) in the gas phase. We focused our attention on the application of molecular dynamics-based methods. DFT-based molecular dynamics was shown to be able to reproduce InfraRed Multi-Photon Dissociation (IR-MPD) and InfraRed Pre-Dissociation (IR-PD) action spectroscopy experiments, and help assign the vibrational bands, taking into account finite temperature, conformational dynamics, and various anharmonicities. Crucial examples of dynamical vibrational spectroscopy are given on the protonated AlanH" series (related to IR-MPD in the 800-4,0(X) cm domain), ionic clusters (related to IR-PD in the 3,000-4,(XX) cm region), and neutral peptides (related to IR-MPD in the far-lR). We give examples from simple (e.g., cationized urea) to more complex (e.g., peptides and carbohydrates) molecular systems where molecular dynamics was particularly suited to understanding CID experiments. 

The chapter IR Spectroscopic Techniques to Study Isolated Biomolecules gives an overview of some of the most common experimental practices currently in use to characterize the strucmre of isolated biomolecules by infrared spectroscopy. We address especially two main categories of experimental approaches conformation-selective infrared spectroscopy of jet-cooled neutral species and infrared (multiple-photon) dissociation spectroscopy of mass-selected ionized biomolecules. Molecular beam laser spectroscopy methods form the experimental basis for the topics covered in the sixth to eighth chapters. Mass spectrometry-based ion spectroscopy provided the experimental data for the studies reviewed in fourth and fifth chapters (and seventh inpart). 

A number of surface-sensitive spectroscopies rely only in part on photons. On the one hand, there are teclmiques where the sample is excited by electromagnetic radiation but where other particles ejected from the sample are used for the characterization of the surface (photons in electrons, ions or neutral atoms or moieties out). These include photoelectron spectroscopies (both x-ray- and UV-based) [89, 9Q and 91], photon stimulated desorption [92], and others. At the other end, a number of methods are based on a particles-in/photons-out set-up. These include inverse photoemission and ion- and electron-stimulated fluorescence [93, M]- All tirese teclmiques are discussed elsewhere in tliis encyclopaedia. 

Various techniques and equipment are available for the measurement of particle size, shape, and volume. These include for microscopy, sieve analysis, sedimentation methods, photon correlation spectroscopy, and the Coulter counter or other electrical sensing devices. The specific surface area of original drug powders can also be assessed using gas adsorption or gas permeability techniques. It should be noted that most particle size measurements are not truly direct. Because the type of equipment used yields different equivalent spherical diameter, which are based on totally different principles, the particle size obtained from one method may or may not be compared with those obtained from other methods. 

Future development of spectroscopic structure-determination methods will depend on the availability of more powerful photon and particle sources as well as advances in photon and particle detectors. Impressive progress has been made in molecular structure determinations based on advances in computation power and in computational algorithms, such as fast Fourier-transform techniques, for nearly every form of spectroscopy and diffraction analysis. Hajdu and co-work-... 

Soukka T, Rantanen T, Kuningas K (2008) Photon upconversion in homogeneous fluorescence-based bioanalytical assays. Ann N Y Acad Sci 1130 188-200, Fluorescence Methods and Applications Spectroscopy, Imaging, and Probes... 

In fluorescence spectroscopy, however, diffuse reflectance correction of spectral distortions in biological media has been studied extensively. Analytical models based on photon migration theory,44 diffusion theory46,60,61 as well as empirical models,62 have been reported to obtain intrinsic fluorescence. In the following, we will review a particular correction method based on photon migration theory for fluorescence spectroscopy and introduce its Raman counterpart. 

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