Coherence experimental techniques

Vibrational dephasing provides us with a powerful method to probe the interaction of a chemical bond with the surrounding medium. Over the years, many experimental techniques have been developed to study dephasing of bonds in many molecular systems at various temperatures, pressures, and concentrations [122-124]. One popular experimental technique is the isotropic Raman lineshape. The other methods involve a coherent excitation of the vibration with a laser pulse and monitoring the decay of the phase coherence,... 

This section of the Annual Reports comprises five contributions from experts in their fields. These contributions attempt to meld advanced technical aspects of biopolymer selection with more theoretical treatments. As a result, the interplay between the applied and basic scientific aspects of an emergent field becomes apparent. The opus by Dr. Bennett Levitan (Santa Fe Institute) provides one of the first coherent frameworks for understanding selection techniques. This chapter provides a firm connection between the experimental techniques used and the probabilistic models that of necessity underlie these techniques, and should prove to be a benchmark both in understanding why selections... 

The years from 1960 to 1975 represented a golden era in the radiofrequency and microwave spectroscopy of open shell diatomic molecules. Molecular beam electric resonance was one of the most important experimental approaches, but microwave, far-infrared and magnetic resonance studies of bulk gaseous samples were equally important and our understanding of these open shell species is derived from a combination of different experimental approaches. In this book we have chosen to organise our descriptions according to the experimental techniques employed, but as with any such scheme, we run the risk, which we wish to avoid, of not connecting the results from different types of experiment in a coherent manner. As we shall see, the OH radical is the example par excellence which illustrates the pitfalls of an approach which is technique-oriented, rather than molecule-oriented. 

An alternate approach is to perform coherent Raman spectroscopy in the time domain rather than in the frequency domain. In this case, a single laser that produces short pulses with sufficient bandwidth to excite all of the Raman modes of interest is employed. One pulse or one pair of time-coincident pulses is used to initiate coherent motion of the intermolecular modes. The time dependence of this coherence is then monitored by another laser pulse, whose timing can be varied to map out the Raman free-induction decay (FID). It should be stressed at this point that the information contained in the Raman FID is identical to that in a low-frequency Raman spectrum and that the two types of data can be interconverted by a straightforward Fourier-transform procedure (12-14). Thus, whether a frequency-domain or a time-domain coherent Raman technique should be employed to study a particular system depends only on practical experimental considerations. 

From an experimental point of view, it is quite evident that for the four nonlinear coherent Raman techniques discussed until now, one either measures the radiation generated at anti-Stokes frequency (CARS, ll)as = 2ui-cvs) or at Stokes frequency (CSRS, 2cJs - or one determines the change AS in the laser beam power (o/ z, IRS uJs -SRGS). In order to get full Raman information of the medium, it is necessary to tune the frequency difference ojl-ujs, then, successively all Raman-active vibrations (or rotations, or rotation-vibrations) will be excited and a complete nonlinear Raman spectrum is then obtained. 

In the case of batteries, the key challenge is to better control the electrode-electrolyte interface and possibly to design new solid-solid or solid-liquid interfaces. This is very difficult without using the aforementioned experimental techniques to dynamically and locally probe the evolution of the electrode-electrolyte interfaces. Posthumous rather than in situ analysis is still widely used, thus missing key information. This is, therefore, another good example of the potential advantages offered by coherent x-ray microradiology [1],... 

The application of lasers in optical experimental techniques has led to a rapid development of research into the properties of elementary excitations in solids. In addition to the conventional methods of linear crystal optics, Raman scattering of light (RSL) has become one of the principal research methods, as have its various modifications, such as coherent active Raman spectroscopy and others. 

A number of books and reviews which describe some of the new experimental techniques have appeared. Topics covered include hole burning spectroscopy,8 various forms of picosecond and femtosecond spectroscopy, femtosecond coherent spectroscopy, ultrafast time-... 

The second volume of Laser Spectroscopy covers the different experimental techniques, necessary for the sensitive detection of small concentrations of atoms or molecules, for Doppler-free spectroscopy, laser-Raman-spectroscopy, doubleresonance techniques, multi-photon spectroscopy, coherent spectroscopy and time-resolved spectroscopy. In these fields the progress of the development of new techniques and improved experimental equipment is remarkable. Many new ideas have enabled spectroscopists to tackle problems which could not be solved before. Examples are the direct measurements of absolute frequencies and phases of optical waves with frequency combs, or time resolution within the attosecond range based on higher harmonics of visible femtosecond lasers. The development of femtosecond non-collinear optical parametric amplifiers (NOPA) has considerably improved time-resolved measurements of fast dynamical processes in excited molecules and has been essential for detailed investigations of important processes, such as the visual process in the retina of the eye or the photosynthesis in chlorophyl molecules. 

For nonspecialists, however, or for people who are just starting in this field, it is often difficult to find from the many articles scattered over many journals a coherent representation of the basic principles of laser spectroscopy. This textbook intends to close this gap between the advanced research papers and the representation of fundamental principles and experimental techniques. It is addressed to physicists and chemists who want to study laser spectroscopy in more detail. Students who have some knowledge of atomic and molecular physics, electrodynamics, and optics should be able to follow the presentation. 

In the first part of the paper we discuss the Raman heterodyne detection of rf-generated sublevel coherence in atomic Sm vapor in the presence of rare gas perturbers. In a second part we report on the observation of novel Ramsey-type resonances due to collisional velocity diffusion in Sm vapor here the experimental technique uses counterpropagating laser fields and relies on coherent Raman processes to optically excite and detect Hertzian coherence. 

Quasi-elastic Neutron Scattering. Coherent and incoherent inelastic neutron scattering are unique experimental techniques to characterize molecular motions on a time scale between 10 and 10 s. The continued development of high resolution inelastic scattering techniques in the past two decades (157-159) enables measurement of the dynamic structure factor S(Q, co) and the... 

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