Coherent technique

Since VSFS is a coherent technique, and as Equation (25) implies, the oscillating electric field from each vibrational state involved in the generation of sum-frequency light can interfere with that from every other state and with that from the non-resonant response. This is quite different from IR spectroscopy where spectra are simple superpositions of intensity from individual vibrational modes. As such, VSFS leads to interesting line shapes that can be interpreted incorrectly if spectral intensities are compared visually without fitting the spectrum. 

Coherent Raman spectroscopy Coherent Raman spectroscopy is a term that refers to a series of closely related nonlinear Raman techniques in which the scattered Raman radiation emerges from the sample as a coherent beam -coherent meaning that the photons are all in phase with one another. The coherent techniques include Stimulated Raman Scattering (SRS), Coherent anti-Stokes Raman Spectroscopy (CARS), Coharent Stokes Raman Spectroscopy (CSRS), and Stimulated Raman Gain Spectroscopy (SRGS). Although most of the nonlinear Raman techniques are also coherent techniques, there is one incoherent nonlinear Raman process called Hyper Raman. 

In the weakly anharmonic molecular crystal the natural modes of vibration are collective, with each internal vibrational state of the molecules forming a band of elementary excitations called vibrons, in order to distinguish them from low-frequency lattice vibrations known as phonons. Unlike isolated impurities in matrices, vibrons may be studied by Raman spectroscopy, which has lead to the establishment of a large body of data. We will briefly attempt to summarize some of the salient experimental and theoretical results as an introduction to some new developments in this field, which have mainly been incited by picosecond coherent techniques. 

The second harmonic generation is a coherent technique giving the fast, electronic in origin, second-order NLO susceptibility (-2co co,co) at a given, measurement frequency co. Here, we limit the discussion to poled films, with °o mm symmetry, which exhibit two nonzero x tensor components the diagonal A zzz( 2co co,co) and the off diagonal xzz( 2co co,co), where Z is the poling (preferential orientation) direction. Usually, thin films are deposited on one side of substrate only (thin film deposited on both sides is discussed in Swalen and Kaj-... 

Other Coherent Techniques for the Detection of Chemical Species.278... 

Experimental geometry for (a) incoherent techniques and (b) coherent techniques. 

The foregoing favorable characteristics are, however, tainted with critical disadvantages. The coherence is generated and controlled by more than one laser beam. Typically, three laser fields are needed for the purposes of the main popular combustion diagnostics. For this reason, it is commonly observed that coherent techniques are synonym of dielectric nonlinearity (i.e., dependence on two or more electric fields). The feature is not without consequences. On the experimental side, the minimum requirement of two laser systems and the crucial sensitivity to the optical alignment renders the measurements difficult. On the theoretical side, the data interpretation is very elaborate and much more sophisticated than the theoretical analysis of incoherent techniques. 

Another coherent technique sensitive to combustion conditions is polarization spectroscopy (PS) [7,9]. Ffere, a weak linearly polarized probe laser beam is crossed... 

For high accuracy short range applications time of flight measurements are frequently replaced by coherent techniques (interferometry). Systems with frequency modulation, where range rates or speed rates are converted into frequency changes and the speed - and length rates are measured in terms of frequency, are of special interest. These systems provide for a lower sensitivity to air turbulences and vibrations. 

Either two or more molecular levels of a molecule are excited coherently by a spectrally broad, short laser pulse (level-crossing and quantum-beat spectroscopy) or a whole ensemble of many atoms or molecules is coherently excited simultaneously into identical levels (photon-echo spectroscopy). This coherent excitation alters the spatial distribution or the time dependence of the total, emitted, or absorbed radiation amplitude, when compared with incoherent excitation. Whereas methods of incoherent spectroscopy measure only the total intensity, which is proportional to the population density and therefore to the square ir of the wave function iff, the coherent techniques, on the other hand, yield additional information on the amplitudes and phases of ir. 

Within the density-matrix formalism (Vol. 1, Sect. 2.9) the coherent techniques measure the off-diagonal elements pab of the density matrix, called the coherences, while incoherent spectroscopy only yields information about the diagonal elements, representing the time-dependent population densities. The off-diagonal elements describe the atomic dipoles induced by the radiation field, which oscillate at the field frequency radiation sources with the field amplitude Ak(r, t). Under coherent excitation the dipoles oscillate with definite phase relations, and the phase-sensitive superposition of the radiation amplitudes Ak results in measurable interference phenomena (quantum beats, photon echoes, free induction decay, etc.). 

The situation changes, however, if instead of the intensity (9.71), the amplitude (9.68) or an intensity representing a coherent superposition of amplitudes can be measured, where the phase information and its development in time is preserved. Such measurements are possible with one of the coherent techniques discussed in Chap. 7. 

Another coherent technique that can be combined with gated detection is levelcrossing spectroscopy (Sect. 7.1). If the upper atomic levels are excited by a pulsed... 

The D-HMQC (dipolar heteronuclear multiple-quantum coherence) technique is a recently developed NMR pulse sequence particularly suitable for the investigation of spatial proximity between quadrupolar and spin-1/2 nuclei. Compared to the crosspolarisation magic-angle spinning technique applied to a quadrupolar nucleus, D-HMQC does not require time-consuming optimisations and exhibits on the quadrupolar spin a better robustness to irradiation offset and to Cq values and... 

Coherent techniques, on the other hand, yield additional information on the amplitudes and phases of the wave functions j/ involved in the measurement. 

The non-scattered part of the transmitted radiation can also be detected by coherent techniques, as illustrated in Fig. 10.59. Using heterodyne... 

Many experimental variations of these coherent techniques have been developed but basically the experiments may be divided into two classifications, i.e. coherent transient technique and photon echo phenomena, the former requires high resolution, high stability sources while the latter relies on sequential pulse excitation with controlled delays. 

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