Nonlinear spectroscopic signals

I. Introduction Nonlinear Spectroscopic Signals for Microscopic Imaging... 168... 

I. INTRODUCTION NONLINEAR SPECTROSCOPIC SIGNALS FOR MICROSCOPIC IMAGING... 

The most recent publications on the S -Sq transition of TRN(OH) are by Braca-monte and Vacarro ]37, 38], who applied their polarization resolved DFWM spectroscopy method [37] to TRN vapor at its room temperature sublimation pressure. This remarkable nonlinear spectroscopic method simplifies the spectrum by allowing Q branch (A/ = 0) transitions to be observed with greatly diminished R and P branch (AJ = 1) signal and, with rotation of a polarization element, vice versa. This selective nonlinear optical technique also produces a large reduction in the signal generated by hot bands, as noted in the above discussion for MA. How-... 

Further structural information has been derived from the recent use of optical spectroscopy as a probe of structure at the L-L interface the two general approaches pursued are (i) total internal reflectance (TIR) fluorescence spectroscopy of adsorbed solute species and (ii) nonlinear spectroscopic methods to generate interface-sensitive spectra of the solvent molecules in the interfacial region. The former approach uses the polarization of the emitted signal, from an s-polarized excitation, to determine the effective dimensionality of the interface. The anisotropy of the fluorescent response from an interface that is flat on the length-scale of the adsorbed probe ( lnm) is [33]... 

Note that the application of the convolution scheme in the simple form (44) requires that the nonlinear polarization contains only a single interaction with the probe laser field. Apart from the transient transmittance spectrum considered above, this condition is also fulfilled for related detection schemes such as time-resolved fluorescence, ionization, and excited-state absorption. Coherent spectroscopic signals such as the photon-echo, on the other hand, contain two interactions with the probe laser field, thus requiring the calculation of the full three-time response function, followed by a double convolution. 

The connection of this nonlinear optical effect with spectroscopy lies in the fact that and hence the signal strength, will be enhanced whenever a linear combination of a subset oi mcomponents approaches the energy difference of two molecular states of proper symmetry. This wi-photon resonant enhancement — which can in principle be observed even in the absence of real transitions, as will be the case for states with no thermal population — has been exploited to develop a number of nonlinear spectroscopic techniques that differ in order n, order of the used resonance m, number of colours (i.e. different actual laser fields that provide the frequency components), spectral and temporal resolution and the actual method used to detect the resonances by monitoring either the power generated at (o or the change in amphtude, polarization or phase at some of the input frequencies. 

A general objective in any in situ spectroscopic technique is to maximize the signal that arises specifically from the electrode surface. Nonlinear optical techniques such as second-harmonic generation (SHG) and sum frequency generation (SFG) are of interest because they involve optical signals that by definition can only arise at the electrode-solution interface [65],... 

Diagnostic information can be obtained to determine whether the calibration model provides an adequate fit to the standards, e.g., nonlinearity or other kinds of model errors can be detected, or whether an unknown sample is adequately fitted by the calibration model. A large lack of fit is usually due to background signals different from those present in the calibration standards. This is what some people have called the false sample problem. For example, suppose a calibration model was developed for the spectroscopic determination of iron in dissolved carbon steel samples. This model might be expected to provide a poor performance in the determination of iron in stainless steel samples. In this case, a figure-of-merit calculated from the biased model would detect the false sample. ... 

To perform the VES calculations it is necessary to consider a finite duration pulse, which has a finite bandwidth. In addition, the actual shape of the vibrational echo spectrum depends on the bandwidth of the laser pulse and the spectroscopic line shape. Several species with different concentrations, transition dipole moments, line shapes, and homogeneous dephasing times can contribute to the signal. Therefore, VES calculations require determination of the nonlinear polarization using procedures that can accommodate these properties of real systems. 

The three methods taken together allow time-resolved spectroscopic characterization of propagating modes with far greater ease and completeness than has been possible in the past. They facilitate a number of important possibilities including spatiotemporal control over propagating modes, excitation and characterization of nonlinear lattice responses, tunable terahertz-frequency spectroscopy, and terahertz frequency and bandwidth signal processing. 

A fmther step is to spectroscopically monitor adsorbed and reacting molecules on the surface of the nanoparticles under technically relevant conditions [59]. A few techniques with high-pressme capability are illustrated in Fig. 15.9b. Ambient pressure vibrational spectra of adsorbed molecules can be obtained by IR-vis SFG or PM-IRAS [51, 55, 60, 61]. Both methods can be applied from UHV to ambient pressure. Nonlinear optical SFG spectroscopy is inherently interface specific (i.e., there is no gas-phase SFG signal), and PM-IRAS allows for an accurate... 

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