On-resonance excitation

In the ion cyclotron resonance (ICR) cell CID is performed by resonant excitation of the precursor ion and subsequent collisions with the background gas. One serious drawback with on-resonance excitation in the ICR cell is that the risk for ion losses due to... 

Under such circumstances, the evolution of a spin system has to be calculated in different rotating frames defined by the corresponding PIPs and a special case may arise, where a spin experiences an on-resonance excitation but off-resonance evolution in the conventional rotating frame. Unpredictable results may occur if the phase coherence in PIPs fails. Unfortunately, to date, no... 

Core excitation of clusters is reviewed as a unique approach to probe element- and site-speciflcally size-dependent properties of free clusters in the gas phase. The fundamental characteristics of core level spectroscopies that rely on resonant excitation are briefly reviewed. Specifically, this includes primary photoabsorption as well as subsequent processes, such as photoionization, electronic relaxation, radiative relaxation, and finally fragmentation of the singly or multiply charged clusters. Elxper-imental techniques as well as selected results on simple model systems, such as variable size rare gas clusters, are presented. 

Resonant excitation of core levels involves the use of tunable soft- and hard-X-rays. This energy regime has widely been used since decades for a variety of investigations on atomic, molecular, surface and condensed matter physics, where synchrotron radiation is currently the primary X-ray source for experimental work in this field. Earlier work on resonant excitation of core levels has already shown that there are numerous unique properties, such as e.g. element specific excitations. Methods for determining structural properties, as well as site-selective fragmentation, promised important applications with respect to fundamental and applied research on size effects of matter. 

As is the case for QTT, in FTICR instraments isolation and excitation take place in the same confined space, where the ions are trapped for a specific time in a combined magnetic and electrostatic field [26]. On-resonance excitation in the FTICR... 

The above equations show the importance of resonance, but are unrealistic since will be limited by relaxation processes in the excited state. Thus a damping factor inversely related to the excited-state lifetime must be introduced. On resonance, excited-state relaxation will deter-... 

Sekatskii, S. and Letokhov, V. (1996). Scanning optical microscopy with a nanometer spatial resolution based on resonant excitation of fluorescence from one-atom excited center. JETP Letters, 63 319-323. 

One of the most interesting features of the Raman spectmm is its dependence on tire incident light frequency, coj. When Wj is on resonance with the excited electronic state, the scattering process closely resembles a process of absorption followed by emission. However, as Uj is detuned from resonance there are no longer... 

Figure Al.6.15. Schematic diagram, showing the time-energy uncertainty principle operative in resonance Raman scattering. If the incident light is detuned from resonance by an amount Aco, the effective lifetime on the excited-state is i 1/Aco (adapted from [15]).

A third pumping method (Fig. Ic) uses an electrical discharge in a mixture of gases. It relies on electronic excitation of the first component of the gas mixture, so that those atoms are raised to an upper energy level. The two components are chosen so that there can be a resonant transfer of energy by collisions from the upper level of the first component to level 3 of the second component. Because there are no atoms in level 2, this produces a population inversion between level 3 and level 2. After laser emission, the atoms in the second component return to the ground state by collisions. 

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

As indicated in Fig. 21.3, for both atomic absorption spectroscopy and atomic fluorescence spectroscopy a resonance line source is required, and the most important of these is the hollow cathode lamp which is shown diagrammatically in Fig. 21.8. For any given determination the hollow cathode lamp used has an emitting cathode of the same element as that being studied in the flame. The cathode is in the form of a cylinder, and the electrodes are enclosed in a borosilicate or quartz envelope which contains an inert gas (neon or argon) at a pressure of approximately 5 torr. The application of a high potential across the electrodes causes a discharge which creates ions of the noble gas. These ions are accelerated to the cathode and, on collision, excite the cathode element to emission. Multi-element lamps are available in which the cathodes are made from alloys, but in these lamps the resonance line intensities of individual elements are somewhat reduced. 

In our tip-enhanced near-field CARS microscopy, two mode-locked pulsed lasers (pulse duration 5ps, spectral width 4cm ) were used for excitation of CARS polarization [21]. The sample was a DNA network nanostructure of poly(dA-dT)-poly(dA-dT) [24]. The frequency difference of the two excitation lasers (cOi — CO2) was set at 1337 cm, corresponding to the ring stretching mode of diazole. After the on-resonant imaging, CO2 was changed such that the frequency difference corresponded to none of the Raman-active vibration of the sample ( off-resonant ). The CARS images at the on- and off- resonant frequencies are illustrated in Figure 2.8a and b, respectively. 

Ion trap MS is particularly suited for chemical structure elucidation, as it allows for simultaneous ion storage, ion activation and fragmentation, and product ion analysis. The fragmentation pathway of selected ions and the fragmentation products provide information on the molecular structure. Compared with triple-quadrupole and especially with sector instruments, the ion trap instrument provides more efficient conversion of precursor ion into product ions. However, the CID process via resonance excitation, although quite efficient in terms of conversion yield, generally results in only one (major) product ion in the product-ion mass spectrum. MS/MS with a quadrupole ion trap offers a number of advantages ... 

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

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