Coherent ion motion

As a result of this excitation step, the net coherent ion motion produces a time-dependent signal on the receiver plates, termed the image current , which represents aU ions in the FT-ICR cell. The image current is converted to a voltage, ampMed, digitized, and Fourier transformed to yield a frequency spectrum that contains complete information about frequencies and abundances of all ions trapped in the cell. A mass spectrum can then be determined by converting frequency into mass because frequency can be measured precisely, the mass of an ion can be determined to one part in 10 or better. 

As shown in figure 2, examples of a stable optical frequency standard of similar performance to the best microwave oscillators mentioned above are those with Ba [30] and Hg [31] trapped in Paul and Penning traps. In many cases, uncertainties due to coherent ion motion present limitations similar to those of the microwave standards discussed above. 

FTICR should be very low to minimise ion—molecule reactions and ion—neutral collisions that dampen the coherent ion motion. A variety of external ion source designs have been developed to deal with this problem, and each design has its own performance characteristics [42]. 

Assuming coherent ion motions continue for the duration of the acquired signal, the maximum obtainable resolution is Fourier-limited and is defined by the equation... 

Figure 4. Ions undergoing coherent cyclotron motion induce image currents in the plates of the FTMS analyzer cell. Reproduced with permission from Ref. 18. Copyright 1985, North-Holland Physics Publishing.

D. M. Neumark We are interested in generating coherent vibrational motion in negative ions, which typically do not have bound excited electronic states. Does your Impulsive Stimulated Raman Scattering (ISRS) scheme work if the excited state is not bound ... 

Damping Loss of coherent ion cyclotron motion primarily due to... 

We have produced coherent states of ion motion fi om the 1010) state by applying either a resonant (fi-equency coj classical driving field or a moving standing wave of laser radiation which resonantly drives the ion motion through the optical dipole force [21,24]. In Fig. 3, we show a measurement of P,(t) after... 

As we have described, a great deal of experimental data on the q 0 vibrational energy levels in the metal azides have been obtained in the last few years. A consistent picture of the internal and external modes in different azide lattices has emerged from these studies. The question of the degree of lattice ionicity in different azide lattices has been raised as a consequence of the available data, but needs detailed theoretical analysis. Particularly noteworthy has been the recent measurement by coherent neutron inelastic scattering of vibrational modes in different directions of the Brillouin zone in a metal azide crystal—KNa- More sophisticated analyses of diffraction data have now yielded precise inter- and intraionic bond lengths, thermal amplitudes of azide ion motions and, most recently, the valence electron density about N3 in KN3, all of which provide important additional input for the characterization of interatomic forces in azides. 

The heart of this instrument is a cell (also known as a Penning trap), which is placed in a strong magnetic field. Ions are confined laterally in this trap by a static magnetic field and axially by a static electric field. Ions are excited by a broadband rf pulse to a coherent orbital motion, and from the frequency of this motion, the miq of the trapped ions can be obtained. Over a dozen designs of cell geometry exists [64]. The cubic cell is shown in Figure 3.21. 

Note In ICR cells, the ions circulate like separate swarms of birds rather than like matter in the rings of Saturn. If ions of the same m/z noncoherently circulated at the same frequency and radius, but occupied the total orbit rather than a small sector of it, there would be no image current induced upon their passage at the detector plates, i.e., coherence of the circulating ion motion is crucial. 

Fig. 20. Coherent circular motion of ions with the same m z ratio in a cubic ICR cell results in a pure sinusoidal signal. For a better view, the planes perpendicular to the magnetic field are omitted.

Ions generated by an electron beam from a heated filament are passed into a cubic cell where they are held by an electric trapping potential and a constant magnetic field. Each ion assumes a cycloidal orbit at its own characteristic frequency, which depends on mJz the cell is maintained under high vacuum. Originally, these frequencies were scanned by varying the electric field until each cycloidal frequency was, in turn, in resonance with an applied constant radiofrequency. At resonance, the motion of the ions of the same frequency is coherent and a signal can be detected. 

In studying this system, the first femtosecond pulse takes the ion pair M+X to the covalent branch of the MX potential at a separation of 2.7 A. The activated complexes [MX], following their coherent preparation, increase their intemuclear separation and ultimately transform into the ionic [M+ X ] form. With a series of pulses delayed in time from the first one the nuclear motion through the transition state and all the way to the final M + X products can be followed. The probe pulse examines the system at an absorption frequency corresponding to either the complex [M X] or the free atom M. 

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