Off-resonance excitation

The energy imparted to the ions depends on the energy of the rf pulse and the duration of the pulse. The energy does not have to be raised in one event but may be absorbed by the ion in small portions. A technique called sustained off-resonance excitation (SORT) (82) uses a low-amplitude rf pulse that is off-resonance to the ion cyclotron frequency. The difference of the cyclotron frequency and the excitation frequency (-500 Hz) causes the ion to experience in- and out-of-phase excitation that has the effect of a repeated expansion and shrinkage of the cyclotron orbit. In this process, the ion undergoes a large number of low-energy collisions and the Ecom slowly increases until the ion dissociates. 

Fig. 9.1. (A) Gaussian (a) and sine (b) excitation profiles. (B) Composite (G3) Gaussian pulse. (C) Train of soft pulses modified after the DANTE sequence to achieve selective off-resonance excitation. (D) Redfield 21412 sequence. (E) Binomial 11, 121, 1331, 14641 sequences. (F) JR (a) and compensated JR (or 1111) (b) sequences. (G) Watergate sequence. (H) Weft (Superweft) sequence. (I) Modeft sequence. (J) MLEV16 sequence. (K) NOESY sequence with trim pulse. (L) MLEV17 sequence with trim pulses. (M) Clean-TOCSY sequence.

FIGURE 9.5 Trajectories of magnetizations from 0 to 900 Hz off resonance, excited by a rectangular pulse of width 1 ms with yB /2ir = 250 Hz, which is a 90° pulse on resonance. A magnetization at 1000 Hz from resonance would complete almost a full circle and give no signal. From Freeman.106... 

Figure 12 Vibrational enhancement selectivity available from resonance Raman spectroscopy. The UV-visible spectrum of a P. aeruginosa azurinis shown together with two different Raman spectra (frozen solution at 77 K) that derive from laser excitation within the S(Cys) — Cu(II) charge-transfer absorption band at 625run (647.1 nm) and away from the absorption (488.Onm). Excitation within resonance leads to dramatically increased Raman scattering from the Cu active site, whereas off-resonance excitation produces a spectrum dominated by bands of nonchromophoric ice...

Figure 3.5. Excitation of magnetisation for which the rf is on-resonance (a) results in the rotation of the bulk vector about the applied rf field, B. Those spins which experience off-resonance excitation (b) are instead driven about an effective rf field, Beff, which is tipped out of the x-y plane by an angle 6, which increases as the offset increases.

Figure 9.1. Imperfections in pulse excitation, (a) A perfect 90° pulse applied at equilibrium, (b) the effect of B)-inhomogeneity and (c) the effect of off-resonance excitation.

Figure 9.2. Off-resonance excitation causes the bulk vector to rotate about an effective rf field, tipped out of the transverse plane.

The binomial sequences aim to improve the zero excitation profile and provide schemes that are less sensitive to spectrometer imperfections. The series may be written 1-1, 1-2-1, 1-3-3-1. .. and so on, where the numbers indieate the relative pulse widths, each separated by a delay x, and the overbar indicates phase inversion of the pulse. For off-resonance spins the pulse elements are additive at the exeitation maximum so for example, should one require 90° off-resonance excitation, 1-1 corresponds to the sequence 45x-t-45 x. Of this binomial series, it turns out that the 1-3-3-1 sequence [66] has good performance and is most tolerant of pulse imperfections by virtue of its symmetry [67]. The trajectory of spins with frequency offset l/2x from the transmitter for a net 90° pulse (1 = 11.25°) is shown in Fig. 9.26. During each... 

On- and off-resonance excitation of Rydberg states and the theme of angular momentum wavepackets... 

SORI-CID, introduced by Gauthier and co-workers [28], is not beset by these problems. As the name suggests, ions are excited slightly off-resonance (500-2000 Hz). Such excitation results in acceleration and deceleration of the ions with a period equal to the difference between the excitation frequency and the ion cyclotron frequency. The periodic decrease in cyclotron radius means that ions are not ejected from the ICR cell. Prior to off-resonance excitation of the precursor ions, inert gas is leaked into the ICR cell. As the ions are excited, collisions with the gas resnlt in conversion of translational energy to internal energy. Again, as a resnlt of the periodic decrease in cyclotron radius, the product ions are formed close to the center of the cell, eliminating resolution issues. It is possible that the product ions have a cyclotron frequency equal to that of the applied excitation waveform. If this were the case, those product ions would be ejected from the ICR cell (resonant ejection). To avoid this occurrence, off-resonance excitation is performed in both directions, for example, 500 Hz. 

Heck, A.J.R. de Koning, L.J. Pinske, F.A. Nibbering, N.M.M. Mass-specific selection of ions in Fourier transform ion cyclotron resonance mass spectrometry-unintentional off-resonance excitation of selected ions. Rapid Commun. Mass Spectrom. 1991, 5,406-414. 

Two-photon absorption can be formally described by a two-step process from the initial level i) via a virtual level v) to the final level /> (Fig. 2.30b). This fictitious virtual level is represented by a linear combination of the wave functions of all real molecular levels kn) that combine with i) and f) by allowed one-photon transitions. The excitation of w) is equivalent to the sum of all off-resonance excitations of these real levels kn). The probability amplitude for a transition i) v)... 

Fig. 9.28. Generation of an oscillation of ion kinetic energy as defined by a beat frequency due to off-resonance excitation vertical connections are to locate points of positive and negative interferences, respectively. The larger the amplitude of fi, the more kinetic energy is contained in the circulating ions. One SORI-CID experiment consists of hundreds of beats.

Multiple-pulse sequences based on off-resonance excitation - where the effective interaction frame rotates at the magic angle and dipolar interactions can be made to vanish. Imaging in this interaction frame requires a combination of matched RF and d.c. gradients. 

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