Double-peak lineshape

II. 1 - In most case, the double-Peak lineshape is a signature of the dispersive character of the saturated susceptibility. This character has been demonstrated b.y means of a heterod.yne detection of the PC field. which allows one to monitor its amplitude and phase. With increasing intensities, the dispersion component takes over the absorption component. and becomes predominant at full saturation (Experiments performed in excited Ne. transitions = 640 nm, 1 s - 2pg and A = 607 nm. 1 s - 2P3 [4]). The PC intensit.y lineshape is thus the square of a dispersion curve. 

To observe a 7s — 9 transition requires that there be a 9p admixture in the 9 state. For odd this admixture is provided by the diamagnetic interaction alone, which couples states of and 2, as described in Chapter 9. For even states the diamagnetic coupling spreads the 9p state to all the odd 9( states and the motional Stark effect mixes states of even and odd (. Due to the random velocities of the He atoms, the motional Stark effect and the Doppler effect also broaden the transitions. Together these two effects produce asymmetric lines for the transitions to the odd 9t states, and double peaked lines for the transitions to even 9( states. The difference between the lineshapes of transitions to the even and odd 9i states comes from the fact that the motional Stark shift enters the transitions to the odd 9( states once, in the frequency shift. However, it enters the transitions to the even 9( states twice, once in the frequency shift and once in the transition matrix element. Although peculiar, the line shapes of the observed transitions can be analyzed well enough to determine the energies of the 9( states of >2 quite accurately.25... 

Figure 5.19. The phase-sensitive COSY for a coupled two-spin AX system. Diagonal peaks have broad, in-phase douh e-dispersion lineshapes (D) whereas crosspeaks have narrow, antiphase double-absorption lineshapes (A), as further illustrated in the row extracted from the spectrum.

Schematic view of the diagonal peak from a COSY spectrum. The squares are supposed to indicate the two-dimensional double dispersion lineshape illustrated below...

Two peaks in Fx are expected at Ox +nJX2, these are just the two lines of the spin 1 doublet. In addition, since these are sine modulated they will have the dispersion lineshape. Note that both components in the spin 1 multiplet observed in F2 are modulated in this way, so the appearance of the two-dimensional multiplet can best be found by "multiplying together" the multiplets in the two dimensions, as shown opposite. In addition, all four components of the diagonal-peak multiplet have the same sign, and have the double dispersion lineshape illustrated below... 

The double quantum filter eliminates or at least suppresses the strong signals from protons that do not experience J-coupling, e.g. the solvent signal, which would otherwise dominate the spectrum and possibly be a source of troublesome tl noise. Compared to a phase-sensitive but non-DQ-filtered COSY with pure absorption lineshapes for the cross peaks but mixed lineshapes for the diagonal peaks, the phase-sensitive, DQ-filtered COSY has pure absoiption lineshapes throughout. 

Previous sections have already made the case for acquiring COSY data such that it may be presented in the phase-sensitive mode. The pure-absorption lineshapes associated with this provide the highest possible resolution and allow one to extract information from the fine-structure within crosspeak multiplets. However, it was also pointed out that the basic COSY-90 sequence suffers from one serious drawback in that diagonal peaks possess dispersion-mode lineshapes when crosspeaks are phased into pure absorption-mode. The broad tails associated with these can mask crosspeaks that fall close to the diagonal, so there is potential for useful information to be lost. The presence of dispersive contributions to the diagonal may be (largely) overcome by the use of the double-quantum filtered variant of COSY [37], and for this reason DQF-COSY is the experiment of choice for recording phase-sensitive COSY data. 

Check it 3.3.2.2 illustrates the phase correction of a phase sensitive IR COSY TPPI spectrum whilst in Check it 3.3.2.3 a phase sensitive COSY spectrum with double quantum filter is simulated. The double quantum filter effects the lineshape in the spectrum such that the diagonal and cross peaks have a narrower absorptive lineshape. 

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