Accidental resonances

As can be seen by inspecting Figure 23, the coherence transfer efficiency is overwhelmingly good thanks to the very efficient 15N— -13C INEPT step employed for transferring magnetization exclusively from 15N to the sequential 13C nucleus. In addition, the possibility of an accidental resonance... 

For HCN the situation is somewhat better, because the data on DCN are much more effectively independent of the HCN data. This molecule has also been the subject of much high-resolution spectroscopic study, so that the vibration-rotation energy levels are particularly well known and its vibrational spectrum is free of accidental resonances. Table 8 compares the results of three quite different calculations. The calculation by Strey and Mills is the most recent, and was based on the latest spectroscopic data the refinement was made to a and x values rather than to the vibrational levels and rotational constants as used by both the earlier workers. Strey and Mills also constrained 3 of the quartic interaction constants to zero, and refined to cubic and quartic force constants in a separate calculation to the quadratic refinement. The level of agreement between the calculations leads to conclusions rather similar to those made above for C02 in particular, standard errors should be multiplied by at... 

In all of these studies, the cross sections for reaction with 0+ were also measured. Mg and Na were found to yield no measurable cross section while cross sections exceeding 100 and 10 A were measured below ion energies of 10 eV for Ca and Fe, respectively. While accidental resonances are not surprising in the case of Fe, given the high density of ionic and... 

E.E.Nikitin, Charge exchange in the accidental resonance case, Izvesiya AN SSSR, 27, 996 (1963)... 

In order to investigate intact neutral ablated products one has to ionize the molecules as gently as possible to avoid secondary decomposition by laser. In this sense, often used MPI techniques are not suited, as mentioned briefly in Section 9.2 intense electric fields of MPI pulses tend to cause extensive fragmentation of the ablated products. Moreover, accidental resonance enhancement of some specific mass peaks may distort the true mass distribution pattern. 

The rate of conventional (single-center) two-photon absorption depends on the square of the focussed laser intensity, and as long ago as 1968 Gontier and Trahin showed that in the absence of accidental resonances an intensity factor of (/// ) is introduced for each additional photon involved in a multiphoton atomic excitation process. The constant / is a characteristic irradiance whose value depends on the sample, and corresponds to the situation where perturbation theory breaks down and all multiphoton processes become equally feasible. A similar trea ment of molecules leads to an intensity factor per photon of y = where If is an irradiance that... 

There are three types of simple charge transfer (a) resonant charge transfer, (b) accidental resonant charge transfer and (c) non-resonant charge transfer. 

Accidental resonant charge transfer is an electron exchange between unlike species also represented by reaction (47), but the energy defect AE is exactly or very close to zero. 

The nomenclature is confusing and its use confused, as an examination of the literature reveals. By exoergic charge-transfer reaction, we imply that the formal chemical reaction A B,A)B is exoergic for ground-state products. If the transfer of the electron is resonant or accidentally resonant, the ionic product will normally be formed in an excited electronic and/or vibrational state such that the actual electron transfer will be thermoneutral or essentially so. The exoergicity is released subsequently by the decay of the excited B product ion. 

The ready fulfilment of the resonance condition, sometimes called accidental resonance (A being small), is a basic feature of charge exchange involving slow polyatomic species. 

The primary H+ ions can react with O atoms by accidental resonance charge transfer producing ions, which react rapidly with H2 producing OH". The H 2 primary ions react rapidly with H2 producing H, which transfers a proton to O atoms also producing OH+... 

Figure 4 ZEKE spectrum of I2 recorded through the prij/sjcore 5d 2g state the arrows indicate diagonal transitions, and the asterisks accidental resonances with A <- X transitions.

The vibrational progressions can be adequately simulated through the calculation of Franck-Condon factors however the observed spin-orbit branching ratio, along with the intensity distribution, reflects a considerable contribution from both spin-orbit and field-induced resonant autoionization processes. Also, accidental resonances at the two-photon level with ion-pair states further perturb the distribution of peak intensities. 

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