Field ionization adiabatic

Fig. 7.7 (a) Field ionization data for Na nd states of n = 30, 32, 34, and 36. (b) Light lines extreme members of m = 0 Stark manifolds (fourth order perturbation theory) dotted lines adiabatic paths to ionization for n = 30, 32, 34, and 36 dark lines diabatic paths to ionization for lowest members of m = 2 manifolds for n = 30, 32, 34, and 36. The lines indicating the classical ionization fields are calculated on the basis of Ref. 5 (from ref. 4). 

It is useful to present visually the difference between adiabatic and diabatic field ionization. In Fig. 7.9 we show schematically how adiabatic and diabatic ionizations occur for three n = 15 states. Diabatic ionization, shown by the solid bold lines, is exactly like hydrogen. Only the red state ionizes at the classical ionization limit the fields for other states are higher. In adiabatic ionization, shown by the bold broken lines, the n = 15 levels are trapped between the n = 14 and n = 16 levels and ionize at the classical ionization limit. In reality the true adiabatic levels, are not field independent, as they are shown in Fig. 7.9, but exhibit the avoided crossings shown in Fig. 7.3. However this simplification in the drawing... 

Equally as interesting as the size of the total cross section is the distribution of the final states subsequent to mixing. Examining the adiabatic field ionization signals of Kachru et al., 30 it appears that only the lowest Stark states nearest to the... 

The most easily observed process with ionic, as with neutral, collisions partners is mixing. When the Na nd states are exposed to an ion beam, the field ionization signal changes from one which is predominantly adiabatic to one which is predominantly diabatic. By measuring the fraction R of signal transferred from the adiabatic to the diabatic peak of the field ionization signal MacAdam et al. measured the depopulation cross section of the Na nd states by He+ ions.1 In the limit of small values of R the depopulation cross section is given by1... 

T, the absolute value of the cross section for 450 eV He+ is determined to be 2.6 x 108 A2 for n = 28. The fact that the higher n cross sections at the ion energy of 450 eV fall below the n5 dependence was later found to be an artifact due to insufficient resolution of the diabatic and adiabatic field ionization signals.2 In later experiments with other ions the n5 dependence shown in Fig. 13.2 was also observed.2,3 The later measurements also verified that the cross section was independent of ion species as long as the ions had the same velocity. Using ions of... 

One of the most interesting aspects of the study of the Na ns and np states is the distribution of final states. In Fig. 13.5 we show the field ionization signals obtained when the 39p, 40s, 39d, and 40p states are exposed to 43 eV Na+ ions.10 There is an initial adiabatic peak and a later broader diabatic feature. The Na+ current is more than adequate to depopulate the 39d state, and the 39d signal presumably reflects substantial population of the higher , m states of n = 39, due to both non-dipole low velocity collisions and multiple collisions. As shown by... 

Fig. 13.5 Adiabatic and diabatic selective field ionization (SFI) for -changed ensembles produced from Na 39p, 40s, 39d, and 40p states by 43 eV Na+ impact. The adiabatic peaks occur at 170-180 V/cm, and the diabatic features occur above 250 V/cm (note change of vertical scale). The diabatic SFI from -changed 50s targets most closely resembles that from 39d. In contrast, 40p and 39p targets yields SFI that indicates a different distribution of Stark sublevels lying high in the n = 39 and 38 manifolds, respectively (from ref. 10).

To convert the observed field ionization spectra to n distributions two procedures were employed.30 One, termed the SFI centroid approach, was simply to calculate the average field at which each n state is ionized, including both adiabatic, m 2, and diabatic, m > 2, contributions. This procedure yields30... 

Fig. 13.13 Typical selective field ionization data for laser excited Na 50d atoms (a) data with electron beam gated off ( ), data following collisions with 25 eV electrons (+) corrected for electron-induced background signals (b) net signal due to electron impact. The horizontal bars indicate the range of field strengths over which n = 50 atoms are expected to ionize adiabatically and diabatically (from ref. 36).

ADE = adiabatic detachment energies ESC A = electron spectroscopy for chemical analysis HOMO = highest occupied molecular orbitals MAES = metastable atom electron spectroscopy MIES = metastable ionization electron spectroscopy OAT = oxygen atom transfer PES = photoelectron spectra PEI = pulsed field ionization PIES = Penning ionization electron spectroscopy QM = quantum-mechanical REMPI = resonantly enhanced multiphoton ionization SC = semiclassical VDE = vertical detachment energies XPS = x-ray photoelectron spectroscopy ZEKE = zero electron kinetic energy Cp = cyclopentadienyl, Ph = phenyl, CeHs Tp =... 

To our knowledge, 46 has never been observed in solution under stable conditions, even at low temperature. Pulse radiolysis " of benzyl chloride as well as flash photolysis ° of several derivatives in HHP have allowed the observation of the electronic absorption spectra of benzyl and its 4-methyl and 4-methoxy derivatives. The and NMR spectra of the 2,4,6-trimethylbenzyl cation and other more heavily substituted benzyl cations, however, have been studied at low temperature in superacid media. In the gas phase, cold benzyl radical has been probed by two-color, resonant two-photon ionization techniques, thus providing very accurate vibrational frequencies below 650 cm for the benzyl cation. Furthermore, the adiabatic ionization energy of benzyl radical and several isotopomers in the ground state were determined from their threshold photoionization spectra using resonant two-photon excitation and detection of electrons by pulsed field ionization. This information, combined with Af//° (CgH5CH2) from Ref. 212 leads to the value of Af//°m(46) reported in Table 9. 

In adiabatic ionization what is important is the ordering of the zero field energy levels, not their precise zero field energies. 

What conditions must be fulfilled for the ionization process to occur in the adiabatic fashion we have just described First, the transition from the zero field ntm states to the intermediate field Stark states must be adiabatic. Second, the traversal of the avoided crossings in the strong field regime, E > l/3n5 must be adiabatic as well. Finally, ionization only occurs at E > W2/4 if the ionization rate exceeds the inverse of the time the pulse spends with E > W2/4. 

---