Field ionization diabatic

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... 

Fig. 7.9 Adiabatic and diabatic paths to ionization for n = 15 states in the center and on the edges of the Stark manifold. The diabatic paths are shown by solid bold lines and the adiabatic paths by broken bold lines. In both cases ionization occurs at the large black dots. The diabatic paths are identical to hydrogenic behavior. The adiabatic n = 15 paths are trapped between the adiabatic n = 14 and n = 16 levels. Adiabatic ionization always occurs at lower fields than diabatic ionization.

Fig. 11.7 Collisionally induced diabatic ionization features observed following collisions of laser-excited Xe 31f atoms with Xe target gas at a pressure of 1(T5 Torr for 8 fis in the presence of several applied static fields. The arrows indicate the ranges of ionizing field strengths over which n = 30 and 31 states are expected to ionize diabatically (from ref. 43).

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... 

Fig. 13.4 Diabatic field ionization signals for i changing under increasing incident beam intensities. For clarity the successive curves are displaced upward by one scale unit. Data points are taken from the transient digitizer records, and a small sloping background has been subtracted. Full curves are fits to the model of MacAdam et al. (from ref. 8).

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).

The ZEKE detection scheme is equivalent to ionization of high-n Rydberg states by Pulsed Field Ionization (PFI). If one assumes that the pulsed field ionization of the Rydberg electron follows a diabatic process (Chupka, 1993), then the ionization threshold is lowered by... 

The H atom is a special case. Since the states of the same n and m are all degenerate in zero field, no matter how slowly we apply a field we project the nim states onto the nnxn2m states, i.e. the transition is always diabatic. On the other hand, as long as the field rises slowly compared to the An interval, a H atom in an nx state remains in the same nx state and ionization always occurs at the same field, irrespective of the rise time of the pulse. 

The passage from the zero field nd state to a single Stark state is evidently adiabatic. A hydrogen nd /n =2 state would pass diabatically from zero field to many nnjn22 Stark states and exhibit multiple ionization fields. 

Exposure of blue Stark states to rapidly rising fields also results in diabatic, or H like, ionization, at fields far above the classical field for ionization. This point has been demonstrated by Neijzen and Donszelmann12 using high lying, n = 66, states of In and by Rolfes et al.13 using Na atoms of n = 34 and m = 2. 

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