Ionization threshold static

Pillet et al. observed that adding small static fields dramatically reduces the microwave fields required for the ionization of Li.19 For example the application of a static field of 1 V/cm lowers the 15 GHz ionization threshold of the Li 42d state from 200 V/cm, to a broad threshold centered at 20 V/cm, a field only slightly in excess of E = 1/3n5 = 13 V/cm. The threshold field 200 V/cm corresponds to the hydrogenic threshold field of 1/9 n4, which will be described shortly. A small field has virtually no effect in a single cycle Landau-Zener model, but its dramatic... 

The first measurements of microwave ionization in any atom were carried out with a fast beam of H by Bayfield and Koch1, who investigated the ionization of a band of approximately five n states centered at n = 65. Using microwave and rf fields with frequencies of 9.9 GHz, 1.5 GHz, and 30 MHz, to ionize the atoms they found that the same field was required at 30 MHz and 1.5 GHz to ionize the atoms, but that a smaller field was required at 9.9 GHz. The measurements showed that at n = 65 frequencies up to 1.5 GHz are identical to a static field. Later, more systematic measurements have confirmed the initial measurements and have allowed significant refinements of our understanding. In Fig. 10.16 we show the ionization threshold fields (in this case the field at which there is 10% ionization) of H in a 9.9 GHz field.21 The ionization fields are plotted as n4E vs n3a>, and they bring out two factors. First, at low frequencies the field required is l/9n4, the static field required to ionize the red n Stark state of m n. Second, as shown by the scaling of the horizontal axis, the required field drops below l/9n4 as the microwave frequency approaches the interval between adjacent n states, 1 In3. 

A much higher circularly polarized field is required to ionize the atoms than a linearly polarized field, as shown by Fig. 10.21, a plot of the threshold fields, where 50% ionization occurs, for linearly and circularly polarized 8.5 GHz fields. As shown by Fig. 10.21, the circularly polarized microwave ionization threshold field is very nearly E = l/16n4, the same as the the static field required to ionize a Rydberg Na atom and much higher than the field required for ionization by... 

Work on Al has included magnetic properties, [37] ionization thresholds and reactivities, [38], static polarizabilities [39], and measurements of collision-induced dissociation of Al [40]. The dimer is the best studied of the aluminium... 

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