Rydberg levels microwave ionization

In alkali atom experiments no explicit resonances have been observed in microwave ionization. However, there are indirect confirmations of the multiphoton resonance picture. First, according to the multiphoton picture the sidebands of the extreme n and n + 1 Stark levels should overlap if E = 1/3n5. In the laser excitation spectrum of Na Rydberg states from the 3p3/2 state in the presence of a 15 GHz microwave field van Linden van den Heuvell et al. observed sidebands spaced by 15.4 GHz, as shown in Fig. 10.15.18 The extent of the sidebands increases linearly with the microwave field, as shown in Fig. 10.15, and the n = 25 and n = 26 sidebands overlap at microwave fields of 150 V/cm or higher, matching the observation that the 25d state has an ionization threshold of 150 V/cm in a 15 GHz field. 

Fig. 5.26 Sensitive detection of microwave radiation by microwave-photon ionization of Rydberg levels...

On the other hand, the large dipole moment of Rydberg atoms offers the possibility to use them as sensitive detectors for microwave and submillimeter-wave radiation [566]. For the detection of radiation with frequency to, a Rydberg level n) is selectively excited by stepwise excitation with lasers in an external electric dc field. The field strength is adjusted in such a way that the energy En of the Rydberg level n) is just below the critical value Eq for field ionization, but E -i- hco is just above. Every absorbed microwave photon tuo then produces an ion that can be detected with 100 % efficiency (Fig. 5.26). 

Fig. 10.23. Sensitive detection of microwave radiation by electric>field-assisted microwave-photon ionization of Rydberg levels. The level populated by MW absorption from the optically pumped level m) is ionized in the electric field...

When a) l/n3, the field required for ionization is E = 1/9n4, and as a> approaches l/n3 it falls to E=0.04n. These observations can be explained qualitatively in the following way. At low n, so that a> 1/n3, the microwave field induces transitions between the Stark states of the same n and m by means of the second order Stark effect. With only a first order Stark shift a state always has the same dipole moment and wavefunction, as indicated by the constant slope dW/d of the energy level curve. Thus when the field reverses, — — , the Rydberg electron s orbit does not change. With a second order Stark shift as well, the slope dW/d is not the same at E and —E, and as a result the dipole moment and wavefunction are not the same. If the field is reversed suddenly a single Stark state in the field E is projected onto several Stark states of the same n and m when E — - E. Since all the Stark states of the same n make transitions among themselves they ionize once the field is adequate to ionize one of them, the red one, at E = 1/9n4 for m n. 

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