Rydberg collisional

Purely optical excitation is possible for alkali and alkaline earth atoms. For most other atoms the transition from the ground state to any other level is at too short a wavelength to be useful. To produce Rydberg states of such atoms a combination of collisional and optical excitation is quite effective. A good example is the study of the Rydberg states of Xe by Stebbings et al.24 As shown in Fig. 3.5, a thermal beam of Xe atoms is excited by electron impact, and a reasonable fraction of the excited atoms is left in the metastable state. Downstream from the electron excitation the atoms in the metastable state are excited to a Rydberg state by pulsed dye laser excitation. 

There are three methods which have been used to study collisions of Rydberg atoms with neutrals. They are direct measurement of collisionally induced population changes, line shift and broadening measurements, and photon echo measurements.23 In this chapter we describe the first of these. The last two are described in the chapter immediately following. 

One of the more well studied collision processes involving Rydberg atoms is collisional angular momentum mixing, or i mixing, the collisional transfer of population among the nearly degenerate states of the same n.28 The process has... 

The depopulation cross sections of the Rb nd states of 25 < n < 40 are 1000 A2, which is the same as the cross section of the Rb ns state if the ns —> (n - 3)1,1 > 3 contribution is subtracted. For the Rb nd states the calculated contribution of the scattering of the nd state to nl S 3 and (n—1)1 s 3 states with no change in the rotational state of the CO is <100 A2, so 90% of the cross section is due to the inelastic transitions leading to rotational excitation. Presumably it is because the resonant transfer accounts for 90% of the observed cross section that the structure in the cross section is more visible in the nd cross sections than in the ns cross sections. For both the ns and nd states minimal collisional ionization is observed and calculated in this n range, principally because there are too few CO molecules with energetic enough A/ = -1 rotational transitions. For example, only CO 7 > 18 states can ionize an n = 42 Rydberg state by a A7 = -1 transition, and only 3% of the rotational population distribution is composed of 7 > 18 states. 

Fig. 11.18 Arrival time spectra of the products of the collisional ionization of Xe 26f high Rydberg atoms by CH3I, and C6F6. As shown, collisions with CH3 lead only to r. Collisions with C F14 lead to both C7F14 and e, as shown by the large signal at early times due to electrons. C6F6 leads to the production of a long lived autodetaching state of QF6 which produces a nearly continuous electron signal at early times (from ref. 79).

It is interesting to note that the pressure broadening described here is in essence the non-resonant manifestation of the same interaction responsible for resonant Rydberg atom-Rydberg atom collisional energy transfer.37... 

The logical extension of the state changing M and An collisions is collisional ionization, for Na Rydberg atoms and Ar+ the process... 

One of the most striking aspects of resonant collisional energy transfer between Rydberg atoms is the magnitude of the cross sections. Accordingly, the first... 

Using a simple, three level model we can develop a feeling for the microwave powers required to observe radiatively assisted collisional energy transfer between Rydberg atoms.3 Consider the dipole-dipole atomic system shown in Fig. 15.1(a). In the Na ns + ns— np + (n - l)p resonant collisions described in the previous chapter the ns state corresponds to both s and s of Fig. 15.1(a) and the n — 1 and np states correspond to p and p of Fig. 15.1(a), respectively. The collisions occurs via the interaction... 

Another interesting and important feature of collisional reduction of cations is that the reducing electron can enter a high molecular orbital corresponding to an excited state of the neutral molecule, radical, or biradical. The types of excited states are depicted in Fig. 3. Electron capture in a Rydberg-type orbital can give... 

An interesting aspect of the collisional resonances shown in Fig. 2 is that they are quite easy to observe. In fact, most of the Rydberg atoms undergo collisions, and we can estimate the cross section rather easily using... 

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