Excitation to the Rydberg states

Rydberg atoms have been produced by charge exchange, electron impact, and photoexcitation, the processes 

In general crr depends on the energy, W0, of the incident electron. However, once Wo is fixed it is a good approximation to assume that do Woj/dWis constant for Vk 0 and given by 

Below the limit, W 0, the energy is no longer a continuous variable, and it is more useful to write the cross section per principal quantum number 

Similar arguments apply to charge exchange and photoexcitation, and the basic result is the same the cross section for the production of Rydberg atoms is the continuation below the limit of the ionization cross section, leading to an n 3 dependence of the excitation cross section. 

Using these assumptions they could then fit their data to determine excitation cross sections, o(n, Wo), for the excitation of a Rydberg state of principal quantum number n by an electron of energy W0. They found the values of o (W0) given in 

---

While ionization by linearly polarized fields has been well studied, there is only one report of ionization by a circularly polarized field, the ionization of Na by an 8.5 GHz field.36 In the experiment Na atoms in an atomic beam pass through a Fabry-Perot microwave cavity, where they are excited to a Rydberg state using two pulsed tunable dye lasers tuned to the 3s — 3p and 3p —> Rydberg transitions at 5890 A and —4140 A respectively. The atoms are excited to the Rydberg states in the presence of the circularly polarized microwave field which is turned off 1 fis after the laser pulses. Immediately afterwards a pulsed field is applied to the atoms to drive any ions produced by microwave ionization to a microchannel plate detector. To measure the ionization threshold field the ion current is measured as the microwave power is varied. 

The most commonly used method is the direct measurement of a decay rate by pulsed excitation and time resolved detection. The most straightforward example of this technique is laser induced fluorescence applied to alkali Rydberg atoms. Alkali atoms are typically contained in a glass cell, which also holds a known pressure of perturber gas. The alkali atoms are excited to the Rydberg state at time t = 0 and the time resolved fluorescence from the Rydberg atoms is detected... 

Figure 18.2 Schematic diagram showing the potential curves for the ground state, the repulsive intermediate states and two of the Rydberg states of CH3I. The vertical arrows show the one-colour, non-resonant (dashed arrows), and the two-colour, resonant (solid arrows) routes for two-photon excitation to the Rydberg states. Note that resonance with the repulsive intermediate state (two-colour excitation) leads to stretching of the C-I bond and this changes the Franck-Condon window for excitation to the Rydberg state, favouring the C-I vibrational mode V3. Reproduced from Min etal, J. Photochem. Photobiol., 1996, 100 9, with permission of Elsevier...

In all cases a two-color multiphoton process is used to excite the molecules to the high-n Rydberg states. In the remainder of this text we use a primed notation to refer to the transitions from the ground state to the intermediate state (e.g., O branch, O branch, etc.), whereas an unprimed notation is used for transitions from the intermediate state J to the Rydberg states with core rotation quantum number for example, 5(2) implies the transition J = 2- N+ = 4, whereas S (2) implies J" = 2 — J = 4. 

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. 

A beam of metastable Xe atoms is excited to a Rydberg state or photoionized and the product Xe+ ions are expelled by a field pulse from 2-5 /is later and impinge upon a position sensitive detector. When the metastable Xe is photoionized the pattern reflects the geometry of the metastable Xe beam, and the same is true when Xe+ is produced by attachment of the electron from a Xe 60f state to SF6. Apparently, there is no deflection of the Xe + by the SF6 . However, at low n,n< 40, the pattern of Xe+ detected is twice as broad as the metastable beam, indicating deflection of the Xe+ ion by the SF6-. As expected, the deflections of the Xe+ become more pronounced at lower n. 

While electrons in conventional beams have velocities too high to have large cross sections, thermal electrons have large cross sections for state changing collisions with Rydberg atoms, and these collisions have been studied in a systematic fashion. Specifically, metastable He atoms in a stationary afterglow have been excited to specific Rydberg states with a laser.37 38 The populations of... 

A third resonance method which has been used to measure the intervals of alkali and Ba Rydberg atoms is delayed field ionization which takes advantage of the increase of the lifetime with L The method used by Safinya et al.15 in the study of Cs nf rch and ni —> ni transitions is typical. Atoms are excited to the ni states in a manner similar to the one used by Fredriksson et al.,1 except that pulsed... 

If the sample of N0 bound 6sni Rydberg atoms is exposed to an integrated photon flux , then the number of atoms excited to the autoionizing states is given by... 

Atomic cryocrystals which are widely used as inert matrices in the matrix isolated spectroscopy become non-inert after excitation of an electronic subsystem. Local elastic and inelastic lattice deformation around trapped electronic excitations, population of antibonding electronic states during relaxation of the molecular-like centers, and excitation of the Rydberg states of guest species are the moving force of Frenkel-pairs formation in the bulk and desorption of atoms and molecules from the surface of the condensed rare gases. Even a tiny probability of exciton or electron-hole pair creation in the multiphoton processes under, e.g., laser irradiation has to be taken into account as it may considerably alter the energy relaxation pathways. 

I lence, (V1I-193) appears to be a major primary process in contradiction to the spin conservation rules. The C( S) may be formed by the photolysis nf CzO, since its concentration does not increase linearly with the flash intensity but shows a higher order dependence on the intensity. More detailed lescription of process (VII-196) may be the following. By absorption of light near 1600 A the C302 molecule is excited to a Rydberg state (838)... 

Due to the selection rule f =0, l, 2, 3 for three photon dipole transitions the excitation of a state is possible. As shown in the simplified potential diagram in fig. 3 two almost resonant intermediate steps (b ttCOJ) and ZTg ) are involved in the three photon excitation of the Rydberg states. This explains the comparatively strong appearance of the triplett bands and leads to the assumption that one of... 

---