Alkali atoms excitation

The advent of tunable dye lasers has opened a new era in light sources for the excitation of atomic fluorescence. An increasing number of descriptions of these may be found in the literature [42], and populations of alkali atoms excited to their 2P resonance states, approaching the theoretical limit equilibrium of 50 %, have been reported [43]. 

Studies of excitation transfer induced in collisions with molecules were not limited to fine-structure states of alkali atoms. Excitation transfer from the 621>3/2 to the 6 2 >5/2 state in thallium was investigated in a Hanle experiment... 

This approach was used by Elliott and co-workers to control the ionization of alkali atoms by one- and two-photon excitation. Wang and Elliott [72] measured the interference between outgoing electrons in different angular momentum states. They showed, for example, that the angular flux of the p2P and the d2D continua of Rb is determined by the phase difference... 

Hooymayers HP, Alkemade CTJ (1966) Quenching of excited alkali atoms and related effects in flames Part II. Measurements and discussion. J Quant Spectrosc Radiat Transfer 6 847-874... 

In spite of the fact that in alkali vapors, which contain about 1 % diatomic alkali-molecules at a total vapor-pressure of 10 torr, the atoms cannot absorb laser lines (because there is no proper resonance transition), atomic fluorescence lines have been observed 04) upon irradiating the vapor cell with laser light. The atomic excited states can be produced either by collision-induced dissociation of excited molecules or by photodissociation from excited molecular states by a second photon. The latter process is not improbable, because of the large light intensities in the exciting laser beam. These questions will hopefully be solved by the investigations currently being performed in our laboratory. 

The alkali atoms may be excited by either being irradiated with resonance radiation... 

The latter method has the advantage of low alkali-atom densities, thus avoiding radiation trapping and chemical reactions and allows selection of the initial kinetic energy of the A atom—subject, however, to some discussion about the velocity distribution and its relaxation before quenching. The excited atoms will loose their excitation energy be either spontaneous emission... 

Figure 1. Graphic display of atomic properties of metastable noble gases. Solid lines correspond to metastables, dashed to analogous alkali atom (He corresponds to Li, etc.) -o-, excitation energies ionization potentials -A-, polarizabilities, with angular momentum substates of metastables shown separately (see Table I for values).

In Chapter 3 we considered briefly the photoexcitation of Rydberg atoms, paying particular attention to the continuity of cross sections at the ionization limit. In this chapter we consider optical excitation in more detail. While the general behavior is similar in H and the alkali atoms, there are striking differences in the optical absorption cross sections and in the radiative decay rates. These differences can be traced to the variation in the radial matrix elements produced by nonzero quantum defects. The radiative properties of H are well known, and the radiative properties of alkali atoms can be calculated using quantum defect theory. 

Now let us consider alkali atoms. We begin by considering the K 4s-np excitations analogous to the excitation from the ground state of H. In K the situation is in fact quite different, as shown by Fig. 4.2(a), a plot of the radial... 

The fine structure intervals of the alkali atoms often fall in the 1-10 MHz range, in which case the transition between spin orbit and uncoupled states can be made either diabatically or adiabatically. Jeys et al.16 have observed the transition from an adiabatic to a diabatic passage from the coupled fine structure states to the uncoupled states. With a pulsed laser, they excited Na atoms from the 3p1/2 state to the 34d3/2 state with o polarized light, which leads to 25% my = 1/2 atoms and... 

The second approach is to use thermal beams of alkali atoms as shown in Fig. 10.2.4 A beam of alkali atoms passes into a microwave cavity where the atoms are excited by pulsed dye lasers to a Rydberg state. A1 /zs pulse of microwave power is then injected into the cavity. After the microwave pulse a high voltage pulse is applied to the septum, or plate, inside the cavity to analyze the final states after interaction with the microwaves. By adjusting the voltage pulse it is possible to detect separately atoms which have and have not been ionized or to analyze by selective field ionization the final states of atoms which have made transitions to other bound states. 

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. 

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... 

Similar coupled-state methods, both with and without the inclusion of positronium terms, have been applied to the excitation of other alkali atoms. The results of McAlinden, Kernoghan and Walters (1994, 1997) and Hewitt, Noble and Bransden (1994) for the dominant resonant excitation cross sections for sodium, rubidium and caesium all exhibit a similar energy dependence to that for lithium. Also, the neglect of positronium terms in the expansion, as in the work of McEachran, Horbatsch and Stauffer (1991), again has the effect of increasing the low energy excitation cross sections over those obtained when such terms are included. 

A pseudo potential approach was adopted by Hickman et al. [259] to calculate the excited metastable states of a He atom under liquid He. The density functional approach developed by Dupont-Roc et al. [260] was applied subsequently [261] for the description of the nature of the cavity formed around an alkali atom in the excited state of non-zero angular momentum. The resulting form of the cavity differs very much from the spherical shape. A similar approach was adopted by De Toffol et al. [262] to find qualitatively the first excited states of Na and Cs in liquid He. Earlier work in this direction was given in detail in Ref. [263]. 

Surface ionisation on a hot wire or ribbon is restricted to substances with low ionisation potentials and has been mainly used for detecting alkali atoms, dimers and halides and a few other similar species [30]. There is evidence that the efficiency of surface ionisation is dependent on the degree of internal excitation [104]. 

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