Perturbed Rydberg series One of the most intensively studied manifestations of channel interaction in the bound states is the perturbation of the regularity of the Rydberg series, which is evident if one simply measures the energies of the atomic states. Although measurements of Rydberg energy levels by classical absorption spectroscopy show the perturbations in the series which are optically accessible from the ground state, the tunable laser has made it possible to study series which are not connected to the ground state by electric dipole transitions as well. One of the approaches which has been used widely is that used by Armstrong et al . As shown in Fig. 22.1, a heat pipe oven contains Ba vapor at a pressure of ∼1 Torr. Three pulsed tunable dye laser beams pass through the oven. Two are fixed in frequency, to excite the Ba atoms from the ground 6s 2 1 S o state to the 3 P 1 state and then to the 6s7s 3 S 1 state. The third laser is scanned in frequency over the 6s7s 3 S 1 → 6s n p transitions. The Ba atoms excited to the 6s n p states are ionized either by collisional ionization or by the absorption of another photon. The ions produced migrate towards a negatively biased electrode inside the heat pipe. The electrode has a space charge cloud of electrons near it which limits the emission current.
Perturbed Rydberg series One of the most intensively studied manifestations of channel interaction in the bound states is the perturbation of the regularity of the Rydberg series, which is evident if one simply measures the energies of the atomic states. Although measurements of Rydberg energy levels by classical absorption spectroscopy show the perturbations in the series which are optically accessible from the ground state, the tunable laser has made it possible to study series which are not connected to the ground state by electric dipole transitions as well. One of the approaches which has been used widely is that used by Armstrong et al . As shown in Fig. 22.1, a heat pipe oven contains Ba vapor at a pressure of ∼1 Torr. Three pulsed tunable dye laser beams pass through the oven. Two are fixed in frequency, to excite the Ba atoms from the ground 6s 2 1 S o state to the 3 P 1 state and then to the 6s7s 3 S 1 state. The third laser is scanned in frequency over the 6s7s 3 S 1 → 6s n p transitions. The Ba atoms excited to the 6s n p states are ionized either by collisional ionization or by the absorption of another photon. The ions produced migrate towards a negatively biased electrode inside the heat pipe. The electrode has a space charge cloud of electrons near it which limits the emission current.
Introduction Series of atomic spectral lines had been studied by emission from excited vapors even before Balmer described the regularity of the visible lines of hydrogen in 1884. However, direct excitation had been restricted to the few lowest-lying levels, particularly those accessible by electric dipole transitions from the ground state using classical light sources. The advent of tunable lasers in the 1970s changed such spectroscopic limitations enormonsly, allowing excitation to a very much larger number of states. Atoms in highly excited states, typically with principal quantum numbers n ≥ 10, are called Rydberg atoms. They are oversize, fragile states of significant intrinsic interest. They are very rare in nature because of their size and fragility, but they are found in interstellar space (H I regions). One of the best sources for comprehensive descriptions of Rydberg atoms and references to the literature describing their exploration is Ref. [120]. It describes their characteristic properties and behavior both classically and quantum mechanically, and especially their strongly enhanced interactions with external fields. The first thing to note is that Rydberg atoms are usually characterized by a single, highly excited electron while all the other electrons remain in their original states. Thus their properties are very nearly classical, and the atoms are much closer to the ideal hydrogen-like model of Chap. 7, along with appropriate quantum defects (see Chap. 10). The spectroscopy of such states is especially interesting because the active electron is much less perturbed by the other electrons in the atom than when it is in lower states. Moreover, there are some special circumstances in which the energy of a Rydberg state is degenerate with an atomic state having two electrons excited to lower levels, and such configurations yield further new information. Second, Rydberg atoms do not usually decay back to the ground state by emission of characteristic radiation, so their study is mediated by other techniques that reveal new and interesting properties. Such properties arise because Rydberg atoms have a huge dipole moment and very low binding energy resulting from the large average distance between the excited electron and the nucleus-plus-core of the rest of the atom with net charge e . Third, transitions between Rydberg states are quite different from those connecting to the ground state for at least three reasons.
Three-photon ionization spectra of Ba were studied which result from two-photon resonances between the 6s21S0 ground state and the 6snd J=2,6s(n+1)s 1S0 (16\ensuremath{\le}n40) Rydberg states and 5d7d 1D2,3P0,2 perturber states. A tunable laser with field strength ${10}^{\mathrm{\ensuremath{-}}(4\mathrm{\ensuremath{-}}5)}$ a.u. was used to drive the excitation in a low-density atomic beam (n\ensuremath{\lesssim}3\ifmmode\times\else\texttimes\fi{}108 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). Resonant line shapes were systematically studied as a function of light polarization and intensity. Studies with linearly polarized light find all resonant line shapes manifest the same peak shift and asymmetry which is traced to the ac Stark shift of the ground state. The resonant peak height, shift, and linewidth scale linearly with light intensity which is accounted for with a rate-equation formulation.For the case of circular polarization, a distinct minimum of ionization occurs between each J=2 doublet of the 6snd series. One level (generally 3D2) is significantly broadened in comparison to the laser linewidth (0.1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) with increasing intensity while its sister level remains narrow (\ensuremath{\sim}0.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). Further, light shifts of the 3D2 states are generally \ensuremath{\gtrsim}1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to the red while shifts of 1D2 states are negligible. This behavior switches for the n=27 doublet. Lastly, the dynamic shifts are not quadratic with the field strength. These effects are attributed in part to cancellation due to channel interference and a two-photon Raman process which mixes each J=2 doublet. These processes act in conjunction with the ac Stark effect to produce the observed profile behavior. The shifts and coupling of the excited states are attributed to the influence of the 6pnd ${J}^{\ensuremath{\pi}}$${=3}^{\mathrm{\ensuremath{-}}}$ autoionization series and are shown to be sensitive to the singlet-triplet mixing coefficient \ensuremath{\beta} of the J=2 doublets. A perturbation analysis is introduced which accounts for the salient features of the data, although it does not include bandwidth and saturation effects. Calculations of light shifts of the 6snd J=2 levels based on this analysis require \ensuremath{\beta} be modified from previous assignments based on hfs measurements for n in the vicinity of the 5d7d 1D2 perturber.
Blackbody radiation is expected to produce a temperature-dependent Stark shift in atomic Rydberg states. Unfortunately, in "one-electron" atoms, all the Rydberg levels experience the same shift, and the effect is only observable on an optical transition from the ground state. However, in a many-electron atom the presence of valence-state perturbers leads to unequal shifts of the Rydberg levels. This should make it possible to observe the frequency shift of a transition between two neighboring Rydberg states at a frequency in the radio-frequency range. This eliminates the need for a very stable visible laser, which so far has been the major technical obstacle to the observation and utilization of the effect. We present as an example a specific case in Ba.
An apparatus has been developed for studying the interaction of Rydberg atoms with surfaces, particularly the perturbation of Rydberg states by a nearby surface and the distance from the surface at which resonance ionization occurs. For this purpose, a source of metastable Xe(3P 0,2) atoms was constructed and characterized. Using single photon laser excitation from the 6s'[1/2]0 state, the nf[3/2]1 Rydberg series for n = 16--20 has been observed with signals well above the background. Measurements of the energy levels of these Rydberg states has identified systematic errors in quoted energy levels from earlier work. The isotopic structure of the 20f Rydberg state was resolved, and the effect of Rydberg excitation in an electric field was investigated. Studies are underway to better understand the behavior of Xe(nf) Rydberg atoms in an electric field. This information will be used in subsequent experiments to study Rydberg-surface interactions.
Les énergies typiques de l’interaction dipolaire entre atomes de Rydberg sont plusieurs ordres de grandeur au-dessus des énergies d’interaction d’atomes et molécules dans le niveau fondamental. Une échelle de distance de plusieurs micromètres découle de cette interaction, ce qui est à l’origine du phénomène de blocage dipolaire, ou la suppression d’excitations d’atomes de Rydberg dans des nuages atomiques denses. Dans une première partie de cette thèse, nous étudions l’application de ce phénomène à l’excitation déterministe d’un atome unique à partir d’un condensat de Bose-Einstein piégé magnétiquement devant une puce à atomes. Une deuxième partie est consacrée à l’étude de l’interaction dipolaire d’ensembles denses d’atomes de Rydberg par spectroscopie microonde des transitions vers les niveaux de Rydberg proches en énergie. Ces ensembles sont créés par excitation laser à partir d’un nuage froid d’atomes de Rb87 dans l’état fondamental. Les spectres des transitions microonde sont élargis et déplacés par l’interaction dipolaire. L’étude de ces spectres permet ainsi d’inférer plusieurs aspects de la distribution spatiale des atomes de Rydberg créés, ce qui révèle différents processus d’excitation selon que la lumière laser est à résonance ou désaccordée. L’évolution mécanique du nuage d’atomes de Rydberg en fonction de leur interaction répulsive a aussi été observée, grâce à une série de spectres microonde à différents délais de l’excitation. Nous montrons ainsi que, pour des échelles temporelles supérieures à 10µs, leur mouvement doit être pris en compte pour la compréhension de la dynamique d’excitation de Rydberg dans des nuages atomiques denses.
The method of space-charge ion detection has been extended to the case of lithium. This was accomplished by the insertion of a space-charge-limited diode into a heat-pipe oven. Lithium vapor was probed with a N2 laser-pumped tunable dye laser from 7150 to 4400 \AA{} and with a Xe arc lamp from 7500 to 2300 \AA{}. One-through four-photon events leading to ionization were observed. The atomic transitions observed were the principal series to n=10 and the 2s to 3d, 4s, and 4d two-photon transitions. The two-photon absorption signals depended upon the third and fourth powers of the light intensity, indicating a new ionization process not heretofore considered important in diode signals. This new process appears to be excitation transfer from excited-state atoms to other excited-state atoms or molecules. For the high excited-state densities achieved in this experiment, this process appears to predominate over photoionization of excited-state atoms and other collisional processes involving excited-state and ground-state species. The $B\ensuremath{\leftarrow}X$, $C\ensuremath{\leftarrow}X$, and $D\ensuremath{\leftarrow}X$ molecular transitions, investigated by previous workers, were observed here.