The field ionization process of the Eu 4f 7 6s n p Rydberg states, converging to the first ionization limit, 4f 7 6s 9 S 4 , is systematically investigated. The spectra of the Eu 4f 7 6s n p Rydberg states are populated with three-step laser excitation, and detected by electric field ionization (EFI) method. Two different kinds of the EFI pulses are applied after laser excitation to observe the possible impacts on the EFI process. The exact EFI ionization thresholds for the 4f 7 6s n p Rydberg states can be determined by observing the corresponding EFI spectra. In particular, some structures above the EFI threshold are found in the EFI spectra, which may be interpreted as the effect from black body radiation (BBR). Finally, the scaling law of the EFI threshold for the Eu 4f 7 6s n p Rydberg states with the effective quantum number is built.
The field ionization process of the Eu 4f 7 6s n p Rydberg states, converging to the first ionization limit, 4f 7 6s 9 S 4 , is systematically investigated. The spectra of the Eu 4f 7 6s n p Rydberg states are populated with three-step laser excitation, and detected by electric field ionization (EFI) method. Two different kinds of the EFI pulses are applied after laser excitation to observe the possible impacts on the EFI process. The exact EFI ionization thresholds for the 4f 7 6s n p Rydberg states can be determined by observing the corresponding EFI spectra. In particular, some structures above the EFI threshold are found in the EFI spectra, which may be interpreted as the effect from black body radiation (BBR). Finally, the scaling law of the EFI threshold for the Eu 4f 7 6s n p Rydberg states with the effective quantum number is built.
The field ionization process of Eu 4f76 snp Rydberg states converging to the first ionization limit 4f76s9S4 was systematically investigated. Firstly,the spectra of Rydberg states 4f76 snp of Eu atom were measured using the three-step laser excitation and electric field ionization( EFI) method.Secondly,the EFI pulse( pulse width of 0. 2 μs) was applied after laser excitation. Then,we continuously scan field ionization voltage to observe the EFI process,and to realize the potential of field ionization we need to understand what happens to the atoms as the pulse field rises from 0 to 3 kV.The exact location of ionization threshold can be obtained when the peak of ionization field changes.Especially,above the threshold,some structures are found due to black body radiation( BBR).
The 4f 7 6s( 9 S)np 8 PJ (J = 5/2, 7/2, 9/2) Rydberg series converging to the first ionization limit 4f 7 6s 9 S4 of the Eu atom using the three-step laser excitation and electric-field-ionization (EFI) method are studied.First, the Eu atom is excited from the 4f 7 6s 2 8 S o 7/2 ground state to the 4f 7 6s7s 8 S o 7/2 state through the 4f 7 6s6p 10 P 9/2 state by the first two dye lasers.Next, it is populated to many higher-n members of the 4f 7 6s( 9 S)np 8 PJ Rydberg series by the third dye laser whose wavelength is scanned within a certain range.Finally, the atom in these higher-n states is ionized by the external pulsed electric field.With the field strength up to 2 kV/cm, we can detect the atom in 4f 7 6s( 9 S)np 8 PJ states with n 40.With the given laser line width, the level energies of Rydberg states with n as high as 72 can be determined.We not only confirm the previous data on the 4f 7 6s( 9 S) np 8 PJ Rydberg series, but also extend the n-value assignment significantly by detecting more states.
The radiative lifetimes of the Eu 4f 7 6s n p ( 8 P J or 10 P J ) Rydberg states with J = 5/2 and 11/2 are investigated with a combination of multi-step laser excitation and pulsed electric field ionization, from which their dependence on the effective principal quantum number is observed. The lifetimes of 21 states are reported along with an evaluation of their experimental uncertainty. The influence of blackbody radiation, due to the oven temperature, on the lifetime of the higher- n states is detected. The non-hydrogen behavior of the investigated states is also observed.
The quantum statistical weight (G) of an atomic energy level is an important spectroscopic parameter, its effect on the atomic ionization process is, however, usually neglected for simplicity. In this work, the influences of the G parameters of the lithium atomic energy levels are taken into account explicitly for the first time in the study on the process of three-step photo-excitation (PE) + electric field ionization (EFI), which yields a significant effect on overall EFI efficiency of the PE+EFI process. With a set of specially designed PE+EFI schemes, the expected results are obtained. First, with a three-step PE scheme, the Li atom is excited by three pulsed lasers with different wavelengths, which are fired simultaneously, to one of the Rydberg states from its ground state, from which the Li atom is ionized by an electric-field pulse with a time delay in order to avoid the Stark effect. Based on the three physical processes the atom experiences the PE, none field, and the EFI processes, and a set of universal rate equations are established according to the conservation law of particle number with the knowledge of physical mechanism of the three different processes and the physical model set up for them, respectively. The G parameters of the four relevant bound energy states are displayed explicitly in the rate equations for the PE process to offer a clear viewabout their effect on the overall EFI efficiency of the PE+EFI process. Secondly, the overall efficiencies of PE+EFI process are calculated with the Matlab programming for the two specified excitation schemes, 2S1/22P1/23S1/225P1/2, 3/2 and 2S1/22P3/23D5/225F5/2, 7/2. The overall EFI efficiency of PE+EFI process is investigated not only by adjusting the laser parameters but also by comparing the results between the two different excitation schemes. In order to establish the relationship between the overall EFI efficiency and external field quantitatively, the dependence of population rate of the relevant bound states on various factors, such as laser and atomic parameters, is calculated systematically. The role of the G parameters of the relevant atomic energy levels played in the population rates is observed to determine which excitation scheme is better in terms of the population rate of the Rydberg state. Meanwhile, the spontaneous emission of the Rydberg state during the time delay between the pulses of electric field and laser is also evaluated to make a balance between avoiding the Stark effect and minimizing the spontaneous emission. Based on the analysis of the calculations, some new results are achieved. An enhancement of the overall EFI efficiency can be obtained by making a sophisticated design on the excitation scheme of the PE+EFI process to optimize the G parameters. The time delay between the pulses of electric field and laser not only lets the atom experience a field-free time period, but also makes an upper limit for the population rate of Rydberg state due to the redistribution of atom among the four relevant bound states in the period. The upper limit is found to be dependent on neither laser parameters nor the absolute values of the G parameters, while only on the branching ratio of the G parameters among those bound states. The overall EFI efficiency is dominated by the population rate of Rydberg state, as the EFI process may ionize all Rydberg atoms once the strength of the EFI field reaches the EFI threshold of the Rydberg state. Hence, the key factor for raising the overall EFI efficiency is to enhance the population rate of Rydberg state in the PE process, which is a hard challenge due to the upper limit for the population rate of Rydberg state.
The outer electron of a Rydberg atom is so weakly bound to the atomic core that a relatively small electric field may cause the electron to be stripped from the atom. The field strength at which this ionization happens is strongly dependent on the state of the Rydberg atom. Pulsed electric field ionization serves, therefore, not only as a means by which a Rydberg atom may be efficiently detected, but also a means by which the state of the Rydberg atom may be analyzed.\nThe first observation of predominantly diabatic passage to field ionization of a non-hydrogenic Rydberg atom has resulted in an improved understanding of the adiabatic and diabatic ionization processes. The magnitudes of the various field ionization features have been measured and found to agree with theory, and by detailed analysis, it is now possible to label the various field ionization signal features with the appropriate (VBAR)m(,l)(VBAR) quantum numbers. A much better understanding of the laser excitation process, the Rydberg state production, and the response of the excited state to an increasing electric field has enabled measurement of some Rydberg fine-structure intervals, through observation of field ionization quantum beats.
We have studied ionization of alkali-metal Rydberg atoms by blackbody radiation (BBR). The results of theoretical calculations of ionization rates of Li, Na, K, Rb and Cs Rydberg atoms are presented. The calculations have been performed for n S, n P and n D states for principal quantum numbers n =8–65 at ambient temperatures of 77, 300 and 600 K. The calculations take into account the contributions of BBR-induced redistribution of population between Rydberg states prior to photoionization and field ionization by extraction electric field pulses. The obtained results show that these phenomena affect both the magnitude of the measured ionization rates and their n dependence. A Cooper minimum for BBR-induced transitions between bound Rydberg states of Li has been found. The calculated ionization rates are compared with our earlier measurements of BBR-induced ionization rates of Na n S and n D Rydberg states with n =8–20 at 300 K. Good agreement for all states except n S with n >15 is observed. Useful analytical formulae for quick estimates of BBR ionization rates of Rydberg atoms are presented. Application of BBR-induced ionization signal to measurements of collisional ionization rates is demonstrated.