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AIP conference proceedings

Effects of dc electric fields on multiphoton ionization of rubidium atoms at low and high densities

Nathan I. Hammer

Published Jan 1, 2001• 0 citations
Publisher

Abstract

Multiphoton ionization (MPI) of rubidium atoms at both low (atomic beam) and high (heat pipe) densities is studied using a tunable OPO laser. At high Rb densities ionization of the laser excited ns, np, and nd states occurs both through photoionization and collisional ionization. Excitation of the np states is found to be induced by the external electric field at both low and high densities. In addition, np signal is also seen at very low (E→0) fields in the heat pipe, providing evidence for collision mixing as well as field mixing. At low densities, signal for the high np states initially increases with applied field, but soon saturates (i.e. becomes field independent) while the signal for high nd states decreases with increasing field. At low Rb densities strong resonance features are observed in the energy region between the zero field limit (IP) and the field ionization limit. These features, as well as the field ionization threshold, are found to be dependent upon the angle between the laser polarization and the direction of the applied dc field. Evidence for tunneling through the barrier created by the −e2/r−eEr potential is also presented for ns and nd states.

DOI
10.1063/1.1405596

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We report wavelength spectra of multiphoton ionization of rubidium, in which the intermediate state is a Rydberg state (35n45). The photoelectron yield is measured for a free atom, for an atom placed in an external electric field, and for an atom in a magnetic field. Both the magnetic- and the electric-field spectra show a dramatic decrease in yield above n\ensuremath{\approxeq}40. Our analysis centers on the field-induced mixing of states. The threshold occurs when the resonant state, which has a large quantum defect, mixes with the manifold. The analysis is also applied to experiments reported previously.

Physical Review A•Jun 1, 1990•6 citations
Multiphoton ionization (MPI) of rubidium atoms at both low (atomic beam) and high (heat pipe) densities is studied using a tunable OPO laser. At high Rb densities ionization of the laser excited ns, np, and nd states occurs both through photoionization and collisional ionization. Excitation of the np states is found to be induced by the external electric field at both low and high densities. In addition, np signal is also seen at very low (E→0) fields in the heat pipe, providing evidence for collision mixing as well as field mixing. At low densities, signal for the high np states initially increases with applied field, but soon saturates (i.e. becomes field independent) while the signal for high nd states decreases with increasing field. At low Rb densities strong resonance features are observed in the energy region between the zero field limit (IP) and the field ionization limit. These features, as well as the field ionization threshold, are found to be dependent upon the angle between the laser polarization and the direction of the applied dc field. Evidence for tunneling through the barrier created by the −e2/r−eEr potential is also presented for ns and nd states.
Multiphoton ionization (MPI) via two-photon resonance with high-lying Rydberg levels is observed on Na and K atoms in modest (12--120 V/cm) dc electric fields. The field-induced MPI signals are manifested to such an extent that the nominally forbidden MPI via two-photon resonance with the np levels is induced at the same signal levels as or more intensely than the allowed MPI via the ns and nd Rydberg levels. The intensity inversion between the allowed and the forbidden MPI signals is observed on the K atoms in the wavelength region where the two-photon energy is very close to the field-ionization limit.
We describe experimental studies of resonantly enhanced multi‐photon ionization (MPI) of cesium atoms in the presence and absence of an external electric field. In the zero‐field studies, photo‐electron angular distributions for one‐ and two‐photon resonantly enhanced MPI are compared with the theory of Tang and Lambropoulos. Deviations of experiment from theory are attributed to hyperfine coupling effects in the resonant intermediate state. The agreement between theory and experiment is excellent. In the absence of an external electric field, signal due to two‐photon resonant three‐photon ionization of cesium via np states is undetectable. Application of an electric field mixes nearby nd and ns levels, thereby inducing excitation and subsequent ionization. Signal due to two‐photon excitation of ns levels in field‐free experiments is weak due to their small photoionization cross section. An electric field mixes nearby np levels which again allows detectable photo‐ionization signal. For both ns and np states the ‘‘field induced’’ MPI signal increases as the square of the electric field for a given principal quantum number and increases rapidly with n for a given field strength.Finally, we note that the classical two‐photon field‐ionization threshold is lower for the case in which the laser polarization and the electric field are parallel than it is when they are perpendicular.
A frequency stabilized ring dye laser, with a linewidth of (TURN)250 kHz, has been utilized to selectively excite Rb(N('2)S, N('2)D) states in the range N = 20 - 100 by a two photon process. The highly excited atoms were detected by the technique of electric field ionization. Excited states with (VBAR)M(,L)(VBAR) < 3 have been observed to ionize along predominantly adiabatic paths and states with (VBAR)M(,L)(VBAR) (GREATERTHEQ) 3 have been observed to ionize along predominantly diabatic paths for the range of N investigated. The electric field dependence of the ionization threshold has been determined to be in good agreement with that calculated from a simple model based on the energies and ionization rates of hydrogen.\nCollisions between highly excited rubidium atoms and neutral xenon atoms, which result in a change in the N and L of the excited state of the rubidium atom, have been investigated. The N of the final state has been shown to be limited to those states which lie closest in energy to the initial state, in good agreement with theory. The distribution of L of the final states has been shown to be completely statistical or biased towards production of high L. The distribution of final L is definitely not biased toward production of states with low values of L.
This paper summarizes a long series of experiments that have been done on Rydberg states of Rb. The basic experimental technique that has been used is atomic-beam laser spectroscopy which allows both low-resolution and high-resolution studies. Excitation of Rydberg states is provided by single-photon absorption from a suitably frequency-doubled rhodamine 6G6G6G tunable laser. Detection is done by field ionization and ion counting. These techniques have allowed us to study the critical ionizing field, which has been found to follow classical, hydrogenic behavior closely. Rydberg states of Rb have been observed from n=28n=28n=28 up to n>100n>100n>100. High-resolution spectroscopic studies have led to measurement of the fine structure for np\mathrm{np}np levels located between n=28n=28n=28 and n=60n=60n=60, as well as the isotope shift of the ground state, $\ensuremath{\delta}\ensuremath{\nu}=(172\ifmmode\pm\else\textpm\fi{}15)$ MHz. Anomalies in the intensity ratio of the doublet $5s\ensuremath{-}np$ transitions have been observed and interpreted as relativistic effects. Perturbations by weak and strong dc electric field are reported. In the case of weak electric fields it is possible to evaluate the polarizability. In the case of strong dc electric fields, new phenomena have been observed and analyzed. We also find experimental evidence of selective photoionization in the presence of an appropriate dc electric field, which could have some applications in laser isotope separation. Finally, we give the preliminary results of an experiment on the effect of a strong nonresonant electromagnetic field on high-lying Rydberg states.
We report wavelength spectra of multiphoton ionization of rubidium, in which the intermediate state is a Rydberg state (35n45). The photoelectron yield is measured for a free atom, for an atom placed in an external electric field, and for an atom in a magnetic field. Both the magnetic- and the electric-field spectra show a dramatic decrease in yield above n\ensuremath{\approxeq}40. Our analysis centers on the field-induced mixing of states. The threshold occurs when the resonant state, which has a large quantum defect, mixes with the manifold. The analysis is also applied to experiments reported previously.