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arXiv:physics/0505055

Two photon ionization of condensate atoms

Marco Anderlini, E. Arimondo

Published May 8, 2005• 0 citations
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Abstract

The efficient photoionization of a Bose-Einstein condensate requires the creation of ions with the smallest possible transfer of atoms from the condensate into the thermal phase. The spontaneous decay from excited states into atomic states with momentum different from the initial one should be reduced. We investigate theoretically the two-photon ionization of a rubidium condensate using near resonant excitation to the 6P state and second photon at 421 nm or 1002 nm into the continuum. Different ionization schemes with coherent control of the first excitation and reduction of the spontaneous decay are presented.

DOI
10.48550/arxiv.physics/0505055

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The efficient photoionization of a Bose-Einstein condensate requires the creation of ions with the smallest possible transfer of atoms from the condensate into the thermal phase. The spontaneous decay from excited states into atomic states with momentum different from the initial one should be reduced. We investigate theoretically the two-photon ionization of a rubidium condensate using near resonant excitation to the 6P state and second photon at 421 nm or 1002 nm into the continuum. Different ionization schemes with coherent control of the first excitation and reduction of the spontaneous decay are presented.
By means of a simple theoretical model and experimental results, we analyse the two-photon ionization of cold rubidium atoms. We consider a two-step process in which ground state atoms are ionized by two blue (421 nm) photons, chosen to be quasi-resonant with the 5S → 6P transition. We show that for a good description of the process we have to take into account not only the three states coupled by the laser radiation, but also all the atomic states involved in the spontaneous emission cascade from the 6P to the 5S state, since optical pumping from 5S 1/2 ( F = 2) to the 5S 1/2 ( F = 1) states modifies the ionization efficiency when ionizing near resonance. The experimental and theoretical determination of ionization cross sections for excitations near resonance with the 6P 1/2 and the 6P 3/2 intermediate levels is presented.
Two-photon ionization cross sections from the rubidium ground state have been measured with both a cw 421-nm laser and a combination of cw 421- and 1002-nm lasers. The measurements were performed within a high-vacuum magneto-optical trap while the trapping lasers were switched off, exploiting the long trap lifetime and also the trap laser confinement. The two-photon cross sections were determined for the blue laser near resonance with the 6P1/2 and 6P3/2 states and compared with the estimates of a theoretical model. In near resonance with the 6P states, large two-photon photoionization cross sections were measured.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
Photoionization of a cold atomic sample offers intriguing possibilities for observing collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap (MOT) with laser pulses ionizing through one-photon and two-photon absorption processes was performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample were examined as functions of the ionizing laser parameters. Ionization cross sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.