Photoelectron angular distributions for two-photon ionization of atomic rubidium

Abstract

In this paper we report measurements of photoelectron angular distributions for two-photon ionization of atomic rubidium. These measurements are carried out over a range of laser wavelengths from 591 to 532 nm. Relative cross sections for the various photoionization channels and continuum-wavefunction phase differences are derived from the data. These phase differences are not in good agreement with calculations of phase differences based on quantum-defect data and Coulomb phase shifts. PACS number{s): 32.80.Fb Photoelectron angular distributions have become a powerful too1 in the study of atomic structure and atomfield interactions.The photoionization processes for these measurements are often of very high order, due to large atomic ionization potentials and long-wavelength laser sources. In this report, we will discuss photoelectron angular distributions for two-photon, nonresonant photoionization of atomic rubidium. Our primary interest in these measurements stems from our related studies of interference between different low-order ionization processes, and the spatial asymmetry induced in the photoelectron Aux by this interference [1]. These measurements are also of interest, however, because they are of only second order in the optical field, and are performed on an alkali atom, making possible detailed calculation of these energy dependent angular distributions. In the following sections of this report, we will brieAy discuss our measurement technique, compare our results with previously reported related results, and present a simple analysis of relative cross sections and phase differences for the various photoionization channels for this atom.The experimental system for measuring photoelectron angular distributions, shown in Fig. 1, is similar to that described previously E2, 3]. The number of electrons ejected in the direction 8, where 8 is defined with respect to the electric-field polarization of the laser beam, are counted using an electron detector and a pulse count-ing system. The electron detector is fixed in space, and the electron count is recorded as the laser polarization is rotated. The dye laser system in this experiment consists of a tunable oscillator with three stages of amplification, all pumped by the frequency doubled output of a neodymium-doped yttrium aluminum garnet (Nd: YAG) laser. Using difFerent dye solutions (R590, R610, and DCM) we obtained TEMOO mode output at seven different wavelengths in the range from 550.5 to 591 nm. We also measured the angular distribution at 532 nm using the second harmonic of the Nd:YAG laser directly. The kinetic energy of the photoelectrons produced in the two-photon process ranges from 19.6 meV (for X=591.0 nm) to 485 meV (for A, =532 nm). The average laser power was 10 mJ, weakly focused to a beam diameter of 0.7 mm. We used a half-wave Fresnel rhomb to rotate the polarization of the laser beam. Through careful alignment of the rhomb and its axis of rotation, displacement of the laser beam in the interaction region was less than 0.2 mm. The polariz...

An Approach To “Quantumness” In Coherent Control

“…Note that these states are defined in terms of the one-and twophoton scattering probability amplitudes, T 1 and T 2 . For monochromatic cw light, these amplitudes are well known [32,33]. However, here we deal with quasi-monochromatic light pulses.…”

“…This control scenario is an extension of Elliott's twocolor photoionization experiments on alkali atoms [14,15,32,33], which demonstrated that the angular distribution of an emitted photoelectron can be controlled by the relative phase between two ionizing laser beams. The sensitivity to phase was attributed to quantum interference between competing ionization processes.…”

“…In particular, they rely on the functional form of this equation, which is reminiscent of double slit interference, the phase sensitivity of the control, and the display of nonadditivity of the contributions. Early two-color photoionization experiments with alkali atoms, [14,15,32,33], serve as seminal examples of this scenario, as do experiments on current directionality in quantum wells [5] and semiconductors [20]. However, phase sensitivity and nonadditivity can also be of classical origin arising, for example, in transport processes [34].…”

Certifying the quantumness of a generalized coherent control scenario

“…., multiplied by corresponding powers of the field amplitudes α l 35-41 . In the present case, only the first two transition amplitudes are relevant 26,27 ; t 1 resolves the 1-photon process and connects only to P continuum states, whereas t 2 accounts for 2-photon ionization and accesses S and D orbitals. The amplitudes contain radial integrals, D 1 and D 2 , that serve here as complex empirical parameters.…”

“…Let a heavy alkali atom be ionized by weak coherent (ω+2ω) radiation [26][27][28] . For purposes of simplification (rather than physical necessity), we assume a tight confinement that fully suppresses decoherence due to recoil 29 .…”