Photoionization of polarized (4 p,J= 3) atoms near threshold

Abstract

Using a crossed atomic and laser beams set-up with mass spectrometric ion detection we have studied the photoionization of laser-excited, aligned Ar * (3p 5 4p 3 D 3 ) atoms from the Ar + ( 2 P 3/2 ) threshold up to photoelectron energies of ε = 0.846 eV. Absolute cross sections are reported for parallel (η = 0 • ) and perpendicular (η = 90 • ) polarization directions of the linearly polarized exciting and ionizing CW lasers over the range ε = 0-0.2 eV. The cross sections exhibit nd (J = 2, 3) autoionization resonances whose combined appearance varies with angle η; this behaviour is traced to the η dependence of the partial cross sections for the final (ion + electron) channels with different total angular momentum, J 1 . Using theoretical formulae whose derivation is outlined we determine the energy dependence of these partial cross sections from least squares fits to the data measured at η = 0 • and 90 • . Experimental results are also presented for the asymmetry function, A LD (η), which describes the linear dichroism for the photoionization of aligned Ar * (4p 3 D 3 ) atoms, and for the full η dependence of the ionization cross section as obtained for 10 selected photoelectron energies in the range (0.017-0.846) eV. Many-electron calculations have been carried out which provide additional insight into the photoionization dynamics. In contrast to the situation for Ne * (3p, J = 3), photoionization of Ar * (4p, J = 3) is not well described by just two reduced matrix elements D s , D d . For Ar * the term dependence of the Hartree-Fock wavefunctions and correlation effects lead to substantial differences between the reduced matrix elements for the five d-wave channels associated with the Ar + ( 2 P 3/2 ) formation. Satisfactory overall agreement is observed between the experimental and theoretical cross sections and their polarization dependence. The calculations also provide information on the weak g-wave contributions and on the cross section for leaving the ion in the excited Ar + ( 2 P 1/2 ) level; the latter amounts to about 1% relative to Ar + ( 2 P 3/2 ) formation in agreement with experimental and MQDT estimates.

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

“…39 K(4s 1/2 , F ¼ 1, 2) atoms to the intermediate levels 40 Ar * (4p 3 D 3 ) or 39 K * (4p 3/2 , F 0 ¼ 2, 3) (quasi-stationary excited state population nearly 50%), is optimized in the sense that the cross-section for ionization of Ar * (4p 3 D 3 ) (Schohl et al, 1997) as well as for K * (4p 3/2 ) atoms (Petrov et al, 2000) is substantial (around 10 À21 m 2 ) and three to four orders of magnitude higher than that for ionization of metastable Ar * (4s 3 P 2 ) atoms (Kau et al, 1998;Petrov et al, 1999) or ground state K(4s) atoms (Sandner et al, 1981), respectively. We note that an analogous two-step photoionization scheme involving ground state Na(3s 1/2 ) atoms has been recently applied by Keil et al (1999) to study laser photoelectron attachment to vibrationally excited Na 2 molecules (see Section IV.B).…”

Near threshold photoionization of excited alkali atoms Ak(np) (Ak = Na, K, Rb, Cs; n = 3-6)

“…The polarization dependent ion data and the electron angular distributions were analyzed on the basis of formulae, given explicitly in [27,28] for photoionization of atoms with total angular momentum J = 3, resulting from the coupling of two subsystems i (i = 1, 2) which both possess angular momenta j i = 3/2. In the present case i = 1 corresponds to the excited 4p (j 1 = 3/2) electron and i = 2 to the nuclear spin of 39 K (j 2 = 3/2).…”

“…We assume the nuclear spin to be frozen in the photoionization process (see the corresponding discussion in the literature [29]); moreover, we neglect -as corroborated by the theoretical results -the effects of spin-orbit coupling which could result in a difference between the phase shifts for the d 3/2 and the d 5/2 waves. Thus we obtain simplified formulae [25,27,28] which only contain the relevant two reduced matrix elements D s and D d , the phase shift difference δ d − δ s and -as an experimental parameter -the effective quadrupole alignment of the 4p 3/2 electron. The alignment was determined from the angular distribution of the resonance fluorescence using formulae given in [29].…”

“…The alignment was determined from the angular distribution of the resonance fluorescence using formulae given in [29]. For the analysis of the ion data we used equations (18,19) from [27]. The electron angular distributions were analysed on the basis of formulae (6a, 6b) in [28] in conjunction with equations (A10a-A10e) in [28].…”

Photoionization of Synchrotron-Radiation-Excited Atoms: Separating Partial Cross Sections by Full Polarization Control

“…The importance of this initial polarization, which is determined by the excitation conditions, has been demonstrated and exploited in the studies of laserexcited states [3][4][5][6]. For example, by varying the polarization of the exciting laser and ionizing photon beams and by taking advantage of the dipole transition selection rules, it is possible to separate photoionization into different atomic continua when detecting simply the total ion yield [7][8][9][10]. Such data can be used, for example, to identify resonances in the photoionization cross section with respect to their angular momentum [10,11] and provide an important step towards the ''perfect'' experiment on photoionization [12] aiming at the complete quantum mechanical knowledge about the process.…”