1. The Kr-Rb+-Sr2+-Y3+ series: Experiment and Theory
Trends in autoionization of Rydberg states converging to the 4s threshold in
The Kr-Rb+-Sr2+-Y3+ series: Experiment and Theory
A. Neogi1, E. Kennedy1, J-P. Mosnier1, P. v Kampen1, G. O’Sullivan2, M. Mansfield3,
Ph. V. Demekhin4, B.M. Lagutin4, I. D. Petrov4, V.L. Sukhorukov4 and J. T. Costello1
1. NCSPT & School of Physical Sciences, Dublin City University. 2. Department of Physics, University College Dublin
3. Department of Physics, University College Cork. 4. Rostov State University of TC, Rostov-on-Don, RUSSIA
Introduction
Closed shell atoms are prototypical systems for the detailed investigation of the atomic photoelectric effect [1]. Using the Dual Laser Plasma technique we undertook some time ago a study of 3s-subshell photoabsorption in the Ar-like ions K+ and Ca2+ [2]. On moving from Ar to K+ and on to Ca2+ dramatic changes in the Fano q-parameter including q-reversal for each of the 3s – np resonances was observed demonstrating a strong interplay between the direct and resonant photoamplitudes with increasing ionization. Theoretical studies have emphasised the important role played by doubly excited states in these spectra [2, 3] and also how the presence of a Cooper minimum can act an additional q altering mechanism. We show here the results of a corresponding experimental and theoretical study of the Kr-like ions Rb+, Sr2+ and Y3+.
REFERENCES
1. V Schmidt, Rep.Prog.Phys (1992)
2. P VanKampen et al.Phys.Rev.Lett 78, 3082 (1997)
3. B M Lagutin et al., J. Phys. B: At. Mol. Opt. Phys (1999)
Main Conclusions
1. Strong configuration interaction between 4s-1(nl) singly and 4p-2(n’l’n”l”) doubly excited states along the isoelectronic sequence is manifest by the rich doubly excited structure observed in Rb+ and Sr2+.
2. Computation performed in the Kr-Rb+-Sr2+-Y3+ series within the configuration interaction Pauli-Fock approach shows that the main reason of the behavior of the Fano ‘q-parameter’ is a strong interplay between direct and correlational amplitudes describing photoexcitation of Rydberg states which converge to the 4s- threshold.
3. This work should prompt further experiments at crossed ion beam-synchrotron experiments (e.g., at ALS, LURE and Aarhus) where absolute cross sections may be measured.
Main Results
1. We have been able to extract relative cross sections from vacuum-UV photoabsorption spectra recorded on a 2.2m grazing incidence spectrometer equipped with intensified photodiode array (MCP/PDA) readout.
2. It is clear that the ‘Fano’ profile parameters for the main 4s – np resonances in each spectrum are very sensitive to degree of ionization.
3. It is also clear that complex multiple-component doubly excited resonances persist at least in the early members of the isoelectronic sequence.
4. Computed cross sections show good agreement with measured spectra.
Slight differences in the slope of the underlying continua between theory and experiment are due to residual scattered light which is a problem at grazing incidence.
Experimental Parameters
Experimental Line Profile Parameters
Rubidium(1+)
Grazing Incidence Spectrometer
Fano1
Res-1
Res-2
Res-3
Res-4
Res-5 E0
35.71
39.44
40.29
40.96
42.35 G
0.15
0.04*
0.03* q
0.22
-ive
+ive r2
0.40
0.23 -
Plasma Generation
Strontium(2+)
F (W/cm2)
Dx (mm)
Dt (ns)
Rb+
5 x 109
200
Sr2+ 50
Y3+
2 x 1010
Fano1
Res-1
Res-2
Res-3
Res-4
Res-5 E0
46.89
53.68
56.53
57.73
58.38 G
0.12
0.10
0.08*
0.06*
0.07* q
0.25
-1.1
-ive r2
0.22
0.07 -
Dual Laser Plasma Experiment
F-On target irradiance
Dx-(Absorbing) Plasma displacement from optic axis of spectrometer
Dt-Inter-Plasma time delay
1 Fano U and Copper J W, Phys. Rev 137, A1364 (1965)/* Upper limits (instrumentally limited)
Computed cross section (folded with instrument function) compared to 65
experimentally derived absorbance 34 36 38 40 42 44 9 45 50 55 60 65
0.8 60 65 70 75 80 85 20 14
0.09
0.5 8 3+ Rb + 7 Sr 2+
0.7 12 Y
0.08 15
0.4
Absorbance
0.6 10 6
0.3 5
0.5 8
0.07 10 4
0.4 6
0.06
0.2 5 3
0.3 4
0.1 2 2
0.05
0.2 1 34 36
E(eV) 38 40 42 44 45 50 55 60 65
E(eV) 60 65 70 75 80 85
0.04
E(eV)
Theory Outline
Photoionization cross sections were computed as described in e.g., Lagutin et al. J. Phys. B: At.Mol.Opt.Phys (1999)
The partial photoionization cross sections are given by:
where D and Hee are the electric dipole and Coulomb operators, respectively. |EJ> are the (Br-like) target ionic states and |EJ> are the doubly excited states.
The total photoionization cross sections of each ionic level EJ is then:
This equation was applied for the calculation of the 4s cross section (with |EJ> = 4s1Jp) and also for the 4p cross section section (with |EJ> = 4p5Jp).
The width of each resonance is given by:
where ∑ is taken over all the final continuum states including the Auger channels.
The ‘Fano’profile parameter is given by: