Measurements of photoionization cross section of the 4p levels and oscillator strength of the 4p-nd 2 D 3/2,5/2 transitions of potassium M.A. Kalyar a,1 , A. Yar a , J. Iqbal a,2 , R. Ali a , M.A. Baig b,n a Atomic and Molecular Physics Laboratory, Quaid-i-Azam University, Islamabad 45320, Pakistan b National Centre for Physics, Quaid-i-Azam University Campus, 45320 Islamabad, Pakistan article info Article history: Received 13 July 2015 Received in revised form 29 August 2015 Accepted 1 September 2015 PACS Numbers: 32.80 Fb 32.80 Rm 32.70.Cs 32.70.Fw abstract We have carried out measurements of absolute photoionization cross sections of the 4p excited levels and oscillator strengths of the 4p-nd Rydberg transitions in potassium using a two-step photo-ex- citation and ionization technique in conjunction with a thermionic diode ion detector. The measure- ments were conducted using the linearly polarized laser light and the absolute values of the cross sec- tions from the 4p 2 P 3/2 and 2 P 1/2 excited levels have been determined at the ionization threshold as (6.3 70.9) Mb and (5.4 70.8) Mb respectively. In addition, photoionization cross sections have been determined at various ionizing wavelengths above the first ionization threshold to explore different energy regions of the continuum. The oscillator strengths for the 4p 2 P 1/2 -nd 2 D 3/2 and 4p 2 P 3/2 -nd 2 D 3/2,5/2 Rydberg transitions have been deduced by using the measured cross sections of the 4p 2 P 1/2 and 2 P 3/2 levels at the ionization threshold. The new results are in good agreement with the available the- oretical and experimental data. & 2015 Published by Elsevier Ltd. 1. Introduction Photoionization is a physical process in which a photon of en- ergy more than the binding energy of an optical electron of an atom interacts and yields a positive ion and a free electron whereas the probability of photoionization is related to photo- ionization cross section. Natural occurrence of photoionization is always in progression in many physical systems like upper atmo- sphere and astrophysical systems. Photoionization of the atomic excited states plays an important role in radiative transfer which occurs both in the laboratory plasma and in astrophysical systems. Oscillator strength or f-value of a transition is a dimensionless quantity which is proportional to the square of the dipole matrix element connecting the two participating states. Oscillator strengths have numerous applications in many fields of physics, such as astrophysics, space physics, plasma physics and laser iso- topes separation [1–3]. Accurate knowledge of oscillator strengths and transition probabilities is of imperative importance to deduce the elemental abundance in the solar photosphere. Potassium makes up about 1.5% of weight of the Earth's crust and is the seventh most abundant element. The oscillator strengths of the potassium spectral lines are needed in different fields of science such as fusion research, laser-produced plasmas and soft X-ray lasers but very little data are available in the literature. Measurements of photoionization cross sections of potassium have attracted much interest both theoretically and experimen- tally. Moskvin [4] pioneered the calculations of the photoioniza- tion cross section for the 4p level using the quantum defect method whereas, Nygaard et al. [5] measured the photoionization cross section of potassium from the 4p excited state using the two- step photoionization technique. Applying the non-relativistic sin- gle-electron model and using a parametric central potential, Ay- mar et al. [6] calculated the photoionization cross sections for the s, p and d Rydberg states of Li, Na and K in the near threshold regions respectively. Burkhardt et al. [7] measured the absolute values of photoionization cross sections from the first excited states of sodium, potassium and barium at 355 nm ionizing laser wavelength. Feng and Zhang [8] reported the photoionization cross section from the 7p excited state at two ionizing laser wa- velengths using the ionization saturation technique. Kupliauskiene [9] calculated the single and shake up photoionization cross sec- tions of sodium and potassium from the ground and first excited states using the relaxed-orbital Multi-Channel Hartree–Fock ap- proach. Kjeldsen et al. [10] reported the absolute photoionization cross section of potassium ion from the 3p threshold to above the 3s threshold using VUV radiation. Petrov et al. [11] calculated the partial and total photoionization cross sections of potassium from Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optlastec Optics & Laser Technology http://dx.doi.org/10.1016/j.optlastec.2015.09.001 0030-3992/& 2015 Published by Elsevier Ltd. n Corresponding author. E-mail addresses: baig@qau.edu.pk, aslam@ncp.edu.pk, baig77@gmail.com (M.A. Baig). 1 Department of Physics, University of Sargodha, Sargodha 40100, Pakistan. 2 Department of Physics, University of Azad Jammu and Kashmir, 13100 Mu- zaffarabad, Pakistan. Optics & Laser Technology 77 (2016) 72–79