Eur. Phys. J. D 37, 23–28 (2006) DOI: 10.1140/epjd/e2005-00261-y T HE EUROPEAN P HYSICAL JOURNAL D Measurement of the photoionization cross-section of the 3p 2 P 1/2,3/2 excited levels of sodium N. Amin, S. Mahmood, M. Anwar-ul-Haq, M. Riaz, and M.A. Baig a Atomic and Molecular Physics Laboratory, Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan Received 6 January 2005 / Received in final form 18 May 2005 Published online 27 September 2005 –c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2005 Abstract. The photoionization cross-section and number density of the 3p 2 P 1/2,3/2 excited levels of sodium have been measured as a function of the laser energy using two-step laser excitation in conjunction with a thermionic diode working in the space charge limited mode. Employing the saturation technique, the cross- sections for the 3p 2 P 1/2 and 3p 2 P 3/2 levels are determined as 2.16 (43) Mb and 3.74 (74) Mb respectively. PACS. 32.30.Jc Visible and ultraviolet spectra – 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) 1 Introduction The photoionization cross-sections of atoms from the ground state are well-known for many atoms [1,2] but comparatively little information is available about the cross-sections from the excited states. Laser spectroscopic techniques have augmented studies of multi-step and multi-photon photoionization processes. The higher in- tensity, narrow bandwidth and tunable radiation of dye lasers play a vital role for the selective excitation of any atomic level, and subsequent photoionization from a given excited state to measure the cross-section of the perti- nent state. Rothe et al. [3,4] pioneered the photoion- ization cross-section for the excited states of Na and Li based on studies of the radiative electron-ion recombina- tion and shock-heated plasma. Ambartzumian et al. [5] pioneered the work on the measurement of the photoion- ization cross-section using the saturation technique and measured the photoionization cross-section of the 6p 2 P 1/2 and 2 P 3/2 levels of rubidium. This technique was extended by Heinzmann et al. [6] to measure the photoionization cross-section of the 7p 2 P 1/2 and 2 P 3/2 levels of cesium. Duong et al. [7] discussed the importance of atomic align- ment and the light polarization and measured the relative cross-section of the sodium 3p level. Subsequently, Smith et al. [8] measured the absolute cross-section for the pho- toionization of the 4d and 5s excited states of sodium us- ing 1.064 μm laser radiation. Effects of polarization in the measurement of the cross-section were also investi- gated. Garver et al. [9] reported the atomic densities of the 3p 2 P 1/2,3/2 levels of sodium by using the radiation- trapping technique. They also measured the apparent life times of the 3p 2 P 1/2 and 2 P 3/2 levels as a function of a e-mail: baig@qau.edu.pk atomic density. Preses et al. [10] reported the photoion- ization of the 3p excited state of sodium using a laser to populate the initial state and broad band synchrotron radiation for the ionizing process. Saha et al. [11] cal- culated the photoionization cross-section for the 4d ex- cited state of sodium employing the multi-configuration Hartree-Fock method. Their results are in good agreement with the experimental results of Smith et al. [8], which were performed at single photon ionization. Burkhardt et al. [12] simultaneously determined the magnitude of the photoionization cross-section and the atomic density for the 3p 2 P 3/2 level of sodium. They used one laser beam to excite the atoms to the resonance level and a second laser beam for the ionization. The ionizing laser beam was focused to a diameter much smaller than that of the exciting laser by a lens. Beterov and Ryabtsev [13] re- ported the single and two-photon ionization of the 4s- level of sodium. The experimental two-photon ionization cross-section of the 4s level at 1064 nm was obtained as σ exp =5.6 ± 2.9 × 10 -47 cm 4 s. The theoretical cross- section for the 4s state was also calculated within the framework of the three level model. Beterov et al. [14] reported the photoionization of the excited sodium atoms of the 4d state in the field of a train of ultra short pulses of a Nd:YAG laser. Kupliauskiene [15] calculated the sin- gle and shake up photoionization cross-section of sodium atom in the ground and first excited states using the relaxed-orbital MCH approach. Petrov et al. [16] reported the partial and total photoionization of the excited al- kali atoms (Na, K, Rb and Cs) using configuration inter- action technique with Pauli-Fock orbitals (CIPF). They also discussed the effect of the polarization of the atomic core with the valence electron. The relative photoion- ization cross-section of potassium (4p 2 P 3/2 ) over the photoelectron energy range 0–0.22 eV for parallel and