J. Phys. B: At. Mol. Opt. Phys. 32 (1999) 5669–5679. Printed in the UK PII: S0953-4075(99)05110-X Two-step laser spectroscopy of the even-parity Rydberg levels of neutral tin Ali Nadeem†, A Ahad†, S A Bhatti†, Nisar Ahmad†, Raheel Ali‡ and M A Baig‡ † Applied Physics division, PINSTECH, PO Box Nilore, 45320 Islamabad, Pakistan ‡ Atomic and Molecular Physics Laboratory, Department of Physics, Quaid-i-Azam University, 45320 Islamabad, Pakistan Received 10 June 1999, in final form 17 August 1999 Abstract. We report the first measurements of the even-parity J = 1 and 2 levels of neutral tin using a two-step laser excitation technique in conjunction with an atomic beam apparatus. The Rydberg series 5pnp ( 1 2 , 3 2 ) 2 (11  n  45) and 5pnp ( 1 2 , 3 2 ) 1 (12  n  18) have been observed converging to the first ionization limit. In addition, five p-levels; 5p8p ( 3 2 , 1 2 ) 1,2 , 5p8p ( 3 2 , 3 2 ) 0,1,2 and three f-levels 5p5f ( 3 2 , 5 2 ) 1,2 and 5p5f ( 3 2 , 7 2 ) 2 converging to the second ionization threshold have been identified. The perturbations among the J = 2 levels have been studied using three-channel quantum defect theory. 1. Introduction Atoms with two valence electrons have been studied extensively by both conventional spectrographic techniques and by using laser spectroscopic methods. These include all the alkaline earth atoms and the group-IIB elements Zn, Cd, Hg and Yb, all these atoms possess closed inner-shell cores and a valence electron in the s 2 configuration. However, relatively little attention has been paid to the fourth-group elements which also have closed inner-shell cores but with valence electrons in the p 2 configuration. In particular, in studies using laser spectroscopic techniques the only element which received attention was lead (Young and Mirza 1980, Farooqi et al 1995, Buch et al 1988, Hasegawa and Suzuki 1996, Bhatti et al 1997 and references therein). The apparent difficulty with the fourth-group elements is that their resonance lines lie in the ultraviolet region, which is inaccessible with dye-laser photons without frequency doubling. Atoms other than lead have the added difficulty that one needs a high-temperature source to obtain their beams, or sufficient vapour density in a vapour containment system. The only available spectroscopic works on tin are by Wilson (1964) and Brown et al (1977) using the conventional photo-absorption technique. Brown et al (1977) investigated the spectrum by a microwave-excited xenon-lamp source providing continuum from 255–190 nm. Subsequently, Connerade and Martin (1977) reported the inner-shell excitation spectra of tin in the energy region from 90–40 nm using a 1 m spectrograph in conjunction with synchrotron radiation. Baig (1986) reported the level structure pertinent to the 5p 3 configuration and the level identifications were supplemented by the Hartree–Fock calculations. All these studies concentrated on the odd-parity levels because the even-parity levels are not accessible from the ground state with conventional absorption techniques. 0953-4075/99/245669+11$30.00 © 1999 IOP Publishing Ltd 5669