IOP PUBLISHING JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS J. Phys. B: At. Mol. Opt. Phys. 41 (2008) 125703 (9pp) doi:10.1088/0953-4075/41/12/125703 Lifetimes of metastable levels of singly ionized titanium: theory and experiment P Palmeri 1 , P Quinet 1,2 , ´ E Bi´ emont 1,2 , J Gurell 3 , P Lundin 3 , L-O Norlin 4 , P Royen 3 , K Blagoev 5 and S Mannervik 3 1 Astrophysique et Spectroscopie, Universit´ e de Mons-Hainaut, B-7000 Mons, Belgium 2 IPNAS, Universit´ e de Li` ege, Sart Tilman B15, B-4000 Li` ege, Belgium 3 Department of Physics, Stockholm University, AlbaNova University Center, SE-10691 Stockholm, Sweden 4 Department of Physics, Royal Institute of Technology, AlbaNova University Center, SE-10691 Stockholm, Sweden 5 Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, BG-1784 Sofia, Bulgaria Received 22 February 2008, in final form 13 May 2008 Published 6 June 2008 Online at stacks.iop.org/JPhysB/41/125703 Abstract This paper presents new theoretical lifetimes of metastable levels in singly ionized titanium, Ti II. Along with the lifetimes, transition probabilities for several decay channels from these metastable levels are presented. The calculations are supported by experimental lifetime determinations of the 3d 3 b 2 D 5/2 and 3d 2 ( 3 P)4s b 2 P 3/2 levels along with revised values of the previously published lifetimes of the 3d 2 ( 3 P)4s b 4 P 5/2 and 3d 2 ( 3 P)4s b 2 P 1/2 levels originating partly from a reanalysis utilizing a recently developed method applied on the previously recorded data and partly from new measurements. The presented theoretical investigation of lifetimes of metastable levels in Ti II shows that the HFR calculations are in general compatible with measurements performed using the ion storage ring CRYRING of Stockholm University. The transition probabilities of forbidden lines derived from the new lifetime values will be useful for the diagnostics of low density laboratory or astrophysical plasmas, particularly those encountered in the strontium filament found in the ejecta of η Carinae. 1. Introduction Metastable states of atoms and ions, generally sensitive to collisional effects, have lifetimes of the order of seconds or more while most excited states are characterized by much shorter lifetimes, usually of the order of ns. These metastable levels can generally decay to the ground state or to lower energy levels via magnetic dipole (M1), electric quadrupole (E2) or higher order magnetic or electric multipole transitions. Due to their sensitivity to collisional effects, the forbidden transitions are generally interesting candidates for density diagnostics in dilute astrophysical plasmas like those encountered in stellar coronae or in planetary nebulae. Publications containing transition probabilities for forbidden lines of singly ionized titanium are very sparse. The National Institute of Standards and Technology (NIST) compilations (Kaufman and Sugar 1986, Wiese and Musgrove 1989, NIST 2007a) for example contain A-values for forbidden lines of some titanium ions but not of Ti II. Such data are, however, relevant and needed for the analysis of laboratory or astrophysical plasmas. A first attempt to measure a very long lifetime (28 ± 10 s) in Ti II is due to Hartman et al (2003a). They measured the lifetime of the 3d 2 ( 3 P)4s b 4 P 5/2 level using the laser probing technique (LPT) at the CRYRING ion storage ring of Stockholm University. This measurement, however, resulted in a lifetime more than a factor of 2 longer than the theoretical estimate (12 s) obtained by the same authors using Cowan’s (1981) code package. The large uncertainty associated with this value was due to the lack of an accurate method for correcting for the systematic effect of repopulation, a method which has now been developed (Mannervik et al 1997, 2005, Royen et al 2007). This new method made a reanalysis possible which changed the previously published value by 1.2 standard deviations. The work on lifetimes of metastable levels in Ti II was later extended by Hartman et al (2005) who used a combination of laboratory and astrophysical measurements to derive transition 0953-4075/08/125703+09$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK