Line strength factors for E,F 1   g (v ′ = 0, J ′ = J′′ )– X 1   g ( v ′′ , J′′ ) (2 + 1) REMPI transitions in molecular hydrogen 1 Andrew E. Pomerantz, Florian Ausfelder, Richard N. Zare, and Winifred M. Huo Abstract: Experimentally and theoretically determined line strengths are presented for E,F 1 Σ + g (v ′ = 0, J ′ = J ′′)– X 1 Σ + g (v ′′, J ′′) (2 + 1) REMPI transitions in H 2 , HD, and D 2 . The experimental technique employs a hot filament source of internally excited hydrogen that allows experimental determination of line strengths for the low rotational states of highly excited vibrational manifolds (v ′′ ≤ 4). The line strengths are found to depend only weakly on J ′′ for the states measured here, and theoretical results indicate that the line strengths depend strongly on v ′′. These values are combined with previously measured and calculated line strengths for these transitions (K.-D. Rinnen, M.A. Buntine, D.A.V. Kliner, R.N. Zare, and W.M. Huo. J. Chem. Phys. 95, 214 (1991)), resulting in a more complete compilation of REMPI line strengths for molecular hydrogen. Key words: hydrogen, spectroscopy, REMPI, line strength, multiphoton. Résumé : Nous avons déterminé théoriquement et expérimentalement les forces de raies pour les transitions REMPI E,F 1 Σ + g (v ′ = 0, J ′ = J ′′)–X 1 Σ + g (v ′′, J ′′) (2 + 1) de H 2 , HD et D 2 . La technique expérimentale, qui utilise un filament chauffé comme source d’hydrogène rovibrationnellement excité, permet de mesurer les forces de raies pour les états ro- tationnels bas des niveaux vibrationnels excités (v ′′ ≤ 4). Nos mesures montrent que les forces de raies ne dépendent que faiblement de J ′′, et les résultats théoriques indiquent que les forces de raies dépendent fortement de v ′′. Ces va- leurs sont combinées aux forces de raies mesurées et calculées antérieurement pour ces mêmes transitions (K.-D. Rin- nen, M.A. Buntine, D.A.V. Kliner, R.N. Zare, and W.M. Huo. J. Chem. Phys. 95, 214 (1991)), ce qui conduit à une compilation plus complète des forces de raies REMPI pour l’hydrogène moléculaire. Mots clés : hydrogène, spectroscopie, REMPI, force de raie, multiphoton. Pomerantz et al. 729 Introduction The electronic spectroscopy of molecular hydrogen has been studied extensively over the past century, beginning with the measurement of emission spectra by Lyman (1) and Werner (2). The classic investigation by Herzberg and Howe (3) on the H 2 B–X system exemplified the richness of infor- mation that can be obtained from photographic spectroscopy. More recent work has employed laser-based techniques that sacrifice the ability to observe many transitions simulta- neously while allowing quantitative measurements of quan- tum state selective molecular concentrations. In particular, rovibrational state selective populations of hydrogen mole- cules resulting from gas-phase chemical reactions (4–7) or gas-surface interactions (8–10) have been studied using reso- nant enhanced multiphoton ionization (REMPI). The inter- pretation of experimental data requires an understanding of the relationship between the measured signals (ion currents) and populations of rovibrational states. Accurate line strengths for these REMPI transitions are essential for per- forming this transformation. Most previous attempts to measure state specific concen- trations of molecular hydrogen using REMPI have employed Q-branch members of the E,F 1 Σ + g – X 1 Σ + g transitions. Poñgainer et al. (11) have measured line strengths for the (0,0) and (1,1) transitions by analyzing state distributions produced by a Knudsen source. Marinero et al. (4) have measured line strengths for the (0,1) and (0,2) transitions in HD by calibrating against a rotationally relaxed microwave discharge source. Rinnen et al. (12) have employed a hot nozzle source to measure line strengths for these (0,0), (0,1), and (0,2) transitions in H 2 , HD, and D 2 . They were able to measure line strengths for 102 rovibrational levels, typically including Q(0)–Q(15) for each molecule at each vibrational level, although they were unable to determine line strengths for the low J ′′ levels of the v ′′ = 1, 2 manifolds. Huo et al. Can. J. Chem. 82: 723–729 (2004) doi: 10.1139/V04-074 © 2004 NRC Canada 723 Received 19 December 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 19 June 2004. A.E. Pomerantz, F. Ausfelder, and R.N. Zare. 2 Department of Chemistry, Stanford University, Stanford, CA, 94305–5080 USA. W.M. Huo. NASA Ames Research Center, Mail Stop T27B-1, Moffett Field, CA, 94035–1000 USA. 1 This article is part of a Special Issue dedicated to the memory of Professor Gerhard Herzberg. 2 Corresponding author (e-mail: zare@stanford.edu).