Spectroscopic studies of jet-cooled AgAu and Au, Gregory A. Bishea and Michael D. Morse Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received 25 March 1991; accepted12July 1991) Resonant two-photon ionization spectroscopy hasbeen applied to the jet-cooled Au, and AgAu molecules.Three new band systems of Au, and two new systems of the poorly characterized AgAu moleculehavebeenobserved. Excited state lifetime measurements have been made, and assignments of the excited states are suggested. For Au, the a ‘Z;t +X ‘2: transition has beendetected, and vibrational levelsof the a “2;: statehavebeenobserved up to v’ = 33, which lies only 120cm - ’ below the convergence limit of the system.This allows a precise confirmation of previous high temperatureKnudsen effusionmeasurements of the bond strength of Au, asD z (Au, ) = 2.290f 0.008eV. In addition, the excited states of Au, of 0: symmetry are shown to have significantly shorter fluorescence lifetimes than the 1, states, and this is explainedas resulting from an admixture of Au + Au- ion pair character in these a = 0: states. The ionization potential of Au, has been bracketedas IP( Au, ) = 9.20 f 0.21 eV, which may be combined with the valuesof D g(Au, ) and IP( Au) to provide the dissociationenergyof the Au: ion as D g (Au,+ ) = 2.32 f 0.21 eV. Detailed comparisons with theoretical resultsare madefor Au,, and assignments of the A and B states of AgAu to a = 0 + and a’ = 1, respectively, are proposed. I. INTRODUCTION In the precedingtwo paperswe have presented results on the resonant two-photon ionization spectra of CuAg’ and CuAu.’ This work wasundertakento provide a detailed understanding of the filled d subshell transition metal dimers, so that a basis for comparison would be available when considering the question of d-orbital contributions to the chemical bonding in the open d-subshell molecules.In the present work we extendthese studies to the heaviercoin- age metal diatomic molecules, AgAu and Au,. In some ways an accurate knowledge of theseheavierspecies will be more important than such a knowledge of their lighter counter- parts, sincethe possibility of significant d-orbital contribu- tions to the chemicalbonding is greater amongthe 4d and 5d transition metals3p4 This results because there is less of a discrepancyin size betweenthe 4d and 5s (or 5d and 6s) orbitals than thereis between the 3d and 4sorbitals, making the d orbitals more accessible for chemical bonding in the second and third transition metal series.3*4 Previous experimentalwork on the AgAu and Au, mol- ecules has been quite limited. In two published works’*6 Ruampsmentions band spectra of AgAu, but no analyses or listings of band positions are provided. As far as we can de- termine, the only other experimentalwork on this molecule consists of the mass spectrometricdeterminationof the bond strengthby Ackerman, Stafford, and Drowart in 1960.7 Pre- vious work on Au, hasbeensomewhatmore thorough. For example,the bond strength of this molecule has beenmea- sured by high temperature mass spectrometry many times,7- l3 making it among the best known of the bond strengths measured by this technique. In addition, theA-X6v16’7 and B-X6+‘4*‘6 band systemsof Au, are well known, and both vibrational and rotational’6*‘7 analyses havebeen provided. Absorption spectra of gold clusters isolated in a low tem- perature argon matrix have also beenreported,‘* with spe- cific features at 365, 317,208, and 198nm assigned to Au,. Theoretical investigations of AgAu have been almost totally lacking; to our knowledge only three studies exist.‘” 21 These provide information about the ground states of Ag-‘h 19-2’ AgAu + ,21 and AgAu- .20,21 Diatomic gold has been subjected to a much more completetheoretical investi- gation, sinceboth the ground state’9-23 and severalexcited electronic states24’25 havebeen studied. Following presenta- tion of our experimentalresults,a comparisonto these theo- retical findings will be made. Experimentaldetailsareprovided in Sec. II, followed by a presentation of resultsin Sec. III. SectionIV then provides a discussionof theseresults and a comparison to previous theoretical work, while Sec.V concludes the paper with a summary of our most important findings. II. EXPERlMENT The spectraof AgAu and Au, were investigatedusing the resonanttwo-photon ionization apparatusdescribed in the previous two papers in this Journal, where results on CuAg’ and CuAu2 are reported. A metal target consisting of Cu, Ag, and Au in equimolar proportions, approximately 2 mm in thicknessand 2.5 cm in diameter, prepared as de- scribed in the preceding paper,2was usedfor production of diatomic AgAu. This was also used for studies of CUAU,~ Cu, Au, and CuAgAu. Because the bond strength of AgAu is weakerthan either CuAu or Au,, this source wasnot ideal for the preparationof the diatomic AgAu molecule (see Sec. IV E), but it was,nevertheless, adequate for the observation of two band systems in this molecule. A 1:l or 2:l alloy of Ag:Au would probably be much better for the production of AgAu. A 99.95% pure gold foil (25 X25 x0.5 mm, Alfa Products) wasused for the studies of Au,. Both targetswere used in a rotating disk vaporizationassembly similar to that described by O’Brien et a12’j Pulsed laservaporization of the 5646 J. Chem. Phys. 95 (8), 15 October 1991 0021-9606/91/205646-14$03.00 @ 1991 American Institute of Physics Downloaded 02 Apr 2001 to 128.110.196.147. Redistribution subject to AIP copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp