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Submitted on 1 Jan 1987
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LASER SPECTROSCOPY OF METAL CLUSTERS
M. Broyer, G. Delacrétaz, P. Labastie, R. Whetten, J. Wolf, L. Wöste
To cite this version:
M. Broyer, G. Delacrétaz, P. Labastie, R. Whetten, J. Wolf, et al.. LASER SPECTROSCOPY OF METAL CLUSTERS. Journal de Physique Colloques, 1987, 48 (C7), pp.C7-591-C7-593.
�10.1051/jphyscol:19877144�. �jpa-00226965�
JOURNAL DE PHYSIQUE
Colloque C7, supplement au n 0 1 2 , Tome 48, ddcembre 1987
LASER SPECTROSCOPY OF METAL CLUSTERS
M. BROYER, G. D E L A C R ~ T A Z * , P. LABASTIE, R.L. WHETTEN", J.P. WOLF* and L. WOSTE*
Laboratoire de Spectrom6trie Ionique et Moleculaire
CNRS LA-171, Universite Lyon I, Bdtiment 205, 43, Bd du 11 novembre 1918, F-69622 Villeurbanne Cedex, France
"Institut de Physique Experimentale, Ecole Polytechnique
FBdbrale de Lausanne, PHB-Ecublens, CH-1015 Lausanne, Switzerland .* Department of Chemistry and Biochemistry, Solid State Science Center, University of California, Los Angeles, CA 90024, U.S.A.
Conventional spectroscopy are not easily applied to clusters systems, because these particules are generally produced in size distributions in molecular beams The two-photon ionization spectroscopy has been demonstrated to be an effident method to select a particular size. Results are available in the literature on Nag 11 1 and Cu3 f21. Recently the depletion spectroscopy [2,31 has been used to investigate predissociated states : an ultraviolet laser directly ionizes the cluster and monitors the remmning population in the molecular beam. Whenever a predissociated level is simultaneously excited by the exciting laser, a depletion of the ion cluster intensity is observed. By this method a completely predissociated highly excited state of N% (the D state) has been recently observed [31. The depletion experiment has been used to probe all the other known states of Nitj (A, B, B' and C states). No evidence of predissociation has been found for the A, 0, B' states while the C state appears strongly predissociated except the two first bands. Figure 1 shows the comparison between the two-photon ionization and the depletion spectra in the C state. The whole spectrum observed by depletion aliows us to determine the symmetry of the C state ( 2 ~ ) and to assigne all the bands by solving the Jahn-Teller hamiltonian.
Theoretically the Na3 ground state is also very interesting. By altering the clustec source, we have been able to measure the ground state vibrational frequencies [41 by observing the hot bands in the spectroscopy of the
B
state.However hot band measurements are useful only for the lowest lying levels of the ground state. To explore the higher lying energy levels, we have performed Stimulated Emission Pumping (S.E.P.) experiments using the C state as an intermediate level : A first laser hv, excites the molecules in the C state and has sufficient energy to perform resonant two-photon ionization. A second laser hq2 stimulates transitions from the intermediate state back to the ground state. It results in a decrease of the two-photon ionization signal. Figure 2 shows the S.E P
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877144
JOURNAL DE PHYSIQUE
Fig.1. C state spectrum obtained by two-photon ionization (lower) and depletion spectroscopy (upper)
spectrum observed using the two first bands (0,1/2) and (0,3/2) of the C state as intermediate level. The rich structure of the ground state is being explored.
Finally we have built special oven designs 151 that combine metal vapors at temperature up to 2000 K and carrier gas pressures up to 10 bars. This design allows to generate cooled cluster beams for a wide choice of materials. Lithium clusters have been obtained by this method and excited states have been observed i n Lig I51 by two-photon ionization (Fig. 3).
660 670 680 690 [nm]
Fig.3. Two-photon ionization spectum of Lig
111 A. Herrmann. M. Hoffmann. S. Leutwyler. E. Schumacher and L. Waste. Chem.
Phys. Lett. 62 (1979) 216 :
G . Delacretaz. L. Woste, Surf. Sci. 156 (1985) 770 ;
G. Delacdtaz, E.R. Grant, R.L. Whetten,
L.
Waste, J.W. Zwanziger, Phys. Rev.Lett. 56 (1986) 2598.
121 M.D. Morse, J.B. Hopkins, P.R.R. Langridge-Smith and R.E. Smalley, J. Chem.
Phys. 79 (198315316.
I31
M.
Brayer, G. Delacrbtat, P. Labastie. J.P. Wolf and L. Waste, Phys. Rev. Lett.57 (1986) 1851.
141
M.
Broyer, G. Delacr&z, P. Labastie, J.P. Wolf and L. Waste, J. Phys. Chem.(to be published).
[51 G. Delacrbtaz, J.P. Wolf and L. Waste, to be published.
