Volume 2 13, number 5,6 CHEMICAL PHYSICS LETTERS I5 October 1993 Time-resolved observation of molecular pseudorotation in Na3 K. Kobe, H. Kiihling, S. Rutz, E. Schreiber, J.P. Wolf, L. Wijste zyxwvutsrqponmlkjihgfedcbaZYXWVU Institutftir Experimentalphysik, Freie Universitiit Berlin, Arnimallee 14, D-14195Berlin, Germany M. Broyer and Ph. Dugourd Laboratoire de Spectromhrie Ionique et Molhlaire (ussocie au CNRSNo. 171). Universitt! Lyon I, B&iment 205.43 Boulevard du I1 Novembre 1918..69622 Vilieurbanne Cedex, France Received 22 July 1993; in final form 19 August 1993 The temporal behaviour of the pseudorotating B state of Nas was investigated in a two-photon-ionization (TPI) experiment, employing picosecond laser pulses and a variable delay between the excitation and ionization step. The results exhibit the pseu- dorotational motion as a temporal sequence. The detailed analysis of the vibronic function time evolution afftrms a pseudo-dahn- Teller (PJT) model for the interpretation of the B state spectroscopic data. Metal clusters of different sizes exhibit distinctive features, which range from purely molecular behav- iour in very small particles to solid-like properties in large aggregates. The size-dependent phenomena can be related to surface-volume ratios, structural ar- rangements and electron-electron interactions in- side the cluster. The pronounced dynamic properties of metal clusters, however, are mainly due to vi- bronically coupled internal degrees of freedom, which result in efficient energy transfer channels. Since the quantity of coupled vibrations increases drastically with the number of participating atoms, an identi- fication of individual energetic pathways inside the “vibronic soup” of the cluster becomes extremely difficult, unless simple systems are chosen. The sodium trimer is a simple system which is a well-known metal cluster. In the equilibrium ge- ometry of the ground state Na, adopts - due to Jahn- Teller distortion - a well-localized obtuse-angled tri- angular shape [ 1,2]. TPI spectra of the electroni- cally excited B state, however, revealed pseudoro- tation features [ 3-51. Under the assumption of vibronic coupling (E@e) inside the doubly degen- erate excited ‘E’ electronic state, the spectra could be reproduced with a quadratic Jahn-Teller model [ 41. Ab initio calculations performed later pointed out the existence of a non-degenerate electronic state of 2A{ symmetry in the dose vicinity of the 2E’ state [ 61. Another interpretation assuming vibronic cou- pling of type (E +A) @e between the non-degenerate 2A’, state and the doubly degenerate excited ‘E’ state provided an equally well-matching reproduction of the experimental data. However, while the first interpretation was based on the assumption of frac- tional quantization and a quadratic Jahn-Teller model, the second interpretation was based on in- teger J values and a pseudo-Jahn-Teller model. No final conclusion can be drawn, unless further exper- imental information is made available. Two possible approaches can be followed to solve the problem: The first one was carried out by Ernst and co-workers [ 7 ] who investigated the Na3 B-X system at rotational resolution and found their results in favour of the pseudo-Jahn-Teller effect interpretation. Our ap- proach was to use time-resolved experiments which have recently been demonstrated as being efficient at studying the dynamics of molecules and clusters [ 8- 131. We measured the temporal evolution of the TPI of the B state of sodium trimers by use of a tunable dye laser (rhodamine 6G) synchronously pumped by the green line of a mode-locked Ar+ laser. The repetition rate of the laser pulses being continuously controlled 554 0009-2614/93/S 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.