Vibronic structure of theNa3ground state by stimulated emission spectroscopy

Abstract

We report the first observation of the high vibronic levels of the Na3 ground state, performed by stimulated emission spectroscopy. The highest cross sections of the process, which uses the C 2 E" state as an intermediate state, are obtained for combinations of symmetric stretch and bending modes. These experimental results are of primary importance for the understanding of energy-level structures in systems exhibiting strong vibronic coupling.PACS numbers: 36.40.+d, 31.50,+w, 35.20.-i Energy levels of diatomic molecules are a well-known quantum-mechanical problem, except for highly excited Rydberg states, in which quantum chaos 1 may appear. For molecules having three atoms or more, the energy levels in the whole potential surface cannot be easily analytically calculated, except in certain circumstances, like the well-defined vibronic-mode approximation or the free pseudorotation in Jahn-Teller systems. 2 Because of their simplicity, the alkali-metal trimer systems are the most promising candidates for achieving deeper insight in this very active field. 34 Among these, Na3 is the best known system: Numerous theoretical calculations have been carried out over the last ten years 5 " 8 and, with the advent of highly cooled supersonic beams, well-resolved spectra of the electronically excited states are available. 9 " 13 However, experimental information about the theoretically better known ground state was, up to now, rather scarce. Lindsay and Thompson 14 have analyzed the matrix spin resonance in terms of pseudorotation. Broyer et a/. 15 investigated the hot bands that appear just below the origin of each of the visible systems. In this way they were able to observe three fundamental frequencies (49, 87, and 139 cm -1 ), and these were associated in a straightforward way with the normal-mode frequencies of vibration computed for the shallow minima of Civ symmetry found in ab initio calculations. 6,7 At present, we are not aware of any precise computations, based on ab initio potential-energy surfaces, of the ground-state energy levels. However, recent classical and quantum calculations have been performed on H3 in a chaotic region. 4 We report in this paper a stimulated emission pumping (SEP) experiment 16 on Na 3 , which has, in the past, been very successfully used to study highly excited vibrational levels at high resolution. The stimulated emission rate is detected by depletion of the photoionization signal. Under these conditions, this technique becomes highly sensitive and cluster-size specific. Our experiment is the first SEP experiment performed on clusters. We have chosen the well-resolved bands of the Na 3 C system as intermediate state for stimulated emission back to ex-cited vibronic levels of the ground state.The principle of the SEP experiment on Na 3 uses the ion-depletion technique 17 (Fig. 1). The Na 3 molecules, produced in a molecular-beam apparatus, are excited in an intermediate level of the C system 11,13 by a first laser co\. A second laser (02 is then used for inducing two competitive p...

Theoretical studies of the potential energy surface and wavepacket dynamics of the Li 3 system

“…8 same as in the preceding two cases. The recurrences in the autocorrelation functions of Li 3 are large in comparison with those of Na 3 [4,5]. This is because the pseudorotation is essential in Li 3 and the barriers are very low.…”

“…In the SEP spectrum of Na 3 [4], the electronically excited C state was taken as an intermediate state (the 2 2 E¢¢ state in h 3h ). Because the PESs of the A (1 2 E) and C states resemble each other in the g 2v cut of Na 3 2) , it is of interest to perform simulations based on the A state.…”

“…Subsequently, Meiswinkel and KoÈ ppel [3] explained theoretically the spectrum based on a pseudo Jahn-Teller model. A congested stimulated emission pumping (SEP) spectrum was observed on a chaotic background for Na 3 by Broyer et al [4] and the vibrational motion in the chaotic region was then investigated theoretically and assigned based on unstable periodic orbits [5,6]. Recently, continuous wave two-photon ionization and fs pump-probe spectroscopy [7,8] have yielded preferential excitations of the pseudorotations and the symmetric stretch in Na 3 (B), respectively.…”

Point-charge model for vibronic coupling constants of metal atom trimers

“…Because of the presence of a single degenerate, JT active mode these trimers constitute actually a good testing ground for existing JT theories such as the concept of 'linear plus quadratic' JT couplings [12]. Many of the aforementioned spectra could be assigned in this way [4][5][6][7][8][9][10], one of the first being the so-called B-system in the two-photon ionization spectrum of Na 3 [ 1 ].Later theoretical work, however, has questioned this interpretation of the B-system and, instead, proposed a significant pseudo Jahn-Teller (PJT) coupling to a nearby nondegenerate state as the dominating dynamic effect [ 13,14]. The linear JT coupling terms, on the other hand, were implied to be very small.…”

A diabatic representation of the two lowest electronic states of Li3

“…The simplest example of such a situation is given by the E ⊗ ε Jahn-Teller (JT) effect [1] with a conical intersection involving the two-fold degenerate electronic states E with the doubly degenerate nuclear vibration modes ε. Alkali metal trimers X 3 with a conical intersections at the geometric D 3h symmetries are special cases of the E ⊗ ε JT systems [2]. As a result, in recent years small metal clusters have received great attention as the subject of many experimental [3][4][5][6][7][8][9][10][11][12] and theoretical [2,[13][14][15][16][17][18][19][20] publications. These systems are especially convenient for experimental studies due to their possibility of vibrational excitation in the visible or near infrared regions [21].…”