Trimer pseudorotating

Viant M R, Cruzan J D, Luoas D D, Brown M G, Liu Kand Saykaiiy R J 1997 Pseudorotation in water trimer isotopomers using terahertz iaser speotrosoopy J. Phys. Chem. 101 9032-41... 

Transient two-photon ionization experiments on trimer systems were, of course, motivated by a need for time-resolved verification of the pseudorotation motion, which can be considered as a superposition of the asymmetric stretch (Qx) and the bending vibration (Qy) [12]. The triatomic molecule with its three modes is quite different from an isolated oscillating dimer, which vibrates in its single mode until eventually it radiates or predissociates. The interplay of vibrational modes in a trimer system can be considered as the prototype of IVR. 

L. Woste In relation to Prof. Tannor s comment, let me remark that the pseudorotating B state of Na3 is bound and there are no indications for fragmentation processes. In U3, however, there are predissociated pseudorotating states. We have observed and identified them. Lis has the advantage to be a homonuclear molecule, which however can have different isotopes. This induces an asymmetry, which can be exploited for pump and control. Later we intend to apply this to heteronuclear trimers like Na2K, where we hope to split off selectively either an Na or a K atom as a function of the initial conditions. 

Figure 1. Normal coordinate and real space description of pseudorotating K,. Circles represent energy contours for the trimer potential surface (3).

In Reference 4, this "alternating linewidth" effect (8) was shown to be consistent with a temperature dependent pseudorotation in Kj. A correlation diagram, which relates transition fields for the three obtuse angled trimer geometries, qualitatively predicts line broadening for M = + and h when pseudorota-... 

Figures 3a and 3b show, respectively, observed and simulated spectra for the pseudorotating trimer at T = 4.2 K. Absorption linewidths for the "bar spectrum" were derived from Equation (10),...

Figure 3(b) shows the ESR spectrum of the trimer Lis in an argon matrix at 28.5 K. Despite complications due to the presence of the heptamer Lij and an unknown impurity LiX, seven equally spaced signals can be seen. These have the correct intensity ratio to belong to a molecule with three equivalent Li nuclei (I = 1). It appears that the Lis trimer is rapidly pseudorotating in an argon matrix at this temperature thus the system behaves as an isotropic one and only one set of g values are seen. 

TVansient two-photon ionization experiments on trimer systems were motivated by a need for time-resolved verification of the pseudorotation motion, which can be considered as a superposition of the asymmetric stretch qx) and the bending vibration In this respect, the situation of a... 

In the second part of this chapter (Sect. 3.2), different wave packet propagation phenomena in excited alkali trimers are discussed. The time-resolved pseudorotation of the sodium trimer is presented in Sect. 3.2.2. Last but not least, applying laser pulses of the same wavelength but of different pulse width enables a mode-selective preparation of the trimer, hence controlling its dynamics (Sect. 3.2.4). Wave packet propagation on a repulsive PES (Sect. 3.2.5), studied on the potassium trimer, leads to the phenomena of ultrafast photodissociation, which then is the topic of the subsequent chapter. 

Real-Time Pseudorotation of the Sodium Trimer. A typical real-time spectrum obtained with picosecond excitation is shown in Fig. 3.41. The excitation wavelength is 620 nm, which corresponds to an intense resonant peak in the TPI spectrum. A clear beat strucure, symmetrical about the zero of time, is observed. The period amounts to 3 ps. The oscillation is damped with a time constant of about 3.5 ps. A constant offset is present, due to TPI... 

The first theoretical calculations to describe these real-time spectra were based on a simple two-dimensional model of the vibrating/pseudorotating sodium trimer [380]. Ab initio energy surfaces served as guidance for constructing model surfaces. The molecular dynamics of the Naa B system were simulated by the representative time-dependent wave packet Its norm... 

In the case of the sodium trimer (see Sect. 3.2.4) the relevant excitation regions in the B state have been determined as between T.Oao and 7.2ao in the Qs coordinate the centers of the excitation regions in the pseudorotation coordinates lie at < = 60°, 180°, 300° and q = 0.56ao. These regions have a diameter of 0.24ao. 

The vibrational excitation is high enough for the excited molecular modes to mix, and after the rebound, the still-unionized neutral trimers are left with enough energy to, for example, pseudorotate through their three equivalent... 

Viant, M. R., Cruzan, J. D., Lucas, D. D., Brown, M. G., Liu, K., Saykally, R. J.(1997). Pseudorotation in water trimer isotopomers using terahertz laser spectroscopy. Journal of Physical Chemistry A, 101, 9032-9041. 

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