Vibronic instability

At the same time, as it is known, the molecular systems in the degenerated states are dynamically unstable with respect to symmetry distortions due to the Jahn-Teller effect [9]. Such kinds of distortions are expected both in the electronic and nuclear subsystems (vibronic instability). The Jahn-Teller effect destabilizes the tetrahedral geometry of the nuclear system, which results in the bond order s changes too. In particular, according to the theory of the Jahn-Teller effect [9] the tetrahedral molecular systems in the 2Ti(P ),lT2(Pi ) or 37 (P4 ) states are dynamically unstable to the e- and t2-type nuclear distortions, one of which is presented in Fig. 7.1 Evidently such a kind of distortion to the butterfly geometry... 

If the mechanical degrees of freedom are coupled strongly to the environment (dissipative vibron), then the dissipation of molecular vibrations is determined by the environment. However, if the coupling of vibrations to the leads is weak, we should consider the case when the vibrations are excited by the current flowing through a molecule, and the dissipation of vibrations is also determined essentially by the coupling to the electrons. Here, we show that the effects of vibron emission and vibronic instability are important especially in the case of electron-vibron resonance. 

Now let us consider several examples of vibron emission and vibronic instability. 

Fig. 34 Vibronic instability in an asymmetric multilevel model voltage-current curve, differential conductance, and the number of excited vibrons (crosses). Dashed line show the voltage-current curve without vibrons (details see in the text).

As in the original treatment of Jahn and Teller our result attributes the vibronic instability to the terms in the Hamiltonian which are linear in the nuclear displacements. Higher order contributions will of course occur as well but they cannot be responsible for the conical instability at the high symmetry origin. 

The harder the AX3 molecules and their atomic constituents are, the more susceptible they become towards distortion (hardness rule). With increasing coordination number and negative charge, a complex becomes softer and displays a less pronounced vibronic instability. The hardness rule is able, within certain limits, to predict trends for lone-pair distortions as a function of chemical parameters. 

In this communication we will give a description of the vibronic E-e interaction in an optical center in a crystal near one of the minima of the trough of the deformed (due to the quadratic vibronic coupling) Mexican-hat-type AP. We will also present a derivation of the nonperturbative formula describing the temperature dependence of the ZPL in the case of an arbitrary change of the elastic springs on the electronic transition. Then we will study a case when the excited state is close to the dynamical instability. Finally, we will apply the obtained general results to the ZPLs in N-V centers in diamond. 

In this article for the simulation of the phosphorus molecule in coordinated states the quantum chemical calculations of P4 were carried out with values of charge q = 0, — 1, —2. Special attention is given to the instabilities of the electronic and nuclear subsystems with the possible reduction of symmetry, resulting in the breaking of the chemical bonds in the tetrahedral phosphorus molecule. Results are analyzed using vibronic theory. 

Not infrequently, gaseous ions are referred to as excited species . Such a description is, in some ways, misleading in the context of unimolecular reactions. The implied transience and instability exists only with respect to collisions and, in the absence of collisions, most ions in their ground vibronic state are stable species. The excited species need to be highly vibrationally excited before they react unimolecularly. If a degree of frivolity is permissible, the ionic unimolecular reaction... 

Our recent observation (jO of electronic state fluorescence from laser-excited PuFe(g) and NpFe(g) has marked the beginning of systematic studies of the photophysics and photochemistry of transuranic hexafluorides and has provided the key to further exploration of the complex vibronic structure characteristic of these compounds. While the photochemistry of UFe has been the object of numerous investigations, only a few remarks are found in the literature concerning the electronic state photochemistry of the transuranic hexafluorides. Given the dense electronic energy level structures of PuFe and NpFe and their relative instability in comparison with UFe, we can anticipate that their photochemistry will involve a rich and complex set of interactions. The work we report here deals primarily with PuF6(g). We turn first to a very brief review of PuFe studies... 

Thus, with these proofs two important additions to the previous traditional understanding of the JTE emerged (1) Any polyatomic system may be subject to the JTE, and (2) if there are instabilities and distortions of high-symmetry configurations, they are due to, and only to the JTE. Together with the previously achieved understanding of the role of quadratic terms of the vibronic coupling, the extended formulation of the JTE that includes the latest achievements in this field is as follows [1,8] ... 

---