Electronic predissociation relaxation

Obviously, the initial orientation of the iodine molecules within the crystal cages has a strong influence on the vibrational relaxation as well as the predissociation of the molecules in the electronic B state. The other states only play a minor role for the dynamics in the first 20 ps. 

The simulations demonstrated, that after the pump excitation of the iodine molecules into their B state three elementary dynamical processes determine the further reaction course (i) the predissociation of the iodine molecules caused by the coupling of the electronic B state to the repulsive ajo states, (ii) the electronic transitions from these states to the A, A, and X states due to the caging effect, and (iii) the vibrational relaxation in all electronic states involved in the reaction. Additionally, an energy shift of the potential curves due to the influence of the crystalline DDR cage could be observed. 

Here we are chiefly interested in the intrinsic causes of line broadening. We do not include among these dissociation, ionization, predissociation, autoionization, and pieisomerization, since few unambiguous examples of their occurrence have been reported for the low-lying excited electronic states. Attention is devoted to broadening through anharmonicity (vibrational relaxation) and in particular to electronic relaxation. 

In cases where the yield of molecular ions is higher than 10% and where the fragmentation pattern depends upon the atomic site of the core hole, the dissociation processes clearly depend upon the electronic structure of the molecule and the details of the electronic relaxation, i.e. not all pathways produce essentially the same result. The mechanism then may involve vibrational dissociation or electronic or vibrational predissociation as well as direct dissociation. Even in these cases, some of the electronic relaxation channels may rupture all the bonds in a molecule and high-kinetic-energy fragments can be produced. Such channels sometimes are labeled a Coulomb explosion, but this terminology should not be confused with the more specific use of the term that is proposed above. 

There exists, in the past few years an increasing interest in the influence of external (magnetic and electric) fields on the dynamics of excited molecular states. This interest is not surprising if we are reminded of the role played by this kind of studies in the development of the atomic physics. We will limit our discussion to the phenomena related to the collisional electronic relaxation application of magnetic fields in the studies of predissociation and of dephasing processes in isolated molecules will not be treated here. 

An elegant molecular-beam study of the photofragmentation of aryl halides and methyl iodide has permitted extraction of excited-state lifetimes from a measured anisotropy parameter which depends upon the lifetime of excited state, the rotational correlation time of the molecule, and the orientation of the electronic transition dipole with respect to the —X bond.38 The lifetimes obtained were methyl iodide 0.07 ps, iodobenzene 0.5 ps, a-iodonaphthalene 0.9 ps, and 4-iodobiphenyl 0.6 ps, from which it was concluded that, whereas methyl iodide dissociates directly, the aryl halides predissociate. A crossed-beam experiment using electron-beam excitation has yielded the results for the Si Tt intersystem-crossing relaxation time in benzene, [sHe]benzene, fluorobenzene, and... 

The investigation of fast processes, such as electron motions in atoms or molecules, radiative or collision-induced decays of excited levels, isomerization of excited molecules, or the relaxation of an optically pumped system toward thermal equilibrium, opens the way to study in detail the dynamic properties of excited atoms and molecules. A thorough knowledge of dynamical processes is of fundamental importance for many branches of physics, chemistry, or biology. Examples are predissociation rates of excited molecules, femtosecond chemistry, or the understanding of the visual process and its different steps from the photoexcitation ofrhodopsin molecules in the retina cells to the arrival of electrical nerve pulses in the brain. 

The problem lies in the assumption required to derive the selection rule that the u=l and u=0 surfaces are the same shape and are merely displaced vertically as we have illustrated in Fig. 2. For HF HF on the contrary, the intermolecular potential is highly anisotropic and rotational excitation of the fragments results in an effective potential which is shallow and may actually cross other surfaces. This has been demonstrated in calculations of Halberstadt et al.. The surfaces taken from their work are shown in Fig. 4. The curve crossing yields relaxation times orders of magnitude more efficient than those calculated by our selection rule. It is a challenge to the theorists to model the predissociation process, consistent with experiment, that allows both HF molecules to rotate on fragmentation. Clearly anisotropic effects will play an important role in understanding vibrational predissociation in other systems as well-for example, in the electronically excited state of OH Ar by Lester et al.. ... 

Spectroscopic Basics of Nas C. In the late 1980s special interest was focused on the C(2) E" state (in Dsh symmetry) of Nas. Energy-resolved spectroscopy allowed the observation of lower vibrational levels of this electronic state by means of TPI, whereas the upper levels require the use of DS to probe dissociative states [369, 374, 393]. The spectrum of the C state is characterized by a vibrational band structure with pseudorotational features, as shown in Fig. 4.3. These investigations confirmed the C state to be partially predissociated. Therefore, the dissociation channel was proposed to be the main relaxation process for states higher in energy than the C state. This could also be demonstrated for the D state by the depletion technique with a few nanoseconds time resolution [375], as well as for Rydberg states close to the ionization limit [124]. 

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