Repulsive excited states

Accurate determination of excited state repulsive potential functions of diatomic molecules in the gaseous phase... 

Figure 28. The HCl excited-state repulsive curve is plotted adjacent to the H-OCO curve obtained by having the H atom approach CO along its axis, with its nuclei fixed at the equilibrium positions (see Figure 18a). The initial Cl-O separation is that of the COj-HCl weakly bonded complex. Note the overlap of the curves.

Rydberg-Klein-Dunham potential curves have been constructed for the states X rir and A 2A using the molecular constants of [7] and [37] (publication in [8]) these have been combined with theoretical (ab initio MCSCF-CI) curves for the X and A states to obtain also the short- and long-range parts of the curves MCSCF-CI potential curves have also been calculated for the unobserved excited states (repulsive) and (quasidissociative) [14]. Potential curves have been calculated for the states X rir, a 2", (quasidissociative), and (repulsive) with the dissociation limit P+(3P) + H(2S) and for the states A A, 2S+ (quasidissociative), and (repulsive) with the dissociation limit P+( D) + H(2S) using the MRD Cl method the results are shown in Fig. 2 along with further possible dissociation limits and corresponding excited states of PH+ [18, 19]. 

Altliough an MOT functions as a versatile and robust reaction cell for studying cold collisions, light frequencies must tune close to atomic transitions and an appreciable steady-state fraction of tire atoms remain excited. Excited-state trap-loss collisions and photon-induced repulsion limit achievable densities. 

Figure 7.24 (a) The repulsive ground state and a bound excited state of Hc2. (b) Two bound states... 

An Xc2 excimer laser has been made to operate in this way, but of much greater importance are the noble gas halide lasers. These halides also have repulsive ground states and bound excited states they are examples of exciplexes. An exciplex is a complex consisting, in a diatomic molecule, of two different atoms, which is stable in an excited electronic state but dissociates readily in the ground state. In spite of this clear distinction between an excimer and an exciplex it is now common for all such lasers to be called excimer lasers. 

In an aqueous solution containing 26 and 27 the excited state of the Ru(II) complex in 26 essentially has no chance to be directly quenched by the donor quencher in 27, because a strong electrostatic repulsion acts between 26 and 27. Sassoon and Rabani added methoxydimethylaniline (MDMA, 28) to this system... 

In some cases the excited state is entirely dissociative (Fig. 7.3), that is, there is no distance where attraction outweighs repulsion, and the bond must cleave. An example is the hydrogen molecule, where a ct 0 promotion always results in cleavage. 

Figure 1.3. Real-time femtosecond spectroscopy of molecules can be described in terms of optical transitions excited by ultrafast laser pulses between potential energy curves which indicate how different energy states of a molecule vary with interatomic distances. The example shown here is for the dissociation of iodine bromide (IBr). An initial pump laser excites a vertical transition from the potential curve of the lowest (ground) electronic state Vg to an excited state Vj. The fragmentation of IBr to form I + Br is described by quantum theory in terms of a wavepacket which either oscillates between the extremes of or crosses over onto the steeply repulsive potential V[ leading to dissociation, as indicated by the two arrows. These motions are monitored in the time domain by simultaneous absorption of two probe-pulse photons which, in this case, ionise the dissociating molecule.

An initial, ultrafast pump pulse promotes IBr to the potential energy curve Vj, where the electrostatic nuclear and electronic forces within the incipient excited IBr molecule act to force the I and Br atoms apart. contains a minimum, however, so as the atoms begin to separate the molecule remains trapped in the excited state unless it can cross over onto the repulsive potential VJ, which intersects the bound curve at an extended... 

Figure 1. Schematic of the radial cuts of the ground- and excited-state potential energy surfaces at the linear and T-shaped orientations. Transitions of the ground-state, T-shaped complexes access the lowest lying, bound intermolecular level in the excited-state potential also with a rigid T-shaped geometry. Transitions of the linear conformer were previously believed to access the purely repulsive region of the excited-state potential and would thus give rise to a continuum signal. The results reviewed here indicate that transitions of the linear conformer can access bound excited-state levels with intermolecular vibrational excitation.

As we have reviewed here, the linear region is not fully repulsive, and transitions of the ground-state, linear conformer access vibrationally excited intermolecular levels that are delocalized in the angular coordinate. As depicted in Fig. 1, however, the internuclear distance is significantly longer in the excited state at the linear geometry. Consequently, there is favorable Franck-Condon overlap of the linear conformer with the inner-repulsive wall of the excited-state potential. It is therefore possible for the linear Rg XY conformers to be promoted to the continuum of states just above each Rg - - XY B,v ) dissociation limit. 

Sobolewski AL, Domcke W, Dedonder-Lardeux C, Jouvet C (2002) Excited-state hydrogen detachment and hydrogen transfer driven by repulsive (l)pi sigma states a new paradigm for nonradiative decay in aromatic biomolecules. J Phys Chem Chem Phys 4 1093—1100... 

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