Excited states dissociation

In radiolysis, a significant proportion of excited states is produced by ion neutralization. Generally speaking, much more is known about the kinetics of the process than about the nature of the excited states produced. In inert gases at pressures of a few torr or more, the positive ion X+ converts to the diatomic ion X2+ very rapidly. On neutralization, dissociation occurs with production of X. Apparently there is no repulsive He2 state crossing the He2+ potential curve near the minimum. Thus, without He2+ in a vibrationally excited state, dissociative neutralization does not occur instead, neutralization is accompanied by a col-lisional radiative process. Luminescences from both He and He2 are known to occur via such a mechanism (Brocklehurst, 1968). 

The photochemistry of ethylene is fairly well understood, but not the radiation chemistry. UV-photolysis shows that the excited states dissociate mainly by elimination of an H atom or a H2 molecule as follows ... 

All features below 15 eV in Fig. 20 are characteristic of DBA to I, II, and III. The monotonic rising signal in the H yield with an energetie threshold near 14.5 eV is characteristic of nonresonant DD of C-H bonds in I, II, and III it could also partially arise from DD of the O-H bond in II and III. However, the broad feature centered around 22 eV has been attributed to DBA and/or resonance decay into an electronically excited state dissociating into and the corresponding cation. The formation of H via DBA... 

In solution when iodine is excited to the bound B excited state, dissociation and recombination processes occur. The dissociation is the result of solvent-induced curve crossing to the dissociative a state, the recombination a result of momentum reversals arising from collisions with the surrounding solvent molecules. Eigenstates of the B state will decay in a continuous manner, whereas wavepackets—if the curve-crossing probability is less than unity—decay in a stepwise manner, giving rise to successive pulses of product. The B and a curves cross near the center of the B state, whereas the B state wavepacket is initially created near the left turning point thus there... 

Tables 2-8 contain pA-values for a range of aromatic compounds. The compounds are mostly classified according to their ring systems, but carbonyl compounds are grouped together for convenience. It has proved impossible to include some material in tabular form and a few molecules with more than one ionizable site have not been included (see e.g. Hercules and Rogers, 1959 Ellis and Rogers, 1964 Mayer and Himel, 1972). Where such polyfunctional molecules are included, the tables may be ambiguous as to the position of protonation or dissociation to which the quoted pA-values refer. For example, the excited state dissociation of protonated hydroxy-...

Rate Constants for the Excited State Dissociation Reaction of Naphthols... 

Since coiled chains of proteins are known to uncurl because of ionic repulsions when ionization occurs, Reid (1957) suggested that excited state dissociation acts as a trigger in rapid biological processes. The 7-azaindole dimer, which undergoes photo-induced double proton transfer (see Section 4), has similarities to the adenine-thymine and guanine-cytosine base pairs of DNA. Its excited state proton transfers have been proposed as possible mechanisms of mutagenesis (Ingram and El-Bayoumi, 1974). 

All these compounds are active participants in environmental photochemistry. Nitrate radicals absorb solar radiation at approximately 600-700 nm and in the excited state dissociate to NOx and oxygen. There are two possible sets of the photodissociation products ... 

Gaydon gives 4 93 0 25 eV, Herzberg2t7 4-1 eV, doubtful. Gaydon s figure is based on excited state dissociation. 

In a rigid medium, /c, t and no solvent motion occurs prior to decay of the excited state. Dissociation of some of the MLJ complexes would be inhibited by the rigid solvent while others could still dissociate. ML5 would either recombine with L or remain trapped. In neither case would there be a net photosolvation while the environment remained rigid, although ML5S could be produced upon warming. Photosolvation may or may not be completely inhibited by low solvent mobility in an associative mechanism. 

A simple estimate of pK has been proposed by Weller (4), based on the so-called FSrster cycle (3) (Fig. 2). The difference between the ground state and excited state dissociation enthalpy is given by the difference of the energy levels, as measured by the absorption or emission spectra of the protonlzed (AE) and deprotonized (AE ) species. 

FIGURE 2. Forster cycle of ground and excited state dissociation neglecting vibrational levels ... 

The observation that the reaction requires an induction time of tens of picoseconds can be used to differentiate between proposed mechanisms of how shock wave energy localizes to cause chemical reaction. This induction time is expected for mechanisms that involve vibrational energy transfer, such as multiphonon up-pumping [107], where the shock wave excites low frequency phonons that multiply annihilate to excite the higher frequency modes involved in dissociation. It is also consistent with electronic excitation relaxing into highly excited vibrational states before dissociation, and experiments are underway to search for electronic excitations. On the other hand, prompt mechanisms, such as direct high frequency vibrational excitation by the shock wave, or direct electronic excitation and prompt excited state dissociation, should occur on sub-picosecond time scales, in contrast to the data presented here. 

A number of unimolecular photoredox reactions have been studied. Kirk and Porter [130] reported a AF value of +4.8 cm3 mol"1 for the charge-transfer photolysis of Co(NH-,)5Br24 and suggested the formation of a caged radical pair, Con(Br ), from the LMCT excited state. Dissociation of this radical pair to the reaction products was suggested to account for the increase in volume as reflected by the positive volume of activation. Charge-transfer photolysis of trans -Pt(CN)4(N3)2 results in the reductive elimination of azide to produce Pt(CN) and dinitrogen [131,132], On... 

Figure 1. Potential energy curves of the Nal molecule. Two excitation processes are indicated, where a first field E (t) induces a 11) <— 0) electronic transition, and a second field E2(t) triggers an excited-state dissociation with yield B (t). Due to the nonadiabatic coupling in the region around Rc, predissociation can also occur, leading to ground-state atomic fragments (yield B0(t)).

A restricted model of the Nal molecule, where predissociation is excluded, was discussed in the last subsection. In what follows, we investigate the possibility of controlling the excited-state dissociation in the presence of the predissociation channel [111] (see Fig. 1). Again, the first step is a femtosecond excitation from the ground state, preparing an excited-state vibrational wavepacket. 

Figure 4. Dissociation and predissociation dynamics in a heating field. The lower panel shows the energy expectation value of the system for two values of the strength parameter (k = s x 10 6 a.u.), as indicated. The middle and upper panels contain the excited-state dissociation (B (t)) and predissociation yields ( oW)> respectively. The dashed line curve in the upper panel represents the field free case.

To conclude this subsection, we demonstrated that heating and cooling LCT fields can be employed to modify the branching ratio of ground-state predissociation and excited-state dissociation. Furthermore, a stabilization of predissociating molecules can temporarily be maintained. 

We now combine various aspects of the problems discussed in the previous sections, taking the Nal dynamics as an example. In Section III.A, the excited-state dissociation of this molecule was treated, excluding the possibility of... 

Figure 27. Successive interaction of two LCT fields where the first (Fi(r)) induces a population transfer and the second acting after 5 ps (E2(t)) triggers excited-state dissociation. These fields are shown in the lower panel, whereas the predissociation ( o(0) and excited-state fragmentation ( i(V)) yields are contained in the upper panel.

Figure 28. Simultaneous interaction of two LCT fields (E (t) + E2(t)) constructed to induce population transfer and excited-state dissociation, respectively. The two field components are displayed separately in the lower panels. The predissociation ( o(f)) and excited-state (B (t)) fragmentation yields are shown in the upper panel.

We now come back to the initially studied problem of Nal fragmentation where the predissociation channel is not taken into account (Section III.A). As is sketched in Fig. 1, we aim at a field-induced excited-state dissociation after an initial femtosecond excitation from the ground state, but now take the rotational degree of freedom into account. Thus the molecular Hamiltonian in the electronic state n) is written explicitly as... 

Figure 35. Field-induced excited-state dissociation of Nal including the rotational degree of freedom. The upper and lower panels contain the dissociation yield and the expectation value of the vibrational Hamiltonian, respectively.

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