Dissociation of Excited Molecules

The excited product CHDCD2 can be collisionally stabilized, but can it otherwise dissociate as follows  

In the case of CH3CD3, various other dissociation modes exist, including C—C and C—H bond breakage, either separately or in various combinations (Freeman, 1968). A few other examples follow of dissociation of excited inorganic and organic molecules  

Generally speaking, the dissociation modes are established by photolysis, isotope studies, the electric field effect, and, to some extent, by special mass spectrometric methods. In addition, polymerization and isomerization studies have been helpful. 

Many investigators consider neutralization as a poorly understood process in the gas phase. While in the best possible cases it may be considered as a homogeneous second-order process, detailed experimentation in specific cases sometimes fails to establish that (Freeman, 1968 Meisels, 1968). At low dose rates and low gas pressures, wall effects can be seen as a major inhibiting factor, as most neutralizations would then be expected to occur on the walls. Coating the wall with specific chemicals has not lead to a uniform conclusion. On the other hand, wall effects are also present at high dose rates. In such cases, and with gas pressure greater than about 0.1 atm, normal positive ions cannot reach the walls if the size of the vessel is 10 cm or more (Freeman, 1968). Even for electrons, it is hard. Large-scale convection is supposed to be the chief transport mechanism this, however, is difficult to establish experimentally. 

There are basically two kinds of neutralization processes for the cation, reaction with the electron and with a negative ion. In each case, it may be assumed that neutralization will occur with the parent or fragment ion of lowest energy. It is believed that the various degradation processes for the cation-fragmentation, ion-molecule reaction, and so forth—are much faster than the neutralization process. In addition, one considers charge transfer, without decomposition, from the cation formally as a neutralization of that species. To effect that, of course, one 

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In spite of the fact that in alkali vapors, which contain about 1 % diatomic alkali-molecules at a total vapor-pressure of 10 torr, the atoms cannot absorb laser lines (because there is no proper resonance transition), atomic fluorescence lines have been observed 04) upon irradiating the vapor cell with laser light. The atomic excited states can be produced either by collision-induced dissociation of excited molecules or by photodissociation from excited molecular states by a second photon. The latter process is not improbable, because of the large light intensities in the exciting laser beam. These questions will hopefully be solved by the investigations currently being performed in our laboratory. 

Summarising, there is unambiguous evidence from esr measurements that free radicals are produced in many systems on irradiation and a wealth of information from scavenger experiments is consistent with this. The radicals may arise from dissociation of excited molecules... 

Zener, C. (1933) Dissociation of excited molecules by external perturbation, Proc. Royal. Soc. A140, 660- 668. 

Free radicals are produced by the dissociation of excited molecules. High-energy-irradiation-induced polymerization is especially important for graft polymerization and polymerization in the solid state. Because of high investment costs, high-energy-radiation-initiated polymerization of gaseous, liquid, or dissolved monomers has not become established. However, to a small extent ( 2000 t/a), the polymerization of methyl methacrylate is radiation initiated. 

Dissociation of excited molecules, either homolytically to form neutral free radicals, or heterolytically to form anions and cations, and ion-molecule reactions provide the next step in the degradation mechanism(3). These reactions are shown in Fig. 1. 

The molecular time scale may be taken to start at 10 14 s following energy absorption (see Sect. 2.2.3). At this time, H atoms begin to vibrate and most OH in water radiolysis is formed through the ion-molecule reaction H20+ + H20 H30+ + OH. Dissociation of excited and superexcited states, including delayed ionization, also should occur in this time scale. The subexcitation electron has not yet thermalized, but it should have established a quasi-stationary spectrum its mean energy is expected to be around a few tenths of an eV. 

Photolysis of CO has been conducted18-20 only at wavelengths greater than the threshold for dissociation ( 1100 A) where reactions of excited molecules... 

The potential energy curves of excited electronic states need not have potential energy minima, such as those shown in Fig. 3.6. Thus Fig. 3.7 shows two hypothetical cases of repulsive states where no minima are present. Dissociation occurs immediately following light absorption, giving rise to a spectrum with a structureless continuum. Transition a represents the case where dissociation of the molecule AB produces the atoms A and B in their ground states, and transition b the situation where dissociation produces one of the atoms in an electronically excited state, designated A. ... 

Reversal-temperature measurements of the Na and Cr lines in simple molecular gases, shock-heated to 2000-3000°K and to 0,2-2 atmospheres agree excellently with temperatures calculated from the measured shock velocity. Thus in these cases, collision processes are rapid enough to maintain effective equilibrium between ground and excited state populations despite radiatio n losses. In some shock tube work, however, the reversal temperature is initially above the equilibrium value, probably owing to delay in dissociation of the molecules, so that the temperature in translation and in internal degrees of freedom of the molecules is initially too high... 

This experimental work on the dissociation of excited Nal clearly demonstrated behavior one could describe with the vocabulary and concepts of classical motions.The incoherent ensemble of molecules just before photoexcitation with a femtosecond laser pump pulse was transformed through the excitation into a coherent superposition of states, a wave packet that evolved as though it represented a single vibrationally activated molecule. 

There can be a difference between the dissociation of polyatomic molecules and delayed ionization in the nature of the initial excitation. In ZEKE spectroscopy the state that is optically accessed (typically via an intermediate resonantly excited state) is a high Rydberg state, that is a state where most of the available energy is electronic excitation. Such a state is typically directly coupled to the continuum and can promptly ionize, unlike the typical preparation process in a unimolecular dissociation where the state initially accessed does not have much of its energy already along the reaction coordinate. It is quite possible however to observe delayed ionization in molecules that have acquired their energy by other means so that the difference, while certainly important is not one of principle. 

Diffusion of molecules in rigid glasses is negligible within the lifetimes of excited molecules, so that the main reactions are unimolecular dissociations and isomerization. These are rather similar to liquid state reactions, but the fragments cannot separate through diffusion and often recombine to restore the reactants. There are exceptions when the photoproducts are in fact more stable than the reactants, as in the case of photoeliminations. 

The photochemical dissociation of a molecule AB often leads to the formation of a pair of radicals A 4- B, e.g. as in Figure 4.33. If the reaction takes place from the lowest triplet excited state of AB, the radicals will have parallel spins and cannot recombine unless a spin flip takes place to bring them to the singlet state of the geminate radical pair. 

LI Laser-induced Fluorescence. The probe wavelength Ap can be adjusted to excite one of the photofragments or the excited complex in the process of dissociation. Consider for instance the dissociation of a molecule AB according to... 

From the preceding discussion it may be inferred that in frozen aqueous solutions there are basically two different mechanisms by which H atoms can be formed (1) by reaction of the electron with acid anions according to Equation 4, and (2) by direct radiolysis of water molecules according to Equation 1. Earlier investigations of aqueous solutions at room temperature have also lead to the same conclusion (70). Although there are no experimental data from either the room temperature studies or from those on the frozen solutions, as to the actual nature of the second process, it is believed (70) that it is the dissociation of excited water molecules formed in the tracks of the fast electrons. 

Even if two vibrational states are degenerate they can yield completely different cross sections. The dissociation of excited vibrational states samples a considerably wider region of the upper-state PES than dissociation of the ground vibrational state. However, because the two quantum mechanical wavefunctions both have an oscillatory behavior, the interpretation of the various cross sections is not always obvious. The photodissociation of excited vibrational states is closely related to the emission spectroscopy of the dissociating molecule which is the topic of the following chapter. 

Freed, K.F. and Band, Y.B. (1977). Product energy distributions in the dissociation of polyatomic molecules, in Excited States, Vol. 3, ed. E.C. Lim (Academic Press, New York). 

Atmospheric pressure plasmas, just like most other plasmas, are generated by a high electric field in a gas volume. The few free electrons which are always present in the gas, due to, for example, cosmic radiation or radioactive decay of certain isotopes, will, after a critical electric field strength has been exceeded, develop an avalanche with ionization and excitation of species. Energy gained by the hot electrons is efficiently transferred and used in the excitation and dissociation of gas molecules. In a nonequilibrium atmospheric pressure plasma, collisions and radiative processes are dominated by energy transfer by stepwise processes and three-body collisions. The dominance of these processes has allowed many... 

Since dissociation of the molecule involves stretching of the H-H bond, it is natural to ask about the change in the PES when the bond extends. Far from the surface, this is simply the vibration of the molecule, and it is by changing the initial vibrational state of the molecule that experiment can probe the PES in this dimension. In gas-surface dynamics, this was done first by Hayden and Lamont [13], They showed that for the H2/Cu system when the temperature of a molecular beam is increased but the translational temperature is kept constant, the dissociation probability increases. Increasing the temperature of the beam increases the Boltzmann population of the vibrationally excited states of the molecule, therefore the conclusion of this work is that vibrationally excited molecules dissociate more readily. 

Another type of reaction that may be used to create reactants is the collisional dissociation of a molecule by some electronically excited species, usually an inert gas atom [28]. Uranium atoms have been generated [29] by the collisional dissociation of uranocene by metastable argon atoms. This is an attractive alternative to the problems associated with the vaporisation of uranium. 

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