Radiative and predissociation

Quenching half-pressure is equal lo (, t) 1 where kt is the rate constant for quenching reaction and r is the mean lifetime (radiative and predissociative) of excited NO. 

Figure 2.3 Perturbations and predissociations affect absorption and emission line intensities in quite different ways. Two pairs of absorption and emission spectra are shown. The first pair illustrates the disappearance of a weakly predissociated line in emission without any detectable intensity or lineshape alteration in absorption. The second pair shows that emission from upper levels with slow radiative decay rates can be selectively quenched by collision induced energy transfer. The opposite effect, selective collisional enhancement of emission from perturbed, longer-lived levels, is well known in CN B2 +—X2 +(u = 0,v") emission spectra (see Fig. 6.14 and Section 6.5.5). (a) the CO B1S+—X1S+(1,0) band in emission (top) and absorption (bottom). The last strong lines in emission are 11(16) and P(18). Emission from levels with J > 17 is weak because the predissociation rate is larger than the spontaneous emission rate. (Courtesy F. Launay and J. Y. Roncin.) (6) The CO A ll—X1 + (0,0) band in emission (bottom) and absorption (top). The a 3 + —X1 +(8,0) band lines appear in absorption because the A1 FI a 3 + spin-orbit interaction causes a small amount of A1 character to be admixed into the nominal a 3 + levels. These a —X lines are absent from the emission spectrum because collisional quenching and radiative decay into a3II compete more effectively with radiative decay into X1 + from the long-lived a 3 + state than from the short-lived A1 state. In addition, collisions and radiative decay into a3II cause the P(31) extra line (E) (arising from a perturbation by d3A v = 4) to be weakened in emission relative to the main line (M). (Courtesy F. Launay, A. Le Floch, and J. Rostas.)...

Only the total lifetime r of a level can be measured, r is related to the rate of decrease of the number of molecules initially in a given level via both radiative and nonradiative routes. Let kr be the radiative rate constant (the probability per unit time that a molecule will leave the level as a result of emission of a quantum of light) and knr the predissociation rate (the dissociation probability per unit time). Recall that the pressure is assumed to be low enough that the rates are not affected by collisions. The number of molecules leaving the initial state during the time interval dt is given by... 

Energy disposal in the photodissociation of formaldehyde has been reported by Houston and Moore. ° They used monochromatic laser sources to excite near single vibronic levels in the and monitored the vUnational oi gy distribution in the CO produced following predissociation under collision-free conditions, using a CO laser probe. Their results, taken together with those of Yeung and Moore on the radiative and non-radiative decay out of the A" state, lead to the scheme... 

Predissociation of molecules in excited states plays a very important role in atmospheric processes. Details of the competition between the radiative and radiationless transitions for 02 and OH are described in Hess et al. The potential curves for H states of NO are shown in Figure 12 together with the important perturber state a n which causes predissociation via spin-orbit coupling. The calculated data for the predissociation rate k, the lifetime, and the line width F for the low vibrational levels of Fl and two rotational levels N are given in Table 2. The calculations predict the predissociation process even at the v = 0 level to be distinctly faster than the radiative transition (r = 70 ns calculated. 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

A relaxation process will occur when a compound state of the system with large amplitude of a sparse subsystem component evolves so that the continuum component grows with time. We then say that the dynamic component of this state s wave function decays with time. Familiar examples of such relaxation processes are the a decay of nuclei, the radiative decay of atoms, atomic and molecular autoionization processes, and molecular predissociation. In all these cases a compound state of the physical system decays into a true continuum or into a quasicontinuum, the choice of the description of the dissipative subsystem depending solely on what boundary conditions are applied at large distances from the atom or molecule. The general theory of quantum mechanics leads to the conclusion that there is a set of features common to all compound states of a wide class of systems. For example, the shapes of many resonances are nearly the same, and the rates of decay of many different kinds of metastable states are of the same functional form. 

The measured lifetime t can be expressed by the pure radiative lifetime and the rate of predissociation kp... 

If one adopts the correct point of view that the complete wave function of any state of a diatomic molecule has contributions from all other states of that molecule, one can understand that all degrees of perturbation and hence probabilities of crossover may be met in practice. If the perturbation by the repulsive or dissociating state is very small, the mean life of the excited molecule before dissociation may be sufficiently long to permit the absorption spectrum to be truly discrete. Dissociation may nevertheless occur before the mean radiative lifetime has been reached so that fluorescence will not be observed. Predissociation spectra may therefore show all gradations from continua through those with remnants of vibrational transitions to discrete spectra difficult to distinguish from those with no predissociation. In a certain sense photochemical data may contribute markedly to the interpretation of spectra. 

Bound electronic states exhibit a discrete spectrum of rovibrational eigenstates below the dissociation energy. The interaction between discrete levels of two bound electronic states may lead to perturbations in their rovibrational spectra and to nonradiative transitions between the two potentials. In the case of an intersystem crossing, this process is often followed by a radiative depletion. Above the dissociation energy and for unbound states, the energy is not quantized, that is, the spectrum is continuous. The coupling of a bound state to the vibrational continuum of another electronic state leads to predissociation. 

In an actual experiment, it is frequently not possible to work under conditions where there are no relaxation effects. The usual reason for this is that the intensity of the fluorescence becomes too weak to observe as the concentration of excited molecules is reduced. The lowest pressures which can be used are defined by a number of parameters the strength of the transition, the power of the laser and the detection efficiency of the system are among the most important. It therefore follows that, in interpreting the results of lifetime measurements, one must consider carefully the possible effects of rotational and vibrational redistribution in the excited state. In a regular unperturbed state where there is little or no change in radiative lifetime with changes in rotational and vibrational level, the effects of relaxation are not observable so long as the fluorescence is still detected with the same efficiency. However, if the excited state is perturbed, for example by predissociation, then the effects of redistribution must be carefully studied. 

To take one example, let us consider the effects of rotational relaxation in BrF. The excited 53FI(0+) state in BrF is crossed by another 0+ state which leads to predissociation of the B state in vibrational levels 7 and 6. The initial study of the dynamics of the B state was carried out in a discharge flow system where the minimum operating pressure was 50 m Torr. The gas-kinetic collision rate coefficient at 298 K for He + BrF(B) collisions is 4.4 x 10-10 cm3 molecule-1 s-1. Thus, at the minimum pressure of 50 m Torr, the average time between collisions of excited BrF molecules and helium buffer gas is 1.5/us. This time is short compared with the radiative lifetime of BrF (42—56/ns [43]) and therefore significant redistribution in the excited state can occur before it radiates. 

Figure I. Schematic representation of the multiphoton process for studying the selective photolysis of Oj. Typical molecular potentials are plotted as functions of internuclear separation with radiative transitions shown by arrows. The first transition causing photolysis is designated la for predissociation or lb for direct dissociation. The second transition used to excite the dissociation products, Cs, to the readily ionized state, Cs is 2 and 2 shows a possible transition for fluorescent...

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