On predissociation

Spectroscopic evidence depends largely on predissociations and is difficult to interpret. It has been discussed at length by Gaydon and by Herzbergand from it four values of Z)(CO) can be obtained ITll, 9 85, 9-61, and 9 14 eV, leading to... 

If the dissociation process is not direct (see the section on predissociation below) and the molecule has time to rotate (a few picoseconds), then the angular distribution will be smeared out (i.e. it becomes isotropic) and the jS value will be zero. Intermediate values of jS can also arise from a breakdown of the axial recoil approximation (i.e. when the tangential velocity of the rotating parent molecule is greater than the recoil velocity), which can occur close to the threshold for dissociation, or when mixed parallel/ perpendicular transitions are encountered. 

Early theoretical work on predissociation employed a vibration-to-translation (V-T) mechanism, which predicted 8 x 105 ns,208 which is clearly inconsistent with Table V. In later work, Ewing introduced the rotational channels of the HF fragments with a vibration-to-translation-and-rotation (V-T,R) mechanism. A first-order perturbation theory treatment involving decoupled channels, which is not particularly reliable, showed an increase of only one order of magnitude in the predissociation rate with inclusion of rotation 209 in fact, however, x was now calculated209,210 to be 2 x ns due to other changes in... 

Sloane C S and Hase W L 1977 On the dynamics of state selected unimolecular reactions chloroacetylene dissociation and predissociation J. Chem. Phys. 66 1523-33... 

Rice O K 1971 On the relation between unimolecular reaction and predissociation J. Cham. Phys. 55 439-46... 

One of the early examples for kinetic studies on the femtosecond time scale is the photochemical predissociation of Nal [74] ... 

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]. 

The preceding discussion was limited mostly to VP processes occurring by direct coupling of the quasibound state of the complex to the dissociative continuum, which is the simplest and most commonly observed decay route for the complexes. However, these systems also serve as ideal venues for studying an array of more complicated dynamical processes, including IVR, and electronic predissociation. This brief section will focus on the former, underscoring some of the inherent dynamical differences between Rg XY complexes by discussing the IVR behavior of a few systems. 

The lowest adiabatic state is completely dissociative. The second and third are bound, but an efficient predissociation of the associated vibronic states can be predicted, on the basis of the strong couplings and small energy gaps in the region of the minima. 

Hydroxyl radical (OH) is a key reactive intermediate in combustion and atmospheric chemistry, and it also serves as a prototypic open-shell diatomic system for investigating photodissociation involving multiple potential energy curves and nonadiabatic interactions. Previous theoretical and experimental studies have focused on electronic structures and spectroscopy of OH, especially the A2T,+-X2n band system and the predissociation of rovibrational levels of the M2S+ state,84-93 while there was no experimental work on the photodissociation dynamics to characterize the atomic products. The M2S+ state [asymptotically correlating with the excited-state products 0(1 D) + H(2S)] crosses with three repulsive states [4>J, 2E-, and 4n, correlating with the ground-state fragments 0(3Pj) + H(2S)[ in... 

Two groups have studied the pyrolysis of CF2HC1 and have calculated AHj-o (CF2) to be-43 and-39.1 kcal. mole 1, respectively 34 Shock waves were also used to study the formation of CF2 from C2F4 and CHF3 values of AHj° (CF2) =-.39.7 3.0 and-40.2 4.0 kcal. mole respectively 18>1were obtained. Other methods that have been used to determine A Hf (CF2) include the pyrolysis of CF4 on graphite 35 and the observation of predissociation in ultraviolet absorption spectra 36 ... 

Warneck et al.38S have presented evidence that absorption in the 1800-2400 A range consists of discrete structure superimposed on a continuum which they attribute to the dissociation to ground state SO and O. The overlying bands are predissociated at X < 1900 A, but the nature and states of the resulting fragments are not established. On the basis of their measured quantum yield, relative importance of the possible primary processes. There is as yet, however, insufficient information for the evaluation of quantitative rate data for the photolysis in this region. 

Budde et have recently observed the ultraviolet laser-induced desorption of NO from oxidized Ni(lOO). The 193 nm excitation wavelength used was resonant with gas phase NO transitions to a predissociative upper state. Desorption yields of NO from clean Ni(lOO) were essentially zero. Comparison of TPD results from clean and oxidized nickel surfaces indicated that an oxidized nickel surface could support a weakly bound NO state not found on clean Ni(100). 

The electronic predissociation from different rovibrational levels of the A and B electronic states has been evaluated, to study the effect of the initial excitation on the process. The bounds states chosen are 1,2,3 and 6 for A, and fc=l,2,3,6 and 12 for B, which correspond to the bending progression of states appearing in the... 

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