Rotational predissociation

I.r. study of hetero-dimers formed from HCl, DCl, HF, and DF with (CH3)20, CH3OH, and (CH3)3COH Vibrational predissociation in HF.O(CH3)2 t I.r. absorption spectra of (H20)2 measured in the region of the O-H stretch by monitoring predissociation Rotational spectra and constants for HjO.HCN t Two-photon MPI/m.s. study of NHj clusters t VUV photoionization/m.s. study. Observation of unprotonated cluster ions (NHjlJ and (NHj) in addition to (NH3)2 and... 

The vibrational bands are diffuse, the diffuseness increasing with vibrational quantum number, again indicating strong predissociation Rotational struc-... 

Pine A S, Lafferty W J and Howard B J 1984 Vibrational predissociation, tunneling, and rotational saturation in the HF and DF dimers J. Chem. Phys. 81 2939-50... 

Unfortunately, predissociation of the excited-state limits the resolution of our photodissociation spectrum of FeO. One way to overcome this limitation is by resonance enhanced photodissociation. Molecules are electronically excited to a state that lies below the dissociation limit, and photodissociate after absorption of a second photon. Brucat and co-workers have used this technique to obtain a rotationally resolved spectrum of CoO from which they derived rotational... 

Rotational Predissociation above the Rg -I- XYfBV) Dissociation Limit... 

Figure 12. Potential energy contour plots for He + I Cl(B,v = 3) and the corresponding probability densities for the n = 0, 2, and 4 intermolecular vibrational levels, (a), (c), and (e) plotted as a function of intermolecular angle, 0 and distance, R. Modified with permission from Ref. 40. The I Cl(B,v = 2/) rotational product state distributions measured following excitation to n = 0, 2, and 4 within the He + I Cl(B,v = 3) potential are plotted as black squares in (b), (d), and (f), respectively. The populations are normalized so that their sum is unity. The l Cl(B,v = 2/) rotational product state distributions calculated by Gray and Wozny [101] for the vibrational predissociation of He I Cl(B,v = 3,n = 0,/ = 0) complexes are shown as open circles in panel (b). Modified with permission from Ref. [51].

Gray and Wozny [101, 102] later disclosed the role of quantum interference in the vibrational predissociation of He Cl2(B, v, n = 0) and Ne Cl2(B, v, = 0) using three-dimensional wave packet calculations. Their results revealed that the high / tail for the VP product distribution of Ne Cl2(B, v ) was consistent with the final-state interactions during predissociation of the complex, while the node at in the He Cl2(B, v )Av = — 1 rotational distribution could only be accounted for through interference effects. They also implemented this model in calculations of the VP from the T-shaped He I C1(B, v = 3, n = 0) intermolecular level forming He+ I C1(B, v = 2) products [101]. The calculated I C1(B, v = 2,/) product state distribution remarkably resembles the distribution obtained by our group, open circles in Fig. 12(b). 

Dimers. It is well known that H2 pairs form bound states which are called van der Waals molecules. The discussions above based on the isotropic interaction approximation have shown that for the (H2)2 dimer a single vibrational state, the ground state (n = 0), exists which has two rotational levels f = 0 and 1). If the van der Waals molecule rotates faster ( > 1), centrifugal forces tear the molecule apart so that bound states no longer exist. However, two prominent predissociating states exist which may be considered rotational dimer states in the continuum (/ = 2 and 3). The effect of the anisotropy of the interaction is to split these levels into a number of sublevels. 

Besides a transition to a continuum level of an excited electronic state, dissociation can occur by another mechanism in electronic absorption spectroscopy. If the potential-energy curve of an excited electronic state A that has a minimum in UA(R) happens to be intersected by the U(R) curve of an unstable excited state B with no minimum in U, then a vibrational level of A whose energy lies near the point of intersection of UA and UB has a substantial probability to make a radiationless transition to state B, which then dissociates. This phenomenon is called predissociation. Predissociation shortens the lifetimes of those vibrational levels of A that are involved, and therefore by the uncertainty principle gives broad vibrational bands with rotational fine structure washed out. 

D. M. Neumark We make no effort to produce vibrationally cold O2, since the B < — X transitions to predissociating upper state levels are rotationally resolved and completely understood. In the case of CH3O, we detach the CH3O- just above the detachment threshold so that we do not produce vibrationally excited CH3O. 

Ricks and Barrow (830) have obtained the predissociation limit from a rotational analysis of the emission and absorption bands of the B X3TU system of S2 vapor. The limit is at 35,999 2.5 cm corresponding to the products S(3P2) + S(3I ). The predissociating state (similar to R in Fig. II 5) is identified as the lu state. 

The fluorescence yield as a function of incident wavelength has been measured by Lee and Uselman (619). The yield starts to increase from 0 at 3979 A to nearly 100 % above 4150 A. The decrease of the yield below 4150 A is attributed to an increase of predissociation supplemented by the rotational energy of the molecule, since incident light of wavelengths above 3979 A does not have sufficient energy to dissociate the molecule at OK (550, 619). 

Ackerman and Biaume (37) have observed that rotational lines become diffuse at if = 4, 8, and 11 for the Schumann-Runge system. They attribute the difluseness to predissociation. 

In some molecules there is another, slower dissociation path known as predissociation. In this case the crossing to the dissociative state is the rate-limiting step, and this may take place after many vibrations in the absorption spectrum the vibrational sub-levels remain sharp, but the rotational levels are blurred (Figure 4.28). 

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