Photodissociation states

It is clear that one of the major challenges in the experimental studies of free radicals is the preparation of radicals. The experimental designs (production of radicals and detection of radicals and photoproducts) are largely dependent on the particular radicals of interest. Nevertheless, many approaches have been taken, as seen in this review, to study the free radical photodissociation, and a great number of systems have been examined during the last couple of years. The sophistication in the experimental studies of free radical photochemistry has reached the level that has been available for the stable molecules. State-to-state photodissociation dynamics of free radicals have been demonstrated for a few small systems. Many more advances in the field of photodissociation dynamics of radicals are expected, and it is hoped that a more systematic and sophisticated understanding of free radical photochemistry can be developed. 

In particular, Shapiro and others calculated state-to-state photodissociation cross sections from vibrationally excited states of HCN and DCN [58], N2O [59], and O3 [60]. Eor instance, the detailed product-vibrational state distributions and absorption spectra of HCN(DCN) were compared [58]. These results were obtained employing a half-collision approximation, where the photodissociation could be depicted as consisting of two steps, that is, absorption of the photon and the dissociation, as well as an exact numerical integration of the coupled equations. In particular, it was predicted that large isotope effects can be obtained in certain regions of the spectrum by photodissociation of vibrationally excited molecules. 

The terms mode-selective and bond-selective dissociation refer to the control of the dissociation products in VMP. The terms are usually used as synonyms although, strictly speaking, the former should refer to selective preexcitation of a vibrational mode and the latter to the resulting selective bond cleavage. Control of the dissociation products in VMP has been extensively reviewed [28-31] and our discussion will focus on molecules studied (or continued to be smdied) after the latest comprehensive review was published [31], An exception will be a short overview on the VMP of water isotopologues since it was the extensive theoretical and experimental investigations of these molecules, in particular H2O and HOD, that opened a new era of detailed smdies of state-to-state photodissociation out of specific rovibrationally excited states of polyatomic molecules. 

Burak, J. W. Hepburn, N. Sivakumar, G. E. Hall, G. Chawla, and P. L. Houston. State-to-state photodissociation dynamics of trans-glyoxal. J. Chem. Phys., 86(3) 1258-1268 (1987). 

Besides its practical importance, photodissociation — especially of small polyatomic molecules — provides an ideal opportunity for the study of molecular dynamics on a detailed state-to-state level. We associate with molecular dynamics processes such as energy transfer between the various molecular modes, the breaking of chemical bonds and the creation of new ones, transitions between different electronic states etc. One goal of modern physical chemistry is the microscopical understanding of molecular reactivity beyond purely kinetic descriptions (Levine and Bernstein 1987). Because the initial conditions can be well defined (absorption of a single monochromatic photon, preparation of the parent molecule in selected quantum states), photodissociation is ideally suited to address questions which are unprecedented in chemistry. The last decade has witnessed an explosion of new experimental techniques which nowadays makes it possible to tackle questions which before were beyond any practical realization (Ashfold and Baggott 1987). 

Dayton, D.C., Jucks, K.W. and Miller, R.E. (1989). Photofragmentation angular distributions for HF dimer Scalar J-J correlations in state-to-state photodissociation, J. Chem. Phys. 90, 2631-2638. 

Guo, H. and Murrell, J.N. (1988a). A classical trajectory study of the A-state photodissociation of the water molecule, J. Chem. Soc., Faraday Trans. 2 84, 949-959. 

Hausler, D., Andresen, P., and Schinke, R. (1987). State to state photodissociation of H2O in the first absorption band, J. Chem. Phys. 87, 3949-3965. 

Shan, J.H., Vorsa, V., Wategaonkar, S.J., and Vasudev, R. (1989). Influence of intramolecular vibrational dynamics on state-to-state photodissociation Trans DONO (A) versus HONO (A), J. Chem. Phys. 90, 5493-5500. 

As already stated, photodissociation of XeF6 is a convenient route to clean XeF4, and photodecomposition is also an effective tool elsewhere. A facile and high yield (99%) preparation of S2Fio is achieved via photodecomposition of SF5Br [264], in which the S-Br bond is easily cleaved by light from a halogen lamp ... 

Oudejans, L. and Miller, R. E., State-to-state photodissociation of oriented HF HCl complexes Isotopic and isomeric effects, J. Phys. Chem. 99, 13670-13679 (1995). 

These experiments gave very different results than those for photodissociation of water in the A state. Photodissociation via the B state is an example of predissociation and the observed internal energy distribution of the OH is found to peak near 20kcal mol-1 of rotational energy [22], Ab initio studies have found that the bound B state possesses a deep potential well associated with a linear HOH minimum [23-26], Excitation from the well-known bent ground state to the linear B state imparts large amounts of bending excitation into the electronically excited water molecule. The... 

The photodissociation study of radicals, especially polyatomic radicals, remains essentially an unexplored research area. Detailed state-to-state photodissociation cross sections for radicals in the UV and VUV provide challenges not only for dynamical calculations, but also for ab initio quantum chemical studies. The measurements of absolute UV and VUV photodissociation cross sections of organosulfur radicals are relevant to the modeling of atmospheric sulfur chemistry. 

State-to-state photodissociation cross sections for molecules are expected to depend on their internal energy distributions. The photodissociation cross sections for CH3S and HS, estimated in the photodissociation studies of H2S, CH3SH, and CH3SCH3, represent cross sections averaged over the... 

If AB is a metastable electronic state, photodissociation is delayed ratha- than direct, and the path is predissociative. Step (S) may thus represent vibrational (or rotational) predissociation from a prepared initial state, or it may take place through radiationless transfer into a repulsive electronic state. 

The experimental idea for the study of state to state photodissociation is described in the figure below. 

Although the formation of products in different states is often analysed almost completely, only very few experiments yield state to state photodissociation cross sections. Usually photodissociation experiments are done with a gas at room temperature, where many rotational states are populated in the parent molecule. In this case product formation originates from many different initially populated rotational states of the parent molecule. Even in experiments, in which jet cooling is used to prepare low rotational states, usually more than one state is populated, in particular, if nuclear spins are important, like in H2 or H2O. 

To understand the features that are described in the Franck-Condon-limit, we introduce the definition of the state to state photodissociation cross sections ... 

This is a special solution for the full scattering problem in the trivial case where the asymptotic scattering wavefunction is not changed at all by the interaction potential, i.e., for the case of zero final state interaction. This implies that the full scattering solution can be replaced by the asymptotic wavefunction f>. This yields the state to state photodissociation cross section in the FC-limit ... 

The breakdown can be seen by going back to Fig.5, where the results for the photodissociation of the Oqq state have been compared with those for the photodissociation of jet cooled water. Because the predictions of Eq.24 agree almost quantitatively with the data for jet cooled H2O (compare Fig.3), Fig.5 yields also the comparison of the predictions of the one electron overlap model with the state to state photodissociation from the Oqo state. Obviously there is bad agreement between experiment and the predictions of this one electron overlap model on the state to state level. This is true for other... 

The photochemistry of O3 has attracted widespread interest primarily due to its importance to atmospheric chemistry (Sato, 2001 Matsumi and Kawasaki, 2002 and references cited therein). O3 is bent(l 16.8°) in its ground electronic state and absorbs weakly in the visible and near-IR regions (this accounts for the deep blue/violet colour of O3 in condensed phases). The visible absorption systems are predissociated due to the low dissociation energy of O3, Dq(0 — O2) = 101 2 kJ mol, and the presence of numerous low-lying repulsive electronic states. Photodissociation in the near-IR and visible (the so-called Chappuis band) regions leads to ground electronic state products, i.e. 

As pointed out in our previous paper on the state-to-state photodissociation of HF dimer , photofragment angular distributions can be used in favorable cases to determine the final internal state distribution of the fragments. The kinematics also can be used to determine the correlations between the rotational states of the two fragments. That is to say the relative rates can be determined for... 

The sodium trimer excited to the electronic C state can be regarded as a fascinating model system, which manifests ultrafast predissociation dynamics. While stationary and nanosecond-pump probe spectroscopy gave the first hints that this excited state photodissociates rather fast, real-time TPI spectroscopy opens a window to directly observe these ultrafast processes. But let us first start with a short review of the spectroscopy of this excited electronic state. 

Lai W, Lin SY, Xie D, Guo H (2008) Full-dimensional quantum dyntunics of A-state photodissociation of ammonia. Absorption spectra. J Chem Phys 129 154311... 

D. C. Dayton, K. W. Jucks, and R. E. Miller,/. Chem. Phys., 90,2631 (1989). Photofragment Angular Distributions for HF Dimer. Scalar J-J Correlations in State-to-State Photodissociation. 

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