Photodissociation rotational state distribution

Nitrosobenzene was studied by NMR and UV absorption spectra at low temperature146. Nitrosobenzene crystallizes as its dimer in the cis- and fraws-azodioxy forms, but in dilute solution at room temperature it exists only in the monomeric form. At low temperature (—60 °C), the dilute solutions of the dimers could be obtained because the thermal equilibrium favours the dimer. The only photochemistry observed at < — 60 °C is a very efficient photodissociation of dimer to monomer, that takes place with a quantum yield close to unity even at —170 °C. The rotational state distribution of NO produced by dissociation of nitrosobenzene at 225-nm excitation was studied by resonance-enhanced multiphoton ionization. The possible coupling between the parent bending vibration and the fragment rotation was explored. 

The final rotational state distributions of the products in the fragmentation of a polyatomic molecule contain additional clues about the intra- and intermolecular dynamics, especially about the coupling in the exit channel. In realistic as well as model studies it has been observed that the rotational state distributions of the photodissociation products reflect the angular dependence of the wave function at the transition state and the anisotropy of the PES in the exit channel [4, 9, 10]. HO2 is no exception. 

Fig. 1.8. Rotational state distributions of OH(2II) and OH(2E) following the photodissociation of H2O via the A state (A = 157 nm Andresen, Ondrey, Titze, and Rothe 1984) and via the B state (A = 121.6 nm Carrington 1964), respectively. The heavy arrow marks the highest rotational state of OH(2E) which can be populated at 121.6 nm.

Fig. 5.6. Comparison of the quantum mechanical and the classical rotational state distribution of NO following the photodissociation of C1NO in the S absorption band. Reproduced from Schinke et al. (1990).

Fig. 6.8. Measured rotational state distributions of NO(X2II) produced in the photodissociation of ClNO(5i). The various curves represent the distributions for the four different electronic sublevels of NO. The fitted lines are drawn as guidelines. Reproduced from Ticktin et al. (1988).

A much clearer picture evolves when one decomposes the total spectrum into the partial photodissociation cross sections a(, n,j) for absorbing a photon with wavelength A and producing NO in a particular vibrational-rotational state with quantum numbers (n,j). Experimentally this is accomplished by measuring so-called photofragment yield spectra. The idea is, in principle, simple the NO product is probed by laser-induced fluorescence (LIF). However, instead of scanning the wavelength Alif of the probe laser (in order to determine the final rotational state distribution) one fixes Alif to a particular transition NO(2n, nj) —>... 

Rotational excitation as a consequence of overall rotation of the parent molecule before the photon is absorbed does not reveal much dynamical information about the fragmentation process. It generally increases with the magnitude of the total angular momentum J and thus increases with the temperature of the molecular sample. In order to minimize the thermal effect and to isolate the dynamical aspects of photodissociation, experiments are preferably performed in a supersonic molecular beam whose rotational temperature is less than 50 K or so. Broadening of final rotational state distributions as a result of initial rotation of the parent molecule will be discussed at the end of this chapter. 

Fig. 10.6. Time-dependent rotational state distribution, defined in Equation (10.10), in the photodissociation of C1NO through the Si state. The times are given in femtoseconds. Adapted from Untch, Weide, and Schinke (1991b).

Fig. 10.10. Final rotational state distribution of the OH products following the photodissociation of H2O2 at 193 nm for three initial temperatures, 7h2o2 = 0 (beam, lower curve), 150 K (middle curve), and 300 K (bulk, upper curve). The experimental data (open circles) for dissociation in the beam and in the bulk are taken from Grunewald, Gericke, and Comes (1988) and Jacobs, Wahl, Weller, and Wolfrum (1987), respectively. The theoretical results (filled circles) for the dissociation in the A and in the B states are averaged according to a ratio of 3 1. Adapted from Schinke (1988c).

Fig. 10.18. Rotational state distributions of CN(X2E+) (upper part) and NO(X2IIi/2) respectively NO(X2n3/2) (lower part) following the photodissociation of NCNO at three wavelengths. The middle part depicts the absorption spectrum. Comparison of experimental data (open circles) with the predictions of the statistical theory (filled circles). Reproduced from Qian et al. (1985).

Fig. 10.20. Rotational state distribution of CN following the photodissociation of C1CN at 191.5 nm. Comparison between exact close-coupling calculations using the full ab initio PES of Waite and Dunlap (1986) (solid curve) and the impulsive model (IM, dashed curve). Adapted from Schinke (1990).

Fig. 10.23. Rotational state distributions of NO following the photodissociation of CH3ONO through the Si state in the molecular beam and in the 300 K bulk. Adapted from Bruhlmann and Huber (private communication).

Figure 11.2 illustrates the scheme of rotational energies for H2O in the electronic ground state and J = 4. In Section 11.3 we will elucidate the photodissociation of single rotational states and the resulting final rotational state distributions of the OH fragment. For this purpose it is important to stress that ... 

Up to now we have exclusively considered the scalar properties of the photodissociation products, namely the vibrational and rotational state distributions of diatomic fragments, i.e., the energy that goes into the various degrees of freedom. Although the complete analysis of final state distributions reveals a lot of information about the bond breaking and the forces in the exit channel, it does not completely specify the dissociation process. Photodissociation is by its very nature an anisotropic process — the polarization of the electric field Eo defines a unique direction relative to which all vectors describing both the parent molecule and the products can be measured. These are ... 

Kiihl, K. and Schinke, R. (1989). Time-dependent rotational state distributions in direct photodissociation, Chem. Phys. Lett. 158, 81-86. 

Schinke, R. (1988b). Rotational state distributions following direct photodissociation of triatomic molecules Test of classical models, J. Phys. Chem. 92, 3195-3201. 

Schinke, R. (1990). Rotational state distributions following the photodissociation of Cl-CN Comparison of classical and quantum mechanical calculations, J. Chem. Phys. 92, 2397-2400. 

Spiglanin, T.A. and Chandler, D.W. (1987). Rotational state distributions of NH(a1A) from HNCO photodissociation, J. Chem. Phys. 87, 1577-1581. 

Waite, B.A. and Dunlap, B.I. (1986). The photodissociation of C1CN A theoretical determination of the rotational state distribution of the CN product, J. Chem. Phys. 84, 1391-1396. 

NCCN Photolysis. Cyanogen (NCCN) photodissociation has also been considered a prototypical statistical dissociation reaction. After absorption to the 11,, 1 Au manifolds, the dissociation is thought to proceed via internal conversion to the ground-state surface. Previous studies have measured the rotational state distribution of the CN fragments from photolysis of cyanogen at 193.3 nm [75-77], The threshold photon energy for CN formation was found to be 47,000 200 cm -1 [78]. 

An impressive example of large rotational excitation is the photodissociation of water in the second absorption band (the B-state), where OH rotational states are populated up to N=45. In contrast, in the first absorption band of the same molecule, very little rotational excitation is found in the OH product, indicating an extremely small anisotropy in the excited state potential surface. This demonstrates, that the rotational state distribution in the products is very sensitive to the featurels of the excited state potential surface, in this case to its anisotrbpy with respect to Y. The large difference in the rotational distributions for the same molecule demonstrates also that dynamics and not kinematical constraints are responsible for this effect. 

Figure 11 Comparison of the theoretical and experimental rotational state distribution for the photodissociation of H2O from the Oqo state using Eq.20...

Fig. 16. The total rotational state distribution of OH(X n) in vibrational state i = 0 from the photodissociation at 121.6 nm. Reprinted, with permission of the American Institute of Physics, from Ref. 84.

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