Rotational energy distribution

MSS Molecule surface scattering [159-161] Translational and rotational energy distribution of a scattered molecular beam Quantum mechanics of scattering processes... 

Mizutani et al. (710) have photolyzed HCN at 1849 A. They have found cyanogen and hydrogen as major products and methane, ammonia, ethane, hydrazine, and methylamine as minor products. Mele and Okabe (692) have found CN(/l2n) and CN(B2E) radicals when HCN was irradiated in the vacuum ultraviolet. The vibrational and rotational energy distributions of CN(B2 ) have been measured. 

Fukutani et al. [8] also observed NO desorption from on-top species at X = 193 nm. The results are very similar to those obtained by Buntin et al. at X = 532 and 355 nm. The decay of the desorption yield on this surface at X = 532 nm gives a desorption cross section of 1 x 10-22 cm2 [6]. A fit to the TOF spectrum by the non-Boltzmann form gives Tt = 910 K. Rotational energy distributions of... 

Figure 14 Rotational energy distribution for desorbed NO (v = 0) of on-top species from Pt(l 1 1) at X = 193 nm. Filled circle 2 = 1/2, open circle 2 = 3/2 [60],...

Tt denotes the mean translational temperature and is obtained from the TOF spectrum for NO (/ = 1/2-912) and CO. These values of Tt are different from the translational temperature obtained from the molecular velocity observing the rotational energy distribution, from which Tr and Fv are obtained, cr desorption cross section. 

It is also important that the influence of the desorption dynamics on the nuclear motion is discussed. Zimmermann and Ho [65] discuss rotational excitation in photodesorption using a simple impulse model. In particular, rotational energy distributions with two spin-orbit states of NO desorption of on-top species from the Pt( 111) surface and from the oxidized Ni(0 01) surface are analyzed. Furthermore, they also discuss velocity distributions [66] and rotational-translational correlations [67]. Murata and Fukutani [28, 68, 69] analyze the experimental data using a simple impulse model without any fitting parameter. The results are described in the present text in detail, and the procedure is quite different from that derived by Zimmermann and Ho [65]. 

The rotational energy distributions by the REMPI spectra of desorbed NO from the hep hollow species on Pt(l 1 1), which is induced by 2.3-6.4 e V laser irradiation, are represented by a Boltzmann distribution such as in Fig. 16a. It is impossible to understand that molecules desorbed by a non-thermal process reach thermal equilibrium in the rotational energy distribution during the very short residence time in the excited state for chemisorbed species on metal and semiconductor surfaces (lifetime = 10 16-10-14 s [62, 72, 73]). Besides, no rotational freedom exists in the chemisorbed state and the desorption is... 

The gradient A and the translational energy I. ], which is calculated from the molecular velocity observing the rotational energy distribution, are listed in Table 5. 

The rotational energy distribution of desorbed NO from on-top species on Pt(l 1 1) at A. = 192 nm is observed to have a non-Boltzmann form, as shown in Fig. 14. Furthermore, the population in the two spin-orbit states is substantially inverted, since the population ratio of 2 = 1/2 and 3/2 is 1 2.2 in low J region. For desorption of hep hollow species at = 193 nm, on the other hand, the population ratio... 

Figure 25 Boltzmann plots for the rotational energy distributions of the fast (filled circle and square) and slow (open circle and square) channels for NO (v = 0) desorption from NiO(0 01) at 100 K [64],...

NO desorption from the alloy surface saturated at 80 K is observed at X = 193 nm and is a singlephoton process. The rotational energy distributions in the Boltzmann plot are shown in Fig. 30 and satisfy an almost linear relation, the gradient of which gives Tt 350 K. The two spin-orbit states look... 

The rotational temperature obtained from a linear relation in the Boltzmann plot of the rotational energy distribution is an index of the lifetime in the intermediate excited state and decreases with decreasing lifetime. The rotational temperature of CO desorbed from Pt(l 1 1) is very low as compared with that of NO desorption, i.e. the lifetime of the excited CO is supposed to be much shorter than that of NO. In the case of CO desorption from Pt(l 11), however, the lifetime is not obtained from the rotational energy distribution, since desorbed molecules are detected by the (2 + 1 )REMPI method in the experiment [ 12] and then the single rotational states are not resolved. On the other hand, the rotational temperature of NO desorbed from Pt(l 1 1)-Ge surface alloy is lower than that from Pt(l 1 1). Then, it is speculated that the lifetime of the excited CO on the alloy is shorter than that on Pt( 111) and the residence time of the excited CO on the alloy is too short to be desorbed. As a consequence, the excited CO molecules are recaptured in the relaxation without desorption. However, it has not been understood why the lifetime of the excited CO molecule (or the excited CO-Pt complex) on Pt( 1 1 1) is shorter than that of the excited NO molecule (complex) on Pt(l 11), and further on the Pt-Ge alloy as compared with Pt(l 1 1). 

In order to obtain information about the energy distributions of reaction products, it is necessary to use a detection method that can determine either the internal state populations of the products or their recoil velocities. The methods employed to measure electronic, vibrational or rotational energy distributions are generally based on a form of emission or absorption spectroscopy, although there are other techniques that are sensitive to internal excitation. A variety of methods are used to measure recoil energy distributions these are commonly based on a mass spectrometric detection system used with some form of velocity analyser. 

A widely-used model in this class is the direct-interaction with product repulsion (DIPR) model [173—175], which assumes that a generalised force produces a known total impulse between B and C. The final translational energy of the products is determined by the initial orientation of BC, the repulsive energy released into BC and the form of the repulsive force as the products separate. This latter can be obtained from experiment or may be assumed to take some simple form such as an exponential decay with distance. Another method is to calculate this distribution from the quasi-diatomic reflection approximation often used for photodissociation [176]. This is called the DIPR—DIP model ( distributed as in photodissociation ) and has given good agreement for the product translational and rotational energy distributions from the reactions of alkali atoms with methyl iodide. 

Excluding some details on the rotational energy distribution of the products, a general agreement has been found between the classical and quantum results when performed on the same PES. It s important to notice that the final attribution... 

Fig- 4. Rotational energy distribution of dissociating hydrogen molecules under conditions of thermal equilibrium (from Wadsworth and Wysong ). 

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