Nitrogen recombination, emission from

The decay of first positive emission from a pulsed nitrogen afterglow has been observed over an intensity range of almost 106 and it has been postulated in this connection that recombination into the B state involves N atoms complexed in some sense with impurity molecules168. 

To study the photolysis of azo compounds, CIDNP was only recently introduced in the field of photochemistry. The CIDNP-effect consists of generating a geminate radical pair which still remembers the spin state of its precursor. So the multiplicity of the precursor can be determined from enhanced absorption or emission signals in azoalkane photolysis. The benzophenone sensitized photolysis of dia-zirine in deuteriochloroform leads to the triplet azo compound 24 which decomposes under elimination of a ground state nitrogen molecule and a triplet methylene 38>. This abstracts deuterium from deuteriochloroform to form the geminate radical pair 25. This can now recombine to give 26 or dissociate to afford the free radical products. 

Further downstream the second emission region ascribed to the decay of these excited nitrogen molecules is noted. Whereas other studies have shown N2(v4 E ) to arise from recombination or processes in the discharge itself, this system gives rise to these excited species by means of a sequence of bimolecular reactions. 

Campbell and Thrush have also investigated the decomposition of H2O, N2O, and CO2 by active nitrogen produced in a discharge flow system. Reaction of these three species was second-order in the nitrogen atom concentration and the rate was also shown to be dependent on the total pressure and not the pressure of H2O, N2O, or CO2 alone. This implies that the active species in the decomposition of these molecules is an excited state of N2 formed by the recombination of nitrogen atoms and furthermore this recombination step is rate-controlling. From the kinetic data (emission intensities) Campbell and Thrush extract the overall three-body recombination rate coefficients for M = CO2, N2O, and H2O at 196 °K, viz. 

Demonstration of the photorelease has been done in particular with Sr + [46]. This process was monitored on several time scales providing evidence for (1) the delayed formation in 9 ps of the charge transfer state of the merocyanine chromophore following ultrafast photodisruption of the nitrogen - cation interaction, (2) the cation movement away from the excited chromophore into the bulk in 400 ps, (3) recombination of the complex in the ground in about 120 ns. These three steps are respectively illustrated in Fig. 7.17a, b, c (see caption for details). Similar transient absorption studies have been carried out on a PDS-crown-Ca + complex, where PDS is an aza-crown derivative of a substituted stilbene [47]. The spectrodynamics observed on the short time scale are very similar to those found in step (1) of the above description, with in particular a delayed rise of a stimulated emission band attributed to a solvent-separated cation-probe pair. Although the full scenario of the cation photoejection from the DCM-crown-Sr, is complex [46], the spectra shown in Fig. 7.17 demonstrate that at least part of the photoexcited complexes does eject the ion into the bulk. 

Figure 14 shows the PL spectrum for Al-doped 3C-SiC around 2.2 eV at 6 K (63). Choyke (59) attributed the sharp peaks observed in the figure to nitrogen donor-A1 acceptor pair recombinations and obtained Eo + Ef, = 310 meV from Eg = 2.4034 meV and E = 2.0934 eV. From the measurements at higher temperatures, an emission peak due to the recombination of a free... 

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