J. Phys. Chem. 1993,97, 4326-4330
4326
Photolysis of Gas-Phase NFC12: Ultraviolet Emission from Clz and ClF Deborah B. Exton, Sarah A. Williams, and Julanna V . Gilbert’ Department of Chemistry, University of Denver, Denver, Colorado 80208 Received: November 2, 1992; In Final Form: January 18, 1993
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Gas-phase mixtures of NFCl2 in argon were photolyzed at 249 and 193 nm with an excimer laser. Photolysis at 249 nm generated emission in the UV, consisting of two bands at 258 and 284 nm, assigned to the D’ A’ transitions in Cl2 and ClF, respectively. Visible emission was also observed, and potential candidates for this emission are discussed. Photolysis at 193 nm generated emission assigned to the ClF(D’ A’ ) transition and emission assigned to transitions in Cl2 originating from electronic states that lie above the D’ state. For both photolysis wavelengths, the emitting species were not products of secondary reactions.
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Introduction The halogen amines are an interesting group of compounds which have the ability to store energy. Studies carried out in our laboratory have been directed toward examining the pathways by which this energy can be released and toward determining the amount of energy stored in these compounds via photolysis experiments. We have found, for example, that excited metastable states of the halogen nitrenes (NX alA, blZ+) are produced efficiently and rapidly when the parent amine is reacted with hydrogen atoms.Iv2 The UV absorption spectra of the halogen amines are characterized by broad structureless features, indicative of dissociative state^.^,^ If the fragments associated with these dissociative states can be identified, then, from energy balance considerations, the energy content of the parent amine can be obtained. Photolysis studies of NC13in low-temperature argon matrices4 and in gas-phase mixtures with helium5 have been carried out. From the FTIR spectra obtained before and after photolysis, it was apparent that NCl (and presumably Cl2 or two C1 atoms) was generated in the low-temperature matrix photolysis experiments. In the gas-phase studies, visible emission was observed, but since the source of the emission could not be correlated with NCl or C12 (or any other known species), it appeared that the matrix photoproducts had been the result of secondary reactions involving initial photoproducts (e.g., NCl2 and a C1 atom) and that the visible emission in the gas-phase studies might be from NC12. Photolysisstudies of NFCl2 in low-temperatureargon matrices generated NF6 and in this respect were analogous to the NC13 matrix photolysis studies. This paper reports the results of experiments carried out to further probe the photolytic processes of NFC12. When gas-phase mixtures of NFCl2 in argon were photolyzed at 193and 249 nm with an excimer laser, UV emission assigned to Clz and ClF was observed. Visible emission was also observed in the 249-nm experiments; however, its source has not been identified. A mechanism for the generation of the excitedstate diatomic halogens is presented, and the implications of these results are discussed. Experimental Section NFCl2 is readily synthesized following the method reported in ref 4 and purified following the modifications discussed in ref 2. After purifying the NFC12, bulbs containing from 1% to 10% mixtures of NFCl2 in argon were prepared. The gas samples were flowed through a stainless steel photolysiscell (20.3 cm long and 2.54 cm in diameter), and the total pressure in the cell was measured with a capacitance manometer. The photolysislight source was a Questek Model 2 110excimer laser, and experiments were carried out at both 249 (KrF) and 0022-3654193/2097-4326%04.0O/0
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Figure 1. (A) UV emission and (B) visible emission observed following pulsed photolysis of NFC12 in argon at 249 nm. 8% mixture of NFCl2 in argon, Ptot,l= 6 Torr. * is the tail of the 249-nm photolysis pulse. 193 nm (ArF). The laser beam was directed axially through the quartz entrance and exit windows of the photolysis cell. The diameter of the laser beam was reduced to 1 cm by light baffles in the photolysiscell. Emission resulting from the photolysiswas viewed at right angles to the axis of the cell through a 2.5-cm quartz window and was dispersed with a 0.25-m Jarrel-Ash monochromator equipped with 500-pm slits for the 249-nm photolysisstudiesand with 250-pm slits for the 193-nmphotolysis studies. For observation of visible emission, a glass filter was placed between the cell and the monochromator to block secondorder UV emission. Both UV and visible emissions were detected with a cooled GaAs photomultiplier tube (Hamamatsu R943-02). Response from the PMT was amplified by a factor of 5, and sent to a gated boxcar integrator (Stanford Research Systems), interfaced to a 80286 microcomputer. Thedata collection system was triggered externally utilizing an RCA 3 1034 PMT which detected the laser 0 1993 American Chemical Societv
Photolysis of Gas-Phase NFC12
The Journal of Physical Chemistry, Vol. 97, No. 17. 1993 4327 /
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Figure 2. Emission intensity at (A) 284 and (B) 258 nm as a function of laser fluence for photolysis of NFClz at 249 nm. The straight lines are linear least-squares fits to the data.
flash. Clz was sometimes present in the NFCl2 samples;however, the emission could not be ascribed to direct excitation of C12since it was not observed when mixtures of Cl2 in argon were photolyzed.
ReSultS Photdysisat249 nm. In the UV absorption spectrumof NFCl2, there is a structureless peak with a maximum at 275 nm which overlaps the wavelength of the KrF excimer laser at 249 nm. When gas mixtures of NFC12 in argon were photolyzed with the KrF excimer laser, both UV and visible emissions were observed. Spectra of the emission are shown in Figure 1, and features with maxima at 258,284, 365, and 428 nm are apparent. The time profiles, as viewed with a 200-MHz oscilloscope, were 20 ns wide (base width), indicating that the species responsible for the emission are initially formed photofragments and not due to secondary chemical reactions. For the features with sufficient intensity, the laser fluence dependence and the NFClz density dependence were measured. Figure 2 shows that the UV peaks at 258 and 284 nm were linear in the laser fluence, and Figure 3 shows that the UV peaks and the 428-nm peak were linear in the NFCl2 density. The energiesand the short lifetimes (
