J . Phys. Chem. 1986, 90, 1931-1934 to construct a plot of l / vs. ~
photochemistry ensues. Thus ketenimines are often good quenchers of excited states but only in the case of ketone sensit i z e r ~ does ~ ~ -any ~ ~photoreaction take place.
1931
[Q],giving the k, values in Table
I. Fluorescence quenching was carried out on a Hitachi-PerkinElmer MPF-2A in the usual manner, using known concentrations of 3 and 5 to diminish the fluorescence intensity of sensitizers in benzene. Quantum yields of ketenimine disappearance were determined by monitoring the characteristic IR band of these compounds at 2020 cm-I. Cyclic voltammetry experiments were conducted using a PAR Model 175 as described in the text.
Experimental Section Ketenimines 34and 527are known compounds. Flash photolysis experiments were carried out in degassed, purified benzene using a Lambda Physik EMG-101 laser to excite the sensitizers. The analyzing light from a quartz-halogen lamp was passed through a Bausch and Lomb monochromator and was detected by a 1P28 photomultiplier. The decay curves were collected and averaged by a LeCroy 20 MHz transient digitizer and a MIK 1 1/2 computer. First-order fits to the decay curves yielded sensitizer triplet lifetimes. Approximately five quencher concentrations were used
Acknowledgment. The authors thank Professor Lawrence A. Singer for helpful discussion and encouragement. Financial support was provided by the National Science Foundation and The Robert A. Welch Foundation.
Photolysis of FN, at 193 nm D. Patel,? A. T. Pritt, Jr., and D. J. Benard* Rockwell International Science Center, Thousand Oaks, California 91 360 (Received: September 20, 1985; In Final Form: December 5, 1985)
The photolysis of FW,by an ArF excimer laser was found to yield both excited NF(b) and N2(A) photofragments in approximate yields of 5% and 25%, respectively. The photodissociation cross section at 193 nm was determined to be 2.8 X lo-'' cm2 and the absorption spectrum was recorded from 190 to 450 nm. The rates of NF(b) and N,(A) quenching by FN3 and NO were determined to be 8.7 X lo5 and 1.1 X lo6 torr-' s-', respectively, and the heat of formation of FN3 was found to lie in the range of 120 to 135 kcal/mol.
Introduction During the past decade, a number of authors have investigated the photochemistry of gaseous azides. Baronavski' studied the photolysis of H N 3 at 266 nm and characterized the photofragment energy distributions. Piper2 investigated the formation of electronically excited states produced by photolysis of HN3 a t 290 nm. In both of these studies NH(alA) was the primary excited product of the photolysis reaction. Coombe et aL3 studied the photolysis of CIN3 at 193 and 249 nm and found both NCl(b'2) and N2(A32)reaction products, while Coombe and Lam4 found only N2(A) from the photolysis of BrN3 at 248 and 308 nm. Fluorine azide (FN,) was frist synthesized by Haller5 in 1942. Since then Milligan and Jacox6 have recorded the IR absorption spectra of FN, and have studied its decomposition by UV photolysis in an Ar matrix. Gipstein and Haller' later established optimum conditions for generating FN3 and were able to obtain a low-resolution relative absorption spectrum of FN3 from 378 to 444 nm. This paper extends the work of the previous authors by reporting a calibrated absorption spectrum of FN3from 190 to 450 nm, the yields of NF(b) and N,(A) produced by photolysis at 193 nm, the rates of quenching of the excited photofragments in the photolysis medium, and the approximate heat of formation of FN3. Experimental Section Experimenters are cautioned that FN3is highly explosive and unstable. Several spontaneous detonations ocurred during our early attempts to produce a useable flow of the material. Therefore, we carried out the FN3 generation process inside a I/&. lexan blast shield. The FN, was produced by reacting a stream of H N 3 entrained in N, carrier gas with a stream of 20% F2 in N 2 diluent in a 3/8 in. 0.d. X 12 ft long coiled copper tube. The H N 3 was generated by dropwise addition of 75% H2S04to NaN, crystals under an atmospheric pressure N 2 purge. A small 'Present address: Litton Guidance and Control Systems, Woodland Hills, CA 91367.
0022-3654/86/2090-193 1$01.50/0
fraction of the HN3/N2 stream was dried by passage over CaS04 prior to the addition of the F2/N2 stream. The effluent of the copper coil reactor was passed through a N a F trap to remove the H F byproduct and then through a Beckman IR or a Pye Unicam UV spectrophotometer prior to admission to the photolysis cell. The IR spectrometer was used to monitor the relative absorptions of FN3 and H N 3 at 2050 and 2150 cm-', respectively, while the UV spectrometer was used to measure the absolute concentration of FN,at 21 1 nm once all of the HN, was reacted by the F, flow. The pressures in the absorption and photolysis cells were monitored by capacitance manometers. The photolysis experiments were carried out at total pressures in the range of 10 to 200 torr. The FN3 was photolyzed by the output of an ArF excimer laser using a fixed aperture and a Gen-Tec power meter to define the laser fluence. Emissions from the photolysis cell were collected at right angles to the laser beam and were dispersed by a 0.25-m monochromator onto a cooled GaAs photomultiplier tube. The photocurrents were passed to a wide-band preamplifier and a transient digitizer while a boxcar integrator was used to average the output of the transient digitizer on successive shots of the excimer laser. A fraction of the pump beam was split off to a pyroelectric detector which was monitored by a second boxcar integrator. The intensity of the photofragment emission was normalized to the pulse energy of the excimer laser by a ratiometer which monitored the outputs of the two boxcar integrators. Spectral and temporal profiles were taken by repetitively firing the excimer laser with a slow flow of FN, through the photolysis (1) A. P. Baronavski, R. G. Miller, and J. R. McDonald, Chem. Phys., 30, 119 (1978) and reference therein. (2) L. G. Piper, R. H. Krech, and R. L. Taylor, J . Chem. Phys., 73, 791 (1980). ' ( 3 j R . D. Coombe, D. Patel, A. T. Pritt, Jr., and F. J. Wodarczyk, J . Chem. Phys., 75, 2177 (1981). (4) R. D. Coombe and C. H. T. Lam, J . Chem. Phys., 79, 3746 (19831, and references therein. ( 5 ) J. F. Haller, Ph.D. dissertation, Cornell University, Ithaca, NY, 1942. (6) D. E. Milligan and M. E. Jacox, J . Chem. Phys., 40, 2461 (1964). (7) E. Gipstein and J. F. Haller, Appl. Spectrmc., 20, 417 (1966).
0 1986 American Chemical Society
1932
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986
cell while scanning either the monochromator wavelength or the delay time of the boxcar integrator. The temporal resolution of our equipment was limited by noise to between 100 and 500 ns. Similar instrumentation was employed in our previous study of UV photodissociation of ClN,. Reference 3 contains a schematic diagram of the apparatus and a detailed account of the experimental methodology. Absolute calibration of the detector was obtained by photolyzing known concentrations of NH,, recording the resulting NH2(A-X) emission, and comparing the results to the photon yield data published by Donelly et aL8 These authors found rapid quenching of ",(A) by residual N H , which we also o b ~ e r v e d . ~To counteract this effect the initial NH3 concentration was kept below 100 mtorr and a series of time-resolved NH,(A+X) spectra were recorded with varied time delays following the laser pulse. The ",(A-X) spectra were extrapolated back to t = 0 at which point the calibration was anchored. The fraction of the NH, that was dissociated was determined by performing a saturation experiment in which the intensity of the NH2(A-X) emission was correlated with the A r F laser energy. The absolute calibration of the detector to the NH,(A-X) bands in the 600-900-nm region was extended to shorter wavelengths by correcting for the instrument response, which was determined from the manufacturers specifications on grating response and photomultiplier tube sensitivity. The major source of error in the detector calibration was the uncertainty factor of 2.5 quoted by Donelly et al. in their measurement of the ",(A-X) yield.
Results Spectral scans of the visible emission produced by ArF laser photolysis of FN, revealed both NF(b+X) emission a t 528 nm and N,(B+A) emission from 600 to 800 nm.lo The vibrational distribution of the NF(b) state was inferred from the relative intensities of the 0 0, 1 1 , and 2 2 transitions and the corresponding Franck-Condon factors." During the first 500 ns after the laser pulse, the vibrational distribution of the NF(b) state was found to be approximately Boltzmann at a temperature of 1200 K, down to pressures below 20 torr, where it became difficult to observe the NF(b-X) emission, due to declining concentrations of FN3. In this regime, the NF(b) experiences approximately ten gas kinetic collisions with the buffer gas during the measurement. Therefore, barring any highly efficient vibrational relaxation by the buffer gas, the observed distribution is approximately nascent. The ArF laser photolysis of ClN, under similar condition^,^ however, yields a highly nonthermal distribution of the NCl(b) state with significant population up to u' = 10. Prompt emissions from as high as v' = 8 in the N2(B) state were also readily observed from the photolysis of FN,. At low fluences the initial intensities of the NF(b-*X) and N2(B-A) emissions both scaled linearly with the laser power. As the laser power increased, the initial yields of both NF(b) and N2(B) saturated. Following the laser pulse, the NF(b) and N2(B) both decayed exponentially. The rate of NF(b) decay was faster than its (45 s-l) radiative rate'* and varied with the pressure of FN,. At low fluences and low FN3 concentrations the N,(B) decayed to l / e of its initial intensity at 8 ks consistent with its known radiative rate.I3 At high laser intensity and high FN3 concentrations, the N,(B) decay was slower than its radiative rate, indicating at least some production by secondary reactions. Upon adding N O to the photolysis cell NO(A-X) emission rose to its peak in less than 500 ns and decayed exponentially at a rate that
-
+
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(8) V. M. Donelly, A. P. Baronavski, and J. R. McDonald, Chem. Phys., 43. 271- (1979). -, - 1
\ - -
(9) V. M. Donelly, A. P. Baronavski, and J. R. McDonald, Chem. Phys., 43, 283 (1979). (10) B. Rosen, Spectroscopic Data Relative to Diatomic Molecules, Pergammon Press, New York, 1970. (1 1) K. A. Mohamed, B. N. Khana, and K. N. Lal, Znd. J . Pure Appl. Phys., 12, 243 (1974). (12) P. H. Tennyson, A. Fontijn, and M. A. A. Clyne, Chem. Phys., 62, 171 (1981). (13) M. Junnehomme and A. B. F. Duncan, J . Chem. Phys., 41, 1692 (1964).
Patel et al.
'6
15
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LASER FLUENCE (MJ/S(1 CM)
Figure 1. Saturation of NF(b) produced by photolysis of FN, at 193 nm.
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Alnrn)
Figure 2. Absorption spectrum of FN, from 190 to 450 nm.
increased in proportion to the N O concentration. To quantify these results further, the partial pressure of FN3 in the absorption cell was determined by recording a relative absorption spectrum of FN3 from 190 to 450 nm, calibrating the spectrum at 193 nm by a saturation experiment, and then using the absorption coefficient a t 21 1 nm to determine the absolute FN, concentration by UV absorption. The saturation experiment was performed by measuring the yield of NF(b) at early times to avoid quenching effects while varying the laser fluence. The results, shown in Figure 1, were fit to an equation of the form I = Zo(l - exp(-uE/hv)) (1) where I is the initial NF(b) intensity, Io is the limiting value of I at high laser fluence, u is the photodissociation cross section, E is the laser fluence, and hv is the photon energy. The best fit curve drawn in Figure 1 corresponds to u(193 nm) = 2.8 X cm2with an uncertainty of 2~30%based on the scatter of the data. Figure 2 shows the calibrated UV absorption spectrum of FN,. The typical FN3concentrations in the photolysis cell corresponded to approximately 0.1% of the total cell pressure. By raising the laser fluence to its maximum value to photolyze nearly all of the FN, in the cell, the yield of NF(b) was determined from its initial intensity to be approximately 5% from knowledge of the detector calibration, the radiative rate of the NF(b-X) transitions, and the FN3concentration in the photolysis cell. The accuracy of this determination is thought to be within a factor of four on the basis of accumulated errors in the measurement. The decay of the NF(b) following the laser pulse was measured at low fluences so that the FN3 concentration following the laser pulse approximated its initial concentration. Figure 3 shows a
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986 1933
Photolysis of FN, at 193 nm 12(
9.6 ,"8.4 2 7.2 -
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PARTIAL PRESSURE OF FN3 (rntorr)
Figure 3. Stern-Volmer plot of NF(b) decay rate vs.
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PARTIAL PRESSURE OF NO (rntorrl
Figure 4. Stern-Volmer plot of NO(A) decay rate vs. NO concentration.
Stern-Volmer plot of the NF(b) decay rate vs. the FN, concentration. The slope of the plot corresponds to quenching of NF(b) by FN3 at a rate of (8.7 f 3.0) X lo5 torr-' s-], the major uncertainty deriving from the determination of the absorption cross section used to measure the FN, concentration. The near zero intercept of the Stern-Volmer plot is consistent with the slow radiative decay of NF(b). The scatter of our data, however, is too large to permit any reliable check on the NF(b+X) transition probability. Figure 4 shows a Stern-Volmer plot of the rate of decay of the NO(A+X) emission vs. the concentration of N O added to the photolysis cell with a slope of (1.1 f 0.2) X lo6 torr-' s-], the major uncertainty deriving from the determination of the decay rates. The NO(A-X) emission is believed to derive from N2(A) photofragments by energy transfer. Since the NO(A-X) transitions are highly allowed, the rise of the N O emission is governed by the NO(A) radiative rate while the decay of the N O emission is governed by the rate of energy transfer from N2(A). The 178 f 19 ns radiative lifetime of NO(A) reported by ZareI4 was too short to be resolved by our equipment in this experiment. The rate of energy transfer from N2(A) to N O reported in the literature ranges from 0.5 X lo6 to 4.0 X IO6 torr-' s-I, possibly depending on the vibrational distribution of the N2(A) state. Our result, which lies in this range, is approximately a factor of three smaller than the latest measurement by Clark and Setser, who found no dependence on the vibrational distribution of the N2(A) state.15 The yield of N2(A) was determined to be 5.0 f 2.0 times the yield of NF(b) from the measured N2(A) to N O transfer rate, the NF(b) radiative decay rate, the N O concentration, and the relative initial intensities of the NF(b+X) and NO(A-X) bands. The N,(A) yield is therefore approximately 25% within a factor of four due primarily to the uncertainty of the NF(b) yield. The ratio of the N2(A) to NF(b) yield, however, is much more accurate since it requires only knowledge of the relative instrument response as a function of wavelength rather than absolute calibration of the detection system. The factors contributing to the uncertainty (14) E.M. Winestock, R. N. Zare, and L. A. Nelton, J . Chem. Phys., 56, 3456 (1972). (15) W. G.Clark and D. Setser, J . Phys. Chem., 84,2225 (1980). and references therein.
in the ratio of the N2(A) to NF(b) yields are uncertainty in the N2(A) to N O transfer rate (f15%), the NF(b) radiative rate (f7%) and the measurements of the intensity ratio (f13%), and the N O concentration (f5%). The yield of N2(B) was not determined because a significant portion of the N2(B-A) bands lied outside the range of our detector. The N2(B-+A)bands that were observed in the range of 600-800 nm were approximately 50 times as intense as the NF(b-X) emissions. Since the radiative rate of N,(B) is roughly three to four orders of magnitude larger than the radiative rate of NF(b) the yield of N2(B+A) emission that was observed was only a small fraction of 1%. The NF(a-X) emission at 875 nm was too weak to detect. Other weak signals included N2(C+B) emission at 337 and 358 nm and various N H bands from 300 to 350 nm. The N H bands undoubtedly were caused by the presence of trace H N , as an impurity in the photolysis cell. The possibility that the N2(B) was produced by photolysis of the HN, impurity can be ruled out on the basis of the 160-nm threshold for this reaction.I6
Discussion Because the FN-N2 bond in FN, is ~ e a k e s tphotolysis ,~ of FN, may be expected to yield N F and N 2 fragments. Both H N 3 and ClN, are thought to have singlet ground states.'^^ Presuming the ground state of FN, to also be a singlet state suggests that all singlet or all triplet state products, such as NF(a,b) + N2(X) or NF(X) + N,(A,B) should be formed. When NF(a,b) + N2(X) is formed upon photodissociation of FN3at 193 nm a large amount of electronic energy is converted into vibrational, rotational, and translational modes of excitation, which may account for the preferred formation of N2(A) + NF(X). Considering the low yields of NF(b) that were observed, one cannot discount the possibility of the nearly resonant formation of NF(b) + N2(A) in violation of spin conservation. Production of NF(a) in yields substantially greater than NF(b) may also have occurred in our experiments and escaped detection due to the extreme (0.18 s-') metastability" of the NF(a) state. If true, this result could account for the yields of the observed photolysis products adding to less than unity. Two factors which may account for the differing nascent vibrational disributions of the NF(b) and NCl(b) states are the tighter binding of the halogen atom6 in FN, and, depending on the heat of formation of FN,, the possibility of a nearly resonant exit channel (to NF(b) + N2(A)) that does not exist, in ClN,. The production of NF(b) + N2(A) in violation of spin conservation may also require a longer-lived excited FN, intermediate in which the excess vibrational energy is distributed prior to dissociation. On the other hand, the absence of structure in the UV absorption spectrum of FN, suggests that there are no bound excited states of FN, involved in the photodissociation process. Since the excited products of the photolysis reaction scaled linearly with the laser fluence, multiphoton excitation can be ruled out. Therefore the energetically available products are limited by the heat of formation of FN,. An upper limit on the heat of formation of FN, of 135 kcal/mol can be inferred from the heats of formation of H N 3 and H F which are 70 and -65 kcal/mol, r e ~ p e c t i v e l y . ' ~ ~The ' ~ heat of formation of ground-state N F is approximately 60 kcal/m01;'~thus with addition of an ArF laser photon (148 kcal/mol), photolysis products with up to 223 kcal/mol excitation over ground-state N2(X) + NF(X) can be obtained, including both NF(b) + N2(A) and vibrationally excited N,(B) + NF(X). Therefore, the N2(B) that was observed may be either a direct product of the photolysis reaction, a secondary product formed by subsequent reaction of residual FN, with one of the primary reaction products, or the result of energy-pooling reactions between excited primary reaction products. Secondary reactions of FN, can be ruled out as a potential source of N2(B) since the yield of N 2 ( B 4 A ) emission did not decline at laser (16) H.Okabe, J . Chem. Phys., 49,2726 (1968). (17) R. J. Malins and D. W. Setser, J . Phys. Chem., 85, 1342 (1981). (18) D.D.Wagman, Null. Bur. Stand. Tech. Note, No. 270-3 (1968). (19) D.R.Stull and H.Prophet, Natl. Stand. Ref. Data Ser., Null. Bur. Stand., No.(1970).
1934 The Journal of Physical Chemistry, Vol. 90, No. 9, 1986 = v ' = 8
I
Patel et al. production of N2(B) was weak enough to allow the observation of a prompt N,(B) signal followed by d decay at the known radiative rate. Considering the low initial concentrations of FN, and assuming a gas kinetic rate of reaction, we should have been able to detect a resolvable rise of the Nz(B) concentration, particularly at low fluences, if the N,(B) was generated as a secondary reaction product. The prompt appearance of the N2(B) signal therefore shows that N2(B) is also a direct product of the photolysis reaction. Direct production of NZ(B,d=8) implies that the heat of formation of FN3 is greater than 120 kcal/mol, which locates the ground state of FN, in the range of 6 to 21 kcal/mol above the energy of the NF(b) + N2(X) states as shown in Figure 5. Coombe et aL3 and Coombe and Lam4 have similarly located the ground states of CIN, and BrN, in proximity to the corresponding NCl(b) + N2(X) and NBr(b) + N2(X) states. Decomposition of these unstable halogen azides is necessarily hindered by an activation barrier; however, a suitable transfer of vibrational energy or optical pumping of the halogen azide vibrational transitions may induce the decomposition. Hartford2*has observed the analogous production of "(a) upon COz laser multiphoton decomposition of HN,. The best overlap of the C 0 2 laser with the halogen azides occurs in FN3 where the R(32-52) transitions22 of the 001 020 band pump the asymmetric stretching mode6 at 1086 cm-'. In the case of CO, laser decomposition of FN, the only energetically available singlet product states are NF(a,b) + N2(X). Therefore, CO, laser multiphoton dissociation of FN, may be an efficient way to generate NF(a,b) metastables with N2(X) byproducts. If the FN3 is completely consumed by the photolysis reaction, then the NF(a,b) metastables will have a prolonged kinetic lifetime since N 2 is a slow quencher of electronically excited stares. Such a clean source of NF(a,b) could find numerous applications in quenching studies and potential energy-transfer laser system^.^^-^^ Further research is required to check out these possibilities. R e t r y NO.FNg, 14986-60-8; NF, 13967-06-1; NO, 10102-43-9; N,,
-
N2(X)
+
NF(X)
Figure 5. Energy levels involved in photolysis of FN, at 193 nm.
fluences that were high enough to significantly reduce the concentration of residual FN,. The most likely energy-pooling reaction occurs between two N2(A) molecules which produces both Nz(B) and N,(C). Our spectra, however, showed a much higher ratio of N,(B) to N2(C) than was found by Hays and Oskam in a pulsed N, discharge.,O Therefore, some but not all of the N,(B) that was observed may have originated from N,(A) energy pooling. Energy pooling between N,(A) and NF(b) is also capable of producing the N2(B) that was observed at high fluences and high initial FN, concentrations. These results, however, do not rule out direct production of Nz(B) by the photolysis reaction. Indeed, at low fluences and low initial FN3 concentrations, the secondary (20) G. N. Hays and H. J. Oskarn, J . Chem. Phys., 59, 1507 (1973).
1721-37-9. (21) (22) (23) (1984). (24) (1983).
A. Hartford, Chem. Phys. Lett., 57, 352 (1978). T. Y. Chang, Opt. Commun., 2, 77 (1970). J. M . Herbelin and R. A. Klingberg, Int. J . Chem. Kiner., 16, 849 A. T. Pritt, D. Patel, and D. J. Benard, Chem. Phys. Lett., 97, 471
