J . Phys. Chem. 1985, 89, 1561-1564 crowave cavity is well-known24 but it would appear from the chemiluminescence spectrum that highly excited chlorine atoms are not products of the metal chloride-active nitrogen reaction. It should be noted that the 4S 4S atom recombination energy of nitrogen is only -80000 cm-' (-9.9 eV) and that most of the observed active nitrogen emissions occur at lower energies than this. All of the spectra are strongly contaminated by metal emission (vide supra) but there are changes in the relative intensities of some of the lines which will be discussed in a later communication. There are also changes in the relative intensities of the various molecular features in the case of niobium. The red system which is very bright in the cavity emission and which is a 30 3A
+
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(24) K.M. Evenson, J. L. Dunk and H. P. Broida, Phys. Rev. A, 136, 1556 (1964).
1561
system?,25is extraordinarily weak in chemiluminescence, whereas the three shorter wavelength systems at 5840-5860, 5740, and 5582 8, appear much more strongly. These systems have not yet been completely analyzed" and their completion will undoubtedly yield new evidence on the mechanism of N b N formation in the chemiluminescent states.
Acknowledgment. T.M.D. acknowledges receipt of a seed grant from the Office of the Vice President for Research of the University of Michigan, under whose auspices this work was done. D.V.V. thanks the National Research Council for her postdoctoral research associateship. (25) T. M.Dunn and K.M. Rao, Nature (London), 222,266-267 (1969). (26) T. M. Dum, Louise K.Hanson, and K.A. Rubinson, Can. J . Phys. 48, 1657-1663 (1970).
Flowing-Afterglow Source of NF(b'Z+): Measurement of Quenching Rate Constants Daimay Lin and D. W. Setser* Chemistry Department, Kansas State University, Manhattan, Kansas 66506 (Received: November 26, 1984)
The reaction of Ar('Po,,) metastable atoms with NF2 has been employed as a flowing-afterglowsource for NF(b) molecules in Ar carrier gas. The flowing-afterglowsource is characterized and used to measure some quenching rate constants at 300 K. In the absence of added reagents, the principal loss of NF(b) molecules is via radiative decay and the NF(b) radiative lifetime is confirmed as -20 ms. For most reagents the quenching mechanism is E-V transfer and the quenching rate constants are in the 10-13-10-15cm3molecule-l s-l range. The quenching rate constants with Clz and Br, are - 4 X lo-" cm3molecule-' s-I, and the mechanism probably is excitation transfer.
Introduction Being isoelectronic with 0,but with higher energy metastable states,' N F is of current interest as a possible source for visible laser systems. Since the first two excited states, NF(aIA) (1.42 eV) and NF(b'Z+) (2.34 eV), are long-lived, 5.6, s and 233 ms, respectively, utilization of the flowing-afterglow technique for their studies is attractive. In this work we report a flowing-afterglow source of NF(b), the N F z + Ar(3Po,z)reaction, that is suitable for kinetic and spectroscopic studies. The N F z radicals were produced by the thermal dissociation4 of N2F4, and the Ar(3Po,2) (1 1.73 and 11.55 eV) atoms were generated by flowing Ar through a hollow cathode discharge.' Using this NF(b) source, we observed the Av = -1, 0, 1 sequences of NF(b-X) emissions from v ' l 7; however, most of the population is in low v' levels. The decay kinetics of NF(b,v'=O,l) at room temperature with 24 reagent gases were studied in the flow reactor. Our NF(b) lifetime measurement of 2 17 ms is in agreement with previously reported values. Unless the reagent has an acceptor state which facilitates fast excitation transfer of the electronic energy, the quenching rate constants are relatively small, iO-15-iO-'3 cm3 molecule-' s-l, and the quenching mechanism is E-V transfer. Comparison is made with quenching rate constants of 02(b) and NH(b); although the magnitudes of the rate constants are similar, there are large differences for specific reagents. The Ar(3Po,2)reaction with NzF4 also generates usable quantities of NF(b), but the NF2 reaction is preferable because the NF(b) concentration is higher and there (1) Kolts, J. H.; Setser, D.W. "Reactive Intermediates in the Gas Phase"; Setser, D. W., Ed.; Academic Press: New York, 1979; Chapter 3. (2) Malins, R. J.; Setser, D.W. J. Phys. Chem. 1981, 85, 1342. (3) Tennyson, P. H.; Fontijn,A.; Clyne, M. A. A. Chem. Phys. 1981,62, 171. (4) Evans, P. J.; Tschuikow-Roux, E. J . Phys. Chem. 1978, 82, 182.
0022-3654/85/2089-1561$01.50/0
are fewer product fragments to cause possible complications.
Experimental Method The Ar(3Po,2)atoms were generated by a hollow cathode discharge in flowing Ar, and the NFz was added to the Ar* flow via the conventional concentric inlet design of our laboratory.] The flowing afterglow consisted of three sections: (i) the hollow cathode discharge, (ii) the Ar*/NF2 prereactor, and (iii) the flow reactor where the decay measurements of NF(b) were made. The electrodes for the hollow cathode discharge were in a 14-mmdiameter tube. The discharge tube was attached to the 28-mmdiameter Ar* NF, prereactor (8 cm in length), which had quartz windows to observe NF(b-X) and other emissions from the NFz and N2F4 Ar* reactions. The NF(b) reactor was a 60-cm-long Pyrex tube of 41-mm diameter which was attached at a right angle to the Ar*/NF2 prereactor. The NF(b) emission was monitored with either a monochromator and RCA-C31034 PM tube or with a movable PM tube (Hamamatsu R212) with interference filter (530 nm). The NFz precursor, N2F4, was purified by pumping away the noncondensible and more volatile portions of a sample at 77 K and stored in a 10-L reservoir at 100 torr. The NF2 radicals were produced by passing N2F4 through a 60-cm length of 7mm-diameter Pyrex tubing that was heated to 500 K before the flow entered the Ar*/NF2 prereactor. The complete dissociation of N2F4 was noted by observing the disappearance of the ArF(C-A) emission at -250 nm, which is generated by Ar(3P2,0)+ N2F4.5 The NF2 concentration, which was deduced from the NzF4
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+
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( 5 ) Kolts, J. H.; Setser, D. W. J. Phys. Chem. 1978, 82, 1766. The rArF = 0.08 entry is a misprint; the correct value is 0.18. Also the XeF(B)/XeF(C) plot in this paper is incorrect; see ref 6.
0 1985 American Chemical Society
1562 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985
flow rate (measured by a calibrated capillary flowmeter), was optimized by monitoring the NF(b-X) emission intensity: a N2F4 flow of -3 X mol min-’ was found to be optimum. The prepurified Ar’ flow was metered by a needle valve and measured by a calibrated triflat flowmeter. The pumping capacity was 2000 L min-’, which gave a flow velocity of 16 m s-l in the NF(b) reactor at 1.6 torr for an Ar flow of -0.1 mol m i d . The plug flow approximation was used to convert the flow distance to At for the kinetic measurements. The greatest uncertainty in the rate constants is from the Ar flow calibration (wet test meter), which has an estimated uncertainty of f15%. The reagent gases were added to the NF(b) reactor 5-cm downstream of the position where the NF(b) flow from the Ar*/NF2 prereactor entered the main reaction tube. The reagent flow rates were measured with a capillary flowmeter, which was calibrated for Ar and corrected according to the viscosity of the reagent gas. The reagent concentrations were 1013-1014molecules ~ m - except ~, for C1, and Br2 which had much faster quenching rates and their concentrations were 2 orders of magnitude smaller. The reagent gases were taken from standard commercial sources, degassed, distilled, and stored in 5-L reservoirs. Except for C1, and Br,, the reagents were stored as pure gases, since high concentrations were needed to compensate for the small rate constants.
Letters
-
-
Experimental Results The reactions of NzF4 and NF, with Ar(3Po,2)both generated NF(b), as could be easily determined from the green NF(b-X) emission that extended for more than 1 m downstream of the Ar*/NF2 mixing zone. The ArF(B-X) and ArF(C-A) emissions are observed from N2F4Sbut not from NF2. This permits the use of the ArF(C-A) emission as a monitor for the presence of N2F4. The ArF(C-A) emission intensity decreased with increasing temperature of the N2F4 flow line, while the opposite trend was found for the NF(b-X) emission intensity. When the temperature reached -500 K, the ArF(C-A) emission disappeared and the NF(b-X) intensity was a maximum. The NF(b-X) emission from NF, was about 4 times as intense as that from NzF4. The NF(b-X) bands were the only emission detected in the 200-700-nm range from Ar* NF2. Although very weak NF(a-X) emission at 874 nm was observed, it was too weak to be useful for studies of NF(a). The absence of observable N,(B-A) emission does not necessarily mean that the system is entirely free of N atoms. The ~ , a small fractional conversion total [Ar*] is 1Oloatoms ~ m - and to N by N F 2 quenching could give such a low [N] that recombination may not be detectable. The larger [NF(b)], as well as the lesser chance that interfering products would be present, caused us to adopt the Ar* + NF2 reaction as the better NF(b) source. Experiments also were done with Xe(3P2) + N2F4 and NF2.6 Although NF(b) is formed from the Xe(3P2)+ NF2 reaction, this reaction has no advantage relative to Ar* + NF, as a NF(b) source. Since the Ar(3Po,2)concentration is of the order of 1Olo atoms cmW3,and since the generation of NF(b) from NF, is moderately efficient (from comparison to the N2(A-X) emission intensity from the Ar(3Po,2) N 2 reaction), the [NF(b)] must . Ar(3P2)quenching rate conbe - 5 X lo9 molecules ~ m - ~The cm3 molecule-I s-I and that for NF2 stants for N2F4 is 3.1 X should be similar; therefore, the Ar(3P,,o) atoms are totally removed in the prereactor and there is no downstream generation of NF(b). Possible problems from this NF(b) source for kinetic studies are associated with the simultaneous presence of (i) F atoms a t a similar concentration to NF(b), (ii) [NF,] at lo’, molecules ~ m - and ~ , (iii) the possible presence of N atoms. The reactions of N and F with NF2 and NF(b) are thought to be too slow79*to be of importance in the flow reactor, but reactions of N, F, or NF2 with reagents or products from quenching reactions may cause problems. For most (but not all) reagents studied in
+
-
1I) i’I1I
(6) Lin, D.; Yu, Y. C.; Setser, D. W. J . Chem. Phys. 1984, 81, 5830. (7) Cheah, C. T.; Clyne, M. A. A,; Whitefield, P. D. J . Chem. SOC., Faraday Trans. 2 1980, 76, 711, 1543. (8) Cheah, C.T.;Clyne, M. A. A. J . Photochem. 1981, 15, 21.
a
Figure 1. NF(b,u‘-X,u”) spectrum showing the Au = -1,O, 1 sequences from the Ar(3Po,2)+ NF2 reaction as observed in the Ar*/NF2 mixing region. The spectrum was rec‘orded with a 0.3-m monochromator with a slit width of 200 pm corresponding to a bandpass of 0.5 nm; the peak position could be assigned with f0.2-nm reliability. At the entrance to the NF(b) flow reactor only NF(b,u’=O-2) could be observed, and the u’ = 2 population was very low.
this work, we found simple first-order decay kinetics for NF(b). Several bands corresponding to the Av = -1, 0, 1 sequences of the NF(b,v’-X,u”) transition could be observed (Figure 1) in the Ar*/NF2 mixing zone. The wavelength corresponding to the peak position of each band was measured and compared to the predicted locations from the published spectroscopic constant^;^ the general agreement was satisfactory (for our low-resolution measurements). The relative intensities and the FC factors calculated from Morse potentials were used to find the relative vibrational populations of NF(b). In the Ar/NF2 reaction zone the relative populations for v ’ = 0-7 are 100, 18, 11, 8, 5 , 4,3, and 2. By the time the flow reached the NF(b) reactor, considerable vibrational relaxation has occurred and at the reagent/NF(b) mixing window only NF(b,v’=G2) could be observed. Thus, the quenching rate constants pertain mainly to NF(b,v’=O) with a small contribution from v’ = 1. The decay of NF(b) consists of radiative decay, wall deactivation, and quenching by Ar, NF,, and other added reagents, [Q]. - d[NF(b)I
-
dt
IT-’ + kw
kAr[Arl + kNF2[NF21 + ~ Q [ Q I I [ N F ( ~ )(1) I
The quenching rate constant for collisions with the wall, k,, is small (vide infra), and hence it is written as independent of pressure. In eq 1 T-’ is the radiative decay constant, k,, and kNF2 are the quenching rate constants by Ar and NF,, respectively, and kQ is the quenching rate constant by added reagent. Since [NF(b)] is small, pseudo-first-order kinetics are followed and we obtain
+
-
AV=-1
In ktota,
=
7-l
[NF(b)l [NF(b)lO
-= ktotalt
+ kw + kAr[Arl + kNF2[NFZI + k ~ [ Q l (2)
The NF(b-X) emission intensity is proportional to [NF(b)] and can be used to monitor the relative concentration. Experiments first were done to characterize the decay without added reagent moving the PM tube detector along the flow reactor and monitoring [NF(b)]. Experiments for 1.5 torr of Ar with variable [NF,] gave pseudo-first-order decay plots, and those first-order rate constants are plotted vs. [NFJ in Figure 2. There is no obvious dependence of the decay constant on [NF,] and ICNF2 < cm3 molecule-’ s-l. The intercept in Figure 2 corresponds to the sum of k , + 7-l + kAr[Ar]. We found no evidence for quenching by Ar and kArhas been reported as
