J . Phys. Chem. 1987, 91, 6090-6092
6090
Observation of N:, by Laser- Induced Fluorescence R. A. Beaman, T. Nelson, D. S. Richards, and D. W. Setser* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: June 29, 1987)
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Gas-phase N3was generated by the F +-NH, reactionand detected by using laser-induced fluorescence in a flow reactor. The (AZZ,+,OOO g211,,000) and the (AZZ,+,O1O X211,,010) excitaGons were investigated. Evidence was obtained for an upper-state lifetime of [HN,], the secondary reaction with N3 from either (1) or (2) gives NF(alA). Habdas, Wategaonkar, and Setser3 concluded that reaction 3 could be discarded as a primary step because the conversion of HN3 to NF(a) was high for excess [F]. Studies of the reaction using vacuum ultraviolet photoelectron spectroscopy (PES) by Dyke, Jonathan, Lewis, and M0rris~9~ provided PES spectra of both N F and N3. They also concluded that the major pathway occurs via either (1) or (2). Based upon the nascent HF(v) vibrational distribution, the direct abstraction reaction has been inferred to be more important than addition-eliminati~n.~ The F/HN3 reaction system, thus, can serve as a chemical source of N 3 or NF(a) depending upon the ratio of [F]/[HN3]. Due to the present interest in N, as a chemical source for generation of electronically excited products,6 there is a need for a direct monitor of [N,] in reactive systems. We wish to report the first laser-induced fluorescence (LIF) observation of N3. Apart from the recent PES s t ~ d yand ~ , ~the observation of N 3 in absorption during an investigation of the halogen atom N3reactions,2c only two previous investigations have been reported. The azide radical was first observed by Thrush’ using flash photolysis of H N , to record a number of absorption bands around 270 nm. A later high-resolution study by Douglas and Jones8 of the bands reported by Tkrush provided a,“ important rotational analysis of the N3(A2Z,+,000 X211,,000) bands, in addition to the tentative assignment of transitio_ns from X(OJ0) to A(010). Since the bond lengths in the N3(X) and N3(A) states are very similar, there were no vibrational progressions observed in the absorption spectra. Thrush
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(1) (a) Sloan J. J.; Watson, D. G.; Wright, J. S. Chem. Phys 1979, 43, 1. (b) Sloan, J. J.; Watson, D. G.; Wright, J. S . Chem. Phys. 1981, 63, 283. (2) (a) Coombe, R. D.; Pritt, Jr., A. T. Chem. Phys. Left. 1978,58,606. (b) Pritt, Jr., A. T.; Coombe, R. D. Inf. J . Chem. Kiner. 1980, 12, 741. (c) Pritt, Jr., A. t.; Patel, D.; Coombe, R. D. Inr. J . Chem. Kinef. 1984, 16, 977. (3) Habdas, J.; Wategaonkar, S.; Setser, D. W. J . Phys. Chem. 1987, 90, 451. (4) Dyke, J. M.; Jonathan, N. B. H.; Lewis, A. E.; Morris, A. J . Chem. SOC.,Faraday Trans. 2 1982, 78, 1445. (5) Dyke, J. M.; Jonathan, N. B. H.; Lewis, A. E.; Morris, A. Mol. Phys. 1982, 47, 1231. (6) (a) David, S. J.; Coombe, R. J. J . Phys. Chem. 1986, 90, 3260, 6594. (b) Piper, L. G.; Krech, R. H.; Taylor, R. L. J . Chem. Phys. 1979, 71, 2099. (c) David, S. J.; Onstad, A. P.; Macdonald, M. A,; Coombe, R. D. Chem. Phys. Lett. 1987, 136, 352. (7) Thrush, B. A. Proc. R. SOC.London 1956, A235, 143. (8) Douglas, A. E.; Jones, W. J. Can. J . Phys. 1965, 43, 2216.
reported two weak bands approximately 1700 cm-’ beyond the main part of the spectrum, which possibly could be the q’ vibrational frequency, and Dyke et al.-provided a tentative assignment of vl” = 1170 cm-’ for N3+(X3Z-), but the vibrational frequencies of N3(%) still remain to be assigned.
Experimental Methods Experiments were done in a fast flow reactor similar to that described by Habdas et aL3 The 5 cm diameter, 45 cm long Pyrex flow tube was pumped by a Roots blower with mechanical backing pump giving bulk flow velocities of up to 91 m s-’; the flow velocity was controlled by a gate valve. Two 5 cm diameter SI-UV quartz windows (ESCO Inc.) on either side of the flow tube, 20 cm from the entrance to the reactor, allowed observation of laser-induced fluorescence generated by a laser beam passing verticially through the observation area. Light baffles above and below the observation zone reduced the scattered laser light. Commercial tank grade H e and Ar were purified by passage through cooled (195 K) molecular sieve traps. The preparation of H N 3 (stored as 15% N H 3 in dry argon) has been fully de~ c r i b e d .The ~ buffer gas flow was established by a mass flowmeter (Teledyne Hastings-Raydist Inc.). Reagent flows were monitored by the rate of pressure rise in a calibrated volume. Coating the flow reactor with halocarbon wax was necessary to prevent the loss of F and N 3 radicals by wall reactions. Uncoated walls also resulted in troublesome background fluorescence from the products of the reactions of N atoms with N 3 and HN3.3*6 The HN3, F-atom source, and buffer gas flows were introduced to the flow reactor via three separate gas inlet ports. The pressure was monitored with a Baratron pressure gauge (MKS instruments). The buffer gas (helium or argon) was divided and added through all three inlet ports to obtain good mixing of reagents. The atom source (8% F2 in He) flow passed through a microwave discharge prior to being introduced into the flow tube via 10 mm diameter quartz tubing. The main buffer gas was introduced through a perforated ring placed upstream of the F-atom inlet to ensure good mixing of both flows, and the reagent inlet position was varied in distance from the observation zone so as to adjust the reaction time to optimize [N,]. Tunable UV laser light was generated by a Questek XeCl excimer laser pumping a Lambda Physik 2002 dye laser; the output was frequency doubled by using an Inrad Autotracker I1 system. The frequency-doubled output of Coumarin 540A (Exciton Inc.) dye gave pulses in the 256-294-nm range. The typical output was 0.3-0.5 mJ at 10 or 25 Hz repetition with a 0.3-cm-’ bandwidth. The pulse duration was ca. 15 ns fwhm. The fluorescence was observed with a 0.5-m monochromator (Minuteman) with 1200-mm-’ grating blazed at 250 nm and photomultiplier tube (Hammamatsu R212 UH in a Pacific Instruments Model 3150 RF housing). Signals from the photomultiplier tube were processed by either a fast digitizer (Tektronix 7912 AD) or boxcar averager (EG & G Model 162 with two Model 166 plug-ins) with the output stored by computer or strip chart recorder, respectively.
0022-3654187 I209 1-6090SO 1.5010 0 1987 American Chemical Societv
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6091
Letters i n
approximately the same decay time as the laser scattered light. The main features of this excitation spectrum, the rotational transitions extending to shorter wavelength and the sharp feature at 271.96 nm, correspond closely to the absorption spectrum reported by Douglas and Jones8 The conditions for Figure 1 were and [F] = 15 X l o i 2and 4 a reaction time of 2 ms with [HN,] x 10” molecules cm-,, respectively (assuming 50% fluorine dissociation by the microwave discharge), which should correspond to [N3]of (2-4) X 10l2Eolecules ~ m - ~After . learning, vide infra, that the lifetime of N3(A) was 5 2 0 ns, an improved excitation spectrum of (A,000 X2113,2,000)showing resolved rotational lines in the 271.5-271.8-nm region was recorded by monitoring X2111/2,000)band with 0.1-mm slits (0.16 nm the (A,000 resolution) for [N,]= 2 X 10” molecules cm-,. A fluorescence spectrum of the (A,OlO-z,OlO) transition was recorded via pumping the 211 2Z+ (270.86 nm) subband and observation (0.13 nm resolution) of the three other transitions to longer wavelength; see Figure 2. The flow system was throttled to give a flow velocity of 31 m s-l (reaction time of 1.6 ms) with 4.4 Torr of He pressure. The spectrum was obtained with [HN,] and [F] = 6.9 X 10l2 and 3.6 X 10l2 cm-3 (assuming 50% dissociation by the microwave discharge), respectively, with a laser power of 0.6 mJ per pulse at 270.86 nm. The width of each subband in Figure 2 is largely due to the re_olution of the monochromator. The excitation spectrum for the X(2Z+,010)level, g(2Z-,010) which was observed by monitoring the A(010) transition, is also shown in Figure 2. Allowing for the spectral resolution of 0.13 nm for the fluorescence spectrum, the positions of all four subbands correspond very well with those quoted by Douglas and Jones. The 211-2Z- band seems unusually broad and may have interestin& rotational structure. The subband presumed8 to fall under the (A2Zu+,000-X2113i2,000)transition is at 272.0 f 0.1 nm, corresponding very well with the head at 272.02 nm (36 761.5 cm-’) that was reported in the absorption spectrum (and is shown in our excitation spectrum, Figure 1). In-the course of obtaining the excitation spectra from w(OO0) and X(OlO), it became evident that the N3(X,000) concentration was, at least, tenfold larger than the N,(%,OIO) concentration. Reaction 1 therefore, does not produce very much bending mode excitation in N3. Experimcnts based upon 010-010 excitation with the ability to pump one subband while observing emission to another, which was free of scattered light, provided the best way to estimate the lifetime of N3(A). The waveforms are shown in Figure 3. The data, which were taken under similar conditions
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WAVELENGTH (A)
Figure 1. Laser excitation spectrum of N3 in the region of the (A2Zt,000-%2113/2,COO)band, recorded at 1.0 Torr of argon. The [HN,] and [F] were 15 X 10l2 and 4 X 10I2 respectively.
Experimental Results The N, absorption spectrums indicates that the (AZZ,+,OOOw211,,000) transition consists of two subbands, 271.9 nm (2Z+ 21132) and 272.5 nm (%+ 2111/2). Each subband consists of six dranches, but the spin splitting of the 2Zu+state was too small to resolve and only four branches with rather open rotational structures were observed.8 Several _appareQt band heads were observed within the region of the N3(A2Z,+-X211,) spectrum; the strongest was reported as 36761.5 cm-’ (272.02 nm). Dzuglas and Jones also assigned bands arising from the (A,010 X,O10) transition, which splits into four subbands because of RennerTeller interactions. The three identified subbands were measured at 270.9, 271.4, and 272.4 nm and assigned to 211-2Z+, 211’Asi2; and 211-22-,respectivdy. The fourth, 211-2A3/2, was thought to be located within the (A2Z,+,000 w2IIy2,OO_O) band. With the addition of one quantum of v i excitation, the A state designation becomes 211 rather than 2Z+. We first recojded an excitation spectrum, see Figure 1, in the region of the (A2Zu+,000-XZI13i2,000)band, using a 1-mm slit (1.6 nm rejolution) on the monochromator to record fluorescence from the A(000 and 010) levels. The_strong feature at 271.96 nm corresponds to overlapping of the (A,000 X2113/2) and the (A211,010 X2A3/2,010)excitations. A background trace, which consisted of the scattered light signal taken with no H N 3 in the reactor, has been subtracted from the spectrum displayed in Figure 1. The fluorescence from N,(A) was prompt and the signal had
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Excitation
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N3(010)
WAVELENGTH ( A )
Figure 2. Laser-induced fluorescence spectrum of the (A,OlO-%,OlO) transition, recorded at 4.4 Torr of He pressure for [HNJ = 6.9 X 10I2 and [F] = 3.6 X 10l2molecules cm-). The 211-2Z+ subband at 270.86 nm was pumped and the 211-2A5/2, 211-2A3/2; and 211-2Z- subbands at 271.9,272.0, and 272.4 nm, respectively, were recorded. The fluorescence was observed with 0.08-mm slit width corresponding to a spectral resolution of 0.13 nm. The excitation spectrum for X(2Zt,010) also is shown; this spectrum was obtained at 0.004 nm resolution with observation of the A(010)-X(2Z-,010) fluorescence.
J . Phys. Chem. 1987, 91, 6092-6094
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lifetimes for the OOO, 010,020, and 001 levels of 351-361 ns, while an earlier study by Reisler, Mangir, and Wittig’O reported lifetimes between 390 and 435 ns. If allowance is made for the v 3 dependence of the radiative lifetime, the NCO lifetime would correspond to about 50 ns for N3,because the NC-0 transition is at -450 nm. Thus, a lifetime of 120 ns for N3(A2Z,+)_isnot unreasonable. Although the rotational lines of the N3(A-2) absorption band are sharp, this does not preclude the possibility of some predissociation. Experiments were done to search for A(OOO)-A(high Y”) transitions in order to assign the vibrational frequencies in the lower state. The monochromator with 0 . 4 4 . 6 mm slit width was scanned from 279 to 290 nm while exciting the A(000) states at 27 1.93 nm and recording the signal by using the boxcar averager with a 20-11s gate. Weak fluorescence signals were recorded at 279.2 f 0.4 and 282.4 f OL4nm. The intensities of the transitions tentatively assigned”-as X(020) and X(100) were less than 5% of the transitions to X(OO0).
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Figure 3. Comparison of the N3(A,010) curve a, and N,(A,OOO), curve b, decay profiles. These profiles were obtained in 4.2 Torr of He by excitation of A(010) from the %(22+,010)band at 270.86 nm with obto the X(2Z-,010)band at 272.6 nm, and servation of the!uorescence by excitation of A(000)Jrom %(2113/2,000) at 271.86 nm with observation of the fluorescence to X(*IIli2,0O0)at 272.6 nm. To within the uncertainty of the measurements, both profiles are the same as the dye-laser pulse shape, curve c. The dip in the waveforms at -10 ns is due t o , electrical noise.
to those used _to record the N,(A,OlO) fluorescence spectrum, suggest a N3(A,010) lifetime of the same order of magnitude as :he laser pulse or shorter. A similaz value w_as obtained for the A(2E:,+,000)state by observing the (A2Z+7X2IIIl2)fluorescence X2113/2) band at 271.85 at 272.5 nm while exciting the (A%+ nm. These decay times were measured for a variety of conditions in both He and Ar buffer and there was no evidence of significant quenching by the reagent gases. A LIF study of the isoelectronic molecule, NCO, by Charlton, Okamura, and Thrushg reported
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(9) Charlton, T. T.; Okamura, T.; Thrush, B. A. Chem. Phys. Lett. 1982, 89, 98.
Conclusion The recording of the laser-induced (A,OlO 3,010) Cuorescence spectrum, as well as the excitation spectra from X(2Z+,010)and X(2113,2,000),provides very strong evidence that the spectral features recorded are indeed due to N,. The sensible dependence of these features upon the [F] and [HN,] is further confirmation. The addition of excess HN, results in a constant or slight decrease in LIF intensity, while addition of excess fluorine results in diminution o the LIF injensity, bezause of the rapid F N3 reaction., Transitions from A(000) to X(100 and 020) were observed following excitation from 3(2113/2,000). The approximate values for w2/1 and wl”ar_e457 and 1320 cm-l. After the short lifetime (120 ns) for N,(A) was recognized, observation of LIF from IN3] = 10” molecules cm-3 with 0.1-mm slits for the monochromator was routine. A number of spectroscopic and kinetic questions about N, can be answered by further study, including the influence of possible predissociation upon higher vibrational levels, the magnitude of the rate constant for selfdestruction by N3, and more accurate measurement of the vibrational frequencies and other spectroscopic constants of N,.
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Acknowledgment. This work was supported by the National Science Foundation (CHE-85-43609) and the U.S. Air Force Weapons Laboratory. We also acknowledge an N S F instrument award for purchase of the excimer laser and the LIF detection equipment used in this work. ( I O ) Reisler, H.; Mangir, M.; Wittig, C. Chem. Phys. 1980, 47, 49. (1 I ) Selection rules for molecules with a center of symmetry preclude transitions involving Av3 = & I . Therefore, the bands must terminate in X(020 and 100). By analogy to BO2, these levels will interact by Fermi resonance.
Reflection-Absorption Infrared Spectroscopy of Tetracyanoethyiene Adsorbed on Cu( 111): Observation of Vibronic Interaction Wulf Erley Institut fur Grenzflachenforschung und Vakuumphysik, Kernforschungsanlage Julich, 0 - 5 1 7 0 Jiilich, West Germany (Received: July I , 1987) The adsorption of tetracyanoethylene (TCNE) on a Cu(ll1) single-crystal surface at 100 K has been measured by Fourier transform reflection-absorption infrared spectroscopy (RAIRS). CN stretching vibrations observed at 2039 and 2192 cm-l are attributed to multiply and singly charged TCNE species in the first and second layer, respectively. A band at 1371 cm-’ observed during the formation of the second layer is interpreted as a C=C stretching vibration of the TCNE anions. This relatively strong band is probably activated by a charge fluctuation between the TCNE anions and the substrate. Multilayer adsorption of TCNE results in an infrared spectrum identical with that reported for crystalline TCNE. In a previous paper’ vibrational spectra of tetracyanoethylene (TCNE) adsorbed on a Cu( 11 1) single-crystal surface at 100 K 0022-3654/87/2091-6092$01.50/0
have been measured by high-resolution electron energy loss spectroscopy (EELS). Evidence has been found that TCNE 0 1987 American Chemical Society
