J. Phys. Chem. 1983, 8 7 , 1286-1288
1286
Laser- Induced Excitation and Emission Spectra of NO3 H. H. Nelson,+Loulse Pasternack, and J. R. McDonald' Chemistry Division, CWe 61 10, Naval Research Laboratory, Washington, D.C. 20375 (Received: November 75, 1982; I n Final Form: January 7, 1983)
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The fluorescence excitation and emission spectra of NO3 (2E' (2B2) 2AZ/ (2B2))are reported. Assignments are made for excited- and ground-state vibrational frequencies. A lower limit of 23 p s is measured for the fluorescence lifetime which is at least a factor of 40 longer than predicted from absorption. Spectroscopic observations are consistent with a CZuground-state geometry.
Introduction The visible absorption spectrum of the nitrate radical, NO3, consists of a series of intense bands in the wavelength range 680-400 nm.1-3 Walsh4 predicted that the molecule has Dsh symmetry and assigned the spectrum to the 2E' 2 A i transition. Olsen and Burnelle5 have performed semiempirical calculations and predict a Y-shaped molecule with the spectrum arising from a ?B2 2B2transition. Ramsay2 photographed approximately 20 bands in the region 665-500 nm and reported them to be completely diffuse under the resolution afforded by a 21-ft spectrograph. More recently, Marinelli, Swanson, and Johnston6 recorded a portion of the absorption spectrum around the peak of the band assigned by h a y 2 as the 0transition (661.90 nm) using a single-mode dye laser (line width < 200 MHz) and found the spectrum to be continuous. Magnotta and Johnston' and more indirectly Graham and Johnston3 found that a portion of the spectrum is photochemically inactive; however, Magnotta and Marinellis,g observed no visible fluorescence from laser excitation of NO3. In this paper, we report observation of laser-induced fluorescence from the nitrate radical. Both the fluorescence excitation spectrum and the dispersed emission spectrum after excitation at the origin have been recorded. From these spectra we are able to derive some ground-state vibrational frequencies for NO3 and to correlate them with previously unassigned frequencies in the excited states. We have also obtained a lower limit for the emission lifetime of the vibrationless level of the excited electronic state.
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Experimental Section Nitrate radical emission is recorded by using a system in which NO3 is generated chemically. Fluorine atoms, produced by passing F2/He mixtures through a microwave driven plasma (2450 MHz), react in a fast flow system with HNO, to produce the nitrate radical. Approximately 10ms reaction time is available before the flowing mixture enters the spectral cell which consists of a Pyrex, 6-way (5 cm diameter) cross with windows on three of the arms. A flashlamp pumped dye laser (Chromatix CMX-4) operating with rhodamine 640 in methanollwater, rhodamine 590 in soaplwater, or rhodamine 590 in methanollwater and producing pulses of -5 mJ is used to excite NO3 fluorescence. For the excitation spectrum, fluorescence is isolated with a combination of colored glass and interference filters and detected with an RCA 31034 PMT. The resulting signal is averaged by a boxcar integrator and digitized and stored by a Nicolet 1180 computer for later analysis. A portion NRL/NRC Resident Research Associate.
of the excitation laser pulse is collected in the same manner and used to normalize the spectrum. Lifetime measurements are made in a similar system; the fluorescence decay is collected by a transient digitizer (Biomation 8100) and transferred to the computer for storage and analysis. So that the emission spectrum could be measured the fluorescence is collected and focused onto the entrance slit of a 0.5-m monochromator (Acton VM-505) with a telescope. The monochromator slits are typically set at 1.0 mm resulting in a bandpass of 1.7 nm (fwhm). Absolute monochromator calibration to 0.05 nm is accomplished with a low-pressure Hg lamp. The fluorescence transmitted by the monochromator is detected by a cooled (260K) RCA 31034 PMT and collected with a gated photon-counting system.1° The total pressure in the spectral cell ranges from 20 to 200 mtorr with the nitric acid pressure at 1-10 mtorr, the F2pressure 2-10 mtorr, and the balance He. Nitric acid is distilled from mixtures of KN03 in 96% H2S04under vacuum. The product is collected a t 250 K with only the middle portion saved. Before each use the HNO:, sample is evacuated to remove any NO2 impurity. He (Air Products, 99.995%) and 10% F2 in He (Ideal Gas Products) are used as received. Results The NO3 excitation spectrum was recorded as four partial scans with three dye solutions. The composite spectrum, normalized for excitation laser power, along with the dye laser output curves is shown in Figure 1. Also shown in this figure as curve b is a portion of the NO2 fluorescence excitation spectrum recorded under similar conditions. As can be seen, NO2 is not a serious interference at frequencies below 16800 cm-'; above this energy all the signal we observe can be attributed to NO2 impurity in the reaction mixture. This spectrum agrees well with the absorption spectrum obtained by Graham and Johnton.^ The relative intensity of the transition at 15 108 cm-' (assigned by Ramsay2as the 0band) is slightly decreased in our spectrum. This presumably arises from a shift of the emission into the bandpass of the filters used as we excite at higher frequencies. The position of the band at 16479 cm-' is shifted with respect to that observed by (1) Sprenger, G. 2.Elecktrochem. 1931, 37, 674-8. (2) Ramsay, D. A., Proc. Colloq. Spectrosc. Int., IOth, 1962 1963, 583-96. (3) Graham, R. A.; Johnston, H. S. J . Phys. Chem. 1978,82, 254-68. (4) Walsh, A. D. J. Chem. SOC.1953, 2301-6. (5) Olsen, J. F.; Burnelle, L. J. Am. Chem. SOC.1970, 92, 3659-64. (6) Marinelli, W. J.; Swanson, D. M.; Johnston, H. S. J . Chem. Phys. 1982, 76, 2864-70. (7) Magnotta, F.; Johnston, H. S. Geophys. Res. Lett. 1980, 7,769-72. (8) Magnotta, F., Ph.D. Thesis, 1979, University of California, Berkeley, and Lawrence Berkeley Laboratory Report LBL-9981. (9) Magnotta, F.; Marinelli, W. J., private communications. (10)Donnelly. V. M.; Pith, W. M.; McDonald, J. R. Chem. Phys. 1980, 49, 289-93.
This article not subject to U.S. Copyright. Published 1983 by the American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 8, 1983
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1287
Wovelength i n m )
1
I
L
l
l
l
l
l
l
l
l
l
l
l
h
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Emission
Frequency ( c m - ' )
Figure 2. Laser-induced fluorescence spectrum of NO3 following excitation at 15 108 cm-'. The signal at the excitation wavelength is approximately 50% NO3 fluorescence and 50% scattered laser light.
TABLE 11: Dispersed Fluorescence Following Excitation at 15 108 cm-' U , cm-' AT; intensity assignment' Figure 1. Fluorescence excitation spectrum of the nitrate free radical: (a) composite, spectrum of NO3, normalized by using the dye laser power curves in the upper portion of the figure; (b) spectrum of NOp under the same conditions. Dye laser output power curves for (c) rhodamine 640 in methanoVwater, (d) rhodamine 590 in soap/water, and (e) rhodamine 590 in methanollwater.
TABLE I: NO, Fluorescence Excitation Spectrum AT; assignment Y , cm-'
15 loga 15 526 15 676 15 928 16 039 16 274 16 47gd
0 418 568 820 931 1166
0-Ob '6
v ,-36OC v2 Y1
96 0-0 28 '6 2 15 YZ 100 u1 53 Y5 5 "1 + Y 2 5 8 11 2Vl 4 10 2Y1+ U 6 ' See the text for a discussion of the proposed assign15 108 14 748 14 429 14 354 14 057 13 619 13 312 13 198 13 108 12 982 12 772 12 622
0 360 679 7 54 1051 1489 1796 1910 2000 2126 2336 24 86
ments.
b
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All values relative t o this value obtained from ref 6. Assignments from ref 2. Assignment from ref 8. This band position does not agree with the absorption data. See the text for a discussion of this. a
Graham and J ~ h n s t o nthis ; ~ is a consequence of the steeply falling fluorescence quantum yield and will be discussed in more detail in the next section. The positions of the six bands that we can clearly observe are listed in Table I. The emission spectrum we observe after excitation a t 15 108 cm-', not corrected for monochromator and PMT response, is shown in Figure 2. The signal shown at the excitation frequency corresponds to approximately 50% scattered laser light and 50% NO3 fluorescence. The relative intensities of the bands observed mirror those of the absorption spectrum. The positions of the bands in this spectrum are listed in Table 11. We have also measured a lower limit to the emission lifetime following excitation of NO3 at 661.8 nm. At the lowest HN03 concentration for which a fluorescence signal can be digitized under our experimental conditions, the measured lifetime is 23 ps, exponential over four lifetimes. Under these conditions, the partial pressures are as follows: H N 0 3 + NO,, p I3 mtorr; F + F2 + HF, p I1.5 mtorr; He, p I20 mtorr. Since HN03 quenches NO3 fluorescence very efficiently and since under our experimental conditions we are unable to accurately monitor partial pressures, it is not possible to extract a zero pressure limit from our
data. Modifications in experimental design are underway to enable more accurate pressure measurements and hence a Stern-Volmer plot to extract the collision-free lifetime. These results will be reported in an expanded paper. Because of the similarity of the NO3 fluorescence decay to that of NO2 we have carefully tried to exclude the possibility that we are monitoring emission from NOz in these experiments. These precautions include monitoring of both excitation and emission spectra, observation of the behavior of the decay with pressure, and response of the system to added NO2. These experiments will be reported in greater detail in the expanded report. We attempted to obtain lifetimes and emission spectra in a system in which NO3was generated photolytically. We used both 193- (ArF) and 249-nm (KrF) radiation to photolyze both N205and ClN03 in search of NO3. In all cases the detection electronics were overwhelmed with emission from NO2which extends from approximately 410 nm to the infrared. The excited NO2 observed is presumed to arise from primary photolysis in the case of N205and secondary reactions ( T i=~10 ~ps) ~in the ~ case of ClN03. Under these circumstances the laser-induced fluorescence signal from NO3 is marginally detectable. We feel that under the conditions of these experiments the majority of the NO3 produced by photolysis is either photolyzed or created with sufficient internal energy to render it undetectable by LIF. Discussion As noted above, a t frequencies below 16400 cm-' the excitation spectrum we obtain agrees well with the ab-
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The Journal of Physical Chemistry, Vol. 87, No. 8, 1983
TABLE 111: Fundamental Frequencies for Some Nitryl Halides and Tentative Assignments for NO, __ ______.___ mode approx description FN02a ClN02b NO,' NO,d,e
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1 2 '3 -1
>
6
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( a , ) symmetric stretch ( a , ) NO, bend ( a , ) N-X stretch ( b , ) out-of-plane bend ( b , ) asymmetric stretch ( b , ) asymmetric bend
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1309.6 822.4 565 742.0 1791.5 560
1293 7 94 411 651 1685 367
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NO,* d,f ___________
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1319 750
1050 754
931 8 20
1617 ( u 3 )
1489 360
1166 418
a From ref 11. From ref 12. From ref 13. Data for NO, represent u l r u 2 , and u , respectively and are listed next t o Tentative assignments from this work. See the text for further discussion. their analogous freqeuncy in the nitryl halides. e The assignments of u , and are inverted, according to the calculations of ref 1 4 . NO,* represents the 2B,upper state of this transition. See ref 5. IJ,
sorption spectrum of Graham and J ~ h n s t o n .We ~ see no NO, fluorescence at excitation frequencies above 16 800 cm-l although the NO, absorption extends to at least 25 0oO cm-'. This result is consistent with a fluorescence quantum yield that falls from a constant value below 16000 cm-' to near zero around 16 600 cm-l. Magnotta and Johnston7 and Magnottas report the quantum yield for the two open NO, photolysis channels (NO, + hv 0 + NOz or Oz + NO) as a function of wavelength in this region. They observe a combined photolysis quantum yield for the two channels which is approximately constant above 17 000 cm-' and then falls to near zero at approximately 15 400 cm-'. These results are in excellent agreement with the qualitative picture of the fluorescence quantum yield obtained in this work. From the dispersed emission spectrum we are able to make some tentative assignments for vibrational frequencies of the NO, ground state and by analogy some excited-state frequencies from the excitation spectrum. In Table 111 we have listed the ground-state vibrational frequencies for FN02, C1NOZ,and NO2 along with our assignments for the ground and excited states of NO,. The nitryl halides are Y-shaped molecules of Czvsymmetry",l2 and have similar electron configuration (bonding) to NO,. As can be seen from Table 111, the vibrational modes that involve predominately NOp motions are well described by the frequencies of NOz itself. This should be less the case for NO, as the bonding has been calculated to differ ~lightly.~J* The nitrate ion, which might be expected to be the closest analogue to NO,, is a planar, symmetric ion of D 3 h symmetry.', The frequency of the symmetric stretch, vl, in NOi- is 1050 cm-' which agrees well with our assignments for the two states of NO3. In the nitrate ion, however, both vz, the out-of-plane bend, and v4 are greater than 700 cm-' while v3 is 1390 cm-'. We are confident of our measurement of 360 cm-' as a fundamental frequency in the NO, ground state; in addition it appears as a hot band in the absorption spectrum (see ref 3 and Table I) and its analogue appears in the excited state at 418 cm-l. For this reason we prefer the assignments listed in Table I11 and used in Table I1 which imply a Y-shaped, Czusymmetry for NO,, in agreement with recent c a l c ~ l a t i o n s . ~ J ~ We have measured a lower limit to the NO, radiative lifetime of 23 gs. This measurement is limited by the efficient quenching of the fluorescence by HNO,. Olsen and Burnelle5 have calculated a fluorescence lifetime of 0.5 gs from their calculated oscillator strength. Using the
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(11) Tanaka, T.; Morino, Y. J . Mol. Spectrosc. 1969, 32, 430-5. (12) Ryason, R.; Wilson, M. Kent, J . Chem. Phys. 1954,22, 2000-3. (13) Herzberg, G. "Molecular Spectra and Molecular Structure"; van Nostrand: New York, 1945; Vol. 11. (14) Swanson, D. M.. private communications.
method of Strickler and Berg,15 the tabulated absorption data of Graham and J o h n ~ t o nthe , ~ mean value of v-, from our fluorescence spectrum, and assuming zBz symmetry for both states, we have calculated a fluorescence lifetime of 0.55 gs. This is inconsistent with the values obtained in this study and is presumably due to coupling to a metastable level or to the ground state as described by DouglaslGand observed in NOz, SOz,etc. This would imply that our fluorescence decay measurement of 23 gs may be far from the collision-free radiative lifetime since the decays we observe are clearly single exponential. The observed long lifetime and efficient quenching by HNO, also explains the failure of Magnotta and Marinelli8v9to observe NO, fluorescence in an earlier experiment. If we assume that N205quenches NO, fluorescence as efficiently as does HNO,, the fluorescence quantum yield under the conditions of the earlier work (100 mtorr of Nz05 in 10 torr of Nz) would be much less than 1%. In conclusion, we have observed laser-induced fluorescence of the nitrate radical in the wavelength region 665-595 nm. The spectra we observe are consistent with earlier absorption3 and p h o t o l y ~ i sstudies ~ - ~ of NO3 and have led to the identification of some vibrational frequencies in the NO, ground and excited states. We have also measured a lower limit of 23 K S for the NO, fluorescence lifetime. The coupling of excited-state levels with those of the ground state likely contributes to the complexity of the absorption spectrum which renders it apparently diffuse. This is likely further exacerbated by severe sequence congestion which results form a very close rotational level spacing. This dichotomy of apparently diffuse absorption and apparently large fluorescence quantum yield will be treated in a future paper. In addition, experiments are in progress in our laboratory designed to obtain (1) emission spectra following excitation of other features of the absorption spectrum, (2) a more complete understanding of the excited state dynamics of NO3 excited in the photochemically inactive region, and (3) a study of the excited state decay in the region of 16500-17500 cm-' where unimolecular dissociation is in competition with radiative decay. Acknowledgment. J. R. M. gratefully acknowledges discussions with Dr. Ikuzo Tanaka of Tokyo Institute of Technology and his co-workers who were studying NO, fluorescence concurrently with this work and H. H. N. acknowledges helpful discussions with Diane Swanson, University of California, Berkeley. Registry No. NO,, 12033-49-7. (15) Strickler, S.J.; Berg, R. A. J . Chem. Phys. 1962, 37, 814-22. 116) Douglas, A. E. J . Chem. Phys. 1966, 45, 1007-15.
