2304
J . Phys. Chem. 1987, 91, 2304-2309
Photodissociatlon of NO2 at 157.6 nm M.-R. Taherian, P. C. Cosby, and T. G. Slanger* Chemical Physics Laboratory, SRI International, Menlo Park, California 94025 (Received: November 7 , 1986)
The photodissociation of NO2 has been studied at 157.6 nm, using an F2laser. The principal products are ground-state NO and 0,4.8 eV of the available energy therefore being disposed of in translational, vibrational, and rotational energy. A total energy balance has not yet been performed, but NO vibrational levels have been detected by laser-induced fluorescence (LIF) in the B211-X211 system in the range u = 4-21; the thermodynamic limit is u = 25. Extremely high rotational levels are found in all ground-state vibrational levels. In the most extensively studied level, u = 5 , most of the rotational population is to be found in the N = 50-70 region. Even at the highest vibrational levels, there is little nascent population at the bottom of the rotational manifold. Rotational relaxation of these high levels, where rotational quanta are typically 200 cm-', occurs at approximately every collision with helium. NO(B-X) emission, independent of the LIF process, is quite strong and appears to be a consequence of resonances between the F2 laser lines and transitions involving the nascent NO levels produced in the photodissociation. N2(B3n,-A32,+) emission is detected, possibly a result of N atom recombination; the most likely source of N atoms is photodissociation and/or photoionization of the excited NO product. Diffuse bands between 770 and 800 nm are also seen, but they have not yet been identified. An attempt was made to determine the selectivity of A-doublet production in the NO2 photodissociation process. For unperturbed levels, the splitting seems to be less than our spectral resolution, 0.5 cm-I, irrespective of rotational level. Much larger effects are expected in transitions to the u = 1 level of the NO(B) state, which is strongly perturbed by the NO(b4Z-) state at N = 35-45. Due to spectral contamination by undesired NO(B-X) bands, we have not yet fully resolved the question but are leaning toward the conclusion that both A-doublets are produced during photodissociation.
Introduction NO2 is a popular molecule for photochemical studies for a variety of reasons. The challenge of understanding its absorption spectrum in the visible spectral region has been a classical problem for many years.14 Its atmospheric role in the ozone chemical cycle and in air pollution makes its study a matter of great practical i m p ~ r t a n c e . ~The , ~ fact that it dissociates into a product, NO, that can be easily characterized in terms of its internal energy states has made NO2 photodissociation studies important sources of dynamical We have previously investigated the distribution of internal energy following N O 2 photodissociation in the 248-290-11111 region.I0 The principal conclusion was that there was a great propensity for the excess available energy to go into N O vibrational excitation. At 248 nm, where only ground-state products can be generated, the available excess energy is 1.9 eV, corresponding to a maximum N O vibrational level of u = 8; the vibrational distribution peaks sharply at u = 7. It has been shown by Morrison et a1.* that exceeding the O(lD) production threshold at 243 nm results in an opening of this channel and, thus, an abrupt decrease in the energy available for NO vibrational excitation. The threshold for the next oxygen excited state, O('S), is 170 nm, and we felt it of interest to determine whether in fact NO2 photodissociation below this wavelength, specifically at the F2 laser line at 157.6 nm, might proceed by the O(lS) + N O pathway. If so, the maximum N O vibrational level is u = 2. If the principal pathway results in formation of O(lD) + NO, then the maximum N O level is u = 13, whereas if products are formed in their ground state, N O levels up to u = 25 can be made. (1) Douglas, A. E.; Huber, K. P. Can. J . Phys. 1965, 43, 74. (2) Brand, J. C. D.; Hardwick, J. L.; Pirkle, R. J.; Celiskar, C. J. Can. J . Phys. 1973, 51, 2184. (3) Haas, Y.; Houston, P. L.; Clark, J. H.; Moore, C. B.; Rosen, H.; Robrish, P. J. Chem. Phys. 1975, 63, 4195. (4) Donnelly, V. M.; Kaufman, F. J . Chem. Phys. 1977, 66, 4100. (5) Noxon, J. F. Science 1975, 189, 547. (6) Tucker, A. W.; Birnbaum, M.; Fincher, C. L. Appl. Opt. 1975, 14, 1418. . ..
A study by Matsumi et al." on multiphoton dissociation of NO, at 500-530 nm had indicated that a four-photon process, equivalent in energy to a single photon at 128 nm, resulted in direct production of very highly vibrationally excited 02.An alternative explanation was that the 02(vib) resulted from a secondary reaction involving o ( % ) , for which we have recently found evidence.I2 In any case, it appeared that UV-vis photodissociation of NO2 was an area in which much could be learned. A previous study by Welge,13carried out at 123.6 nm, had shown that small yields of rotationally hot NO(B211) and NO(A22+) were generated, but the major pathways were not investigated.
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Experimental Section These experiments were carried out with a Lumonics 861-T excimer laser, operated on F2 to give 10 mJ of 157.5-157.6-nm emission, in conjunction with counterpropagating radiation from a Quanta-Ray dye laser, pumped by a Nd:YAG laser. The physical arrangement was standard for a pump-and-probe experiment. The dye laser output was doubled in KDP for investigation of the lower N O vibrational levels and was not doubled for the highest; excitation on the 5-20 NO(B-X) transition, for example, is at 547 nm. The two dyes that were used were Exciton Fluorescein 548 and R590. As a Pellin-Broca prism was not used for separating the two beams, it was necessary to check on each excitation scan whether the observed emission was caused by the UV or the visible beams. Excitation spectra were taken with a filtered photomultiplier, using a set of Dietrich 10-nm filters. Because of the great array of vibrational and rotational levels that are populated in this system, subsequent measurements should be made with a monochromator to avoid spectral ambiguities. The signals from the EM1 9558 photomultiplier were processed by an S R S 250 boxcar averager and monitored on a strip chart recorder. A digital delay generator controlled the delay times between the two lasers, and the averager utilized a 100-ns gate. For UV fluorescence scans, a 0.3-m monochromator was interspersed between the stainless steel reaction cell and the photo-
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(7) Zacharias, H.; Geilhaupt, M.; Meier, K.; Welge, K. H. J. Chem. Phys. 1981, 74, 918. (8) Morrison, R. J. S.; Rockney, B. H.; Grant, E. R. J. Chem. Phys. 1981, 75, 2643. (9) Morrison, R. J. S.;Grant, E. R. J . Chem. Phys. 1982, 77, 5994. (10) Slanger, T. G.; Dyer, M. J.; Bischel, W. K. J . Chem. Phys. 1983, 79,
2231.
0022-365418712091-2304$01.50/0
(1 1) Matsumi, Y.; Murasawa, Y.; Obi, K.; Tanaka, I. Laser Chem. 1983, 1, 113. (12). Jusinski, L. E.; Sharpless, R. L.; Slanger, T.G., accepted for publication i n J. Chem. Phys. ( 1 3) Welge, K. H. J. Chem. Phys. 1966, 45, 11 13.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2305
Photodissociation of NOz a t 157.6 nm 54 J"
- 112
53 48
54
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52
50
55
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58
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TABLE I: Molecular Constants' of NO(B211) (v = 2) Determined from the B-X 2-5 Bandb
p22
38366.880 (127) 34.10 (27) 1.323 (133) 1.09301 (36) [1.09206]c 5.584 (172) [5.179] 63.5 (21.3) [5.615] -4.2 (2.5) -1.34 (28)
2,
AD' (Xlo-')
B' D'(Xl0") H' (Xl0-l2) q' (xio-5) p' ( X 10-2)
'Units are cm-I. Numbers in parentheses are two standard deviations of the constant expressed in terms of its last digits. bConstants for N O ( X 2n)( u = 5) used in the fit are A" = 121.6762, AD" = 2.88 X lo4, B"= 1.607946, D"= 5.56488 X lo", H"= 2.19 X lo-", q" = -7.7 X and p" = -1.31 X CValuescomputed from the potential energy curve.
,
20 2700
210 5
271 0
271 5
272 0
272 5
,
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273 0
WAVELENGTH inml
Figure 1. Excitation spectrum of NO(B-X) 2-5 band. [NO2] = 60 [He]= 6 Torr, Xab = 420 nm (2-14 band), no delay between
mTorr, lasers.
multiplier. The considerably weaker fluorescence spectra in the visible region were recorded with an image tube intensified transmission grating spectrograph. The NO2was used as a 1% mixture in helium in the -0.5 L/s flow system, and pressures were measured with a Baratron gauge.
50
52
Ed
(14) Schmid, R.; Farkas, D.; Konig, T. Z . Phys. 1930, 64, 84. (IS) Amiot C.; VtrgC, J. J. Mol. Spectrosc. 1980, 81, 424. (16) Taherian, M. R.; Cosby, P. C.; Slanger, T. G. J . Chem. Phys. 1985, 83, 3878. (17) Engleman, R., Jr.; Rouse, P. E.; Peek,H. M.; Baiamonte, V. D. Beta and Gamma Band Systems of Nitric Oxide; Los Alamos Scientific Laboratory: Los Alamos, NM, 1970; p 4364. (18) Zare, R. N. J. Chem. Phys. 1964, 40, 1934. (19) Kaiser, E. W. J . Chem. Phys. 1970, 53, 1686. (20) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure, IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (21) Hutson, J. M. J. Phys. B 1981, 14, 851.
58
60
62
64
66
58
70
J"-lI2
Figure 2. Fit of calculated to observed positions [NO(B-X) 2-5 band, P22and R22branches]. 20
Results and Discussion Rotational Distributions. Upon carrying out excitation scans above 270 nm and into the visible spectral region following 157.6-nm dissociation of NOz, we found evidence for copious production of vibrationally and rotationally excited ground-state NO. Figure 1 shows a portion of a scan of the NO(B-X) 2-5 band, taken with no temporal delay between the F2and dye lasers. For a 300 K rotational distribution, this band has a width of 2-3 nm. The present spectrum, from the head to the last identified line, is 22 nm in width. In fact, we did not observe the head of the band a t 260 nm-the spectrum in Figure 1 starts with the PZ2(48.5) line. Furthermore, the last clearly identified line, P11(73.5), is not weak. Overlapping with other bands makes it difficult to continue the sequence to longer wavelengths, although with another choice of detection wavelength this problem could be ameliorated. Line tabulations of the NO(B-X) 2-5 band have been reported by Schmid et al.,14 but their data include only J"I 20.5. In order to identify the high rotational lines in the present spectrum, the line positions and intensities were calculated from the molecular constants of the two states. The constants for NO(X),,s, including the centrifugal distortion constants, are well-known.'s~16 The constants for NO(B),=2 were taken from those given by Engelman et al." These latter constants were based on sparse data, exclusively at low J. We therefore generated the RKR potential energy ~ u r v e ~for~ the , ' ~B state from the vibrational and rotational constants of this state,20 and we used this curve to compute2' the rotational constants B, D, and H for u = 2. The A-doubling constants q and p for the B state were estimated by extrapolation of these constants from u = 0 and 1. The line positions for the NO(B-X) 2-5 band predicted by these constants were consistent
56
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I
1
54
58
66
60
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,
70
72
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1
62
84 J
65
68
74
112
Figure 3. As in Figure 2 (P,, and R,,branches).
with the data of Schmid et al.14 and allowed identification of features in the present spectrum to within several cm-' for J in the range of 50.5-70.5. Improved molecular constants for the B-X 2-5 band were obtained by simultaneously fitting2*the 67 lines23 reported by Schmid et al. and the 83 high-J lines observed in the present work to the effective Hamiltonian for the B211state. All constants in the Hamiltonian of the NO ground state were frozen in the fit. The B-state constants determined in the fit, together with their estimated uncertainties (two standard deviations), are listed in Table I. Also listed in this table are the constants used for the ground state. In this manner, all of the observed lines were fit with a standard deviation of 0.3 cm-', which is comparable to the estimated uncertainties of both sets of data. Comparisons of the experimental and calculated data are shown in Figures 2 and 3. Although no absolute calibration is available, it was assumed that the dye laser readings were correct in a relative sense, whereupon an absolute adjustment of only 0.8 cm-I was needed to equalize the positive and negative scatter in Figures 2 and 3. Without increasing the value of the second-order centrifugal distortion constant, H,over the value obtained from the RKR potential, discrepancies of up to 4 cm-' result. It should be noted that no resolved A-doublets are observed in either the work of Schmid et al.14 or the present spectra. Such splittings are found to be quite small (
