J. phys. Chem. 1082, 86, 7427-1428
1427
Formation of the Vlbratlonally Excited Hydroxyl Radkal by Electron Impact of Water Vapor Iwao FuJRa,' Osaka Electro-Communication Univwslty, 18-8. Hatsumachi, Neyap wa, Osaka 572. Japan
Jun Tamaki, Toshlo Kasal, Klyoshl Fukul,+ and KeiJlKuwata' Department of Chemistry. Faculty ot Scknce, Osaka University, Toyonaka, Osaka 560, Japan (Received: August 7, 1981; I n Final Form: November 16, 1981)
Infrared emission spectra were obtained in the electron impact of water vapor at about 1.3 Pa (1 X torr). An emission band in the wavelength region of 2.6-3.1 pm was assigned to the vibrationally excited OH radical. An order of magnitude estimation was done of the emission cross section for OH (Au = 1).
Introduction The vibrationally excited OH radical in the electronic ground state (X211)has been observed in night airglow in the visible and infrared regi0ns.l It has also been observed in chemiluminescence experiments involving H O3 OH + 02,where many strong overtone emissions (Au L 2) appear.2 In photon impact and electron impact of H20, the formation of the OH radical has been spectroscopically studied extensively, but most of the studies deal with the excited A22+state of the OH radical. However, fragment molecules in the electronic ground state should also be produced, which can be detected by light absorption? laser-induced fluorescence, or other methods. If the molecules are formed in the vibrationally excited state, they could be detected by the observation of infrared emission. This method would be easy to apply under electron impact where strong emission occurs in the near-ultraviolet region. In the present study, infrared emission spectra were observed for the OH radical formed by electron impact of water vapor. The emission cross section of OH (Au = 1) was roughly estimated, and the mechanisms of ita formation are discussed.
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Experimental Section The electron impact system used in previous work4-e was modified for the experiments at higher pressures. An electron beam was generated by a triode electron gun, which used a rhenium filament that is chemically inactive and enabled a long time measurement at a pressure as high as 1 X torr (1torr N 133.3 Pa). Differential pumping was done for reducing the pressure of the chamber of the electron gun. The electron energy was 100-120 eV (1eV N 1.602 X J),and the beam current was about 1mA. For the observation of the emitted light, a quartz window for the infrared light was used, which has a nearly constant transparency up to 3.4 pm. The light was detected by use of a monochromator blazed at 3 pm and a PbS cell composed of one or four elements of Hamamatsu P81P cooled a t -78 "C. The bandwidth of the monochromator was 0.05 pm. The spectral response in the region of 2.5-3.3 pm was calibrated by a blackbody radiation source. For the elimination of the high-order diffraction, one of two long-pass filters were used, the cutoff wavelengths being about 1.0 pm (IRDlA) and 2.0 pm. The electron beam was chopped by applying a square wave voltage of 100 Hz on the grid of the electron gun. A 'New Cosmos Electric Co., 5-4, Mitauya-Naka, 2-Chome, Yodogawa-ku, Osaka, Japan. 0022-3654/82/2086-142?$07.25/0
TABLE I: Observed Lines of the H Atom in Figure 3 Paschen series
n 4-3 5-3 6-3
Brackett series
wavelength, p m
n
wavelength, p m
1.875 1.282" 1.094a
6-4 7-4 8-4 9-4
2.626 2.166 1.945 1.818
a Denotes that the line appeared a t the position of the second-order diffraction as well as the first-order.
modulated signal at the same frequency was introduced to a lock-in amplXer. The electron beam chopping method wm chosen because the effect of thermal radiation was very large when a light chopper was used. The signal intensity in the infrared region is generally much lower than that in the visible and ultraviolet regions because of the lower sensitivity of a photoconductive detector such as PbS and the lower probability of spontaneous emission from the excited molecules. For this reason, the measurement was carried out with a long time constant of the amplifier (10 s) and accumulation by a minicomputer was carried out. Results Figure l,a and b, shows the emission spectra obtained by electron impact of H 2 0 and D20, respectively, in the wavelength region of 2.6-3.2 pm. The sharp peak which appeared at 2.63 pm in both spectra was assigned to the hydrogen (or deuterium) atom corresponding to the transition n = 6 4. In Figure l a (H20),a broad band appeared in the 2.6-3.1-pm region, but this band did not appear in Figure l b (D20). We assigned this to the transition of Au = 1 of the vibrationally excited OH radical. If the emission corresponds to some atomic transition, D20 should give nearly the same spectrum as H20. Although the emission of OD radical should appear at about 3.8 pm, this could not be observed because of the spectral response of the PbS detector. The vertical arrows in the figure indicate the positions of the band origin,'^^ which ap-
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(1) R. MacDonald, H. L. Buijs, and H. P. Gush,Can.J. Phys., 46,2575 (1968). (2) For example, G. E. Streit and H. S. Johnston,J. Chem. Phys., 64, 96 (1976). (3) K. H. Welge and F. Stuhl, J. Chem. Phys., 46, 2440 (1967). (4) K. Fukui, I. Fujita, and K. Kuwata, Shitsuryo Bumeki, 23, 105 (1975). (5) K. Fukui, I. Fujita, and K. Kuwata, J. Phys. Chem., 81, 1252 (1977); M. T. H. Liu, T. Tanaka, T. Hirotsu, K. Fukui, I. Fujita, and K. Kuwata, ibid.,84, 3184 (1980). (6) I. Fujita, Z. Phys. Chem. (Frankfurtam Main), in press. (7) P. E. Charters and J. C. Polanyi, Can. J. Chem., 38, 1742 (1960). (8) W. S.Benedid, E. K. Plyler, and C. J. Humphreys, J.Chem. Phys., 21, 398 (1953).
0 1982 American Chemical Society
1420
The Journal of Physlcal Chemistry, Vol. 86,No. 8, 1982
Fujita et al.
z
0
~
w
--fy--Jw+ I
1.0
1.5
t
2.0 2,5 WAVELENGTH
3,O
pm
Flgure 3. Emission spectrum obtained by electron impact of H20: electron energy, 100 eV; current, 1.3 mA; pressure, 1 X torr; band-pass, 0.045 pm; filter, cutoff wavelength of about 1 pm. Flgrve 1. Emlssion spectra of (a) H20 and (b) D20 in the wavelength region from 2.5 to 3.1 pm: electron energy, 100 eV; beam current, 0.8 mA; pressure of colllsion chamber, 1 X lo-* torr. A long-pass filter of 2-pm cutoff wavelength was used. (a)
0
d(6-41
z
i314 t x
.-+
+
626p
0
0 3i(v=1-01 2 a p m
U
t1(5-.ii 2 - 2 6 p J~~v=.-O
2 5um
't
: 20
60
Electron Energy
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140
100
(ev)
Flgure 4. Excitatlon functlon of OH: wavelength, 2.8 pm; band-pass, 0.045 pm. See text for the absolute cross section.
BEAP CLRREIIT (n:
Flguro 2. (a) Dependence of intensity of ( 0 )OH ( v = 1-0) and (0) H (n = 6 4) on the beam current: pressure, 1 X torr. (b) Pressure dependence of the same emissions: beam current, 0.6 mA.
proximately agree with the positions of the Q branches. Although spectral resolution has not been realized because of low emission intensity, this spectrum suggests that the OH radical in at least u = 1and 2 and probably also u = 3 states would be formed. Figure 2a shows the dependence of the emission intensities of OH (Au = 1) and H (n = 6 4) on the electron beam current. This indicates a linear relationship between the current and the intensity for both emissions. Also a linear relationship between the emission intensity and the pressure of the sample gas is shown in Figure 2b. These results indicate that both excited species were formed in single collisions of the H20 molecule with an electron. Figure 3 shows an emission spectrum in the wavelength region of 1-3 pm. As summarized in Table I, all spectral lines except the very weak OH band at 2.7 pm were assigned to the emissions of the hydrogen atom. h almost identical spectrum, except for the OH band, was obtained by electron impact of DzO. This supports the conclusion that the whole spectrum consisb of atomic lines and the single OH band, and it follows that any molecular spectrum except for that of the OH was not observed in this wavelength region.
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Discussion An estimation of the emission cross section of the band at 2.6-3.1 pm, which has been assigned to the OH radical (X211,Au = 11, was done by comparison with that of the hydrogen atom as follows. The emission intensity of the Brackett-8 line (n = 6 4,2.626 pm) was estimated from the cross section of the Balmer-6 line (n = 6 2), where a common upper state (n = 6) is concerned with both transitions. As seen in ref 9, the branching ratio of the n
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=6 4 transition to the n = 6 2 transition depends on the azimuthal quantum number of the n = 6 state. The calculated values n(6 4)/n(6 2) of 0.48,0.19,0.18, and infinity have been given for 6s, 6p, 6d, and 6f states. A mean value of 0.3 is assumed at the present time. According to Tsurubuchi et al., the emission cross section of the Balmer-6 line from HzOhas been measured to be about 9X cm2at an electron energy of 100 eV.l0 It follows that the cross section of the Brackett-P line is about 3 X cm2. From the ratio of areas in Figure la, the emission cross section ofthe OH radical was roughly estimated to be 4 X cm2,where quenching by the wall has been neglected. If the value of Vroom et al.ll is used for the cross section of Balmer-6, the cross section of OH is calculated to be smaller than the present value by a factor of 6. Not many studies have been done on the dissociation of H,O to form the OH(X211)state. In the photolysis of H20, it has been reported that the intensity of the u = 1 state was only a few percent of the (0,O)band in the absorption spectrum of the OH(X211)r a d i ~ a l .In ~ electron impact the amount of OH radical formed in the v = 1 state of X211 may be as small as with photolysis. From a correlation diagram for the dissociation of HzO, the OH(X211)ground state is correlated with the AIBl state of HzO at 7.4 eV, and also the 3B1and 3A, states.12 At electron energies of about 100 eV, singlet states would have a major contribution. The excitation function of OH (XQ, u 2 1)in Figure 4 gradually increases to about 80 eV, and this feature is consistent wjth that for the optically allowed transition from XIAl to AIB1. (9)H. A. Bethe and E. E. Salpeter, Handb. Phys., 35, 352 (1951). (IO) S.Tsurubuchi,T. Iwai,and T. Horie, J.Phys. SOC.Jpn., 36,537 (1974). (11)D.A. Vroom and F. J. de Heer, J. Chem. Phys., 50, 1883 (1969). (12)I. Yamashita, Bull. Mech. Eng. Lab. (Tokyo), 17,1 (1975).
