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Electron-excited optical emission from H2O adsorbed on potassium halides at low temperatures
525
Nuclear Instruments and Methods in Physics Research B43 (1989) 525-528 North-Holland, Amsterdam
EPSON-EX~~D AT LOW ~~PE~~~S M. KAMADA
OPTICAL EMISSION FROM H,O ADSORBED
ON POTASS~~
HALIDES
* and E.T. ARAKAWA
Health and Safety Research Diuision, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6123,
USA
Received 22 March 1989
Electron-excited optical emissions from Hz0 adsorbed on KC1 and KBr crystals at 78 K have been investigated. A vibronic structure with constant energy intervals of about 0.12 eV was observed in the wavelength region of 440-740 nm. Exposure of the cooled surfaces to water vapor increased the emission intensity, while exposure to oxygen gases suppressed it. The temperature dependence of the vibronic emission intensity showed an anti-correlation with that of the OH emission intensity. These results indicate that the vibronic emission may be ascribed to 0; ions which are produced through dissociation of adsorbed H,O by electron excitation.
1. Introduction In recent years, optical emission spectroscopy has been applied in surface studies because it gives useful information about excited atoms and molecules in adsorption and desorption phenomena [1,2]. However, most previous works have been carried out at ambient or higher temperatures, where both electronic and thermal processes play important roles in the phenomena. Here, we report the optical emission spectra obtained at 78 K, where the electronic process is dominant. The potassium halides (KC1 and KBr) and H,O were chosen because of their simple structures and vast literature on electronic structures, including defects produced by electron excitation. We could detect small amounts of 0; ions produced by the interaction of adsorbed Hz0 with incident electrons, because of their large radiative yield at low temperatures.
2. Experimental procedures
The measurements were carried out in a vacuum system equipped with a turbomolecular pump and a cold trap. A single crystal of KCI, nominally ultrapure, was kindly supplied by the Crystal Growth Laboratory of the University of Utah. A single crystal of KBr was grown in our laboratory by the Kyropoulos technique in a dry nitrogen atmosphere. The specimens were heated to about 520 K and quenched in situ before measure-
* Permanent address: College of Engineering, Osaka Prefecture, Sakai 591, Osaka, Japan. 0168-583X/89/$03.50 (North-Holland
Physics
Q Elsevier Publishing
Science
University
Publishers
Division)
of
B.V.
ments. Specimens about 8 X 8 X 1.5 mm3 in size were cleaved in the vacuum chamber at 7 X 10e9 Torr with a knife edge. No vibronic emission spectra were observed before the introduction of water vapor into the chamber. The distilled water was boiled in order to remove dissolved 0, and CO,. The water vapor was introduced through a variable leak-valve onto the sample surface, which was cooled down to liquid nitrogen temperature. A cold trap surrounding the samples was used to prevent ~nta~nation onto the sample surface from residual gases. Pure oxygen and CO, gases were introduced into the chamber through a water trap. The emission spectrum of a single crystal of KBr containing 0; impurity, which was obtained from R. Kato of Kyoto University, was also measured for comparison. The specimens were bombarded with 1.5keV electrons at a current density of about 5 l.~A/mm*. The optical emission was collected with a quartz lens through a sapphire window and imaged on the entrance slit of a spectrometer with a spectral bandwidth of about 4 nm. All spectra reported here have been corrected for the spectral response of the detecting system. The temperature was monitored with a chromel-alumel therm~ouple attached to the sample holder.
3. Results and discussion Fig. l(a) shows the emission spectrum of KC1 taken at 78 K. The broad bands appearing at 480 and 600 nm are of bulk origin [3]. The small band around 270 nm, which appears as a vibronic structure at room temperature, may be due to adsorbed species such as CN- ions 141. A vibronic structure is also seen in the wavelength
M. Kumada, E. T. Arakuwu / Electron-e~cifed optical emission from Hz0
526 $ =2 2
Table 1 Vibronic structures from H,O/KCl K by electron excitation
and H,O/KBr
at T = 78
c
H,O/KCI
#I
Wavelength
Spacing
Wavelength
Spacing
ii iFi
(nm)
(ev)
(nm)
(ev)
445*4 467 491 518 545 576 610 648 689 736
0.13 0.13 0.13 0.12 0.12 0.12 0.12 0.11 0.11
80 z
----.
200
400
W
H,O/KBr
__
600
000
WAVELENGTH (nm)
Fig. 1. Optical emission spectra at T = 78 K excited by I.5keV electrons from KC1 crystal without H,O exposure (a) and with lo4 L H,O exposure (b). The emission spectra were taken with the resolution of 4 nm.
of 480-720 nm in the spectrum (b) taken after the introduction of lo4 L Ha0 (1 L = 10m6 Torrs). The energy spacing of the vibronic hues is almost constant at 0.12 eV, indicating that it arises from vibrational levels of molecular species. Fig. 2(a) shows the optical emission spectrum of KBr taken at 78 K. The broad bands around 300 and 540 nm are of bulk origin IS]. After the introduction of lo4 L H,O, a vibronic structure appears in the wavelength region of 440-740 nm, and a small peak is seen at 308 nm (b). The small peak at 308 nm is attributed to the transition A ‘X+ --, X ‘II in OH molecules (61 desorbed by electron st~m~ation. The energy spacing of the vibronic lines in the emission spectrum of HZO/KBr is nearly identical to that in the spectrum of H&/KC1 as listed in table 1. Prince et al. [7] have investigated the optical emission from ice bombarded by low-energy electrons. They observed three broad bands at 400, 440, and 495 nm and the OH emission line. Beenakker et al. [S] observed
484jz4 508 535 565 599 636 677 720
0.12 0.12 0.12 0.12 0.12 0.12 0.11
region
/
400 WAVELENGTH
600 (nm)
t
Fig. 2. Optical emission spectra at T = 78 K excited by I.$-keV electrons from KBr crystal without Ha0 exposure (a) and with 104 L H,O exposure (b). The emission spectrum of KBr : 0; taken with the same resoiution of 4 nm under the excitation of 260-nm photons is also shown for comparison (c).
OH emission lines and hydrogen Balmer lines from electron-impact excitation of water vapor. Lew and Iieiber [9] observed HzOc emission lines in the visible region from an electrical discharge in water vapor. The present vibronic structures do not agree with any of these emission lines or with the emission lines originating from the interaction of electrons with gaseous CO,
[61. Rolfe and co-workers [lO,ll] have reported the emission spectra from 0; impurities in alkali halides consisting of a vibronic structure with energy intervals of about 0.12 eV. The spacing of the vibronic lines in our emission spectra from H,O/KCl and H,O/KBr agrees with that obtained in KC1 : 0; and KBr : 0; [lo]. In order to compare the present vibronic structure with the 0; emission lines in more detail, the emission spectrum of KBr : 0; was measured with the same spectral resolution under the excitation of 260~MS photons. The wavelengths and gross features of the vibronic lines in the emission spectrum of H,O/KBr are in good agreement with those in KBr : 0; , as seen in figs. 2(b) and 2(c). Thus, the vibronic lines observed in the emission spectra from H,O/KCl and H,O/KBr can be attributed to transitions in 0; ions produced by electron excitation. Fig. 3 shows the dependences of the 0; emission intensity on exposure of KBr at T = 78 K to varying doses of Ha0 (a) and 0, (b). Since the intensity changed with time (first increasing and then decreasing slightly), each data point was obtained one minute after exposure. As seen in this figure, the emission intensity increases and then reaches saturation with increasing exposure to H,O vapor, while exposure to 0, produces a continuous decrease in intensity. The introduction of CO, showed no change in the emission intensity. These results indicate that 0; ions are produced by the interaction of adsorbed H,O with incident electrons.
M. Kamada, E. T. Arakawa / Electron-excited optical emission from H,O
3 1.5!‘E 3
KBr
e
(a) Hz0
(b) O2
J
I 100
I 50
00
0
EXPOSURE
150
(L)
Fig. 3. Dependences of the vibronic line intensity of T = 78 K on exposure of the KBr surface to varying doses of H,O (a) and 0, (b).
When water vapor is excited by electrons or photons, it decomposes through dissociation channels as follows [7,12]: H,O =!%
OH+H,
(I)
O+H,,
(2)
H20+ + e.
(3)
However, optical emission due to HzO+ or 0 atoms in the visible region were not observed in the present experiment. Instead, the major optical emission observed from H,O adsorbed in potassium halides, aside from the bands due to bulk origin from the alkali halides, was the 0; vibronic lines. Therefore, adsorbed water appears to decompose through dissociation channels different from those for water vapor. Fig. 4 shows the temperature dependences of the 0; optical emission line at 545 nm (a) and OH emission
line at 308 nm (b) from H,O/KBr and that of the 0; optical emission line at 545 nm in KBr : 0; (c). As seen in this figure, the intensity of the 0; emission line decreases, while that of the OH emission line increases, with increasing temperature. A similar temperature dependence of the 0; emission lines was observed for H,O/KCl. However, the OH emission line was not clearly observed because of the overlapping CN- band. The radiative yields of 0; emission lines in KCl: 0; ([13] and KBr : 0; are almost constant in the temperature range concerned, showing that the temperature dependence of the 0; emission from H,O/KCl and H,O/KBr is not due to thermal quenching of the radiative yield. Thus, it appears that some H,O adsorbed on potassium halides decompose into 0; ions and others into OH molecules under electron excitation and that the branching ratio between these channels depends on the temperature. Temperature dependences of H,O adsorbed on the sample surfaces and H,O desorbed by electron excitation were also monitored by Auger spectroscopy and with a quadrupole mass spectrometer in another conventional Auger chamber. In these experiments, specimens were cleaved in air and introduced to the chamber within one hour. The intensity of the oxygen KLL Auger line decreased in accord with the temperature dependence of the 0; emission lines, while the partial pressure of H,O showed an inverse dependence with the oxygen KLL Auger line, indicating that some of the adsorbed H,O desorb as neutral molecules by electron excitation. Thus, the adsorbed H,O could decay by the following channels: H,O,,:
OH + H,
(4)
:02,
(5)
H,O,
3 .% 3 1.5- -O-O-~--.~ Ji
.-.-.-.-.-
s > lt % F z g 0.5ii 5 0
100
150 TEMPERATURE
200 PK)
Fig. 4. Temperature dependences of the 0, emission line at 545 nm (a) and the OH emission line at 308 nm (b) from H,O/KBr and that of the 0; emission line at 545 nm in KBr : 0; (c).
521
+ H, >
(6)
where channels (4) and (6) result in the desorption of OH and H,O molecules and channel (5) leads to the 0; optical emission. It should be mentioned that lo-20% of the 0; emission intensity still exists after the cycle of heating to 520 K and subsequent cooling to 78 K. This indicates that some adsorbed 0; ions may be embedded in the surface layer of the potassium halides during electron excitation. The possibility that 0; ions may be produced from OH molecules through channel (4) seems unlikely [14] since the luminescence bands due to OH impurities in KC1 and KBr were not clearly observed in the wavelength region of 300-400 nm [15,16]. On the other hand, it is known that the co-adsorption of oxygen atoms with molecules adsorbed on metallic surfaces plays an important role in adsorption phenomena [1,2,17]. Therefore, if a small amount of oxygen is present in the water vapor and oxygen atoms
528
M. Kamada, E. T. Arakawa
co-adsorb ions
H,O,,
with
could
H,O
be formed
+ O,, :
on the KC1 and
KBr
surfaces,
/ Electron-excited 0,
optical emission from H,O
References
as follows:
02., + H,.
(7)
However, the probability for this latter process to account for the 0; emission is small, since the wavelengths of the emission lines in H,O/KCl are shifted from those in H,O/KBr. Further investigations, such as m~surements of the excitation spectra around excitation thresholds of valence electrons and optical emission measurements with high resolution under various surface conditions, are desired. In summary, we have measured electron-excited emission spectra from H,O adsorbed on KC1 and KBr at 78 K. The spectra showed a vibronic structure with constant energy intervals of about 0.12 eV. Exposure of the surfaces to water vapor increased the emission intensity, while exposure to oxygen suppressed it. The temperature dependence of the intensity of the vibronic structure showed an anti-correlation with that of the OH emission line. From the above observations and from a comparison with the spectrum of 0, impurity in alkali halides, we attribute the present vibronic structure to 0; ions formed by electron excitation of H,O adsorbed on the sample surface. The authors would like to thank Professor R. Kato of Kyoto University for supplying a single crystal of KBr containing 0; impurity. This research was sponsored by the Office of Health and Environmental Research, US Department of Energy, under contract DEAC05840R21400 with Martin Marietta Energy Systems, Inc.
(11Desorption
Induced by Electronic Transitions (DIET I), eds. N.H. Tolk, M.M. Traum, J.C. Tully and T.E. Madey (Springer, Berlin/Heidelberg/New York, 1983). PI Desorption Induced by Electronic Transitions (DIET II), eds. W. Brenig and D. Menzel (Springer, Berlin/Heidelberg/New York, 1985). 131 M.N. Kabler, Phys. Rev. 136 (1964) 245. 141 See, for example, D. Cherry et al., Nucl. Instr. and Meth. B13 (1986) 533, and refs. cited therein. PI C.T. Butler, Phys. Rev. 141 (1966) 750. of [61 R.W.B. Pearse and A.G. Gaydon, The Identification Molecular Spectra (Chapman & Hall, Ltd., London, 1976). f71 R.H. Prince, G.N. Sears and F.J. Morgan, J. Chem. Phys. 64 (1976) 3998. F.J. DeHeer, H.B. Krop and G.R. PI C.I.M. Beenakker, Mohlmann, Chem. Phys. 6 (1974) 445. [91 H. Lew and I. Heiber, J. Chem. Phys. 58 (1973) 1256. WI J. Rolfe, F.R. Lipsett and W.J. King, Phys. Rev. 123 (1961) 447. ill1 J. Rolfe, M. Ikezawa and T. Timusk, Phys. Rev. B7 (1973) 3913. I. Nenner, H. Frohbch WI 0. Dutuit, A. Tabche-Fouhaile, and P.M. Guyon, J. Chem. Phys. 83 (1985) 584. P31 S.R. Wilk, R.W. Boyd and K.J. Teegarden, Optics Commun. 47 (1983) 404. [I41 Physics of Color Centers, ed. W.B. Fowler (Academic Press, New York and London, 1968) p. 169. 4 Wt D.A. Patterson and M.N. Kabler, Solid State Co~un. (1965) 75. 1161 H. Kbstlin, Solid State Commun. 4 (1965) 81. [17] C. Noble and C. Bemrdorf, Surf. Sci. 182 (1987) 499.
Nuclear Instruments and Methods in Physics Research B43 (1989) 525-528 North-Holland, Amsterdam
EPSON-EX~~D AT LOW ~~PE~~~S M. KAMADA
OPTICAL EMISSION FROM H,O ADSORBED
ON POTASS~~
HALIDES
* and E.T. ARAKAWA
Health and Safety Research Diuision, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6123,
USA
Received 22 March 1989
Electron-excited optical emissions from Hz0 adsorbed on KC1 and KBr crystals at 78 K have been investigated. A vibronic structure with constant energy intervals of about 0.12 eV was observed in the wavelength region of 440-740 nm. Exposure of the cooled surfaces to water vapor increased the emission intensity, while exposure to oxygen gases suppressed it. The temperature dependence of the vibronic emission intensity showed an anti-correlation with that of the OH emission intensity. These results indicate that the vibronic emission may be ascribed to 0; ions which are produced through dissociation of adsorbed H,O by electron excitation.
1. Introduction In recent years, optical emission spectroscopy has been applied in surface studies because it gives useful information about excited atoms and molecules in adsorption and desorption phenomena [1,2]. However, most previous works have been carried out at ambient or higher temperatures, where both electronic and thermal processes play important roles in the phenomena. Here, we report the optical emission spectra obtained at 78 K, where the electronic process is dominant. The potassium halides (KC1 and KBr) and H,O were chosen because of their simple structures and vast literature on electronic structures, including defects produced by electron excitation. We could detect small amounts of 0; ions produced by the interaction of adsorbed Hz0 with incident electrons, because of their large radiative yield at low temperatures.
2. Experimental procedures
The measurements were carried out in a vacuum system equipped with a turbomolecular pump and a cold trap. A single crystal of KCI, nominally ultrapure, was kindly supplied by the Crystal Growth Laboratory of the University of Utah. A single crystal of KBr was grown in our laboratory by the Kyropoulos technique in a dry nitrogen atmosphere. The specimens were heated to about 520 K and quenched in situ before measure-
* Permanent address: College of Engineering, Osaka Prefecture, Sakai 591, Osaka, Japan. 0168-583X/89/$03.50 (North-Holland
Physics
Q Elsevier Publishing
Science
University
Publishers
Division)
of
B.V.
ments. Specimens about 8 X 8 X 1.5 mm3 in size were cleaved in the vacuum chamber at 7 X 10e9 Torr with a knife edge. No vibronic emission spectra were observed before the introduction of water vapor into the chamber. The distilled water was boiled in order to remove dissolved 0, and CO,. The water vapor was introduced through a variable leak-valve onto the sample surface, which was cooled down to liquid nitrogen temperature. A cold trap surrounding the samples was used to prevent ~nta~nation onto the sample surface from residual gases. Pure oxygen and CO, gases were introduced into the chamber through a water trap. The emission spectrum of a single crystal of KBr containing 0; impurity, which was obtained from R. Kato of Kyoto University, was also measured for comparison. The specimens were bombarded with 1.5keV electrons at a current density of about 5 l.~A/mm*. The optical emission was collected with a quartz lens through a sapphire window and imaged on the entrance slit of a spectrometer with a spectral bandwidth of about 4 nm. All spectra reported here have been corrected for the spectral response of the detecting system. The temperature was monitored with a chromel-alumel therm~ouple attached to the sample holder.
3. Results and discussion Fig. l(a) shows the emission spectrum of KC1 taken at 78 K. The broad bands appearing at 480 and 600 nm are of bulk origin [3]. The small band around 270 nm, which appears as a vibronic structure at room temperature, may be due to adsorbed species such as CN- ions 141. A vibronic structure is also seen in the wavelength
M. Kumada, E. T. Arakuwu / Electron-e~cifed optical emission from Hz0
526 $ =2 2
Table 1 Vibronic structures from H,O/KCl K by electron excitation
and H,O/KBr
at T = 78
c
H,O/KCI
#I
Wavelength
Spacing
Wavelength
Spacing
ii iFi
(nm)
(ev)
(nm)
(ev)
445*4 467 491 518 545 576 610 648 689 736
0.13 0.13 0.13 0.12 0.12 0.12 0.12 0.11 0.11
80 z
----.
200
400
W
H,O/KBr
__
600
000
WAVELENGTH (nm)
Fig. 1. Optical emission spectra at T = 78 K excited by I.5keV electrons from KC1 crystal without H,O exposure (a) and with lo4 L H,O exposure (b). The emission spectra were taken with the resolution of 4 nm.
of 480-720 nm in the spectrum (b) taken after the introduction of lo4 L Ha0 (1 L = 10m6 Torrs). The energy spacing of the vibronic hues is almost constant at 0.12 eV, indicating that it arises from vibrational levels of molecular species. Fig. 2(a) shows the optical emission spectrum of KBr taken at 78 K. The broad bands around 300 and 540 nm are of bulk origin IS]. After the introduction of lo4 L H,O, a vibronic structure appears in the wavelength region of 440-740 nm, and a small peak is seen at 308 nm (b). The small peak at 308 nm is attributed to the transition A ‘X+ --, X ‘II in OH molecules (61 desorbed by electron st~m~ation. The energy spacing of the vibronic lines in the emission spectrum of HZO/KBr is nearly identical to that in the spectrum of H&/KC1 as listed in table 1. Prince et al. [7] have investigated the optical emission from ice bombarded by low-energy electrons. They observed three broad bands at 400, 440, and 495 nm and the OH emission line. Beenakker et al. [S] observed
484jz4 508 535 565 599 636 677 720
0.12 0.12 0.12 0.12 0.12 0.12 0.11
region
/
400 WAVELENGTH
600 (nm)
t
Fig. 2. Optical emission spectra at T = 78 K excited by I.$-keV electrons from KBr crystal without Ha0 exposure (a) and with 104 L H,O exposure (b). The emission spectrum of KBr : 0; taken with the same resoiution of 4 nm under the excitation of 260-nm photons is also shown for comparison (c).
OH emission lines and hydrogen Balmer lines from electron-impact excitation of water vapor. Lew and Iieiber [9] observed HzOc emission lines in the visible region from an electrical discharge in water vapor. The present vibronic structures do not agree with any of these emission lines or with the emission lines originating from the interaction of electrons with gaseous CO,
[61. Rolfe and co-workers [lO,ll] have reported the emission spectra from 0; impurities in alkali halides consisting of a vibronic structure with energy intervals of about 0.12 eV. The spacing of the vibronic lines in our emission spectra from H,O/KCl and H,O/KBr agrees with that obtained in KC1 : 0; and KBr : 0; [lo]. In order to compare the present vibronic structure with the 0; emission lines in more detail, the emission spectrum of KBr : 0; was measured with the same spectral resolution under the excitation of 260~MS photons. The wavelengths and gross features of the vibronic lines in the emission spectrum of H,O/KBr are in good agreement with those in KBr : 0; , as seen in figs. 2(b) and 2(c). Thus, the vibronic lines observed in the emission spectra from H,O/KCl and H,O/KBr can be attributed to transitions in 0; ions produced by electron excitation. Fig. 3 shows the dependences of the 0; emission intensity on exposure of KBr at T = 78 K to varying doses of Ha0 (a) and 0, (b). Since the intensity changed with time (first increasing and then decreasing slightly), each data point was obtained one minute after exposure. As seen in this figure, the emission intensity increases and then reaches saturation with increasing exposure to H,O vapor, while exposure to 0, produces a continuous decrease in intensity. The introduction of CO, showed no change in the emission intensity. These results indicate that 0; ions are produced by the interaction of adsorbed H,O with incident electrons.
M. Kamada, E. T. Arakawa / Electron-excited optical emission from H,O
3 1.5!‘E 3
KBr
e
(a) Hz0
(b) O2
J
I 100
I 50
00
0
EXPOSURE
150
(L)
Fig. 3. Dependences of the vibronic line intensity of T = 78 K on exposure of the KBr surface to varying doses of H,O (a) and 0, (b).
When water vapor is excited by electrons or photons, it decomposes through dissociation channels as follows [7,12]: H,O =!%
OH+H,
(I)
O+H,,
(2)
H20+ + e.
(3)
However, optical emission due to HzO+ or 0 atoms in the visible region were not observed in the present experiment. Instead, the major optical emission observed from H,O adsorbed in potassium halides, aside from the bands due to bulk origin from the alkali halides, was the 0; vibronic lines. Therefore, adsorbed water appears to decompose through dissociation channels different from those for water vapor. Fig. 4 shows the temperature dependences of the 0; optical emission line at 545 nm (a) and OH emission
line at 308 nm (b) from H,O/KBr and that of the 0; optical emission line at 545 nm in KBr : 0; (c). As seen in this figure, the intensity of the 0; emission line decreases, while that of the OH emission line increases, with increasing temperature. A similar temperature dependence of the 0; emission lines was observed for H,O/KCl. However, the OH emission line was not clearly observed because of the overlapping CN- band. The radiative yields of 0; emission lines in KCl: 0; ([13] and KBr : 0; are almost constant in the temperature range concerned, showing that the temperature dependence of the 0; emission from H,O/KCl and H,O/KBr is not due to thermal quenching of the radiative yield. Thus, it appears that some H,O adsorbed on potassium halides decompose into 0; ions and others into OH molecules under electron excitation and that the branching ratio between these channels depends on the temperature. Temperature dependences of H,O adsorbed on the sample surfaces and H,O desorbed by electron excitation were also monitored by Auger spectroscopy and with a quadrupole mass spectrometer in another conventional Auger chamber. In these experiments, specimens were cleaved in air and introduced to the chamber within one hour. The intensity of the oxygen KLL Auger line decreased in accord with the temperature dependence of the 0; emission lines, while the partial pressure of H,O showed an inverse dependence with the oxygen KLL Auger line, indicating that some of the adsorbed H,O desorb as neutral molecules by electron excitation. Thus, the adsorbed H,O could decay by the following channels: H,O,,:
OH + H,
(4)
:02,
(5)
H,O,
3 .% 3 1.5- -O-O-~--.~ Ji
.-.-.-.-.-
s > lt % F z g 0.5ii 5 0
100
150 TEMPERATURE
200 PK)
Fig. 4. Temperature dependences of the 0, emission line at 545 nm (a) and the OH emission line at 308 nm (b) from H,O/KBr and that of the 0; emission line at 545 nm in KBr : 0; (c).
521
+ H, >
(6)
where channels (4) and (6) result in the desorption of OH and H,O molecules and channel (5) leads to the 0; optical emission. It should be mentioned that lo-20% of the 0; emission intensity still exists after the cycle of heating to 520 K and subsequent cooling to 78 K. This indicates that some adsorbed 0; ions may be embedded in the surface layer of the potassium halides during electron excitation. The possibility that 0; ions may be produced from OH molecules through channel (4) seems unlikely [14] since the luminescence bands due to OH impurities in KC1 and KBr were not clearly observed in the wavelength region of 300-400 nm [15,16]. On the other hand, it is known that the co-adsorption of oxygen atoms with molecules adsorbed on metallic surfaces plays an important role in adsorption phenomena [1,2,17]. Therefore, if a small amount of oxygen is present in the water vapor and oxygen atoms
528
M. Kamada, E. T. Arakawa
co-adsorb ions
H,O,,
with
could
H,O
be formed
+ O,, :
on the KC1 and
KBr
surfaces,
/ Electron-excited 0,
optical emission from H,O
References
as follows:
02., + H,.
(7)
However, the probability for this latter process to account for the 0; emission is small, since the wavelengths of the emission lines in H,O/KCl are shifted from those in H,O/KBr. Further investigations, such as m~surements of the excitation spectra around excitation thresholds of valence electrons and optical emission measurements with high resolution under various surface conditions, are desired. In summary, we have measured electron-excited emission spectra from H,O adsorbed on KC1 and KBr at 78 K. The spectra showed a vibronic structure with constant energy intervals of about 0.12 eV. Exposure of the surfaces to water vapor increased the emission intensity, while exposure to oxygen suppressed it. The temperature dependence of the intensity of the vibronic structure showed an anti-correlation with that of the OH emission line. From the above observations and from a comparison with the spectrum of 0, impurity in alkali halides, we attribute the present vibronic structure to 0; ions formed by electron excitation of H,O adsorbed on the sample surface. The authors would like to thank Professor R. Kato of Kyoto University for supplying a single crystal of KBr containing 0; impurity. This research was sponsored by the Office of Health and Environmental Research, US Department of Energy, under contract DEAC05840R21400 with Martin Marietta Energy Systems, Inc.
(11Desorption
Induced by Electronic Transitions (DIET I), eds. N.H. Tolk, M.M. Traum, J.C. Tully and T.E. Madey (Springer, Berlin/Heidelberg/New York, 1983). PI Desorption Induced by Electronic Transitions (DIET II), eds. W. Brenig and D. Menzel (Springer, Berlin/Heidelberg/New York, 1985). 131 M.N. Kabler, Phys. Rev. 136 (1964) 245. 141 See, for example, D. Cherry et al., Nucl. Instr. and Meth. B13 (1986) 533, and refs. cited therein. PI C.T. Butler, Phys. Rev. 141 (1966) 750. of [61 R.W.B. Pearse and A.G. Gaydon, The Identification Molecular Spectra (Chapman & Hall, Ltd., London, 1976). f71 R.H. Prince, G.N. Sears and F.J. Morgan, J. Chem. Phys. 64 (1976) 3998. F.J. DeHeer, H.B. Krop and G.R. PI C.I.M. Beenakker, Mohlmann, Chem. Phys. 6 (1974) 445. [91 H. Lew and I. Heiber, J. Chem. Phys. 58 (1973) 1256. WI J. Rolfe, F.R. Lipsett and W.J. King, Phys. Rev. 123 (1961) 447. ill1 J. Rolfe, M. Ikezawa and T. Timusk, Phys. Rev. B7 (1973) 3913. I. Nenner, H. Frohbch WI 0. Dutuit, A. Tabche-Fouhaile, and P.M. Guyon, J. Chem. Phys. 83 (1985) 584. P31 S.R. Wilk, R.W. Boyd and K.J. Teegarden, Optics Commun. 47 (1983) 404. [I41 Physics of Color Centers, ed. W.B. Fowler (Academic Press, New York and London, 1968) p. 169. 4 Wt D.A. Patterson and M.N. Kabler, Solid State Co~un. (1965) 75. 1161 H. Kbstlin, Solid State Commun. 4 (1965) 81. [17] C. Noble and C. Bemrdorf, Surf. Sci. 182 (1987) 499.