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Temperature dependence of absorption cross-section of H2O, HOD, and D2O in the spectral region 140–193 nm
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1572–1576
Temperature dependence of absorption cross-section of H2O, HOD, and D2O in the spectral region 140–193 nm Chao-Yu Chunga, Eh Piew Chewa, Bing-Ming Chenga,*, Mohammed Bahoub, Yuan-Pern Leeb,1 a
Synchrotron Radiation Research Center, Hsinchu Science-Based Industrial Park, No. 1 R&D Road VI, Hsinchu 300, Taiwan b Department of Chemistry, National Tsing Hua University 101, Sec. 2, Kuang Fu Road, Hsinchu 30013, Taiwan
Abstract Measurements of absolute photoabsorption cross-sections of H2O, D2O and HOD in the spectral range 140–195 nm were performed at 295, 275, and 250 K using a double-beam absorption cell with a synchrotron radiation source. The absorption cross-sections in the spectral range 150–174 nm are nearly invariant upon isotopic substitution, but become smaller for heavier isotopomers in the region l>174 nm. For each isotopomer, absorption cross-sections are nearly temperature independent in the spectral range 140–173 nm, but exhibit a small negative temperature dependence for wavelengths greater than 173 nm. # 2001 Elsevier Science B.V. All rights reserved. PACS: 33.20.Ni; 96.30.Ge Keywords: Absorption cross-section; H2O; HOD; D2O
1. Introduction Owen et al. [1] discovered that Martian water is enriched in deuterium; the deduced D/H ratio is enriched in deuterium by a factor of 6 relative to the terrestrial value. It is generally agreed that this isotopic fractionation is a result of escape processes; the lighter (H) species is removed over long periods [2–4]. Recent detection of Lyman a emission of deuterium from the upper atmosphere of Mars [5] leads to an isotopic fractionation
*Corresponding author. Tel.: +886-3-578-0281 ext. 7323; fax: +886-3-578-9816. E-mail addresses: [email protected] (B.-M. Cheng), [email protected] (Y.-P. Lee). 1 Jointly appointed by the Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan.
factor much lower than that predicted by a theoretical model of Yung et al. [6]. Water is the ultimate source of hydrogen in Martian atmosphere and its photolysis is the primary mechanism for destruction of H2O and HOD to form H or D [6]. It is important to determine absolute absorption cross-sections of various isotopomers of water in order to assess the importance of the photoinduced isotopic fractionation effect (PHIFE) [7]. Laboratory measurements of absorption crosssection of H2O, HOD and D2O in the range 140– 195 nm were performed recently. Preliminary results at 295 K were applied to a modified model of Martian atmosphere to support the importance of PHIFE and to reconcile a discrepancy between recent observations and past modeling [7]. Because the temperature of Mars is low, it is important to determine whether there are significant changes in
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 7 6 2 - 8
C.-Y. Chung et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1572–1576
absorption cross-sections at temperatures characteristic of the atmosphere of Mars.
2. Experiments The light source was the synchrotron radiation dispersed with a 1-m Seya-Namioka monochromator located at the Synchrotron Radiation Research Center (SRRC) in Taiwan with a 1.5 GeV storage ring [7]. The monochromator was equipped with a grating of 600 grooves/mm blazed at 140 nm. The slit width was typically 0.05 mm, corresponding to a spectral bandwidth of 0.1 nm. The monochromator was scanned in either 0.1- or 0.2-nm steps with signal averaging period of 3 s at each step. Fig. 1 shows a schematics of the dual beam photoabsorption cell. The absorption cells with an inner diameter of 39.5 mm and a path length of 113.3 cm was used. To avoid variation in pressure due to irradiation or surface adsorption/desorption, a reservoir of 1382 cm3 in volume was connected to the absorption cell. The absorption cell has a pressure port connected with four MKS Baratron pressure meters (model 127AA, range: 0.1, 1, 10 and 100 Torr). A small fraction of the light beam was reflected with a CaF2 plate and, after passing one additional CaF2 plate, irradiated onto a glass window coated with sodium salicylate; the fluorescence signal subsequently detected was employed for normalization. The light transmitted from the CaF2 beamsplitter passed through an
Fig. 1. Experimental setup.
1573
absorption cell equipped with two CaF2 endwindows, and irradiated onto a glass window coated with sodium salicylate. The temperature was controlled by circulating refrigerated ethanol through the cooling coil. The vapor pressure of water at 275 and 250 K are 4.58 and 0.58 Torr, respectively. Hence, the pressure was kept below 3.5 and 0.35 Torr at 275 and 250 K, respectively, to avoid condensation on the windows. In addition to pure H2O and D2O (Merck Sharp and Dome, isotopic purity 99.8%) samples, two mixtures were prepared from deionized H2O and pure D2O with molar ratios 1 : 2.81 and 1 : 4.02, respectively. Relative concentrations of H2O, D2O, and HOD in the gas sample mixture were determined from the equilibrium constant H2 Oð‘Þ þD2 Oð‘Þ $ 2HODð‘Þ
ð1Þ
with Keq ’3.74 at 250–295 K [8,9]. The gaseous molar fractions of H2O, HOD, and D2O of these two samples are thus determined to be (0.079, 0.394, 0.526) and (0.046, 0.328, 0.625). For experiments involving deuterated samples, at least 10 cycles of passivation were carried out.
3. Results and discussions Absorbance was calculated according to Beer’s law. Data with absorbance greater than 2.0 were discarded to avoid saturation effects. At each wavelength, absorbance was measured at 8 to 14 different pressures and plotted against number densities; they are fitted to a line to yield the absorption cross-section. For samples containing H2O, HOD and D2O, absorption cross-sections of H2O and D2O measured separately in this work were used to derive that of HOD. The resultant absorption cross-sections of H2O, HOD, and D2O at 250 K in the spectral region 140–195 nm are shown in Fig. 2; those determined at 295 K has been shown previously [7]. Crosssections at 1-nm intervals for each species at 250 and 295 K are also listed in Table 1. A complete listing of data at 0.2-nm intervals is available from the web site http://ams-bmc.srrc.gov.tw. Variations in absorption cross-sections of HOD determined from two mixtures containing different
1574
C.-Y. Chung et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1572–1576
fractions of HOD are typically within 5%; averaged values are listed. Cross-sections of H2O determined in this work are compared with recent measurements. Our spectrum is nearly identical to that reported by Chan et al. [10]. Our results are also in agreement with those of Yoshino et al. [11] (using a 12-cm cell and synchrotron radiation) in the spectral region 140–182 nm. For wavelength greater than 182 nm, Yoshino et al. overestimated the cross-section. Because we employed a much longer absorption cell, our measurements are expected to be more reliable in this region. We also extended the measurements from 185 to 195 nm. Absorption cross-sections of D2O shows a trend similar to those of Laufer et al. [12]. Our
Fig. 2. Cross-sections of H2O (solid line), HOD (dashed line), and D2O (dotted line) at 250 K.
Table 1 Absorption cross-section (in 1019 cm2) of various isotopomers of H2O at 250 and 295 K 295 K
250 K
nm
H2O
HOD
D2O
H2O
HOD
D2O
140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171
7.99 8.03 5.99 5.93 5.66 6.00 6.60 7.90 9.09 10.7 12.1 14.3 16.9 19.4 22.0 24.6 28.2 31.1 34.6 37.2 40.9 43.6 46.3 49.0 51.5 53.2 54.1 54.1 53.8 53.2 52.3 49.8
6.04 4.82 3.89 3.77 3.43 4.08 5.05 5.96 7.27 8.46 11.3 11.9 16.7 20.1 23.3 27.3 30.2 34.4 38.6 42.4 46.5 49.5 52.4 55.4 59.5 60.1 61.9 62.6 58.2 57.4 55.8 51.0
3.82 2.66 2.41 2.46 2.84 3.51 4.33 5.43 6.68 8.38 10.4 13.3 16.6 19.8 23.8 27.3 32.3 36.8 41.5 46.6 50.8 54.9 59.7 62.3 64.5 66.3 67.1 64.5 61.8 59.2 55.4 48.4
7.81 8.42 6.32 6.20 6.09 6.42 7.32 8.20 9.67 11.5 13.3 15.5 17.6 20.7 23.5 26.7 29.3 32.6 36.3 38.9 43.3 46.7 49.1 52.0 53.8 56.4 57.3 57.5 56.7 56.0 55.4 52.2
6.72 5.33 3.27 3.77 3.95 4.04 6.73 5.60 7.56 11.5 11.0 12.9 18.4 23.8 24.8 27.3 31.5 36.5 41.3 47.7 49.2 53.1 55.2 58.5 61.7 62.4 61.9 62.9 61.2 58.0 56.8 51.8
3.47 2.76 2.68 2.07 3.20 3.67 4.18 5.62 7.17 8.73 11.5 13.5 16.2 20.0 24.0 28.5 33.1 37.9 42.7 47.5 52.6 57.1 61.6 64.9 66.4 68.4 69.4 67.4 64.2 61.3 57.7 51.1
C.-Y. Chung et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1572–1576
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Table 1 (continued) 295 K
250 K
nm
H2O
HOD
D2O
H2O
HOD
D2O
172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195
45.7 41.4 36.2 33.2 29.9 25.4 19.6 13.7 8.72 5.45 3.16 1.87 1.11 0.671 0.407 0.246 0.154 0.0956 0.0588 0.0385 0.0238 0.0162 0.0095 0.0058
45.0 37.8 34.2 29.2 23.2 17.1 11.6 6.88 3.89 2.22 1.31 0.783 0.464 0.282 0.175 0.106 0.0648 0.0408 0.0248 0.0145 0.0077 0.0051 0.0032 0.0017
40.5 33.9 28.8 22.3 14.9 9.31 5.09 2.63 1.40 0.644 0.354 0.199 0.113 0.0656 0.0385 0.0242 0.0148 0.0095 0.0066 0.0036 0.0026 0.0016 0.0010 0.0006
47.9 43.1 38.9 33.7 30.1 25.1 19.6 13.4 8.42 5.04 2.96 1.77 1.03 0.637 0.368 0.200 0.117 0.079
45.0 39.0 35.4 28.6 23.6 17.6 10.9 6.37 4.04 1.50
42.6 36.1 30.2 24.3 16.3 9.51 4.98 2.65 1.24 0.56
measurements are higher (+18%) than theirs near the maximum peak at 167 nm and lower (35%) near 142.5 nm. These differences are better explained by assuming H2O impurities present in D2O samples. Water contamination reduces the cross-section values at the maximum peak and increases those at 142.5 nm as shown in Fig. 2. There is no previous measurements for HOD. The absorption cross-section of HOD is identical to that of H2O at 172.0 nm, decreases to half of that of H2O near 179.3 nm, and further decreases to one third near 195 nm. The theoretical reason for the difference between the cross-sections of H2O and its heavier isotopomers is the difference in zero point energies [13]. Fig. 3 shows ratios of cross-sections of H2O at 275 and 250 K to that at 295 K. Absorption crosssections at three temperatures are nearly identical in the range 140–172 nm, with deviations within 8% for H2O and D2O, and within 12% for HOD. For l5173 nm the absorption cross-section at 250 K relative to that at 295 K decreases from
Fig. 3. Absorption cross-sections of H2O at 275 K and 250 K relative to those at 295 K.
unity to 0.86 at 190 nm. Because smaller crosssections at longer wavelengths result to larger errors, we are unable to distinguish the small differences in temperature dependence of crosssections among various isotopomers, nor between 250 and 275 K. But the general trend of 15%
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C.-Y. Chung et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1572–1576
decrease in absorption cross-section near 190 nm at low temperature is obvious. The small temperature effect in absorption is expected to have negligible effects on modeling of Martian atmosphere. In conclusion, the absolute absorption crosssections of water and its isotopomers at 295, 275, and 250 K were determined in the spectral range 140–195 nm. There is a distinct isotopic effect on absorption near threshold; the variation in absorption cross-section between H2O and HOD supports proposed photo-induced isotopic fractionation effect in Mars. The temperature dependence of absorption cross-section is small, with a small negative dependence for wavelengths greater than 173 nm, reaching 15% near 190 nm.
Acknowledgements We thank the National Science Council and the Synchrotron Radiation Research Center for financial support.
References [1] T. Owen, J.P. Maillard, C. Debergh, B.L. Lutz, Science 240 (1988) 1767. [2] M.B. McElroy, Science 175 (1972) 443. [3] M.B. McElroy, Y.L. Yung, Planet. Space Sci. 24 (1976) 1107. [4] B.M. Jakosky, Geophys. Res. Lett. 20 (1994) 1591. [5] V.A. Krasnopolsky, M.J. Mumma, G.R. Gladstone, Science 280 (1998) 1576. [6] Y.L. Yung, J.S. Wen, J.P. Pinto, M. Allen, K.K. Pierce, S. Paulson, Icarus 76 (1988) 146. [7] B.-M. Cheng, E.P. Chew, C.-P. Liu, M. Bahou, Y.-P. Lee, Y.L. Yung, M.F. Gerstell, Geophys. Res. Lett. 26 (1999) 3657. [8] J.W. Pyper, R.S. Newbury, G.W. Barton Jr., J. Chem. Phys. 46 (1967) 2253. [9] I. Kirshenbaum, in: H.C. Urey, G.M. Murphy (Eds.), Physical Properties and Analysis of Heavy Water, McGraw-Hill, New York, 1951. [10] W.F. Chan, G. Cooper, C.E. Brion, Chem. Phys. 178 (1993) 387. [11] K. Yoshino, J.R. Esmond, W.H. Parkinson, K. Ito, T. Matsui, Chem. Phys. 211 (1996) 387. [12] A.H. Laufer, J.R. McNesby, Can. J. Chem. 43 (1965) 3487. [13] Y.L. Yung, C.E. Miller, Science 278 (1997) 1778.
Temperature dependence of absorption cross-section of H2O, HOD, and D2O in the spectral region 140–193 nm Chao-Yu Chunga, Eh Piew Chewa, Bing-Ming Chenga,*, Mohammed Bahoub, Yuan-Pern Leeb,1 a
Synchrotron Radiation Research Center, Hsinchu Science-Based Industrial Park, No. 1 R&D Road VI, Hsinchu 300, Taiwan b Department of Chemistry, National Tsing Hua University 101, Sec. 2, Kuang Fu Road, Hsinchu 30013, Taiwan
Abstract Measurements of absolute photoabsorption cross-sections of H2O, D2O and HOD in the spectral range 140–195 nm were performed at 295, 275, and 250 K using a double-beam absorption cell with a synchrotron radiation source. The absorption cross-sections in the spectral range 150–174 nm are nearly invariant upon isotopic substitution, but become smaller for heavier isotopomers in the region l>174 nm. For each isotopomer, absorption cross-sections are nearly temperature independent in the spectral range 140–173 nm, but exhibit a small negative temperature dependence for wavelengths greater than 173 nm. # 2001 Elsevier Science B.V. All rights reserved. PACS: 33.20.Ni; 96.30.Ge Keywords: Absorption cross-section; H2O; HOD; D2O
1. Introduction Owen et al. [1] discovered that Martian water is enriched in deuterium; the deduced D/H ratio is enriched in deuterium by a factor of 6 relative to the terrestrial value. It is generally agreed that this isotopic fractionation is a result of escape processes; the lighter (H) species is removed over long periods [2–4]. Recent detection of Lyman a emission of deuterium from the upper atmosphere of Mars [5] leads to an isotopic fractionation
*Corresponding author. Tel.: +886-3-578-0281 ext. 7323; fax: +886-3-578-9816. E-mail addresses: [email protected] (B.-M. Cheng), [email protected] (Y.-P. Lee). 1 Jointly appointed by the Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan.
factor much lower than that predicted by a theoretical model of Yung et al. [6]. Water is the ultimate source of hydrogen in Martian atmosphere and its photolysis is the primary mechanism for destruction of H2O and HOD to form H or D [6]. It is important to determine absolute absorption cross-sections of various isotopomers of water in order to assess the importance of the photoinduced isotopic fractionation effect (PHIFE) [7]. Laboratory measurements of absorption crosssection of H2O, HOD and D2O in the range 140– 195 nm were performed recently. Preliminary results at 295 K were applied to a modified model of Martian atmosphere to support the importance of PHIFE and to reconcile a discrepancy between recent observations and past modeling [7]. Because the temperature of Mars is low, it is important to determine whether there are significant changes in
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 7 6 2 - 8
C.-Y. Chung et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1572–1576
absorption cross-sections at temperatures characteristic of the atmosphere of Mars.
2. Experiments The light source was the synchrotron radiation dispersed with a 1-m Seya-Namioka monochromator located at the Synchrotron Radiation Research Center (SRRC) in Taiwan with a 1.5 GeV storage ring [7]. The monochromator was equipped with a grating of 600 grooves/mm blazed at 140 nm. The slit width was typically 0.05 mm, corresponding to a spectral bandwidth of 0.1 nm. The monochromator was scanned in either 0.1- or 0.2-nm steps with signal averaging period of 3 s at each step. Fig. 1 shows a schematics of the dual beam photoabsorption cell. The absorption cells with an inner diameter of 39.5 mm and a path length of 113.3 cm was used. To avoid variation in pressure due to irradiation or surface adsorption/desorption, a reservoir of 1382 cm3 in volume was connected to the absorption cell. The absorption cell has a pressure port connected with four MKS Baratron pressure meters (model 127AA, range: 0.1, 1, 10 and 100 Torr). A small fraction of the light beam was reflected with a CaF2 plate and, after passing one additional CaF2 plate, irradiated onto a glass window coated with sodium salicylate; the fluorescence signal subsequently detected was employed for normalization. The light transmitted from the CaF2 beamsplitter passed through an
Fig. 1. Experimental setup.
1573
absorption cell equipped with two CaF2 endwindows, and irradiated onto a glass window coated with sodium salicylate. The temperature was controlled by circulating refrigerated ethanol through the cooling coil. The vapor pressure of water at 275 and 250 K are 4.58 and 0.58 Torr, respectively. Hence, the pressure was kept below 3.5 and 0.35 Torr at 275 and 250 K, respectively, to avoid condensation on the windows. In addition to pure H2O and D2O (Merck Sharp and Dome, isotopic purity 99.8%) samples, two mixtures were prepared from deionized H2O and pure D2O with molar ratios 1 : 2.81 and 1 : 4.02, respectively. Relative concentrations of H2O, D2O, and HOD in the gas sample mixture were determined from the equilibrium constant H2 Oð‘Þ þD2 Oð‘Þ $ 2HODð‘Þ
ð1Þ
with Keq ’3.74 at 250–295 K [8,9]. The gaseous molar fractions of H2O, HOD, and D2O of these two samples are thus determined to be (0.079, 0.394, 0.526) and (0.046, 0.328, 0.625). For experiments involving deuterated samples, at least 10 cycles of passivation were carried out.
3. Results and discussions Absorbance was calculated according to Beer’s law. Data with absorbance greater than 2.0 were discarded to avoid saturation effects. At each wavelength, absorbance was measured at 8 to 14 different pressures and plotted against number densities; they are fitted to a line to yield the absorption cross-section. For samples containing H2O, HOD and D2O, absorption cross-sections of H2O and D2O measured separately in this work were used to derive that of HOD. The resultant absorption cross-sections of H2O, HOD, and D2O at 250 K in the spectral region 140–195 nm are shown in Fig. 2; those determined at 295 K has been shown previously [7]. Crosssections at 1-nm intervals for each species at 250 and 295 K are also listed in Table 1. A complete listing of data at 0.2-nm intervals is available from the web site http://ams-bmc.srrc.gov.tw. Variations in absorption cross-sections of HOD determined from two mixtures containing different
1574
C.-Y. Chung et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1572–1576
fractions of HOD are typically within 5%; averaged values are listed. Cross-sections of H2O determined in this work are compared with recent measurements. Our spectrum is nearly identical to that reported by Chan et al. [10]. Our results are also in agreement with those of Yoshino et al. [11] (using a 12-cm cell and synchrotron radiation) in the spectral region 140–182 nm. For wavelength greater than 182 nm, Yoshino et al. overestimated the cross-section. Because we employed a much longer absorption cell, our measurements are expected to be more reliable in this region. We also extended the measurements from 185 to 195 nm. Absorption cross-sections of D2O shows a trend similar to those of Laufer et al. [12]. Our
Fig. 2. Cross-sections of H2O (solid line), HOD (dashed line), and D2O (dotted line) at 250 K.
Table 1 Absorption cross-section (in 1019 cm2) of various isotopomers of H2O at 250 and 295 K 295 K
250 K
nm
H2O
HOD
D2O
H2O
HOD
D2O
140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171
7.99 8.03 5.99 5.93 5.66 6.00 6.60 7.90 9.09 10.7 12.1 14.3 16.9 19.4 22.0 24.6 28.2 31.1 34.6 37.2 40.9 43.6 46.3 49.0 51.5 53.2 54.1 54.1 53.8 53.2 52.3 49.8
6.04 4.82 3.89 3.77 3.43 4.08 5.05 5.96 7.27 8.46 11.3 11.9 16.7 20.1 23.3 27.3 30.2 34.4 38.6 42.4 46.5 49.5 52.4 55.4 59.5 60.1 61.9 62.6 58.2 57.4 55.8 51.0
3.82 2.66 2.41 2.46 2.84 3.51 4.33 5.43 6.68 8.38 10.4 13.3 16.6 19.8 23.8 27.3 32.3 36.8 41.5 46.6 50.8 54.9 59.7 62.3 64.5 66.3 67.1 64.5 61.8 59.2 55.4 48.4
7.81 8.42 6.32 6.20 6.09 6.42 7.32 8.20 9.67 11.5 13.3 15.5 17.6 20.7 23.5 26.7 29.3 32.6 36.3 38.9 43.3 46.7 49.1 52.0 53.8 56.4 57.3 57.5 56.7 56.0 55.4 52.2
6.72 5.33 3.27 3.77 3.95 4.04 6.73 5.60 7.56 11.5 11.0 12.9 18.4 23.8 24.8 27.3 31.5 36.5 41.3 47.7 49.2 53.1 55.2 58.5 61.7 62.4 61.9 62.9 61.2 58.0 56.8 51.8
3.47 2.76 2.68 2.07 3.20 3.67 4.18 5.62 7.17 8.73 11.5 13.5 16.2 20.0 24.0 28.5 33.1 37.9 42.7 47.5 52.6 57.1 61.6 64.9 66.4 68.4 69.4 67.4 64.2 61.3 57.7 51.1
C.-Y. Chung et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1572–1576
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Table 1 (continued) 295 K
250 K
nm
H2O
HOD
D2O
H2O
HOD
D2O
172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195
45.7 41.4 36.2 33.2 29.9 25.4 19.6 13.7 8.72 5.45 3.16 1.87 1.11 0.671 0.407 0.246 0.154 0.0956 0.0588 0.0385 0.0238 0.0162 0.0095 0.0058
45.0 37.8 34.2 29.2 23.2 17.1 11.6 6.88 3.89 2.22 1.31 0.783 0.464 0.282 0.175 0.106 0.0648 0.0408 0.0248 0.0145 0.0077 0.0051 0.0032 0.0017
40.5 33.9 28.8 22.3 14.9 9.31 5.09 2.63 1.40 0.644 0.354 0.199 0.113 0.0656 0.0385 0.0242 0.0148 0.0095 0.0066 0.0036 0.0026 0.0016 0.0010 0.0006
47.9 43.1 38.9 33.7 30.1 25.1 19.6 13.4 8.42 5.04 2.96 1.77 1.03 0.637 0.368 0.200 0.117 0.079
45.0 39.0 35.4 28.6 23.6 17.6 10.9 6.37 4.04 1.50
42.6 36.1 30.2 24.3 16.3 9.51 4.98 2.65 1.24 0.56
measurements are higher (+18%) than theirs near the maximum peak at 167 nm and lower (35%) near 142.5 nm. These differences are better explained by assuming H2O impurities present in D2O samples. Water contamination reduces the cross-section values at the maximum peak and increases those at 142.5 nm as shown in Fig. 2. There is no previous measurements for HOD. The absorption cross-section of HOD is identical to that of H2O at 172.0 nm, decreases to half of that of H2O near 179.3 nm, and further decreases to one third near 195 nm. The theoretical reason for the difference between the cross-sections of H2O and its heavier isotopomers is the difference in zero point energies [13]. Fig. 3 shows ratios of cross-sections of H2O at 275 and 250 K to that at 295 K. Absorption crosssections at three temperatures are nearly identical in the range 140–172 nm, with deviations within 8% for H2O and D2O, and within 12% for HOD. For l5173 nm the absorption cross-section at 250 K relative to that at 295 K decreases from
Fig. 3. Absorption cross-sections of H2O at 275 K and 250 K relative to those at 295 K.
unity to 0.86 at 190 nm. Because smaller crosssections at longer wavelengths result to larger errors, we are unable to distinguish the small differences in temperature dependence of crosssections among various isotopomers, nor between 250 and 275 K. But the general trend of 15%
1576
C.-Y. Chung et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1572–1576
decrease in absorption cross-section near 190 nm at low temperature is obvious. The small temperature effect in absorption is expected to have negligible effects on modeling of Martian atmosphere. In conclusion, the absolute absorption crosssections of water and its isotopomers at 295, 275, and 250 K were determined in the spectral range 140–195 nm. There is a distinct isotopic effect on absorption near threshold; the variation in absorption cross-section between H2O and HOD supports proposed photo-induced isotopic fractionation effect in Mars. The temperature dependence of absorption cross-section is small, with a small negative dependence for wavelengths greater than 173 nm, reaching 15% near 190 nm.
Acknowledgements We thank the National Science Council and the Synchrotron Radiation Research Center for financial support.
References [1] T. Owen, J.P. Maillard, C. Debergh, B.L. Lutz, Science 240 (1988) 1767. [2] M.B. McElroy, Science 175 (1972) 443. [3] M.B. McElroy, Y.L. Yung, Planet. Space Sci. 24 (1976) 1107. [4] B.M. Jakosky, Geophys. Res. Lett. 20 (1994) 1591. [5] V.A. Krasnopolsky, M.J. Mumma, G.R. Gladstone, Science 280 (1998) 1576. [6] Y.L. Yung, J.S. Wen, J.P. Pinto, M. Allen, K.K. Pierce, S. Paulson, Icarus 76 (1988) 146. [7] B.-M. Cheng, E.P. Chew, C.-P. Liu, M. Bahou, Y.-P. Lee, Y.L. Yung, M.F. Gerstell, Geophys. Res. Lett. 26 (1999) 3657. [8] J.W. Pyper, R.S. Newbury, G.W. Barton Jr., J. Chem. Phys. 46 (1967) 2253. [9] I. Kirshenbaum, in: H.C. Urey, G.M. Murphy (Eds.), Physical Properties and Analysis of Heavy Water, McGraw-Hill, New York, 1951. [10] W.F. Chan, G. Cooper, C.E. Brion, Chem. Phys. 178 (1993) 387. [11] K. Yoshino, J.R. Esmond, W.H. Parkinson, K. Ito, T. Matsui, Chem. Phys. 211 (1996) 387. [12] A.H. Laufer, J.R. McNesby, Can. J. Chem. 43 (1965) 3487. [13] Y.L. Yung, C.E. Miller, Science 278 (1997) 1778.