Comment pubs.acs.org/JPCA
Reply to “Comment on '308 nm Photolysis of Nitric Acid in the Gas Phase, on Aluminum Surfaces, and on Ice Films'” Chengzhu Zhu,† Bin Xiang,‡ Liang T. Chu,§ and Lei Zhu*,§ †
School of Resources & Environmental Engineering, Hefei University of Technology, Hefei 230009, P. R. China Department of Earth and Planetary Sciences, School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States § Wadsworth Center, New York State Department of Health and Department of Environmental Health Sciences, State University of New York, Albany, New York 12201-0509, United States ‡
J. Phys. Chem. A 2010, 114 (7), 2561−2568. DOI: 10.1021/jp909867a J. Phys. Chem. A 2012, 116. DOI: 10.1021/jp307052w surface density on Al, ice, and fused silica surfaces, and e−σ′(s)n(s) represents the percentage of transmitted photolysis beam following its absorption by adsorbed HNO3 on each of the two interior fused silica window surfaces (σ′(s) is the HNO3 absorption cross-section on the fused silica surface). Equation 11 of the original article1 should be replaced by the following equation:
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n response to Dr. Jovan Tadic’s comments concerning our article1 entitled “308 nm Photolysis of Nitric Acid in the Gas Phase, on Aluminum Surfaces, and on Ice Films”, we inadvertently left out a term describing the absorption of the photolysis beam by adsorbed HNO3 on the inner surfaces of both entrance and exit fused silica windows of the surface-study cell (this omission should not affect later publications of this group as we had made proper corrections; the correction itself was also small). In our original article,1 we determined the absorption cross-sections of HNO3 adsorbed on Al surfaces and on ice films by measuring the transmitted photolysis fluence exiting the surface-study cell following its absorption by the HNO3 vapor at 10 regions inside the cell and by adsorbed HNO3 at nine cell surfaces struck by the photolysis beam, as a function of HNO3 pressure. The relationship between the photolysis beam energy exiting the surface-study cell and the incoming photolysis beam energy was approximated by eq 10 in our original article.1 After considering the absorption of the photolysis beam by the inner surfaces of the entrance and exit fused silica windows, eq 10 in our original article1 is replaced by the following equation:
ln(Es,out) = ln(Es,in) + 2 ln Tw − 10α(g)l(g)P(HNO3) − 9σ(s)n(s) − 2σ ′(s)n(s) + 9 ln R = −10α(g)l(g)P(HNO3) − (9σ(s) + 2σ ′(s))n(s) + C (2)
As can be seen from eq 2, a plot of ln(Es,out) versus P(HNO3) would be linear at all HNO3 pressures, if the photolysis beam only experienced the gas-phase HNO3 absorption. We suggested a slope change in the plot of ln(Es,out) versus P(HNO3) at an HNO3 pressure of about 0.020 Torr (Figure 4 of ref 1) to be a change in the nature of adsorption from monolayer to multilayer because n(s) is linearly proportional to P(HNO3) at low pressure according to Langmuir isotherm. Once we substitute Es,out and α(g)l(g)P(HNO3) into eq 2 with those corresponding to the change of slope in Figure 4 and also use the experimentally determined C value, we can obtain a value for (9σ(s) + 2σ′(s))n(s) (rather than 9σ(s)n(s) in the original article1), which corresponds to monolayer coverage of HNO3 on Al (or ice film) and fused silica surfaces. Using σ′(s) for HNO3 adsorbed on the fused silica surface at 308 nm determined from our previous study,2 the 308 nm adsorbed HNO3 absorption cross-sections are revised to (3.96 ± 0.45) × 10−18 cm2/molecule (295 K, Al), (3.92 ± 0.44) × 10−18 cm2/ molecule (253 K, Al), and (9.41 ± 2.25) × 10−19 cm2/molecule (253 K, ice film). Thus, cross-section corrections are 6% (295 K, Al), 6% (253 K, Al), and 22% (253 K, ice film) from our original reported values.1 Note, n(s) was estimated using a van der Waals radius at monolayer coverage;1 n(s) was treated as the same on fused silica window surfaces and on Al or ice surfaces. Furthermore, a fixed C value for each experiment was
Es,out = Es,inTw 2(e−α(g)l(g)P(HNO3))10 (e−σ(s)n(s))9 (e−σ ′ (s)n(s))2 R9 (1)
where Tw represents the transmission coefficient of the fused silica window, Es,in and Es,out denote the photolysis beam energy measured prior to the beam’s entry into the cell and after the beam exited the cell, R represents the reflectivity of Al or icefilm surfaces toward the photolysis beam (we treated R as a constant following adsorption of a molecular layer of HNO3 on the surface as a first-order approximation), e−α(g)l(g)P(HNO3) denotes the percentage of transmitted photolysis light following its absorption by the HNO3 vapor at each of the 10 regions inside the cell (α(g) is the gas-phase absorption coefficient of HNO3 at 308 nm, l(g) is the path length traveled at each of the 10 regions), e−σ(s)n(s) represents the percentage of transmitted photolysis beam following its absorption by adsorbed HNO3 on Al or on ice-film surfaces at each of the nine cell surfaces struck by the photolysis beam (σ(s) represents an “apparent HNO3 surface absorption cross section” obtained with an incident angle of 45° in the experiments, n(s) represents the HNO3 © 2012 American Chemical Society
Received: July 27, 2012 Revised: September 26, 2012 Published: September 27, 2012 10465
dx.doi.org/10.1021/jp307455q | J. Phys. Chem. A 2012, 116, 10465−10466
The Journal of Physical Chemistry A
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used in calculating cross-sections. These approximations could be a source of uncertainty for σ(s), particularly on ice. After accounting for the contribution to NO2* absorption measured at the center of the surface-study cell by HNO3 photolysis on the inner surfaces of both fused silica windows and using the revised 308 nm HNO3 cross-section values on Al surfaces and on ice films, we have obtained the revised NO2* quantum yields from the HNO3 photolysis on the Al surface to be 0.77 ± 0.15 and 0.86 ± 0.20 at 295 and 253 K, respectively, and the revised NO2* quantum yield from the HNO3 photolysis on the ice film to be 0.59 ± 0.22. Thus, the quantum yield corrections are 4% (Al, 295 K), 7% (Al, 253 K), and 2% (ice film, 253 K). Although the analysis of the interaction of the laser beam with fused silica surfaces at 45° incident angle is substantially simpler than that of the interaction of the laser beam with an adsorbed layer of HNO3 on Al or ice-film surfaces, the analysis does provide an insight that can help us to understand more complex processes. In the case of light interaction with a fused silica window at 45° incident angle, the light is reflected by the front surface of the fused silica window and by the back surface of the fused silica window. The total energy of the two outgoing, reflected laser beams is equal to the incoming laser beam energy, if there are no transmission/absorption losses of the laser beam on the fused silica window. On the basis of our previous work,2 we know that the laser beam experiences transmission/absorption losses through fused silica windows. For the laser beam entering the interior of the fused silica window, it passes through the fused silica window twice before it exits the window. The reflection of the incoming laser beam by the front fused silica window surface takes away about 4% of the beam energy with a clean fused silica window. Thus, the light incident on the rear surface of the fused silica window is about 96% of the incoming laser beam energy minus the laser energy loss associated with transmission/absorption loss through the fused silica window. For the interaction between an incoming laser light and a layer of HNO3 adsorbed on Al or ice, a portion of the laser light can be reflected off the top of the adsorbed HNO3 layer (with an unknown percentage of reflection contribution); another portion can first enter the adsorbed HNO3 layer (refraction) and be absorbed by this HNO3 layer with unknown thickness and cross section (the cross section value may not resemble that of solid HNO3), before the light beam is reflected by Al or ice surfaces. This process may be expressed in terms of light in layered media (Airy’s formula). However, there are many unknowns. Since the adsorbed HNO3 is an ultrathin molecular layer (the molecular layer thickness is much smaller than the wavelength of the light), the refraction contribution is negligible, and we can treat the interaction of light with HNO3 adsorbed on Al or ice surfaces as a point interaction.
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Comment
REFERENCES
(1) Zhu, C.; Xiang, B.; Chu, L. T.; Zhu, L. J. Phys. Chem. A 2010, 114, 2561. (2) Zhu, C.; Xiang, B.; Zhu, L.; Cole, R. Chem. Phys. Lett. 2008, 458, 373.
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Drs. Juan Du and Li Huang for recalculating experimental results concerning the HNO3 absorption crosssections on Al surfaces and on ice films, and the NO2* quantum yields from the HNO3 photolysis on Al surfaces and on ice films. 10466
dx.doi.org/10.1021/jp307455q | J. Phys. Chem. A 2012, 116, 10465−10466
