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Absorption Cross Sections of Surface-Adsorbed H2O in the 295−370 nm Region and Heterogeneous Nucleation of H2O on Fused Silica Surfaces Juan Du, Li Huang, and Lei Zhu* 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 ABSTRACT: We have determined absorption cross sections of a monolayer of H2O adsorbed on the fused silica surfaces in the 295−370 nm region at 293 ± 1 K by using Brewster angle cavity ring-down spectroscopy. Absorption cross sections of surfaceadsorbed H2O vary between (4.66 ± 0.83) × 10−20 and (1.73 ± 0.52) × 10−21 cm2/ molecule over this wavelength range, where errors quoted represent experimental scatter (1σ). Our experimental study provides direct evidence that surface-adsorbed H2O is an absorber of the near UV solar radiation. We also varied the H2O pressure in the surface study cell over the 0.01−17 Torr range and obtained probe laser absorptions at 295, 340, and 350 nm by multilayer of adsorbed H2O molecules until the heterogeneous nucleation of water occurred on fused silica surfaces. The average absorption cross sections of multilayer adsorbed H2O are (2.17 ± 0.53) × 10−20, (2.48 ± 0.67) × 10−21, and (2.34 ± 0.59) × 10−21 cm2/molecule at 295, 340, and 350 nm. The average absorption cross sections of transitional H2O layer are (6.06 ± 2.73) × 10−20, (6.48 ± 3.85) × 10−21, and (8.04 ± 4.92) × 10−21 cm2/molecule at 295, 340, and 350 nm. The average thin water film absorption cross sections are (2.39 ± 0.50) × 10−19, (3.21 ± 0.81) × 10−20, and (3.37 ± 0.94) × 10−20 cm2/molecule at 295 nm, 340 nm, and 350 nm. Atmospheric implications of the results are discussed.

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etry. Water uptake on a hydrophilic silicon oxide surface18 and water uptake on silicon-containing dust particles23−25 were determined using attenuated total reflection-Fourier transform infrared spectroscopy. Transmission FTIR was used to probe the interaction19 of water vapor with glass and quartz surfaces whereas atomic force microscopy and X-ray photoelectron spectroscopy were used to measure surface topography and surface chemical composition following surface interaction with water vapor. Adsorption of water vapor on highly hydrophilic amorphous SiO2 film on Si was studied between 263 and 294 K using ambient pressure photoelectron spectroscopy6 and Kelvin probe microscopy;26 about four to five water layers were needed to reach a structure similar to that of a water film above 273 K. Despite the wealth of information obtained from the previous studies of water adsorption on various SiO2 surfaces, it is unknown whether water molecules or water films adsorbed on SiO2 surfaces exhibit near UV absorption. Investigation of the optical properties of the surface-adsorbed water in the near UV region is necessary as SiO2 is an important component of the surfaces of the ground and of dust aerosols, and thin water films are pervasive on these environmental surfaces under the average humidity conditions of the atmosphere. Although water molecules do not exhibit an electronic transition in the near UV region (the energy

INTRODUCTION Water vapor is abundant in the earth’s atmosphere. It is a major absorber of solar radiation and an important greenhouse gas.1 Under conditions of ambient temperature and relative humidity, water vapor can be adsorbed on various environmental surfaces2,3 such as the ground surfaces, building and window surfaces, salt and aerosol surfaces, and leaf surfaces. Aerosol particles coated with a thin film of water are possible precursors to cloud formation in the atmosphere.4 Coating surfaces of salt particles with water has been found to affect chemical reactivity of these particles.5 The growth of water films on the surface of materials also plays an important role in corrosion, dissolution, catalysis, and biological processes.3,6−8 One important material that has widespread presence in silicon technology and in natural minerals is SiO2.9−11 The structures of water in contact with various SiO2 surfaces have been calculated using theoretical methods. 12−16 An ordered hexagonal water layer has been predicted on the hydroxylated surface of quartz (0001). As the water layer increases, a highly stable bilayer membranelike structure is likely to appear on quartz surface.16 A variety of experimental techniques have been used to investigate the adsorption of H2O on different SiO2 surfaces under ultrahigh vacuum conditions or under low vacuum to ambient pressure conditions.17−25 For example, the adsorption kinetics of H2O on a well-defined, fully hydroxylated SiO2 surface was studied22 at surface temperatures of 130−250 K under ultrahigh vacuum conditions and using laser-induced thermal desorption technique combined with mass spectrom© 2013 American Chemical Society

Received: June 5, 2013 Revised: August 13, 2013 Published: August 15, 2013 8907

dx.doi.org/10.1021/jp405573y | J. Phys. Chem. A 2013, 117, 8907−8914

The Journal of Physical Chemistry A

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difference between the first electronically excited singlet state, 1 1B1, and the ground electronic state, 1 1A1, of water is about 7.4 eV or equivalent excitation wavelength of 167 nm),27,28 theoretical calculations indicate that water molecules exhibit vibrational overtone and combination bands in this wavelength region.29−36 The theoretically calculated near UV band origins30,36 for the highly excited water overtone and combination bands are consistent with band origins determined from water vapor multiphoton excitation studies.37,38 In a separate study, our group recently measured39 the single photon absorption cross sections of water vapor in the 290− 350 nm region. Water vapor near UV spectrum displays structure in this wavelength region, suggesting highly excited vibrational overtone and combination bands are the origin of water vapor near UV absorption. The experimentally measured dissociation threshold40 of water vapor following its overtone excitation is 243 nm. Thus, absorption of the near UV light is unlikely to photodissociate surface-adsorbed water molecules, but surface-adsorbed H2O can act as a chromophore for absorbing near UV radiation and subsequently transfer such excitation energy to nearby surface-adsorbed molecules or to nearby gas-phase molecules through energy transfer processes. Given that there are various types of surfaces in the atmosphere and the large abundance of water vapor, determination of the absorption cross sections of surface-adsorbed water molecules in the near UV region will allow an evaluation of the impact of surface-adsorbed water to absorption of solar radiation in this wavelength region. Reported in this paper are results obtained from the measurement of the UV absorption cross sections of a monolayer of H2O adsorbed on fused silica (SiO2) window surfaces in the 295−370 nm region, as well as results obtained from a determination of the multilayer adsorption of water on fused silica surfaces until the occurrence of the heterogeneous nucleation at selected wavelengths. These studies were carried out by exploring the application of a novel variant of cavity ring-down technique,41,42 Brewster angle cavity ring-down spectroscopy.43

Figure 1. Schematics using Brewster angle cavity ring-down spectroscopy to determine probe laser absorption by surface-adsorbed H2O.

The probe beam inside the cavity decayed as a result of absorption/scattering in the gas phase and absorption/ scattering by adsorbed species on surfaces, absorption/ transmission loss through fused silica window(s), and mirror transmission loss. The photon intensity decay inside the cavity was monitored with a photomultiplier tube (PMT) placed after the rear cavity mirror. The PMT output was amplified, digitized, and transferred to a computer. The decay curve was fitted to a single-exponential decay function, from which the ring-down time constant (τ) and the total loss (Γ) per roundtrip pass were extracted. Water sample was purified by pumping deionized water inside a glass bubbler for 30 min to remove dissolved air. Water vapor was subsequently introduced into the ring-down cavity. The H2O vapor pressure inside the cavity was measured using an MKS 622A Baratron capacitance manometer (1 Torr full scale, measurement uncertainty is ≤0.25% of the pressure reading), which can measure pressures down to 10−4 Torr, and an Edwards 600AB Barocel pressure sensor (1000 Torr full scale, measurement uncertainty is ≤0.15% of the pressure reading). The cavity was evacuated before another pressure of H2O was added. Absorption of the probe beam by surfaceadsorbed H2O was determined under static conditions. All experimental measurements were made at an ambient temperature of 293 ± 1 K.

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EXPERIMENTAL TECHNIQUE Absorption cross sections of surface-adsorbed H2O in the 295− 370 nm region were obtained by introducing the linearly polarized probe laser beam into the ring-down cavity containing single or a pair of mutually compensating fused silica Brewster window(s) (1 in. diameter and 1 mm thickness, Brewster angle for the vacuum/fused silica interface was ∼56°) placed in the path of the main optical axis inside the cavity. When p-polarized light is incident on an optical window oriented at Brewster’s angle, the reflection loss is zero, and almost all of the light is transmitted.44 A brief sketch of the experimental scheme is shown in Figure 1. Detailed description can be found from the publications of our group elsewhere.45,46 Only essential features are depicted here. A pair of high-reflectance cavity mirrors vacuum-sealed both ends of the cell. Several pairs of cavity mirrors with reflectivity of 99.7%-99.9% were used to cover the 295−370 nm range. The tunable probe laser beam in the 295− 370 nm region was provided by the fundamental or the second harmonic output of a dye laser pumped by a 308 nm excimer laser. Laser dyes used were Rhodamine 590, Rhodamine 610, Rhodamine 640, DCM, p-terphenyl, and butyl PBD. The pulse energy of the ring-down probe beam was attenuated to ≤0.25 mJ/pulse and subsequently p-polarized before the probe beam entered the ring-down cavity through the front cavity mirror.

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RESULTS AND DISCUSSION 3.1. Absorption Cross Sections of a Monolayer of Surface-Adsorbed H2O in the 295−370 nm Region. When Brewster angle cavity ring-down spectroscopy was used to measure absorption cross sections of a monolayer of H2O adsorbed on fused silica surfaces, the probe beam inside the cavity experienced mirror transmission loss, absorption/ scattering of the probe beam by H2O in the gas phase and by H2O adsorbed on the front and back surfaces of each fused silica window, and absorption/transmission loss through fused silica window(s). Round-trip absorption/scattering of the probe beam by surface-adsorbed H2O was acquired by subtracting mirror transmission loss, absorption/transmission loss through fused silica window(s), and round-trip absorption/scattering by H2O in the gas phase, from the total cavity losses. Mirror transmission loss and absorption/transmission loss through fused silica window(s) can be obtained by measurement of cavity losses in the absence of H2O inside the ring-down cavity containing fused silica Brewster window(s). Gas-phase H2O 8908



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absorption was calculated for a given H2O pressure inside the cavity as we have measured39 gas-phase absorption cross sections of H2O in the 290−350 nm region (H2O gas-phase absorption was not detectable in the 355−370 nm region). Literature47 water vapor scattering cross section is on the order of 0.16 × 10−27 to 0.07 × 10−27 cm2/molecule in the 290−350 nm region, which are several orders of magnitude smaller than the water vapor absorption cross sections over this wavelength range. Literature48 liquid water scattering cross sections are about 1−2 orders of magnitude smaller than liquid water absorption cross sections in the 300−370 nm region. A literature water vapor Raman backscattering cross section49−51 of 3.0 × 10−29 cm2·sr−1 was reported for an incident wavelength of 266 nm. As the estimated H2O Raman backscattering cross sections are small (2.0 × 10−29 to 0.80 × 10−29 cm2·sr−1) in the 295−370 nm region, and the probe laser pulse energy used in our study was ≤0.25 mJ/pulse, which is much smaller than the pulse energy on the order of 80−120 mJ/pulse used in literature H2O Raman scattering studies,49−51 we are not expected to detect Raman scattering by surface-adsorbed H2O. Or in other words, light scattering by surface-adsorbed H2O is negligible compared to light absorption by surface-adsorbed H2O in the 295−370 nm region. Plotted in Figure 2 is absorption of the 340 nm probe beam by adsorbed H2O as a function of the H2O vapor pressure (PH2O) in the cell.

Figure 3. 1/(absorption by adsorbed H2O) as a function of 1/PH2O at λ = 340 nm. A linear least-squares fit of the data up to saturation monolayer coverage (filled circles) is also shown.

As seen from this figure, our experimental data can be fitted to a linear plot for H2O pressures up to about 0.075 Torr, and the slope of the plot changes at higher H2O pressures. Assuming the adsorption of H2O on fused silica surfaces fits the Langmuir adsorption isotherm, we would expect 1/(H2O surface concentration) to be proportional to 1/PH2O. Because absorption of the probe beam by adsorbed H2O is proportional to the H2O surface concentration for monolayer adsorption, our experimental data in Figure 3 suggested the occurrence of monolayer adsorption for H2O pressures up to about 0.075 Torr. Fitting the experimental data up to saturation monolayer coverage using linear least-squares analysis is also shown in the same figure. The reciprocal of the intercept gives absorption of the probe beam at 340 nm by H2O molecules that have saturated the monolayer adsorption sites. A similar approach was adopted to obtain maximum absorption of the probe beam by a monolayer of adsorbed H2O at other wavelengths. As each Brewster window has both front and rear surfaces and light passes through Brewster window twice during a round-trip, we divided maximum round-trip absorption of the probe beam by a monolayer of adsorbed water on one Brewster window by a factor of 4 to obtain maximum absorption of the probe beam by a monolayer of adsorbed water per surface. Listed in Table 1 are values of the probe laser absorption (per surface) by a saturated monolayer of H2O (Amax) in the 295−370 nm region (probe laser absorption by surface-adsorbed H2O at wavelengths longer than 370 nm was below the detection limit). To convert absorption by a saturated monolayer of surfaceadsorbed H2O into the H2O surface absorption cross section, we need to know the H2O surface concentration. The H2O surface concentration to form monolayer adsorption on fused silica surfaces (Csurf,H2O) is estimated at about 1.6 × 1015 molecule/cm2, if we used a van der Waals radius52 of 1.41 Å for H2O. This calculation was made by assuming water molecules are close-packed on the surface; the H2O surface concentration will be smaller if they are not closely packed on surface. Absorption cross sections of H2O on fused silica surfaces were obtained by dividing absorption of the probe beam (per surface) by a saturated monolayer of adsorbed H2O by H2O surface concentration (i.e., σsurface = surface absorption/ surface concentration); the H2O surface cross section data are listed in Table 1 and shown in Figure 4.

Figure 2. Round-trip absorption of the 340 nm probe beam by H2O adsorbed on one Brewster window for PH2O up to 0.20 Torr.

As seen from this figure, absorption of the probe beam by surface-adsorbed H2O initially increases rapidly with increasing H2O pressure up to a pressure of about 0.012−0.015 Torr, is nearly invariant with H2O pressure in the 0.020−0.075 Torr range, and exhibits further increase at H2O pressures higher than 0.075 Torr. To exclude the chamber wall as a source of absorption in Figure 2, we thoroughly cleaned the chamber wall, heated and simultaneously pumped the chamber using a diffusion pump for several hours. We continued the evacuation of the cell using a diffusion pump when the temperature of the cell was cooled, and we remeasured absorption by adsorbed H2O at 340 nm as a function of water vapor pressure. The results were not affected by this additional cleaning and pumping of the chamber, suggesting the absorption we measured indeed came from that of the surface-adsorbed H2O molecules. Illustrated in Figure 3 is 1/(absorption by adsorbed H2O) as a function of 1/PH2O at 340 nm. 8909



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determination of ring-down losses, with and without H2O, in the Brewster angle ring-down cavity (∼1% total); accuracy in the determination of the H2O pressure (0.25%) and the H2O gas-phase absorption cross sections; and accuracy in the determination of the surface concentration of H2O. In addition to acquiring absorption cross sections of surface-adsorbed H2O as a function of wavelength, we extracted Langmuir adsorption constant of H2O (bH2O) on fused silica surfaces. Probe laser absorption by adsorbed H2O (per surface) at a given wavelength (Asurf,H2O) is related to the H2O pressure in the cell (PH2O) through the following relationship:

Table 1. Surface Absorption Cross Sections (σsurf) of H2O in the 295−370 nm Region λ (nm) 295 300 305 310 315 320 325 330 335 340 345 350 355 360 365 370

σsurf (cm2/molecule)a

Amax (7.45 (6.54 (6.20 (6.02 (4.25 (2.57 (4.31 (3.57 (2.10 (1.02 (9.46 (1.29 (9.83 (5.37 (3.72 (2.77

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.34) 1.04) 0.30) 0.31) 0.83) 0.33) 0.78) 1.62) 0.52) 0.13) 1.81) 0.18) 1.77) 0.80) 1.77) 0.84)

× × × × × × × × × × × × × × × ×

−5

10 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−6 10−5 10−6 10−6 10−6 10−6

(4.66 (4.08 (3.87 (3.76 (2.66 (1.60 (2.69 (2.23 (1.31 (6.34 (5.91 (8.06 (6.15 (3.36 (2.32 (1.73

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.83) 0.65) 0.19) 0.19) 0.52) 0.21) 0.48) 1.01) 0.33) 0.80) 1.13) 1.12) 1.09) 0.50) 1.10) 0.52)

× × × × × × × × × × × × × × × ×

10−20 10−20 10−20 10−20 10−20 10−20 10−20 10−20 10−20 10−21 10−21 10−21 10−21 10−21 10−21 10−21

A surf,H2O = σsurf,H2O × Csurf,H2O × (bH2OPH2O/(1 + bH2OPH2O))

(1)

Using the experimentally determined Asurf,H2O versus PH2O profile and the σsurf,H2O value at a given wavelength and using the estimated Csurf,H2O value of 1.6 × 1015 molecule/cm2, we derived bH2O value of 553 ± 277 Torr−1. 3.2. Adsorption of H2O on Fused Silica Surfaces from Multilayer to Heterogeneous Nucleation. In addition to the determination of the absorption cross sections of a monolayer of surface-adsorbed H2O in the 295−370 nm region, we investigated the multilayer adsorption of H2O on fused silica surfaces until the occurrence of the heterogeneous nucleation of H2O on surfaces at 295, 340, and 350 nm. To separate probe laser absorption by H2O adsorbed on fused silica surfaces from probe laser absorption by H2O in the gas phase and by H2O adsorbed on cavity mirrors particularly at water vapor pressures higher than 15 Torr, we removed the fused silica Brewster window from the ring-down cavity and measured the cavity loss as a function of the water vapor pressure in the cell from 0.01 Torr up to 17 Torr pressure at 293 K. (The literature47 water vapor scattering cross section is on the order of 0.15 × 10−27 to 0.07 × 10−27 cm2/molecule in the 295−350 nm region. Round-trip water vapor scattering contribution to cavity loss is estimated to be less than 1 ppm even at an H2O pressure of 17 Torr in the cavity and a cavity length of 47 cm.) Absorption of the probe beam by water vapor adsorbed on fused silica surfaces was obtained by subtracting absorption of the probe beam by water vapor in the gas phase and by water vapor adsorbed on cavity mirrors from the total probe laser absorption. Shown in Figure 5 is absorption of the 340 nm probe beam by adsorbed H2O as a function of the relative humidity (RH) in the cell. We discuss Figure 5 by dividing this figure into four regions corresponding to RH < 0.43% (H2O pressures of up to 0.075 Torr), RH over the 0.43−39% range (H2O pressures of 0.075− 6.9 Torr), RH over the 46−80% range (H2O pressures of 8−14 Torr), and RH over the 86−97% range (H2O pressures of 15− 17 Torr). Absorption of the probe beam shows an initial fast increase with water vapor pressure suggesting a rapid uptake of H2O on fused silica surfaces to saturate the monolayer adsorption sites at H2O vapor pressures in the 0.020−0.075 Torr range (see Figure 2 for an expansion of the horizontal axis of Figure 5 for H2O pressures of less than 0.20 Torr). Absorption of the 340 nm probe beam by adsorbed H2O showed a modest increase with RH in the 0.43−39% range, corresponding to multilayer adsorption of H2O on fused silica surfaces. Saturation of the second and third layer adsorption sites on SiO2 surfaces6,18 were reported at about 15% RH and

a

Errors quoted (1σ) represent experimental scatter from three repeated cross section measurements.

Figure 4. Absorption cross sections of surface-adsorbed H2O in the 295−370 nm region.

The H2O surface absorption spectrum shown in Figure 4 appears to be on the tail of an electronic absorption band as the surface may have shifted the water vapor 1 1A1 → 1 1B1 electronic absorption band toward the longer wavelength. Also noticeable are absorption peaks attributable to highly excited vibrational overtone and combination bands of adsorbed water that are superimposed on the adsorbed water electronic absorption spectrum. These peaks are less pronounced and are red-shifted by about 15−20 nm from those in the gas phase determined from a separate study of this group.39 The H2O surface absorption cross sections vary between (4.66 ± 0.83) × 10−20 and (1.73 ± 0.52) × 10−21 cm2/molecule over this wavelength range. Relative uncertainties (1σ) in the determination of surface absorption cross sections of H2O in the 295− 370 nm region are included in Table 1. They are within 5% at 305 and 310 nm; 15% at 320, 340, 350, and 360 nm; 20% at 295, 300, 315, 325, 345, and 355 nm; 25% at 335 nm; 30% at 370 nm; 45% at 330 nm; and 50% at 365 nm. In addition to experimental scatter from a minimum of three repeated measurements, systematic errors in the determination of the H2O surface cross sections include absolute accuracy in the 8910



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SiO2 film was coated with the first one to two monolayer(s) of H2O, indicating the dipole moment of the H2O molecules are parallel to the surface of the adsorbed layer(s). For RH in the 20−40% range, the surface potential showed a gradual increase, indicating that the average dipole orientation started to point toward the gas phase when the third layer of H2O was adsorbed to the SiO2 surface. For the sake of convenience in presenting the results, we derived an H2O multilayer absorption cross section that was averaged over the second and the third water layer; we obtained a cross section value of (2.48 ± 0.67) × 10−21 cm2/molecule at 340 nm. We also derived average H2O multilayer absorption cross sections of (2.17 ± 0.53) × 10−20 and (2.34 ± 0.59) × 10−21 cm2/molecule at 295 and 350 nm. The average H2O multilayer absorption cross sections at 295, 340, and 350 nm are listed in Table 3, along with monolayer, transition layer, thin film water, and literature48,53 liquid H2O absorption cross sections at these wavelengths. For RH over the 46−80% range (referred to as transition layers), the literature26 surface potential for a SiO2 surface with adsorbed H2O layers exhibits a large increase, in strong contrast to the gradual increase in probe laser absorption at 340 nm by adsorbed H2O with RH shown in Figure 5, suggesting the average H2O dipolar orientation changed and pointed toward the gas phase at this RH range. Saturation of the fourth layer and the fifth layer adsorption sites on SiO2 surfaces6,18 was reported at about 57% RH and 85% RH. Absorption cross sections for the fourth layer and the fifth layer of adsorbed water were acquired by dividing the 340 nm probe laser absorptions by the fourth layer and the fifth layer of adsorbed H2O per surface by the estimated H2O surface concentration of 1.6 × 1015 molecules/cm2. The H2O surface absorption cross sections thus obtained are (3.76 ± 0.37) × 10−21 and (9.21 ± 1.09) × 10−21 cm2/molecule at 340 nm for the fourth layer and the fifth layer of adsorbed H2O. Similarly, we obtained the H2O surface cross section values for the fourth layer and the fifth layer at 295 and 350 nm. The layer-specific transition layer H2O cross section values are listed in Table 2. For the sake of convenience in reporting the results, we also provided here H2O transition layer absorption cross section values that have been averaged over the fourth and the fifth layer of adsorbed H2O; they are (6.06 ± 2.73) × 10−20, (6.48 ± 3.85) × 10−21, and (8.04 ± 4.92) × 10−21 cm2/molecule at 295, 340, and 350 nm, respectively. These average H2O transition layer cross section values are listed in Table 3. For RH over the 86−97% range, probe laser absorption at 340 nm by adsorbed H2O exhibits a fast increase with increasing H2O pressure, suggesting rapid heterogeneous nucleation of H2O and growth of a water film on fused silica surfaces. The H2O pressure at which the heterogeneous

Figure 5. Round-trip absorption of the 340 nm probe beam by adsorbed H2O on one Brewster window as a function of the RH in the cell.

35% RH. Using literature6,18 water layer thickness as a function of the relative humidity and assuming the thickness6,18 of each water layer is 0.3 nm, we can convert round-trip absorption by adsorbed water on one Brewster window as a function of the RH into round-trip absorption by adsorbed water as a function of the number of water layers. To convert round-trip absorption by surface-adsorbed water on one Brewster window into absorption by adsorbed water per surface, we divided round-trip absorption by a factor of 4 as light interacted with the Brewster window twice during a round trip, and each window has both front and rear surfaces. Absorption cross sections for the second and third layer of adsorbed water were acquired by dividing the 340 nm probe laser absorptions by the second and the third layer of adsorbed H2O per surface by the estimated H2O surface concentration of 1.6 × 1015 molecules/ cm2 for each layer. The H2O surface absorption cross sections thus obtained are (2.00 ± 0.26) × 10−21 and (2.95 ± 0.48) × 10−21 cm2/molecule at 340 nm for the second and the third layer of adsorbed H2O. Similarly, we obtained the second and the third layer H2O surface cross section values at 295 and 350 nm. These layer-specific multilayer H2O surface cross section values are listed in Table 2. The layer-specific H2O surface cross sections showed some increase from the second to the third layer at 295, 340, and 350 nm, which are in line with results from literature26 adsorbed water layer surface potential measurements on a SiO2 film. Verdaguer et al.26 measured the surface potential for SiO2 film with adsorbed H2O layer(s) as a function of the RH using AFM operating in the Kelvin probe mode at 294 K. They found that the surface potential of the SiO2 film did not change when the

Table 2. Absorption Cross Section versus Wavelength for the Second to Eighth Layers of Adsorbed Water σ (cm2/molecule) layer no. 2nd 3rd 4th 5th 6th 7th 8th a

295 nm (1.80 (2.54 (4.13 (7.99 (1.82 (2.63 (2.73

± ± ± ± ± ± ±

0.78) 0.75) 0.86) 2.34) 0.34) 0.22) 0.14)

× × × × × × ×

340 nm 10−20a 10−20 10−20 10−20 10−19 10−19 10−19

(2.00 (2.95 (3.76 (9.21 (2.28 (3.58 (3.76

± ± ± ± ± ± ±

0.26) 0.48) 0.37) 1.09) 0.14) 0.22) 0.24)

× × × × × × ×

350 nm 10−21 10−21 10−21 10−21 10−20 10−20 10−20

(1.93 (2.76 (4.56 (1.15 (2.30 (3.71 (4.08

± ± ± ± ± ± ±

0.41) 0.74) 0.59) 0.32) 0.17) 0.34) 0.25)

× × × × × × ×

10−21 10−21 10−21 10−20 10−20 10−20 10−20

Errors quoted (1σ) represent experimental scatter. 8911



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Table 3. Absorption Cross Sections versus Wavelength for Monolayer, Multilayer, Transition Layer, Thin Film, and Liquid Water λ (nm) 295 340 350

σmono (cm2/molecule) −20 a

(4.66 ± 0.83) × 10 (6.34 ± 0.80) × 10−21 (8.06 ± 1.12) × 10−21

σmulti (cm2/molecule) −20 b

(2.17 ± 0.53) × 10 (2.48 ± 0.67) × 10−21 (2.34 ± 0.59) × 10−21

σtrans (cm2/molecule) −20 c

(6.06 ± 2.73) × 10 (6.48 ± 3.85) × 10−21 (8.04 ± 4.92) × 10−21

σfilm (cm2/molecule) −19 d

(2.39 ± 0.50) × 10 (3.21 ± 0.81) × 10−20 (3.37 ± 0.94) × 10−20

σliquid (cm2/molecule)d,e ∼1.0 × 10−24 e ∼8.3 × 10−25 f ∼4.8 × 10−25 f

a

Errors quoted (1σ) represent experimental scatter from three repeated cross section measurements. bAverage H2O multilayer cross section value over the second and third layer of adsorbed H2O. cAverage H2O transition layer cross section value over the fourth and fifth layer of adsorbed H2O. d Average thin H2O film cross section value over the sixth to the eighth layer of adsorbed H2O. eLiquid water cross section value at 295 nm from ref 53. fLiquid water cross section values at 340 and 350 nm from ref 48.

fused silica surface and the first layer of H2O molecules than the interaction between the multilayer adsorbed water molecules. The average absorption cross sections of transition water layers are (6.06 ± 2.73) × 10−20, (6.48 ± 3.85) × 10−21, and (8.04 ± 4.92) × 10−21 cm2/molecule at 295, 340, and 350 nm, respectively. The respective average absorption cross sections of multilayer adsorbed water are (2.17 ± 0.53) × 10−20, (2.48 ± 0.67) × 10−21, and (2.34 ± 0.59) × 10−21 cm2/molecule at 295, 340, and 350 nm. The average transition layer water cross section values are 2.6−3.4 times those of multilayer water cross section values. The average absorption cross sections of thin water films are (2.39 ± 0.50) × 10−19, (3.21 ± 0.81) × 10−20, and (3.37 ± 0.94) × 10−20 cm2/molecule at 295, 340, and 350 nm, respectively. The average thin water film absorption cross sections are 11.0−14.4 times those for the multilayer adsorbed H2O at 295, 340, and 350 nm. The increase in the adsorbed water layer cross section values from multilayer to transition layer to thin film water is directly parallel to the increase in surface potential of the SiO2 film with adsorbed water layers. As the probe laser we used is p-polarized, the direction of the laser polarization is aligned with the direction of the surface potential that is pointed toward the gas phase. When the adsorbed water layer changes from multilayer to the transition layer to thin film water, there is increasing alignment between the dipole moment of surface water layer and the laser polarization, which leads to an increase in H2O cross section values from multilayer to transition layer to thin water film. The thin film water absorption cross section values reported here were obtained from surface absorption measurements using a polarized laser beam. The natural sunlight is randomly polarized. To calculate absorption of sunlight by thin water films using our thin water film cross section values, one needs to divide our water film cross section values by square root of 2 to account for the random polarization of the natural sunlight. Literature liquid water absorption cross section values48,53 are about 1.0 × 10−24, 8.3 × 10−25, and 4.8 × 10−25 cm2/ molecule at 295, 340, and 350 nm. Thus, the liquid water cross section values are several orders of magnitude smaller than those of monolayer, multilayer, transition layer, and thin film water. The adsorbed water molecules form hydrogen bonded structure, and the adsorbed water layers were reported to be flat and homogeneous on SiO2 surface at any humidity.26 On the other hand, water molecules undergo random motions in the liquid phase; the water dipole is randomly oriented in the macroscopic volume and there is no alignment of the laser polarization with dipole of liquid water. That explains why liquid water absorption cross sections are much smaller than those of monolayer, multilayer, transition layer, and thin film water. In addition to the surface-adsorbed water molecules studied in this paper, our group previously determined the near UV

nucleation of H2O occurs on fused silica surfaces obtained by Brewster angle cavity ring-down spectroscopy determined in this work is consistent with that obtained using X-ray photoelectron spectroscopy (XPS) by Salmeron et al.6 As Salmeron et al.6 did not look at monolayer H2O adsorption on SiO2 film, we could not compare our monolayer water adsorption data with results from the study by Salmeron et al.6 For RH over the 86−97% range, the surface potential of the SiO2 film with adsorbed water layers26 increased very little with increasing RH, suggesting it was approaching the surface potential of the liquid water surface. In such a circumstance, the average water layer dipolar orientation no longer changes and water layers on the SiO2 film reach a bulklike structure. To convert experimentally measured probe laser absorption at 340 nm by adsorbed H2O at RH over the 86−97% range into probe laser absorption by adsorbed H2O as a function of the number of thin film water layers, we used literature6,18 water layer thickness as a function of the relative humidity and assumed each water layer has a thickness of 0.3 nm. Saturation of the sixth, seventh, and eighth layer of thin water film occurs at 92%, 96%, and 97% RH. From the layer-specific probe laser absorptions at 340 nm by the sixth to the eighth layer of adsorbed water molecules and the estimated H2O surface concentration of 1.6 × 1015 molecules/cm2 per layer, we obtained absorption cross sections for the sixth to the eighth layer of thin water film. The H2O surface absorption cross sections thus obtained are (2.28 ± 0.14) × 10−20, (3.58 ± 0.22) × 10−20, and (3.76 ± 0.24) × 10−20 cm2/molecule at 340 nm for the sixth, seventh, and eighth layer of thin H2O film, where errors quoted represent 1σ scatter. Similarly, we obtained the H2O surface cross section values for the sixth to the eighth layer for thin film water at 295 and 350 nm. The layer-specific thin film water cross section values at 295, 340, and 350 nm are listed in Table 2. For the convenience in presenting the results, we obtained a water film absorption cross section that was averaged over the sixth to the eighth layer, which is (3.21 ± 0.81) × 10−20 cm2/molecule at 340 nm. The average water film absorption cross sections are (2.39 ± 0.50) × 10−19 and (3.37 ± 0.94) × 10−20 cm2/molecule at 295 and 350 nm, respectively. Absorption cross sections of a monolayer of adsorbed H2O on fused silica surfaces are (4.66 ± 0.83) × 10−20, (6.34 ± 0.80) × 10−21, and (8.06 ± 1.12) × 10−21 cm2/molecule at 295, 340, and 350 nm, respectively. The respective average absorption cross sections of multilayer-adsorbed H2O are (2.17 ± 0.53) × 10−20, (2.48 ± 0.67) × 10−21, and (2.34 ± 0.59) × 10−21 cm2/ molecule at 295, 340, and 350 nm. The monolayer H2O absorption cross sections at 295, 340, and 350 nm are 2.1−3.5 times those of the multilayer adsorbed H2O. The larger monolayer absorption cross sections of H2O on fused silica surfaces compared to those of multilayer adsorbed H2O may be the result of the stronger attractive interaction between the 8912



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absorption cross sections45,46 of a monolayer of surfaceadsorbed nitric acid (HNO3). The HNO3 surface cross section values are at least 3 orders of magnitude larger than the liquid HNO3 cross section values in the wavelength region studied.46 There is strong attractive interaction between the fused silica surface and the first layer of H2O or HNO3 molecules. Such attractive interaction is manifested through the formation of a hydrogen bond between the SiO2 and H2O or between SiO2 and HNO3 on the surface. Thus, H2O or HNO3 adsorbed on the fused silica surfaces is very different from the H2O or HNO3 molecules in the liquid phase where liquid H2O and HNO3 undergo randomized motion in the macroscopic volume. That helps to explain why absorption cross sections of surface-adsorbed H2O and surface-adsorbed HNO3 are several orders of magnitude larger than those of H2O and HNO3 in the liquid phase. Under the average humidity conditions of the atmosphere, the adsorbed water layers we are likely to encounter on the surfaces of the roads and bridges, on the surfaces of windows and buildings, and on the surfaces of silicate containing solid particles are those of multilayer and transition layer water molecules. Although the multilayer and transition layer water molecules are invisible to the naked eyes, they are, nonetheless, present. Given that the near UV absorption cross sections of multilayer and transition layer water molecules are several orders of magnitude larger than those of the literature liquid water absorption cross section values, it is necessary to use near UV absorption cross section data obtained for adsorbed water molecules to model the near UV optical properties of surfaces coated with water layers under the average humidity conditions of the atmosphere.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Drs. Chris Judd, Liang T. Chu, and Richard Cole for many discussions. We are grateful for the support provided by the National Science Foundation under grant AGS-0969985.

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(1) Arking, A. Bringing Climate Models into Agreement with Observations of Atmospheric Absorption. J. Clim. 1999, 12, 1589− 1600. (2) Grassian, V. H. Surface Science of Complex Environmental Interfaces: Oxide and Carbonate Surfaces in Dynamic Equilibrium with Water Vapor. Surf. Sci. 2008, 602, 2955−2962. (3) Ewing, G. E. Thin Film Water. J Phys. Chem. B 2004, 108, 15953−15961. (4) Pruppacher, H. R.; Klett, J. D. Microphysics of Clouds and Precipitation; Kluwer Academic Publishers: The Netherlands, 1997. (5) DeHann, D. O.; Brauers, T.; Oum, K.; Stutz, J.; Nordmeyer, T.; Finlayson-Pitts, B. J. Heterogeneous Chemistry in the Troposphere: Experimental Approaches and Applications to the Chemistry of Sea Salt Particles. Int. Rev. Phys. Chem. 1999, 18, 343−385. (6) Salmeron, M.; Schlögl, R. Ambient Pressure Photoelectron Spectroscopy: A New Tool for Surface Science and Nanotechnology. Surf. Sci. Rep. 2008, 63, 169−199. (7) Thiel, P. A.; Madey, T. E. The Interaction of Water with Solid Surfaces: Fundamental Aspects. Surf. Sci. Rep. 1987, 7, 211−385. (8) Henderson, M. A. The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited. Surf. Sci. Rep. 2002, 46, 1−308. (9) Iler, R. K., The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica; Wiley: New York, NY, 1979. (10) Bergna, H. E.; Roberts, W. O. Colloidal Silica: Fundamentals and Applications; CRC Press: Boca Raton, FL, 2006. (11) Legrand, A. P., Ed. The Surface Properties of Silicas; Wiley: New York, NY, 1998. (12) Yang, J.; Meng, S.; Xu, L. F.; Wang, E. G. Ice Tessellation on a Hydroxylated Silica Surface. Phys. Rev. Lett. 2004, 92, 146102. (13) Yang, J.; Meng, S.; Xu, L. F.; Wang, E. G. Water Adsorption on Hydroxylated Silica Surfaces Studied Using the Density Functional Theory. Phys. Rev. B 2005, 71, 035413. (14) Lu, Z.-Y.; Sun, Z.-Y.; Li, Z.-S.; An, L.-J. Stability of TwoDimensional Tessellation Ice on the Hydroxylated β-Cristobalite (100) Surface. J. Phys. Chem. B 2005, 109, 5678−5683. (15) Yang, J.; Wang, E. G. Water Adsorption on Hydroxylated αQuartz (0001) Surfaces: From Monomer to Flat Bilayer. Phys. Rev. B 2006, 73, 035406. (16) Chen, Y.-W.; Cheng, H.-P. Structure and Stability of Thin Water Films on Quartz Surfaces. Appl. Phys. Lett. 2010, 97, 161909. (17) Aarts, I. M. P.; Pipino, A. C. R.; Hoefnagels, J. P. M.; Kessels, W. M. M.; van de Sanden, M. C. M. Quasi-Ice Monolayer on Atomically Smooth Amorphous SiO2 at Room Temperature Observed with a High-Finesse Optical Resonator. Phys. Rev. Lett. 2005, 95, 166104. (18) Asay, D. B.; Kim, S. H. Evolution of the Adsorbed Water Layer Structure on Silicon Oxide at Room Temperature. J. Phys. Chem. B 2005, 109, 16760−16763. (19) Sumner, A. L.; Menke, E. J.; Dubowski, Y.; Newberg, J. T.; Penner, R. M.; Hemminger, J. C.; Wingen, L. M.; Brauers, T.; Finlayson-Pitts, B. J. The Nature of Water on Surfaces of Laboratory Systems and Implications for Heterogeneous Chemistry in the Troposphere. Phys. Chem. Chem. Phys. 2004, 6, 604−613. (20) Du, Q.; Freysz, E.; Shen, Y. R. Vibrational Spectra of Water Molecules at Quartz/Water Interfaces. Phys. Rev. Lett. 1994, 72, 238− 241. (21) Ostroverkhov, V.; Waychunas, G. A.; Shen, Y. R. New Information on Water Interfacial Structure Revealed by PhaseSensitive Surface Spectroscopy. Phys. Rev. Lett. 2005, 94, 046102.

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CONCLUSIONS In this paper, we reported measurements of the absorption spectrum and absorption cross sections of a monolayer of surface-adsorbed water in the 295−370 nm region. The H2O surface absorption cross section varies between (4.66 ± 0.83) × 10−20 and (1.73 ± 0.52) × 10−21 cm2/molecule over this wavelength range. Our experimental results provide direct evidence that surface-adsorbed water absorbs near UV solar radiation. Because water vapor can be adsorbed on various surfaces of the environment under the average humidity condition of the atmosphere, surface-adsorbed water can serve as a chromophore to absorb near UV light and subsequently transfer absorbed photon energy to nearly gaseous or surface-adsorbed molecules to initiate photochemical transformations. In addition to the determination of the absorption cross sections of a monolayer of surfaceadsorbed H2O, we investigated multilayer adsorption of water vapor on fused silica surfaces and the heterogeneous nucleation of H2O on surfaces. We directly measured absorption cross sections of multilayer, transition layer, and thin film water at selected UV wavelengths. Our work has also demonstrated that Brewster angle cavity ring-down spectroscopy is a sensitive, enabling technique that not only can be used to study monolayer molecular adsorption on surfaces but also can be applied to investigate the multilayer molecular adsorption on the surface and the heterogeneous nucleation processes.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*L. Zhu: tel, 518-474-6846; fax, 518-473-2895; e-mail, zhul@ wadsworth.org. 8913



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