Determination of absorption cross sections of
surface-adsorbed HNO3 in the 290-330 nm region by
Brewster angle cavity ring-down spectroscopy
Chengzhu Zhu, Bin Xiang, Richard Cole, Lei Zhu
Wadsworth Center, New York State Department of Health
School of Public Health, SUNY-Albany
June 23, 2009
Introduction
A. Why study nitric acid photolysis on surfaces?
 A major atmospheric oxidation product of NOx
 Slow photolysis rate in the gas phase
 The surface photolysis rate of HNO3 to form HONO and NOx
has been reported to be 1-2 orders of magnitude faster than
that from the gas phase photolysis
 The photolysis of HNO3 adsorbed on ground surfaces has
been proposed as a major daytime source of HONO.
 photolysis (surface-adsorbed HNO3)  1, and NO2 = 1 from 308
nm photolysis of gaseous HNO3
 UV absorption spectrum of surface-adsorbed HNO3 may be
red-shifted compared with the spectrum of HNO3 vapor
B. Experimental challenges
Experimental techniques that can be used to measure UV
absorption cross sections of surface-adsorbed species are
few.
As physical chemists, we respond to the challenge by
exploring the use of the Brewster angle cavity ring-down
technique to measure the UV absorption cross sections of
HNO3 adsorbed on fused silica surfaces.
C. What is Brewster angle cavity ring-down spectroscopy?
A novel variant of cavity ring-down spectroscopy
Generally involves the insertion of one or two optically
transmissive elements into the ring-down cavity at
Brewster’s angle to decrease reflection losses.
When P-polarized light is incident on an optical element set
at Brewster's angle, the reflection loss is zero.
D. Previous applications of Brewster angle CRD technique
Muir and Alexander measured absorption features of films of
oxazine and malachite green dyes adsorbed on thin
borosilicate substrates in the 580-700 nm region.
Xu et al. inserted either one or two quartz cuvettes into the
ring-down cavity at Brewster’s angle in their liquid phase
study of vibrational overtones of benzene in the 591-621 nm
region.
Experimental Apparatus
Frequency
Doubler
Dye
Laser
Excimer
Laser
Polarizing beamsplitter cube
Ring-down cavity
PMT
Digitizer
Brewster windows
Computer
How to measure absorption by surfaced-adsorbed species?
The probe beam inside the cavity experienced losses due to
mirror transmission loss
absorption by HNO3 in the gas phase
absorption by HNO3 adsorbed on the front and rear surfaces
of each fused silica window
absorption by/transmission through fused silica windows
Results and Discussion
A. Gas-phase absorption cross sections of HNO3 in the 290-330 nm
region.
Round-trip cavity loss (ppm)
9000
a
8500
8000
7500
7000
6500
6000
5500
0.00
0.05
0.10
0.15
0.20
0.25
0.30
330
340
8
b
6
Cross section (10
-21
2
cm /molecule)
P(HNO3, Torr)
4
2
0
-2
280
290
300
310
320
Wavelength (nm)
a) Round-trip cavity loss versus
HNO3 pressure at 300 nm.
b) Gas phase absorption cross
sections of HNO3 at 295 K
(circles, this work; triangles:
Burkholder et al.; squares:
Rattigan et al. ).
B. Absorption cross sections for HNO3 on fused silica surfaces
in the 290-330 nm region.
Absorption by surface-adsorbed HNO3 (ppm)
1600
 Absorption of the 300 nm probe
laser beam by adsorbed HNO3,
plotted against HNO3 pressure.
 Absorption of the probe laser
radiation by surface-adsorbed
HNO3 initially increased rapidly,
with increasing HNO3 pressure,
up to a pressure of about 15
mTorr; then, the rate of
1200
800
absorption increase slows, at
higher HNO3 pressures.
400
0
0.00
0.05
0.10
HNO3 pressure (Torr)
0.15
0.20
 A plot of 1/(absorption by
adsorbed HNO3) as a function of
1/PHNO3 at a probe laser
1/(Absorption by adsorbed HNO3)
4000
3000
wavelength of 300 nm.
 Our experimental data points can
be fitted to a straight line for
HNO3 pressures up to about 15
mTorr, but the slope of the plot
would change at higher HNO3
pressures.
2000
1000
0
0
100
200
300
-1
1/PHNO3 (Torr )
400
If the adsorption of HNO3 on fused silica surfaces fits the
Langmuir adsorption isotherm, a plot of 1/(HNO3 surface
concentration) will be proportional to 1/PHNO3.
Since absorption of the probe laser radiation by adsorbed HNO3 is
proportional to the surface HNO3 concentration for monolayer
adsorption, our experimental data suggested the occurrence of
monolayer adsorption for HNO3 pressures up to about 15 mTorr,
whereas multilayer adsorption of HNO3 on fused silica surfaces
occurred at higher HNO3 pressures.
The reciprocal of the intercept gives the absorption of the probe
laser beam at 300 nm, by HNO3 molecules that have saturated
the monolayer adsorption sites.
Absorption of the probe laser beam (Amax) by a saturated monolayer
of HNO3 as a function of wavelength, .
 (nm)
290
295
300
305
310
315
320
325
330
Amax
(1.630.18)×10-3
(1.400.16)×10-3
(1.300.13)×10-3
(0.960.15)×10-3
(1.150.04)×10-3
(1.140.11)×10-3
(0.880.20)×10-3
(0.410.04)×10-3
(0.390.04)×10-3
How to convert absorption by a saturated monolayer of surfaceadsorbed HNO3 into the surface absorption cross section?
 We need to know the maximum surface HNO3
concentration.
The maximum HNO3 surface concentration to form
monolayer adsorption on fused silica surfaces is estimated
to be 1.1x1014 molecule/cm2, if a van der Waals radius of 5.5 Å
is used for HNO3.
Absorption cross sections of HNO3 on fused silica surfaces
were determined through division of the absorption from the
saturated monolayer of surface-adsorbed HNO3 by the
maximum HNO3 surface concentration.
24
20
-19
2
HNO3 surface absorption cross sections (10 , cm /molecule)
Absorption cross sections of HNO3 on fused silica surfaces
versus wavelength obtained using Brewster angle CRD
16
12
8
4
0
280
290
300
310
320
Wavelength (nm)
330
340
A comparison of surface and gas-phase absorption cross
sections of HNO3 as a function of wavelength, .
 (nm)
surface (cm2molecule-1)
vapor (cm2molecule-1)
290
295
300
305
310
315
320
325
330
(1.850.21)×10-18
(1.590.18)×10-18
(1.480.15)×10-18
(1.090.17)×10-18
(1.310.05)×10-18
(1.300.12)×10-18
(1.000.23)×10-18
(0.470.05)×10-18
(0.450.05)×10-18
(5.980.27)×10-21
(4.090.28)×10-21
(2.590.18)×10-21
(1.680.19)×10-21
(0.950.01)×10-21
(0.510.05)×10-21
(0.470.06)×10-21
(0.210.05)×10-21
(0.190.04)×10-21
Summary and conclusions
 We have successfully applied Brewster angle cavity ring-down
spectroscopy to determine the absorption cross sections of HNO3
on fused silica surfaces in the 290-330 nm region.
 Our work extends the application of Brewster angle CRD to the UV
region, and demonstrates further the sensitivity and the capability
of this technique in the study of surface photochemical processes.
 Our work has shown that Brewster angle cavity ring-down
spectroscopy can easily distinguish between monolayer and
multilayer molecular adsorption on the surface; thus, this
technique is a valuable addition to the arsenal of sensitive surface
analysis techniques.
Summary and conclusions (continued)
 The larger HNO3 absorption cross sections on fused silica
surfaces in the 290-330 nm region, as compared to cross
sections in the gas phase, are possibly due to red-shift of the
HNO3 absorption spectrum under the influence of the fused
silica surfaces.
 The much larger surface absorption cross sections of HNO3 in
the 290-330 nm region, compared with those in the gas phase,
corroborate results from photolysis rate measurements in field
studies.
Acknowledgments
Fabrication of the cell
Mr. Steve Meyer
Discussions
Dr. Liang Chu
Dr. Andrew Alexander
Funding
NSF-Atmospheric Chemistry Program