Investigation of the photolysis of the surface-adsorbed HNO3 by combining laser photolysis with Brewster angle cavity ring-down spectroscopy

Article abstract of DOI:10.1016/j.cplett.2012.03.034

We have investigated the photolysis of HNO₃ adsorbed on fused silica surfaces by using excimer laser photolysis at 308 and 351 nm combined with direct probing excited NO₂ product retained on surfaces using Brewster angle cavity ring-

Full text of DOI:10.1016/j.cplett.2012.03.034

Chemical Physics Letters 534 (2012) 77–82
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Investigation of the photolysis of the surface-adsorbed HNO3 by combining
laser photolysis with Brewster angle cavity ring-down spectroscopy
⇑
Otman Abida, Juan Du, Lei Zhu
Wadsworth Center, New York State Department of Health and Department of Environmental Health Sciences, State University of New York, Albany, NY 12201-0509, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
We have investigated the photolysis of HNO3 adsorbed on fused silica surfaces by using excimer laser
photolysis at 308 and 351 nm combined with direct probing excited NO2 product retained on surfaces
using Brewster angle cavity ring-down spectroscopy. The sensitivity and the capability of this technique
in the study of surface photochemical processes have been demonstrated. The quantum yields of the
excited NO2 produced from the HNO3 photolysis on surfaces are (1.1 0.2) and (1.0 0.3) at 308 and
351 nm. The disappearance rate of excited NO2 in the presence of H2O vapor was measured to be
(2.9 0.8) ꢀ 10ꢁ13 cm3 moleculeꢁ1 sꢁ1 at 295 K.
Received 25 November 2011
In final form 12 March 2012
Available online 19 March 2012
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Al surfaces and on ice films was conducted by photolyzing ad-
sorbed HNO3 on Al and ice surfaces but there was ꢃ1.2 cm distance
Nitric acid (HNO3), formed in the atmosphere through oxidation
[1,2] of NOx (NOx = NO + NO2), is removed from the atmosphere
mainly by wet and dry deposition [1]. Nitric acid deposited on
ground, building, and vegetation surfaces was long thought to be
a permanent sink for NOx, as the gas phase tropospheric HNO3 pho-
tolysis rate is slow [3,4], and the photolysis rate of adsorbed nitric
acid was assumed to be slow. However, field study reported [5]
that the photolysis rate for HNO3 deposited on the ground and veg-
etation surfaces to form HONO and NOx was one to two orders of
magnitude faster than that in the gas phase. The photolysis of
HNO3 adsorbed on ground surfaces has been proposed as a major
daytime source of HONO in low-NOx environments [5–7]. To eluci-
date the difference between the nitric acid photolysis rates in the
gas phase and on surfaces, the Zhu group recently determined
[8,9] the UV absorption cross sections of HNO3 adsorbed on fused
silica surfaces in the 290–365 nm region using Brewster angle cav-
ity ring-down spectroscopy [10]. The surface absorption cross sec-
tions of HNO3 are found to be at least two orders of magnitude
higher than the cross section values of the nitric acid vapor, in
the wavelength region studied. Our group also investigated [11]
the photolysis of HNO3 in the gas phase at 253 and 295 K, on alu-
minum surfaces at 253 and 295 K, and on ice films at 253 K by
using 308 nm excimer laser photolysis combined with cavity
ring-down spectroscopy [12,13]. Formation of OH þ NOꢂ2, where
NOꢂ2 represents the electronically excited NO2, was found to be a
predominant photolysis pathway from the gas phase photolysis
of HNO3 at 308 nm. The 308 nm photolysis of HNO3 adsorbed on
between Al or ice surfaces and the center of the cell where the
probe beam was located. The excited NO2 formed from the photol-
ysis of the adsorbed HNO3 needed to desorb from the Al and ice
surfaces and travel to the center of the cell to be detected by cavity
ring-down spectroscopy. Since the probe beam did not directly
interact with either Al or ice surface, and it only monitored excited
NO2 that was diffused into the gas phase and in the probe beam
area, the experimental technique and configuration we used in
the study of the HNO3 photolysis on Al or ice surfaces had reduced
sensitivity, and we had to use higher HNO3 pressures (HNO3 pres-
sures on the order of 0.1–0.5 Torr) in the previous surface photol-
ysis study. As a result, surface photolysis study was not conducted
by photolyzing a monolayer or several layers of adsorbed HNO3 on
surfaces.
Despite the recent works by our group, there are still unre-
solved questions concerning the HNO3 photolysis on surfaces. For
example, (1) Can surface photolysis study be conducted under con-
ditions of monolayer or at most several layers of adsorbed HNO3?
(2) What is the 308 nm photolysis quantum yield of adsorbed
HNO3 on fused silica surface at 295 K? Does surface type affect ad-
sorbed HNO3 photodissociation quantum yield? (3) How does pho-
tolysis quantum yield of surface-adsorbed HNO3 vary with
wavelength? In this Letter, we report results obtained by exploring
the use of Brewster angle cavity ring-down spectroscopy for direct
and time-resolved monitoring of excited NO2 formed from 308 to
351 nm photolysis of HNO3 molecules adsorbed on fused silica
Brewster window surfaces. The approach that we developed has
allowed us to directly probe excited NO2 retained on the surface
following the photolysis of adsorbed HNO3 at close to monolayer
condition. The disappearance rates of excited NO2 (formed from
⇑
Corresponding author. Fax: +1 518 473 2895.
E-mail address: zhul@wadsworth.org (L. Zhu).
0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cplett.2012.03.034

78
O. Abida et al. / Chemical Physics Letters 534 (2012) 77–82
Figure 1. Schematics combining excimer laser photolysis with probing surface photolysis products using Brewster angle cavity ring-down spectroscopy.
the 308 nm photolysis of adsorbed HNO3) in the presence of excess
H2O vapor were also measured.
The inner surfaces of the surface-study cell, and the stainless
steel joints on the gas transport line, were treated with halocarbon
wax or grease (Series 1500 wax and 25-5S grease, Halocarbon
Products Corp.) to minimize decomposition of HNO3 on cell sur-
faces and on stainless steel joints.
2. Experimental technique
Since the 308 nm absorption cross section of HNO3 on fused silica
surface [8] (1.2 ꢀ 10ꢁ18 cm2 moleculeꢁ1) is 1140 times larger than
that in the gas phase [11,14,15] (1.1 ꢀ 10ꢁ21 cm2 moleculeꢁ1), and
since the surface photolysis study was carried out at HNO3 pressures
on the order of tens of mTorr, the absorption of the probe beam at
552.84 nm by the excited NO2, obtained through measurement of
the cavity losses with and without introducing the 308 nm excimer
laser beam into the surface study cell should be that resulting from
the HNO3 photolysis on surfaces. A pulse/delay generator was used
to vary the delay time between the firing of the photolysis laser
and the firing of the probe laser. Only fresh HNO3 sample was used
in the surface photolysis study, the sample was pumped away after
averaging results over 20–50 photolysis pulses at a given pressure
and delay time. We usually evacuated the cell for several minutes
to get rid of residual HNO3 in the cell and we ensured no degassing
of HNO3 from the cell wall before we added new HNO3 sample to
the surface study cell. All experiments were carried out at an ambient
temperature of 295 K, and at a repetition rate of 0.5 Hz.
Purified HNO3 was prepared by vacuum distillation of a 3:2 mix-
ture of sulfuric acid (98%, Mallinckrodt Baker) and nitric acid (70%,
Mallinckrodt Baker) at 273 K into a trap cooled at ethanol/dry ice
temperature (195 K) [14]. Repeated distillations were conducted,
to purify the sample. To further reduce NO2 impurity before each
experiment, we purged NO2 from the liquid HNO3 bubbler, by flow-
ing N2 carrier gas through the bubbler for about 30 min [16], we
then pumped the liquid HNO3 bubbler for 10 min in the absence
of N2 flow. Previous measurements by our group [8,9] indicate that
HNO3 thus purified has an NO2 impurity of less than 0.05%.
An excimer laser (Lambda Physik LPX110i) operating at 308 or
351 nm was used to photolyze HNO3 adsorbed on fused silica Brewster
windows, the products formed from the photolysis of adsorbed HNO3
were then probed by Brewster angle cavity ring-down spectroscopy. A
diagram of the experimental setup is shown in Figure 1. The custom
stainless-steel cell, with shape of a hollow rectangular prism, was vac-
uum-sealed by a pair of high-reflectance cavity mirrors at both ends. A
plate holding two mutually compensating Brewster windows (1 mm
thick) were mounted inside the cell along its main optical axis. Two
pairs of fused silica windows were attached to the front and back of
the cell for transmission of the photolysis beam. The output from the
photolysis laser was split into two beams (by using a 50% reflectance
beam splitter) which subsequently entered the reaction cell at 90° an-
gle to the main cell axis through two fused silica windows on the front
side, the photolysis beams hit the Brewster windows inside the cell be-
fore they exited the cell through two fused silica windows on the back
side. The photolysis beams did not interact with cell wall, there was no
stray light, and the photolysis beam shapes were the same before and
after the cell. The probe laser pulse from a dye laser (Continuum
ND6000 with Rhodamine 590 laser dye) pumped by a Nd:YAG laser
(Continuum Surelite II), delayed relative to the photolysis laser pulse
and used to detect excited NO2 [11], was introduced into the cell along
the main optical axis. The probe laser beam overlapped with the pho-
tolysis beams on Brewster windows. A fraction of the probe laser pulse
was injected into the cavity through the front mirror, and the intensity
decay of this fraction inside the cavity was measured by monitoring
the weak transmission of light through the rear mirror, with a photo-
multiplier tube (PMT). The amplified PMT signal was fitted to a single-
exponential decay function, from which the ring-down time constant
and the total loss per optical pass was calculated. The surface-study
cell is equipped with reagent ports, a pumping port, and pressure mea-
surement port, located on top of the cell. A rectangular viewport, made
of transparent plastic and installed on the top, along the main optical
path, allowed us to view the photolysis beam. The pressure inside the
surface-study cell was measured with an MKS Baratron capacitance
monometer.
3. Results and discussion
3.1. Time-resolved study of the photolysis of HNO3 on fused silica
surfaces at 295 K
We investigated the 308 and 351 nm photolysis of HNO3 ad-
sorbed on fused silica surfaces at 295 K, and we directly probed

O. Abida et al. / Chemical Physics Letters 534 (2012) 77–82
79
for excited NO2 formed from the HNO3 photolysis on fused silica
surfaces using Brewster angle cavity ring-down spectroscopy.
Based upon the previous published results by Goodman et al.
[17] that HNO3 molecularly and reversibly adsorbed on SiO2 parti-
cles, we believe that HNO3 deposited on fused silica surfaces re-
tained its molecular form prior to its photolysis by 308 nm/
351 nm excimer laser beam. Shown in Figure 2 is a product absorp-
tion spectrum in the 551.50–558.00 nm region after the 308 nm
photolysis beams stroke on Brewster windows and with 100 mTorr
HNO3 inside the surface study cell. The observation of product
absorption in a spectral region that the excited NO2 exhibits strong
absorption [11,18] suggests that the excited NO2 is a product from
the HNO3 photolysis inside the surface study cell. To find out
whether the excited NO2 absorption resulted from the HNO3 pho-
tolysis in the gas phase or resulted from the photolysis of adsorbed
HNO3 on Brewster windows, we added 100 mTorr of HNO3 into the
surface study cell but removed Brewster windows from the ring-
down cavity. We could not detect excited NO2 absorption after
we fired excimer laser at 308 nm, suggesting that the excited
NO2 formed from the HNO3 photolysis in the gas phase was not
detectable at an HNO3 pressure of 100 mTorr inside the surface-
study cell. As a result, the excited NO2 absorption shown in Figure
2 originates from the 308 nm photolysis of adsorbed HNO3. A sim-
ilar product absorption spectrum was obtained from the photolysis
of adsorbed HNO3 at 351 nm.
The cavity ring-down spectrometer was tuned to the excited
NO2 absorption maximum at 552.84 nm, and the absorption result-
ing from the photolysis of adsorbed HNO3 was measured as a func-
tion of time. Shown in Figure 3 is a transient round-trip absorption
profile measured at 552.84 nm following the 308 nm photolysis of
adsorbed HNO3 at an HNO3 pressure of 30 mTorr in the cell (The
peak absorption wavelength reported here was obtained with a
Continuum Nd:YAG pumped dye laser system which offers wave-
length accuracy of 0.05 nm. The peak wavelength of 552.57 nm
for the excited NO2 reported by our group [11] was obtained using
Figure 3. Time profile of the round-trip absorption measured at 552.84 nm
following the 308 nm photolysis of HNO3 adsorbed on fused silica surfaces with
30 mTorr of HNO3 inside the surface-study cell at 295 K. Circles denote experi-
mental data. Curve shown is the kinetic simulation using the ACUCHEM software. See
text for details.
the ground state NO2 absorb probe laser beam at 552.84 nm with
respective absorption cross sections [11] of 9.4 ꢀ 10ꢁ19 and
8.9 ꢀ 10ꢁ20 cm2 moleculeꢁ1 in the gas phase at 295 K. In the ab-
sence of literature absorption cross sections of the excited NO2
and the ground-state NO2 on surfaces at 552.84 nm, we assumed
the absorption cross sections of the excited NO2 and the ground-
state NO2 on surfaces to be similar to their gas phase cross section
values at the same wavelength. To find out what can be the max-
imum difference between the absorption cross section of the
ground state NO2 on fused silica surface versus that in the gas
phase, we added NO2 into the surface-study cell, tuned the ring-
down probe beam to NO2 absorption maximum at 446.10 nm,
and measured total absorption as a function of the NO2 pressure
in the surface-study cell. We then removed Brewster windows
from the ring-down cavity and determined the probe laser absorp-
tion as a function of the NO2 pressure in the cell. By comparing the
absorption of the probe beam in the presence of NO2 but with and
without Brewster windows inside the cavity, we found that the
majority of the probe laser absorption resulted from the NO2
absorption in the gas phase for NO2 pressures in the mTorr range.
We also found that NO2 had little tendency of sticking on fused sil-
ica surfaces at room temperature. The surface absorption cross sec-
tion of ground state NO2 at 446.10 nm is estimated at most twice
its gas phase cross section value at this wavelength.
a
Molectron DL14 dye laser which had lower wavelength
accuracy).
The probe laser absorption at 552.84 nm initially increases with
delay time, between the firing of the photolysis laser and the probe
laser, and reaches a maximum at about 12–15 s. It subsequently
showed rapid decrease with delay time in the 15–60 s range fol-
lowed by residual low absorption that is nearly independent of
time at delay times longer than 60 s. Both the excited NO2 and
l
l
l
The following kinetic scheme was used to fit the experimental
temporal round-trip absorption profiles at 552.84 nm:
NOꢂ2 ꢁ S ¼ NOy2 ꢁ S
ð1Þ
NOy ꢁ S ¼ NO2 ꢁ S
ð2Þ
ð3Þ
2
NO2 ꢁ S ¼ NO2 þ S
AbsorptionðtÞ ¼ 8rNOy
ꢀ ½NOy2 ꢁ Sꢄ þ 8rNO
ꢀ ½NO2 ꢁ Sꢄ ð4Þ
2;surf
2;surf
where NOꢂ ꢁ S represents the electronically-excited NO2 that
2
was generated from the photolysis of adsorbed HNO3, S represents
substrate, NOy2 ꢁ S represents the surface-bound electronically-ex-
cited NO2 that has undergone some degree of relaxation, NO2 ꢁ S
denotes the surface-bound ground state NO2, NO2 represents the
ground state NO2 that was desorbed from the surface and escaped
Figure 2. Cavity ring-down absorption spectrum of the product in the 551.50–
558.00 nm region, after the 308 nm photolysis of adsorbed HNO3 at 100 mTorr
HNO3 pressure inside the surface-study cell at 295 K. The spectrum was recorded at
from the probe beam area, rNOy
and rNO
denote the respec-
2;surf
a wavelength interval of 0.10 nm. The photolysis/probe laser delay was 15
ls.
tive surface absorption cros2s;sursf ections of NO2y and NO2 at

80
O. Abida et al. / Chemical Physics Letters 534 (2012) 77–82
552.84 nm, ½NOy ꢁ Sꢄ and [NO2 ꢁ S] represent the respective con-
following the HNO3 photolysis is that of the electronically-excited
NO2 that has undergone some degree of relaxation.
2
centrations of NOy2 and NO2 on surfaces. Transient absorption pro-
files at 552.84 nm from the photolysis of adsorbed HNO3 at HNO3
pressures of 10, 20, and 30 mTorr inside the cell were compared
with the values calculated by the ACUCHEM simulation program
[19]. The vibrational redistribution rate constant of surface-bound
The temporal absorption profile at 552.84 nm from the photol-
ysis of HNO3 adsorbed on fused silica surfaces is also compared to
the temporal absorption profile at this wavelength from the pho-
tolysis of HNO3 adsorbed on Al surfaces and on ice films [11]. A dif-
ferent cell configuration was used to study the photolysis of HNO3
on Al surfaces and on ice films. In that study, the surface photolysis
temporal absorption profile showed an absorption maximum [11]
ꢂ
electronically-excited NO2ðk ꢁSÞ, the quenching rate constant of
NO2
surface-bound, redistributed electronically-excited NO2ðkNOy ꢁSÞ,
and the desorption rate constant of ground-state NO2 from the 2sur-
ꢂ
face ðkNO ꢁSÞ were used as input parameters. Initial values of kNO
,
at photolysis/probe laser delay of ꢃ30
ls since the surface photol-
ysis generated excited NO2 needed to travel to the center of the cell
2ꢁS
2
kNO , and kNO
were given to the program, and the simulated
absorption profiles were compared with the experimental results.
y ꢁS
2ꢁS
2
to be detected by cavity ring-down spectroscopy.
ꢂ
Numerical values of kNO ꢁS, k
, and kNO
were subsequently
The average excited NO2 quantum yields from 308 to 351 nm
photolysis of HNO3 adsorbed on fused silica surfaces at 295 K were
derived from the ratio of the number of excited NO2 formed from
the photolysis of adsorbed HNO3 in the surface area where the
photolysis and the probe laser overlapped to the number of photol-
ysis photons absorbed by adsorbed HNO3 in the same area.
The number of photolysis photons absorbed by surface-adsorbed
HNO3 (Nabs,photon) in the photolysis/probe laser overlap area on each
window was determined from (i) the photolysis beam energy strik-
ing on each fused silica Brewster window (Ephotolysis), (ii) the individ-
ual photon energy (hc/k) at the photolysis wavelength (k), (iii) the
absorption cross section of the surface-adsorbed HNO3 at the pho-
NOy2ꢁS
2ꢁS
2
adjusted so as to optimize the fit. The following rate constant val-
ues are extracted: kNO
of (1.4 0.3) ꢀ 105 sꢁ1
,
kNO
of
ꢂ ꢁS
y ꢁS
(7.5 0.6) ꢀ 104 sꢁ1, and2 kNO
of (7.4 2.7)ꢀ103 sꢁ1 at 295 K.
2
2ꢁS
of 1.4 ꢀ 105 sꢁ1 suggests that the electronically-excited
ꢂ ꢁS
kNO
NO22 formed from the adsorbed HNO3 photolysis gets vibrationally
redistributed to lower vibrational levels of electronically-excited
NO2 at time scale on the order of 7
ls on surfaces. The extracted
kNO
value corresponds to an electronic quenching lifetime on
y ꢁS
the2 order of 13
l
s, which is shorter than the literature reported
lifetimes [20] for quenching of electronically-excited NO2 of 55–
90 s in the gas phase. Thus, the relaxation rate for electroni-
l
cally-excited NO2 adsorbed on surface is faster than that in the
tolysis wavelength ðrHNO Þ, (iv) the surface concentration of
3;surf
gas phase. kNO
of 7.4 ꢀ 103 sꢁ1 would correspond to surface-
HNO3 ðnHNO Þ, and (v) the ratio of probe beam area to photolysis
2ꢁS
3;surf
desorption lifetime for the ground state NO2 on the order of
beam area on each fused silica Brewster window (Aprobe/Aphotolysis),
135
ls (Inclusion of desorption of ground-state NO2 from surface
via the equation:
in the simulation makes the simulated temporal absorption profile
and the measured temporal absorption profile agree better at
t > 30 ls). Temporal absorption profiles at 552.84 nm from the
Nabs;photon ¼ Ephotolysis
ꢀ
rHNO
ꢀ nHNO
ꢀ ðAprobe=AphotolysisÞ
3;surf
3;surf
ꢀ 2=ðhc=kÞ
ð5Þ
photolysis of adsorbed HNO3 at 10, 20, and 30 mTorr HNO3 pres-
The incident photolysis beam energy entering the cell was mea-
sured by a calibrated Joulemeter placed in front of the cell prior to
its entrance through one front cell window. Once the photolysis
beam entered the surface-study cell (Figure 1) through a fused silica
window at the front side, it subsequently struck a fused silica Brew-
ster window, before it exited the cell through a fused silica window
on the back side. The photolysis beam energy striking on each fused
silica Brewster window was corrected for photolysis beam transmis-
sion loss at the front cell window, and for reflection of the photolysis
beam from the exit cell window. The absorption cross sections of
HNO3 adsorbed on fused silica surfaces at 308 and 351 nm were pre-
viously determined [8,9] by our group using Brewster angle cavity
ring-down spectroscopy. They are 1.2 ꢀ 10ꢁ18 and 1.6 ꢀ 10ꢁ19
cm2 moleculeꢁ1 at 308 and 351 nm, respectively. Previous measure-
ments [8,9] of the absorption of the probe laser beam by adsorbed
HNO3 on fused silica surfaces as a function of HNO3 pressure in the
cell by our group indicates that saturation of monolayer adsorption
sites on fused silica surfaces occurred at HNO3 pressures of about
14 2 mTorr inside the cell. The maximum HNO3 surface concentra-
tion to form monolayer adsorption on fused silica surfaces can be
estimated using a van der Waals radius [21] of 5.5 Å for HNO3. It
was about 1.1 ꢀ 1014 molecule cmꢁ2. Assuming the surface concen-
tration of HNO3 varies linearly with HNO3 pressure in the cell over
the 6–14 mTorr range, we can estimate HNO3 surface concentration
before the saturation of monolayer surface sites.
ꢂ
sures in the cell are well fitted by the extracted kNO ꢁS, k
,
NOy2ꢁS
and kNO ꢁS values at 295 K. The quality of the fit is shown2in Figure 3
2
with HNO3 pressure of 30 mTorr in the surface-study cell.
The temporal absorption profiles from the photolysis of HNO3 ad-
sorbed on fused silica surfaces are compared to the temporal absorp-
tion profiles from the gas phase HNO3 photolysis. As described in the
previous publication of our group, [11] the temporal absorption pro-
file at 552.84 nm following the gas phase photolysis of HNO3 in the
0.5–1.5 Torr pressure range displayed an absorption maximum at
15
ls after the photolysis pulse, showed decrease in absorption with
delay time in the 15–100
l
s range due to the quenching of the ex-
cited NO2 (with rate constant of 1.8 ꢀ 104 sꢁ1) and the reaction be-
tween the excited NO2 and HNO3 to form an NO2ꢅHNO3 adduct
(with rate constant of 1.1 ꢀ 10ꢁ13 cm3 moleculeꢁ1 sꢁ1), and the
absorption value is nearly independent of time at delay times in
the 150–400
l
s range. In that study, we did not obtain absorption
s because
values at photolysis/probe laser delay of shorter than 15
l
the PMT sometimes collected reflected photolysis beam inside the
cell and we did not have a data analysis program that would reject
photolysis pulse pick-up as ring-down curve at that time.
To find out whether temporal absorption at 552.84 nm from the
gas phase HNO3 photolysis also increases with delay time at
t < 15 ls similar to that observed from the photolysis of HNO3 ad-
sorbed on fused silica surfaces, we measured the temporal absorp-
tion profiles at 552.84 nm from the gas phase HNO3 photolysis in
this work and found that absorption also increases with delay time
The number of the excited NO2 molecules generated from 308 to
351 nm photolysis of adsorbed HNO3 per window for a given HNO3
pressure inside the surface-study cell were obtained from the mea-
surements of the round-trip absorption by excited NO2 at
at t < 15
l
s. Thus, the excited NO2 absorption increases with delay
time at t 6 15
l
s and decreases with delay time at t > 15 s follow-
l
ing the gas phase HNO3 photolysis suggesting that the excited NO2
we observed at 552.84 nm was fed from another excited state of
NO2. The 308 nm photolysis of gas phase (or adsorbed) HNO3 di-
rectly leads to the formation of the electronically-excited NO2.
The excited NO2 we monitored in the 551.50–558.00 nm region
552.84 nm at a photolysis laser-probe laser delay of 15
ls, the
absorption cross section of the excited NO2 on surface rNO
y
at
2;surf
552.84 nm, and the size of the area (Aprobe) where the photolysis
and the probe beam overlapped, via NNOy ¼ Absorptionꢂ
2

O. Abida et al. / Chemical Physics Letters 534 (2012) 77–82
81
A
probe=ð4rNO
y
Þ. The absorption cross section of the excited NO2 at
surface-study cell and depositing HNO3 on fused silica Brewster
window surfaces, and then introducing water vapor into the sur-
face-study cell. Once the 308 nm photolysis beam struck on the
fused silica Brewster window surfaces, the excited NO2 generated
from the photolysis of the adsorbed HNO3 could either react with
water vapor or be quenched by water vapor.
2;surf
552.84 nm on surfaces is assumed to be similar to the cross section
value [11] of the excited NO2 in the gas phase (9.4 ꢀ 10ꢁ19 cm2 mol-
eculeꢁ1). Shown in Figure 4 is round-trip absorption by excited NO2
generated from the 308 nm photolysis of HNO3 adsorbed on fused
silica surfaces measured as a function of HNO3 pressure in the sur-
face-study cell at 295 K. As seen from Figure 4, the excited NO2
absorption increased with increasing HNO3 pressure up to HNO3
pressure of about 10 mTorr inside the surface-study cell, the ex-
cited NO2 absorption reached a plateau with HNO3 pressures of
about 12–14 mTorr in the surface study cell, and the excited NO2
absorption showed further increase with increasing HNO3 pressure
for HNO3 pressures higher than 16 mTorr. The plateau in the ex-
cited NO2 absorption versus P(HNO3) plot likely corresponds to
absorption by excited NO2 at steady-state concentration which
was formed from the photolysis of adsorbed HNO3 at saturation
monolayer coverage. A plot similar to Figure 4 was obtained from
the photolysis of adsorbed HNO3 at 351 nm.
The excited NO2 quantum yields from 308 to 351 nm photolysis
of HNO3 adsorbed on fused silica surfaces were determined as a
function of HNO3 pressure in the surface-study cell in the 6–
14 mTorr range. The excited NO2 quantum yields were found to be
independent of HNO3 pressure in the 6–14 mTorr range to within
experimental measurement uncertainty. The average excited NO2
quantum yields from 308 to 351 nm photolysis of HNO3 adsorbed
on fused silica surfaces are 1.1 0.2 and 1.0 0.3, respectively,
NOy þ H2O ! HONO þ OH
ð6Þ
2
NOy þ H2O ! NO2 þ H2O
ð7Þ
2
The total rate constant for the quenching/reaction of the excited
NO2 with H2O can be determined by measuring the pseudo-first or-
der disappearance rate constants of the excited NO2 in the pres-
ence of varying excess H2O concentrations. The pseudo-first
order disappearance rate constant of the excited NO2 at a given ex-
cess H2O vapor concentration can be determined by tuning the
probe laser wavelength to the excited NO2 absorption maximum
at 552.84 nm and measuring the probe laser absorption as a func-
tion of time delay between the firing of the photolysis and the
probe laser. The bimolecular rate constant for the reaction/deacti-
vation of the excited NO2 with H2O is (2.9 0.7) ꢀ 10ꢁ13 cm3
moleculesꢁ1 sꢁ1 at 295 K, where error quoted represents 1
r mea-
surement uncertainty. Due to the absorption of the ring-down
probe beam by water vapor at 552.84 nm, the range of excess
water vapor pressure that can be varied was limited in the current
study. Thus, the water vapor pressure inside the cell was kept at
where errors quoted (1r) represent measurement precision. Our
1.0 Torr and lower, and the NOy =H2O rate constant measurement
2
group previously obtained [11] the excited NO2 quantum yields of
0.80 0.15 and 0.92 0.26 from the 308 nm photolysis of HNO3
on Al surface at 295 and 253 K, and an excited NO2 quantum yield
of 0.60 0.34 from 308 nm photolysis of HNO3 on ice film at
253 K. The excited NO2 quantum yield from the HNO3 photolysis
on fused silica surface obtained here is similar to those obtained
from the HNO3 photolysis on Al surfaces at 308 nm after considering
combined uncertainties of both measurements. The excited NO2
quantum yield from the HNO3 photolysis on fused silica surface is
higher than that from the HNO3 photolysis on ice film possibly
due to some degree of salvation of the excited NO2 by ice.
was subject to significant uncertainty.
Li et al. [22] reported the generation of OH radical from the reac-
tion of the electronically-excited NO2 with water vapor, H2O, using
laser induced fluorescence technique. In that study, the electroni-
cally-excited NO2 was formed by exciting the ground state NO2 with
a visible light source. Li et al. monitored OH formation rates in the
presence of varying excess H2O concentrations and obtained a
rate constant [22] on the order of (1.7 0.8) ꢀ 10ꢁ13 cm3 mole-
culeꢁ1 sꢁ1. The rate constant between the electronically-excited
NO2 and H2O obtained by Li et al. agrees with our NOy =H2O reac-
tion/deactivation rate constant within the combined measurement
uncertainty of both studies.
2
3.2. Quenching/reaction rate constant of excited NO2 with H2O at
295 K
4. Conclusions
The quenching/reaction rate constant of the excited NO2 and
water vapor was determined by first introducing HNO3 into the
In this Letter, we have investigated the 308 and 351 nm
photolysis of HNO3 adsorbed on fused silica surfaces by combining
excimer laser photolysis with Brewster angle cavity ring-down
spectroscopy, and we obtained the excited NO2 quantum yields
from the photolysis of the adsorbed HNO3 at both wavelengths.
The surface photolysis study was conducted by doping a monolayer
or several monolayers of adsorbed molecules on the Brewster win-
dow surfaces and then directly probing photolysis intermediates/
products that are retained on the surfaces. The sensitivity of com-
bining laser photolysis with Brewster angle cavity ring-down spec-
troscopic probe to study photodissociation of surface-adsorbed
molecules at low surface coverage is clearly demonstrated in the
current study. Time-resolved probing of the excited NO2 following
the photolysis of adsorbed HNO3 also sheds light on the nature of
the excited NO2 formed from the HNO3 photolysis. We also deter-
mined the reaction/deactivation rate constant between the excited
NO2 and H2O.
Acknowledgements
Figure 4. The NOy2 round-trip absorption measured at 552.84 nm from the 308 nm
photolysis of adsorbed HNO3 plotted as a function of HNO3 pressure in the surface-
study cell. The photolysis/probe laser delay was set at 15 ls.
We thank Mr. Steve Meyer and Mr. Tim Hoffman for constructing
the cell for surface photolysis study, and we thank Dr. Liang T. Chu

82
O. Abida et al. / Chemical Physics Letters 534 (2012) 77–82
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