Photodissociation Cross Sections of N2O3 Adsorbed on Au(111)

Article abstract of DOI:10.1021/jp993217s

The photochemistry of N₂O₃ adsorbed on a Au(111) surface has been studied at <100 K. N₂O₃ is prepared by a reaction of NO₂ chemisorbed on a Au surface with gas-phase NO. The adsorption states of N₂O₃ are characterized by IR reflection - absorption spectroscopy. Adsorbed N₂O₃ is found to undergo rapid photodissociation to form chemisorbed NO₂ and gas-phase NO under UV and visible illumination. The photodissociation cross section is determined to be (3.0 ± 1.8) x 10^-17 cm² at 350 nm, which is much greater than the maximum absorption cross section of gas-phase N₂O₃ and one of the highest values ever observed for photochemical reactions on a metal surface. The spectral profile of the NO yield is significantly shifted to longer wavelengths as compared to the absorption spectrum of gas-phase N₂O₃. The significant red shift of N₂O₃ absorption in aromatic solvents suggests that electron transfer from the surface to N₂O₃ enhances the absorption cross section of adsorbed N₂O₃ at longer wavelengths.

Full text of DOI:10.1021/jp993217s

J. Phys. Chem. B 2000, 104, 4863-4866
4863
Photodissociation Cross Sections of N2O3 Adsorbed on Au(111)
,†
Shinri Sato,* Dai Yamaguchi, Takehito Senga, and Masahiro Kawasaki
Catalysis Research Center and Graduated School of EnVironmental Earth Science, Hokkaido UniVersity,
Sapporo 060-0811, Japan, and Department of Molecular Engineering, Kyoto UniVersity,
Kyoto 606-8501, Japan
ReceiVed: September 10, 1999; In Final Form: March 20, 2000
The photochemistry of N
by a reaction of NO chemisorbed on a Au surface with gas-phase NO. The adsorption states of N
characterized by IR reflection-absorption spectroscopy. Adsorbed N is found to undergo rapid
photodissociation to form chemisorbed NO and gas-phase NO under UV and visible illumination. The
photodissociation cross section is determined to be (3.0 ( 1.8) × 10
greater than the maximum absorption cross section of gas-phase N
observed for photochemical reactions on a metal surface. The spectral profile of the NO yield is significantly
shifted to longer wavelengths as compared to the absorption spectrum of gas-phase N . The significant red
shift of N absorption in aromatic solvents suggests that electron transfer from the surface to N enhances
the absorption cross section of adsorbed N at longer wavelengths.
O
2 3
adsorbed on a Au(111) surface has been studied at <100 K. N
2
O
3
is prepared
2
2 3
O
are
2
O
3
2
-
17
2
cm at 350 nm, which is much
3
O and one of the highest values ever
2
2
O
3
O
2 3
2 3
O
2
O
3
1
. Introduction
ciation is attributable to enhancement in the absorption cross
section of adsorbed N2O3 due to electron transfer from the
surface.
A photochemical reaction of molecules adsorbed on a solid
surface is often different in kinetics and dynamics from its gas-
phase counterpart, depending on chemical and physical proper-
ties of the surface.1-3 In general, a photoexcited state on a metal
surface undergoes rapid quenching due to the coupling of
photoexcited states to the electron bath in the substrate, even
when reactants are not bonded chemically to the surface. In some
cases, however, a cross section of adsorbed molecule is com-
parable to or greater than that of the gas-phase counterpart. Metal
substrate excitation, which involves photoelectron emission and
photoinduced electron transfer from a metal surface to adsor-
bates, is thought to be responsible for such a large cross
2
. Experimental Section
All measurements were carried out in an ultrahigh vacuum
7
system that has been described in detail previously. An Au-
(111) single crystal (12 mm in diameter, 1.5 mm thick, 99.999%
purity) used as a substrate was cleaned by repeating Ar-ion
sputtering at 800 K and annealing at 900 K. Ozone was
sometimes used for surface cleaning. Gases were analyzed with
a mass spectrometer (ANELVA AQA-200). For IR reflection-
absorption spectroscopy (IRAS) measurements, an IR beam
from FTIR (BIO-RAD FTS-155) was p-polarized by a wire grid
polarizer and then focused by a BaF2 lens onto the sample
through a BaF2 window, the cutoff wavelength of which is
3
-6
7
section.
ciation cross section of N2O4 adsorbed on a NO2-covered Au-
111) surface is as large as those in the gas and condensed
In our recent study, we found that a photodisso-
(
7
phase. Since the yield of the N2O4 photodissociation was
significantly reduced when the surface was covered with a thin
water-ice film, we concluded that the N2O4 photolysis is
enhanced by photoexcitation of the surface. The spectral profile
of the photodissociation cross sections is, however, not similar
to the absorption spectrum of the Au surface but rather resembles
-1
around 900 cm . The diameter of the IR beam at the sample
position was ca. 8 mm. The incident angle of the IR beam was
∼
85°. The paths of the IR beam were purged with dry air to
reduce the effects of moisture. IRA spectra were recorded with
-1
4
cm resolution and 200 scans. The light source was a
paraboloidal-reflector Xe lamp (ILC LX300UV, 300 W), which
was monochromatized by a grating monochromator (RITSU
MC-20L) and focused by a quartz lens onto the whole sample
area through an MgF2 window. The photon flux of monochro-
matized light was measured after the quartz lens with chemical
actinometry using ferrioxalate. Substrate temperature was
virtually unchanged under irradiation with monochromatized
light. To reduce light intensity neutral density filters (Toshiba)
were used.
7
the absorption spectrum of N2O4 adsorbed on LiF. On the basis
of these results, we have proposed enhancement in the absorp-
tion cross section of adsorbed N2O4 due to electron transfer from
Au surface to adsorbed N2O4.7
1
0
In the present paper, we report remarkably rapid photodis-
sociation of N2O3 adsorbed on a Au surface, which was formed
by a reaction of chemisorbed NO2 with gas-phase NO at <130
8
,9
K. The photodissociation cross section was determined to be
17
2
(
3.0 ( 1.8) × 10- cm at 350 nm, one of the highest values
NO2 was stored in a glass reservoir in the presence of an
excess of O2 to prevent its decomposition to NO and O2 during
storage. The gas mixture was frozen at 77 K, and O2 was
pumped away before use. NO was used after adsorption on well-
outgassed molecular sieve 13X at 77 K. Gases were introduced
into the chamber through a long nozzle, the end of which was
3
ever observed for photochemical reactions on metal surfaces,
while the absorption cross section of gas-phase N2O3 is nearly
zero at 350 nm. The large cross section of N2O3 photodisso-
†
E-mail: shinri@cat.hokudai.ac.jp. Fax: +81-11-709-4748.
1
0.1021/jp993217s CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/27/2000

4864 J. Phys. Chem. B, Vol. 104, No. 20, 2000
Sato et al.
Figure 2. Relation between the relative coverages of chemisorbed NO
and adsorbed N . Relative coverage is defined as the intensity ratio
of the IRAS band to the saturation intensity.
2
2
O
3
TABLE 1: Vibrational Frequencies (cm- )a of N
1
O
2 3
Adsorbed on Au(111) and N in CH Cl
2
O
3
2
2
N2O3 in CH2Cl2
N2O3/Au(111)
IRASd
mode
IR (transmission)b HREELSb IRASc
Figure 1. IRA spectra of adsorbed N
NO chemisorbed on Au(111) with gas-phase NO at 90 K. The inset
shows the adsorption states of N (see text).
2 3
O formed by the reaction of
ν(O2N-NO)
F(O2NNO)
δ(NO2)
253
614
772
1291
1600
1849
250
450-650
800
1180
1270
1890
n.d.
n.d.
806 n.d.
1182 1274
1272 (IRAS inactive)
1897 1900
2
2
O
3
νs(NO2)
ca. 1 cm away from the substrate. A pulsed valve (General
Valve) was used to control exposures of gases to the surface.
ν (NO )
a
2
ν(NO)
a
ν, stretching; F, rocking; δ, bending; n.d., no data due to the cutoff
windows. b Reference 8. c Reference 9. d This work.
3. Results and Discussion
of the BaF
2
3
.1. Characterization of Adsorption States. Since NO2
by HREELS and IRAS. They assigned the HREELS and IRAS
bands of adsorbed N2O3 with reference to the IR transmission
spectrum of N2O3 in CH2Cl2. Their assignments are listed in
Table 1 together with our IRAS results. Their assignments are
in good agreement with the IR transmission spectrum of N2O3
adsorbed on Au surfaces at >150 K undergoes thermal
decomposition upon desorption at around 210 K, we refer to
this species as chemisorbed NO2. In the present study, N2O3
7
was prepared on the Au(111) surface at <100 K by the reaction
of gas-phase NO with chemisorbed NO2.7
-9
Figure 1 shows
-
1
in CH2Cl2 except for the 1180 and 1274 cm bands, which
were assigned to the symmetric and asymmetric stretching of
chemisorbed NO2, respectively. Since our result indicates that
part of chemisorbed NO2 remains unreacted with gas-phase NO,
IRA spectra of chemisorbed NO2 and N2O3. The band at 1184
-
1
cm is assigned to the NO2 symmetric stretching of chemi-
sorbed NO2. The NO2 asymmetric stretching of chemisorbed
NO2 cannot be observed by IRAS because IRAS is sensitive
only to vibrational modes perpendicular to the surface (surface
-
1
it is reasonable to assign the 1180 cm band to the NO2
-
1
selection rule).1
1,12
2
symmetric stretching of chemisorbed NO and the 1274 cm
The chemisorbed NO2 band decreased in
band to the NO2 symmetric stretching of adsorbed N2O3. For
intensity upon NO exposure with growth of the new bands at
274 and 1900 cm and remains even after the new bands are
8
-
1
2 3
adsorbed N O , Koel and co-workers had originally proposed
1
the adsorption state (a) in the inset of Figure 1. Recently, they
revised it to the adsorption state (b), since the symmetric and
asymmetric NO2 stretching bands have comparable intensity and
the NO stretching band is stronger in intensity than the NO2
saturated after 2 langmuir exposure of NO. These new bands
are assigned to NO stretching and NO2 asymmetric stretching
of N2O3, respectively. The intensities of these bands at saturation
were found to depend on the initial coverage of chemisorbed
NO2. Figure 2 shows a relation between the relative coverages
of chemisorbed NO2 and adsorbed N2O3. N2O3 coverage
decreases with decreasing NO2 coverage and drops to zero
before NO2 coverage becomes zero, indicating that part of
9
stretching bands. Their recent revision is, however, inadequate,
since the absence of asymmetric NO2 stretching band indicates
that this vibrational mode is parallel to the surface as in the
case of chemisorbed NO2, which has the structure of chelating
-1
NO . In addition, the strong NO stretching band of adsorbed
chemisorbed NO2 does not react with NO. The 1184 cm band
remained after saturation of the N2O3 bands is, therefore,
attributable to chemisorbed NO2 remaining unreacted. Thus, the
asymmetric NO2 stretching band of adsorbed N2O3 is not
observed by IRAS due to the surface selection rule,1 similarly
to the chemisorbed NO2 case. In addition, the total intensity of
2
N2O3 could arise from charge transfer from the Au surface to
NO. Therefore, the adsorption state (a) is much more likely than
the adsorption state (b).
1,12
When the substrate is heated, adsorbed N2O3 undergoes
thermal decomposition at >110 K, as shown in Figure 3. The
decomposition is completed at <160 K with the complete
recovery of chemisorbed NO2. In agreement with these results,
NO desorption occurred without NO2 desorption during tem-
perature-programmed desorption (TPD). Although the N-N
bond breaking of adsorbed N2O3 thus occurs at temperatures
-
1
the 1184 and 1274 cm bands of adsorbed N2O3 does not
change with NO exposure, suggesting that the former band
converts into the latter band.
Koel and co-workers8 studied the adsorption states of
,9
chemisorbed NO2 and adsorbed N2O3 on the Au(111) surface

Cross Sections of N2O3 Adsorbed on Au(111)
J. Phys. Chem. B, Vol. 104, No. 20, 2000 4865
2 3
Figure 3. Change in the IRA spectrum of adsorbed N O as a function
of temperature.
Figure 5. IRAS change during the 350 nm photolysis of adsorbed
N
2
O
3
at 93 K. A 5% neutral density filter was used to reduce light
intensity. The inset shows semilogarithmic plots for the amounts of
residual N during the photolysis.
2 3
O
linear with irradiation time; the cross section seems to decrease
with a decrease in N2O3 coverage. In fact, the cross section
becomes small when the initial coverage of N2O3 was reduced.
The reason for this is, however, not clear at present. It should
be noted that the nonlinearity of the plots is not due to a
nonlinear relation between N2O3 coverage and IR band intensity,
since the desorption amount of NO during TPD was proportional
to the IRAS absorbance of adsorbed N2O3. The cross section is
-
17
determined by least-squares fitting to be (3.0 ( 1.8) × 10
Figure 4. Temporal profile of NO yield during the 350 nm photolysis
of adsorbed N at 93 K: (a) no filter; (b) 70% neutral density filter.
2
cm . This value is one of the highest cross sections ever reported
for surface photochemical reactions on metal substrates.1
The wavelength dependence of the NO yield is shown in
Figure 6. The yield is normalized by photon flux at each
wavelength. The photodesorption of NO occurred to some extent
even at 700 nm probably due to an n f π* transition.13 The
inset of Figure 6 shows the UV-region absorption spectra of
N2O3 in the gas phase, an aqueous solution, and an n-hexane
2
O
3
as low as 160 K, its N-N bond energy could be close to that
of N2O3 in CH2Cl2 (9.7 kcal/mol), since the N-N stretching
frequencies of both N2O3 molecules are very close to each other,
8
as shown in Table 1. Bartram and Koel estimated the N-N
bond energy of adsorbed N2O3 to be 10 kcal/mol from the
maximum temperature of NO desorption.
1
3
solution. The absorption maximized at around 250 nm is
3.2. Photodissociation of Adsorbed N2O3. Photodissociation
1
3
attributed to a π f π* transition. The absorption of N2O3
increases upon dissolution in a solvent, and its peak shifts to
longer wavelengths. A maximum cross section increases from
of adsorbed N2O3 at 350 nm was so efficient that NO
photodesorption was almost completed within 30 s, as shown
in Figure 4. The initial rise of NO yield was almost proportional
to light intensity. For IRAS measurements, light intensity was
reduced by the use of a 5% neutral density filter to obtain a
moderate photolysis rate. IRA spectra of adsorbed N2O3 during
photolysis are shown in Figure 5. The bands at 1900 and 1274
-
18
2
-17
9
.6 × 10
cm at 225 nm for gas-phase N2O3 to 1.6 × 10
2
cm at 240 nm for N2O3 in an aqueous solution. The maximum
cross section observed for the photodissociation of adsorbed
N2O3 is, however, much larger than the absorption cross sections
-1
2 3
of N O in the gas-phase and the solutions. Furthermore, the
cm assigned to adsorbed N2O3 decreased with irradiation time,
-
1
absorption for the latter declines sharply toward longer wave-
lengths and becomes nearly zero at 350-400 nm. The large
cross section of adsorbed N2O3, therefore, may be ascribed to
substrate photoexcitation. Although photoelectron emission from
the Au surface does not occur under irradiation at <234 nm
due to its large work function (5.3 eV), photoinduced electron
transfer from the surface to adsorbates can occur by tunneling
through the metal/adsorbate barrier. Whether or not a surface
photoreaction on a metal surface occurs by substrate photoex-
citation can be determined by a polarization probe method, in
which the polarization-dependent yield is measured as a function
while the band at 1184 cm assigned to chemisorbed NO2
increased due to the conversion of adsorbed N2O3 into chemi-
sorbed NO2 and NO. No other bands were detected during
3
irradiation. The residual amount of N2O3, N, is given as
N ) N exp(-σn )
(1)
0
hν
where N0 is the initial amount of N2O3, σ is the photodissociation
cross section, and nhν the number of photons irradiated. The
-1
semilogarithmic plots of the absorbance of the 1900 cm band
are shown in the inset of Figure 5. The plots are not exactly

4866 J. Phys. Chem. B, Vol. 104, No. 20, 2000
Sato et al.
propose a mechanism involving enhancement in the direct
absorption of adsorbed N2O3 by electron transfer from the
13
surface. Shaw and Vosper observed that the visible absorption
of N2O3 in organic solvents increases with a decrease in the
ionization potential of the solvent. For instance, the absorption
-
1
2
maximum of N2O3 was 97 mol dm at 730 nm in carbon
-
1
2
tetrachloride, while it was 275 mol dm at 690 nm in
m-dimethoxybenzene. Their results indicate that electron dona-
tion from the solvent to N2O3 enhances the absorption of N2O3.
Although the absorption maximum in the UV region could not
be measured due to strong absorption of the solvents, it is sure
that the UV absorption also increases with an increase in the
donor ability of the solvent, judging from the significant increase
13
in the tail of the π f π* transition in the visible region. For
NO2/Au and N2O3/Au, Bartram and Koel observed a large
8
increase in work function (1.6 eV) upon NO2 adsorption on the
Au(111) surface and the lowering of the work function (-0.8
eV) upon formation of adsorbed N2O3. The increase in work
function indicates electron transfer from the Au surface to
chemisorbed NO2, and the decrease in work function after N2O3
formation would arise from electron transfer from NO2 to NO
in adsorbed N2O3, which can reduce the shielding effect of NO2.
The strong NO stretching of adsorbed N2O3 may indicate this
electron transfer toward NO. It is therefore reasonably assumed
that electron transfer from the Au surface to adsorbed N2O3
greatly enhances its absorption in the UV region as well as in
the visible region. Thus, the wavelength dependence of the
photodissociation of N O adsorbed on the Au(111) surface is
Figure 6. Wavelength dependence of initial NO yield in the photolysis
of adsorbed N at 93 K and the reflectivity of an evaporated Au
film. The inset is the absorption spectra of N in the gas phase (a),
2
O
3
1
6
2
O
3
1
3
an aqueous solution (b), and an n-hexane solution (c).
of the incident angle of light. Hasselbrink et al.14 applied this
method to the photodissociation of N2O4 adsorbed on a NO-
saturated Pd(111) surface at 193 and 248 nm and concluded
that the photoabsorption by the metal is the dominant primary
step in the photodissociation. Because photoelectron emission
from a Pd surface does not occur at >220 nm due to its work
function (5.6 eV), they attributed the photodissociation to
electron transfer from the surface to the adsorbate. We have
found a large cross section in the photodissociation of N2O4
3
2
3
similar in shape to the absorption spectrum of N O in the gas
2
3
phase or in a solvent, as observed in the present study.
References and Notes
(1) Chuang, T. J.; Seki, H.; Hussla, I. Surf. Sci. 1985, 158, 525.
(2) King, D. S.; Cavanagh, R. R. AdV. Chem. Phys. 1989, 76, 45.
(3) Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Surf. Sci. Rep. 1991, 13, 73.
(4) Gadzuk, J. W.; Richter, L. J.; Buntin, S. A.; Kingand, D. S.;
7
adsorbed on a NO2-covered Au(111) surface. The yield of the
Cavanagh, R. R. Surf. Sci. 1990, 235, 317.
(
5) Antoniewicz, P. R. Phys. ReV. B 1980, 21, 3811.
N2O4 photodissociation was significantly reduced when the
surface was covered with a thin water-ice film so that we
concluded enhanced photolysis due to electron transfer from
(6) Miesewich, J. A.; Heinz, T. F.; Newns, D. M. Phys. ReV. Lett. 1992,
6
8, 3737.
(7) Sato, S.; Senga, T.; Kawasaki, M. J. Phys. Chem. B 1999, 103,
5063.
7
the surface to adsorbed N2O4. Since photoinduced electron
(
(
8) Bartram, M. E.; Koel, B. E. Surf. Sci. 1989, 213, 137.
9) Wang, J.; Koel, B. E. J. Phys. Chem. A 1998, 102, 8573.
transfer from a metal surface to adsorbates has been reported
for NO adsorbed on Cu(111) using the two-photon photoemis-
sion spectroscopy,15 similar photoinduced electron transfer may
be responsible for the enhanced photodissociation of N2O4 over
the Au surface.
(10) Gordon, A. J.; Ford, R. A. The Chemist’s Companion A Handbook
of Practical Data, Techniques, and References: John Wiley & Sons: New
York, 1972; p 362.
(
2.
11) Greenler, R. G.; Rahn, R. R.; Schwartz, J. P. J. Catal. 1971, 23,
4
The absorption spectra of Pd and Au, however, cannot explain
the wavelength dependence of the N2O4 photodissociation yields,
as discussed in our previous paper. The same is true for the
(12) Suetaka, W. Surface Infrared and Raman Spectroscopy; Prenum
Press: New York and London, 1995; p 13.
(13) Shaw, A. W.; Vosper, A. J. J. Chem. Soc., Dalton Trans. 1972,
7
9
61.
(14) Hasselbrink, E.; Jakubith, S.; Nettesheim, S.; Wolf, M.; Cassuto,
A.; Ertle, G. J. Chem. Phys. 1990, 92, 3154.
15) Kinoshita, I.; Misu, A.; Munakata, T. J. Phys. Chem. 1995, 102,
970.
16) Chronological Scientific Tables; National Astronomical Observa-
tory: Maruzen, Tokyo, 1997; p 521.
present N2O3/Au(111) system. As shown in Figure 6, the
16
reflectivity of an evaporated Au film varies to a small extent
in the wavelength range observed, while the N2O3 photodisso-
ciation yield changes drastically. Therefore, a simple mechanism
(
2
(
7
involving electron attachment would be ruled out, and we