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Infrared-visible sum frequency generation study of HCOOH on an MgO(001) surface
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.‘....
“.v.:.:.:.?
.. . . . :.:.:
. . . . .. . . . . . ../...._..............................
:::::;......
‘kurface science‘ ..
Surface Science 283 (1993) 468-472 North-Holland
‘:‘.‘.y.... ., .....:.>:q&; “” ‘::‘-Y: ‘.....,.,./. ..,,, ..,, ..,.,.. ..,_ ..,:.:.:::::~~~~~,~.~:~,~~~ (,,,/ ,“‘,‘,,,,,,,,., Y...’ :‘:: ‘.11’.... ....‘i.....: ,,,, ,,:,.. (,: ,.,,,,, ‘X” ““” .i.‘. .‘..../. ,_ ,/, “:‘.‘.:.::o:.:.:.:.::.:.>:.:..:.:.::::: :.::::::,:,,:.,.,., ..‘i.. ...... .._.( .
Infrared-visible sum frequency generation on an MgO(001) surface Kazunari Domen *, Naotoshi and Chiaki Hirose
Akamatsu,
Hiroyoshi
study of HCOOH
Yamamoto,
Akihide
Wada
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan Received
21 April
1992; accepted
for publication
30 July 1992
Adsorption and decomposition of HCOOH on an MgO(OO1) single crystal surface were studied by means of infrared-visible sum frequency generation (SFG) spectroscopy and temperature programmed desorption (TPD) in UHV. In the SFG experiment, tunable ultrashort infrared pulses were produced by parametric oscillation/amplification in two pieces of LiNbO, single crystals and the tunable range was 2600-4000 cm-‘. HCOOH was adsorbed on MgO(001) as formate (HCOO) species at room temperature. The TPD study revealed that HCOOH mainly decomposed into CO and H,O at _ 510 K. By SFG, the C-H stretching band was observed at 2870 cm-’ with a shoulder at - 2850 cm-‘. The in situ measurement of the change of the SFG signal intensity (2870 cm-‘) was carried out during the dosage of HCOOH.
1. Introduction Recently, infrared-visible sum frequency generation (SFG) at interfaces has been demonstrated to be a new probe of surface vibrational states [l-12]. It has some remarkable characteristics distinct from other vibrational spectroscopies such as IR, Raman, and HREEL spectroscopies. Surface SFG is a second-order nonlinear optical process caused by two intense coherent beams of light with different wavenumbers [131. In the case of infrared-visible SFG, the beam intensity of the sum frequency is resonantly enhanced when the frequency of the incident infrared beam coincides with that of a vibrational band of the surface species. Surface SFG is highly surface specific if the interface is located between two centro-symmetric media because the second-order nonlinear electric susceptibility tensor is nonzero only at the interface. It is applicable to various types of surfaces; insulators, semiconductors, and metals as far as they are optically flat.
* To whom correspondence
0039-6028/93/$06.00
should
be addressed.
0 1993 - Elsevier
Science
Publishers
In this study, formic acid (HCOOH) adsorption and decomposition on an MgO(001) single crystal surface was investigated by temperature programmed desorption (TPD) and SFG. The C-H stretching mode of the adsorbed species was monitored by SFG. MgO is an insulator, and other vibrational spectroscopic techniques are not readily applicable to the adsorbed molecules on such a single crystal surface.
2. Experimental The experimental apparatus of SFG is shown in fig. 1. The optical setup was similar to the previously reported one [ll]. Picosecond laser pulses were generated by a mode-locked Nd : YAG laser (repetition rate 10 Hz, pulse width 35 ps). Tunable IR pulses were generated in a two-stage LiNbO, (10 mm X 10 mm‘X 50 mm) optical parametric amplifier [14]. The band width was N 13 cm-‘. A visible laser beam (532 nm) was generated by second-harmonic generation. The fluencies of IR and visible beams were about 0.2 and 0.5 mJ per pulse, respectively. The inci-
B.V. All rights reserved
R Domen et al. / SFG study of HCOOH on MgO(OO1) Ft : 1064tm
469
n
cut Sllert
Fz : 53Znra cutNter LN: LiNbO3
HCOOH/M@O
MC : monocbromater PM : photomultiplier S
: Mg0(001)ssmple
Fig. 1. Setup of experimental apparatus. UHV chamber is equipped with LEED-AES and MS.
I
I
400
600
700
600 Temp./K
Fig. 2. TPD spectra of CO (m/e = 28) from Mg~~l) various dosages of HCOOH at room temperature.
dent angle of the IR beam was 45” from the surface normal and that of visible beam was 40”. The two beams hit the sample simultaneously in a common plane of incidence through a CaF, window. The generated SFG signal came out through another CaF, window and was detected by a photomultiplier after passing through a cut-off filter and a monochromator to eliminate stray light. Both incident IR and visible beams were p-polarized. An Mg~~l) single crystal (10 mm x 10 mm x 1 mm> was held in a UHV chamber (base pressure = 1.5 x lo- lo Torr). The backface of the sample piece was coated with Ni by evaporation to several pm of thickness for the sake of resistive heating. The cleaning of the sample was carried out by oxidation with 0, at 900 K until the carbon peak disappeared from the AES signal followed by annealing at 900 K to produce clear (1 X 1) LEED spots. Neat or Ar-diluted HCOOH gas was dosed through a gas doser tube (1.8 mm in diameter) of stainless steel.
with
tive heating and the peak at 510 K was assigned to desorption from the MgO surface. The TPD spectra of HCOOH (m/e = 46, parent molecule) was also monitored but the signal intensity was negligible under the experimental condition. From the mass spectrum of HCOOH in the gas phase, the signal intensity ratio of m/e = 46 (HCOOH) versus m/e = 28 (CO) was determined to be 1: 1.2 in our mass spectrometer. It was, therefore, concluded that the observed signal of CO was not due to the fragment of HCOOH molecules but desorbed from the MgO surface via decomposition of adsorbed HCOOH. After approximate separation of the two peaks (510 and 580 K), the integrated peak areas of 510 K was plotted as a function of dose time in fig. 3. The TPD spectra of CO, (m/e = 441, H,O (m/e = 181, and H, (m/e = 2) were also measured.
3. Results and discussion The TPD spectra of CO (m/e = 28) from MgO(OO1) with various dosages of HCOOH at room temperature is shown in fig. 2. The background pressure increased by N 1 x 10m9 Torr during the dosage. Although there appeared two peaks at 510 and 580 K, the latter peak was tentatively assigned to desorption from Ni or NiO on the backside of the sample for electric resis-
P-.-_ 0
-e
120
Dose
I
l
240
time I 8
Fig. 3. Dependences of the amounts of CO (m/e = 28) and CO, (m/e = 44) from HCOOH/MgO(OOl) on dose time.
K Domen et al. / SFG study of HCOOH on MgO(OO1)
470
I
I
2750
2950
II
I
2950 ( cm-’ )
2750
I
I
2950
2950 ( cm-’ )
Fig. 4. SFG spectra of the C-H stretching region of adsorbed formate on MgO(OO1).HCOOH was adsorbed at room temperature for 15 s, and (b) 120 s (see figs. 2 and 3). Both incident IR and visible beams were p-polarized.
Although the desorption of CO, was observed at around 450 and 510 K, the signal intensity was much less than that of CO as shown in fig. 3. H,O mainly desorbed at 510 K with a shoulder peak at a lower temperature. The desorption of H, was not observed clearly because of rather high background signal of m/e = 2. From these results, it is concluded that HCOOH mainly decomposes into CO and H,O on the MgO(001) surface at around 510 K according to the following scheme: HCOOH --f CO + H,O.
(1)
A similar result concerning the decomposition of HCOOH on the MgO(001) surface was previously reported by Peng and Barteau [15,16]. The SFG spectra of the C-H stretching region, which was observed after adsorption of HCOOH on the MgO(001) surface at room temperature, are shown in fig. 4. Figs. 4a and 4b were obtained after the dosage for - 15 s and 120 s, respectively (see figs. 2 and 3). In each case, a main peak was observed at 2870 cm-’ with a shoulder on the lower wavenumber side. The background signal was sufficiently low. Neither the change of the spectral line shape nor the surface damage took place during the SFG measurements. According to the literature [17], the C-H stretching band of neat HCOOH is located at 2960 cm- ‘. On the other hand, our group also measured by FT-IR the adsorption of HCOOH on MgO powder in the transmission mode [18]. In this case, it was found that HCOOH adsorbed dissociatively to produce a formate ion (HCOO-1
and surface hydroxyl species (OH) even at room temperature. The C-H stretching band of the formate species appeared at 2866 cm-‘, which was at almost the same position as was observed by SFG. Therefore, the peak observed by SFG is assigned to the C-H stretching mode of the formate species which was produced by the dissociative adsorption of HCOOH at room temperature on the MgO(001) surface as follows: HCOOH(g)
+ MgO --, HCOO( a) + OH(a). (2)
It has been known that adsorbed formate species assume several structures on metal oxide surfaces; bridging (I), bidentate (II), and unidentate (III) species as shown in fig. 5 are conceivable [19-211. Judging from the literature and the FT-IR spectrum containing other vibrational modes, the formate species on MgO powder is assigned to the bridging type. Therefore, we tentatively assign the formate species on the MgO(001) surface to the same structure, i.e. type (I). According to eq. (2), the surface hydroxyl group is also produced by adsorption of HCOOH. On
(1)
(II)
(III)
Fig. 5. Structures assumed for adsorbed formate species on MgO@Ol).
471
K Domen et al. / SFG study of HCOOH on MgO(OO1)
-~
cn
r I
I
I
I
I
0
4
8
12
18
Dose Time
I
mln
Fig. 6. Change of the SFG signal intensity of the main peak of C-H stretching mode during the dosage of HCOOH on MgO(001) at room temperature. IR beam was tuned at 2870 cm-‘. HCOOH was diluted by Ar (HCOOH/Ar = l/25).
MgO powder, the infrared absorption peak of the O-H stretching band was observed at 3746 cm-‘. The SFG measurement was also carried out on MgO(001) by scanning the infrared light between 3000 and 4000 cm-‘. Not a remarkable vibrationally resonant SFG signal but a rather weak nonresonant background signal was observed under the present experimental condition. Comparing figs. 4a and 4b, the SFG signal intensities of the main peaks at 2870 cm-’ were not so different, although the amount of adsorbed HCOOH depended on the dose time as indicated in fig. 3. Note that SFG signal intensities were plotted on the same scale for both spectra in fig. 4. To monitor the change of the SFG signal intensity during the dosage of HCOOH, the infrared beam was tuned at 2870 cm-i and the in situ measurement was carried out. In this experiment, HCOOH gas was diluted by Ar (HCOOH : Ar = 1: 25) to dose HCOOH slowly on the MgO(001) surface. The result is shown in fig. 6. After introduction of the HCOOH/Ar gas, the SFG signal intensity stayed at the background level for - 4 min. During this period, HCOOH probably adsorbed preferentially on the inner wall of the doser made of stainless steel tube and no efficient exposure of HCOOH onto the MgO surface took place. After this induction period, the intensity of the SFG signal increased nonlinearly and it leveled off after - 12 min. The intensity of the vibrationally resonant SFG signal is proportional to the square of the population density. The time-dependent
increase of the signal intensity from 4 to 8 min after introduction is approximately proportional to the square of the exposure time. This suggests that the amount of adsorbed formate species increased almost linearly with the dose time during that period. After 8 min of dose time, the increase of the signal intensity became slower and after 12 min the intensity was almost constant within experimental error. This behavior seems to indicate the saturation of the amount of adsorbed formate species. However, the dose time (12 min) of diluted HCOOH in the SFG experiment shown in fig. 6 is qualitatively comparable to the 15 s exposure time of neat HCOOH which gave the results as shown in fig. 3, judging from the TPD peak areas of both experiments. This means that even after saturation of the SFG signal intensity, the amount of formate species still kept increasing during the dosage. This discrepancy leads us to conclude that at the initial stage HCOOH adsorbs on Mg0(001) to produce the surface formate species which gives rise to the vibrationally resonant SFG signal at 2870 cm-‘, and HCOOH keeps adsorbed (or absorbed) on MgO(001) after the period but this time the adsorbed species does not contribute to the increase of the SFG signal intensity at 2870 cm-‘. At present, we do not have any experimental data which enable us to decide whether the saturation of the SFG signal intensity indicates the completion of monolayer adsorption or not. It might be noteworthy that the intensity of the shoulder peak as shown in fig. 4 appears to increase with the dose time from 15 s (a) to 120 s (b). To make more detailed discussion in this context, the higher resolution of the SFG measurements is indispensable. Different combinations of polarizations for visible and infrared beams will also provide useful information on this system. Further work including the improvement of the apparatus is in progress.
References [II X.D. Zhu, H. Suhr and Y.R. Shen, Phys. Rev. B 35 (1987) 3047. [21 J.H. Hunt, P. Guyot-Sionnest Phys. Lett. 133 (1987) 189.
and
Y.R.
Shen,
Chem.
472
X Domen et al. / SFG study of HCOOH on MgO(OO1)
[3] P. Guyot-Sionnest, J.H. Hunt and Y.R. Shen, Phys. Rev. Lett. 59 (1987) 1597. [4] P. Guyot-Sionnest, R. Superfine, J.H. Hunt and Y.R. Shen, Chem. Phys. Lett. 144 (1988) 1. [5] A.L. Harris, C.E.D. Chidsey, N.J. Levinos and D.N. Loiacono, Chem. Phys. Lett. 41 (1987) 350. [6] A.L. Harris, L. Rothberg, L.H. Dubois, N.J. Levinos and L. Dhar, Phys. Rev. L&t. 64 (1990) 2086. [7] J. Miragriotta, R.S. Polizzotti, P. Rabinowitz, S.D. Cameron and R.B. Hall, Appl. Phys. A 51 (1990) 221. [8] J. Miragriotta, R.S. Polizzotti, P. Rabinowitz, S.D. Cameron and R.B. Hall, Chem. Phys. 143 (1990) 123. [9] P. Guyot-Sionnest, P. Dumas, Y.J. Chabal and G.S. Higashi, Phys. Rev. Lett. 64 (1990) 2156. [lo] J.Y. Huang, R. Superfine and Y.R. Shen, Phys. Rev. A 42 (1990) 3660. [ll] N. Akamatsu, K. Domen, C. Hirose, T. Onishi, H. Shimizu and K. Masutani, Chem. Phys. Lett. 181 (1991) 175.
[12] C. Hirose, N. Akamatsu and K. Domen, J. Chem. Phys. 96 (1992) 997. [13] Y.R. Shen, in: The principles of Nonlinear Optics (Wiley, New York) ch. 25 (1984). [14] A. Seilmeier, K. Spanner, A. Laubereau and W. Kaiser, Opt. Commun. 24 (1978) 237. [15] X.D. Peng and M.A. Barteau, Catal. Lett. 7 (1990) 395. [16] X.D. Peng and M.A. Barteau, Langmuir 7 (1990) 1426. [17] Y. Miyazawa, J.W. Brasch and R.J. Jackobsen, J. Mol. Spectrosc. 24 (1967) 314. [18] J. Kondo, Y. Sakata, K. Maruya, K. Tamaru and T. Onishi, Appl. Surf. Sci. 28 (1987) 475. [19] K. Nakamoto, in: Infrared and Raman Spectra of Inorganic and Coordinated Compounds, 4th ed. (Wiley, New York 1986) ch. 3, p. 231. [20] N.R. Avery, Appl. Surf. Sci. 14 (1982-83) 149. [21] S. Tanaka, M. Onchi and M. Nishijima, J. Chem. Phys. 91 (1989) 2712, and the references therein.
.‘....
“.v.:.:.:.?
.. . . . :.:.:
. . . . .. . . . . . ../...._..............................
:::::;......
‘kurface science‘ ..
Surface Science 283 (1993) 468-472 North-Holland
‘:‘.‘.y.... ., .....:.>:q&; “” ‘::‘-Y: ‘.....,.,./. ..,,, ..,, ..,.,.. ..,_ ..,:.:.:::::~~~~~,~.~:~,~~~ (,,,/ ,“‘,‘,,,,,,,,., Y...’ :‘:: ‘.11’.... ....‘i.....: ,,,, ,,:,.. (,: ,.,,,,, ‘X” ““” .i.‘. .‘..../. ,_ ,/, “:‘.‘.:.::o:.:.:.:.::.:.>:.:..:.:.::::: :.::::::,:,,:.,.,., ..‘i.. ...... .._.( .
Infrared-visible sum frequency generation on an MgO(001) surface Kazunari Domen *, Naotoshi and Chiaki Hirose
Akamatsu,
Hiroyoshi
study of HCOOH
Yamamoto,
Akihide
Wada
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan Received
21 April
1992; accepted
for publication
30 July 1992
Adsorption and decomposition of HCOOH on an MgO(OO1) single crystal surface were studied by means of infrared-visible sum frequency generation (SFG) spectroscopy and temperature programmed desorption (TPD) in UHV. In the SFG experiment, tunable ultrashort infrared pulses were produced by parametric oscillation/amplification in two pieces of LiNbO, single crystals and the tunable range was 2600-4000 cm-‘. HCOOH was adsorbed on MgO(001) as formate (HCOO) species at room temperature. The TPD study revealed that HCOOH mainly decomposed into CO and H,O at _ 510 K. By SFG, the C-H stretching band was observed at 2870 cm-’ with a shoulder at - 2850 cm-‘. The in situ measurement of the change of the SFG signal intensity (2870 cm-‘) was carried out during the dosage of HCOOH.
1. Introduction Recently, infrared-visible sum frequency generation (SFG) at interfaces has been demonstrated to be a new probe of surface vibrational states [l-12]. It has some remarkable characteristics distinct from other vibrational spectroscopies such as IR, Raman, and HREEL spectroscopies. Surface SFG is a second-order nonlinear optical process caused by two intense coherent beams of light with different wavenumbers [131. In the case of infrared-visible SFG, the beam intensity of the sum frequency is resonantly enhanced when the frequency of the incident infrared beam coincides with that of a vibrational band of the surface species. Surface SFG is highly surface specific if the interface is located between two centro-symmetric media because the second-order nonlinear electric susceptibility tensor is nonzero only at the interface. It is applicable to various types of surfaces; insulators, semiconductors, and metals as far as they are optically flat.
* To whom correspondence
0039-6028/93/$06.00
should
be addressed.
0 1993 - Elsevier
Science
Publishers
In this study, formic acid (HCOOH) adsorption and decomposition on an MgO(001) single crystal surface was investigated by temperature programmed desorption (TPD) and SFG. The C-H stretching mode of the adsorbed species was monitored by SFG. MgO is an insulator, and other vibrational spectroscopic techniques are not readily applicable to the adsorbed molecules on such a single crystal surface.
2. Experimental The experimental apparatus of SFG is shown in fig. 1. The optical setup was similar to the previously reported one [ll]. Picosecond laser pulses were generated by a mode-locked Nd : YAG laser (repetition rate 10 Hz, pulse width 35 ps). Tunable IR pulses were generated in a two-stage LiNbO, (10 mm X 10 mm‘X 50 mm) optical parametric amplifier [14]. The band width was N 13 cm-‘. A visible laser beam (532 nm) was generated by second-harmonic generation. The fluencies of IR and visible beams were about 0.2 and 0.5 mJ per pulse, respectively. The inci-
B.V. All rights reserved
R Domen et al. / SFG study of HCOOH on MgO(OO1) Ft : 1064tm
469
n
cut Sllert
Fz : 53Znra cutNter LN: LiNbO3
HCOOH/M@O
MC : monocbromater PM : photomultiplier S
: Mg0(001)ssmple
Fig. 1. Setup of experimental apparatus. UHV chamber is equipped with LEED-AES and MS.
I
I
400
600
700
600 Temp./K
Fig. 2. TPD spectra of CO (m/e = 28) from Mg~~l) various dosages of HCOOH at room temperature.
dent angle of the IR beam was 45” from the surface normal and that of visible beam was 40”. The two beams hit the sample simultaneously in a common plane of incidence through a CaF, window. The generated SFG signal came out through another CaF, window and was detected by a photomultiplier after passing through a cut-off filter and a monochromator to eliminate stray light. Both incident IR and visible beams were p-polarized. An Mg~~l) single crystal (10 mm x 10 mm x 1 mm> was held in a UHV chamber (base pressure = 1.5 x lo- lo Torr). The backface of the sample piece was coated with Ni by evaporation to several pm of thickness for the sake of resistive heating. The cleaning of the sample was carried out by oxidation with 0, at 900 K until the carbon peak disappeared from the AES signal followed by annealing at 900 K to produce clear (1 X 1) LEED spots. Neat or Ar-diluted HCOOH gas was dosed through a gas doser tube (1.8 mm in diameter) of stainless steel.
with
tive heating and the peak at 510 K was assigned to desorption from the MgO surface. The TPD spectra of HCOOH (m/e = 46, parent molecule) was also monitored but the signal intensity was negligible under the experimental condition. From the mass spectrum of HCOOH in the gas phase, the signal intensity ratio of m/e = 46 (HCOOH) versus m/e = 28 (CO) was determined to be 1: 1.2 in our mass spectrometer. It was, therefore, concluded that the observed signal of CO was not due to the fragment of HCOOH molecules but desorbed from the MgO surface via decomposition of adsorbed HCOOH. After approximate separation of the two peaks (510 and 580 K), the integrated peak areas of 510 K was plotted as a function of dose time in fig. 3. The TPD spectra of CO, (m/e = 441, H,O (m/e = 181, and H, (m/e = 2) were also measured.
3. Results and discussion The TPD spectra of CO (m/e = 28) from MgO(OO1) with various dosages of HCOOH at room temperature is shown in fig. 2. The background pressure increased by N 1 x 10m9 Torr during the dosage. Although there appeared two peaks at 510 and 580 K, the latter peak was tentatively assigned to desorption from Ni or NiO on the backside of the sample for electric resis-
P-.-_ 0
-e
120
Dose
I
l
240
time I 8
Fig. 3. Dependences of the amounts of CO (m/e = 28) and CO, (m/e = 44) from HCOOH/MgO(OOl) on dose time.
K Domen et al. / SFG study of HCOOH on MgO(OO1)
470
I
I
2750
2950
II
I
2950 ( cm-’ )
2750
I
I
2950
2950 ( cm-’ )
Fig. 4. SFG spectra of the C-H stretching region of adsorbed formate on MgO(OO1).HCOOH was adsorbed at room temperature for 15 s, and (b) 120 s (see figs. 2 and 3). Both incident IR and visible beams were p-polarized.
Although the desorption of CO, was observed at around 450 and 510 K, the signal intensity was much less than that of CO as shown in fig. 3. H,O mainly desorbed at 510 K with a shoulder peak at a lower temperature. The desorption of H, was not observed clearly because of rather high background signal of m/e = 2. From these results, it is concluded that HCOOH mainly decomposes into CO and H,O on the MgO(001) surface at around 510 K according to the following scheme: HCOOH --f CO + H,O.
(1)
A similar result concerning the decomposition of HCOOH on the MgO(001) surface was previously reported by Peng and Barteau [15,16]. The SFG spectra of the C-H stretching region, which was observed after adsorption of HCOOH on the MgO(001) surface at room temperature, are shown in fig. 4. Figs. 4a and 4b were obtained after the dosage for - 15 s and 120 s, respectively (see figs. 2 and 3). In each case, a main peak was observed at 2870 cm-’ with a shoulder on the lower wavenumber side. The background signal was sufficiently low. Neither the change of the spectral line shape nor the surface damage took place during the SFG measurements. According to the literature [17], the C-H stretching band of neat HCOOH is located at 2960 cm- ‘. On the other hand, our group also measured by FT-IR the adsorption of HCOOH on MgO powder in the transmission mode [18]. In this case, it was found that HCOOH adsorbed dissociatively to produce a formate ion (HCOO-1
and surface hydroxyl species (OH) even at room temperature. The C-H stretching band of the formate species appeared at 2866 cm-‘, which was at almost the same position as was observed by SFG. Therefore, the peak observed by SFG is assigned to the C-H stretching mode of the formate species which was produced by the dissociative adsorption of HCOOH at room temperature on the MgO(001) surface as follows: HCOOH(g)
+ MgO --, HCOO( a) + OH(a). (2)
It has been known that adsorbed formate species assume several structures on metal oxide surfaces; bridging (I), bidentate (II), and unidentate (III) species as shown in fig. 5 are conceivable [19-211. Judging from the literature and the FT-IR spectrum containing other vibrational modes, the formate species on MgO powder is assigned to the bridging type. Therefore, we tentatively assign the formate species on the MgO(001) surface to the same structure, i.e. type (I). According to eq. (2), the surface hydroxyl group is also produced by adsorption of HCOOH. On
(1)
(II)
(III)
Fig. 5. Structures assumed for adsorbed formate species on MgO@Ol).
471
K Domen et al. / SFG study of HCOOH on MgO(OO1)
-~
cn
r I
I
I
I
I
0
4
8
12
18
Dose Time
I
mln
Fig. 6. Change of the SFG signal intensity of the main peak of C-H stretching mode during the dosage of HCOOH on MgO(001) at room temperature. IR beam was tuned at 2870 cm-‘. HCOOH was diluted by Ar (HCOOH/Ar = l/25).
MgO powder, the infrared absorption peak of the O-H stretching band was observed at 3746 cm-‘. The SFG measurement was also carried out on MgO(001) by scanning the infrared light between 3000 and 4000 cm-‘. Not a remarkable vibrationally resonant SFG signal but a rather weak nonresonant background signal was observed under the present experimental condition. Comparing figs. 4a and 4b, the SFG signal intensities of the main peaks at 2870 cm-’ were not so different, although the amount of adsorbed HCOOH depended on the dose time as indicated in fig. 3. Note that SFG signal intensities were plotted on the same scale for both spectra in fig. 4. To monitor the change of the SFG signal intensity during the dosage of HCOOH, the infrared beam was tuned at 2870 cm-i and the in situ measurement was carried out. In this experiment, HCOOH gas was diluted by Ar (HCOOH : Ar = 1: 25) to dose HCOOH slowly on the MgO(001) surface. The result is shown in fig. 6. After introduction of the HCOOH/Ar gas, the SFG signal intensity stayed at the background level for - 4 min. During this period, HCOOH probably adsorbed preferentially on the inner wall of the doser made of stainless steel tube and no efficient exposure of HCOOH onto the MgO surface took place. After this induction period, the intensity of the SFG signal increased nonlinearly and it leveled off after - 12 min. The intensity of the vibrationally resonant SFG signal is proportional to the square of the population density. The time-dependent
increase of the signal intensity from 4 to 8 min after introduction is approximately proportional to the square of the exposure time. This suggests that the amount of adsorbed formate species increased almost linearly with the dose time during that period. After 8 min of dose time, the increase of the signal intensity became slower and after 12 min the intensity was almost constant within experimental error. This behavior seems to indicate the saturation of the amount of adsorbed formate species. However, the dose time (12 min) of diluted HCOOH in the SFG experiment shown in fig. 6 is qualitatively comparable to the 15 s exposure time of neat HCOOH which gave the results as shown in fig. 3, judging from the TPD peak areas of both experiments. This means that even after saturation of the SFG signal intensity, the amount of formate species still kept increasing during the dosage. This discrepancy leads us to conclude that at the initial stage HCOOH adsorbs on Mg0(001) to produce the surface formate species which gives rise to the vibrationally resonant SFG signal at 2870 cm-‘, and HCOOH keeps adsorbed (or absorbed) on MgO(001) after the period but this time the adsorbed species does not contribute to the increase of the SFG signal intensity at 2870 cm-‘. At present, we do not have any experimental data which enable us to decide whether the saturation of the SFG signal intensity indicates the completion of monolayer adsorption or not. It might be noteworthy that the intensity of the shoulder peak as shown in fig. 4 appears to increase with the dose time from 15 s (a) to 120 s (b). To make more detailed discussion in this context, the higher resolution of the SFG measurements is indispensable. Different combinations of polarizations for visible and infrared beams will also provide useful information on this system. Further work including the improvement of the apparatus is in progress.
References [II X.D. Zhu, H. Suhr and Y.R. Shen, Phys. Rev. B 35 (1987) 3047. [21 J.H. Hunt, P. Guyot-Sionnest Phys. Lett. 133 (1987) 189.
and
Y.R.
Shen,
Chem.
472
X Domen et al. / SFG study of HCOOH on MgO(OO1)
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