J. Phys. Chem. 1995,99, 13659-13663
SF6
+ Ba Beam-Surface
13659
Ionization Induced by Infrared Radiation?
J. Castaiio, V. Zapata, G. Makarov,s and A. Gonzalez Ureiia" Unidad de Lheres y Haces Moleculares, Instituto Pluridisciplinar, Universidad Complutense de Madrid, 28040 Madrid, Spain Received: March 20, 1995@
A C02 laser-induced SF6 4-Ba beam-surface ionization process has been studied in which vibrational excitation of SF6 molecules was carried out at the polished surface of polycrystalline Ba electrically heated to 675 K. Both electron emission and negative molecular ion signal were detected. The dependence of the molecular ion signal on laser fluence and frequency (on SF6 molecular absorption) as well as on the SF6 gas pressure in the nozzle was studied. The results suggest a clear vibrational selectivity, i.e., vibrational enhancement, of the SF6 Ba beam-surface ionization process. Several possible mechanisms for the negative molecular ion formation in the IR laser-induced beam-surface ionization process are discussed.
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I. Introduction Recently, laser-induced processes at gas-surface interfaces have attracted considerable attenti~nl-~ due to both the basic scientific interest and the potential technological applications of these processes. Among these, the photoinduced ionization or charge transfer (CT) process constitutes a good example of interest to scientists from many field^^.^ because the CT often plays an essential role in subsequent chemical reactions at the gas-surface interfa~e.~.' At an adsorbate-solid surface, the photoinduced CT occurs either between the substrate and adsorbate or among adsorbate molecules.* The former process may be facilitated by substrate photoelectron or subvacuum hot carriers, which arise due to UV ex~itation.~ In the majority of photoinduced CT experiments at the gassurface interface carried out up to now, UV or visible lasers were used as the photon s0urces.4-~ These lasers are characterized by a large photon energy (several electronvolts), that is, enough for electronic excitation of adsorbates, and a rather short pulse duration and high intensity, which helps to compete with a fast energy relaxation to substrate. On the other hand, the interest in using infrared laser radiation for studying CT processes at surfaces relies on the possibility of exciting vibrational modes of adsorbate molecules. It is well-known that vibrational excitation is very important in promoting endoergic gas-phase chemical reactionslo.' I as well as in controlling chemical processes occurring in adsorbed layers.'* The use of IR lasers for initiating gas-surface reactions when the gas consists of polyatomic molecules allows us to put, at the gas-surface interface, a large amount of energy due to IR multiphoton absorption.13 In addition, the use of an IR laser for the gas-surface interaction studies leads to a high efficiency of an IR multiphoton excitation process. As is known from gas-phase experiments, at rather moderate for IR C02 laser energy fluences of about 1 Jlcm2, it is possible to excite to high vibrationally excited states (up to energy levels E, 2 1 eV) practically all irradiated molecule^.'^ Therefore, in spite of a rather small value of C02 laser quantum (0.1-0.12 eV) and longer pulse duration ( 2 100 ns), one can induce effective CT
'
This work received financial support from the DGICYT of Spain (Grant PBg1/357) and The Fundacidn Ramdn Areces. On sabbatical leave from Institute of Spectroscopy, Russian Academy of Sciences, 142092, Troitzk-Moscow, Russia. * To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, August 15, 1995. 7
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0022-365419512099-13659$09.00/0
processes at the gas-surface interface. A TEA C02 laser was used in an earlier experiment described in ref 15 for the study of laser-induced reaction of SF6 with silicon surfaces. In these experiments the reaction of vibrationally excited SF6 molecules with the Si surface to produce the etching of silicon was studied. Recently, in the Instituto Pluridisciplinar, we have initiated experiments to study the laser-induced ionization processes at the gas-surface interface using molecular beams and IR lasers for the excitation of molecules. The surface used was polycrystalline Ba, which has a rather low work function (-2.5 eVI6),and the absorbate was the SFg molecules (electron affinity 1.05 eVI6), whose IR multiphoton absorption in gas phase" and in molecular has been studied in detail. Note that although Ba has a higher work function than, for example, Cs (1.25 eV), it has a larger density of states at the Fermi level, which increases the charge transfer rate. In ref 20 it was demonstrated that Ba is a very good choice for the highefficiency production of H- ions. Our preliminary results in the study of the ionization process at the Ba SF6 gas-surface interaction were published in refs 21 and 22. In those studies a C02 laser was used for the initiation of the reaction at the gas-surface interface, and the lock-in amplifier technique was used for the detection of the ion signal. In experiments described in refs 21 and 22, we were unable to clearly distinguish between a negative molecular ion signal and electron emission from the laser-heated surface. Nevertheless, the frequency dependence of the ion yield indicated that negative molecular ions were formed. In contrast to the earlier experiments in the present study, a pulsed TEA CO2 laser was used for the excitation of SF6 molecules coming to the Ba surface. It was installed inside the differentially pumped ultrahigh-vacuum chamber, and an independent electrical heating of it was accomplished. The use of a pulsed TEA C02 laser allowed us to apply the time-offlight (TOF) technique for the detection of ion signal and, as a result, to distinguish between molecular negative ions and electron emission. The present paper reports on the main features of the experimental technique together with the SF6 Ba ionization data induced by a TEA C02 laser pulse.
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11. Experimental Setup
The experimental setup is schematically shown in Figure 1. It consists of three main parts: (1) the differentially pumped
0 1995 American Chemical Society
13660 J. Phys. Chem., Vol. 99, No. 37, 1995 r
Castafio et al. SFi Negative Ion Signal
PDD 4
-5.0~10~
Figure 1. Schematic view of the experimental setup.
high-vacuum (HV) and ultrahigh-vacuum (UHV)chambers with a SFg molecular beam and a Ba surface installed inside the UHV chamber; (2) a line-to-line tunable TEA C02 lasers for the excitation of molecules; (3) the registration system (including digital oscilloscope and the computer) for measuring the ion signal, parameters of the laser pulses, and data processing. The high-vacuum chamber (HVC) was pumped by the oil diffusion pump (Leybold 2.200, 2000 Us) to give pressures down to 1 x Torr. The ultrahigh-vacuum chamber (UHVC) was installed inside the HVC and was pumped by a turbomolecular pump (Balzers TPU 240,200 Us)to pressures around Torr. The Ba crystal was installed in the center of the UHV chamber in the holder made from metallic and ceramic pieces. The surface of the Ba crystal was mechanically polished. It was possible to vary the Ba surface angle with respect to the laser and molecular beams coming to the crystal. The Ba crystal could be heated resistively by a nicrom wire, allowing the Ba surface to be heated to 675 K. In between the outer side of the Ba and a ceramic holder, a thermocouple (NiCrNiAl) was placed for measuring the Ba surface temperature. This heat treatment was employed to eliminate surface impurities because they may alter the electronic properties of the surface and consequently the negative ion yield. Although in our experiments the surface was not characterized, it was ablated by the laser under ultrahigh-vacuum conditions (p -= mbar) for extensive periods before experiments were undertaken. The SFg molecular beam was formed by nozzle with a diameter of 0.1 mm (made from Ruby) and a skimmer (diameter 0.5 mm) mounted at the wall of the UHV chamber. A little downstream of the skimmer, a high-vacuum valve (VAT, SVV 040 HM)was installed in the UHV chamber. When this valve was fully opened, the SFg molecular beam reached the Ba surface: otherwise in the case of not fully position, leak gas was allowed into the UHV chamber so that the desired SFg gas pressure was maintained inside the chamber. The wire ion detector (Ni wire) was placed in front of the Ba surface at a distance of 3.8 cm. A continuously variable dc voltage up to f 5 V was applied to the wire detector by the current amplifier (Keythley Model 428). A rather large ion path from the Ba surface to the detector allowed us to resolve the negative molecular ion signal from the electron signal. A TEA C02 laser (Type XL-370 TS, Manufacturer Coherent Hull) line tunable in the range 9-11 ,um was used for the excitation of molecules at the Ba surface. The laser beam was directed to the Ba surface perpendicular to the SFg beam direction via a ZnSe pindow in the HVC and a KBr window in the UHVC. A nearly Gaussian in cross section laser beam was selected by the orifice installed inside the laser resonator.
4
0.0
5.0~10-~
1.0~10~
1.5~10~
2.0~10~
Time (sec)
Figure 2. A typical time-of-flight (TOF) signal obtained in a TEA C02,laser-induced SF6 4- Ba beam surface ionization process. The peaks associated with the electron emission and negative molecular ions are indicated. The negative and positive peaks at the beginning are electrical noise due to the C02 laser discharge.
TABLE 1: Experimental Conditions laser system
vacuum
electronics
wavelength/pm energy p”r pulse/mJ temporal profile per pulse fwhm peak/ns tail/ps background presdmbar SF6 predmbar b a ~ u mtemp range/K barium-detector distancekm bias voltageN amplification factorN A-’
9-1 1 250 100 3 6 x 0-9 2 x 0-6 300- 675 3.8
f5 107
The energy in the pulse was about 250 mJ with a total pulse length of about 3 ,us (100 ns fwhm peak 3 ,us tail). The energy of the laser pulse was measured by a pyroelectric detector (time constant -0.5 ms). To monitor the temporal behavior of the laser pulses, a “photon drag” detector (noncooled Ge: Au crystal) with a time resolution of ca. 1 ns was used. The calibration of C02 laser lines was carried out by using a M I 3 absorption cell. It is well-known that ammonia has very strong absorption lines23in the region of C02 laser generation belonging to the v2 (umbrella) vibrational mode. Some of these lines coincide well with the C02 laser transitions. The signals from the ion detector, “photon drag”, and pyroelectric detectors were directed to the digital oscilloscope (Tektronix Mod TDS 540) and then to the computer (486150) for data storage and further analysis. Table 1 lists the most relevant experimental conditions of the present work.
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In. Results and Discussion In the present work the dependence of the molecular ion signal on the energy and frequency of C02 laser pulses as well as on the SFg pressure in the nozzle were studied. The typical shape of the negative ion signal is shown in Figure 2. Notice the electron signal that appears just after the laser pulse and extends about 20 ,us and the negative molecular ion signal, which peaks around 75 ,us. When the SFg pressure in the nozzle was increased from about 0.1 to 1 atm, the molecular ion signal followed a linear dependence. This means that (a) the observed signal is associated with the SFg beam and (b) no saturation of Ba surface by the SFg molecules took place. In contrast, the electron signal decreased with increasing SF6 pressure in the nozzle. Similar behavior of the photoelectron emission signal versus the surface coverage was observed earlier in refs 6 and 7 for the case of chlorinated methanes on the Ag (1 11) surface. As suggested in ref 6, this is probably due to electron scattering
J. Phys. Chem., Vol. 99, No. 37, 1995 13661
SF6 f Ba Beam-Surface Ionization TABLE 2: Energetic9 of the Different Electron Attachment (and Dissociation) Processes for the SFs System Drocess reaction AHoJeV 1 sF6- SF5 + F 4.05 2 SF5 e- SF5-3.7 3 F e- F-3.4 4 SF6+ e- SF5- + F 0.4 5 SF6 + e- SF5 + F0.65 sF6 + e- SF6-1.0 6
+
a
+
t
1
1
P(28)
1
1
P(24)
1
1
P(20)
1
1
P(16)
1
1
P(12)
1
----
Values taken from refs 16 and 24.
1
-'' -2
-5.0
-
-5,5
-
A
.-w
o
o
Laser Line: 10P20 n=3.8 Laser Line: 10R20 n=4.3
I
-"OI -7.5 -8.0 I 0.;
I
1
945
950
I
955
Figure 4. Linear absorption spectrum of 32SF6. Figures on top mark COz P-branch transitions (adapted from ref 25).
e s -6.5 -
E -b.O
,
I
940
Frequency (cm-')
M
g
1
935
,
A
+
,
0,'s
0.5
These two lines are very different because of their distinct IR linear and multiphoton excitation efficiency of SF6 mole c u l e ~ . ' ~The - ' ~ 10P-branch C02 laser lines coincide well with a very strong v3 absorption band of SF6, while in the region of 10R-branch lines of COZonly a weak combination band a 9 V6 of SF6 appears. The linear absorption cross section for the v2 v6 band of sF6 is about 2 orders of magnitude smaller than for v3. In ref 19 it was shown that the IR multiphoton absorption of SFs in a beam is about 1 order of magnitude smaller when the vz v6 combination band is excited by TEA CO2 laser pulses at energy fluences from 0.1 to 2 J/cm*, compared with when v3 mode is pumped. From Figure 3 one can see that the molecular ion signal for the lOP(20) C02 laser line is about 10 times larger than for lOR(20) line at the small energy fluences studied and, on the other hand, about 8 times larger for high-energy fluences. This means that the gas-surface molecular ionization yield is very sensitive to the vibrational excitation of molecules. In addition, one can also see that the molecular ion signal depends very strongly on the laser pulse energy fluence. The slopes for the dependence studied are equal to n = 3.8 for the lOP(20) laser line and n = 4.3 for the lOR(20) line. This probably indicates the nonlinear character of the mechanism of the molecular ion formation; that is, absorption of more than one IR photon from the C02 laser pulse is responsible for the formation of negative SF6 molecular ions. It is interesting to compare the molecular ion signal dependence on the laser pulse energy fluence with that of the electron emission. For the latter we have observed a clear thermal behavior well described by the Richardson-Dushman lawI6 given by J = A F exp(-w/kr) where J is the electron emission, A is a constant, w is the work function, and T is the electron temperature which was proportional to the laser fluence. A full account of this electron emission behavior will be described in a forthcoming paper; however, the interesting point to be emphasized is that the electron yield is entirely thermal in character, with no multiphoton dependence. To investigate further the possible multiphoton character of the absorption-mediated SF6 ionization, we selected two co2 laser lines of the P branch. The absorption cross section of the SF6 gas as a function of the wavelength is displayed in Figure 4. We canied out the gas-surface ionization experiment this time using both P16 and P22 lines. The molecular ion signal was measured as a function of the laser power for these two lines. Figure 5 shows the results in a log-log plot. The strong power dependence can be seen for both laser lines. The main difference between Figures 3 and 5 is that whereas in the latter the signal corresponds to the peak of the time-of-flight spectrum,
1.0
1.1
I.'2
1.3
1.4
Log (Laser Pulse Energy Fluence)
Figure 3. Dependences of the negative molecular ion signal on exciting laser pulse energy fluence for the lOP(20)and lOR(20) COZlaser lines. The SF6 pressure in the nozzle is 0.5 atm.
in the adsorbed layers that return the electrons back to the metal, decreasing the total electron emission. When SF6 gas in the nozzle was replaced by NZor C2H4, the molecular negative ion signal was not observed, although photoelectron emissions were observed in both these cases. NZ can be considered as a neutral nonabsorber of COz laser radiation gas while C Zhas~an absorption band in the 10.6 p m region. Note that C2H4 has a smaller electron affinity than SF6, ca. 0.49 eV. Most likely, the observed negative molecular ion signal can be associated with the SF6- ion, although the time resolution of our detection system was not accurate enough to resolve molecular fragment ions (like SFs-). The appearance of smaller than SF5- fragments ions in these experiments seems unlikely, because the C02 laser pulse energy fluence ( 5 1 J/cm2) was not high enough for the dissociation of SF6 molecules in a beamI8 and probably at the Ba surface. Nevertheless the SF5- formation cannot be ruled out since it can appear as a product of a subsequent dissociation of initially vibrationally excited formed SF6- ions by the remainder of the same laser pulse. The dissociation energy of the SF6- ion is about 1 eV.24 Table 2 lists the energetics of these attachment processes. In principle the observed time-of-flight (TOF) negative molecular ion signal may consist of both the SF6- and SF5ions which were not resolved in the present experiments because of the small difference in TOF (ca. 7%). Additional evidence to support the SFs- and SFs- ions as the predominant components in the observed low-resolution TOF data are provided by the analysis of the shift in the TOF peak as the accelerating voltage is increased. The time peak evolution is consistent with a heavy ion of mle = 146 f 25. Nevertheless, in view of the additional energy required for the SF5- ions to be formed, it is likely that SF6- is the major component. The ion signal intensity versus the COz laser power and frequency are displayed in Figure 3, which shows the dependences of the molecular negative ion signal on the laser pulse energy fluence for the lOP(20) and lOR(20) C02 laser lines.
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Castafio et al.
13662 J. Phys. Chem., Vol. 99, No. 37, 1995
1.9 1.8
P2 6
1.7
1.6
1.5
s" 1.4 1.3
I .2 1.1
Log (Laser Pulse Energy Fluencc)
Figure 5. SF6- signal as a function of the laser energy fluence for the lOP16 and 10P22 laser C02 lines. Notice the enhancement factor between both signals.
in Figure 3 it represents the total (integrated) area of the timeof-flight distribution. This difference, due to experimental convenience, has little, if any, effect in the conclusion obtained on the fluence and frequency dependence of the ion yield. The observation of a higher signal for the P16 than for P22 at the same laser power rules out a significant contribution from gasphase multiphoton process in the ionization process. However, the overall enhancement factor of ca. 4 when the signal of the P16 line is compared with P22 confirms the vibrational selectivity of the induced gas-surface ionization. An important point in the understanding the observed signal is to distinguish whether the molecules are being excited at the surface or in the gas phase. To clarify this point, several runs were carried out under the same laser conditions, but in a geometry where it propagated parallel to the surface. No ion signal was detected under such experimental conditions. Furthermore, it should be noted that the ion signal can also be detected even for low laser intensities, 5 1 J/cm2 (Le., 1 2 0 MW/ cm2). As mentioned earlier at this fluence level, the SF6 dissociation yield has been determined to be very possibly less than 2 x Therefore, our results seem to favor the reaction of vibrationally excited, e.g. SFs+. Since only ion signal is observed when the laser is directed to the surface, the major pathways that can be considered are the following SF6+
-
SF6-(g)
+ e- (photo- or thermoemitted)
-
SF6+(ad) -k e- (Ba hot surface) SF,'(g)
+ Ba (hot surface)
SF,-(g)
SF6-(g)
(a)
(b) (c)
We think that the predominant mechanism involves the electron attachment of the vibrationally adsorbed molecule, Le. mechanism b rather than mechanism a or c, based on the following arguments. (1) The ion signal is also observed even when the surface has been exposed to SF6 beam but with the SF6 gasheam tumed off, that is, when SF6 adsorbed at the Ba surface (p < lo-* mbar) is present. (2) The ion signal is observed even under gas pressures of SF6 p 9 1 x mbar (see Table 1). Under these conditions it takes 10 ps for SF6(s) to travel ca. 1 mm and collide with some part of the Ba surface. The comparison of this time scale with the actual tof distribution (over 38 mm of flight path) rules out the possibility that the SF6+(g) f Ba(surface) step is the limiting step and it strongly indicates that the laser radiation effect is most likely associated
Figure 6. Schematic view of two possible mechanisms for laserinduced (charge transfer) beam-surface ionization: (left) substrate mediated linear or multiphoton, IR absorption; (right) adsorbate mediated, linear or multiphoton, IR absorption. W, barium work function; EA, SFs electron affinity; E F , barium Fermi level. See text for details.
>u a. z
w
LI
0
Figure 7. Qualitative representation of the potential energy curves for the SFs-SFs- system (adapted from ref 24).
with the absorbate andor adsorbate-substrate excitation. (3) Finally, by comparing the energy fluence dependence of the SF6- signal with that of the electron emission, the lack of correlation between both processes is clearly seen as was mentioned earlier, which rules out mechanism a, so leaving mechanism b as the most likely pathway to explain the chargetransfer process. Summarizing, in principle, we can consider the following two possibilities schematically displayed in Figure 6: 1. Substrate-Mediated Absorption. Essentially, it consists of the excitation (heating) by CO2 laser pulses of the electrons in the substrate. Laser excitation pumps the electron out of the Fermi level, and subsequently they tunnel to the SF6- potential. This mechanism would show a thermal character without a significant frequency selectivity. 2. Adsorbate-Mediated Absorption. Here the main process is the vibrational excitation of SF.5 molecules (and near) the Ba surface followed by the energy transfer to surface, heating of electrons, and the realization of the first mechanism. In contrast to the first mechanism, the present one would clearly show a vibrational, laser frequency, dependence via resonant SF6 IR absorption? In view of these two mechanisms, it is interesting to consider the neutral and ionic potential of the S-F distance.24 These are shown in Figure 7, where the crossing between the covalent and ionic potential as the S-F distance increases can be seen. As a result of this qualitative representation of the potential
SF6
+ Ba Beam-Surface
Ionization
Energy ( SF; - SFa ) (eV)
I
J. Phys. Chem., Vol. 99, No. 37, 1995 13663 on the sF6 gas pressure in the nozzle, while the electron emission signal decreased with increasing SF6 pressure and showed a thermal behavior. The rather steep dependence of the molecular ion signals on the exciting laser pulse energy fluence @ was observed S cPn (where n > 1). This indicates a nonlinear probably multiphoton character of the mechanism of the SFs+(ad) Ba (hot surface) ionization. The negative molecular ion signal was also shown to be very sensitive to the SF6 molecular absorption (to the exciting C02 laser frequency). Thus, enhancement factors r = 10 or 4 were found for lOP(20) versus lOR(20) and 1OP(16) versus lOP(22) lines, respectively. This supports the vibrational selectivity (vibrational enhancement) of the SF6 Ba gas-surface IR laser-photoinduced ionization process. Work is now in progress in which the surface temperature is changed independently of the vibrational excitation of the molecular beam to further clarify the role of the specific vibrational excitation inherent in the SF6 molecule.
-
+
+
Figure 8. Vertical electron affinity of SF6 as a function of the S-F coordinate. To build this one-electron picture of the electron attachment process, the molecular affinity (energy level) was estimated as the difference between the SF6- and SF6 curves in Figure 7. Solid line represents the curve for u = 0; dashed line same representation for u = 1 (see text for details).
energy surface, one can easily recognize the possible role played by the resonant laser excitation. Indeed, as displayed in Figure 6, there is no need for the electrons to surmount the vacuum level in order to produce SF6-. The higher the vibrational excitation, the greater the SF6 electron affinity and so the more intense the ion yield originated by the (resonant) infrared radiation. This is clearly shown in Figure 8 where we have plotted the vertical electron affinity of SF6 as a function of the S-F coordinate. In this one-electron picture the vacuum level of Ba coincides with SF6(a) and its electron affinity, estimated as the difference between the SF6- and SF6 curves in Figure 7 , is drawn as a function of the S-F distance. Notice how SFs-, formed when a hot electron in Ba resonantly tunnels to the molecular affinity level, is most probable at large S-F bond distances. It is interesting to point out that for example at 1.68 A the vertical electron affinity becomes equal to the work function. With this resonant tunneling mechanism the energy transfer from the molecule to the surface (mechanism 2 above) is not strictly necessary. Of course, vibrational excitation of the SF6 would indeed increase the electron affinity and so would enhance the electron attachment yield. In fact, if one compares the spatial extension of a v = 0 Gaussian eigenfunction with that from a vibrationally excited state of SF6, it becomes apparent that the latter state will have a larger probability for attaching an electron because it extends further along the S-F coordinate. It should also be noted that the vertical electron affinity will also increase as the SF6 molecule is brought closer to the surface due to the stabilization of the anion by its image charge. In conclusion, we believe that this resonant tunneling mechanism seems to be the most probable to explain the negative ion formation.
IV. Concluding Remarks
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A TEA COz laser-induced SF6 Ba beam surface ionization process was studied in which vibrational excitation of SF6 molecules was carried out at (and near) the polished surface of the electrically heated polycrystalline Ba. The IR laser pulses induce electron emission and negative molecular ions that were detected and resolved by low-resolution time-of-flight (TOF) measurements. It was reported that the molecular ion signal is associated with the SF6 molecular beam and follows a linear dependence
Acknowledgment. G.M. fully acknowledges a sabbatical fellowship from the Complutense University. The authors acknowledge the constructive criticism of the referee. References and Notes (1) Polanyi, J. C.; Rieley, H. In Dynamics of Gas-Surface Interactions, Rettner, C . T., Ashford, M. N. R., Eds.; Royal Society of Chemistry: London, 1990. (2) Zhou, X. L.; Zhu, S. Y.; White, J. M. Surf. Sci. Rep. 1991, 13, 73. (3) Ho, W. Comments Condens. Mater. Phps. 1988, 13, 293. (4) Chuang, T. J. Surf. Sci. 1986, 178, 763. (5) Ho, W. In Desorption Induced by Electronic Transitions; DIET IV, Betz, G., Varga, P., Eds.; Springer: Berlin, 1990; p 48. (6) Dixon-Warren. St. J.: Jensen. E. T.: Polanvi. J. C. J . Chem. Phvs. 1993, 98,5938. (7) Dixon-Wanen. St. J.: Heidi, D. V.: Jensen, E. T.; Polanvi. J. C. J . Chem. Phys. 1993, 98, 5954. ( 8 ) Sun, Z. J.; Schwaner, A. L.; White, J. M. Chem. Phps. Lett. 1994, 219, 118. (9) Gadzuk, J. W. In Laser Spectroscopy and Photochemistry on Metal Surfaces; Dai, H. L., Ho, W., Eds., to be published. (10) Levine, R. D.: Bemstein, R. B. Molecular Reaction Dynamics and Chemical Reactiuiv; Oxford University Press: Oxford, 1987. (11) Gonzilez Ureiia, A. Adu. Chem. Phys. 1987, 66, 213. (12) See, for example: the Book of Abstracts of the European Conference on Gas-Surface Dynamics; Graz, Austria, Aug 1992. (13) Bagratashvili, V. N.; Letokhov, V. S.; Makarov, A. A.; Ryabov, E. A. Multiple Photon Infrared Laser Photophysics and Photochemistr?;; Harwood Academic: Chur, 1985. (14) Bagratashvili, V. N.; Ionov, S. I.; Makarov, G. N. In Laser Spectroscopy of Highly Vibrationally Excited Molecules; Letokhov, V. S., Ed.; Adam Higler: New York, 1989. (15) Chuang, T. J. J. Chem. Phys. 1980, 72, 6303; 1981, 74, 1453. (16) Handbook of Chemistry and Physics, 74th Linde; David, R., Ed.; CRC Press: Cleveland, OH, 1993-1994. See also: Rudden, N. M.; Wilson, J. Elements of Solid State Physics; Wiley: New York, 1993. (17) Ambartzumian, R. V.; Makarov, G. N.; Puretzky, A. A. Opt. Commun. 1980, 34, 81. (18) Apatin, V. M.; Makarov, G. N. Sou. Phys. JETP 1983, 57, 8. (19) Aoatin. V. M.: Bezudova. T. V.: Makarov. G. N. Oot. Commun. 1982, 42, i55. (20) Van Os, C. F. A.; Heeren, R. M. A,; Van Amersfort, P. W. A .p .p l . Phys. Lett. 1987, 51, 1495. (21) Castaiio Aspas, J.; Heeg, B.; Bescbs, B.; Zapata, V.; Gonzilez Ureiia, A. Faraday Discuss. 1993, 96, 227. See also: CastaAo, J.; Zapata, V.; Makarov, G.; Gonzilez Ureiia, A. J . Chin. Chim. Phys., in press. (221 Castaiio Aspas, J.; Gonzilez Ureiia, A. Laser Chem. 1994, 14, 201. (23) Urban, S.; Papousek. D.; Kauppimen, J.; Yamada. K.; Winnewisser, G.J . Mol. Spectrosc. 1983, 101, 1. (24) Streit. G. E. J . Chem. Phys. 1982, 72, 826 and refs 6-10 cited therein. (25) Nowak, A. V.; Lyman, J. L. J. Quant. Spectrosc. Radiat. Transfer 1975, 15, 945. (26) See for example: Ambansumyan, R. V.; Letokhov, V. S. In Chemical and Biochemical Application of Lasers; Moore, C. B., Ed.; Academic; New York. 1977; Vol. 3, p 167. (27) Schultz, D. A,; Sudbo, Aa. S.: Krajnovich, D. J.; Kwok, H. S.; Shen, Y. R.; Lee, Y . T. Annu. Rev. Phys. Chem. 1979, 30, 379. I
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