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Rotational state populations of no molecules scattered from clean and adsorbate-covered Pt(111) surfaces
273
Surface Science 131 (1983) 273-289 North-Holland Publishing Company
ROTATIONAL SCATTERED SURFACES J. SEGNER
STATE POPULATIONS OF NO MOLECULES FROM CLEAN AND ADSORBATE-COVERED Pt( 111)
*, H. ROBOTA,
Institut fiir Physikalische Rep. of Germany
W. VIELHABER
and G. ERTL
Chemie, Universitiit Miinchen, Sophienstrasse
Il, D - 8GOOMiinchen 2, Fed.
and F. FRENKEL
**, J. HAGER,
W. KRIEGER
Max - Planck - Institut fiir Quantenoptik, Received
3 February
1983; accepted
and H. WALTHER
D - 8046 Garching, Fed. Rep. of Germany
for publication
26 May 1983
A supersonic beam of NO molecules was scattered from a Pt(ll1) surface either clean or covered with NO, C, and 0. Rotational state populations and angular distributions of the scattered molecules were determined at two incident energies and at different surface temperatures T,. The rotational distributions obtained are Boltzmann distributions with rotational temperatures Tr,,. Interaction with the clean and NO-covered Pt surface shows diffuse angular distributions and the independence of T,,, on the initial kinetic energy. Full rotational accommodation was found for scattering from the NO-covered (T, < 300 K), only incomplete accommodation at the clean Pt surface (T, > 450 K). Interaction with the C-covered surface results in broad nearly specular scattering lobes and at higher surface temperatures in a constant value of Tr,, < T, which depends on the initial kinetic energy. Scattering at the oxygen-covered surface presents an intermediate case: diffusely and specularly scattered fractions are observed. The experimental results are discussed in terms of underlying microscopic scattering processes.
1. Introduction The possibility to probe the internal state distribution of molecules with high sensitivity by the use of laser light opened a new area for the investigation of dynamic processes occurring at gas/solid interfaces [ 1- 131. So far, these studies have mainly concentrated on the determination of rotational state distributions and often yielded quite surprising results: In most cases the state populations of molecules leaving the surface can formally be described by a * Present address: Department of Chemistry, University of Toronto, Toronto, Ontario, ** Present address: Siemens AG, Hoffmannstrasse 51, D-8000 Mtinchen 70, Fed. Germany.
0039-6028/83/0000-0000/$03.00
0 1983 North-Holland
Canada. Rep. of
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J. Segner et al. / Rotational state populations
of NO molecules
Boltzmann distribution with a rotational temperature T,,,. However, T,,,is frequently smaller than the surface temperature T,, even if the molecule had a very long residence time (2 10e4 s) on the surface. In the case of direct inelastic scattering (NO/Ag(lll)) at high kinetic energies of the incident molecules the distribution no longer follows the Boltzmann law and indicates a direct transformation of translational into rotational energy (“rotational rainbow”) [3,9]. The test molecule NO exhibits non-zero orbital and spin angular momenta and therefore, due to spin-orbit coupling, two states (II,,2 and II,,,) exist, separated by 123 cm-’ in energy: This offers the additional possibility to probe the redistribution of internal energy in different electronic states. The present work describes the results of experiments with NO scattered at a Pt( 111) surface which was either clean or covered with various adlayers (NO, 0, C) in order to modify the interaction potential. The type of scattering varies between almost pure direct inelastic and pure trapping/desorption. The results with the clean Pt( 111) surface are in qualitative agreement with the findings of Asscher et al. [7,12] who investigated this system by a different method. The carbon-covered surface behaves almost identically to the pyrographite sample investigated previously in this laboratory [ 111.
2. Experimental The experiments were performed with a UHV molecular beam system [14] which was equipped with facilities for laser-induced fluorescence (LIF) [l]. A block diagram of the optical and electronic components is reproduced in fig. 1. A tunable dye laser is pumped by a XeCl excimer laser at a repetition rate of 5-10 s-‘. Frequency doubling provides 5 ns pulses in the desired wavelength range ( - 226 nm) with a linewidth of 0.15 cm- ’ and with a pulse energy up to 40 pJ. The laser beam enters and exits the vacuum system through Brewsterangle quartz windows. Inside the chamber it is deflected by aluminum coated mirrors which can be moved so that the laser beam crosses and analyses either the incoming molecular beam or the molecules leaving the surface. The analysed volume of - 1 mm3 was at a distance of - 12 mm from the sample surface producing an angular resolution of about 20” at 5 mm beam diameter. The fluorescence light is collected by a quartz lens system and focused through a quartz window onto the cathode of a photomultiplier tube. The signal is then amplified and recorded by means of a boxcar integrator. Simultaneously the spectrum of a reference cell ( 10e3 mbar NO at T = 300 K) is recorded, which allows normalization and evaluation of the recorded intensities. Densities of the scattered molecules in the range of lo6 molecules cmm3 per state could still be detected. The sample was mounted on the rotatable axis of a manipulator. The
J. Segner et al. / Rotational state populations Excimer laser
of NO molecules
275
Dye Laser Trigger y Reference B
&it
Y
PM I PA
Molecular
beam
Scattering chamber
Fig. 1. Schematic drawing of the experimental setup (PA = preamplifier, PM = photomultiplier, QMS = quadrupole mass spectrometer).
PD = photodiode,
scattered molecules could be analysed by a quadrupole mass spectrometer which could be rotated around the manipulator axis within the scattering plane. In this way both the (polar) angle of incidence and of detection could be independently varied. With a beam diameter of 2 mm and the detector at 60 mm distance from the sample the angular resolution was about 2”. A supersonic beam of NO molecules was generated by a nozzle-skimmer device within a three-stage differentially pumped source. The translational energy of the molecules Ekin could be enhanced from 80 to 210 meV by seeding with He. Adiabatic expansion of the molecules behind the nozzle causes their rotational energy to relax. It was found by LIF that the population distribution of the rotational states of the primary particles can be described by the rotational temperatures T,,, = 39 K at Ekin = 80 meV and T,,, = 20 K at Ekin = 210 meV [ 151. The formation of (NO), clusters in the beam depends on the product p,d ( p,, = source pressure = 800 mbar, d = nozzle diameter = 7 x lop3 cm) and was negligible under the applied conditions [16]. The flux of NO molecules striking the surface was about 4 x lOI cmm2 SC’ both for Eki, = 80 meV and Ekin = 210 meV. Preparation and characterization (by LEED, AES and He reflection) of the clean Pt( 111) surface has been described in ref. [ 171. The adsorption/desorption properties of this surface, being either clean or covered by oxygen had been studied previously [ 181.
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J. Segner et al. / Rotational state populations of NO molecules
3. Results 3. I. Trapping/desorption:
clean and NO-covered
Pt(l I I) surface
If a clean Pt( 111) surface (at q s 800 K) is exposed to NO, nonactivated chemisorption with a sticking coefficient of about 0.9 takes place [ 18-201. The remaining fraction undergoes direct inelastic scattering. At higher surface temperatures or higher translational energies the directly scattered portion increases [21], for example by about a factor of two by increasing Ekin from 80 to 210 meV [22]. If a continuous flux F of NO molecules strikes the surface an equilibrium coverage will be built up, depending on F and the surface residence time T( T,). Fig. 2 shows this equilibrium coverage as a function of temperature for Relative F= 4 X lOI molecules cm-* s-’ as applied in the LIF experiments. coverages were determined either from TDS data (squares) or from the material balance in transient beam experiments (circles). Calibration of the absolute coverage scale was achieved by comparison with data from a surface saturated with CO at 300 K, where the coverage 8 = 0.5 based on LEED data is generally accepted [23]. The NO molecular beam contains a small fraction (I 0.5%) of NO, which decomposes with almost unit probability at a clean Pt(ll1) surface into NO,, + O,, [24]. The experiments above 400 K were therefore performed with a background pressure of H, (8 x 10e9 mbar) which removes the oxygen atoms by rapid reaction. The steady-state H coverage introduced in this way is negligible [25]. According to fig. 2 under steady-state conditions at q s 300 K scattering of
0
El
I
200
300 LOO Tsurface[K 1
I
500
600
Fig. 2. NO coverage of the Pt( 111) surface in parts of one monolayer (ML) versus surface temperature for a particle flux of 4 X lOI molecules cm 2 SC’. The data were obtained by thermal desorption (squares) and transient adsorption (circles) experiments.
J. Segner et al. / Rotationalstaie populationsof NO molecules
NOlPt
(1111
Ekin:80111e!’ ,_o--o-_o ,A--&_.
-. 0%.
277
15 o 250K A 300K q L50K 0 600K
Fig. 3. Angular distributions of NO molecules scattered from a Pt( 111) surface at different surface temperatures for Eki, = 80 meV and an incidence angle of 60’.
the NO molecules takes place at an NO-covered surface, while at T >= 450 K the interaction with the clean Pt( 111) surface will dominate. Angular distributions of NO molecules scattered at surface temperatures between 250 and 600 K are reproduced in fig. 3. In all cases a slightly distorted cosine distribution is observed. At constant primary flux the signal intensity in the mass spectrorneter decreases proportional to Ts- ‘I2 [22]. Since the latter records the particle density (instead of the flux) which depends on the mean velocity, it may be concluded that the particles leave the surface after accommodation of their mean translational energy. Direct determination of the velocity distributions of desorbing NO molecules through time-of-flirt measurements support this conclusion [20]. (Note that a small fraction of about 10% is scattered through a direct-inelastic channel which causes the slight enhancement of the intensity over the cosine distribution near the specular direction and which is responsible for the sticking coefficient of only 0.9. This effect will only be of negligible importance for the discussion of the LIF measurements.) Trapping/desorption at T, $300 K is determined by the “precursor” potential, i.e. by the interaction of the incoming NO molecules with already adsorbed particles. This mechanism manifests itself also in the observed non-linear variation of the sticking coefficient with coverage 1181. The lifetime in the precursor state in the relevant temperature range was too short to be measured by the modulated beam technique but will certainly be much longer than the average “collision time” ( - IO-l2 s). For T, =( 300 K more than 90% of the incident NO molecules return to the gas phase via the precursor potential. The chemisorbed NO molecules are characterized by a mean surface residence time r = l/k, which is determined by the rate constant for desorption k, = 10i5.5exp(- 138.5 kJ mol-‘,/RT) (in the limit of small coverage) [IS].
278
J. Segner et al. /
8
too
200
300
8
LOO
Rotationaf state populations of NO molecules
500
600
700
i
800
8
900
1000
Ii00
Et”t [cm-‘1 Fig. 4. Rotational state distribution for NO molecules scattered from a Pt( I i 1) surface at 890 K. The straight line represents a Boltzmann distribution fitted to the measured points. Rotational populations of the two electronic ground states are superimposed in the diagram ( Ein, = internal energy).
(This rate constant is essentially governed by the properties of surface defects [l&21].) Direct determination of r through modulated molecular beam measurements over a limited temperature range ($600 K) [1X,22] and extrapolation yields typical values ranging from about 3 s at y? = 400 K to 5 X 10m7 s at TS = 800 K. With increasing coverage (decreasing temperature) the heat of adsorption decreases continuously, simultaneously the fraction of molecules leaving the surface from the “precursor” potential increases: The temperature range between about 300 and 400 K in fig. 2 characterizes this situation where both trapping/desorption processes at the clean and at the NO-covered Pt surface occur with comparable probabilities. The rotational state populations could in all cases be fitted to a Boltzmann distribution yielding a rotational temperature T,,, for which fig. 4 gives an example with NO molecules scattered at a 890 K Pt( 111) surface. The intensity of the fluorescence light allowed data analysis up to a rotational quantum number J” = 47/2. If the incoming molecular beam strikes the surface at an angle of 30” (with respect to the surface normal) those particles leaving the surface in specular direction are analyzed by LIF. In this case the detected NO consists of a mixture of desorbing and direct-inelastically scattered particles. With 56* angle of incidence detection is performed in the direction normal to the surface; this means that nearly all molecules originate from desorption. The value of T,,, could be determined to about f7%, in the worst case, and was (~thin this accuracy) independent of the angle of detection. Consequently the inaccuracy
.I. Segner et al. / Rotational
state populations
of NO moiecuies
279
Trot
[Kl 5oc t-
I-
I-
lIncidence
System
3-
56=’
NO/Ptflll)
0
0
NO-He/Ft(lll)
0
q
NO~Ft(~~~~+O NO-He/Pt(lllkO I
1
, 100
@
I
L
200
angle
30=’
300
&IO
500
TsurraceM~
600
700
600
Fig. 5. Plot of the experimental rotational temperature versus the surface temperature for NO molecules scattered from clean and oxygen-covered Pt( 111) surfaces. Open symbols were obtained at E,,, = 80 meV, closed symbols at E,,, = 210 meV (seeded beam).
of the present results does not allow to decide whether T,,, for the directly scattered molecules is slightly different from that of the desorbing molecules. Therefore the data are representative only for those molecules which leave the surface through a desorption process. The variation of T,,, with the surface temperature T, for the NO/Pt( 111) scattering is reproduced in fig. 5. In the range of desorption from the precursor state, i.e. for T,g 300 K, T,,, = Tsis observed. Above T,= 350 K the rotational temperature begins to deviate from Ts,and in the temperature range characteristic for desorption from the chemisorbed state (2 450 K) T,,, increases only very slightly with T,.At T,= 800 K, for example, the rotational temperature is only 440 f 20 K. These findings are in agreement with the more qualitative results of Asscher et al. [7,12], and also confirm the conclusions of Cavanagh and King [6], whereafter NO molecules thermally desorbing from a Ru surface
280
J. Segnw
et al. / Rotationalstate poFu~aiionsof NO moieties
exhibit a considerably lower rotational temperature than would correspond to complete thermal accommodation with the surface. (This latter experiment, however, could only be performed at a single T,.) Changing the translational energy of the primary molecules from 80 to 210 meV was without measurable effect on the resulting rotational distributions which is shown in fig. 5 for T, between 250 and 550 K. This was to be expected for molecules which had intermediately been trapped at the surface and thus lost their ‘~memory” of their primary momentum. The different orientation of the dipole oscillator for the Q branch transitions as compared with the R and P branches 1261 enables one to detect a rotational polarization of the scattered molecules. Experiments with polarized laser light failed to reveal preferential orientation of the rotational axis. Finally, it was found that the populations within each of the electronic can be described by the same ‘&. That means that the states II,,2 and II,,, overall population ratio N(II,,,,): N(II,,,) is given by exp( -AE/kT,,,), where dE is the fine-s~ucture-spotting (123 cm-’ = 15.2 meV) and T,,, the rotational temperature. This result is analogous to previous findings, e.g. with NO/graphite [ 111. 3.2. Direct-inelastic
scattering:
graphitized
Pt(ll1)
surface
Exposure of the clean Pt( 111) surface at 1000 K to about 0.5 Pa s ethylene caused the formation of a graphite overlayer as characterized by the well-known [27] ring-like LEED pattern, indicating the presence of crystallites with their basal planes parallel to the substrate surface but with random azimuthal ordering. This system is therefore quite similar in nature to the pyrographite sample studied previously [ 111.
Fig. 6. Angular distributions of NO molecules scattered from a 650 K carbon-covered Pt(lll) surface at an incidence angle of 60” and at different initial kinetic energies. (The two distributions have different scales.)
J. Segner et al. / Rotational state popuiations of NO moiecules
281
The surface was characterized by He scattering. At T, = 280 K about 8% of the incoming atoms are specularly reflected. This intensity is somewhat lower than with the pyrograp~te sample, indicating a higher concentration of surface defects. Helium scattering failed to detect reaction of NO, or adsorption of NO, CO, or 0, in the temperature range 240 5 q LS;1000 K. of NO molecules with Fig. 6 shows two “in plane” angular distributions different initial translational energies scattered from the graphitized sample at Ts= 650 K. The distributions are composed of a diffuse (cosine) part and a broad lobe appro~mately peaking in specular direction which is caused by direct inelastic scattering at “flat” portions of the surface. Increasing the translational energy of the incoming molecules from 80 to 210 meV causes an increase in the fraction of directly scattered particles. This is due to a reduction of the influence of the attractive potential whose well depth can be described by an adsorption energy of about 12 kJ/mol [28]. Further investigation of the angular dist~bution dependence on the surface temperature showed that for T,greater than 450 K the particle flux into the lobular part remained nearly constant similar to the results at the pyrographite sample [ 111. Simple considerations (assuming a frequency factor for desorption of IO” s-‘) show that with an adsorption energy of I2 kJ/mol at 500 K the mean residence time of an adsorbed particle will only be of the order of IO-” s corresponding to one collision. That means effective exchange of the translational energy of the incoming particles with the surface is no longer possible and trapping/desorption becomes meaningless. As a consequence, the constant diffuse part in the angular distribution present even at T,> 500 K must be ascribed to direct scattering at surface defects rather than to trapping/deso~tion. Assuming that the sticking coefficient S becomes zero for T, > 500 K, integration of the angular distributions over the entire half-space
SyS&m NO/Pyfographite
NOlPt
Olll+C
Ekim
0
80 meV
0
80moV
-
Q
210meV
-
0
Fig. 7. Plot of the sticking coefficient as obtained from integration of the angular distributions versus the surface temperature for the systems NO/graphite and NO/Pt( 11 I)+ C and an incidence angle of 60°.
282
J. Segner et al. / Rotational siute populations
of NO molecules
yields the variation of S with Ts as reproduced in fig. 7. The sticking coefficient at higher translational energy (210 meV) shows a decrease which can be ascribed to the above mentioned reduction of the influence of the attractive potential. The LIF measurements at the carbon-covered surface were performed at incidence angles of 30’ and of 60”. According to the 30” geometry of the detection system, the particles predominantly excited were those which had been scattered into the specular direction. Since the measurements were restricted to surface temperatures above 350 K, almost exclusively direct-inelastically scattered particles were detected. The previous work with pyrographite was mainly concerned with the effect of q and of the angle of detection on the rotational energy distribution [ 111. The main emphasis in the present
0 &I0
System
incidence 30°
angle 60°
Fig. 8. Plot of the experimental rotational temperature versus the surface temperature for NO molecules scattered from the surface of a pyrographite crystal and a carbon-covered Pt( 111) surface. Open symbols were obtained at Eki, = 80 meV, closed symbols at Ek,” = 210 meV (seeded beam).
J. Segner et al. / Rotational
state populations
of NO molecules
283
study was, therefore, concentrated on the influence of the translational energy of the primary particles. All derived rotational energy distributions can be reasonably well described by Boltzmann distributions with rotational temperatures T,,, which are plotted versus the surface temperature T, in fig. 8. For Ekin = 80 meV and 350 < T, < 750 K a r,,, = 250 of: 15 K was found, independent of T, and in full agreement with the previous results at the pyrographite surface [ 111 (included in fig. 8). If the translational energy is increased by a factor of 2.6 (i.e. Ekin = 210 meV) the rotational temperature of the scattered molecules was found to be increased by a factor of 1.3 to 325 & 20 K. For both values of ELin the distribution of the NO molecules over the two by a Boltzmann electronic states IT,,,, and rlr3,* can again be described distribution with temperature T = T,,,. 3.3. Trupping/desorption surface
plus direct-inelastic
scattering:
oxygen-covered
Pt(ll1)
The last example concerns a more complicated situation where about equal fractions of the molecules leave the surface via trapping/desorption and via direct scattering. The kinetic parameters for NO interaction with a Pt( 111) surface precovered with oxygen (6, = 0.25) had been investigated previously [ 181. In the present work a higher O-coverage, namely 0, = 0.60 f 0.05, was established through interaction with NOz: This molecule dissociates with high at T, 2 500 probability into NO,, + Oad, whereafter NO desorbes immediately K [24]. The oxygen coverage was determined through thermal desorption as described previously [24]. “Clean-off” reactions with H, of CO could be suppressed by cooling the walls of the scattering chamber with liquid nitrogen whereby the total background pressure was lowered to < 3 X lo- ” mbar. The flux of NO molecules which is directly scattered at this 0-precovered surface was determined through the modulated beam technique. At T, = 330 K the mean surface residence time of adsorbed NO molecules was about 0.5 s and thereby much longer than the reciprocal of the applied chopper frequency. As a consequence the signal from the particles undergoing trapping/desorption becomes completely demodulated and does not contribute to the signal recorded by the lock-in amplifier. On the other hand, the steady-state NO coverage established under these conditions is still negligibly small (8,, < 5x 10p3) so that no interference with scattering at adsorbed NO molecules (see section 3.1) occurs. The resulting angular distributions for an incidence angle of 60” and T, = 550 K for two different translational energies (Eki, = 80 and 210 meV) are reproduced in fig. 9. The molecules with the lower initial kinetic energy appear predominantly with a diffuse (i.e. cosine-like) scattering pattern, while the higher kinetic energy causes an increase of the intensity in specular direction. This result must be ascribed to the existence of an attractive
284
J. Segner et al. / Rotationul state populations
of NO molecules
Fig. 9. Angular distributions of the non-sticking fraction of NO molecules scattered from a 550 K Pt( 111)surface covered with 0.6 monolayers of oxygen atoms at different initial kinetic energies. (The two distributions have different scales.)
potential which has a more pronounced effect on the particles with lower translational energy. At higher surface temperature distinction between directly scattered molecules and those interacting via trapping/desorption can no longer be made on the basis of their different surface residence time. However, by comparing the angular distribution of the directly scattered molecules with the total angular distribution, the trapping/desorption fraction can be estimated. For q = 550 K the trapping probability is in this way estimated to be about 0.5. The initial sticking coefficient for NO was found to decrease approximately in proportion to the coverage of preadsorbed oxygen. This contrasts with the findings with preadsorbed NO where very efficient trapping with almost unit probability in the “precursor” state (i.e. on top of the chemisorbed layer) causes a much less pronounced decrease of the sticking coefficient, but is, on the other hand, consistent with the number of 0.5 for the sticking probability of NO at 6, = 0.6 as just outlined. Analysis of the desorption kinetics for NO from this surface was based on modulated beam measurements and follows standard techniques as described elsewhere [29]. The resulting data yield a first-order rate constant k, = These udexp( - E,/RT) with Us = 10’0.6*0.8 SC’ and Ed = 6 1.9 + 5.4 kJ/mol. values are quite similar to those reported earlier for 0, = 0.25 [ 181. At T, = 550 K, as an example, the mean surface residence time of the adsorbed NO molecules is found to be 2 x lop5 s, which is of course independent of the kinetic energy of the primary particles. LIF measurements at the same surface temperature (T, = 550 K) and in the direction of specular reflection yielded a rotational temperature T,,, = 323 + 20 K for particles with an initial kinetic energy of Ekin = 80 meV which increased
J. Segner et al. / Rotational siafe populations of NO molecules
285
by a factor of 1.3 to T,,, = 422 + 25 K upon raising Ekin to 2 10 meV. These results are included in fig. 5. Measurements at an incidence angle of 56” and with Ekin = 80 meV gave a rotational temperature T,,, = 306 f 15 K in fair agreement with the above mentioned value. This is to be expected since the directly scattered and the desorbing fractions are about equal for 80 meV kinetic energy, and have nearly the same isotropic angular distribution (fig. 9). In the case of trapping/desorption (note the results for NO interacting with of the initial kinetic the clean Pt surface) the value of T,,, is independent energy of the molecules; the results for NO/Pt( 111) + 0 therefore demonstrate that the data are strongly influenced by the directly scattered fraction.
4. Discussion The principal results of the present study may be summarized as follows: (i) Under the applied experimental conditions the rotational energy distribution of the NO molecules leaving the surface can always be described by a Boltzmann distribution with rotational temperature T,,,. (ii) The population of the two electronic ground states II,,* and 113,,* can be described by a “spin-orbit” temperature equal to T,,,, i.e. ~(~~~~)/~(~,,~) = exp( - AE,/k[r,,,) where AE = 123 cm-’ is the energy difference between the two systems. Within each of the two spin-orbit systems the rotational energy distribution is described by the same Trot. (iii) At low surface temperatures T, the rotational temperature is equal to T,. This holds for both scattering mechanisms, namely direct inelastic (up to Ts - 200 K, as determined with the pyro~ap~te surface, which is considered equivalent to the graphitized Pt surface) as well as trapping/deso~tion (with the NO precursor potential, up to T, - 300 K). (iv) At higher temperatures T,,, < T,, again for both direct and trapping/desorption scattering. On the other hand, T,,, was never observed to exceed T,. (v) In the case of NO/Pt( 111) + C and NO/C, T,,, reaches a limiting value which depends on the initial translational energy. For the system NO/Pt(l 11) the value for T,,, is almost independent of T, for T, 2 550 K. (vi) Finally no alignment of angular momentum of the scattered molecules could be observed within the experimental limits of error. These findings are in good agreement with reports in the literature as far as comparison is possible: with only one exception [3] all systems investigated so far exhibit rotational dist~butions which can be described by a Boltzmann distribution and a rotational temperature. The mentioned exception concerns scattering of NO at Ag(ll1) with high initial kinetic energies (exceeding the maximum values applied in the present work) as will be discussed in more detail below. For NO molecules scattered (i.e. desorbing) from a clean Pt( 111) surface at c = 580 K, Asscher et al. [12] reported T,,, between 400 and 480 K,
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J. Segner et al. / Rotarional stafe populations
of NO molecules
independent of the incident kinetic energy, in good agreement with the values of fig. 5. King and Cavanagh [lo] investigated NO thermally desorbing from Ru(OO1) and found T,,, = 235 k 35 K at T, = 455 K. This value differs considerably from those for NO/Pt(l 1 l), although the adsorption energy on the clean surface is quite similar in both cases. One has to keep in mind, however, that the comparison of the Ru experiments with the NO/Pt( 111) + 0 system would be more appropriate as the Ru surface could be substantially covered with O,,( + Nad) due to dissociative adsorption of NO. If one assumes a linear relationship between the initial kinetic energy of the molecules and their final rotational energy [3] extrapolation of the NO/Pt(lll)+ 0 data to Eki, = 0 (approaching the situation for thermal desorption) yields roughly T,,, = 250 K which should be in fair agreement with the Ru results. The scattering of NO from a Cu surface [ 131 has yielded previously unobserved and potentially exciting effects, namely a Trot far higher than T, and a conservation of the initial rotational distribution during the scattering event. However, the surface properties were not fully characterized, and therefore a comparison with the present results is difficult. At first sight one might be tempted to attribute the observed incomplete accommodation of the rotational energy (figs. 5 and 8) to mean surface residence times which are too short for complete energy exchange. For NO interacting with a clean Pt( 111) surface 7 varies between 3 s at T, = 400 K and 5 X lo-’ s at T, = 800 K while T,,, remains essentially constant (see fig. 5). An estimate for NO/graphite [ 1 l] suggests that in this case r drops to about lo- I2 s at 500 K while T,,, remains constant above T, - 300 K. Moreover, increasing the translational energy in this case also increases T,,, while the interaction time decreases further. Basically the following two processes may cause a variation of the rotational energy of a diatomic molecule scattered at a surface: 4.1. Adiabatic
transformation
of translational
into rotational energy
If a non-spherical molecule collides with a hard wall part of its translational energy may be transformed into rotational energy without energy exchange with the solid. This effect was first observed with the diffractive scattering of hydrogen (HD) at MgO [30], LiF [3 l] and Pt [32] surfaces. The latter results have recently been the subject of more detailed theoretical analysis ]33]. Briefly, if a non-spherical (with respect to the mass distribution) molecule hits a surface, excepting very rare orientations, the collision will exert a torque on the molecule which increases its rotational energy at the expense of its translational energy. Clearly this effect has only to be expected for direct-inelastic scattering events. Increasing the translational energy will increase the rotational energy,
J. Segner et al. / Rotational state populations
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287
and this is indeed observed in the present experiments similar to the previous work with NO/Ag(l 11) [3]. If adiabatic translational rotational energy transformation is the dominant of T,. With the present systems process, T,,, should be practically independent this is not the case for low surface temperatures. Instead T,,, never exceeds T, which points to the importance of the coupling to the heat bath of the solid (see below). Calculations with hard-cube models [34] as well as with more realistic potentials [35] indicate that with the adiabatic process the rotational temperature no longer follows a Boltzmann distribution: Such an effect (together with rotational polarization [9]) was indeed observed with NO/Ag( 111) for high kinetic energies ( > 300 meV) and for higher rotational quantum numbers (J” > 41/2) and was referred to as “rotational rainbow scattering” [3,9], in analogy to corresponding observations in gas-phase scattering experiments [36]. We may conclude from this section that adiabatic translational-rotational energy transformation may be the dominant process in direct scattering at high kinetic energies for high rotational quantum numbers and with not too low surface temperatures. With those systems of the present study which are dominated by direct scattering, this effect is still present to some extent (explaining the increase of T,,, with ~5,;“) but is, on the other hand, superimposed by phenomena due to coupling to the excitations of the solid. 4.2. Energy exchange with the solid The starting point of this discussion will be the static potential of a molecule bound to the surface. This potential hypersurface will have minima along the surface normal coordinate as well as in the range of angular coordinates of the molecular axis. Free rotations of the gaseous molecule will be transformed into frustrated rotations in front of the surface. As a consequence a Franck-Condon factor given by the overlap of the wave functions of the frustrated rotation of the bound molecule with those of the rotations of the free molecule will affect the rotational state populations of molecules leaving the surface. The similarity of the experimental results for direct scattering as well as for desorption suggests an orientational effect in the transitions from the surface potential to the free molecule. This similarity becomes most pronounced at lower surface temperatures where, even for direct-inelastic scattering, the observed rotational distribution agrees with the surface temperature. A limiting case for frustrated rotation of a molecule bound to a surface was recently treated theoretically along these lines by Gadzuk et al. [37]. Here the orientation of the molecular axis was restricted to a certain cone around the surface normal, thus representing the analogue to the well-known particle-inthe-box problem. One consequence of these constraints is the appearance of a zero-point energy. Transition into the gas phase was modelled by a sudden
288
J. Segner et al. / Rotational state populations
of NO molecules
switch-off of this restriction. The resulting rotational distribution was found to be under certain conditions indeed Boltzmann-like, where T,,, has of course nothing in common with a thermal equilibrium. The only aspect of the gas-solid interaction which enters this analysis is the variation of the (static) potential with the angular orientation of the molecular axis. Since this model provides coupling to the excitations of the solid by means of a thermal bath, it fails to include details of the dynamics of energy exchange. In particular, for low surface temperatures, where only the lowest hindered rotor states are significantly occupied, the zero point energy is sufficiently high to produce rotational temperatures higher than the surface temperature. The experimental observation that T,,, becomes equal to T5 (but never exceeds T,)at low surfce temperatures points to the importance of the details of the energetic coupling between molecule and surface in the transition state. A complete quantum mechanical treatment of this problem is at present being developed by Doyen [38] with first encouraging results. This theory is based on the idea that the interaction dynamics will cause a deformation of the surface and thereby also a variation of the interaction potential, with the consequence that a phononic Franck-Condon factor is of importance for the energy exchange between the molecule and the solid. It is felt that experiments of the described kind will ultimately yield detailed information on the dynamics of gas-solid interactions which in turn are responsible for the kinetic parameters of surface reactions, for energy accommodation coefficients, etc. We are, however, only at the beginning of this new development, and much more experimental and theoretical work will be needed in the coming years.
Acknowledgements We are grateful to Dr. G. Doyen for clarifying discussions. Financial support was obtained from the Deutsche Forschungsgemeinschaft (SFB 128) and from the Alexander von Humboldt foundation which granted a fellowship for H.R.
References 111F. Frenkel, J. Hager, W. Krieger, Segner, Phys. Rev. PI G.M. McClelland, 831. [31A.W. Kleyn, A.C. [41 L.D. Talley, W.A.
H. Walther, Letters 46 (1981) 152. G.D. Kubiak, H.G. Rennagel
C.T. Campbell,
G. Ertl, H. Kuipers
and J.
and R.N. Zare, Phys. Rev. Letters 46 (1981)
Luntz and D.J. Auerbach, Phys. Rev. Letters 47 (1981) 1169. Sanders, D.J. Bogan and M.C. Lin, Chem. Phys. Letters 78 (1981) 500.
J. Segner et ai. / Rotational state populations of”NO molecules
289
[S] J.W. Hepburn, E.J. Northrup, G.L. Ogram, J.C. Polanyi and J.M. Williamson, Chem. Phys. Letters 85 (1982) 127. [6] R.R. Cavanagh and D.S. King, Phys. Rev. Letters 47 (1981) 1829. [7] M. Asscher, W.L. Guthrie, T.H. Lin and G.A. Somorjai, Phys. Rev. Letters 49 (1982) 76. [8] D. Ettinger, K. Honma, M. Keil and J.C. Polanyi, Chem. Phys. Letters 87 (1982) 413. [9] (a) A.C. Luntz, A.W. Kleyn and D.J. Auerbach, Phys. Rev. B25 (1982) 4273; (b) A.C. Luntz, A.W. Kleyn and D.J. Auerbach, J. Chem. Phys. 76 (1982) 737. [lo] D.S. King and R.R. Cavanagh, J. Chem. Phys. 76 (1982) 5634. [ 1I] F. Frenkel, J. Hager, W. Krieger, H. Walther, G. Ertl, J. Segner and W. Vielhaber, Chem. Phys. Letters 90 (1982) 225. 1121 M. Asscher, W.L. Guthrie, T.H. Lin and G.A. Somorjai, J. Chem. Phys. 78 (1983) 6992. 1131 J.S. Hayden and G.J. Diebold, J. Chem. Phys. 77 (1982) 4767. [ 141 T. Engel, J. Chem. Phys. 69 (1978) 373. [ 151 Deviations from a Boltzmann distribution were observed for rotational quantum numbers J” > 5/2 (the states with higher J” numbers exhibit a slightly higher population). The fraction of molecules deviating from a Boltzmann distribution amounts, however, only to about 5% of the total number of particles. f16] D. Golomb, R.E. Good and RF. Brown, J. Chem. Phys. 52 (1970) 1545. [17] C.T. Campbell, G. Ertl, H. Kuipers and J. Segner, Surface Sci. 107 (1981) 207, 220. [ 181 C.T. Campbell, G. Ertl and J. Segner, Surface Sci. 115 (1982) 309. 1191 T.H. Lin and G.A. Somorjai, Surface Sci. 107 (1981) 573. [20] W.L. Guthrie, T.H. Lin, S.T. Ceyer and G.A. Somorjai, J. Chem. Phys. 76 (1982) 6398. [21] J.A. Serri, M.J. Cardillo and G.E. Becker, J. Chem. Phys. 77 (1982) 2175. [22] J. Segner, Thesis, Munich ( 1982). 1231 B.E. Hayden and A.M. Bradshaw, Surface Sci. 125 (1983) 787. [24] 3. Segner, W. Vielhaber and G. Ertl, Israel J. Chem. 22 (1982) 375. [25] K. Christmann, G. Ertl and T. Pignet, Surface Sci. 54 (1976) 365. [26] M.P. Sinha, CD. Caldwell and R.N. Zare, J. Chem. Phys. 61 (1974) 491. [27] R.P. Merrill and D.L. Smith, Surface Sci. 21 (1970) 203. [28] C.E. Brown and D.G. hall, J. Colloid Interface Sci. 42 (1973) 334. [29] D.R. Olander, J. Colloid Interface Sci. 58 (1977) 169. 1301 R.G. Rowe and G. Fhrlich, J. Chem. Phys. 63 (1975) 4648. [31] G. Boato, P. Cantini and L. Mattera, J. Chem. Phys. 65 (1976) 544. [32] J.P. Cowin, CF. Yu, S.J. Sibener and J.E. Hurst, J. Chem. Phys. 75 (1981) 1033. [33] K.B. Whaley, J.C. Light, J.P. Cowin and S.J. Sibener, Chem. Phys. Letters 89 (1982) 89. [34] W.L. Nichols and J.H. Weare, J. Chem. Phys. 62 (19750 3745; 63 (1975) 379; 66 (1977) 1075. [35] R. Schinke, J. Chem. Phys. 76 (1982) 2352. [36] (a) B.E. Wilcomb and P.J. Dagdigian, J. Chem. Phys. 67 (1977) 3829; (b) K. Bergmann, U. Hefter and J. Witt, J. Chem. Phys. 72 (1980) 4777. [37] J.W. Gadzuk, U. Landman, E.J. Kuster, C.L. Cleveland and R.N. Bamett, Phys. Rev. Letters 49 (1982) 426. [38] G. Doyen, in preparation.
Surface Science 131 (1983) 273-289 North-Holland Publishing Company
ROTATIONAL SCATTERED SURFACES J. SEGNER
STATE POPULATIONS OF NO MOLECULES FROM CLEAN AND ADSORBATE-COVERED Pt( 111)
*, H. ROBOTA,
Institut fiir Physikalische Rep. of Germany
W. VIELHABER
and G. ERTL
Chemie, Universitiit Miinchen, Sophienstrasse
Il, D - 8GOOMiinchen 2, Fed.
and F. FRENKEL
**, J. HAGER,
W. KRIEGER
Max - Planck - Institut fiir Quantenoptik, Received
3 February
1983; accepted
and H. WALTHER
D - 8046 Garching, Fed. Rep. of Germany
for publication
26 May 1983
A supersonic beam of NO molecules was scattered from a Pt(ll1) surface either clean or covered with NO, C, and 0. Rotational state populations and angular distributions of the scattered molecules were determined at two incident energies and at different surface temperatures T,. The rotational distributions obtained are Boltzmann distributions with rotational temperatures Tr,,. Interaction with the clean and NO-covered Pt surface shows diffuse angular distributions and the independence of T,,, on the initial kinetic energy. Full rotational accommodation was found for scattering from the NO-covered (T, < 300 K), only incomplete accommodation at the clean Pt surface (T, > 450 K). Interaction with the C-covered surface results in broad nearly specular scattering lobes and at higher surface temperatures in a constant value of Tr,, < T, which depends on the initial kinetic energy. Scattering at the oxygen-covered surface presents an intermediate case: diffusely and specularly scattered fractions are observed. The experimental results are discussed in terms of underlying microscopic scattering processes.
1. Introduction The possibility to probe the internal state distribution of molecules with high sensitivity by the use of laser light opened a new area for the investigation of dynamic processes occurring at gas/solid interfaces [ 1- 131. So far, these studies have mainly concentrated on the determination of rotational state distributions and often yielded quite surprising results: In most cases the state populations of molecules leaving the surface can formally be described by a * Present address: Department of Chemistry, University of Toronto, Toronto, Ontario, ** Present address: Siemens AG, Hoffmannstrasse 51, D-8000 Mtinchen 70, Fed. Germany.
0039-6028/83/0000-0000/$03.00
0 1983 North-Holland
Canada. Rep. of
214
J. Segner et al. / Rotational state populations
of NO molecules
Boltzmann distribution with a rotational temperature T,,,. However, T,,,is frequently smaller than the surface temperature T,, even if the molecule had a very long residence time (2 10e4 s) on the surface. In the case of direct inelastic scattering (NO/Ag(lll)) at high kinetic energies of the incident molecules the distribution no longer follows the Boltzmann law and indicates a direct transformation of translational into rotational energy (“rotational rainbow”) [3,9]. The test molecule NO exhibits non-zero orbital and spin angular momenta and therefore, due to spin-orbit coupling, two states (II,,2 and II,,,) exist, separated by 123 cm-’ in energy: This offers the additional possibility to probe the redistribution of internal energy in different electronic states. The present work describes the results of experiments with NO scattered at a Pt( 111) surface which was either clean or covered with various adlayers (NO, 0, C) in order to modify the interaction potential. The type of scattering varies between almost pure direct inelastic and pure trapping/desorption. The results with the clean Pt( 111) surface are in qualitative agreement with the findings of Asscher et al. [7,12] who investigated this system by a different method. The carbon-covered surface behaves almost identically to the pyrographite sample investigated previously in this laboratory [ 111.
2. Experimental The experiments were performed with a UHV molecular beam system [14] which was equipped with facilities for laser-induced fluorescence (LIF) [l]. A block diagram of the optical and electronic components is reproduced in fig. 1. A tunable dye laser is pumped by a XeCl excimer laser at a repetition rate of 5-10 s-‘. Frequency doubling provides 5 ns pulses in the desired wavelength range ( - 226 nm) with a linewidth of 0.15 cm- ’ and with a pulse energy up to 40 pJ. The laser beam enters and exits the vacuum system through Brewsterangle quartz windows. Inside the chamber it is deflected by aluminum coated mirrors which can be moved so that the laser beam crosses and analyses either the incoming molecular beam or the molecules leaving the surface. The analysed volume of - 1 mm3 was at a distance of - 12 mm from the sample surface producing an angular resolution of about 20” at 5 mm beam diameter. The fluorescence light is collected by a quartz lens system and focused through a quartz window onto the cathode of a photomultiplier tube. The signal is then amplified and recorded by means of a boxcar integrator. Simultaneously the spectrum of a reference cell ( 10e3 mbar NO at T = 300 K) is recorded, which allows normalization and evaluation of the recorded intensities. Densities of the scattered molecules in the range of lo6 molecules cmm3 per state could still be detected. The sample was mounted on the rotatable axis of a manipulator. The
J. Segner et al. / Rotational state populations Excimer laser
of NO molecules
275
Dye Laser Trigger y Reference B
&it
Y
PM I PA
Molecular
beam
Scattering chamber
Fig. 1. Schematic drawing of the experimental setup (PA = preamplifier, PM = photomultiplier, QMS = quadrupole mass spectrometer).
PD = photodiode,
scattered molecules could be analysed by a quadrupole mass spectrometer which could be rotated around the manipulator axis within the scattering plane. In this way both the (polar) angle of incidence and of detection could be independently varied. With a beam diameter of 2 mm and the detector at 60 mm distance from the sample the angular resolution was about 2”. A supersonic beam of NO molecules was generated by a nozzle-skimmer device within a three-stage differentially pumped source. The translational energy of the molecules Ekin could be enhanced from 80 to 210 meV by seeding with He. Adiabatic expansion of the molecules behind the nozzle causes their rotational energy to relax. It was found by LIF that the population distribution of the rotational states of the primary particles can be described by the rotational temperatures T,,, = 39 K at Ekin = 80 meV and T,,, = 20 K at Ekin = 210 meV [ 151. The formation of (NO), clusters in the beam depends on the product p,d ( p,, = source pressure = 800 mbar, d = nozzle diameter = 7 x lop3 cm) and was negligible under the applied conditions [16]. The flux of NO molecules striking the surface was about 4 x lOI cmm2 SC’ both for Eki, = 80 meV and Ekin = 210 meV. Preparation and characterization (by LEED, AES and He reflection) of the clean Pt( 111) surface has been described in ref. [ 171. The adsorption/desorption properties of this surface, being either clean or covered by oxygen had been studied previously [ 181.
216
J. Segner et al. / Rotational state populations of NO molecules
3. Results 3. I. Trapping/desorption:
clean and NO-covered
Pt(l I I) surface
If a clean Pt( 111) surface (at q s 800 K) is exposed to NO, nonactivated chemisorption with a sticking coefficient of about 0.9 takes place [ 18-201. The remaining fraction undergoes direct inelastic scattering. At higher surface temperatures or higher translational energies the directly scattered portion increases [21], for example by about a factor of two by increasing Ekin from 80 to 210 meV [22]. If a continuous flux F of NO molecules strikes the surface an equilibrium coverage will be built up, depending on F and the surface residence time T( T,). Fig. 2 shows this equilibrium coverage as a function of temperature for Relative F= 4 X lOI molecules cm-* s-’ as applied in the LIF experiments. coverages were determined either from TDS data (squares) or from the material balance in transient beam experiments (circles). Calibration of the absolute coverage scale was achieved by comparison with data from a surface saturated with CO at 300 K, where the coverage 8 = 0.5 based on LEED data is generally accepted [23]. The NO molecular beam contains a small fraction (I 0.5%) of NO, which decomposes with almost unit probability at a clean Pt(ll1) surface into NO,, + O,, [24]. The experiments above 400 K were therefore performed with a background pressure of H, (8 x 10e9 mbar) which removes the oxygen atoms by rapid reaction. The steady-state H coverage introduced in this way is negligible [25]. According to fig. 2 under steady-state conditions at q s 300 K scattering of
0
El
I
200
300 LOO Tsurface[K 1
I
500
600
Fig. 2. NO coverage of the Pt( 111) surface in parts of one monolayer (ML) versus surface temperature for a particle flux of 4 X lOI molecules cm 2 SC’. The data were obtained by thermal desorption (squares) and transient adsorption (circles) experiments.
J. Segner et al. / Rotationalstaie populationsof NO molecules
NOlPt
(1111
Ekin:80111e!’ ,_o--o-_o ,A--&_.
-. 0%.
277
15 o 250K A 300K q L50K 0 600K
Fig. 3. Angular distributions of NO molecules scattered from a Pt( 111) surface at different surface temperatures for Eki, = 80 meV and an incidence angle of 60’.
the NO molecules takes place at an NO-covered surface, while at T >= 450 K the interaction with the clean Pt( 111) surface will dominate. Angular distributions of NO molecules scattered at surface temperatures between 250 and 600 K are reproduced in fig. 3. In all cases a slightly distorted cosine distribution is observed. At constant primary flux the signal intensity in the mass spectrorneter decreases proportional to Ts- ‘I2 [22]. Since the latter records the particle density (instead of the flux) which depends on the mean velocity, it may be concluded that the particles leave the surface after accommodation of their mean translational energy. Direct determination of the velocity distributions of desorbing NO molecules through time-of-flirt measurements support this conclusion [20]. (Note that a small fraction of about 10% is scattered through a direct-inelastic channel which causes the slight enhancement of the intensity over the cosine distribution near the specular direction and which is responsible for the sticking coefficient of only 0.9. This effect will only be of negligible importance for the discussion of the LIF measurements.) Trapping/desorption at T, $300 K is determined by the “precursor” potential, i.e. by the interaction of the incoming NO molecules with already adsorbed particles. This mechanism manifests itself also in the observed non-linear variation of the sticking coefficient with coverage 1181. The lifetime in the precursor state in the relevant temperature range was too short to be measured by the modulated beam technique but will certainly be much longer than the average “collision time” ( - IO-l2 s). For T, =( 300 K more than 90% of the incident NO molecules return to the gas phase via the precursor potential. The chemisorbed NO molecules are characterized by a mean surface residence time r = l/k, which is determined by the rate constant for desorption k, = 10i5.5exp(- 138.5 kJ mol-‘,/RT) (in the limit of small coverage) [IS].
278
J. Segner et al. /
8
too
200
300
8
LOO
Rotationaf state populations of NO molecules
500
600
700
i
800
8
900
1000
Ii00
Et”t [cm-‘1 Fig. 4. Rotational state distribution for NO molecules scattered from a Pt( I i 1) surface at 890 K. The straight line represents a Boltzmann distribution fitted to the measured points. Rotational populations of the two electronic ground states are superimposed in the diagram ( Ein, = internal energy).
(This rate constant is essentially governed by the properties of surface defects [l&21].) Direct determination of r through modulated molecular beam measurements over a limited temperature range ($600 K) [1X,22] and extrapolation yields typical values ranging from about 3 s at y? = 400 K to 5 X 10m7 s at TS = 800 K. With increasing coverage (decreasing temperature) the heat of adsorption decreases continuously, simultaneously the fraction of molecules leaving the surface from the “precursor” potential increases: The temperature range between about 300 and 400 K in fig. 2 characterizes this situation where both trapping/desorption processes at the clean and at the NO-covered Pt surface occur with comparable probabilities. The rotational state populations could in all cases be fitted to a Boltzmann distribution yielding a rotational temperature T,,, for which fig. 4 gives an example with NO molecules scattered at a 890 K Pt( 111) surface. The intensity of the fluorescence light allowed data analysis up to a rotational quantum number J” = 47/2. If the incoming molecular beam strikes the surface at an angle of 30” (with respect to the surface normal) those particles leaving the surface in specular direction are analyzed by LIF. In this case the detected NO consists of a mixture of desorbing and direct-inelastically scattered particles. With 56* angle of incidence detection is performed in the direction normal to the surface; this means that nearly all molecules originate from desorption. The value of T,,, could be determined to about f7%, in the worst case, and was (~thin this accuracy) independent of the angle of detection. Consequently the inaccuracy
.I. Segner et al. / Rotational
state populations
of NO moiecuies
279
Trot
[Kl 5oc t-
I-
I-
lIncidence
System
3-
56=’
NO/Ptflll)
0
0
NO-He/Ft(lll)
0
q
NO~Ft(~~~~+O NO-He/Pt(lllkO I
1
, 100
@
I
L
200
angle
30=’
300
&IO
500
TsurraceM~
600
700
600
Fig. 5. Plot of the experimental rotational temperature versus the surface temperature for NO molecules scattered from clean and oxygen-covered Pt( 111) surfaces. Open symbols were obtained at E,,, = 80 meV, closed symbols at E,,, = 210 meV (seeded beam).
of the present results does not allow to decide whether T,,, for the directly scattered molecules is slightly different from that of the desorbing molecules. Therefore the data are representative only for those molecules which leave the surface through a desorption process. The variation of T,,, with the surface temperature T, for the NO/Pt( 111) scattering is reproduced in fig. 5. In the range of desorption from the precursor state, i.e. for T,g 300 K, T,,, = Tsis observed. Above T,= 350 K the rotational temperature begins to deviate from Ts,and in the temperature range characteristic for desorption from the chemisorbed state (2 450 K) T,,, increases only very slightly with T,.At T,= 800 K, for example, the rotational temperature is only 440 f 20 K. These findings are in agreement with the more qualitative results of Asscher et al. [7,12], and also confirm the conclusions of Cavanagh and King [6], whereafter NO molecules thermally desorbing from a Ru surface
280
J. Segnw
et al. / Rotationalstate poFu~aiionsof NO moieties
exhibit a considerably lower rotational temperature than would correspond to complete thermal accommodation with the surface. (This latter experiment, however, could only be performed at a single T,.) Changing the translational energy of the primary molecules from 80 to 210 meV was without measurable effect on the resulting rotational distributions which is shown in fig. 5 for T, between 250 and 550 K. This was to be expected for molecules which had intermediately been trapped at the surface and thus lost their ‘~memory” of their primary momentum. The different orientation of the dipole oscillator for the Q branch transitions as compared with the R and P branches 1261 enables one to detect a rotational polarization of the scattered molecules. Experiments with polarized laser light failed to reveal preferential orientation of the rotational axis. Finally, it was found that the populations within each of the electronic can be described by the same ‘&. That means that the states II,,2 and II,,, overall population ratio N(II,,,,): N(II,,,) is given by exp( -AE/kT,,,), where dE is the fine-s~ucture-spotting (123 cm-’ = 15.2 meV) and T,,, the rotational temperature. This result is analogous to previous findings, e.g. with NO/graphite [ 111. 3.2. Direct-inelastic
scattering:
graphitized
Pt(ll1)
surface
Exposure of the clean Pt( 111) surface at 1000 K to about 0.5 Pa s ethylene caused the formation of a graphite overlayer as characterized by the well-known [27] ring-like LEED pattern, indicating the presence of crystallites with their basal planes parallel to the substrate surface but with random azimuthal ordering. This system is therefore quite similar in nature to the pyrographite sample studied previously [ 111.
Fig. 6. Angular distributions of NO molecules scattered from a 650 K carbon-covered Pt(lll) surface at an incidence angle of 60” and at different initial kinetic energies. (The two distributions have different scales.)
J. Segner et al. / Rotational state popuiations of NO moiecules
281
The surface was characterized by He scattering. At T, = 280 K about 8% of the incoming atoms are specularly reflected. This intensity is somewhat lower than with the pyrograp~te sample, indicating a higher concentration of surface defects. Helium scattering failed to detect reaction of NO, or adsorption of NO, CO, or 0, in the temperature range 240 5 q LS;1000 K. of NO molecules with Fig. 6 shows two “in plane” angular distributions different initial translational energies scattered from the graphitized sample at Ts= 650 K. The distributions are composed of a diffuse (cosine) part and a broad lobe appro~mately peaking in specular direction which is caused by direct inelastic scattering at “flat” portions of the surface. Increasing the translational energy of the incoming molecules from 80 to 210 meV causes an increase in the fraction of directly scattered particles. This is due to a reduction of the influence of the attractive potential whose well depth can be described by an adsorption energy of about 12 kJ/mol [28]. Further investigation of the angular dist~bution dependence on the surface temperature showed that for T,greater than 450 K the particle flux into the lobular part remained nearly constant similar to the results at the pyrographite sample [ 111. Simple considerations (assuming a frequency factor for desorption of IO” s-‘) show that with an adsorption energy of I2 kJ/mol at 500 K the mean residence time of an adsorbed particle will only be of the order of IO-” s corresponding to one collision. That means effective exchange of the translational energy of the incoming particles with the surface is no longer possible and trapping/desorption becomes meaningless. As a consequence, the constant diffuse part in the angular distribution present even at T,> 500 K must be ascribed to direct scattering at surface defects rather than to trapping/deso~tion. Assuming that the sticking coefficient S becomes zero for T, > 500 K, integration of the angular distributions over the entire half-space
SyS&m NO/Pyfographite
NOlPt
Olll+C
Ekim
0
80 meV
0
80moV
-
Q
210meV
-
0
Fig. 7. Plot of the sticking coefficient as obtained from integration of the angular distributions versus the surface temperature for the systems NO/graphite and NO/Pt( 11 I)+ C and an incidence angle of 60°.
282
J. Segner et al. / Rotational siute populations
of NO molecules
yields the variation of S with Ts as reproduced in fig. 7. The sticking coefficient at higher translational energy (210 meV) shows a decrease which can be ascribed to the above mentioned reduction of the influence of the attractive potential. The LIF measurements at the carbon-covered surface were performed at incidence angles of 30’ and of 60”. According to the 30” geometry of the detection system, the particles predominantly excited were those which had been scattered into the specular direction. Since the measurements were restricted to surface temperatures above 350 K, almost exclusively direct-inelastically scattered particles were detected. The previous work with pyrographite was mainly concerned with the effect of q and of the angle of detection on the rotational energy distribution [ 111. The main emphasis in the present
0 &I0
System
incidence 30°
angle 60°
Fig. 8. Plot of the experimental rotational temperature versus the surface temperature for NO molecules scattered from the surface of a pyrographite crystal and a carbon-covered Pt( 111) surface. Open symbols were obtained at Eki, = 80 meV, closed symbols at Ek,” = 210 meV (seeded beam).
J. Segner et al. / Rotational
state populations
of NO molecules
283
study was, therefore, concentrated on the influence of the translational energy of the primary particles. All derived rotational energy distributions can be reasonably well described by Boltzmann distributions with rotational temperatures T,,, which are plotted versus the surface temperature T, in fig. 8. For Ekin = 80 meV and 350 < T, < 750 K a r,,, = 250 of: 15 K was found, independent of T, and in full agreement with the previous results at the pyrographite surface [ 111 (included in fig. 8). If the translational energy is increased by a factor of 2.6 (i.e. Ekin = 210 meV) the rotational temperature of the scattered molecules was found to be increased by a factor of 1.3 to 325 & 20 K. For both values of ELin the distribution of the NO molecules over the two by a Boltzmann electronic states IT,,,, and rlr3,* can again be described distribution with temperature T = T,,,. 3.3. Trupping/desorption surface
plus direct-inelastic
scattering:
oxygen-covered
Pt(ll1)
The last example concerns a more complicated situation where about equal fractions of the molecules leave the surface via trapping/desorption and via direct scattering. The kinetic parameters for NO interaction with a Pt( 111) surface precovered with oxygen (6, = 0.25) had been investigated previously [ 181. In the present work a higher O-coverage, namely 0, = 0.60 f 0.05, was established through interaction with NOz: This molecule dissociates with high at T, 2 500 probability into NO,, + Oad, whereafter NO desorbes immediately K [24]. The oxygen coverage was determined through thermal desorption as described previously [24]. “Clean-off” reactions with H, of CO could be suppressed by cooling the walls of the scattering chamber with liquid nitrogen whereby the total background pressure was lowered to < 3 X lo- ” mbar. The flux of NO molecules which is directly scattered at this 0-precovered surface was determined through the modulated beam technique. At T, = 330 K the mean surface residence time of adsorbed NO molecules was about 0.5 s and thereby much longer than the reciprocal of the applied chopper frequency. As a consequence the signal from the particles undergoing trapping/desorption becomes completely demodulated and does not contribute to the signal recorded by the lock-in amplifier. On the other hand, the steady-state NO coverage established under these conditions is still negligibly small (8,, < 5x 10p3) so that no interference with scattering at adsorbed NO molecules (see section 3.1) occurs. The resulting angular distributions for an incidence angle of 60” and T, = 550 K for two different translational energies (Eki, = 80 and 210 meV) are reproduced in fig. 9. The molecules with the lower initial kinetic energy appear predominantly with a diffuse (i.e. cosine-like) scattering pattern, while the higher kinetic energy causes an increase of the intensity in specular direction. This result must be ascribed to the existence of an attractive
284
J. Segner et al. / Rotationul state populations
of NO molecules
Fig. 9. Angular distributions of the non-sticking fraction of NO molecules scattered from a 550 K Pt( 111)surface covered with 0.6 monolayers of oxygen atoms at different initial kinetic energies. (The two distributions have different scales.)
potential which has a more pronounced effect on the particles with lower translational energy. At higher surface temperature distinction between directly scattered molecules and those interacting via trapping/desorption can no longer be made on the basis of their different surface residence time. However, by comparing the angular distribution of the directly scattered molecules with the total angular distribution, the trapping/desorption fraction can be estimated. For q = 550 K the trapping probability is in this way estimated to be about 0.5. The initial sticking coefficient for NO was found to decrease approximately in proportion to the coverage of preadsorbed oxygen. This contrasts with the findings with preadsorbed NO where very efficient trapping with almost unit probability in the “precursor” state (i.e. on top of the chemisorbed layer) causes a much less pronounced decrease of the sticking coefficient, but is, on the other hand, consistent with the number of 0.5 for the sticking probability of NO at 6, = 0.6 as just outlined. Analysis of the desorption kinetics for NO from this surface was based on modulated beam measurements and follows standard techniques as described elsewhere [29]. The resulting data yield a first-order rate constant k, = These udexp( - E,/RT) with Us = 10’0.6*0.8 SC’ and Ed = 6 1.9 + 5.4 kJ/mol. values are quite similar to those reported earlier for 0, = 0.25 [ 181. At T, = 550 K, as an example, the mean surface residence time of the adsorbed NO molecules is found to be 2 x lop5 s, which is of course independent of the kinetic energy of the primary particles. LIF measurements at the same surface temperature (T, = 550 K) and in the direction of specular reflection yielded a rotational temperature T,,, = 323 + 20 K for particles with an initial kinetic energy of Ekin = 80 meV which increased
J. Segner et al. / Rotational siafe populations of NO molecules
285
by a factor of 1.3 to T,,, = 422 + 25 K upon raising Ekin to 2 10 meV. These results are included in fig. 5. Measurements at an incidence angle of 56” and with Ekin = 80 meV gave a rotational temperature T,,, = 306 f 15 K in fair agreement with the above mentioned value. This is to be expected since the directly scattered and the desorbing fractions are about equal for 80 meV kinetic energy, and have nearly the same isotropic angular distribution (fig. 9). In the case of trapping/desorption (note the results for NO interacting with of the initial kinetic the clean Pt surface) the value of T,,, is independent energy of the molecules; the results for NO/Pt( 111) + 0 therefore demonstrate that the data are strongly influenced by the directly scattered fraction.
4. Discussion The principal results of the present study may be summarized as follows: (i) Under the applied experimental conditions the rotational energy distribution of the NO molecules leaving the surface can always be described by a Boltzmann distribution with rotational temperature T,,,. (ii) The population of the two electronic ground states II,,* and 113,,* can be described by a “spin-orbit” temperature equal to T,,,, i.e. ~(~~~~)/~(~,,~) = exp( - AE,/k[r,,,) where AE = 123 cm-’ is the energy difference between the two systems. Within each of the two spin-orbit systems the rotational energy distribution is described by the same Trot. (iii) At low surface temperatures T, the rotational temperature is equal to T,. This holds for both scattering mechanisms, namely direct inelastic (up to Ts - 200 K, as determined with the pyro~ap~te surface, which is considered equivalent to the graphitized Pt surface) as well as trapping/deso~tion (with the NO precursor potential, up to T, - 300 K). (iv) At higher temperatures T,,, < T,, again for both direct and trapping/desorption scattering. On the other hand, T,,, was never observed to exceed T,. (v) In the case of NO/Pt( 111) + C and NO/C, T,,, reaches a limiting value which depends on the initial translational energy. For the system NO/Pt(l 11) the value for T,,, is almost independent of T, for T, 2 550 K. (vi) Finally no alignment of angular momentum of the scattered molecules could be observed within the experimental limits of error. These findings are in good agreement with reports in the literature as far as comparison is possible: with only one exception [3] all systems investigated so far exhibit rotational dist~butions which can be described by a Boltzmann distribution and a rotational temperature. The mentioned exception concerns scattering of NO at Ag(ll1) with high initial kinetic energies (exceeding the maximum values applied in the present work) as will be discussed in more detail below. For NO molecules scattered (i.e. desorbing) from a clean Pt( 111) surface at c = 580 K, Asscher et al. [12] reported T,,, between 400 and 480 K,
286
J. Segner et al. / Rotarional stafe populations
of NO molecules
independent of the incident kinetic energy, in good agreement with the values of fig. 5. King and Cavanagh [lo] investigated NO thermally desorbing from Ru(OO1) and found T,,, = 235 k 35 K at T, = 455 K. This value differs considerably from those for NO/Pt(l 1 l), although the adsorption energy on the clean surface is quite similar in both cases. One has to keep in mind, however, that the comparison of the Ru experiments with the NO/Pt( 111) + 0 system would be more appropriate as the Ru surface could be substantially covered with O,,( + Nad) due to dissociative adsorption of NO. If one assumes a linear relationship between the initial kinetic energy of the molecules and their final rotational energy [3] extrapolation of the NO/Pt(lll)+ 0 data to Eki, = 0 (approaching the situation for thermal desorption) yields roughly T,,, = 250 K which should be in fair agreement with the Ru results. The scattering of NO from a Cu surface [ 131 has yielded previously unobserved and potentially exciting effects, namely a Trot far higher than T, and a conservation of the initial rotational distribution during the scattering event. However, the surface properties were not fully characterized, and therefore a comparison with the present results is difficult. At first sight one might be tempted to attribute the observed incomplete accommodation of the rotational energy (figs. 5 and 8) to mean surface residence times which are too short for complete energy exchange. For NO interacting with a clean Pt( 111) surface 7 varies between 3 s at T, = 400 K and 5 X lo-’ s at T, = 800 K while T,,, remains essentially constant (see fig. 5). An estimate for NO/graphite [ 1 l] suggests that in this case r drops to about lo- I2 s at 500 K while T,,, remains constant above T, - 300 K. Moreover, increasing the translational energy in this case also increases T,,, while the interaction time decreases further. Basically the following two processes may cause a variation of the rotational energy of a diatomic molecule scattered at a surface: 4.1. Adiabatic
transformation
of translational
into rotational energy
If a non-spherical molecule collides with a hard wall part of its translational energy may be transformed into rotational energy without energy exchange with the solid. This effect was first observed with the diffractive scattering of hydrogen (HD) at MgO [30], LiF [3 l] and Pt [32] surfaces. The latter results have recently been the subject of more detailed theoretical analysis ]33]. Briefly, if a non-spherical (with respect to the mass distribution) molecule hits a surface, excepting very rare orientations, the collision will exert a torque on the molecule which increases its rotational energy at the expense of its translational energy. Clearly this effect has only to be expected for direct-inelastic scattering events. Increasing the translational energy will increase the rotational energy,
J. Segner et al. / Rotational state populations
of NO molecules
287
and this is indeed observed in the present experiments similar to the previous work with NO/Ag(l 11) [3]. If adiabatic translational rotational energy transformation is the dominant of T,. With the present systems process, T,,, should be practically independent this is not the case for low surface temperatures. Instead T,,, never exceeds T, which points to the importance of the coupling to the heat bath of the solid (see below). Calculations with hard-cube models [34] as well as with more realistic potentials [35] indicate that with the adiabatic process the rotational temperature no longer follows a Boltzmann distribution: Such an effect (together with rotational polarization [9]) was indeed observed with NO/Ag( 111) for high kinetic energies ( > 300 meV) and for higher rotational quantum numbers (J” > 41/2) and was referred to as “rotational rainbow scattering” [3,9], in analogy to corresponding observations in gas-phase scattering experiments [36]. We may conclude from this section that adiabatic translational-rotational energy transformation may be the dominant process in direct scattering at high kinetic energies for high rotational quantum numbers and with not too low surface temperatures. With those systems of the present study which are dominated by direct scattering, this effect is still present to some extent (explaining the increase of T,,, with ~5,;“) but is, on the other hand, superimposed by phenomena due to coupling to the excitations of the solid. 4.2. Energy exchange with the solid The starting point of this discussion will be the static potential of a molecule bound to the surface. This potential hypersurface will have minima along the surface normal coordinate as well as in the range of angular coordinates of the molecular axis. Free rotations of the gaseous molecule will be transformed into frustrated rotations in front of the surface. As a consequence a Franck-Condon factor given by the overlap of the wave functions of the frustrated rotation of the bound molecule with those of the rotations of the free molecule will affect the rotational state populations of molecules leaving the surface. The similarity of the experimental results for direct scattering as well as for desorption suggests an orientational effect in the transitions from the surface potential to the free molecule. This similarity becomes most pronounced at lower surface temperatures where, even for direct-inelastic scattering, the observed rotational distribution agrees with the surface temperature. A limiting case for frustrated rotation of a molecule bound to a surface was recently treated theoretically along these lines by Gadzuk et al. [37]. Here the orientation of the molecular axis was restricted to a certain cone around the surface normal, thus representing the analogue to the well-known particle-inthe-box problem. One consequence of these constraints is the appearance of a zero-point energy. Transition into the gas phase was modelled by a sudden
288
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of NO molecules
switch-off of this restriction. The resulting rotational distribution was found to be under certain conditions indeed Boltzmann-like, where T,,, has of course nothing in common with a thermal equilibrium. The only aspect of the gas-solid interaction which enters this analysis is the variation of the (static) potential with the angular orientation of the molecular axis. Since this model provides coupling to the excitations of the solid by means of a thermal bath, it fails to include details of the dynamics of energy exchange. In particular, for low surface temperatures, where only the lowest hindered rotor states are significantly occupied, the zero point energy is sufficiently high to produce rotational temperatures higher than the surface temperature. The experimental observation that T,,, becomes equal to T5 (but never exceeds T,)at low surfce temperatures points to the importance of the details of the energetic coupling between molecule and surface in the transition state. A complete quantum mechanical treatment of this problem is at present being developed by Doyen [38] with first encouraging results. This theory is based on the idea that the interaction dynamics will cause a deformation of the surface and thereby also a variation of the interaction potential, with the consequence that a phononic Franck-Condon factor is of importance for the energy exchange between the molecule and the solid. It is felt that experiments of the described kind will ultimately yield detailed information on the dynamics of gas-solid interactions which in turn are responsible for the kinetic parameters of surface reactions, for energy accommodation coefficients, etc. We are, however, only at the beginning of this new development, and much more experimental and theoretical work will be needed in the coming years.
Acknowledgements We are grateful to Dr. G. Doyen for clarifying discussions. Financial support was obtained from the Deutsche Forschungsgemeinschaft (SFB 128) and from the Alexander von Humboldt foundation which granted a fellowship for H.R.
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