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MgO(100)
irolume 113, number 3
CHEMICAL
L,ARCE ENERGY TRANSFER A STUDY
OF
PHYSICS LETTERS
IN HYPERTHERMAL
HEAVY-ATOM-SURFACE
18 January 1985
SCATTERING:
H&MgO~lOO)
E.KG~~~Y, A. AMlRAV L)eprtrhnent of ChemiwY. Tel-Aviv University. 69 9 78 Tef-Aviv, Ismel R.. ELBER and RB. GERBER Blitz Iiaber Centerfor Moleculllr Dynamics and Department of Physical CBemtity, l7:e Hebrew University of Jerusalem. 91904
Jeruwlem. Isael
Received 28 October 1984
Large energy transfer to the solId was observed for Hg scattering from single-crystat MgO(100) in the i-10 eV rage_ The final velocity distribution was narrow. The scattering angular dis~ution was narrow and slightly suprasp~cula~.Classical trajectory calculations using a model solid constructedout of a hundredlayersresuftedin a double maximumcornpresslontravclbngwave.This model reproduced the kge energy transfer and its dependenceon incident energy.
1. Introduction
2. Experimental
Atom-surface energy transfer has received considerdble attention in the last decade [l--4] _Most effort has been devoted to the dynamics of rare gas molecules colliding with a metal surface, where usually the metal atom is heavier than the rare gas atom [5.6]. Resently, helium scattenng from alkali hahdes [7 J and MgO [8J wasused to study the surface phonon dispersion curves. Jn this Letter ye report the experimental‘study and theoretical model calculation of high kinetic energy heavy atom or molecule scattering from an Mg0 surface where the projectiles are heavier than the individual surface atoms. This scattering results in a large energy transfer from the atomic kinetic energy into a travelling compression wave m the MgO. We have studied in detail mercury, molecular iodine and xenon scattered from single-crystal MgO(100). This Letter focuses on the results for Hg scattering from MgO. while ti study of Iz energy transfer will be discussed in a publication that also deals with the iodine dissociation [9] in collisions with the surface.
Hg atoms were seeded in a hydrogen supersonic beam and accelerated to a high kinetic energy in the range l-9 eV [9-l 11. A 30 m platmum nozzle was used, and the nozzle temperature was 1 LO--130eC, ccrrespozding to 0.3-l 2 Torr partial vapour pressure. The hydrogen backing pressure was scanned in the range of 500-l #IO0 Torr to control the mercury kinetic energy. Additional details on the aerodynamic acceleration are published elsewhere [ 111. The beam was then skimmed and collimated through two differentia: pzzging chambers into an ultra high vacuum (UHV) chamber (base pressure 2 X 1O-y To@_ The accelerated beam was either square wave modulated
for phase sensitive detection or rne~h~i~y chopped for kinetic energy me~remen~ using the tune-offlight (TOF) technique. In the UlW chamber the beam collided with a single crystal MgO(100) slab in its
available (Adolf-Meller) and X-ray analyzed. It was mounted on an UHV manipulator and annealed at about 900°C to givGhighly reproducible sharp specular and f-t-order diffraction peaks indicative of a clean and well ordered surface. (A detailed study of
303
Volume 113, number 3
18 January1985
CHEMICAL PHYSICS LETrERs
the energy dependence of helium and hydrogen diffraction from MgO will be published elsewhere [ 121.)
SPECULAR
HglMgO
Two quadrupole mass spectrometer (QMS) heads were used for detection. One was in line with the di-
rect molecular beam and served for kinetic energy measurements. The second was at 45” to the incoming beam direction and mounted 37 cm From the surface with a hollow liquid-nitrogen cold trap in front of it, served for angular resolution and scattered beam TOF experiments. The rods and ionizer of the QMS were in line with the molecular beam in order to avoid draw-out and TOF distortion problems found in the fly-through mode. Secondary scattering from the ionizer resulted in a weak long tail in the TOF spectra
ANUE
p
A
3
uo z
Gi z w Iz -
,
32 5 125 225 Beam; Surface Angle (deg.) 2.5
that did not effect our results This weak tail was further reduced by the use of external magnets. In the angular scattering experiment the surface was rotating
and the beam detector angle yyas fixed at 4S”. In the TOF experiment the beam-surface angle was about 21a to give maximum reflection intensity.
Time
of Fllghl (psec)
Fig 1. Mercury Kattering and vcIocity distribution from single-crystal MgO. (a) Scattered intensity versus beam-sur-
3. Results In fig. 1 we show both the angular distribution of scattered mercury from MgO (fig. la) and typical TOF spectra of both incoming and scattered mercury from MgO (fig. lb). As shown in fig. la the angular distribution of scattered Hg atoms is narrow (344 and centered in a supraspecular angle. In this case the incident kinetic energy is 3 eV, the incident beamsurface angle is 20.8” and the exit beam-surface angle is 24.2O. This supraspecular scatteringimplies anisotropy in the energy loss to the surface which is higher in its vertical component than in its parallel one. In fig. la the mercury kinetic energy is 3.0 eV and the energy loss to the surface is 1.05 eV. According to this a vertical energy loss should have resulted in ~20” incoming and e5” scattered atom peaks in the angular distribution. Since isotropic energy transfer (in which the vertical and parallel velocity components lose the Same Fraction of their initial magnitudes) should result in specular reflection, our scattering rep
resents an intermediate case. This behaviour was also observed in the past for high-energy argon scattering from silver [6]. The shape of the angular distribution is unchanged with increasing the kinetic energy
but the shift from the specular reflection is gradually 304
face angle. The beam-QMS detector angls is futed at 45”. Mercury is seeded in Hz at bad&g pressure of 700 Torr, resultjng in 3 eV kinetic energy. @) TOF spectxa of mercury seeded in hydrogen with 2 100 Torr nozzle backing prcsrure. TOF resolution is 20 ps, incident (inc.) and scattered(SIX) cncrgiesUe 5.6 and 3.3 eV. The beam-surface angle is 21’ at the peak of Ihe angular scattering The beam pathlength was tither 5 0 cm for the unscattered beam or 25 cm to the surfaoe and 37 cm from the surfa= to the QMS detector far the scattered beam.
decreased and at 8 eV kinetic energy the difference between the incoming and scattered angles is only l-5”, implying almost isotropic energy transfer. In fig. lb typical TOF spectra are shown. In this case the initial kinetic energy was 5.6 eV and the final kinetic energy was 3.3 eV resulting in 2.3 eV or 41% energy loss. Another interesting feature of the TOF spectra is the fmal velocity distribution which is very narrow with its Av/vbeing almost the same as that of the unscattered atoms (12.7% versus 11.5%) demonstrating the single-sollision condition without adsorption [3]. In fig. 2 several results of TOF spectra as in fig. lb are summarized and compared with classical trajectory calculations. The degree of energy transfer gradually increases from 28% at 2.2 eV to 44% at 8.7 eV. At all of OUTkinetic energy range the scattering
Volume 113, number 3
>
CHEMICAL PHYSICS LETl-EZRS
* Expertmenl
a5 z
Hg/MqO
0 Theory
- 2 WP
g4 -t -I I
.
I
I
0
\-
n
o-
+,
I
i
t
I
1
,
I
I
I
1
2
3
4
5
6
7
8
3
INITIAL
KINETIC
ENERGY
IO
teV)
Fig. 2. Mercury energy loss in hrgh kinetic energy scattering From MgO. (I), experimental results. (0). results of the trajectory calculations. Experimental conditions as in fw_ 1. but hydrogen backing pressure is adjusted in the range SOO14000 Torr to give the indicated energes Each point is taken from MO spectra as in fg_ lb. equal or comparable to that of the unscattered beam. Only at our low-energy limit was the velocity ~st~bution slightly larger in the scattered beam compared with the incident beam. In addition, we have checked the surface temperature effect and rt was small. Raising the surface temperature in steps from 275°C (chosen to yield a clean and unadsorbed surface as determined by hydrogen diffraction) to 800°C. resulted in only slightly increased energy transfer and velocity distribution width was supersonic with AV/V
4. Theory To provide a relatively simpIe model of the system, the fo~o~g co~de~tio~ were invoked. By the size of the’Hg atom and the geometry of the MgG(IO0) surface, it is estimated that the mercury strikes directly a surface area of about one unit cell, including 2 Mg and 2 0 atoms. The solid was modelled as consisting of vibrating units of the abovementioned group of atoms. Internal vibrations of each such group were ignored for simplicrty. Considering normal impact of the Hg atom on the surface, only the chain of “units” along the directron of impact was considered to move. Forces with neighboring chains were inchrded, but these units were kept static. To obtain the forces between the units, Mg-0; Mg-Mg and O-O mteractions were parameterized as Morse functions:
[--p(Rt,
18 January1985
-R,
-
%I)1 1 I
where R,b is the distance between atoms 4 and li. We chose 0 = 2.0 bolu-I, and adjusted B to yield the experimental Debye frequency_ R. is determined from crystal geometry. From the above atom-atom potentrals, the forces between the ~olyatomic units comprkkg the model solid were calculated_ The potential between Hg and the surface was taken as a sum of pair&se interactions between mercury and the surface atoms. Each such interaction was taken as a repulsive exponential VS(rS) = A exp(-or,)
,
where rS is the distance between Hg and the surface atom s. a, the steepness parameter of the interactron, was taken as the typical value of 0.8 bohr-l. The strength parameter A does not affect the energy-trausfer results. A detailed description of the model ~-LU be published elsewhere. The amount of energy transfer was obtained numerically by classical trajectory computatrons. We note that calculations were carried out for a whole range of potential parameters. The parameter values quoted gave the best fit with experiment. We stress that the quaiitative features of the excitation dynamics were the same for all potentials used. Given the crudeness of the model, the agreement with experiment (shown in fig. 2) was surprisingly good. The model reproduced farrly well not only the magnitude of the energy transfer, but also its variation with incidence energy. The velocity ~~bution of the scsttered atoms, not shown here, was also in agreement with experiment_ Assuming on this basis that the model is reliable at least on the qualitative level, the trajectories were analyzed for insight into the dynamics. An interestig excitation pattern was revealed. Upon initial impact, a large amount of energy is contained locally in a few edge atoms, and begins to travel as a pulse into the solid. This pulse remains localized over a few atoms (*7) only, and is therefore very anharmonic. As a consequence of the Hg impact, however, the solid atoms (unit@ were compressed and therefore a second energy pulse can be imparted to the sofid, which due to the compression is spread over many more atoms 305
Volume
113. number
3
CHEMICAL PHYSICS LETTERS
.
526 B s x 263
18 J-q
1985
cesses is discussed in ref. [9] _ In xenon about 37% energy transfer was observed at 5 eV and the angular d&r5bution was narrow and narrowed itself with increasing kinetic energy. We note that success of the theoretical model (using the same vibrational Hamiltonian as here) for 18jMg0, X&Kg0 was also very good, giving further support to the validity of the model.
Acknowledgement N Fig.3.Energy
and is thus appr~ately
harmonic. A doubly peaked energy @se thus propagates into the solid, and its qualitative shape is maintained over time scales much longer than that of the collision duration. We emphasize that the qualitative feature of double pulse excitation does not seem to depend on the details of the model used provided the collider mass exceeded that of Mg end of 0. Fig. 3 shlowsthe energy distribution in the solid, as a function of the distance from the edge, at a partkular time instant. We conjecture that the first, anhumanic pulse is soliton-like (pulse shape does not change with time as rt propagates and it is setitive to the anharmonicity). The excitation pattern should
depend on mass ratios of the gas and the solid atoms, and on the wbrational stiffness of the sohd.
This large enew-afer phenomenon was also studied in xenon and in molecular iodine scattered from MgO where the results are described in detail elsewhere [9] _ Briefly, Iz scattering was also accomparued by large energy transfer, over 60%, partially due to rotational energy transfer and mainly as m mercury due to energy transfer to the surface. The TOF width was slightly broader due to rotational excitation, but still supersonic. In addition to energy transfer, coIlisions of the I2 system involve also the channel of molecular dissociation. The relationbetween the two pro-
306
We thank C. Hoti+ for &iUful technical as&stance_ This work was supported by the Petroleum Reby the American Chemical search Fund a~ered Society and the Umted States-Israel Binational Science Foundation grant No. 3209 (to AA) and 3210 (to RBG). The &itz Haber Center is supported by the Minerva Gesellschaft Er die Forschung, Munich, FRG.
References [I] A G Stall., D-L. Smith and RP. MeniU,
J. Chem.. Phys 54 (1971) 163. [2] R Subbarao and D-R. NilIer, J. Chen Phys 58 (1973) 5247. [3] JE. Hurst. C9r. Bedcer. JP. COW&I, KC. Janda, L. Wharton and D-J. Auerbach, Phys. Rev. Letters 43 (1979) 1175. [4] K-C. Janda. J-E;. Hurst, CA_ Becker, JP. Cowin, DJ. Auerbach and L. Wharton, J. Chem_ Phys_ 72 Cl9801
2403. [S] D-R_Miller and RE. Subbarao, 3. Chem Phys 52 (1970) 425_ 161 MJ. Romney
(1969) 2490.
and JB_ Andersolu. J. Chem Phys 5 1
[7] RB. Doak andJP Tc+ennies, SurfaoeSd 117 U982j 1. [S] G. Brusdeylinq RB. Soak, J-G. Skofronick and J P_ Toennies, Surface Sci 128 (1983) 191. E Kolodney. A. Amhav, R. Elder and R B. Gerbex, to be published [IO] E. Kolodncw and A Amirw, J. Chem. Phyr 79 (1983) 464S. Ill] E- Kolodney and k Amizw. Chem. Phys. 82 (19831 269_ [ 121 E. K010dnq and A. Amirav, to be published. [9]
CHEMICAL
L,ARCE ENERGY TRANSFER A STUDY
OF
PHYSICS LETTERS
IN HYPERTHERMAL
HEAVY-ATOM-SURFACE
18 January 1985
SCATTERING:
H&MgO~lOO)
E.KG~~~Y, A. AMlRAV L)eprtrhnent of ChemiwY. Tel-Aviv University. 69 9 78 Tef-Aviv, Ismel R.. ELBER and RB. GERBER Blitz Iiaber Centerfor Moleculllr Dynamics and Department of Physical CBemtity, l7:e Hebrew University of Jerusalem. 91904
Jeruwlem. Isael
Received 28 October 1984
Large energy transfer to the solId was observed for Hg scattering from single-crystat MgO(100) in the i-10 eV rage_ The final velocity distribution was narrow. The scattering angular dis~ution was narrow and slightly suprasp~cula~.Classical trajectory calculations using a model solid constructedout of a hundredlayersresuftedin a double maximumcornpresslontravclbngwave.This model reproduced the kge energy transfer and its dependenceon incident energy.
1. Introduction
2. Experimental
Atom-surface energy transfer has received considerdble attention in the last decade [l--4] _Most effort has been devoted to the dynamics of rare gas molecules colliding with a metal surface, where usually the metal atom is heavier than the rare gas atom [5.6]. Resently, helium scattenng from alkali hahdes [7 J and MgO [8J wasused to study the surface phonon dispersion curves. Jn this Letter ye report the experimental‘study and theoretical model calculation of high kinetic energy heavy atom or molecule scattering from an Mg0 surface where the projectiles are heavier than the individual surface atoms. This scattering results in a large energy transfer from the atomic kinetic energy into a travelling compression wave m the MgO. We have studied in detail mercury, molecular iodine and xenon scattered from single-crystal MgO(100). This Letter focuses on the results for Hg scattering from MgO. while ti study of Iz energy transfer will be discussed in a publication that also deals with the iodine dissociation [9] in collisions with the surface.
Hg atoms were seeded in a hydrogen supersonic beam and accelerated to a high kinetic energy in the range l-9 eV [9-l 11. A 30 m platmum nozzle was used, and the nozzle temperature was 1 LO--130eC, ccrrespozding to 0.3-l 2 Torr partial vapour pressure. The hydrogen backing pressure was scanned in the range of 500-l #IO0 Torr to control the mercury kinetic energy. Additional details on the aerodynamic acceleration are published elsewhere [ 111. The beam was then skimmed and collimated through two differentia: pzzging chambers into an ultra high vacuum (UHV) chamber (base pressure 2 X 1O-y To@_ The accelerated beam was either square wave modulated
for phase sensitive detection or rne~h~i~y chopped for kinetic energy me~remen~ using the tune-offlight (TOF) technique. In the UlW chamber the beam collided with a single crystal MgO(100) slab in its
available (Adolf-Meller) and X-ray analyzed. It was mounted on an UHV manipulator and annealed at about 900°C to givGhighly reproducible sharp specular and f-t-order diffraction peaks indicative of a clean and well ordered surface. (A detailed study of
303
Volume 113, number 3
18 January1985
CHEMICAL PHYSICS LETrERs
the energy dependence of helium and hydrogen diffraction from MgO will be published elsewhere [ 121.)
SPECULAR
HglMgO
Two quadrupole mass spectrometer (QMS) heads were used for detection. One was in line with the di-
rect molecular beam and served for kinetic energy measurements. The second was at 45” to the incoming beam direction and mounted 37 cm From the surface with a hollow liquid-nitrogen cold trap in front of it, served for angular resolution and scattered beam TOF experiments. The rods and ionizer of the QMS were in line with the molecular beam in order to avoid draw-out and TOF distortion problems found in the fly-through mode. Secondary scattering from the ionizer resulted in a weak long tail in the TOF spectra
ANUE
p
A
3
uo z
Gi z w Iz -
,
32 5 125 225 Beam; Surface Angle (deg.) 2.5
that did not effect our results This weak tail was further reduced by the use of external magnets. In the angular scattering experiment the surface was rotating
and the beam detector angle yyas fixed at 4S”. In the TOF experiment the beam-surface angle was about 21a to give maximum reflection intensity.
Time
of Fllghl (psec)
Fig 1. Mercury Kattering and vcIocity distribution from single-crystal MgO. (a) Scattered intensity versus beam-sur-
3. Results In fig. 1 we show both the angular distribution of scattered mercury from MgO (fig. la) and typical TOF spectra of both incoming and scattered mercury from MgO (fig. lb). As shown in fig. la the angular distribution of scattered Hg atoms is narrow (344 and centered in a supraspecular angle. In this case the incident kinetic energy is 3 eV, the incident beamsurface angle is 20.8” and the exit beam-surface angle is 24.2O. This supraspecular scatteringimplies anisotropy in the energy loss to the surface which is higher in its vertical component than in its parallel one. In fig. la the mercury kinetic energy is 3.0 eV and the energy loss to the surface is 1.05 eV. According to this a vertical energy loss should have resulted in ~20” incoming and e5” scattered atom peaks in the angular distribution. Since isotropic energy transfer (in which the vertical and parallel velocity components lose the Same Fraction of their initial magnitudes) should result in specular reflection, our scattering rep
resents an intermediate case. This behaviour was also observed in the past for high-energy argon scattering from silver [6]. The shape of the angular distribution is unchanged with increasing the kinetic energy
but the shift from the specular reflection is gradually 304
face angle. The beam-QMS detector angls is futed at 45”. Mercury is seeded in Hz at bad&g pressure of 700 Torr, resultjng in 3 eV kinetic energy. @) TOF spectxa of mercury seeded in hydrogen with 2 100 Torr nozzle backing prcsrure. TOF resolution is 20 ps, incident (inc.) and scattered(SIX) cncrgiesUe 5.6 and 3.3 eV. The beam-surface angle is 21’ at the peak of Ihe angular scattering The beam pathlength was tither 5 0 cm for the unscattered beam or 25 cm to the surfaoe and 37 cm from the surfa= to the QMS detector far the scattered beam.
decreased and at 8 eV kinetic energy the difference between the incoming and scattered angles is only l-5”, implying almost isotropic energy transfer. In fig. lb typical TOF spectra are shown. In this case the initial kinetic energy was 5.6 eV and the final kinetic energy was 3.3 eV resulting in 2.3 eV or 41% energy loss. Another interesting feature of the TOF spectra is the fmal velocity distribution which is very narrow with its Av/vbeing almost the same as that of the unscattered atoms (12.7% versus 11.5%) demonstrating the single-sollision condition without adsorption [3]. In fig. 2 several results of TOF spectra as in fig. lb are summarized and compared with classical trajectory calculations. The degree of energy transfer gradually increases from 28% at 2.2 eV to 44% at 8.7 eV. At all of OUTkinetic energy range the scattering
Volume 113, number 3
>
CHEMICAL PHYSICS LETl-EZRS
* Expertmenl
a5 z
Hg/MqO
0 Theory
- 2 WP
g4 -t -I I
.
I
I
0
\-
n
o-
+,
I
i
t
I
1
,
I
I
I
1
2
3
4
5
6
7
8
3
INITIAL
KINETIC
ENERGY
IO
teV)
Fig. 2. Mercury energy loss in hrgh kinetic energy scattering From MgO. (I), experimental results. (0). results of the trajectory calculations. Experimental conditions as in fw_ 1. but hydrogen backing pressure is adjusted in the range SOO14000 Torr to give the indicated energes Each point is taken from MO spectra as in fg_ lb. equal or comparable to that of the unscattered beam. Only at our low-energy limit was the velocity ~st~bution slightly larger in the scattered beam compared with the incident beam. In addition, we have checked the surface temperature effect and rt was small. Raising the surface temperature in steps from 275°C (chosen to yield a clean and unadsorbed surface as determined by hydrogen diffraction) to 800°C. resulted in only slightly increased energy transfer and velocity distribution width was supersonic with AV/V
4. Theory To provide a relatively simpIe model of the system, the fo~o~g co~de~tio~ were invoked. By the size of the’Hg atom and the geometry of the MgG(IO0) surface, it is estimated that the mercury strikes directly a surface area of about one unit cell, including 2 Mg and 2 0 atoms. The solid was modelled as consisting of vibrating units of the abovementioned group of atoms. Internal vibrations of each such group were ignored for simplicrty. Considering normal impact of the Hg atom on the surface, only the chain of “units” along the directron of impact was considered to move. Forces with neighboring chains were inchrded, but these units were kept static. To obtain the forces between the units, Mg-0; Mg-Mg and O-O mteractions were parameterized as Morse functions:
[--p(Rt,
18 January1985
-R,
-
%I)1 1 I
where R,b is the distance between atoms 4 and li. We chose 0 = 2.0 bolu-I, and adjusted B to yield the experimental Debye frequency_ R. is determined from crystal geometry. From the above atom-atom potentrals, the forces between the ~olyatomic units comprkkg the model solid were calculated_ The potential between Hg and the surface was taken as a sum of pair&se interactions between mercury and the surface atoms. Each such interaction was taken as a repulsive exponential VS(rS) = A exp(-or,)
,
where rS is the distance between Hg and the surface atom s. a, the steepness parameter of the interactron, was taken as the typical value of 0.8 bohr-l. The strength parameter A does not affect the energy-trausfer results. A detailed description of the model ~-LU be published elsewhere. The amount of energy transfer was obtained numerically by classical trajectory computatrons. We note that calculations were carried out for a whole range of potential parameters. The parameter values quoted gave the best fit with experiment. We stress that the quaiitative features of the excitation dynamics were the same for all potentials used. Given the crudeness of the model, the agreement with experiment (shown in fig. 2) was surprisingly good. The model reproduced farrly well not only the magnitude of the energy transfer, but also its variation with incidence energy. The velocity ~~bution of the scsttered atoms, not shown here, was also in agreement with experiment_ Assuming on this basis that the model is reliable at least on the qualitative level, the trajectories were analyzed for insight into the dynamics. An interestig excitation pattern was revealed. Upon initial impact, a large amount of energy is contained locally in a few edge atoms, and begins to travel as a pulse into the solid. This pulse remains localized over a few atoms (*7) only, and is therefore very anharmonic. As a consequence of the Hg impact, however, the solid atoms (unit@ were compressed and therefore a second energy pulse can be imparted to the sofid, which due to the compression is spread over many more atoms 305
Volume
113. number
3
CHEMICAL PHYSICS LETTERS
.
526 B s x 263
18 J-q
1985
cesses is discussed in ref. [9] _ In xenon about 37% energy transfer was observed at 5 eV and the angular d&r5bution was narrow and narrowed itself with increasing kinetic energy. We note that success of the theoretical model (using the same vibrational Hamiltonian as here) for 18jMg0, X&Kg0 was also very good, giving further support to the validity of the model.
Acknowledgement N Fig.3.Energy
and is thus appr~ately
harmonic. A doubly peaked energy @se thus propagates into the solid, and its qualitative shape is maintained over time scales much longer than that of the collision duration. We emphasize that the qualitative feature of double pulse excitation does not seem to depend on the details of the model used provided the collider mass exceeded that of Mg end of 0. Fig. 3 shlowsthe energy distribution in the solid, as a function of the distance from the edge, at a partkular time instant. We conjecture that the first, anhumanic pulse is soliton-like (pulse shape does not change with time as rt propagates and it is setitive to the anharmonicity). The excitation pattern should
depend on mass ratios of the gas and the solid atoms, and on the wbrational stiffness of the sohd.
This large enew-afer phenomenon was also studied in xenon and in molecular iodine scattered from MgO where the results are described in detail elsewhere [9] _ Briefly, Iz scattering was also accomparued by large energy transfer, over 60%, partially due to rotational energy transfer and mainly as m mercury due to energy transfer to the surface. The TOF width was slightly broader due to rotational excitation, but still supersonic. In addition to energy transfer, coIlisions of the I2 system involve also the channel of molecular dissociation. The relationbetween the two pro-
306
We thank C. Hoti+ for &iUful technical as&stance_ This work was supported by the Petroleum Reby the American Chemical search Fund a~ered Society and the Umted States-Israel Binational Science Foundation grant No. 3209 (to AA) and 3210 (to RBG). The &itz Haber Center is supported by the Minerva Gesellschaft Er die Forschung, Munich, FRG.
References [I] A G Stall., D-L. Smith and RP. MeniU,
J. Chem.. Phys 54 (1971) 163. [2] R Subbarao and D-R. NilIer, J. Chen Phys 58 (1973) 5247. [3] JE. Hurst. C9r. Bedcer. JP. COW&I, KC. Janda, L. Wharton and D-J. Auerbach, Phys. Rev. Letters 43 (1979) 1175. [4] K-C. Janda. J-E;. Hurst, CA_ Becker, JP. Cowin, DJ. Auerbach and L. Wharton, J. Chem_ Phys_ 72 Cl9801
2403. [S] D-R_Miller and RE. Subbarao, 3. Chem Phys 52 (1970) 425_ 161 MJ. Romney
(1969) 2490.
and JB_ Andersolu. J. Chem Phys 5 1
[7] RB. Doak andJP Tc+ennies, SurfaoeSd 117 U982j 1. [S] G. Brusdeylinq RB. Soak, J-G. Skofronick and J P_ Toennies, Surface Sci 128 (1983) 191. E Kolodney. A. Amhav, R. Elder and R B. Gerbex, to be published [IO] E. Kolodncw and A Amirw, J. Chem. Phyr 79 (1983) 464S. Ill] E- Kolodney and k Amizw. Chem. Phys. 82 (19831 269_ [ 121 E. K010dnq and A. Amirav, to be published. [9]