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Measurements of residence times of gold atoms on alkali halide surfaces using time of flight techniques
Surface
Science
0 North-Holland
86 ( 1979) 36 ~4 I Publishing Compan)
MEASUREMENTS OF RESIDENCE TIMES OF GOLD ATOMS ON ALKALI HALIDE SURFACES USING TIME OF FLIGHT TECHNIQUES
Manuscript
received
in final form
1 November
1978
A molecular beam apparatus is described to measure the residence time of atoms on surfaces for the computation of the adsorption energy. For gold on sodium chloride and potassium bromide, respectively, the adsorption energies are estimated and compared with those calculated from nucleation experiments.
1. Introduction
The condensation of metals on alkali halide surfaces is commonly described by kinetic condensation theory [l]. Electron micrographs of thin island films [7,3] were analysed using the method of quantitative electronic image analysis [4]. in connection with AES analysis [5] of the films themselves. Such analysis leads to the conclusion that, during the initial stage of film growth, the main processes are the reevaporation of single adsorbed atoms and the fonnation of biatomic molecules which develop into stable clusters by further accumulation of surface diffusing atoms. Those atoms, which do not collide with existing aggregates or other single atoms during their residence time 7,, leave the surface. The time 71 is given by the Frenkel equation: r, = r,, exp(f:‘,/X-T) .
(1)
where 70 is the reciprocal Debye frequency and B1 the adsorption energy. This equation is valid provided the number of condensed atoms is low. In the case of a high condensation coefficient the residence time distribution will be cut off at times comparable to those needed for collisions with existing aggregates.
2. Experimental The experiments were carried out in a stainless steel, Viton sealed apparatus equipped with a turbomolecular pump and liquid N2 shielding (P 5 2 X 10F9 Torr). 36
M. Harsdorff. E.A. Knabbe /Measurements
of residence time of Au atoms
31
BLl, BL2 apertures; Fig. 1. Experimental arrangement: T vapour source; Ut , Uz shutters; Qul, Qu2 quadrupole mass filters; PO crystal oven; K crystal, S liquid Nz shielding; Ch beam chopper; A is axis of rotation for Qu2 and PO.
The experimental arrangement is shown in fig. 1. The gold beam is coming from the crucible T and is collimated by apertures BLl and BL2. The beam can be stopped by hand-driven shutters Ur and UZ or chopped by a motor driven rotating disk with two slits of 7” angular width. The target crystal K is placed in the crystal oven PO. Quadrupole mass filters Qul and Qu2 are equipped with cross beam ion sources. The signal of Qul is used for the regulation of the beam density [6], whilst Qu2 detects the reevaporated atoms. After amplification by a multiplier and application of counting techniques, the output signal of Qu2 is fed into a multichannel analyser. The heater PO and the quadrupole Qu2 are rotatable around the axis A. Not shown are a carbon sublimator for stabilizing the island films before breaking vacuum and the crystal cleaving device. The rotating chopper disk also intercepts a light beam to produce a reference signal which is used to synchronize the multichannel analyser with the rotation frequency of the chopper. The described technique has already been used by Joyce et al. [7].
3. Results The flight time of the atoms whilst passing from the chopper to the cross beam ionizer of Qu2 is the sum of the time of flight from the chopper to the surface, the residence time on the surface and the time of flight from the surface to the ionizer of Qu2. Since we deal with distributions of those times, the arrival time distribution at the ionizer is the convolution of the corresponding distributions and the chopper function describing the opening and closing of the chopper. So, in order to make
measurements of residence times, the flight time distributions of the direct beam and of the scattered beam must be known. The flight time distribution of the vapour beam was determined by mounting the quadrupole Qu3 in the direct beam at two different distances from the chopper. By recording the output signal of Qu:!for different rotation frequencies at different distances, and comparing this with theoretically computed spectra, it turned out that the flight time distribution of the direct beam is correctly described by deriving it from a MaxwellLBoltzmann energy distribution with the temperature of the beam source. The flight time distribution of the scattered beam was measured in a similar way by mounting Qu2 out of axis from the specimen. Within the intervals of confidence, the flight time distributions of reevaporated atoms for gold on NaU and KBIare well described by a Boltzmann energy distribution. These results were confirmed by angular distribution experiments. which show a cosine dependcncc ot the intensity of atoms scattered from the (100) surfaces of NaCl and KBr (see figs. 3a and ?b). This indicates complete thermal accommodation.
T=371 “C ND=4,9.10’2ctn-2
5-l
iAu/KBri T= 282°C ND= 5,1.10”
a-f2
5-l
b
Fig. 2. Angular distributions of the intensity of reevaporatrd gold atoms t’rom the I IOOJ wrfxxs of (a) NaCl and (b) KHr
M. Harsdor,fj”, EA.
Knabhe
/Measurements
of residence
Fig. 3. Comparison of computed (curves) and measured on (100) NaCl surfaces. Deposition rate ND = 5 X 10”
39
time of Au atoms
(points) time of flight spectra for gold cm-* xc-‘; gold source temperature
1459 K; crystal temperature 427 K.
For
the measurement
of residence
times,
with various beam densities
and different
spectra
spectra
with the theoretical
time
chopper
of flight
spectra
frequencies.
leads to the following
were recorded
Comparison
of these
conclusions:
I
10 Fig. 4. (‘omparison of computed for gold on (100) K3r surfaces. used in the computations.
100
50
(dotted curves) and measured NI) = 5
X
10”
cnC2
set
(points)
tixro-5,)
time of flight
spectra
-” , T= 475 K: ~1 = residence time
40
hf. ffarsdorfj:
B.A. Knabbe /Measurements
of‘residence
time
of Au
atoms
M. Harsdorjx
E.A. Knabbe /Measurements
of residence time ofAu
atoms
41
(1) At low chopper frequencies, no residence time could be detected at all for surface temperatures T in the ranges 340 < T < 690 K for sodium chloride, and 470 < T < 560 K for potassium bromide. (2) At high chopper frequencies (-100 set-‘) and low deposition rates (down to 5 X 10” set-’ cm-‘), no residence times could be detected on sodium chloride in the temperature range 333 to 644 K. From the comparisons shown in fig. 3 the possible residence time must be shorter than 2 X lo-’ sec. In the case of potassium bromide a residence time of about (5 -I 2) X 10m5 set was found as indicated in fig. 4. The cleavage plane was exposed to 2 X lo’* atoms cm-*.
4. Discussion Table 1 shows a comparison of theoretical and different experimental values of the adsorption of gold atoms on NaCl and KBr cleavage planes. The theoretical values in column I are in clear contradiction to those of the present experiments shown in column IV. This statement is also valid for the experimental values in column III for the substrate NaCl. The experimental values for KBr in column II are in agreement with the present experiments. The other values in columns II and III seem to be to high but not in clear contradiction to the present residence time experiments. It must be concluded that the evaluation of adsorption energies for the system Au-NdCl from nucl$ation experiments leads to high values compared with those computed from residence time experiments. Further accurate time of flight experiments are needed, which are capable of measuring shorter residence times.
References [l] [2] [3] [4] [5] [6] [7] [S]
G. Zinsmeister, Vacuum 16 (1966) 529. H. Schmeisser and M. Harsdorff, Z. Naturforsch. 25a (1970) 1896. V.N.E. Robinson and J.L. Robins, Thin Solid Films 20 (1974) 155. A. Puskeppel and M. Harsdorff, Thin Solid Films 35 (1976) 99. R. Anton, Thesis, Hamburg (1974). H. Schmeisser, Vakuum Tech. 21 (1972) 165. B.A. Joyce and C.T. Foxon, J. CrystalGrowth 31 (1975) 122. E.M. Chan, M.J. Buckingham and J.L. Robins, Surface Sci. 67 (1977)
285.
Science
0 North-Holland
86 ( 1979) 36 ~4 I Publishing Compan)
MEASUREMENTS OF RESIDENCE TIMES OF GOLD ATOMS ON ALKALI HALIDE SURFACES USING TIME OF FLIGHT TECHNIQUES
Manuscript
received
in final form
1 November
1978
A molecular beam apparatus is described to measure the residence time of atoms on surfaces for the computation of the adsorption energy. For gold on sodium chloride and potassium bromide, respectively, the adsorption energies are estimated and compared with those calculated from nucleation experiments.
1. Introduction
The condensation of metals on alkali halide surfaces is commonly described by kinetic condensation theory [l]. Electron micrographs of thin island films [7,3] were analysed using the method of quantitative electronic image analysis [4]. in connection with AES analysis [5] of the films themselves. Such analysis leads to the conclusion that, during the initial stage of film growth, the main processes are the reevaporation of single adsorbed atoms and the fonnation of biatomic molecules which develop into stable clusters by further accumulation of surface diffusing atoms. Those atoms, which do not collide with existing aggregates or other single atoms during their residence time 7,, leave the surface. The time 71 is given by the Frenkel equation: r, = r,, exp(f:‘,/X-T) .
(1)
where 70 is the reciprocal Debye frequency and B1 the adsorption energy. This equation is valid provided the number of condensed atoms is low. In the case of a high condensation coefficient the residence time distribution will be cut off at times comparable to those needed for collisions with existing aggregates.
2. Experimental The experiments were carried out in a stainless steel, Viton sealed apparatus equipped with a turbomolecular pump and liquid N2 shielding (P 5 2 X 10F9 Torr). 36
M. Harsdorff. E.A. Knabbe /Measurements
of residence time of Au atoms
31
BLl, BL2 apertures; Fig. 1. Experimental arrangement: T vapour source; Ut , Uz shutters; Qul, Qu2 quadrupole mass filters; PO crystal oven; K crystal, S liquid Nz shielding; Ch beam chopper; A is axis of rotation for Qu2 and PO.
The experimental arrangement is shown in fig. 1. The gold beam is coming from the crucible T and is collimated by apertures BLl and BL2. The beam can be stopped by hand-driven shutters Ur and UZ or chopped by a motor driven rotating disk with two slits of 7” angular width. The target crystal K is placed in the crystal oven PO. Quadrupole mass filters Qul and Qu2 are equipped with cross beam ion sources. The signal of Qul is used for the regulation of the beam density [6], whilst Qu2 detects the reevaporated atoms. After amplification by a multiplier and application of counting techniques, the output signal of Qu2 is fed into a multichannel analyser. The heater PO and the quadrupole Qu2 are rotatable around the axis A. Not shown are a carbon sublimator for stabilizing the island films before breaking vacuum and the crystal cleaving device. The rotating chopper disk also intercepts a light beam to produce a reference signal which is used to synchronize the multichannel analyser with the rotation frequency of the chopper. The described technique has already been used by Joyce et al. [7].
3. Results The flight time of the atoms whilst passing from the chopper to the cross beam ionizer of Qu2 is the sum of the time of flight from the chopper to the surface, the residence time on the surface and the time of flight from the surface to the ionizer of Qu2. Since we deal with distributions of those times, the arrival time distribution at the ionizer is the convolution of the corresponding distributions and the chopper function describing the opening and closing of the chopper. So, in order to make
measurements of residence times, the flight time distributions of the direct beam and of the scattered beam must be known. The flight time distribution of the vapour beam was determined by mounting the quadrupole Qu3 in the direct beam at two different distances from the chopper. By recording the output signal of Qu:!for different rotation frequencies at different distances, and comparing this with theoretically computed spectra, it turned out that the flight time distribution of the direct beam is correctly described by deriving it from a MaxwellLBoltzmann energy distribution with the temperature of the beam source. The flight time distribution of the scattered beam was measured in a similar way by mounting Qu2 out of axis from the specimen. Within the intervals of confidence, the flight time distributions of reevaporated atoms for gold on NaU and KBIare well described by a Boltzmann energy distribution. These results were confirmed by angular distribution experiments. which show a cosine dependcncc ot the intensity of atoms scattered from the (100) surfaces of NaCl and KBr (see figs. 3a and ?b). This indicates complete thermal accommodation.
T=371 “C ND=4,9.10’2ctn-2
5-l
iAu/KBri T= 282°C ND= 5,1.10”
a-f2
5-l
b
Fig. 2. Angular distributions of the intensity of reevaporatrd gold atoms t’rom the I IOOJ wrfxxs of (a) NaCl and (b) KHr
M. Harsdor,fj”, EA.
Knabhe
/Measurements
of residence
Fig. 3. Comparison of computed (curves) and measured on (100) NaCl surfaces. Deposition rate ND = 5 X 10”
39
time of Au atoms
(points) time of flight spectra for gold cm-* xc-‘; gold source temperature
1459 K; crystal temperature 427 K.
For
the measurement
of residence
times,
with various beam densities
and different
spectra
spectra
with the theoretical
time
chopper
of flight
spectra
frequencies.
leads to the following
were recorded
Comparison
of these
conclusions:
I
10 Fig. 4. (‘omparison of computed for gold on (100) K3r surfaces. used in the computations.
100
50
(dotted curves) and measured NI) = 5
X
10”
cnC2
set
(points)
tixro-5,)
time of flight
spectra
-” , T= 475 K: ~1 = residence time
40
hf. ffarsdorfj:
B.A. Knabbe /Measurements
of‘residence
time
of Au
atoms
M. Harsdorjx
E.A. Knabbe /Measurements
of residence time ofAu
atoms
41
(1) At low chopper frequencies, no residence time could be detected at all for surface temperatures T in the ranges 340 < T < 690 K for sodium chloride, and 470 < T < 560 K for potassium bromide. (2) At high chopper frequencies (-100 set-‘) and low deposition rates (down to 5 X 10” set-’ cm-‘), no residence times could be detected on sodium chloride in the temperature range 333 to 644 K. From the comparisons shown in fig. 3 the possible residence time must be shorter than 2 X lo-’ sec. In the case of potassium bromide a residence time of about (5 -I 2) X 10m5 set was found as indicated in fig. 4. The cleavage plane was exposed to 2 X lo’* atoms cm-*.
4. Discussion Table 1 shows a comparison of theoretical and different experimental values of the adsorption of gold atoms on NaCl and KBr cleavage planes. The theoretical values in column I are in clear contradiction to those of the present experiments shown in column IV. This statement is also valid for the experimental values in column III for the substrate NaCl. The experimental values for KBr in column II are in agreement with the present experiments. The other values in columns II and III seem to be to high but not in clear contradiction to the present residence time experiments. It must be concluded that the evaluation of adsorption energies for the system Au-NdCl from nucl$ation experiments leads to high values compared with those computed from residence time experiments. Further accurate time of flight experiments are needed, which are capable of measuring shorter residence times.
References [l] [2] [3] [4] [5] [6] [7] [S]
G. Zinsmeister, Vacuum 16 (1966) 529. H. Schmeisser and M. Harsdorff, Z. Naturforsch. 25a (1970) 1896. V.N.E. Robinson and J.L. Robins, Thin Solid Films 20 (1974) 155. A. Puskeppel and M. Harsdorff, Thin Solid Films 35 (1976) 99. R. Anton, Thesis, Hamburg (1974). H. Schmeisser, Vakuum Tech. 21 (1972) 165. B.A. Joyce and C.T. Foxon, J. CrystalGrowth 31 (1975) 122. E.M. Chan, M.J. Buckingham and J.L. Robins, Surface Sci. 67 (1977)
285.