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Photoelectron emission from small particles suspended in a gas
Surface Science 106 (1981) 538-543 North-Holland Publishing Company
PHOTOELECTRON EMISSION SUSPENDED IN A GAS A. SCHMIDT-OTT*
FROM SMALL PARTICLES
and B. FEDERER
Atmospheric Physics ETH, CH-8093 Ziirich, Switzerland Received 8 September 1980; accepted for publication 11 November 1980
Photoelectron emission from small particles (d - 100 A) suspended in air and other gases is measured for the first time. The electrons photoemitted from the particles lead to an aerosol photoconductivity I which is measured by means of an AC bridge circuit. S(t) is governed by the photoemission rate, the rate of attachment of electrons (or negative ions in electronegative gases) to particles and the diffusion loss of ions and particles. It is related to the particles’ size, concentration and photoelectric yield Y (electrons emitted per incident photon). Y(hv) is determined near the photothreshold 4 (4 < hv < q5+ 1.5 eV) for particles of Ag, Au and various metaloxides in an Nz-02 mixture. It is demonstrated that Y is abnormally high (e.g. a factor of 10’ for Ag!) compared to photo yields of large surfaces in ambient gases. The sensitivity of Y(hv) to adsorbates on the particle surface is demonstrated for Hz0 adsorption on W oxide particles. Due to the enhanced photoelectric yield, photoelectric charging is very efficient for small particles. The electrical mobility spectrum of aerosol particles photoelectrically charged to a Coulomb limit is related to their size distribution and their photothresholds. Such measurements are performed with atmospheric aerosols and compared with size distributions obtained with the commercially available Withby electrostatic aerosol analyzer based on charging by small ion attachment and mobility analysis. Implications of the high photo yield on atmospheric processes are discussed.
1. Introduction Photoelectron emission is a well-known technique in investigating the basic properties of condensed matter. Here, a method has been developed to observe for the first time photoemission from small particles. The main difficulties consist in the suspension of particles of sufficient concentration and the measurement of extremely small amounts of electric charge formed by the photoelectrons, even if the particles are exposed to the light of the most intensive sources of monochromatic UV light available. Particle suspension in a gas showed to be an elegant solution since the photoemitted electrons diffuse in the gas, being available for conductivity measurements for a long time. Furthermore, in a gas particles can simply be generated by evaporation. The performed measurements of aerosol photoconductivity enabled a determination of the photoelectric quantum yield Y&v), i.e. the escape probability of an electron when a photon is incident on the particle cross section. In addition to that, the l
Former affiliation: Solid-State Physics ETH, Ziirich, Switzerland.
0039-6028/81/00004000/$02.50
@ North-Holland
Publishing Company
A. Schmidf-Off,B. Federer/ Photoelectron emission
539
measurements led to a size determination, even for ultrafine particles (R -20 A) in monodisperse aerosols. Apart from the basic interest, photoemission from small particles can be used in aerosol analysis or for the observation of reactions of particles with gases. It is applicable to the atmospheric aerosol. The photothreshold is very sensitive to surface adsorbates. Therefore, adsorption of atoms or molecules to a particle in a catalytic process or water nucleation is well observable in a photoconductivity measurement. From the mobility spectrum of a photoelectrically charged polydisperse aerosol, the distribution of photothresholds with respect to particle size can be determined, when the size spectrum is known. 2.1. Theory of aerosol phomconductivity When photoionizing a particle, the intrinsic photothreshold 4i, i.e. the energy an electron requires to leave the particle surface is enhanced by the Coulomb force of the positively charged particle. This means, the electron is recaptured if +i < hv < 4, where
R is the particle radius and p the number of elementary charges on the particle before electron emission [12]. For a given photon energy hv, the maximum number of charges a particle can acquire in photoionisation is therefore Pmax= (hi - bi) R 4m&o/e2.
(2)
The Fowler-Nordheim law [l] Y(hv) = C(hV- d? )
(3)
as valid for photoemission near threshold for macroscopic metal surfaces, was found to apply for all investigated particles (see section 2.3). In the photoconductivity measurements performed, the particle radii and hv were chosen such that double particle ionisation could be excluded or neglected using eqs. (2) and (3). The aerosol photoconductivity can then be described by the following equations comprising electron emission (= small ion production), attachment of small ions to particles and diffusion of ions and particles out of the zone under investigation: Z = enb, t eZ+b, , dnldt = japp dZ”/dt = jay-Z-
t juY_Z-
(4) - nq”ZO- nq+Z+ - an ,
t nq+Z+ - juY”p
- nq”ZO- &F’ ,
(5) (6)
and analogously dZ+ldt = . . . . ,
(7)
dZ-ldt = . . . . ,
(8)
z=z+tz-tzo.
(9)
A. Schmidt-Ott,B. Federer 1 Photoelectionemission
540
The superscripts signify the particle charge and the symbols denote the following: 2: aerosol photoconductivity, n: photoelectron concentration, 2: particle concentration, j: photon flux, b, and b,: electrical mobility of negative small ions and particles, respectively, u: geometrical particle cross section (= rR*), 7: coefficient for attachment of small ions to particles, ps and &: diffusion loss constants of small ions and particles, respectively. In the experiments to be presented, the particle size and the experimental conditions were chosen such that either all attachment losses (n) or all diffusion losses (& &,) were negligible. When attachment dominates the photoconductivity saturates when the electron emission rate equals the attachment rate (fig. 1). The photoconductivity in saturation is then proportional to the photoyield and independent of the particle concentration: Z @Z uYlq .
(10)
For details and the determination of r] see ref. [2]. When diffusion dominates, the time dependence shows a behaviour as in fig. 2. The evaluation is most simple if the light is removed when Z is highest. From eqs. (4)-(9) follows [3]: ay
=
J_ h
ln
((dC(LJldt)Ij=o
(11)
dX(O)/dt > ’
also allowing a yield determination independendent of the concentration. u can be obtained through the decay of photoconductivity after the light is removed (t > t,,,). The conductivity decays according to the diffusion losses of small ions and particles. X(t) is then a superpositionof two exponential decays with the decay rates & and /3, that can well be determined. The equation &lb, = &I&
(12)
is valid due to the proportionality of electrical mobility and diffusion coefficient and leads to b,, since 6, is known (for air see ref. [4]). The Stokes-Cunningham equation [5] gives the connection between b, and the particle size. 2.2. The measurement of aerosol photoconductivity The conductivity is measured by means of an alternating electric field (f = 100 Hz) in
Fig. 1. Time dependence of aerosol photoconductivity removed at t,,,.
I: when attachment is dominant. Light is applied at t = 0 and
Fig. 2. Time dependence of aerosol photoconductivity S when diffusion is dominant. Light is applied at t = 0 and removed at t,,,.Dashed line: S(t),as expected for continuous illumination.
A. Schmidt-Ott. B. Federer / Photoelectionemission
541
order to avoid a drift of charge carriers to the electrodes. In a bridge circuit a phase sensitive amplifier detects the current component in phase with the field [2]. For the measurements on particles with R B 50 A the aerosol is confined in a cylindrical chamber with parallel bars as electrodes [2]. A cylindrical region between the bars is exposed to the light. The distance between the electrodes is large enough to assure a negligible diffusion loss of ions. This way, eq. (10) was applicable for the Y determination. Measurements on particles with R d 50 8, are possible only with the diffusion loss method (eq. (11)) because the attachment coefficients are very small. The aerosol is inserted in a cylindrical condenser and the whole space between the electrodes is illuminated [3]. 2.3. Experimental results and implications All experiments were preformed at normal pressure and room temperature with a 4: 1 N&, mixture. The particles were prepared by evaporation which led to narrow size distributions with R d 50 A. This was verified with electron micrographs. The size could be varied with the evaporation temperature. The Fowler-Nordheim equation was verified for (hv - 4) < 1.5 eV with various particle sizes (20 < R < 100 A) and the substances Ag, Au, Moo3 and W03. A drastic enhancement of the slope of Y(hv) near threshold was observed for all aerosols that allowed an absolute yield determination with (10) or (ll), i.e. with particles small enough to fulfil the assumption of single charge (R d 100A). W03 particles (R = 50 A) were prepared by heating an oxidized W wire in a NT02 mixture. The size was obtained by electron microscopy. Y(hv) was determined with (10) and compared with the photoyield of the wire surface in the same atmosphere [6]. An enhancement of c (eq. (3)) of CparticlelCwire > 5 x 10’ resulted. Table 1 shows the results obtained for an aerosol of ultrafine Ag particles by the diffusion loss method (eq. (11)). The particles were produced by heating a Ag wire in Nz. Their size was determined with eq. (12). One observes Cparticle/C
wire z
10’
Table 1 Radius R, photoelectric
Particle
Macroscopic surface
threshold 4 and photoemission
constant c (eq. 3) of Ag particles and macroscopic surface
R (A)
0 (ev)
c
30 21 20
4.55 4.57 4.65
1.8 x 1o-3 3.9 x 1o-3 8.9 x 10-S
m
4.90
7.3 x 1o-5
542
A. Schmidt-Ott,B. Federer
i Photoelectronemission
for R = 20 A and a strong decrease of c with increasing R. The increasing 4 for decreasing R can be ascribed to the Coulomb enhancement (eq. (1)). The wire’s photothreshold is higher than the particle threshold. This can be understood when considering that di is very sensitive to surface crystal orientations and adsorbates causing a dipole layer which may very well be different for particle and wire. The yield enhancement, however, cannot be explained by this phenomenon. The influence of adsorbed molecules on particles was demonstrated qualitatively by adding small amounts of HZ0 vapor to the aerosol. A threshold increase for Ag and a decrease for W03 both in the order of 0.1 eV resulted while c remained constant. An explanation of the high y cannot be found in the different electronic density of states in a small particle, since significant changes from the bulk do not occur above sizes corresponding to a few hundred atoms [7]. In attempting to understand the yield enhancement for small particles it is considered that a surface curvature results in a larger phase space for the escape of an excited electron. A calculation based on a homogeneous probability of electron excitation in the volume of a sphere and an inelastic collision attenuation of the electron in the lattice according to exp(-x/h,) leads to an enhancement factor for electron emission of less than 4 [8]. Evidently, this does not account for the enhancement observed. An explanation must therefore be searched for in connection with the adsorption process of the photon. The uncertainty field of the photon (A = 2300 A) is much larger than the particle dimension. It can therefore be regarded as “incident” on the particle (see yield definition) within an area larger than the particle’s geometrical cross section. Furthermore, the plasmon frequency and optical properties are known to be different from the bulk in a finite medium [9]. Implications of the yield enhancement must be considered in connection with photoemission from interstellar grains [lo]. Furthermore photoelectric charging of low-threshold particles in the atmosphere may result in an enhanced catalytic activity. Studies on the mechanisms of photochemical gas-to-particle reactions (for instance smog production) have, until now, not considered photoelectric charging of particles.
3. Analysis of the atmosphericaerosol Photoelectric charging has shown to be an efficient charging mechanism for some atmospheric aerosols due to the high photoelectric yield. In the commercially available Withby Electrostatic Aerosol Analyzer [ll] the aerosol particles are charged by small ion attachment (diffusion charging) and the resulting charge and size dependent mobility b1 is a monotonous function the size. The mobility distribution determined in the EAA by electrostatic precipitation therefore yields the size distribution. In a second run the same aerosol is photoelectrically charged to the Coulomb limit (eq. (2)) by a strong UV lamp and the mobility distribution is recorded with the same EAA mobility analyzer. The deviation from the first mobility distribution leads to the charge p2 acquired by photoelectric charging for each particle size. Since the photothreshold bi is connected with p2 through eq. (2) the distribution 4i(R) can be determined. This procedure is based on the assumptions that the
543
A. Schmidt-Ott, B. Federer / Photoelectronemission
,,Q ihv-@)leVl .I2
.
.08 .OL’ 0
0
.OL
.08
.I2
.I6
l
RLum]
I
OO
.08
.I6
.21
. .32 Rlum]
Fig 3. Size dependence of (hv - &) for particles of WOs. Fig. 4. Particle size dependence of (hv - $J in a garage aerosol.
particles of a certain size basically have one photothreshold and the mobility of the photoelectrically charged particles is a monotonous function of the size. For the aerosols investigated so far, the assumptions were well fulfilled. This could be concluded from the fact that the two mobility distributions had very similar shapes. The figs. 3 and 4 show (hv - di) as a function of R obtained for a chemically homogeneous W03 aerosol and for a garage aerosol, respectively. We are very much indebted to H.C. Siegmann for helpful discussions and for placing experimental equipment at our disposal. We also thank P.M. Marcus for communicating his calculations of the escape enhancement of photoelectrons from a small sphere and J.P. Kopp for valuable contributions in photoelectric charging of atmospheric aerosols.
References (11 M. Cardona and L. Ley, in: Topics in Applied Physics, Vol. 26 (Springer, Berlin, 1978). [2] A. Schmidt-Ott and H.C. Siegmann, Appl. Phys. Letters 32 (1978) 710. [3] A. Schmidt-Ott, P. Schurtenberger and H.C. Siegmann, Phys. Rev. Letters 4.5 (1980) 1284. [4] H. Riekert, Thesis, Univ. Konstanz (1971) unpublished. [5] R.A. Gussman, J. Appl. Met. 8 (1969) 999. [6] A. Schmidt-Ott, Thesis, ETH Ziirich, No. 6438, unpublished. [7] R.C. Baetzold, M.G. Mason and J.F. Hamilton, J. Chem. Phys. 72 (1980) 366. [8] P.M. Marcus, private communication. (91 C.J. Duthler, S.E. Johnson and H.P. Broida, Phys. Rev. Letters 26 (1971) 1236. [lo] W.D. Watson, Astrophys. J. 176 (1972) 103. [ll] Thermal Systems Inc., Model 3030 EAA, 2500 North Cleveland Ave., St. Paul, Minnesota, USA. [12] A more precise deviation of b(R) including the image force modification due to surface curvature has been carried out by D.M. Wood, Phys. Rev. Letters 46 (1981) 749.
PHOTOELECTRON EMISSION SUSPENDED IN A GAS A. SCHMIDT-OTT*
FROM SMALL PARTICLES
and B. FEDERER
Atmospheric Physics ETH, CH-8093 Ziirich, Switzerland Received 8 September 1980; accepted for publication 11 November 1980
Photoelectron emission from small particles (d - 100 A) suspended in air and other gases is measured for the first time. The electrons photoemitted from the particles lead to an aerosol photoconductivity I which is measured by means of an AC bridge circuit. S(t) is governed by the photoemission rate, the rate of attachment of electrons (or negative ions in electronegative gases) to particles and the diffusion loss of ions and particles. It is related to the particles’ size, concentration and photoelectric yield Y (electrons emitted per incident photon). Y(hv) is determined near the photothreshold 4 (4 < hv < q5+ 1.5 eV) for particles of Ag, Au and various metaloxides in an Nz-02 mixture. It is demonstrated that Y is abnormally high (e.g. a factor of 10’ for Ag!) compared to photo yields of large surfaces in ambient gases. The sensitivity of Y(hv) to adsorbates on the particle surface is demonstrated for Hz0 adsorption on W oxide particles. Due to the enhanced photoelectric yield, photoelectric charging is very efficient for small particles. The electrical mobility spectrum of aerosol particles photoelectrically charged to a Coulomb limit is related to their size distribution and their photothresholds. Such measurements are performed with atmospheric aerosols and compared with size distributions obtained with the commercially available Withby electrostatic aerosol analyzer based on charging by small ion attachment and mobility analysis. Implications of the high photo yield on atmospheric processes are discussed.
1. Introduction Photoelectron emission is a well-known technique in investigating the basic properties of condensed matter. Here, a method has been developed to observe for the first time photoemission from small particles. The main difficulties consist in the suspension of particles of sufficient concentration and the measurement of extremely small amounts of electric charge formed by the photoelectrons, even if the particles are exposed to the light of the most intensive sources of monochromatic UV light available. Particle suspension in a gas showed to be an elegant solution since the photoemitted electrons diffuse in the gas, being available for conductivity measurements for a long time. Furthermore, in a gas particles can simply be generated by evaporation. The performed measurements of aerosol photoconductivity enabled a determination of the photoelectric quantum yield Y&v), i.e. the escape probability of an electron when a photon is incident on the particle cross section. In addition to that, the l
Former affiliation: Solid-State Physics ETH, Ziirich, Switzerland.
0039-6028/81/00004000/$02.50
@ North-Holland
Publishing Company
A. Schmidf-Off,B. Federer/ Photoelectron emission
539
measurements led to a size determination, even for ultrafine particles (R -20 A) in monodisperse aerosols. Apart from the basic interest, photoemission from small particles can be used in aerosol analysis or for the observation of reactions of particles with gases. It is applicable to the atmospheric aerosol. The photothreshold is very sensitive to surface adsorbates. Therefore, adsorption of atoms or molecules to a particle in a catalytic process or water nucleation is well observable in a photoconductivity measurement. From the mobility spectrum of a photoelectrically charged polydisperse aerosol, the distribution of photothresholds with respect to particle size can be determined, when the size spectrum is known. 2.1. Theory of aerosol phomconductivity When photoionizing a particle, the intrinsic photothreshold 4i, i.e. the energy an electron requires to leave the particle surface is enhanced by the Coulomb force of the positively charged particle. This means, the electron is recaptured if +i < hv < 4, where
R is the particle radius and p the number of elementary charges on the particle before electron emission [12]. For a given photon energy hv, the maximum number of charges a particle can acquire in photoionisation is therefore Pmax= (hi - bi) R 4m&o/e2.
(2)
The Fowler-Nordheim law [l] Y(hv) = C(hV- d? )
(3)
as valid for photoemission near threshold for macroscopic metal surfaces, was found to apply for all investigated particles (see section 2.3). In the photoconductivity measurements performed, the particle radii and hv were chosen such that double particle ionisation could be excluded or neglected using eqs. (2) and (3). The aerosol photoconductivity can then be described by the following equations comprising electron emission (= small ion production), attachment of small ions to particles and diffusion of ions and particles out of the zone under investigation: Z = enb, t eZ+b, , dnldt = japp dZ”/dt = jay-Z-
t juY_Z-
(4) - nq”ZO- nq+Z+ - an ,
t nq+Z+ - juY”p
- nq”ZO- &F’ ,
(5) (6)
and analogously dZ+ldt = . . . . ,
(7)
dZ-ldt = . . . . ,
(8)
z=z+tz-tzo.
(9)
A. Schmidt-Ott,B. Federer 1 Photoelectionemission
540
The superscripts signify the particle charge and the symbols denote the following: 2: aerosol photoconductivity, n: photoelectron concentration, 2: particle concentration, j: photon flux, b, and b,: electrical mobility of negative small ions and particles, respectively, u: geometrical particle cross section (= rR*), 7: coefficient for attachment of small ions to particles, ps and &: diffusion loss constants of small ions and particles, respectively. In the experiments to be presented, the particle size and the experimental conditions were chosen such that either all attachment losses (n) or all diffusion losses (& &,) were negligible. When attachment dominates the photoconductivity saturates when the electron emission rate equals the attachment rate (fig. 1). The photoconductivity in saturation is then proportional to the photoyield and independent of the particle concentration: Z @Z uYlq .
(10)
For details and the determination of r] see ref. [2]. When diffusion dominates, the time dependence shows a behaviour as in fig. 2. The evaluation is most simple if the light is removed when Z is highest. From eqs. (4)-(9) follows [3]: ay
=
J_ h
ln
((dC(LJldt)Ij=o
(11)
dX(O)/dt > ’
also allowing a yield determination independendent of the concentration. u can be obtained through the decay of photoconductivity after the light is removed (t > t,,,). The conductivity decays according to the diffusion losses of small ions and particles. X(t) is then a superpositionof two exponential decays with the decay rates & and /3, that can well be determined. The equation &lb, = &I&
(12)
is valid due to the proportionality of electrical mobility and diffusion coefficient and leads to b,, since 6, is known (for air see ref. [4]). The Stokes-Cunningham equation [5] gives the connection between b, and the particle size. 2.2. The measurement of aerosol photoconductivity The conductivity is measured by means of an alternating electric field (f = 100 Hz) in
Fig. 1. Time dependence of aerosol photoconductivity removed at t,,,.
I: when attachment is dominant. Light is applied at t = 0 and
Fig. 2. Time dependence of aerosol photoconductivity S when diffusion is dominant. Light is applied at t = 0 and removed at t,,,.Dashed line: S(t),as expected for continuous illumination.
A. Schmidt-Ott. B. Federer / Photoelectionemission
541
order to avoid a drift of charge carriers to the electrodes. In a bridge circuit a phase sensitive amplifier detects the current component in phase with the field [2]. For the measurements on particles with R B 50 A the aerosol is confined in a cylindrical chamber with parallel bars as electrodes [2]. A cylindrical region between the bars is exposed to the light. The distance between the electrodes is large enough to assure a negligible diffusion loss of ions. This way, eq. (10) was applicable for the Y determination. Measurements on particles with R d 50 8, are possible only with the diffusion loss method (eq. (11)) because the attachment coefficients are very small. The aerosol is inserted in a cylindrical condenser and the whole space between the electrodes is illuminated [3]. 2.3. Experimental results and implications All experiments were preformed at normal pressure and room temperature with a 4: 1 N&, mixture. The particles were prepared by evaporation which led to narrow size distributions with R d 50 A. This was verified with electron micrographs. The size could be varied with the evaporation temperature. The Fowler-Nordheim equation was verified for (hv - 4) < 1.5 eV with various particle sizes (20 < R < 100 A) and the substances Ag, Au, Moo3 and W03. A drastic enhancement of the slope of Y(hv) near threshold was observed for all aerosols that allowed an absolute yield determination with (10) or (ll), i.e. with particles small enough to fulfil the assumption of single charge (R d 100A). W03 particles (R = 50 A) were prepared by heating an oxidized W wire in a NT02 mixture. The size was obtained by electron microscopy. Y(hv) was determined with (10) and compared with the photoyield of the wire surface in the same atmosphere [6]. An enhancement of c (eq. (3)) of CparticlelCwire > 5 x 10’ resulted. Table 1 shows the results obtained for an aerosol of ultrafine Ag particles by the diffusion loss method (eq. (11)). The particles were produced by heating a Ag wire in Nz. Their size was determined with eq. (12). One observes Cparticle/C
wire z
10’
Table 1 Radius R, photoelectric
Particle
Macroscopic surface
threshold 4 and photoemission
constant c (eq. 3) of Ag particles and macroscopic surface
R (A)
0 (ev)
c
30 21 20
4.55 4.57 4.65
1.8 x 1o-3 3.9 x 1o-3 8.9 x 10-S
m
4.90
7.3 x 1o-5
542
A. Schmidt-Ott,B. Federer
i Photoelectronemission
for R = 20 A and a strong decrease of c with increasing R. The increasing 4 for decreasing R can be ascribed to the Coulomb enhancement (eq. (1)). The wire’s photothreshold is higher than the particle threshold. This can be understood when considering that di is very sensitive to surface crystal orientations and adsorbates causing a dipole layer which may very well be different for particle and wire. The yield enhancement, however, cannot be explained by this phenomenon. The influence of adsorbed molecules on particles was demonstrated qualitatively by adding small amounts of HZ0 vapor to the aerosol. A threshold increase for Ag and a decrease for W03 both in the order of 0.1 eV resulted while c remained constant. An explanation of the high y cannot be found in the different electronic density of states in a small particle, since significant changes from the bulk do not occur above sizes corresponding to a few hundred atoms [7]. In attempting to understand the yield enhancement for small particles it is considered that a surface curvature results in a larger phase space for the escape of an excited electron. A calculation based on a homogeneous probability of electron excitation in the volume of a sphere and an inelastic collision attenuation of the electron in the lattice according to exp(-x/h,) leads to an enhancement factor for electron emission of less than 4 [8]. Evidently, this does not account for the enhancement observed. An explanation must therefore be searched for in connection with the adsorption process of the photon. The uncertainty field of the photon (A = 2300 A) is much larger than the particle dimension. It can therefore be regarded as “incident” on the particle (see yield definition) within an area larger than the particle’s geometrical cross section. Furthermore, the plasmon frequency and optical properties are known to be different from the bulk in a finite medium [9]. Implications of the yield enhancement must be considered in connection with photoemission from interstellar grains [lo]. Furthermore photoelectric charging of low-threshold particles in the atmosphere may result in an enhanced catalytic activity. Studies on the mechanisms of photochemical gas-to-particle reactions (for instance smog production) have, until now, not considered photoelectric charging of particles.
3. Analysis of the atmosphericaerosol Photoelectric charging has shown to be an efficient charging mechanism for some atmospheric aerosols due to the high photoelectric yield. In the commercially available Withby Electrostatic Aerosol Analyzer [ll] the aerosol particles are charged by small ion attachment (diffusion charging) and the resulting charge and size dependent mobility b1 is a monotonous function the size. The mobility distribution determined in the EAA by electrostatic precipitation therefore yields the size distribution. In a second run the same aerosol is photoelectrically charged to the Coulomb limit (eq. (2)) by a strong UV lamp and the mobility distribution is recorded with the same EAA mobility analyzer. The deviation from the first mobility distribution leads to the charge p2 acquired by photoelectric charging for each particle size. Since the photothreshold bi is connected with p2 through eq. (2) the distribution 4i(R) can be determined. This procedure is based on the assumptions that the
543
A. Schmidt-Ott, B. Federer / Photoelectronemission
,,Q ihv-@)leVl .I2
.
.08 .OL’ 0
0
.OL
.08
.I2
.I6
l
RLum]
I
OO
.08
.I6
.21
. .32 Rlum]
Fig 3. Size dependence of (hv - &) for particles of WOs. Fig. 4. Particle size dependence of (hv - $J in a garage aerosol.
particles of a certain size basically have one photothreshold and the mobility of the photoelectrically charged particles is a monotonous function of the size. For the aerosols investigated so far, the assumptions were well fulfilled. This could be concluded from the fact that the two mobility distributions had very similar shapes. The figs. 3 and 4 show (hv - di) as a function of R obtained for a chemically homogeneous W03 aerosol and for a garage aerosol, respectively. We are very much indebted to H.C. Siegmann for helpful discussions and for placing experimental equipment at our disposal. We also thank P.M. Marcus for communicating his calculations of the escape enhancement of photoelectrons from a small sphere and J.P. Kopp for valuable contributions in photoelectric charging of atmospheric aerosols.
References (11 M. Cardona and L. Ley, in: Topics in Applied Physics, Vol. 26 (Springer, Berlin, 1978). [2] A. Schmidt-Ott and H.C. Siegmann, Appl. Phys. Letters 32 (1978) 710. [3] A. Schmidt-Ott, P. Schurtenberger and H.C. Siegmann, Phys. Rev. Letters 4.5 (1980) 1284. [4] H. Riekert, Thesis, Univ. Konstanz (1971) unpublished. [5] R.A. Gussman, J. Appl. Met. 8 (1969) 999. [6] A. Schmidt-Ott, Thesis, ETH Ziirich, No. 6438, unpublished. [7] R.C. Baetzold, M.G. Mason and J.F. Hamilton, J. Chem. Phys. 72 (1980) 366. [8] P.M. Marcus, private communication. (91 C.J. Duthler, S.E. Johnson and H.P. Broida, Phys. Rev. Letters 26 (1971) 1236. [lo] W.D. Watson, Astrophys. J. 176 (1972) 103. [ll] Thermal Systems Inc., Model 3030 EAA, 2500 North Cleveland Ave., St. Paul, Minnesota, USA. [12] A more precise deviation of b(R) including the image force modification due to surface curvature has been carried out by D.M. Wood, Phys. Rev. Letters 46 (1981) 749.