Measurement of Photoelectron Emission by  Vacuum Ultraviolet Ray Irradiation Shugo Okamura, Toru Iwao, Motoshige Yumoto (Musashi Institute of Technology), Hiroaki Miyake, Kumi Nitta (JAXA) 
Abstract     Recently, the satellite plays many roles depending on the purpose, including communication, weather observation, 
astronomy observation and space development. The satellite is requisitioned long life and high reliability in such situation. 
However, around an altitude of several 100 km, there is Atomic Oxygen (AO) which is a main ingredient agents. AO has high 
reactivity and its density is about 1015 atoms/m3. Because the satellite collides with AO, surface materials of the satellite are 
deteriorated. As a result, it leads unevenness and oxidation. Accordingly, it is necessary to monitor the surface condition. 
    In this study, photoemission characteristics of several materials, such as metals, glasses and polymers are measured by use
of the deuterium lamp and the band path filters. From the measurement the threshold energy for photoemission and the 
quantum efficiency was evaluated. As a result, the thresholds of samples are 4.9 to 5.7 eV. The quantum efficiency of metals is
about 100 times higher than other samples. The quantum efficiency of PS which includes benzene ring is several times higher 
compared with PP and PTFE. From these results, it is deduced that deteriorated materials may emit large amount of 
photoelectron. 
Key words : photoelectron emission, vacuum ultraviolet ray, polymer, threshold energy for photoemission, quantum efficiency 
1.� Introduction
Recently, the satellite is requisitioned the long life and high 
reliability. By the way, there is Atomic Oxygen (AO) which is a 
main ingredient agents in LEO (low-earth orbit). An altitude of 
several 100 km (about 10-3 Pa) AO is the main constituent of 
the atmosphere in LEO. It is formed by dissociation of O2 by 
ultraviolet irradiation from the sun. AO has high reactivity and 
its density is about 1015 atoms/m3. Because the satellite collides 
with AO, surface materials of the satellite are deteriorated (1).
Therefore it is necessary to monitor the surface materials of 
the satellite. The authors thought that the change of the 
deteriorated situation of surface materials might be detected 
by the measurement of the photoemission characteristic by the 
sun. Therefore, it is necessary to summarize the fundamental 
data concerned with photoemission characteristics related to 
the surface condition of materials. On the other hand, analysis 
of the surface condition by AO irradiation is important. 
Compared with these results, it might be effective for the 
monitoring of the deteriorated situation. In this report, results 
of preliminary experiment are summarized. 
2. Experimental setup 
The experimental setup is shown in Fig.1. The deuterium 
lamp of 200 W and band pass filters (BPF) are used for a source 
of VUV (vacuum ultraviolet) ray. The VUV ray is reflected by an 
aluminum vapor deposition mirror made by the vacuum 
evaporation method on the optical flat glass and is irradiated on 
the sample. Photoelectrons are collected by the semi-spherical 
electrode and are measured at the electrode placed the sample. 
Here, the radiation wavelength range of the deuterium lamp is 
115-400 nm. The transmittance of BPF has 15-20 % at the peak 
wavelength and the half bandwidth is about 20 nm with each 
filters. Negative bias voltage around 7 V is applied to the sample 
side, to prevent the pull back electrons that were emitted from the 
sample and the electrons emitted at the surface of chamber. 
Additionally, irradiated flux of VUV ray is measured by using a 
photomultiplier for each wavelength to evaluate the quantum 
efficiency for the photoemission. The measurement is performed 
by pressure less than 10-2 Pa. The measured samples are metals, 
glasses, films, paints and polymers. 
Fig.1 Experimental setup. 
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3.� Results and discussion
3�1� Threshold energy 
The square root of the magnitude of photoemission current 
value corrected by the irradiated VUV ray intensity is shown at 
the axis of ordinates and energy of the irradiated VUV ray is the 
axis of abscissas, which is so called Fowler plots is shown in 
Fig.2. From the graph, the threshold energy for photoemission is 
evaluated. The results are summarized in Table 1. The metals 
have the same magnitude as the reported one (2). Now, the dispersion of evaluated value was about �0.2 eV. The other 
samples have 5.4-5.7 eV. In the case of polymer, it is reported that 
the bandgap of polymer has about 9 eV (3). On the other hand, it is 
reported that the surface level exists around 4-5 eV in the case of 
polymer (4). Therefore, the reason why the threshold of about 5 
eV was obtained is existing a lot of the surface level around 5 eV. 
The surface depth to emit photoelectrons are extremely shallow, 
accordingly the results obtained by the measurement may be 
influenced by these surface level. 
Fig. 2� Fowler plots. 
3�2� Quantum efficiency 
Quantum efficiency for photoemission was calculated by 
using the flux of VUV ray irradiated on the sample for each BPF 
and magnitude of the photoemission current. The flux of VUV 
ray irradiated on the sample was corrected by the transmittance of 
each BPF by using the sensitivity of the photomultiplier and was 
converted into the intensity at the central wavelength of each one. 
The results are shown in Figs.3-7. The samples with relatively 
high conductivity tend to emit electrons easily from the figures. 
For example, It is clear that volume resistively of metals have about 10-8 ��m and that of polymers have about 1015 ��m. In 
the case of paint, samples of conductivity have 105�106 ��m. 
As well the samples that conductivity is small were not seen in 
the tendency that photoemission currents decreased with time. 
For example, the quantum efficiency of film A from Fig.4 is 
about 10-5 at the photon energy of 7 eV. It is necessary to irradiate 
photons about 105 with energy of about 7 eV for one electron. 
The exposure dose of the photon of about 7 eV by the sun in 
space is measured with about 1011/ m2 per second. Accordingly, 
many electrons might be emitted. Here in polymers, PS which includes the benzene ring in its 
chemical structure emits more electrons in each wavelength than 
PP and PTFE which does not include the benzene ring. Therefore, 
it is thought that having benzene ring may contribute to 
photoemission. When polymers are irradiated by AO, it is 
expected that many polar bonds including oxygen are formed at 
the surface. It is expected in particular that a lot of double bonds 
such as a carbonyl group, the carboxyl group and so on are 
generated. In other words, quantum efficiency of the materials 
which is deteriorated by AO irradiation might increase, because 
photoemission quantity of PS having a benzene ring including a 
lot of double bond increased. On the other hand, unevenness of 
surface is generated by AO irradiation (5). In figure, quantum 
efficiency of the deteriorated glass decreased. It is necessary to 
clarify effect which is dominant for photoemission, one is the 
introduction of double bonds and the other is the formation of 
unevenness at the surface. Samples introduced double bonds and 
Table 1� Threshold energy for photoelectron emission  
of each samples. 
Materials Characteristics Threshold energy (eV)
AuMetal 4.9 
Ag 5.1
Glass A 
Glass A(deteriorated) 5.7 
Glass B 5.7 
Glass B(ITO-coated) 5.7 
Film A 
Film B 5.4 
Film C 5.7 
Paint A  
Conductive 
Paint B 5.4 
Paint C  
Non-Conductive
Paint D 5.7 
Paint E 5.7 
Polymer 
PTFE 5.7
PS 5.4
 宇宙航空研究開発機構特別資料JAXA-SP-07-030
formed unevenness will be prepared and its quantum efficiency 
will be measured in future. In addition, data between 170-210 nm 
will be measured in future, because quantum efficiency is looked 
like discontinuity. 
Fig. 3� Quantum efficiency of metals. 
Fig. 4� Quantum efficiency of glasses. 
Fig. 5� Quantum efficiency of films. 
Fig. 6� Quantum efficiency of paints. 
Fig. 7� Quantum efficiency of polymers. 
第４回「宇宙環境シンポジウム」講演論文集
3�2� Magnitude of photoelectron current irradiated by the 
Comparing with the intensity irradiated by the test and the 
intensity of the sun, magnitude of photoelectron current from the 
sample irradiated by the sun was calculated. The results are 
shown in table 2. More than 10-6 A/m2 is estimated for each 
sample including the insulator, and more than 10-4 A/m2 is 
obtained in case of conduction one. 
4.� Conclusion 
The photoemission characteristics from many materials were 
measured. The result is as follows; 
�The threshold energy of metals were similar magnitude 
to the reported wok function. 
�The threshold energy of other samples have 5.4-5.7 eV. 
�The samples with small magnitude of volume specific 
resistance tend to be easy to emit electrons. As the 
results, the quantum efficiency is high. 
�It is deduced that samples with double bonds emit 
photoelectron easily. 
We are planning to clarify the relation between the 
photoemission characteristics and the deterioration by formation 
of unevenness by introduction of double bonds. 
References 
(�) H. Fujii, K. Sonoda and H. Nishimoto: “Spacecraft charging and material degradation in space enviroment”, Trans. IEEJ, Vol.113-A, No.9, pp.609-617 (1993) (�) V. S. Fomenko., G. V. Samsonov: “Handbook of     Thermionic Properties”, New York, Plenum Press Data     Division, pp.39-41, pp.60-62 (1966) (�) T. Tanaka: “Optical absorption and electrical conduction in polyethylene”, J. Appl, Phys., Vol.44, No.5, pp.2430-2432 (1973) (�) D. K. Davis: “Charge generation on dielectric surfaces”, J. Phys. D : Appl. Phys., Vol.2, pp.1533-1537(1969) (�) Eitan Grossman, Y. Lifshitz : “In situ study of Kapton using novel hyperthermal oxygen atom source”, J. Spacecraft Rockets, Vol.36, No.1, pp.75-78(1999) 
Table 2� Estimated photoelectron current by the sun 
irradiation of each samples. 
Materials Photoelectron current (A/m2)
Au3.6�10-3
Ag3.2�10-3
Glass A 2.8�10-4
Glass A(deteriorated) 9.7�10-6
Glass B 1.3�10-4
Glass B(ITO-coated) 1.6�10-4
Film A 2.5�10-4
Film B 2.9�10-4
Film C 3.4�10-4
Paint A Conductive 3.4�10-4
Paint B 3.2�10-4
Paint C 
Non-Conductive 
3.6�10-5
Paint D 2.6�10-5
Paint E 9.3�10-6
PP1.4�10-5
PTFE 6.9�10-6
PS3.0�10-4
 宇宙航空研究開発機構特別資料JAXA-SP-07-030