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Photoelectron spectroscopy of naphthacene and perylene crystals by the rare gas emission lines
CHEMICAL PHYSICS LETTERS
Volume 20, number 2
PHOTOELECI’RQN
SPECTROSCOPY
OF NA~HT~AC~NE
AND PERYLENE CRYSTALS
BY THE RARE GAS EMISSION LKNES Kazuhiko SEKI, Hiroo IKKUCHI The htitute
for Solid State Physics. The University of Tokyo, Roppottgi, Minato-ktc, Tok_w 106, Japarr
and
Received 31 January 1973
The energy distribution CUWES(EDC’s) of the photoelectrons emitted from nap~t~acc~e (Clgff 12) and pcryfcnc (C2oFIl2) crystals were measured using the rare 83s resonance emission lines (He 21.22 eV, Ne 16.85 and 16.67 eV and Ar 11.83 and 11.62 ev). The peaks in the higher kinetic energy rgicns agree we11 with the pesks ob:ainccI from moIccuhr photoclcctron spectroscopy studies if constant energy shifts xc aaumed. In the lower kinetic encrg) regions, !lowcver, fixed kinetic energy peaks xc found which r\re independent of the incident photon energies.
The observation of photoemission from solids has a powerful tool in the study of their electronic structures [I]. As for the photoelectron spectroscopy of organic moIecular crystals, however, the incident photon energies have been restricted below about 11.8 eV, because the LiF window is located on the optical path of the vacuum ~~travioIct monochromator. With a single exception, Zagrubskii and Vilesov [2J have studied the photoemjssion from some organic compounds under illumination up to 14.5 eV. In this letter we report the preliminary results of the photoelectron spectroscopy of naphthacene (CI&lI~) and perylene (C~~H~~)‘c~stals under illumination of the rare gas resonance lines whose photon energies are 21.22 eV for He, 16.85 and 16.67 eV for Ne and 11.83 and 11.62 eV for Ar. The rare gas discharge lamp was of n-type. The photoelectron energy andyzer was a spherical retarding potential type one, with a gold-plated collector of I50 mm in diameter with an emitter of 20 mm in diameter. The photoelectron energy distribution curves (EDC’s) were measured by an ac modulation become
technique [3J which has been reported previously [“tJ . The total photocurrents were of the order of !O-lolo-l1 A. The aromatic hydrocarbons appfied were purified by the repeating recrystallization and sublimation. in the EDC’s of naphthacene and peryIene crystals after the correction of the contact potential differences between the sampIes and the cohector [2, 41, there are two groups of peaks and shoulders. One is “the moving group” which is shifted with respect to the kinetic energy (KEj by an amount cquai b rhe vahe of the change in the incident photon energies Itv. The other is “the fixed group” which has fixed kinetic energies independent of 1lrv. Ihe “moving” structures appeared in the higher kinetic energy regions of the EDC’s. For these structures, the ionization poterttiaIs (IP’s), determined by lE’=hv--KE, have constant values with respecf to the incident photon energies. The ED& for naphthacene and paylene against Pare shown in figs. la and 2a. 197 :_
‘_
Volume 20, number 2
CHEMICAL PtfYSfCS IJZTfERS
15
May 1973
Fig. 1. The energy distribution curves for naphthaocnc interpreted so as to giv: the ionization potentials. (a) Crystalline state. & t4v = 21;22 eV. B, Izv = 16.67 and 16.85 eV, (The mean value 16.76 eV is ux+d.) C, hv = 11.62 and 11.83 cV. (The mean value II.73 eV is used.) D, hv = 10.33 cV by Hirooka et al. [4]. &) Gaseous state by Clark et al. IS]. The units of the ordinate are arbitrary and not common to the different curves.
These moving structures are considered to be closeiy associated with the valence bands of these organic crystals [2, 41. According to Koop&ns’ theorem, these
Tp’s correspond
to
the valenceband emrgy levels
of these crystals. To compare the energy distribution curves’of the gaseous and solid states of these compounds, the recent results of molecular photoelectron spectroscopy [5,6;I are also shown in figs. Ib and 2b. As for ++hthaq2ne, the nurne~~~ values of ,the IF% are aiso Iisted in table 1 with the results in the gaseous
state as y.vellas the values obtained in previous photoemission work [Z, 41. In table I, the ~ola~~atioll energy
is defined
as the difference
one [71f The agreement among these experimental findings is satisfactory. Further, the agreement of these levels between the gaseous and the crystalline states is fairly good, if the polarization energy is assumed to be constant. Similar findings are also obtained for perylene with B polarization energy of about 1i2 eV. These :
fgg ,: ‘1 :.. ,,’
,’
‘,.
. .. .
_,._.
,._
:. . ..-
of the enerm
betweenthe gaseousstate and the crystafhe
”
:
Volume 20, number 2
Fig. 2. The energy d~st~butia~
CHEKICAL PHYS:CS LETfERS
curves for pcrylenc interpreted
so as to give the ionization potenti&
Ia) Crystalline state.
A, Irv =
21.22 eV. B, hv = 16.67 and 16.85 eV. (The mean value 16.76 CV is used.) C, Itv= 11.62 and 11.83 eV. (The mean value 11.73 eV is used.) D, 11~= 10.33 CV by Hirooka [8]. (b) Gaseous state by Boschi et al. [6]. The units of the ordinate are arbitrary and not common to the different curves. facts sholv that for such molecular crystaIs, the intermokcular interactions are very weak, so the bandwidths of these energy levels are very narrow. The “fixed” structures appear in the lower kinetic energy regions of the EDC’s. The mean kinetic energy values for the three different incident photon energies are 1.7 and 0.4 eV for naphthacene and 0.7 eV for
perylene. These peaks in naphthame
are also found
in the work of Zagrubskii and Viiesov E.21 under ib lumination having energy higher than FZV= I 1 eV, bvt in the study of photoemission in the vacuum uitra- . violet region in our laboratory 143, the 0.4 eV stru& ture has already appeared at hv= 7.75 eV. .4nother aspect found in both co;npounds is that the peaks at
199 ..
Volume 20, number 2
CHEMICAL PHYSICS LETTERS
The energy Icveis of naphthacene results in the solid state
tib)
Crystal 3)
15 May 1973
Table 1 in the crystafline state compared with those in the gaseous state and with other photoemission
Gas [.5J
AE
Polarization energy Cl
Ref. [4]
7.01 8.41 (8.6) 9.56
0.0 1.41 l.S, 2.55
1.i 1.1
10.25 9.7 11.1 g) 12.0 g) 13.4 g) l4.18) 15.7 g)
3.24 1.69 4.1 5.0 6.4 7.1 8.7
1.0 1.1
--.--
AE
Ref. [2]
AE
5.83 7.28
0.0 1.45
5.2 d) 5.76e) 7.15
0.0 1.39
8.29
2.46
8.25
2.49
9.40 8.70
3.57 2.87
9.5 8.6
3.74 2.84
10.7 11.9
4.94 6.14
5.28 d)
5.3 d) 5.9 7.3
0.0 I.4
(9”:;) t-I) 10.0 10.9 12.3 13.2 14.4 IS.3 (16.2)
3.2 2.8 4.1 5.1 6.4 7.3 8.5 9.8 10.7
1.1 1.1 1.1 0.9 1.3
3 ?he mean values obtained from the three photon energies He, Nc and Ar arc hstcd. For the NC and Ar doublets, the mean values are used, but for the calculati,Jn of the threshold energy for photocmission, the higher photon energies arc used. b) ti is the energ difference from the first peak. c) Polarization energy = IF&as) - IPlcrystal). d) The threshold energy for photoemission. e) The value 5.78 in the English translation of ref. [4J should be a misprint. 0 The values in parentheses arc dubious. g) The values of IP(gas) are read from fig. 4b (ref. [ 51).
the Iower kinetic energies become more $Mnant with increasing incident photon energies. The behaviour is characteristic of the crystalline state, because the peaks of the lower kinetic energy regions for the gaseous state under irradiation of hv = 21.22 eV are not so dominant. These results suggest that, as the photon energy increases, a large electron emission in the lower kinetic energy region with fixed peaks is intensified beside the direct photoemission which corresponds to the gaseous one. To discuss these phcnomena, the data of the variation of the quantum yields of the peaks with the change of the photon energies are indispensable [2,4f. When the precise measurements of the light intensities, which are in progress, are carried out, these phenomena will be discussed thoroughIy. ‘The authors uiish to thank Dr. T. Hiraoka for his helpful discussion. They also express their thanks to
Mr. Kiyoo Tsuji of our Institute for preparing the photoelectron spectrometer. References [ 1 J W.E. Spicer, in: Optical propertics of solids, ed. F. Abeiis (North-Holland, Amsterdam, 1972). [2] A.A. Zagrubskii and F.I. Vilcsov, Fiz. Tverd. Tela 13 (lV_l) 2300 [English transl. Soviet Phys. Solid State 13 (1972) 19271. 13) R.C. Eden, Rev. Sci. Instr. 41 (1970) 252. [4] T. Hirooka, K. Tanaka, hf. Fujihira, H. Inokuchi, Y. Harada and K. Kuchitsu, Chem. Phys. Letters 18 (1973) 930. [5] P.A. Clark, F. Brogli and E. Heilbionncr, Hclv. Chim. Acta 55 (1972) 1415. [6] R. Boschi, J.M. Murre!l and W. Schmidt, Discussions Faraday Sot., to be published. (71 L.E. Lyons, J. Chem. Sot. (!957) 5001; LE. Lyons and J.C. Mackie, Proc. Chem. Sot. CI962) 71. [ 81 T. Hirooka, private communication.
:
200
.: ,.‘.
Volume 20, number 2
PHOTOELECI’RQN
SPECTROSCOPY
OF NA~HT~AC~NE
AND PERYLENE CRYSTALS
BY THE RARE GAS EMISSION LKNES Kazuhiko SEKI, Hiroo IKKUCHI The htitute
for Solid State Physics. The University of Tokyo, Roppottgi, Minato-ktc, Tok_w 106, Japarr
and
Received 31 January 1973
The energy distribution CUWES(EDC’s) of the photoelectrons emitted from nap~t~acc~e (Clgff 12) and pcryfcnc (C2oFIl2) crystals were measured using the rare 83s resonance emission lines (He 21.22 eV, Ne 16.85 and 16.67 eV and Ar 11.83 and 11.62 ev). The peaks in the higher kinetic energy rgicns agree we11 with the pesks ob:ainccI from moIccuhr photoclcctron spectroscopy studies if constant energy shifts xc aaumed. In the lower kinetic encrg) regions, !lowcver, fixed kinetic energy peaks xc found which r\re independent of the incident photon energies.
The observation of photoemission from solids has a powerful tool in the study of their electronic structures [I]. As for the photoelectron spectroscopy of organic moIecular crystals, however, the incident photon energies have been restricted below about 11.8 eV, because the LiF window is located on the optical path of the vacuum ~~travioIct monochromator. With a single exception, Zagrubskii and Vilesov [2J have studied the photoemjssion from some organic compounds under illumination up to 14.5 eV. In this letter we report the preliminary results of the photoelectron spectroscopy of naphthacene (CI&lI~) and perylene (C~~H~~)‘c~stals under illumination of the rare gas resonance lines whose photon energies are 21.22 eV for He, 16.85 and 16.67 eV for Ne and 11.83 and 11.62 eV for Ar. The rare gas discharge lamp was of n-type. The photoelectron energy andyzer was a spherical retarding potential type one, with a gold-plated collector of I50 mm in diameter with an emitter of 20 mm in diameter. The photoelectron energy distribution curves (EDC’s) were measured by an ac modulation become
technique [3J which has been reported previously [“tJ . The total photocurrents were of the order of !O-lolo-l1 A. The aromatic hydrocarbons appfied were purified by the repeating recrystallization and sublimation. in the EDC’s of naphthacene and peryIene crystals after the correction of the contact potential differences between the sampIes and the cohector [2, 41, there are two groups of peaks and shoulders. One is “the moving group” which is shifted with respect to the kinetic energy (KEj by an amount cquai b rhe vahe of the change in the incident photon energies Itv. The other is “the fixed group” which has fixed kinetic energies independent of 1lrv. Ihe “moving” structures appeared in the higher kinetic energy regions of the EDC’s. For these structures, the ionization poterttiaIs (IP’s), determined by lE’=hv--KE, have constant values with respecf to the incident photon energies. The ED& for naphthacene and paylene against Pare shown in figs. la and 2a. 197 :_
‘_
Volume 20, number 2
CHEMICAL PtfYSfCS IJZTfERS
15
May 1973
Fig. 1. The energy distribution curves for naphthaocnc interpreted so as to giv: the ionization potentials. (a) Crystalline state. & t4v = 21;22 eV. B, Izv = 16.67 and 16.85 eV, (The mean value 16.76 eV is ux+d.) C, hv = 11.62 and 11.83 cV. (The mean value II.73 eV is used.) D, hv = 10.33 cV by Hirooka et al. [4]. &) Gaseous state by Clark et al. IS]. The units of the ordinate are arbitrary and not common to the different curves.
These moving structures are considered to be closeiy associated with the valence bands of these organic crystals [2, 41. According to Koop&ns’ theorem, these
Tp’s correspond
to
the valenceband emrgy levels
of these crystals. To compare the energy distribution curves’of the gaseous and solid states of these compounds, the recent results of molecular photoelectron spectroscopy [5,6;I are also shown in figs. Ib and 2b. As for ++hthaq2ne, the nurne~~~ values of ,the IF% are aiso Iisted in table 1 with the results in the gaseous
state as y.vellas the values obtained in previous photoemission work [Z, 41. In table I, the ~ola~~atioll energy
is defined
as the difference
one [71f The agreement among these experimental findings is satisfactory. Further, the agreement of these levels between the gaseous and the crystalline states is fairly good, if the polarization energy is assumed to be constant. Similar findings are also obtained for perylene with B polarization energy of about 1i2 eV. These :
fgg ,: ‘1 :.. ,,’
,’
‘,.
. .. .
_,._.
,._
:. . ..-
of the enerm
betweenthe gaseousstate and the crystafhe
”
:
Volume 20, number 2
Fig. 2. The energy d~st~butia~
CHEKICAL PHYS:CS LETfERS
curves for pcrylenc interpreted
so as to give the ionization potenti&
Ia) Crystalline state.
A, Irv =
21.22 eV. B, hv = 16.67 and 16.85 eV. (The mean value 16.76 CV is used.) C, Itv= 11.62 and 11.83 eV. (The mean value 11.73 eV is used.) D, 11~= 10.33 CV by Hirooka [8]. (b) Gaseous state by Boschi et al. [6]. The units of the ordinate are arbitrary and not common to the different curves. facts sholv that for such molecular crystaIs, the intermokcular interactions are very weak, so the bandwidths of these energy levels are very narrow. The “fixed” structures appear in the lower kinetic energy regions of the EDC’s. The mean kinetic energy values for the three different incident photon energies are 1.7 and 0.4 eV for naphthacene and 0.7 eV for
perylene. These peaks in naphthame
are also found
in the work of Zagrubskii and Viiesov E.21 under ib lumination having energy higher than FZV= I 1 eV, bvt in the study of photoemission in the vacuum uitra- . violet region in our laboratory 143, the 0.4 eV stru& ture has already appeared at hv= 7.75 eV. .4nother aspect found in both co;npounds is that the peaks at
199 ..
Volume 20, number 2
CHEMICAL PHYSICS LETTERS
The energy Icveis of naphthacene results in the solid state
tib)
Crystal 3)
15 May 1973
Table 1 in the crystafline state compared with those in the gaseous state and with other photoemission
Gas [.5J
AE
Polarization energy Cl
Ref. [4]
7.01 8.41 (8.6) 9.56
0.0 1.41 l.S, 2.55
1.i 1.1
10.25 9.7 11.1 g) 12.0 g) 13.4 g) l4.18) 15.7 g)
3.24 1.69 4.1 5.0 6.4 7.1 8.7
1.0 1.1
--.--
AE
Ref. [2]
AE
5.83 7.28
0.0 1.45
5.2 d) 5.76e) 7.15
0.0 1.39
8.29
2.46
8.25
2.49
9.40 8.70
3.57 2.87
9.5 8.6
3.74 2.84
10.7 11.9
4.94 6.14
5.28 d)
5.3 d) 5.9 7.3
0.0 I.4
(9”:;) t-I) 10.0 10.9 12.3 13.2 14.4 IS.3 (16.2)
3.2 2.8 4.1 5.1 6.4 7.3 8.5 9.8 10.7
1.1 1.1 1.1 0.9 1.3
3 ?he mean values obtained from the three photon energies He, Nc and Ar arc hstcd. For the NC and Ar doublets, the mean values are used, but for the calculati,Jn of the threshold energy for photocmission, the higher photon energies arc used. b) ti is the energ difference from the first peak. c) Polarization energy = IF&as) - IPlcrystal). d) The threshold energy for photoemission. e) The value 5.78 in the English translation of ref. [4J should be a misprint. 0 The values in parentheses arc dubious. g) The values of IP(gas) are read from fig. 4b (ref. [ 51).
the Iower kinetic energies become more $Mnant with increasing incident photon energies. The behaviour is characteristic of the crystalline state, because the peaks of the lower kinetic energy regions for the gaseous state under irradiation of hv = 21.22 eV are not so dominant. These results suggest that, as the photon energy increases, a large electron emission in the lower kinetic energy region with fixed peaks is intensified beside the direct photoemission which corresponds to the gaseous one. To discuss these phcnomena, the data of the variation of the quantum yields of the peaks with the change of the photon energies are indispensable [2,4f. When the precise measurements of the light intensities, which are in progress, are carried out, these phenomena will be discussed thoroughIy. ‘The authors uiish to thank Dr. T. Hiraoka for his helpful discussion. They also express their thanks to
Mr. Kiyoo Tsuji of our Institute for preparing the photoelectron spectrometer. References [ 1 J W.E. Spicer, in: Optical propertics of solids, ed. F. Abeiis (North-Holland, Amsterdam, 1972). [2] A.A. Zagrubskii and F.I. Vilcsov, Fiz. Tverd. Tela 13 (lV_l) 2300 [English transl. Soviet Phys. Solid State 13 (1972) 19271. 13) R.C. Eden, Rev. Sci. Instr. 41 (1970) 252. [4] T. Hirooka, K. Tanaka, hf. Fujihira, H. Inokuchi, Y. Harada and K. Kuchitsu, Chem. Phys. Letters 18 (1973) 930. [5] P.A. Clark, F. Brogli and E. Heilbionncr, Hclv. Chim. Acta 55 (1972) 1415. [6] R. Boschi, J.M. Murre!l and W. Schmidt, Discussions Faraday Sot., to be published. (71 L.E. Lyons, J. Chem. Sot. (!957) 5001; LE. Lyons and J.C. Mackie, Proc. Chem. Sot. CI962) 71. [ 81 T. Hirooka, private communication.
:
200
.: ,.‘.