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Photoconductivity and photoelectron emission of liquid squalane and squalene induced by vacuum-ultraviolet light
18 August 1995
ELSEVIER
CHEMICAL PHYSICS LETTERS Chemical Physics Letters 242 (1995) 320-324
Photoconductivity and photoelectron emission of liquid squalane and squalene induced by vacuum-ultraviolet light Hitoshi Koizumi a, Ryuzi Katoh b,1, Klaus Lacmann b, Werner F. Schmidt b a Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan b Abteilung Strahlenchemie, Hahn-Meitner-lnstitut Berlin, D-14109 Berlin, Germany Received 23 May 1995
Abstract
The photoconductivity of liquid squalane (2,6,10,15,19,23-hexamethyltetracosane, C3oH62) and liquid squalene C3oH5o) was measured as a function of photon energy. The energy thresholds of photoconductivity and photoemission were determined. From the difference of these two values the energies of the electronic conduction levels Vo(C3oH6o)----0.15 eV and V0(C3oHso)=-0.55 eV were estimated. Absolute quantum yields of free ions were measured at a field strength of 6.7 kV cm-1 in the photoconductivity experiment. The difference in yields of the photoconductivity and the photoelectron emission is discussed taking into account the escape probability from geminate recombination and the transport of photoelectrons in the liquid and through the liquid/vacuum interface.
(2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene,
On the other hand, the photoconductivity threshold, Eth(PC) is given by
1. Introduction
Photoinduced electron emission and photoconductivity have been employed to study the photoionization process, the electron transport and the electronic energy levels in liquids and solids [1-3]. The threshold energy of photoinduced electron emission, Eth(pe) is related to the gas phase ionization potential, IP, and polarization energy of the cation, P+, by Eth(pe ) = IP + P+.
(1)
1 On leave from the Faculty of Science, Gakushuin University, Mejiro, Tokyo 171, Japan. Present address: High Energy Density Laboratory, National Institute of Materials and Chemical Research, Tsukuba, Japan.
Eth(PC) = IP + P+ + V0 ,
(2)
where V0 is the energy of the electronic conduction level of the liquid (it is sometimes called the electron affinity of the liquid). Comparison of Eth(Pe) and Eta(pc) hence gives the value of Vo. We have already made such an estimation of V0 for silicone oils [4]. The values of Etb(pe) were measured by us [4] and the values of Eth(pc) were taken from measurements by Baron et al. [5]. Vo values are usually obtained from measurements of the change in work function of a metal cathode when it is immersed in a liquid [6] or from measurements of the change in the ionization potential of a suitable solute molecule [7]. The work function measurements sometimes suffer from adsorbed layers on the cathode.
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 0 9 - 2 6 1 4 ( 9 5 ) 0 0 7 4 2 - 3
H. Koizumi et al. / Chemical Physics Letters 242 (1995) 320-324
The present method is free from these limitations, however, V0 is obtained as the difference of two values of the order of 10 eV which may affect the accuracy of the determination. Absolute yields of photoconductivity and photoelectron emission give information on the photoexcitation and photoionization in liquids, and the electron transport through the liquid and across the liquid/vapor interface [8-11]. We have determined the absolute yield of free ions, and compared it with the photoelectron emission yield.
2. Experimental A schematic diagram of the photoconductivity and photoelectron emission experiments is shown in Fig. 1. A deuterium discharge lamp (Hamamatsu L1835) followed by a vacuum ultraviolet monochromator (Acton VM502) was used as light source. The bandwidth of the monochromatized light was 1 nm. The light entered the cell through the MgF2 window coated inside with a gold film of 8 nm thickness. In the photoconductivity experiment, the cell was filled with a liquid sample. In the photoelectron emission experiment, a thin liquid film was put on the cathode. The photoconductivity and the emitted electron current were measured with a pico-ammeter (Keithley, model 617). All measurements were performed at room temperature. The samples were squalane (stated purity 99%) by Aldrich and squalene ( > 98%) by Merck. They were MgF2 window VUV.__[..~___........ ._~xmirrOr
,i ht ]U
1
/
I MgF2/ ] Au window pico-ammeter k - ' ~ & voltage
Y 7"-suppl,
.-.~_ Fig. 1. Experimentalapparatusfor the measurementsof photoconductivityand photoelectronemission.
321
purified by percolation through columns of activated silica gel and molecular sieves 4 .~. Subsequently, they were degassed under vacuum.
3. Results and discussions Fig. 2 shows the photoconductivities and photoelectron emission currents of squalane and squalene. The applied field strength for the photoconductivity experiment was 6.7 kV/cm, and that for the photoelectron emission experiment was 670 V/cm. The absolute light intensity was calculated from the photoelectron emission current from a gold cathode for which the absolute quantum yield of the photoelectron emission (qbp~) is known [11]. The ratio of the photoconductivity current and the light intensity gives the quantum yield of free ions (@~) at the applied field of 6.7 kV/cm. Figs. 3 and 4 show the quantum yield of free ions for squalane and squalene, respectively. The figures include the quantum yield of the photoelectron emission for comparison. Near the threshold, @~ is known to depend as a power function on the energy difference between the photon energy and the threshold energy ( h v - E t h ) [12], @fi = C[ h v - E~h(pc)] n,
(3)
where C is a constant. An exponent of n = 5 / 2 gives the best fit for the ionization of neutral molecules in non-polar solvents [13,14]. In Fig. 5, @fi to the 2 / 5 power is plotted as a function of photon energy. Extrapolation to q~ion= 0 yields the value of Eth(pC). For squalane Eth(pc)--8.25 5:0.1 eV, and for squalene Eth(pc) = 6.35 5:0.1 eV were obtained. With the previously reported values of Eth(Pe) and with Eqs. (1) and (2), we can calculate V0 for these liquids. For squalane V0 -- -0.15 eV, and for squalene V0 = - 0 . 5 5 eV were obtained. These values are compared in Table 1 with those reported for other liquid alkanes and alkenes [7,9,13,15,16]. The value of squalane is comparable to the value of 2,2,4-trimethylpentane, also a compound with CH3-side groups. The value for squalene is considerably lower compared to the values of the other alkenes. It is possible that the great number of CH 3-side groups is responsible for this effect.
1t. Koizumi et aL / Chemical Physics Letters 242 (1995) 320-324
322
l.OxlO
43
....
o.s <
~0.6~ ii 0.4 ~
~/
, .........
, .........
, .........
-1
10 , .........
,....,....,
....
,,,r,i
....
, ....
I ....
, ....
t
....
. ....
10 .2
Squalane
(,) .o
l 0 "3
E
g 10-4
0.2
0.0
• Photoconductivity ; - - - Photoemission J
~
100
120
140
160
180
10-6
200
.,l
. . . .
6
Wavelenl~h / n m
i . . . .
i . , . , l
7
. . . .
i
, . . . .
8
i . . . .
i . . . .
9
i . . . .
i , , .
10
11
PhotonEnergy/ eV
2.0Xl0"13 .... , ......... , ......... , . . . . . . . . . . . . . . . . . . , ......... ,....,
Fig. 3. Q u a n t u m yield o f free ions f r o m p h o t o c o n d u c t i v i t y and photoemission, squalane.
Co)
~'
, ....
.o
10 "5
<
I ....
....
1.5
Comparison of the quantum yields is impeded by the fact that the measurements were possible at two values of the applied field only. The discussion must remain qualitative at best. Both the @~ and ~pe values can be related to the probability of escape of the photoelectrons, Peso, from their parent positive ions,
1.0
0.5 a_
0.0
=
100
120
140
160
180
i
, , .
1.
,,
[ , . .
u
lO q
. . ,
u,
..
I
, , , 1 ' ' ' ' 1
. . . .
, . . . .
I . . . .
, . . . .
I . . . .
! . . . .
I . . . .
!
. . . .
• °o
(c) "
(4)
200
Wavelength / nm ,
r/P~c,
. -
o°
~
i0 -2
,°~ua'ene oq
"""
". §
~
-1
I0 "s
10.3
f"
10 4
,,'
s
.~ -2 I0 "s .4
i
10. 5
-3 104 ,
I
,
100
,
t
I
•
120
.
.
i
.
140
.
.
I
.
.
160
.
I
.
180
.
.
I
200
Wavelength [nm] Fig. 2. Photoeurrents as a function o f the w a v e l e n g t h for squalene; (a) photoconduetivity, E = 6.7 × 103 V c m - 1 ; (b) photoemission, E = 6 7 0 V e m - 1; (e) s p e c t r u m o f the D2-1am p.
,,s • '~
• Photoconduetivity - - - Photocmission
t
10-6 6 . . . . . . . . , . . . . . . . . . , . . . . . . . . . , . . . . . . . . . , . . . . . . . 7 8 9 10
11
P h o t o n E n e r g y / eV Fig. 4. Q u a n t u m yield o f free ions f r o m p h o t o c o n d u c t i v i t y a n d photoemission, squalene.
H. Koizumi et al. / Chemical Physics Letters 242 (1995) 320-324
0.15
,,,r
. . . .
i ....
i . . . .
i ....
i . . . .
i , , , , i , t [ . , i
. . . .
'1
....
o
• O
Squalanc Squalene
o oo o o O
O Oo
0.10
C'
o 00%
i,; °oo
0.05
0oo 6
7
8
9
10
11
Photon Energy / eV Fig. 5. Quantum yields of free ions from photoconductivity at 6.7 × 103 V c m - 1 plotted as a function of photon energy.
where ,/ is the quantum yield of initial photoionization. An ejected electron loses its excess energy by collisions with molecules until it becomes thermalized. The mean separation distances in hydrocarbons between the parent positive ion and the thermalized electron are between 50 and 100 ,~. In principle, the photoemission yield should be greater than the photoconductivity yield since superthermal electrons have a higher probability of escape. If the ionization
Table 1 Photoionization threshold and V0 values for liquid hydrocarbons at room temperature Liquid
Eth(PC) ( e V )
Eth(Pe) ( e V )
V0 ( e V )
squalane squalene
8.25 6.35
8.4 a 6.9 a
- 0.15 - 0.55
n-hexane 3-methylpentane 2,2,4-trimethylpentane (isooctane) 2,2,4,4-tetramethylpentane
8.6 b 8.85 d 8.3 b
+0.10 c + 0.01 ¢ - 0.17 ¢
8.2 c
-0.33 c
2,3-dimethyl-2-butene (tetramehylethylene)
6.8 o
-0.25 c
cis-2-butene a Ref. [11]. b Ref. [13]. c Ref. [7]. d Ref. [9]. • Ref. [16]. f Ref. [15].
323
event occurs at a distance from the liquid/vapor interface which is smaller than the thermalization distance, the emission of a superthermal electron into the vapor phase occurs. If a thermalized electron arrives at the interface, thermal activation is necessary in order to overcome the energetic barrier given by V0. The photoconductivity is determined by thermalized electrons and their parent positive ions only. In the case of photoemission the contributions of superthermal and thermal electrons to the observed emission current depend on the energy dependence of the absorption coefficient. An increase in the absorption coefficient with photon energy leads to a shift in the center of gravity of the ionization events towards the interface. More superthermal electrons are able to escape. In the photoconductivity experiment, an increase in the absorption coefficient (the concentration of charge carrier pairs near the window electrode increases) leads to an increased influence of the interaction between electrons, ions and their respective image charges on the escape process. These conditions seems to be met for squalane where the yield of photoemission exceeds the yield of photoconductivity over the whole range of photon energies. In squalene, above 8.5 eV, the same reasoning holds. At lower photon energies the photoconductivity yield exceeds the emission yield. It is possible that due to the more negative value of V0 the transfer of thermal electrons is hampered due to the formation of a space charge layer. Measurements at different applied field strengths could clarify this point.
Acknowledgement RK received financial support from the Nishina Memorial Foundation for a one year stay at the Hahn-Meitner-Institut. HK received financial support for a short stay from the Hahn-Meitner-Institut. Both contributions are gratefully acknowledged.
References - 0.16 f
[1] M. Pope and C.E. Swenberg, Electronic processes in organic crystals (Clarendon Press, Oxford, 1982). [2] K. Seki, Mol. Cryst. Liquid Cryst. 171 (1989) 255.
324
H. Koizumi et al. / Chemical Physics Letters 242 (1995) 320-324
[3] W.F. Schmidt, Electronic energy levels in nonpolar dielectric liquids, in: Excess electrons in dielectric media, eds., C. Ferradini and J.-P. Jay-Gerin (CRC Press, Boca Raton, 1991). [4] H. Koizumi, K. Lacmann and W.F. Schmidt, J. Phys. D. 25 (1992) 857. [5] P.L. Baron, J. Casanovas, J.P. Guelfucci and R.L.S. Hoi, IEEE Trans. Electr. Insul. 23 (1988) 563. [6] R.A. Holroyd and M. Allen, J. Chem. Phys. 54 (1971) 5014. [7] R.A. Holroyd and R.L. Russell, J. Phys. Chem. 78 (1974) 2128. [8] R.D. Birkhoff, J.M. Heller Jr., L.R. Painter, J.C. Ashley and H.H. Hubbell Jr., J. Chem. Phys. 76 (1982) 5208. [9] E.H. B&tcher and W.F. Schmidt, J. Chem. Phys. 80 (1984) 1353.
[10] H. Koizumi, Chem. Phys. Letters 219 (1994) 137. [11] H. Koizumi, K. Lacmann and W.F. Schmidt, J. Electr. Spectry. Rel. Phenom. 67 (1994) 417. [12] U. Sowada and R.A. Holroyd, J. Phys. Chem. 84 (1980) 1150. [13] J. Casanovas, R. Grob, D. Delacroix, J.P. Guelfucci and D. Blanc, J. Chem. Phys. 75 (1981) 4661. [14] R.A. Holroyd, J.M. Preses and N. Zevos, J. Chem. Phys. 79 (1983) 483. [15] R.A. Holroyd, S. Tames and A. Kennedy, J. Phys. Chem. 79 (1975) 2857. [16] K. Buschick and W.F. Schmidt, IEEE Trans. El. lnsul. 24 (1989) 353.
ELSEVIER
CHEMICAL PHYSICS LETTERS Chemical Physics Letters 242 (1995) 320-324
Photoconductivity and photoelectron emission of liquid squalane and squalene induced by vacuum-ultraviolet light Hitoshi Koizumi a, Ryuzi Katoh b,1, Klaus Lacmann b, Werner F. Schmidt b a Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan b Abteilung Strahlenchemie, Hahn-Meitner-lnstitut Berlin, D-14109 Berlin, Germany Received 23 May 1995
Abstract
The photoconductivity of liquid squalane (2,6,10,15,19,23-hexamethyltetracosane, C3oH62) and liquid squalene C3oH5o) was measured as a function of photon energy. The energy thresholds of photoconductivity and photoemission were determined. From the difference of these two values the energies of the electronic conduction levels Vo(C3oH6o)----0.15 eV and V0(C3oHso)=-0.55 eV were estimated. Absolute quantum yields of free ions were measured at a field strength of 6.7 kV cm-1 in the photoconductivity experiment. The difference in yields of the photoconductivity and the photoelectron emission is discussed taking into account the escape probability from geminate recombination and the transport of photoelectrons in the liquid and through the liquid/vacuum interface.
(2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene,
On the other hand, the photoconductivity threshold, Eth(PC) is given by
1. Introduction
Photoinduced electron emission and photoconductivity have been employed to study the photoionization process, the electron transport and the electronic energy levels in liquids and solids [1-3]. The threshold energy of photoinduced electron emission, Eth(pe) is related to the gas phase ionization potential, IP, and polarization energy of the cation, P+, by Eth(pe ) = IP + P+.
(1)
1 On leave from the Faculty of Science, Gakushuin University, Mejiro, Tokyo 171, Japan. Present address: High Energy Density Laboratory, National Institute of Materials and Chemical Research, Tsukuba, Japan.
Eth(PC) = IP + P+ + V0 ,
(2)
where V0 is the energy of the electronic conduction level of the liquid (it is sometimes called the electron affinity of the liquid). Comparison of Eth(Pe) and Eta(pc) hence gives the value of Vo. We have already made such an estimation of V0 for silicone oils [4]. The values of Etb(pe) were measured by us [4] and the values of Eth(pc) were taken from measurements by Baron et al. [5]. Vo values are usually obtained from measurements of the change in work function of a metal cathode when it is immersed in a liquid [6] or from measurements of the change in the ionization potential of a suitable solute molecule [7]. The work function measurements sometimes suffer from adsorbed layers on the cathode.
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 0 9 - 2 6 1 4 ( 9 5 ) 0 0 7 4 2 - 3
H. Koizumi et al. / Chemical Physics Letters 242 (1995) 320-324
The present method is free from these limitations, however, V0 is obtained as the difference of two values of the order of 10 eV which may affect the accuracy of the determination. Absolute yields of photoconductivity and photoelectron emission give information on the photoexcitation and photoionization in liquids, and the electron transport through the liquid and across the liquid/vapor interface [8-11]. We have determined the absolute yield of free ions, and compared it with the photoelectron emission yield.
2. Experimental A schematic diagram of the photoconductivity and photoelectron emission experiments is shown in Fig. 1. A deuterium discharge lamp (Hamamatsu L1835) followed by a vacuum ultraviolet monochromator (Acton VM502) was used as light source. The bandwidth of the monochromatized light was 1 nm. The light entered the cell through the MgF2 window coated inside with a gold film of 8 nm thickness. In the photoconductivity experiment, the cell was filled with a liquid sample. In the photoelectron emission experiment, a thin liquid film was put on the cathode. The photoconductivity and the emitted electron current were measured with a pico-ammeter (Keithley, model 617). All measurements were performed at room temperature. The samples were squalane (stated purity 99%) by Aldrich and squalene ( > 98%) by Merck. They were MgF2 window VUV.__[..~___........ ._~xmirrOr
,i ht ]U
1
/
I MgF2/ ] Au window pico-ammeter k - ' ~ & voltage
Y 7"-suppl,
.-.~_ Fig. 1. Experimentalapparatusfor the measurementsof photoconductivityand photoelectronemission.
321
purified by percolation through columns of activated silica gel and molecular sieves 4 .~. Subsequently, they were degassed under vacuum.
3. Results and discussions Fig. 2 shows the photoconductivities and photoelectron emission currents of squalane and squalene. The applied field strength for the photoconductivity experiment was 6.7 kV/cm, and that for the photoelectron emission experiment was 670 V/cm. The absolute light intensity was calculated from the photoelectron emission current from a gold cathode for which the absolute quantum yield of the photoelectron emission (qbp~) is known [11]. The ratio of the photoconductivity current and the light intensity gives the quantum yield of free ions (@~) at the applied field of 6.7 kV/cm. Figs. 3 and 4 show the quantum yield of free ions for squalane and squalene, respectively. The figures include the quantum yield of the photoelectron emission for comparison. Near the threshold, @~ is known to depend as a power function on the energy difference between the photon energy and the threshold energy ( h v - E t h ) [12], @fi = C[ h v - E~h(pc)] n,
(3)
where C is a constant. An exponent of n = 5 / 2 gives the best fit for the ionization of neutral molecules in non-polar solvents [13,14]. In Fig. 5, @fi to the 2 / 5 power is plotted as a function of photon energy. Extrapolation to q~ion= 0 yields the value of Eth(pC). For squalane Eth(pc)--8.25 5:0.1 eV, and for squalene Eth(pc) = 6.35 5:0.1 eV were obtained. With the previously reported values of Eth(Pe) and with Eqs. (1) and (2), we can calculate V0 for these liquids. For squalane V0 -- -0.15 eV, and for squalene V0 = - 0 . 5 5 eV were obtained. These values are compared in Table 1 with those reported for other liquid alkanes and alkenes [7,9,13,15,16]. The value of squalane is comparable to the value of 2,2,4-trimethylpentane, also a compound with CH3-side groups. The value for squalene is considerably lower compared to the values of the other alkenes. It is possible that the great number of CH 3-side groups is responsible for this effect.
1t. Koizumi et aL / Chemical Physics Letters 242 (1995) 320-324
322
l.OxlO
43
....
o.s <
~0.6~ ii 0.4 ~
~/
, .........
, .........
, .........
-1
10 , .........
,....,....,
....
,,,r,i
....
, ....
I ....
, ....
t
....
. ....
10 .2
Squalane
(,) .o
l 0 "3
E
g 10-4
0.2
0.0
• Photoconductivity ; - - - Photoemission J
~
100
120
140
160
180
10-6
200
.,l
. . . .
6
Wavelenl~h / n m
i . . . .
i . , . , l
7
. . . .
i
, . . . .
8
i . . . .
i . . . .
9
i . . . .
i , , .
10
11
PhotonEnergy/ eV
2.0Xl0"13 .... , ......... , ......... , . . . . . . . . . . . . . . . . . . , ......... ,....,
Fig. 3. Q u a n t u m yield o f free ions f r o m p h o t o c o n d u c t i v i t y and photoemission, squalane.
Co)
~'
, ....
.o
10 "5
<
I ....
....
1.5
Comparison of the quantum yields is impeded by the fact that the measurements were possible at two values of the applied field only. The discussion must remain qualitative at best. Both the @~ and ~pe values can be related to the probability of escape of the photoelectrons, Peso, from their parent positive ions,
1.0
0.5 a_
0.0
=
100
120
140
160
180
i
, , .
1.
,,
[ , . .
u
lO q
. . ,
u,
..
I
, , , 1 ' ' ' ' 1
. . . .
, . . . .
I . . . .
, . . . .
I . . . .
! . . . .
I . . . .
!
. . . .
• °o
(c) "
(4)
200
Wavelength / nm ,
r/P~c,
. -
o°
~
i0 -2
,°~ua'ene oq
"""
". §
~
-1
I0 "s
10.3
f"
10 4
,,'
s
.~ -2 I0 "s .4
i
10. 5
-3 104 ,
I
,
100
,
t
I
•
120
.
.
i
.
140
.
.
I
.
.
160
.
I
.
180
.
.
I
200
Wavelength [nm] Fig. 2. Photoeurrents as a function o f the w a v e l e n g t h for squalene; (a) photoconduetivity, E = 6.7 × 103 V c m - 1 ; (b) photoemission, E = 6 7 0 V e m - 1; (e) s p e c t r u m o f the D2-1am p.
,,s • '~
• Photoconduetivity - - - Photocmission
t
10-6 6 . . . . . . . . , . . . . . . . . . , . . . . . . . . . , . . . . . . . . . , . . . . . . . 7 8 9 10
11
P h o t o n E n e r g y / eV Fig. 4. Q u a n t u m yield o f free ions f r o m p h o t o c o n d u c t i v i t y a n d photoemission, squalene.
H. Koizumi et al. / Chemical Physics Letters 242 (1995) 320-324
0.15
,,,r
. . . .
i ....
i . . . .
i ....
i . . . .
i , , , , i , t [ . , i
. . . .
'1
....
o
• O
Squalanc Squalene
o oo o o O
O Oo
0.10
C'
o 00%
i,; °oo
0.05
0oo 6
7
8
9
10
11
Photon Energy / eV Fig. 5. Quantum yields of free ions from photoconductivity at 6.7 × 103 V c m - 1 plotted as a function of photon energy.
where ,/ is the quantum yield of initial photoionization. An ejected electron loses its excess energy by collisions with molecules until it becomes thermalized. The mean separation distances in hydrocarbons between the parent positive ion and the thermalized electron are between 50 and 100 ,~. In principle, the photoemission yield should be greater than the photoconductivity yield since superthermal electrons have a higher probability of escape. If the ionization
Table 1 Photoionization threshold and V0 values for liquid hydrocarbons at room temperature Liquid
Eth(PC) ( e V )
Eth(Pe) ( e V )
V0 ( e V )
squalane squalene
8.25 6.35
8.4 a 6.9 a
- 0.15 - 0.55
n-hexane 3-methylpentane 2,2,4-trimethylpentane (isooctane) 2,2,4,4-tetramethylpentane
8.6 b 8.85 d 8.3 b
+0.10 c + 0.01 ¢ - 0.17 ¢
8.2 c
-0.33 c
2,3-dimethyl-2-butene (tetramehylethylene)
6.8 o
-0.25 c
cis-2-butene a Ref. [11]. b Ref. [13]. c Ref. [7]. d Ref. [9]. • Ref. [16]. f Ref. [15].
323
event occurs at a distance from the liquid/vapor interface which is smaller than the thermalization distance, the emission of a superthermal electron into the vapor phase occurs. If a thermalized electron arrives at the interface, thermal activation is necessary in order to overcome the energetic barrier given by V0. The photoconductivity is determined by thermalized electrons and their parent positive ions only. In the case of photoemission the contributions of superthermal and thermal electrons to the observed emission current depend on the energy dependence of the absorption coefficient. An increase in the absorption coefficient with photon energy leads to a shift in the center of gravity of the ionization events towards the interface. More superthermal electrons are able to escape. In the photoconductivity experiment, an increase in the absorption coefficient (the concentration of charge carrier pairs near the window electrode increases) leads to an increased influence of the interaction between electrons, ions and their respective image charges on the escape process. These conditions seems to be met for squalane where the yield of photoemission exceeds the yield of photoconductivity over the whole range of photon energies. In squalene, above 8.5 eV, the same reasoning holds. At lower photon energies the photoconductivity yield exceeds the emission yield. It is possible that due to the more negative value of V0 the transfer of thermal electrons is hampered due to the formation of a space charge layer. Measurements at different applied field strengths could clarify this point.
Acknowledgement RK received financial support from the Nishina Memorial Foundation for a one year stay at the Hahn-Meitner-Institut. HK received financial support for a short stay from the Hahn-Meitner-Institut. Both contributions are gratefully acknowledged.
References - 0.16 f
[1] M. Pope and C.E. Swenberg, Electronic processes in organic crystals (Clarendon Press, Oxford, 1982). [2] K. Seki, Mol. Cryst. Liquid Cryst. 171 (1989) 255.
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H. Koizumi et al. / Chemical Physics Letters 242 (1995) 320-324
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