J. Phys. Chem. 1981, 85,4008-4015
4008
Determination of Energies of Quasifree Electron State V o in Organic Solids from Electron Transmission Spectra Kenzo Hlraoka Faculty of Engineering, Yamanashi Universlty, Takeda-4, Kofu 400, Japan (Received: April 6, 1981; In Final Form: August 3, 1981)
The transmission of low-energy electrons (0-15 eV) through 10-100 A films of alkanes, alkenes, and aromatic compounds has been systematically studied. Structures are clearly indicated by electron current Ittransmitted through a thin film as a function of the incident electron energy Vi, displayed as dlt/dVi vs. Vi. With increasing the film thickness, a drastic decrease of the height of the first peak (due to injection of electrons in the film) and an appearance and a strong growth of a second peak are observed for alkanes and alkenes. The energies of quasifree electron state V, for solid pentane, hexane, cyclohexane, methylcyclohexane,heptane, octane, decane, isopentane, 3-methylpentane, 2,2,4-trimethylpentane, 1-hexene,cyclohexene, 1-octene, and 1-decene are determined by measuring the energy of the second peak from the first peak of the spectrum for the metal block. For neopentane, tetramethylsilane, and aromatic compounds, neither a drastic decrease of the first peak nor an appearance of a second peak is observed, indicating that these compounds have negative V, values. The V, values for benzene, toluene, and naphthalene are determined by measuring the positive shifts of the energy scale of the transmission spectra relative to the gas-phase electronic energy levels. From the energy of the onsets of broad negative peaks appearing at -14 eV for alkanes and alkenes, the solid phase ionization energies I , are determined. The polarization energies of cations P+by the solid media are estimated for some compounds.
Introduction Excess electrons are excellent probes of the complex electron-molecule interactions found in condensed media. Electron mobilities and conduction state energy levels relative to vacuum are the major experimental quantities available to probe the weak interactions of excess electrons. Condensed hydrocarbons are complex and electron interactions in them are the focus of much current interest. The energies of quasifree electron state V, have been reported for several liquid hydrocarb~ns.l-~However, attempts to measure Vo directly in polar or nonpolar glassy matrices have failed.5 The V, values in glassy matrices have been estimated by an indirect method using eq 1," Il, I, = I g
+ P + v,
(1)
where I1 and I, are the ionization potential of an impurity molecule A in a liquid and solid solution, respectively, Ig is the gas phase ionization potential of A, PC is the polarization energy of the medium by the positive ion A+, and V, is the energy of quasifree electron state. Recently, Hiraoka and Hamill* and Sanches reported a simple method for measuring the electronic levels of molecules supported as ultrathin films on a metal surface at low temperatures. The film was bombarded by a beam of low-energy electrons, and the current transmitted through the film (It)was measured as a function of the incident electron energy (Vi). The transmission spectra, displayed as dIt/dVi vs. Vi or d21t/dVt vs. Vi, were found particularly useful for detecting optically forbidden elec(1)R. A. Holroyd and M. Allen, J. Chem. Phys., 54, 5014 (1971). (2)R. A. Holroyd, J. Chem. Phys., 57, 3007 (1972). (3)R. A. Holroyd and R. L. Russell, J . Phys. Chem., 78,2128 (1974). (4)R. Schiller, Sz.Vass, and J. Mandics, Int. J. Radiat. Phys. Chem., 5,491 (1973). (5)S.Noda, L.Kevan, and K. Fueki, J. Phys. Chem., 79,2866(1975). (6)J. Bullot and M. Gauthier, Can. J. Chem., 55, 1821 (1977). (7)D.Grand and A. Bemas, J. Phys. Chem., 81, 1209 (1977). (8)K.Hiraoka and W. H. Hamill, J. Chem. Phys., 56,3185(1972);57, 3870, 3881, 4058 (1972);58, 3686 (1973);59,5749 (1973). (9)L. Sanche, Chem. Phys. Lett., 65,61 (1979);J. Chem. Phys., 71, 4860 (1979). 0022-365418112085-4008$01.25/0
tronic transition^.^,^ In the previous work,1° we reported direct measurements of energies of quasifree electron state Vo for solid hexane and benzene by measuring the transmission spectra dIt/dVi vs. Vi for 10-100 A films of benzene and hexane at -80 K. In this work, transmission spectra for a large variety of hydrocarbons are measured and the interactions of injected excess electrons with films are systematicallystudied. The main objective of this work is to relate the energy dependence of the transmission features to specific interactions occurring in films of alkanes, alkenes, and aromatic compounds.
Experimental Section The experimental procedure has been described.8J0The temperature of the stainless steel metal block was -80 K and pressure was SlO-' Pa. The cathode was modulated by 0.3 V a t 78 Hz by the reference signal of a lock-in amplifier. The cathode voltage was swept upward with It I2 X A transmitted through the thin film. The transmission spectra were displayed as dIt/dVi vs. Vi, where eVi is the energy of the incident electron. The zero of the electron energy scale in the present work has been chosen arbitrarily as the onset of the first peak of the spectrum dIt/dVi vs. Vi (i.e., for electron injection). The first choice is convenient when no established sharp transition is available as an internal standard. All samples except neopentane and tetramethylsilane (Me,Si) were Tokyo-Kasei extra pure. Neopentane was Tokyo-Kagaku-Seiki extra pure. Me4% was supplied by Eastman Organic Chemicals. The film thickness was changed by changing the deposition time with the constant vapor pressure at 1.3 X Pa. Results and Discussion In the previous work,1° the transmission spectra dIt/dVi vs. Vi for benzene and hexane were discussed in detail. It was found that there was almost no difference in shape (height and half-width) between the first peak for the metal block and those for 2 to 10 langmuirll thick benzene (10)K.Hiraoka and M. Nara, Bull. Chem. SOC.Jpn., 54,1589(1981).
0 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 26, 798 1
Energies of Quasifree Electron State
1
I S O PENTAN€
4009
P n.PENTANt ,
I
I
> metal block
I
0
2 4 6 9 IC 12 I4 INCICENT ELECTRON E N E R G Y , e V
I6
, 0
Flgure 1. The transmission spectra dIt/dVl vs. V , for metal block and isopentane. The film thickness was changed from 2 to 10 langmuirs.
films. On the contrary, the first peak of transmission spectra for hexane decreased drastically and a second peak appeared and grew strongly as the film thickness was increased. At 30 langmuirs, the first peak showed up as only a small shoulder peak of the second peak. From these experimental results, it was concluded that thin solid films of benzene and hexane have negative and positive Vo, respectively. The Vovalue for hexane, 0.9 eV, was determined by measuring the energy of the second peak from the first peak of the spectrum for the metal block as a reference. In the following sections, the results for alkanes, alkenes, and aromatic compounds are summarized, and the discussion is presented for the determination of the energy of quasifree electron state V,, the solid phase ionization energy I,, and the cation polarization energies by the medium P+. Alkanes. The transmission spectra dIt/dVi vs. Vi for isopentane, pentane, and neopentane are shown in Figures 1-3. A decrease of the first peak and a growth of the second peak are observed for isopentane with an increase of the film thickness, indicating that the Vovalue in solid isopentane is positive. The behavior of pentane spectra is somewhat different from other alkanes (Figure 2). The first peak decreases more drastically than that of isopentane by the deposition of a 2 langmuir thick film, but it does not show any decrease with an increase of the film thickness from 2 to 10 langmuirs. On the other hand, the second peak becomes smaller as the film thickness increases. In the separate experiments, it was observed that the sharp second peak did not show up when the temperature of the metal block became higher than liquid nitrogen temperature by 10 K. This observation suggests that the change of pentane spectra is not due to the increase of the film thickness but to the change of the structure of the condensed pentane film on the metal block. A closer examination of pentane spectra reveals that the energy of the second peak stays almost constant and the intensity of the second peak sim-
-
(11) The amount of the gas admitted in the vacuum chamber is expressed in langmuir units (1 langmuir = 1 X 10" torr s). When the sticking probability is unity, the surface will be covered by approximately one monolayer with 1 langmuir gas admission.
I
2 4 6 8 IO I2 I4 INCIDENT E L E C T R C N ENERGY, e V
'
16
Figure 2. The transmission spectra for metal block and pentane.
0
2
4
6
8
0 1 2
I 4 1 6
IhCIDENT ELECTRON ENERGY, eV
Flgure 3. The transmisslon spectra for metal block and neopentane.
ply becomes smaller with an increase of the film thickness. Considering that the height of the first peak stays almost constant from 2 to 10 langmuirs, the film may gradually form islands on the metal block. Another less probable possibility is the gradual change of the structure of the film with time, possibly from glassy to crystalline structure. Although any further speculation would not be profitable because no detailed information on the structure of the deposited sample can be obtained by the present method, it would be worthwhile to note that the present method is sensitive enough to detect the "change" of the structure of the thin solid film deposited on the metal block. The transmission spectra for neopentane are shown in Figure 3. Contrary to the spectra for isopentane and pentane, the first peak neither decreases nor is a growth of the second peak observed with an increase of the film thickness, indicating that Voin solid neopentane is nega-
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The Journal of Physical Chemistry, Vol. 85, No. 26, 1981
l " " " " " " " " 1
Hiraoka
I
TMS
1
I
0
2 4 6 8 IO 12 14 INCIDENT ELECTRON ENERGY, eV
16
0
2
4
6
8
INCIDENT ELECTRON
Flgure 4. The transmlssion spectra for metal block and tetramethylsilane.
IO
12
14
16
E N E R G Y , eV
Figure 6. The transmission spectra for metal block and 2,2,4-trimethylpentane.
3-METHYLPENTANE
I /I 0
2
4
8
6
IO
I2
14
16
I N C I D E N T E L E C T R O N ENERGY, e V
m e t a l block
Figure 5. The transmission spectra for metal block and 3-methylpentane.
tive. In our experiments,there have been no alkanes found whose Vo values are negative except neopentane. It is known that the electron mobility in liquid and solid neopentane is exceptionally high among other hydrocarbons.12J3 The transmission spectra for tetramethylsilane, which is also known for its high electron mobility, are shown in Figure 4. As in the case of neopentane, no decrease of the first peak is observed. Besides, a sharp negative peak is observed at 1.5 eV which is also present in benzene transmission spectra.1° It is obvious that the solid MelSi also has a negative V,. Unfortunately, no established sharp transitions are available as internal standards for neopentane and Me4%and it is not possible
-
~
~~~
~
(12)W.F. Schmidt, Can. J. Chern., 55, 2197 (1977). (13)H. Namba, K.Shinsaka, and Y. Hatano, J.Chern. Phys., 70,5331 (1979).
I
1 0
4 INCIDENT
2
8 IO 12 14 ELECTRON ENERGY / eV
6
16
Figure 7. The transmission spectra for metal block and heptane.
to estimate Vofor these compounds. Transmission spectra for 3-methylpentane, 2,2,4-trimethylpentane (2,2,4-TMP),heptane, and cyclohexane are presented in Figures 5-8. The appearance of the second peak is observed for all these compounds; i.e., their Vo values are positive. The second peak of the spectra for 2,2,4-TMP appears almost as a shoulder peak of the first peak, indicating that its Vo is smaller than less globular hydrocarbons. When the temperature of the metal block was higher than liquid nitrogen temperature by -15 K during the sample deposition, the sharp second peak could not be observed for 2,2,4-TMP and cyclohexane; instead a broad first peak was observed which showed a decrease with an
The Journal of Physlcal Chemistry, Vol. 85, No. 26, 1981 4011
Energies of Quasifree Electron State
-
-7
I " " " " " "
CYCLOHEXANE
TOLUEhE
I
,
> 1
V
D
I
metal block
'
I
0
2
4
INCIDENT
ic 0
2 4 INCIDENT
m e t a l block
6 8 ELECTRON
10 I2 14 E N E R G Y , eV
I6
Flgure 10. The transmission spectra for metal block and toluene. I
6 8 10 12 14 ELECTRON E N E R G Y , eV
16
NAPHTHALENE
Flgure 8. The transmission spectra for metal block and cyclohexane.
CYCLOHEXENE
THRESHOLD
EXCITATION
'1
I
> u
1 u
I-----__,
metal block
0
2 4 INCIDENT
6 8 ELECTRON
12 I4 E N E R G Y , eV
IO
2 4 6 8 IO 12 14 16 INCIDENT ELECTRON E N E R G Y , eV
I 16
Flgure 9. The transmission spectra for metal block and cyclohexene.
increase of the film thickness. As was suggested for the case of pentane, the structure of the films of these compounds deposited at higher temperatures may be different from that deposited at the liquid nitrogen temperature. Alkenes. The transmission spectra for 1-hexene, cyclohexene, 1-octene, and 1-decene were measured. For all of these compounds, a decrease of the first peak and an appearance of a second peak were observed, indicating that these compounds have positive V,. The transmission spectra for cyclohexene appear in Figure 9. The decrease of the first peak is much less prominent than that for cyclohexane and the second peak appears as only a shoulder peak on the first peak; i.e., the Vovalue for cyclohexene is smaller than that for cyclohexane. Aromatics. Generally, the height of the first peak of the transmission spectra for aromatic compounds such as benzene, toluene, pyridine, aniline, naphthalene, biphenyl,
Figure 11. The transmission spectrum for naphthalene. The threshold excitation spectrum" is also shown for comparison.
and octylbenzene neither shows a marked decrease nor are the appearance and a growth of a second peak observed with an increase of the film thickness. Thus these aromatic compounds are considered to have negative Vovalues. The transmission spectra for toluene and naphthalene are shown in Figures 10 and 11. The energy scale for toluene spectra is calibrated by using spectroscopic data as references.14-16 The features and energies of peaks of toluene spectra are surprisingly similar to those for benzenelo (see also Figure 12). The horizontal axis for naphthalene spectra is the electron energy for the threshold excitation spectrum of gaseous naphthalene measured by Compton et a1.l' The agreement between structures in the transmission spectra and reference data is good except a small (14)DMS U.V.Atlas of Organic Compounds, Butterworth, London, 19fifi
(15)D.F. Evans, J . Chem. SOC.2753 (1959). (16)M.J. S. Dewar and S. D.Worley, J. Chem. Phys., 50, 654 (1969). (17)R. N. Compton, R. H. Huebner, P. W. Reinhardt, and L. G . Christophorou, J. Chen. Phys., 48,901 (1968).
4012
Hiraoka
The Journal of Physical Chemistry, Vol. 85, No. 26, 1981 120
11
OCTYLBENZENE
BENZENE 0‘
0
1
1
I
1
1
I
4
IO
20
30
40
50
60
70
FiLM
0
2
INCIDENT
4
6
8
1 0 1 2 1 4
E L E C T R O N E N E R G Y , eV
Figure 12. The transmission spectra for metal block, benzene, octylbenzene, and octane. The energy scale is for the spectra of benzene. The zero of the energy scale for octane and octylbenzene is chosen arbitrarily as the onset of the first peak.
peak at 1.1 eV for toluene, and a sharp shoulder peak at 1.6 eV for naphthalene. Recently, Jordan and Burrow studied the temporary anion states of unsaturated hydrocarbons in the gas phase by electron transmission spectroscopy.lS They observed the vertical temporary anion states at 1.15 and 4.85 eV for benzene, at 1.11 and 4.88 eV for toluene, and at 0.19,0.90, 1.67, 3.37, and 4.72 eV for naphthalene, respectively. In the transmission spectra for benzene and toluene (Figures 12 and lo), small but sharp peaks at -1.1 eV are observed. In the transmission spectrum of naphthalene (Figure 1 0 , a sharp shoulder peak at 1.6 eV is observed. The agreement between the energies of peaks at transmission spectra and those of temporary anion states measured by Jordan and Burrow is remarkably good. Such good agreement might suggest that the peaks observed below the lowest excited electronic states observed in benzene, toluene, and naphthalene transmission spectra are due to the temporary anion states. However, there are other structures remaining to be explained with peaks at -1.8 and -2.7 eV in benzene (Figure 12) and at -2.2 eV in toluene transmission spectra (Figure 10). Besides, the transmission spectra for saturated hydrocarbons generally have characteristic structures in the energy region of 0-3 eV above the second peak (see Figure 1-8). It is difficult to consider that these structures are due to the temporary anion formation because the angular momentum of the lowest unfilled molecular orbitals for saturated hydrocarbons is zero and no efficient angular momentum barrier can be set to retain the incoming electron at the target molecule. Thus another mechanism must be invoked to explain the low-energy events in the transmission spectra. The transmission spectrum is a convolution of the currentvoltage characteristics of an electron gun I(V), and the energy-dependent acceptance coefficient of electrons for the film F(V).lo F(V) may be affected by so many factors such as (1) the density and the structure of the film, (2) the density and the distribution of the well-depth of (18) K. D. Jordan and P. D. Burrow, Acc. Chem. Res., 11,341 (1978).
THiCKNESS
,
I 80
8
Flgure 13. The dependence of the height of the first peak (due to injection of electrons in the film) on the film thickness. The peak height at zero film thickness (Le., 100%) corresponds to the height of the first peak for the metal block before the sample is deposited. The dotted lines in the figure do not have any physical meanings.
trapping sites,8J9 (3) the energy dependent structure factor: (4) angular and energy-dependentelastic and inelastic cross sections for incident electrons,9p20(5) the energy and the distribution of density of states of the conduction band for excess electrons,21(6) the influence of image potential: (7) the energy-dependent reflection coefficient of slow electrons at the film-vacuum and filmsubstrate interface: (8) the surface potential of the film,22(9) electron transfer processes in the film,23v24etc. From the experimentalpoint of view, it is important to determine which factors play major roles for the events observed in the low-energy region of the transmission spectra. In order to interpret the first low-energy structure in the transmission spectra, Sanche suggested the importance of an enhancement of vibrational excitation related to the energy dependence of the structure factorsg Recently Ueno et a1.21measured the secondary electron emission from the solid n-CzsHss,n-C36H74, n-C4Hw, and polyethylene, and investigated the conduction band structures of these solid films. If the information on the conduction band structures for the solid samples investigated in this experiment were available, it would be very helpful for an understanding of the low-energy events observed in the transmission spectra because the profile of the conduction band must be reflected on the transmission spectra to some extent. At the present stage, the relevant experimental evidences obtained are far from adequate for the full understanding of the transmission spectra, and no further speculation will be given in this paper. It has been described’O that the sign of Vofor benzene is opposite to that for octane. In order to investigate the effect of the structure of molecule on V,, the transmission spectra for octylbenzene were measured (Figure 12). Figure 12 also shows the transmission spectra for benzene and octane for comparison. The film thickness is 10 langmuirs for all samples. The transmission spectra for octylbenzene show neither a decrease of the first peak nor an appearance and a growth of the second peak with an increase of the film thickness. The thin film of octyl(19) K. Funabashi and T. Kajiwara, J.Phys. Chem., 76,2726 (1972). (20) I. Cheng and K. Funabashi, J. Chem. Phys., 59, 2977 (1973). (21) N. Ueno, T. Fukushima, K. Sugita, S. Kiyono, K. Seki, and H. Inokuchi, J. Phys. SOC.Jpn., 48, 1254 (1980). (22) E. G. McRae, Rev. Mod. Phys., 51, 541 (1979). (23) J. J. Huang and J. L. Magee, J. Chem. Phys., 61, 2736 (1974). (24) Y . 4 . Chang and W. B. Berry, J. Chem. Phys., 61, 2727 (1974).
Energies of Quasifree Electron State
benzene must have a negative Vo. Energy of Quasifree Electron State V,. Figure 13 shows the change of the height of the first peak as a function of the film thickness. The peak height at zero film thickness (i.e., 100%) corresponds to the height of the first peak for the metal block before the sample is deposited. The height of the first peak for MelSi neopentane, benzene, and toluene shows only a gradual decrease with an increase of the film thickness. It is interesting to note that the initial increase is observed for Me4Siand neopentane (see Figures 3 and 4). The gradual decrease of the peak height and an absence of a growth of the second peak are the characteristic features for a film whose Vois negative. On the other hand, a film whose Vois positive generally shows a sudden decrease of the peak height after the deposition of the sample and a growth of the second peak, as shown in Figures 1, 2, and 5-9. Vois specifically the energy of the bottom of the conduction band and this band has some distribution of density of states. The second peak of the transmission spectra may approximately correspond to the first derivative of the distribution function of density of states of the conduction band; i.e., the second peak corresponds to the maximum slope near the onset of the conduction band. As Vois defined as the onset of the conduction band measured with respect to the vacuum level, the energy difference between the second peak and the first peak for the metal block (i.e. the approximate vacuum level) could be somewhat above the actual onset energy. In addition, the bottom of the conduction band must have some fluctuation of energy because of a more or less disordered structure of the film. The energy spread of the rising branch of the second peak is 0-0.3 eV larger than that of the first peak (see Figures 1, 5 , 7, and 8). This energy spread may be due to the leading tail near the onset of the conduction band and to the fluctuation of the energy of the bottom of the conduction band. Unfortunately, the resolution in this experiment is not sufficient to determine which effect is more prevalent. In this experiment, the Vovalues are tentatively determined by measuring the energy of the second peak from the first peak of the metal block. The estimated error for Vomay be around +0.2, -0.3 eV. The determined Voare listed in Table I (column 2). Because the energy resolution in this experiment is about 0.4 eV, the second peak may not be resolved for a compound whose Vois 0-0.4 eV. A more detailed information on the energy of the conduction band and its distribution function could only be obtained by using an apparatus with much higher resolution like S a n c h e ' ~ . ~ The negative excess electron energy may be estimated by an amount of the positive shift of the spectrum if the reference peaks are available for the determination of the energy scale. Since the first peak of the spectrum describes a crude electron energy distribution function with the high-energy tail at the onset, the positive shift of the position of the first peak from the zero incident electron energy would correspond to the negative excess electron energy. The calibration of the energy scale shifts the transmission spectra of toluene and naphthalene by 0.3 and 1.1eV, respectively (see Figures 10 and 11). Thus the Vo values in solid toluene and naphthalene are determined as -0.3 and -1.1 eV, respectively. The estimated error for these values may be f0.2 eV. It was evident from our experimental results that the excess electron energies Vo for pyridine, aniline, biphenyl, octylbenzene, neopentane, and Me4Siare negative, but the estimation of their values is difficult because of the lack of reference data or sharp structures in the transmission spectra.
The Journal of Physical Chemistry, Vol. 85, No. 26, 198 1 4013
TABLE I: V , , Vo.liarIs,I,, a n d P
cyclohexane methylcyclohexane heptane octane decane isopentane neopentane Me,Si 3-methyl pentane 2,2,4-TMP 1-hexene cyclohexene 1-octene 1-decene benzene toluene pyridine aniline naphthalene biphenyl octylbenzene
O . O l d 9.0 10.35 -1.9 0.02d 9.0 10.18 -2.1
0.6 0.9 (0.98,' 0.66,b 0 . 6 2 b ) 0.5 (0.48,c 0.47b) 0.5
pentane hexane
O.Ole
8.6
9.81 -1.7
0.08e
8.8
9.85 -1.6
0.13e O.lfje
9 . 0 10.08 - 2 . 1 9.1 10.25 -2.1 9.0 9.0 10.32 -1.8
1.0 1.0 1.0 (0.93') 0.5
