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Photoelectric emission from the valence band of some alkali halides
J. Phys.
Chcm. Solids
Pergamon Press 1957. Vol. 3. pp. l-6.
PHOTOELECTRIC
EMISSION
OF SOME
FROM THE VALENCE
ALKALI
E. A. TAFT
BAND
HALIDES
and H. R. PHILIPP
General Electric Research Laboratory, (Received
26 April
Schenectady,
New York
1957)
Abstract-Photoelectric emission from the valence band, previously measured for CsI, has been investigated in the other alkali iodides and several other halides. As in CsI, the yields rise rapidly from the threshold to values of order 0.1 electron/incident photon. For the iodides, concurrent measurements of reflectivity and optical absorption agree with MARTIENSSEN’Srecent results and show detail consistent with interband transitions. Approximate values for the energy difference between the bottom of the conduction band and the vacuum level are deduced.
INTRODUCTION
sults on absorption at 20°K. Within this limitation, however, they corroborate his findings. The reflectance results are useful for comparing evaporated films with single crystals. Except for interference effects in the films (see Fig. 4), results for the two kinds of surfaces were identical. To facilitate discussion of trends in the electronic energy structures, tentative new data for LiI are included.c4) For purposes of comparison, intrinsic photoemission data are also presented for all the potassium halides and for those cesium salts having the CsCl structure.
the photoIN AN earlier paper,@) we described emission from CsI. Large yields, observed for hv > 6 eV, were independent of defects in the samples and were attributed to emission from the va ence band. Concurrent optical absorption data showed a comparatively small edge consistent with band-to-band transitions if the electron affinity of CsI was estimated to be small (i.e., if the bottom of the conduction band lay only slightly below the vacuum level). This paper reports similar data on the other alkali iodides in which the electron affinity is larger, of order 1 eV. MARTIENSSEN~~)has recently made an elegant and extensive investigation of the optical absorption of the alkali iodides. At 20”K, the absorption edge for CsI shows up more clearly than in reference 1. Furthermore, it appears to be double in structure with components at hv = 6.24 and 6.31 eV. (In addition, the second absorption peak near 6 eV is also doubled.) In this paper, we continue to relate this double edge in CsI to band-to-band transitions and treat it as analogous to the very clear single edges found by MARTIENSSEN(~) at roughly the same photon energy for iodides of Rb, K, and Na.t3) Along with our photoelectric data on the iodides we present optical absorption and reflectance data taken at 300°K and at 80°K. They are point-bypoint measurements and are not as detailed as MARTIENSSEN’S recording-spectrophotometer reA
EXPERIMENTAL Vacuum tubes of three different designs were employed. (1) Cylindrical diodes, fitted with cleaved LiF windows,* were used to determine the spectral distribution of photoelectric yield at 300°K. Emission measurements were made on evaporated films with the spectral range extended to greater than 11 eV with the aid of a vacuum monochromatorc5) kindly placed at our disposal by P. D. * Windows, cleaved from single crystals obtained from the Harshaw Chemical Company, were sealed to a 4 in. diameter extension from the tube envelope with Dennison’s red sealing wax. Bake temperatures were limited but system pressures of lo-’ mm Hg were readily obtainable. Tubes with quartz bubble windows showed the same behavior to 8 eV. The alkali halides do not Beem to be very sensitive to vacuum conditions, but we must avoid any claim that these surfaces, although reproducible, were atomically clean. 1
2
E.
A. TAFT
AND
H.
R.
PHILIPP
over the temperature range 80 to 500’K held single crystals or evaporated films (on Al,O, crystals) in vacuum behind fused-quartz windows, through which measurements were made.
I-
I
RESULTS
Spectral distribution of the photoelectric yield 2
lo-’, 6
1
I
I
7
I
I
0
I
9
I IO
I
I II
hv
eV
FIG. 1. Spectral distribution of the photoelectric yield for the alkali iodides.
I I , KCI
hv
eV
FIG.2. Spectral distribution of the photoelectric
yield for the potassium halides and for several cesium salts.
JOHNSON. (2) A double
window tube, described in a previous paper,(l) provided data on the optical absorption of thin films in the temperature range 80 to 500°K. (3) Tubes used to measure reflectance
The spectral distribution of the photoelectric yield for the alkali iodides is shown in Fig. 1. For comparison purposes, data on other halide samples are shown in Fig. 2. The curves are all similar in shape. There is an initial rapid increase in yield which levels off at values near O.l.* t This emission is an intrinsic property of the emitter, independent of crystal impurities and imperfections, and is attributed here to electrons ejected from the valence band. The threshold for this high-yield process is a measure of the energy difference between the top of the valence band and the vacuum level. The shift in the threshold for corresponding potassium and cesium halides is slightly less than 1 eV and will be referred to in a later discussion of the electron affinity. When F-centers are deliberately added to these salts by irradiation in the first fundamental absorption band, a measurable yield appears at lower the yield for quantum energy. (6) For completeness, a KI sample containing about lOle F-centers cm-3 is shown in Fig. 3. Assuming that each incident photon produces one excited electron, the plateau value for the valence band yield indicates that the excited electrons have roughly a 10 per cent chance of appearing outside the surface. If one assumes the same escape probability for the exciton-induced yield, 2 x 10-S at the first absorption peak, one concludes that the efficiency of energy transfer to F-centers is low in these KI samples, of order 0.02. The energy distribution of the exciton-induced * Emission from the valence band of LiI appears to be roughly coincident with that from the iodides of Rb, K and Na at yields below 10m2; the threshold may be slightly lower. Processes involving F-centers or other defects sometimes complicate the measurements below hv = ‘7.5 eV. For KF the currents are small and the results of Fig. 2 are only qualitative. t Spectral distribution data on single crystals of both NaCl and KC1 have been obtained at Cornell University by J. W. TAYLOR~~~ P. L. HARTMAN to whom we are indebted for discussion of their work. Our KC1 results are in good agreement with theirs.
PHOTOELECTRIC
EMISSION
FROM
VALENCE
BAND
OF SOME
ALKALI
HALIDES
3
“Y
FIG. 3. Spectral distribution of the photoelectric yield for a KI sample containing N 1O’O F-centers cmw8.
FIG. 4b
yield is considerably different from the valence band yield, however, and the actuaf. value may be somewhat higher.(‘)
nv
FIG. 4c
C”
4
E.
pl)
A.
TAFT
AND
K.
R.
PHILIPP
*v
er
FIG. 4d
ev
FIG. 4e
FIG. 4. Reflectance and optical absorption of the alkali iodides at 300°K (dotted curves) and 80% (solid curves). Reflectance data are presented for single crystals, except for RbI and LiI, where measurements were made onlyon
anneated evaporated films. Interference effects appears in these films in the region of optical transparency. Optical trans~ssion data for annealed evaporated films are plotted as estimated absorption constant; no correction has been made for surface effects such as reflectance. ?‘he values were roughly calculated from interference coIors and tube geometry.(“)
Optical properties Absorption constants for evaporated, annealed alkali-iodide films are shown in Fig. 4. (No corrections have been made for reflection loss; the absolute values are rough estimates.) The results agree with ~~~RTI~~SSEN'S. The doubiing of the second absorption peak of CsI (which we previously overlooked(Q) barely shows up at 8O0K, as MARTIENSSEN has pointed out. The weaker double structure of the absorption edge we were not able to resolve with our techniques, but the edge itself is clear. For RbI and KI, the double peak is missing, and the absorption edge (MARTIENSSENlists these as “stufe”) is very pronounced. It moves closer to the first peak in NaI and becomes less distinct. For LiI it appears to have moved under the first peak, which is weaker. The reflectance data are consistent with these results,
showing csI.@*
better
resolution
of
the
doublet
for
DISCUSSION We obviously cannot assign unique threshold photon energies to the valence band photoemission or to the optical absorption edge in the absence of a more quantitative treatment of these effects. Useful comparisons may be made, nevertheless, by arbitrarily noting the values of hv at which the photoemission is 1O-3 electron/photon and at which the optical absorption constant rises more or less step-wise near a value of lo5 cm-l. Table 1 shows such values, the photoelectric thresholds (column 1) being given for 300°K and * The room temperature results for KI are in agreement with early reflectance measurements of HILSCH and POHL.
PHOTOELECTRIC
EMISSION
FROM
VALENCE
Table 1 = Crystal
_-
CSI RbI KI NaI LiI
1 6-4 ev 7.3 7.3 7.3 7.3
---
2
(i*3* 6.1 6.2
I i I
3
4
5
0.1 I.2 1.1
5.76 5.69 5.80 5.55 5.92
0.5 0.4 0.4 0.2 0.0
i
1. Photoelectric threshold for valence band emission at 300’K. 2. Optical absorption edge at 80°K. 3. Electron affinity. 4. First fundamental absorption peak at 80°K. 5. Exciton ionization energy. * At 20” M~~~~~~ss~~ shows two edges at 6.24 and 6.31, respectively. t Taken as coincident with the exciton peak.
the absorption edges (column 2) for 80’K. (Photoelectric thresholds at 80°K would be roughly O-2 eV higher,(l) a correction which we neglect in view of the coarseness of this procedure). The photoelectric threshold fixes the valence band relative to the vacuum level, and the optical absorption edge corresponds to the valence-conduction band gap. The difference is the electron affinity (column 3) of the crystal, which has a very small value for CsI but a larger one for the other salts. The estimates tend to be too small rather than too large. In Fig. 4 of the paper by HARTMAN,NELSONand SIEGFRIED@) there appears an interesting feature in the optical absorption of KC1 which looks somewhat like the absorption edge in KI. Assuming for the moment that the two cases are comparable, we arrive at an electron affinity of O-2 eV for KC1 using the photoelectric threshold 8.7 eV from Fig. 2 above. For LiI, the optical absorption edge is ill-defined and has been taken in Table I as coincident with the first absorption peak. The absorption spectrum bears some resemblance to those of the silver salts, for which the discrete first exciton peak is not clearly separated from the continuum as it is in RbI for example.(g)$ The valence band is the Sp band of the iodine ions, which is broadened as $ Dr. W. M&TIENSSEN also has kindly sent us absorption data taken by Y. OKAMOTO at Giittingen on these salts.
BAND
OF SOME
ALKALI
HALIDES
5
these ions are packed more closely in LiI.@O) Thus, one expects the absorption edge to be less sharp than for the heavier iodides. (We notice incidentally that the high-frequency dielectric constant of LiI is higher than for any other alkali halide; it approaches that of the silver halides.) To display the pertinent trends more clearly in Table 1, we plot the difference in energy (column 5) between the first absorption peak (column 4) and the absorption edge (column 2). Keeping in mind the unsophisticated definitions adopted here, we call this the “exciton ionization energy”. We have not discussed in any precise way the character of the band-to-band optical transitions that we associate with the optical absorption edge and the succeeding continuum. We should remark, however, that a recent calculation of the electronic energy structure of KC1 by HOWLAND shows that both direct and indirect optical transitions could originate near the top, of the valence band without differing greatly in energy. Another subject which is worth further scrutiny is the behavior of the photoemission in the vicinity of the absorption edge and at the position of the absorption peaks lying beyond the edge. At 80’K the iodides show a slight rise (of order 30 per cent) in the photoelectric yield at the absorption edge when F-centers are present in the sample. (This effect is not clear in Fig. 3 for KI since this curve is for 300°K. It appears weakly in Fig. 1 of the last paper of reference 6). At the present time this small rise looks like a peak in the yield near 6 eV rather than like a plateau that follows the optical absorption beyond the edge. There may conceivably be an unresoived optical peak at this point, though we are reluctant to believe this in view of MARTXENSSEN’S detailed work at 20’K. In any case, we summarize by saying that optical absorption in the immediate vicinity of the edge stimulates photoemission from Fcenters for reasons not yet understood. For KI, irradiation in the second optical absorption peak at 6.7 eV, which lies beyond the edge, does not appear to stimulate photoemission from F-centers, at least with efficiency comparable to that found in the first peak. Experiments in the vicinity of the valence band emission threshold are difficult, however, and we have therefore shown this region in Fig. 3 as a dashed line to indicate it as tentative.
E.
6
A.
TAFT
AND
Acknowledgements-We are grateful to Dr. W. MAFCTIENSSEN for an advance copy of his paper and for comments on his work. We are indebted to P. HARTMAN and J. TAYLORfor interesting discussions of their work. We particularly thank L. APKER for many helpful discussions throughout the course of this work.
H. R. PHILIPP
5. 6.
7.
8.
REFERENCES I. PHILIPP H. R. and TAFT E. r. Phys. Chem. Solids 1, 159 (1956). 2. MARTIENSSEN W. r. Phys. Chem. Solids2, 257 (1957). 3. HILXH R. and POHL R. 2. Phys. 59, 812 (1930); SCHNEIDERE. and O’BRYAN H. Phys. Rev. 51,293 (1937); MARTIENSSEN W. Nuchr. Akad. Wiss. Giittingen. Math. Physik 11, 257 (1955); FESEFELDT H. Z. Phys. 64, 626 (1930); FISCHER F. Z. Phys. 139, 328 (1954). 4. HILSCH R. and POHL R. W. Z. Phys. 59,812 (1930);
9. 10.
Il.
SCHNEIDERE. and O’BRYAN H. Phys. Rev. 51, 293 (1937). JOHNSONP. D.3. Opt. Sot. Amer. 42, 278 (1952). APKER L. and TAFT E. Phys. Rev. 79,964 (1950); 81, 698 (1951); 82, 814 (1951); PHILIPP H. R. and TAFT E. Phys. Rev. 106, 671 (1957). In this connection see INCHAUSPEN. and MAURER R. J. Air Force Technical Note OSR-TN-55-281 ; TEEGARDENK. J. Phys. Rev. 105, 1222 (1957). HILSCH R. and POHL R. W. Z. Phys. 57, 145 (1929); HARTMAN,NELSEN, and SIEGFRIEDPhys. Rev. 105. 123 (1957). TUTIHASI S. Phys. Rev. 105, 882 (1957). O’BRYAN H. and SKINNER H. Proc. Rov. Sot. A176,229 (1940); MOTT N. F. and GURNE; R. W. Electronic Processes in Ionic Crystals p. 79. Oxford University Press, London (1940). HOWLAND L. P. Quarterly Progress Report, Solid State and Molecular Theory Group p. 23. M.I.T., Cambridge, Mass. (1957). We are indebted to Dr. HOWLAND for stimulating conversations.
Chcm. Solids
Pergamon Press 1957. Vol. 3. pp. l-6.
PHOTOELECTRIC
EMISSION
OF SOME
FROM THE VALENCE
ALKALI
E. A. TAFT
BAND
HALIDES
and H. R. PHILIPP
General Electric Research Laboratory, (Received
26 April
Schenectady,
New York
1957)
Abstract-Photoelectric emission from the valence band, previously measured for CsI, has been investigated in the other alkali iodides and several other halides. As in CsI, the yields rise rapidly from the threshold to values of order 0.1 electron/incident photon. For the iodides, concurrent measurements of reflectivity and optical absorption agree with MARTIENSSEN’Srecent results and show detail consistent with interband transitions. Approximate values for the energy difference between the bottom of the conduction band and the vacuum level are deduced.
INTRODUCTION
sults on absorption at 20°K. Within this limitation, however, they corroborate his findings. The reflectance results are useful for comparing evaporated films with single crystals. Except for interference effects in the films (see Fig. 4), results for the two kinds of surfaces were identical. To facilitate discussion of trends in the electronic energy structures, tentative new data for LiI are included.c4) For purposes of comparison, intrinsic photoemission data are also presented for all the potassium halides and for those cesium salts having the CsCl structure.
the photoIN AN earlier paper,@) we described emission from CsI. Large yields, observed for hv > 6 eV, were independent of defects in the samples and were attributed to emission from the va ence band. Concurrent optical absorption data showed a comparatively small edge consistent with band-to-band transitions if the electron affinity of CsI was estimated to be small (i.e., if the bottom of the conduction band lay only slightly below the vacuum level). This paper reports similar data on the other alkali iodides in which the electron affinity is larger, of order 1 eV. MARTIENSSEN~~)has recently made an elegant and extensive investigation of the optical absorption of the alkali iodides. At 20”K, the absorption edge for CsI shows up more clearly than in reference 1. Furthermore, it appears to be double in structure with components at hv = 6.24 and 6.31 eV. (In addition, the second absorption peak near 6 eV is also doubled.) In this paper, we continue to relate this double edge in CsI to band-to-band transitions and treat it as analogous to the very clear single edges found by MARTIENSSEN(~) at roughly the same photon energy for iodides of Rb, K, and Na.t3) Along with our photoelectric data on the iodides we present optical absorption and reflectance data taken at 300°K and at 80°K. They are point-bypoint measurements and are not as detailed as MARTIENSSEN’S recording-spectrophotometer reA
EXPERIMENTAL Vacuum tubes of three different designs were employed. (1) Cylindrical diodes, fitted with cleaved LiF windows,* were used to determine the spectral distribution of photoelectric yield at 300°K. Emission measurements were made on evaporated films with the spectral range extended to greater than 11 eV with the aid of a vacuum monochromatorc5) kindly placed at our disposal by P. D. * Windows, cleaved from single crystals obtained from the Harshaw Chemical Company, were sealed to a 4 in. diameter extension from the tube envelope with Dennison’s red sealing wax. Bake temperatures were limited but system pressures of lo-’ mm Hg were readily obtainable. Tubes with quartz bubble windows showed the same behavior to 8 eV. The alkali halides do not Beem to be very sensitive to vacuum conditions, but we must avoid any claim that these surfaces, although reproducible, were atomically clean. 1
2
E.
A. TAFT
AND
H.
R.
PHILIPP
over the temperature range 80 to 500’K held single crystals or evaporated films (on Al,O, crystals) in vacuum behind fused-quartz windows, through which measurements were made.
I-
I
RESULTS
Spectral distribution of the photoelectric yield 2
lo-’, 6
1
I
I
7
I
I
0
I
9
I IO
I
I II
hv
eV
FIG. 1. Spectral distribution of the photoelectric yield for the alkali iodides.
I I , KCI
hv
eV
FIG.2. Spectral distribution of the photoelectric
yield for the potassium halides and for several cesium salts.
JOHNSON. (2) A double
window tube, described in a previous paper,(l) provided data on the optical absorption of thin films in the temperature range 80 to 500°K. (3) Tubes used to measure reflectance
The spectral distribution of the photoelectric yield for the alkali iodides is shown in Fig. 1. For comparison purposes, data on other halide samples are shown in Fig. 2. The curves are all similar in shape. There is an initial rapid increase in yield which levels off at values near O.l.* t This emission is an intrinsic property of the emitter, independent of crystal impurities and imperfections, and is attributed here to electrons ejected from the valence band. The threshold for this high-yield process is a measure of the energy difference between the top of the valence band and the vacuum level. The shift in the threshold for corresponding potassium and cesium halides is slightly less than 1 eV and will be referred to in a later discussion of the electron affinity. When F-centers are deliberately added to these salts by irradiation in the first fundamental absorption band, a measurable yield appears at lower the yield for quantum energy. (6) For completeness, a KI sample containing about lOle F-centers cm-3 is shown in Fig. 3. Assuming that each incident photon produces one excited electron, the plateau value for the valence band yield indicates that the excited electrons have roughly a 10 per cent chance of appearing outside the surface. If one assumes the same escape probability for the exciton-induced yield, 2 x 10-S at the first absorption peak, one concludes that the efficiency of energy transfer to F-centers is low in these KI samples, of order 0.02. The energy distribution of the exciton-induced * Emission from the valence band of LiI appears to be roughly coincident with that from the iodides of Rb, K and Na at yields below 10m2; the threshold may be slightly lower. Processes involving F-centers or other defects sometimes complicate the measurements below hv = ‘7.5 eV. For KF the currents are small and the results of Fig. 2 are only qualitative. t Spectral distribution data on single crystals of both NaCl and KC1 have been obtained at Cornell University by J. W. TAYLOR~~~ P. L. HARTMAN to whom we are indebted for discussion of their work. Our KC1 results are in good agreement with theirs.
PHOTOELECTRIC
EMISSION
FROM
VALENCE
BAND
OF SOME
ALKALI
HALIDES
3
“Y
FIG. 3. Spectral distribution of the photoelectric yield for a KI sample containing N 1O’O F-centers cmw8.
FIG. 4b
yield is considerably different from the valence band yield, however, and the actuaf. value may be somewhat higher.(‘)
nv
FIG. 4c
C”
4
E.
pl)
A.
TAFT
AND
K.
R.
PHILIPP
*v
er
FIG. 4d
ev
FIG. 4e
FIG. 4. Reflectance and optical absorption of the alkali iodides at 300°K (dotted curves) and 80% (solid curves). Reflectance data are presented for single crystals, except for RbI and LiI, where measurements were made onlyon
anneated evaporated films. Interference effects appears in these films in the region of optical transparency. Optical trans~ssion data for annealed evaporated films are plotted as estimated absorption constant; no correction has been made for surface effects such as reflectance. ?‘he values were roughly calculated from interference coIors and tube geometry.(“)
Optical properties Absorption constants for evaporated, annealed alkali-iodide films are shown in Fig. 4. (No corrections have been made for reflection loss; the absolute values are rough estimates.) The results agree with ~~~RTI~~SSEN'S. The doubiing of the second absorption peak of CsI (which we previously overlooked(Q) barely shows up at 8O0K, as MARTIENSSEN has pointed out. The weaker double structure of the absorption edge we were not able to resolve with our techniques, but the edge itself is clear. For RbI and KI, the double peak is missing, and the absorption edge (MARTIENSSENlists these as “stufe”) is very pronounced. It moves closer to the first peak in NaI and becomes less distinct. For LiI it appears to have moved under the first peak, which is weaker. The reflectance data are consistent with these results,
showing csI.@*
better
resolution
of
the
doublet
for
DISCUSSION We obviously cannot assign unique threshold photon energies to the valence band photoemission or to the optical absorption edge in the absence of a more quantitative treatment of these effects. Useful comparisons may be made, nevertheless, by arbitrarily noting the values of hv at which the photoemission is 1O-3 electron/photon and at which the optical absorption constant rises more or less step-wise near a value of lo5 cm-l. Table 1 shows such values, the photoelectric thresholds (column 1) being given for 300°K and * The room temperature results for KI are in agreement with early reflectance measurements of HILSCH and POHL.
PHOTOELECTRIC
EMISSION
FROM
VALENCE
Table 1 = Crystal
_-
CSI RbI KI NaI LiI
1 6-4 ev 7.3 7.3 7.3 7.3
---
2
(i*3* 6.1 6.2
I i I
3
4
5
0.1 I.2 1.1
5.76 5.69 5.80 5.55 5.92
0.5 0.4 0.4 0.2 0.0
i
1. Photoelectric threshold for valence band emission at 300’K. 2. Optical absorption edge at 80°K. 3. Electron affinity. 4. First fundamental absorption peak at 80°K. 5. Exciton ionization energy. * At 20” M~~~~~~ss~~ shows two edges at 6.24 and 6.31, respectively. t Taken as coincident with the exciton peak.
the absorption edges (column 2) for 80’K. (Photoelectric thresholds at 80°K would be roughly O-2 eV higher,(l) a correction which we neglect in view of the coarseness of this procedure). The photoelectric threshold fixes the valence band relative to the vacuum level, and the optical absorption edge corresponds to the valence-conduction band gap. The difference is the electron affinity (column 3) of the crystal, which has a very small value for CsI but a larger one for the other salts. The estimates tend to be too small rather than too large. In Fig. 4 of the paper by HARTMAN,NELSONand SIEGFRIED@) there appears an interesting feature in the optical absorption of KC1 which looks somewhat like the absorption edge in KI. Assuming for the moment that the two cases are comparable, we arrive at an electron affinity of O-2 eV for KC1 using the photoelectric threshold 8.7 eV from Fig. 2 above. For LiI, the optical absorption edge is ill-defined and has been taken in Table I as coincident with the first absorption peak. The absorption spectrum bears some resemblance to those of the silver salts, for which the discrete first exciton peak is not clearly separated from the continuum as it is in RbI for example.(g)$ The valence band is the Sp band of the iodine ions, which is broadened as $ Dr. W. M&TIENSSEN also has kindly sent us absorption data taken by Y. OKAMOTO at Giittingen on these salts.
BAND
OF SOME
ALKALI
HALIDES
5
these ions are packed more closely in LiI.@O) Thus, one expects the absorption edge to be less sharp than for the heavier iodides. (We notice incidentally that the high-frequency dielectric constant of LiI is higher than for any other alkali halide; it approaches that of the silver halides.) To display the pertinent trends more clearly in Table 1, we plot the difference in energy (column 5) between the first absorption peak (column 4) and the absorption edge (column 2). Keeping in mind the unsophisticated definitions adopted here, we call this the “exciton ionization energy”. We have not discussed in any precise way the character of the band-to-band optical transitions that we associate with the optical absorption edge and the succeeding continuum. We should remark, however, that a recent calculation of the electronic energy structure of KC1 by HOWLAND shows that both direct and indirect optical transitions could originate near the top, of the valence band without differing greatly in energy. Another subject which is worth further scrutiny is the behavior of the photoemission in the vicinity of the absorption edge and at the position of the absorption peaks lying beyond the edge. At 80’K the iodides show a slight rise (of order 30 per cent) in the photoelectric yield at the absorption edge when F-centers are present in the sample. (This effect is not clear in Fig. 3 for KI since this curve is for 300°K. It appears weakly in Fig. 1 of the last paper of reference 6). At the present time this small rise looks like a peak in the yield near 6 eV rather than like a plateau that follows the optical absorption beyond the edge. There may conceivably be an unresoived optical peak at this point, though we are reluctant to believe this in view of MARTXENSSEN’S detailed work at 20’K. In any case, we summarize by saying that optical absorption in the immediate vicinity of the edge stimulates photoemission from Fcenters for reasons not yet understood. For KI, irradiation in the second optical absorption peak at 6.7 eV, which lies beyond the edge, does not appear to stimulate photoemission from F-centers, at least with efficiency comparable to that found in the first peak. Experiments in the vicinity of the valence band emission threshold are difficult, however, and we have therefore shown this region in Fig. 3 as a dashed line to indicate it as tentative.
E.
6
A.
TAFT
AND
Acknowledgements-We are grateful to Dr. W. MAFCTIENSSEN for an advance copy of his paper and for comments on his work. We are indebted to P. HARTMAN and J. TAYLORfor interesting discussions of their work. We particularly thank L. APKER for many helpful discussions throughout the course of this work.
H. R. PHILIPP
5. 6.
7.
8.
REFERENCES I. PHILIPP H. R. and TAFT E. r. Phys. Chem. Solids 1, 159 (1956). 2. MARTIENSSEN W. r. Phys. Chem. Solids2, 257 (1957). 3. HILXH R. and POHL R. 2. Phys. 59, 812 (1930); SCHNEIDERE. and O’BRYAN H. Phys. Rev. 51,293 (1937); MARTIENSSEN W. Nuchr. Akad. Wiss. Giittingen. Math. Physik 11, 257 (1955); FESEFELDT H. Z. Phys. 64, 626 (1930); FISCHER F. Z. Phys. 139, 328 (1954). 4. HILSCH R. and POHL R. W. Z. Phys. 59,812 (1930);
9. 10.
Il.
SCHNEIDERE. and O’BRYAN H. Phys. Rev. 51, 293 (1937). JOHNSONP. D.3. Opt. Sot. Amer. 42, 278 (1952). APKER L. and TAFT E. Phys. Rev. 79,964 (1950); 81, 698 (1951); 82, 814 (1951); PHILIPP H. R. and TAFT E. Phys. Rev. 106, 671 (1957). In this connection see INCHAUSPEN. and MAURER R. J. Air Force Technical Note OSR-TN-55-281 ; TEEGARDENK. J. Phys. Rev. 105, 1222 (1957). HILSCH R. and POHL R. W. Z. Phys. 57, 145 (1929); HARTMAN,NELSEN, and SIEGFRIEDPhys. Rev. 105. 123 (1957). TUTIHASI S. Phys. Rev. 105, 882 (1957). O’BRYAN H. and SKINNER H. Proc. Rov. Sot. A176,229 (1940); MOTT N. F. and GURNE; R. W. Electronic Processes in Ionic Crystals p. 79. Oxford University Press, London (1940). HOWLAND L. P. Quarterly Progress Report, Solid State and Molecular Theory Group p. 23. M.I.T., Cambridge, Mass. (1957). We are indebted to Dr. HOWLAND for stimulating conversations.