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Investigating ultraviolet photoelectron spectroscopy of ice
Solid State Communications, Vol. 29, pp. 511—514 Pergamon Press Ltd. 1979. Printed in Great Britain.
INVESTIGATING ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF ICE I.Abbati, L.Braicovich, and B.De Michelis Istituto di Fisica del Politecnico, Milan, Italy and Gruppo Nazionale di Struttura della Materia del CNR, 20133 Milan, Italy (Received on Dec. 13, 1978 by R.Fieschi) Photoelectron energy distribution curves (EDCs) from ice excited by Hel (21.2 eV) and NeI (16.8 eV) radiation are presented. The strict connection between valence density of states and EDC5 forces to rule out the previous suggestion by Shibaguchi et al. that conduction band density of states is of paramount importance in determining the EDC5 excited by ultraviolet light. The results also allow a discussion of bind calculations published till now; the need for a theoretical investigation on photoemission from ice is put forward.
The study of the electron states of hydrogen bonded systems has great interdisciplinary importai~ce and is receiving great attention ; ice is a typical hydrogen bonded system and its electron statQs 4 have studied and been experimentally theoretically~ with photoemission5 and optical ab— sorptionS~6. The informations obtain— ed with photoemission are rather puzzling. In ref. (5) XPS results (Al Ka radiation) and UPS results (at various hv between 13.5 and 19.2 eV) are given; the XPS results do not correla— te with UPS ones. The upper XPS structures are typically due to va— lence states and are correlated with pho 9o~missionfrom free H~Omolecules ‘ . The authors assumed that in UPS spectra any information on initial by the of scatterings suffered by hot out states the transition is washed photoelectrons which eventually reach a kind of equilibrium with the density of empty states, so that they tried t~ compare UPS results with calculations of conduction bands. This result, if confirmed, is rather interesting but seei~s to be contradicted by some sparSe information which can be traced in the literature, although no definite conclusion can be o~awnsince no other UPS ad hoc measurements on ice have been published to t~e author’s knowledge. Page et al. measured the Hell (40,8 eV) EDC from Ni covered “at saturation” with condensed water and found a typical valence state spectrum. A correlation with the structures of the EDCs from isolated water mol~ 8ules has been found also by Yu et al. in the course of their research (hv=21.2 eV) on physisorption (around a mono— layer coverage on M0S2).
We have thus carried out an ad hoc experiment on photoemission from ice samples by using HeI (21.2 eV) and Nel (16.8 eV), with the purpose of clarifying the nature of the information UPS andtheoretical of di— scussingobtained brieflywith available results. The apparatus (base pressure i.i~10 Torr) contained a spherical retarding potential analyzer for the measurement of angle integrated EDCs. The samples were prepared by condensing at ~ Torr highly purified water onto a cooled substrate: we used either the (0001)Zn face or polycri— stalline copper prepared by evaporation in situ. The samples were condensed a ~8O°K, then heated above the tempeandcrystallization then cooled again. Thecubic raturç1 for in the sampl~ phase’ thickness was limited below ~1OO L in order to avoid troubles with the charging of the sample. The EDCs at hu = 21.2 eV and at 16.8 eV are given in Fig. 1 (solid lines) together with the curve obtained from the first EDC by subtraction of the background due to scattered elec— trons (dashed line); the details on this subtraction and some results on the scattering of hot e~ctrons in ice will be given elsewhere’’. The following comments can be done. (i) The information given by the EDCs refer typically to valence states of ice as shown by the three structures A,B,C strictly correlated to those obtained from free molecules7’ . Also the EDC at 16.8 eV gives valence state information, but limited to the shallowest peak A, owing to the presence of background which prevents B to be seen. This fact forces to rule out the UPS 511
512
ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF—ICE
results by shibaguchi et al. and their interpretation in terms of final states. Thus UPS is basically a probe of ice valence states as it happens for the other solids, ii) Our results are in excellent agreemerit with the positions of the structures given by Page with Hell as shown in Fig. 1. The agreement is also sati— sfactory with the XPS results of ref. _________________________________________
~~—NeI
CI /
\
/
~
\
—
/
\
\
/ / / /
I
/ /
~8
6
4
(I) ~gj (h)
eV
Ii
I
II
I —F— +
(b \~(c)
2 Hell
I
xis
results and the available calculations
In Fig.and1 the tions we have widths given of the also bands the posical—
+
H~Omolecule PR theory mol. levels
H— f RR theory \I I mol. levels
lb
1
3a1
The are extent presentto besides which relaxation the variation effects of initial states due to ice formation from water molecules cannot be establish ed in the absence of some ad hoc theo- — retical investigation which could be very interesting. the comparison between the Nevertheless present experimental
(d) (e)
is clearly seen as shown in Fig. 1 where we give the positions of the peaks with the arbitrary assumption of aligning peak A which is the best resolved in all the measurements. The possibility of charging of ice samples in the different experiments prevented the use of other criteria to compare the various results. In the figure we give also the molecular representation the structures belong to. (iii) As far as the modification of the peak positions with respect to the isolated molecules is concerned the following considerations can be made. Relaxation effects are surely present in the measurements; in physisorbed waterl0 this relaxation resulted the same for the different levels within the accuracy of the experiment. Nevertheless it could be argued as in ref. (5) that in ice the relaxation could be different for different levels; this conjecture could receive some support from the fact seen by Salaneck’T3 that relaxation in a molecular crystal like anthracene depends on the environment at the itsurf from fact and that (e.g. is ac~)and different in the bulk relaxation can be different for differ— ent states even in relatively narrow 14 energy range as shown by Bagus et al.
/
/
Vol.29, No.6
lb1
Fig.1 EDC5 of ice for (a) h’/ = 21.2 eV and (b) hv = 16.8 eV; (C )spectrum of ice obtained from curve (a) after subtraction of the background. Positions of the peaks from (d) UPS at hv = 40.8 eV (ref. 9) and (e) XPS (ref. 5). (f) positions of the peaks in the spectrum of H 0 molecule (ref. 7). Upper valence binds of ice with the molecular eigenvalues used by the authors after (g) PR theory (ref. 3) and (h) RR theory (ref. 4). (5). In all cases an increase of the distance between the structures A and C and between the structures A and B with respect to isolated water molecules
of ice electron
states
is interesting.
culated by Pastori 4 and (RR)Resca3 together (PR) with and by Resca
and Resta
the molecular eigenvalues used by the authors. PR disregard the potential terms in the hamiltonian in their non self consistent molecular tight binding calculation while RR go beyond this approximation. The Fig. 1 points out clearly that the PR and RR results are considerably different and this show how crytical the calculations are against the change of the parameters and mainly against the adjustment of the crystal potential. In PR the bands widen with respect to the molecular eigenvalues while in the more refined calculations by RR the opposite happens. The fact that the better treatment gives the less satisfactory agreement with the measurement is a strong indication that the agreement between PR and the present results is probably not fully significant, as far as the position of the bands is concerned. On the contrary as far as the bandwidths are concerned, an interesting point is the qualitative agreement between both theories and the experiment on the fact that the
Vol. 29, No. 6
Table 1.
ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF ICE
513
Comparison between various results on upper valence bands of ice (energy in eV). The shallowest structure has been conventionally assumed at the same energy in all the lines. A (1b
1,1b2)
B
C
(3a1)
(1b2,1b1)
Experimental Hel (present work) Hell (ref. 9) XPS (ref. 5) Water molecule (ref. 7 and 8)
lb1
0
—2.80
—6.35
0
—2.50
-6.20
0
—2.35
—6.20
=
0
—2.15
lb2
=
—5.95
Theoretical PR
ice
H2O mol RR
ice
H2O mol
(+0.19, —0.19)
(—1.40, —2.43)
—0.18 (+0.02,
(—6.14, —6.41)
—1.91
—0.02)
(—0.68, —1.65)
+0.97
—6.22 (—5.19, —5.47)
-0.53
maximum broadening upon con~nsation is suffered by the nonbonding orbital 3a1 (peak B). This is due to the in— teraction of the same orbitals of near molecules, In conclusion we have seen that in the whole ultraviolet range photoemission from ice is related to initial state effects rather than to final sta— tes as suggested in ref. (5); we have also shown t~a~ the available band calculations ‘ can be used as a starting point for the discussion of photo-
-5.32
emission but that more refined calculations, possibly with the inclusion of relaxation, are required to carry out a detailed comparison with the experiment. The present results could stimulate this theoretical effort which will be of great help for a further understanding of the electron states of ice. Aknowledgements. The authors are gra— teful to Prof. G.Pastori-Parravicini and Prof. R.Resta for helpful discus— sions.
REFERENCES I. 2.
ALLEN L.C., J.Am.Chem.Soc. 97,
3. 4. 5.
PASTORI-PARRAVICINI G. and RESCA L., Phys.Rev. B8, 3009 (1973). RESCA L. and RESTA R., Phys.Stat.Sol. (b) 81, 129 (1977). SHIBAGUCHI T., ONUKI H., and ONAKA R., J.Phys.Soc.Japan 42, 152 (1977).
6.
WATANABE N., KITAMURA H., and NAXAI Y., Proc. IV Intern.Conf. VUV Radiat. Phys., p. 70, Pergamon Press, Oxford (1974). TURNER D.W., BAKER C., BAKER A.D., and BRUNDLE C.R., Molecular Photoelectron Spectroscopy. Wiley Interscience, New York (1970). SIEGBAHN R., Esca Applied to Free Molecules, North Holland Publ.Co., Amsterdam (1969). PAGE P.J., TRIMM D.L., and WILLIAMS P.M., J.Chem.Soc. Faraday Trans. I, 70 1769 (1974).
7. 8. 9.
6921 (1975). PASTORI-PARRAVICINI G. and RESCA L., J.Phys.C4, L314
(1971).
10. YU K.!., NC~NAMIN T.C., and SPICER W.E., Surf.Sci. 50, 149 (1975); a good correlation has also been found for chemisorbtion on Pt around a monol&y.r by LX1~DA~I., MILLER J.N., and SPICER W.E. (to be published). 11. SHIM&OKA K., J.Phys.Soc.Japan 15, 106 (1960).
514
ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF ICE
12. ABBATI I., BRAICOVICH L., andDE MICHELIS B., to be published. 1-3. SALANECK W.R., Phys.Rev.Lett. 40, 60 (1970). 14. BAGUS P.S. and HERMAN K., Sol.Stat.Comm. 20, 5 (1976).
Vol.29, No.6
INVESTIGATING ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF ICE I.Abbati, L.Braicovich, and B.De Michelis Istituto di Fisica del Politecnico, Milan, Italy and Gruppo Nazionale di Struttura della Materia del CNR, 20133 Milan, Italy (Received on Dec. 13, 1978 by R.Fieschi) Photoelectron energy distribution curves (EDCs) from ice excited by Hel (21.2 eV) and NeI (16.8 eV) radiation are presented. The strict connection between valence density of states and EDC5 forces to rule out the previous suggestion by Shibaguchi et al. that conduction band density of states is of paramount importance in determining the EDC5 excited by ultraviolet light. The results also allow a discussion of bind calculations published till now; the need for a theoretical investigation on photoemission from ice is put forward.
The study of the electron states of hydrogen bonded systems has great interdisciplinary importai~ce and is receiving great attention ; ice is a typical hydrogen bonded system and its electron statQs 4 have studied and been experimentally theoretically~ with photoemission5 and optical ab— sorptionS~6. The informations obtain— ed with photoemission are rather puzzling. In ref. (5) XPS results (Al Ka radiation) and UPS results (at various hv between 13.5 and 19.2 eV) are given; the XPS results do not correla— te with UPS ones. The upper XPS structures are typically due to va— lence states and are correlated with pho 9o~missionfrom free H~Omolecules ‘ . The authors assumed that in UPS spectra any information on initial by the of scatterings suffered by hot out states the transition is washed photoelectrons which eventually reach a kind of equilibrium with the density of empty states, so that they tried t~ compare UPS results with calculations of conduction bands. This result, if confirmed, is rather interesting but seei~s to be contradicted by some sparSe information which can be traced in the literature, although no definite conclusion can be o~awnsince no other UPS ad hoc measurements on ice have been published to t~e author’s knowledge. Page et al. measured the Hell (40,8 eV) EDC from Ni covered “at saturation” with condensed water and found a typical valence state spectrum. A correlation with the structures of the EDCs from isolated water mol~ 8ules has been found also by Yu et al. in the course of their research (hv=21.2 eV) on physisorption (around a mono— layer coverage on M0S2).
We have thus carried out an ad hoc experiment on photoemission from ice samples by using HeI (21.2 eV) and Nel (16.8 eV), with the purpose of clarifying the nature of the information UPS andtheoretical of di— scussingobtained brieflywith available results. The apparatus (base pressure i.i~10 Torr) contained a spherical retarding potential analyzer for the measurement of angle integrated EDCs. The samples were prepared by condensing at ~ Torr highly purified water onto a cooled substrate: we used either the (0001)Zn face or polycri— stalline copper prepared by evaporation in situ. The samples were condensed a ~8O°K, then heated above the tempeandcrystallization then cooled again. Thecubic raturç1 for in the sampl~ phase’ thickness was limited below ~1OO L in order to avoid troubles with the charging of the sample. The EDCs at hu = 21.2 eV and at 16.8 eV are given in Fig. 1 (solid lines) together with the curve obtained from the first EDC by subtraction of the background due to scattered elec— trons (dashed line); the details on this subtraction and some results on the scattering of hot e~ctrons in ice will be given elsewhere’’. The following comments can be done. (i) The information given by the EDCs refer typically to valence states of ice as shown by the three structures A,B,C strictly correlated to those obtained from free molecules7’ . Also the EDC at 16.8 eV gives valence state information, but limited to the shallowest peak A, owing to the presence of background which prevents B to be seen. This fact forces to rule out the UPS 511
512
ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF—ICE
results by shibaguchi et al. and their interpretation in terms of final states. Thus UPS is basically a probe of ice valence states as it happens for the other solids, ii) Our results are in excellent agreemerit with the positions of the structures given by Page with Hell as shown in Fig. 1. The agreement is also sati— sfactory with the XPS results of ref. _________________________________________
~~—NeI
CI /
\
/
~
\
—
/
\
\
/ / / /
I
/ /
~8
6
4
(I) ~gj (h)
eV
Ii
I
II
I —F— +
(b \~(c)
2 Hell
I
xis
results and the available calculations
In Fig.and1 the tions we have widths given of the also bands the posical—
+
H~Omolecule PR theory mol. levels
H— f RR theory \I I mol. levels
lb
1
3a1
The are extent presentto besides which relaxation the variation effects of initial states due to ice formation from water molecules cannot be establish ed in the absence of some ad hoc theo- — retical investigation which could be very interesting. the comparison between the Nevertheless present experimental
(d) (e)
is clearly seen as shown in Fig. 1 where we give the positions of the peaks with the arbitrary assumption of aligning peak A which is the best resolved in all the measurements. The possibility of charging of ice samples in the different experiments prevented the use of other criteria to compare the various results. In the figure we give also the molecular representation the structures belong to. (iii) As far as the modification of the peak positions with respect to the isolated molecules is concerned the following considerations can be made. Relaxation effects are surely present in the measurements; in physisorbed waterl0 this relaxation resulted the same for the different levels within the accuracy of the experiment. Nevertheless it could be argued as in ref. (5) that in ice the relaxation could be different for different levels; this conjecture could receive some support from the fact seen by Salaneck’T3 that relaxation in a molecular crystal like anthracene depends on the environment at the itsurf from fact and that (e.g. is ac~)and different in the bulk relaxation can be different for differ— ent states even in relatively narrow 14 energy range as shown by Bagus et al.
/
/
Vol.29, No.6
lb1
Fig.1 EDC5 of ice for (a) h’/ = 21.2 eV and (b) hv = 16.8 eV; (C )spectrum of ice obtained from curve (a) after subtraction of the background. Positions of the peaks from (d) UPS at hv = 40.8 eV (ref. 9) and (e) XPS (ref. 5). (f) positions of the peaks in the spectrum of H 0 molecule (ref. 7). Upper valence binds of ice with the molecular eigenvalues used by the authors after (g) PR theory (ref. 3) and (h) RR theory (ref. 4). (5). In all cases an increase of the distance between the structures A and C and between the structures A and B with respect to isolated water molecules
of ice electron
states
is interesting.
culated by Pastori 4 and (RR)Resca3 together (PR) with and by Resca
and Resta
the molecular eigenvalues used by the authors. PR disregard the potential terms in the hamiltonian in their non self consistent molecular tight binding calculation while RR go beyond this approximation. The Fig. 1 points out clearly that the PR and RR results are considerably different and this show how crytical the calculations are against the change of the parameters and mainly against the adjustment of the crystal potential. In PR the bands widen with respect to the molecular eigenvalues while in the more refined calculations by RR the opposite happens. The fact that the better treatment gives the less satisfactory agreement with the measurement is a strong indication that the agreement between PR and the present results is probably not fully significant, as far as the position of the bands is concerned. On the contrary as far as the bandwidths are concerned, an interesting point is the qualitative agreement between both theories and the experiment on the fact that the
Vol. 29, No. 6
Table 1.
ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF ICE
513
Comparison between various results on upper valence bands of ice (energy in eV). The shallowest structure has been conventionally assumed at the same energy in all the lines. A (1b
1,1b2)
B
C
(3a1)
(1b2,1b1)
Experimental Hel (present work) Hell (ref. 9) XPS (ref. 5) Water molecule (ref. 7 and 8)
lb1
0
—2.80
—6.35
0
—2.50
-6.20
0
—2.35
—6.20
=
0
—2.15
lb2
=
—5.95
Theoretical PR
ice
H2O mol RR
ice
H2O mol
(+0.19, —0.19)
(—1.40, —2.43)
—0.18 (+0.02,
(—6.14, —6.41)
—1.91
—0.02)
(—0.68, —1.65)
+0.97
—6.22 (—5.19, —5.47)
-0.53
maximum broadening upon con~nsation is suffered by the nonbonding orbital 3a1 (peak B). This is due to the in— teraction of the same orbitals of near molecules, In conclusion we have seen that in the whole ultraviolet range photoemission from ice is related to initial state effects rather than to final sta— tes as suggested in ref. (5); we have also shown t~a~ the available band calculations ‘ can be used as a starting point for the discussion of photo-
-5.32
emission but that more refined calculations, possibly with the inclusion of relaxation, are required to carry out a detailed comparison with the experiment. The present results could stimulate this theoretical effort which will be of great help for a further understanding of the electron states of ice. Aknowledgements. The authors are gra— teful to Prof. G.Pastori-Parravicini and Prof. R.Resta for helpful discus— sions.
REFERENCES I. 2.
ALLEN L.C., J.Am.Chem.Soc. 97,
3. 4. 5.
PASTORI-PARRAVICINI G. and RESCA L., Phys.Rev. B8, 3009 (1973). RESCA L. and RESTA R., Phys.Stat.Sol. (b) 81, 129 (1977). SHIBAGUCHI T., ONUKI H., and ONAKA R., J.Phys.Soc.Japan 42, 152 (1977).
6.
WATANABE N., KITAMURA H., and NAXAI Y., Proc. IV Intern.Conf. VUV Radiat. Phys., p. 70, Pergamon Press, Oxford (1974). TURNER D.W., BAKER C., BAKER A.D., and BRUNDLE C.R., Molecular Photoelectron Spectroscopy. Wiley Interscience, New York (1970). SIEGBAHN R., Esca Applied to Free Molecules, North Holland Publ.Co., Amsterdam (1969). PAGE P.J., TRIMM D.L., and WILLIAMS P.M., J.Chem.Soc. Faraday Trans. I, 70 1769 (1974).
7. 8. 9.
6921 (1975). PASTORI-PARRAVICINI G. and RESCA L., J.Phys.C4, L314
(1971).
10. YU K.!., NC~NAMIN T.C., and SPICER W.E., Surf.Sci. 50, 149 (1975); a good correlation has also been found for chemisorbtion on Pt around a monol&y.r by LX1~DA~I., MILLER J.N., and SPICER W.E. (to be published). 11. SHIM&OKA K., J.Phys.Soc.Japan 15, 106 (1960).
514
ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF ICE
12. ABBATI I., BRAICOVICH L., andDE MICHELIS B., to be published. 1-3. SALANECK W.R., Phys.Rev.Lett. 40, 60 (1970). 14. BAGUS P.S. and HERMAN K., Sol.Stat.Comm. 20, 5 (1976).
Vol.29, No.6