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Configuration interaction effects in the HeIIα 5p−1 photoelectron spectrum of Cs
Volume 66. number 3
CONFIGURATIOh PHOTOELECTRON
CHEMICAL
1.5 October 1979
PHYSICS LJXTERS
INTERACTION EFFECTS IN THE HeIIcr5p-’ SPECTRUM OF Cs
E.P.F. LEE and A-W_ POTTS
Recrised 12 June 1979
The lieUp phoroekcrron spectrum of atomic caesiam is reported. Ionization from rhe 5p sheII resuuitsin a multiplicity of ionized safes which ciln be understood i.ugeIy in terms of fmal-state configuration interaction.
1. Iutroduction It has become ciear over recent years that electron correfation effects can pI3y an important part both in the determination of photoelectron cross sections [I j and in the photoionization process 3s revealed in the fused-frequency photoelectron [2,3] or absorption spectrum [4] _ In the photoeIectron spectrum the process is nomlslly described in terms of configuration inter3ction (CI) applying in either the initial neutral or foal ionized states. This results in states of mtied configuation and the observation of c&sic&y forbidden states which are said to borrow intensity from the oneelectron ‘%IIowed” processes_ Such states were postulated as the cause of anomalous features observed in the M+ valence p-’ photoelectron spectra observed for the lMfX- gasphase alkali halides [5] _ It has recently been shown that CI pIays ;UI important roIe both in the B315p absorption spectrum [4] and in the Ba (5~)~~ photoeIectron spectrum [6] _ CaIcuIations on CsII h3ve shown that CI also has 3 major effect on the ionized states formed by 5p-’ ionization from Cs [7] - On the basis of these calculations Reader has been able to make assignments of many of the ststes of CsII observed in emission spectra [7,8] _ Against this background it seemed to us logical to spectrum of investigate the 5p-’ photoelectron atomic Cs in an attempt to assess the part played in it by corre!ation effects. The HeIIar photoelectron spec-
trum is reported since although the 5p-’ ionization at c;f. I7 V should be observable within the He1 energy range contamination effects from this reactive metal appeared to reduce the sensitivity of the spectrometer to low energy electrons. This was also our experience in 3 previous He1 investigation of Cs vapour [9] _
3. Experimental The spectra in this work were obtained using an improved version of the spectrometer-furnace arrangement previousIy described [IO] _Increased pumping in the ionization chamber reduced the amount of residual He gas from the lamp in the ionization region. As a result signals due to He Is ionization on the He11 lines were substantially reduced (cf. ref. [S] )_ However a small trace of N2 persisted in aI.I spectra despite considerable efforts to remove it (fig_ L)_ The caesium used was 99.98% pure (Koch-Light) and was loaded into the alumina furnace tube with a layer of cyclohexane to prevent oxidation. Spectra were recorded with a furnace temperature of 500 K and reproducibility was checked with three separate samples.
3. Results and discussion The HeIILv photoelectron
spectrum
of Cs obtained 553
15 October 1379
and art? dccwdfe to +0.0_3 eV for tile relatively intense peaks in the lower ioniation energy region and 20.06 eV for weak peaks in thit high energy :egion. of Cs wxs observed Nthough the (6s)- t ionization with the tieltx tine it wz.ts not detected on the Hella line. This is presunxtbly because of a low s orbital
in this work is shown in l-s_ f _ An espmtsion of the 17 to 19 cV ionirittion energ- regic;rr Is shown in ITS_I?. The ionizatiotl potenthis md the relative intensities of their ztssocirrted features in the spectrum corrected fur trans:issim are given in tab!e I _TIE spectra were dibrated with respect to X7 and He Is peaks
r\Ie;znrrd wrticztt bnization
potenthls
-----__“_I_--
---
This miurk IP (CV
1721 1 1727 ES-65 17.85
reirrrivc intsnsit)’
150 -to 25 100
19.21I
____________I_____
--_----1_1__
Z of 5P56+
oKherconf~$pmktions
1P (CV) 3)
82SfiF,-. _3cp~(~)*:39~~(~)I 6IZ$&,
Sp%d. 3P2. 3D?I S&d.
17.20 17.23;17.1-6
‘PO; ‘Pt. ‘Dt
5$5d,‘P,.
=D1
17.64
I7% Z&, Tie
5p55d, ‘P=. ‘Dz
If.86
97rPiir)*
.Sp55d, 3Po
I9&5
81% I&,
5p55d. 3D1. ‘Pt
19.11
17&$* -_
5p55d. 3Pt, 3Dt
19.21 19.57
1957
5
5ps6p.~(;,,
19.85
6
5~~6~. %,,
19.76
xB_o7
z
70.00: 20.10
20-4 1 X-46
4 I?
5p56p,~&~(~~z __ 566~. &
10.39
5~~6~. @,t
21.44
x.64
6
5~~6~. $4) t ; $(&
21.66;21.67
21.86
2
-
21.81 24JO; 24.12
2139
8
5~~6~. $t&, 5Ps7s2&Z:f(~~l
z4_10
4
5P%&&)*
‘1 From rcviadlist
554
(IP) and rehtire intcmirics of Cs I-- _-_-___ --.-_--_ -_ .__--_ Rexfer I41
X9.06 19-13
--
of energy ieds
for CsiI 161,t&q
(62)-l of C5 to be 3.89eV_
22_39:22_43
vo1umc 66. number 3
CIrcMIcAL
PHYSICS LET-KRS
cross section for the IIeIIa frequency (5p/6s ratio > 150). Simihr beh.rviour ~3s 3Iso observed for I33 [6j _ K3nd Rb [II]_ Eszimining the spectrum in detail. although part of the spectrutil Cf@_ 2) is complicated by residual N, am! :11+ tIe(IS)-1 ionizstion on the HelIp line (app3rent ionir.rtion potential 17.029 eV) two strong peJks 3re observed 3t 17.2 3nd 19. I eV and 3re clertrly due to the overlap of two and three pe.rks respectively. Low resohrtion (=O. I eV fwhm) resu!ting partiy from s3mpIe contamination prevents 3ccurate decorn olution of these pc3ks. From these peaks and the sdditional fe.rtures 3t 17.65 and 17% eV it IS clex that even the 17.0 to 19.0 eV region of the spectrum cannot be interpreted in terms of 3 single 5p”6s’ configuration Using Ifartree-Fock c3lcuIations with inclusion of limited CI, Reader 113s recently 3signed the various states of CsIl determined by emission work [7,S] _ The energies of these states are in excellent ttgreement with the photoeiectron work and are given in table 1 with their assignments_ It seems rertson3bIe to assume similar sssignments for the photoelectron spectrum_ States srising from the Sp’6s configuration are shown to undergo considerabIe CI with 5p55d states. A theoretic31 estim3te of the rel3tive intensities of these brtnds assuming them to be governed by fimd state CI effects is given by the percent3se of the 5~~6s configurrttion associrtble with these states [7] with an 3Ilow3nce for the 2J+ 1 degener3cy of the states. This corresponds to the one electron contribution to t!ie formation of 3 particuIar state_ Intensities deduced on this basis are given in tsbie 2 and are seen to be in esceIIent agreement with the experimental relative intensities_ In both cases the total intensity associated with the 19-I eV band w3s put equ31 to LOO. This indic3tes that for the principal states in the photoeiectron spectrum the ionization is governed by fin31 state configuration interaction effects The ionization potentials of the weak peaks observed in the 19 to 24 eV region of the spectrum also show good agreement with the opticai work (tabIe 1). These features are interpreted in terms of states arising from the 5p56p and 5~~7s configurations_ Unfortunately no caIcuIations are availabIe showing the extent of the particitiation of the one electron 5p56si configuration in these states so it is not possible to comment on the relative intensities in terms of fmal st3te CL
I5 October 1979
Table 2 Comp.trison bcween obsorded and predicted relatire intensities of peaks due to ionization to ionic states with contributions from rbc 5~~6s configuration Predicted rehrire
IP (ev)
Major configra:ion
mtcnsity
17.11 1727 I
150
136
5~~6s ;<:,, ___
17 65
30
47
17.55
25
23
5p’6s?&, __ 5p56d 3Pz
100
100
19 06 19.13
5p’6s $fi>e,r
19.7 1
Considering the possibility of initial state CI only the configuration 5p67s’could make any marked contribution to the 5~~6s’ ground state. This initial configuration ciearly could not give rise to the 5p56p’ ionized configuration but might contribute to the 5ps7s1 states. A point of interest in this respect is that this initial st3te confi_grtiration would not be detectable in the HeI, (6~)~~ spectrum (cf. ref. [9] )_ The observed intensity ratio of 2 to 1 for the bands associated with the 5~~7s’ stzttes is consistent with their assigned _ degeneracy (t3bIe I)_ This situation is only IikeIy to apply where the states are formed through initia1 state CL It therefore appears that while the main part of the Cs 5p-’ roniztltion bands may be adequately accounted for in terms of foal state CI initial state CI may we11 be responsible for some of the weaker features_
Acknowledgement We should like to thsnk the S.R.C_ for fmancid support.
References
111JX. West. P-R_ Woodruff, K. Codling md R-G_ Houk_ate. J. Phys_ B 9 (1976) 407s
PI A-1’. Potts and T-A. Wlliams, J. Electron Spectry. 3 (1974) 3_
CIIFMIC_AL
PIIYSICS
LETI-ERS 171 J_ Redder. Phys. Rr\_ l3A (1976) 507. :SJ J_ Rtider and Cl__ Epstein_ J_ Opt_ See
I.5 October
1979
At-n_ 65 (1975)
638. 191 T-A. Wdhmr ad A.W. Potts. J. Clecrron Spcctr~ _ S (1976) 331. [ 101 E.P.I=. Lee and Ah’. Potts. Proc. Roy_ Sot 36% (1979) 39.5; Chem. PII) s_ Letters 63 (1979) 6 I _ [ I 11 E_P.l=_ Lrs and X.W. Potts. Chrm. Phyr Letters. to be publi&xL
556
CONFIGURATIOh PHOTOELECTRON
CHEMICAL
1.5 October 1979
PHYSICS LJXTERS
INTERACTION EFFECTS IN THE HeIIcr5p-’ SPECTRUM OF Cs
E.P.F. LEE and A-W_ POTTS
Recrised 12 June 1979
The lieUp phoroekcrron spectrum of atomic caesiam is reported. Ionization from rhe 5p sheII resuuitsin a multiplicity of ionized safes which ciln be understood i.ugeIy in terms of fmal-state configuration interaction.
1. Iutroduction It has become ciear over recent years that electron correfation effects can pI3y an important part both in the determination of photoelectron cross sections [I j and in the photoionization process 3s revealed in the fused-frequency photoelectron [2,3] or absorption spectrum [4] _ In the photoeIectron spectrum the process is nomlslly described in terms of configuration inter3ction (CI) applying in either the initial neutral or foal ionized states. This results in states of mtied configuation and the observation of c&sic&y forbidden states which are said to borrow intensity from the oneelectron ‘%IIowed” processes_ Such states were postulated as the cause of anomalous features observed in the M+ valence p-’ photoelectron spectra observed for the lMfX- gasphase alkali halides [5] _ It has recently been shown that CI pIays ;UI important roIe both in the B315p absorption spectrum [4] and in the Ba (5~)~~ photoeIectron spectrum [6] _ CaIcuIations on CsII h3ve shown that CI also has 3 major effect on the ionized states formed by 5p-’ ionization from Cs [7] - On the basis of these calculations Reader has been able to make assignments of many of the ststes of CsII observed in emission spectra [7,8] _ Against this background it seemed to us logical to spectrum of investigate the 5p-’ photoelectron atomic Cs in an attempt to assess the part played in it by corre!ation effects. The HeIIar photoelectron spec-
trum is reported since although the 5p-’ ionization at c;f. I7 V should be observable within the He1 energy range contamination effects from this reactive metal appeared to reduce the sensitivity of the spectrometer to low energy electrons. This was also our experience in 3 previous He1 investigation of Cs vapour [9] _
3. Experimental The spectra in this work were obtained using an improved version of the spectrometer-furnace arrangement previousIy described [IO] _Increased pumping in the ionization chamber reduced the amount of residual He gas from the lamp in the ionization region. As a result signals due to He Is ionization on the He11 lines were substantially reduced (cf. ref. [S] )_ However a small trace of N2 persisted in aI.I spectra despite considerable efforts to remove it (fig_ L)_ The caesium used was 99.98% pure (Koch-Light) and was loaded into the alumina furnace tube with a layer of cyclohexane to prevent oxidation. Spectra were recorded with a furnace temperature of 500 K and reproducibility was checked with three separate samples.
3. Results and discussion The HeIILv photoelectron
spectrum
of Cs obtained 553
15 October 1379
and art? dccwdfe to +0.0_3 eV for tile relatively intense peaks in the lower ioniation energy region and 20.06 eV for weak peaks in thit high energy :egion. of Cs wxs observed Nthough the (6s)- t ionization with the tieltx tine it wz.ts not detected on the Hella line. This is presunxtbly because of a low s orbital
in this work is shown in l-s_ f _ An espmtsion of the 17 to 19 cV ionirittion energ- regic;rr Is shown in ITS_I?. The ionizatiotl potenthis md the relative intensities of their ztssocirrted features in the spectrum corrected fur trans:issim are given in tab!e I _TIE spectra were dibrated with respect to X7 and He Is peaks
r\Ie;znrrd wrticztt bnization
potenthls
-----__“_I_--
---
This miurk IP (CV
1721 1 1727 ES-65 17.85
reirrrivc intsnsit)’
150 -to 25 100
19.21I
____________I_____
--_----1_1__
Z of 5P56+
oKherconf~$pmktions
1P (CV) 3)
82SfiF,-. _3cp~(~)*:39~~(~)I 6IZ$&,
Sp%d. 3P2. 3D?I S&d.
17.20 17.23;17.1-6
‘PO; ‘Pt. ‘Dt
5$5d,‘P,.
=D1
17.64
I7% Z&, Tie
5p55d, ‘P=. ‘Dz
If.86
97rPiir)*
.Sp55d, 3Po
I9&5
81% I&,
5p55d. 3D1. ‘Pt
19.11
17&$* -_
5p55d. 3Pt, 3Dt
19.21 19.57
1957
5
5ps6p.~(;,,
19.85
6
5~~6~. %,,
19.76
xB_o7
z
70.00: 20.10
20-4 1 X-46
4 I?
5p56p,~&~(~~z __ 566~. &
10.39
5~~6~. @,t
21.44
x.64
6
5~~6~. $4) t ; $(&
21.66;21.67
21.86
2
-
21.81 24JO; 24.12
2139
8
5~~6~. $t&, 5Ps7s2&Z:f(~~l
z4_10
4
5P%&&)*
‘1 From rcviadlist
554
(IP) and rehtire intcmirics of Cs I-- _-_-___ --.-_--_ -_ .__--_ Rexfer I41
X9.06 19-13
--
of energy ieds
for CsiI 161,t&q
(62)-l of C5 to be 3.89eV_
22_39:22_43
vo1umc 66. number 3
CIrcMIcAL
PHYSICS LET-KRS
cross section for the IIeIIa frequency (5p/6s ratio > 150). Simihr beh.rviour ~3s 3Iso observed for I33 [6j _ K3nd Rb [II]_ Eszimining the spectrum in detail. although part of the spectrutil Cf@_ 2) is complicated by residual N, am! :11+ tIe(IS)-1 ionizstion on the HelIp line (app3rent ionir.rtion potential 17.029 eV) two strong peJks 3re observed 3t 17.2 3nd 19. I eV and 3re clertrly due to the overlap of two and three pe.rks respectively. Low resohrtion (=O. I eV fwhm) resu!ting partiy from s3mpIe contamination prevents 3ccurate decorn olution of these pc3ks. From these peaks and the sdditional fe.rtures 3t 17.65 and 17% eV it IS clex that even the 17.0 to 19.0 eV region of the spectrum cannot be interpreted in terms of 3 single 5p”6s’ configuration Using Ifartree-Fock c3lcuIations with inclusion of limited CI, Reader 113s recently 3signed the various states of CsIl determined by emission work [7,S] _ The energies of these states are in excellent ttgreement with the photoeiectron work and are given in table 1 with their assignments_ It seems rertson3bIe to assume similar sssignments for the photoelectron spectrum_ States srising from the Sp’6s configuration are shown to undergo considerabIe CI with 5p55d states. A theoretic31 estim3te of the rel3tive intensities of these brtnds assuming them to be governed by fimd state CI effects is given by the percent3se of the 5~~6s configurrttion associrtble with these states [7] with an 3Ilow3nce for the 2J+ 1 degener3cy of the states. This corresponds to the one electron contribution to t!ie formation of 3 particuIar state_ Intensities deduced on this basis are given in tsbie 2 and are seen to be in esceIIent agreement with the experimental relative intensities_ In both cases the total intensity associated with the 19-I eV band w3s put equ31 to LOO. This indic3tes that for the principal states in the photoeiectron spectrum the ionization is governed by fin31 state configuration interaction effects The ionization potentials of the weak peaks observed in the 19 to 24 eV region of the spectrum also show good agreement with the opticai work (tabIe 1). These features are interpreted in terms of states arising from the 5p56p and 5~~7s configurations_ Unfortunately no caIcuIations are availabIe showing the extent of the particitiation of the one electron 5p56si configuration in these states so it is not possible to comment on the relative intensities in terms of fmal st3te CL
I5 October 1979
Table 2 Comp.trison bcween obsorded and predicted relatire intensities of peaks due to ionization to ionic states with contributions from rbc 5~~6s configuration Predicted rehrire
IP (ev)
Major configra:ion
mtcnsity
17.11 1727 I
150
136
5~~6s ;<:,, ___
17 65
30
47
17.55
25
23
5p’6s?&, __ 5p56d 3Pz
100
100
19 06 19.13
5p’6s $fi>e,r
19.7 1
Considering the possibility of initial state CI only the configuration 5p67s’could make any marked contribution to the 5~~6s’ ground state. This initial configuration ciearly could not give rise to the 5p56p’ ionized configuration but might contribute to the 5ps7s1 states. A point of interest in this respect is that this initial st3te confi_grtiration would not be detectable in the HeI, (6~)~~ spectrum (cf. ref. [9] )_ The observed intensity ratio of 2 to 1 for the bands associated with the 5~~7s’ stzttes is consistent with their assigned _ degeneracy (t3bIe I)_ This situation is only IikeIy to apply where the states are formed through initia1 state CL It therefore appears that while the main part of the Cs 5p-’ roniztltion bands may be adequately accounted for in terms of foal state CI initial state CI may we11 be responsible for some of the weaker features_
Acknowledgement We should like to thsnk the S.R.C_ for fmancid support.
References
111JX. West. P-R_ Woodruff, K. Codling md R-G_ Houk_ate. J. Phys_ B 9 (1976) 407s
PI A-1’. Potts and T-A. Wlliams, J. Electron Spectry. 3 (1974) 3_
CIIFMIC_AL
PIIYSICS
LETI-ERS 171 J_ Redder. Phys. Rr\_ l3A (1976) 507. :SJ J_ Rtider and Cl__ Epstein_ J_ Opt_ See
I.5 October
1979
At-n_ 65 (1975)
638. 191 T-A. Wdhmr ad A.W. Potts. J. Clecrron Spcctr~ _ S (1976) 331. [ 101 E.P.I=. Lee and Ah’. Potts. Proc. Roy_ Sot 36% (1979) 39.5; Chem. PII) s_ Letters 63 (1979) 6 I _ [ I 11 E_P.l=_ Lrs and X.W. Potts. Chrm. Phyr Letters. to be publi&xL
556