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Double ionization of fluorobenzene to singlet and triplet states of its dication
15 November 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 262 (1996) 355-361
Double ionization of fluorobenzene to singlet and triplet states of its dication S.E. Silcocks a, N. Jeffreys a, F.M. Harris ~, S.R. Andrews b, D.E. Parry a Mass Spectrometry Research Unit, University of Wales Swansea, Singleton Park, Swansea SA2 8PP. UK b Department o f Chemistry, University of Wales Swansea, Singleton Park, Swansea SA2 8PP. UK Received 20 August 1996
Abstract Double-ionization energies of the fluorobenzene molecule to singlet and triplet electronic states of its dication have been measured using double-charge-transfer spectroscopy. The values obtained are compared with those calculated using a semi-empirical form of the multiple-scattering X a computational method. The calculated data show that the density of electronic states of C6HsF 2+ increases markedly as the energy increases. However, the first six measured energies to triplet states, and the first five to singlet states, closely match the energies of predicted electronic transitions, thereby verifying the association of these measured energies with double ionization to specific states of C6HsF 2÷.
1. Introduction In a previous experimental study [1] of fluorinated benzene molecules, double-ionization energies to electronic states of their dications were measured. Double-charge-transfer (DCT) spectroscopy was used, and it was assumed that triplet states of the dications were populated since OH + and F + were chosen as the projectile ions. This assumption is justified since many double-electron-capture reactions, which are central to DCT spectroscopy, satisfy spin conservation [2-4]. Further, comparison of experimental results with theoretical predictions suggests that triplet states of C6H62+ were populated in a DCT spectroscopy study of benzene using these projectile ions [5]. The spectra obtained for C 6 H s F [1] showed five fairly broad peaks corresponding to double-ionization energies between 25.2 + 0.5 eV and 38.4 + 1.0 eV. Calculated values were not available at that time, so it was not possible to interpret the energies in
terms of electronic transitions. Since that time, however, a theoretical study of C r H s F has been completed, the results of which are presented in this Letter. They suggest that more peaks would have been observed if in the previous experimental study a spectrometer of higher resolving power had been used. Such a spectrometer has been used in the investigation reported here to obtain more details of double-ionization energies to triplet states of C r H s F 2+. In addition, double-ionization energies to singlet states have been measured using H + as the projectile ion in DCT spectroscopy. The experimental results are interpreted in terms of the electronic transitions predicted in the theoretical part of the study.
2. Experimental The spectrometer used is a reversed-geometry double-focusing mass spectrometer (model 8230,
0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0009-2614(96)01072-X
356
S.E. Silcocks et a l . / Chemical Physics Letters 262 (1996) 355-361
Table 1 Calculated and measured double-ionization energies of C 6 H 5F to triplet electronic states of C 6 H5 F 2 + Measured values
Calculated values
present investigation
previous investigation
present investigation
Peak a
DIE (eV)
DIE (eV)
DIE b (eV)
A
25.4 + 0.2
25.2 _+ 0.5
25.4
B
27.8 + 0.4
28.0 ___0.5
mean value (eV)
term
configuration
3B 2
la~- 13b~- t
3A I
2b~-13b~- J
3A 2
10bf 13b~- I
3B2 3B j 3B 1
2b~- J l a ] l 10b~- ~la~ t 1 la~- 13bj i
3A 2
1 la I l l a ] i
3A 2
9b~- 13b/I
3B I
9b~-lla~- I
3A 2
8b~- 13b~- I
3B I 3131 3A 2
10a~- 13b~ i 8 b f Ila2 I 2b~- I lOb2 l
3A 2 3B I 3B 2 3A I 3B 1
10a~ 11a~- i l l a j 12b~- 1 lla lll0b2 i Ibm-13b~- l 9a~ 13b~- i
32.1 32.2 32.4 32.4 32.7
3B 2 3A 2 3A 2 3A i 3B I
lb~- I la 2 z 7b~- 13b~- i 9a~-Ila~l 9b 2 z 10b2 J 7b-Ila2 i
33.3 33.3 33.3 33.4
3A 2 3A I 3B 2 3B I
8b~- 12b~- I 8b~- Jl0b~ -I 9b~- l 1 la~- 2 8a~- )3b~- i
33.8 33.8 34.0 34.0
3B I 3B 2 3B 2 3A 2
10a~- 12b~- l 10a~- J 10b~- i 8b~ 11 la~- i 8a( ~la2-I
34.6 34.6 34.8 34.8
3A 2 3A I 3A I 3B I
lb~- 110b2 10a~- i 1 la~- i lb l- 12b / i 9a 7 '2b~- i
27.8 27.8 27.8
C
28.5 + 0.3
D
29.5 _+ 0.3
28.4 28.5 28.7
28.5
29.2 29.3 29.4 30.1 E
30.2
30.2 5:0.2 30.3
F
31.2 + 0.2
31.2 __. 0.5
30.8 30.8 30.8
30.8
31.4 31.6 31.7 31.7 31.9 G
H
31.9 + 0.3
32.1
33.4 + 0.2
33.6
S.E. Silcocks et a l . / Chemical Physics Letters 262 (1996) 355-361
357
Table 1 (continued) Measured values
Calculated values
present investigation
previous investigation
present investigation
Peak a
DIE (eV)
DIE b (eV)
DIE (eV)
mean value (eV)
term
configuration
3Ai 3B2 3A 1 3A 2
3BI
8b 2 9a t 7b~ 7b, Ib t
35.4 35.7 35.9 36.2 36.2 36.5 36.5 36.5 36.7
3B2 ~A I 3B2 3A2 3B2 3B I 3B 2 3B 2 3A L
10ai 19b ? i 9a L II la~ i 7b 2 i 1 la / i lb I 19b 2 t 10a/18b 2 i 8a~ 12b~ t 8a i II0b2 t 9a] t9b_, 1 7b 2 19b2 I
37.2 37.3 37.3
3A I 3B 2 3A:
3BI
8a/]lla/j 9a~ t8b2 i lb / 18b 2 I Ibt ]10a/I
3AI 3B 2 3B 2 3Al
9a i 8a i 7b~ 7b 2
34.9 34.9 35.2 35.2
35.3
35.1 + 0.8
19b, 1 l l0b ' t 110b_, i 12b i i ~1 la / L
35.5
37.5 38.4 4- 1.0
37.7 37.8 38.1 38.1 38.1
tl0a~ 1 19b2 ] II0ai i 18b 2 k
a See Fig. 1. b These values are the calculated values +0.8 eV (see text).
manufactured by Finnigan MAT, Bremen, Germany). It has been modified [6] for DCT spectroscopy and is capable of resolving peaks which are 0.7 eV apart. The projectile ions (OH + and H ÷) were generated by electron ionization of water, accelerated to 3 keV and mass-selected using the spectrometer's magnet. C6HsF molecules were introduced into a collisiongas cell located close to the intermediate focal point in the second field-free region. The ions were transmitted through the cell, some undergoing the double-electron-capture reaction represented by A + + C6HsF ~ A - + C6H~ + ,
(l)
in which A ÷ represents OH + or H ÷. The negative ions generated were transmitted to the detector by
BD
100.
C E
A
50.
H
@
) 2~
29'80
2~0
En(CdltsF) (eV) Fig. 1. A typical OH + DCT spectrum for C6HsF.
358
S.E. Silcocks et a l . / Chemical Physics Letters 262 (1996) 355-361
Table 2 Calculated and measured double-ionization energies of C 6 H sF to singlet electronic states of C 6 H sF 2÷ Measured values
Calculated values
Peak a
DIE (eV)
A
25.9 + 0.2
DIE b (eV)
mean value (eV)
term
configuration
'A
3b~- 13b~ 1
26.0
~B 2
la~-'3b~- 1
26.9
IA 1
la 2 ~la 2
~A i
2b~- J3b~- I
~A2
10b~ J3b I 1
'B 2 ~B ~B~
2 b . I la 2 l 10b2, la 2 i lla~- 13b~- '
~A 2
lla~- I l a ] I
30.1
IA 2
9b~- '3b~- '
31.2 5:0.4
30.9 31.0 31.4 31.4 31.5 31.5 31.6
31.3
~B~ IA 2 'A~ 'B, IA2 ~B 1 ~A 1
9b 2 i la 2- i 8b~- 13b~- l 2b~- 12b~- J 10a~ 13b~- i 2bj- I10b~- J 8b 2 i la ~- I 10b~- 110b~- i
32.5
IA 2 ' B, IB 2 IB I IA 1
10a~- 'la~- 1
32.3 + 0.3
32.2 32.3 32.4 32.6 32.7
9a~- J3b~- i lb~- 13b~- I
33.2
IA 2 ZAz 'B 2 IA 2 IA I 'B, IA I
7b 2 13b ~- I 9b 2 J2b~- I lbl- I la 2 i 9a~ I la~- i 9b 2 J 10b 2 i 7b~ ~la~- ' l l a ~ - ' l l a ~ -I
34.4
IA tA, 'B2 tB t iB I IB 2 IA 2 IB 2 IA 1
8b 2 J2b l- J 8b~- J l 0b 2 z 9b 2 I I la~- i 8a I- 13b ~- l 10a~- 12b~- i 10a~- I10b2 J 8a~ l l a ] t 8b~- I I la 7 i 9b 2- 19b 2 i
25.7
B
26.6 + 0.3
C
28.0 + 0.3
25.9
28.4 28.5 28.5
D
29.0 + 0.2
E
30.1 + 0.4
29.1 29.2 29.5
29.3
30.0
F
G
H
I
30.0
33.2 ___0.3
33.0 33.1 33.2 33.2 33.3 33.5 33.5
34.1 _+ 0.3
33.9 34.0 34.1 34.1 34.4 34.6 34.7 34.8 35.0
1 la~ 12b~- i 1 la~ 110b~ 1
S.E. Silcocks et a l . / Chemical Physics Letters 262 (1996) 355-361
359
Table 2 (continued) Measured values Peak a
J
Calculated values DIE (eV)
DIE b (eV)
35.2 +_ 0.3
35.4 35.5 35.5 35.6 35.6 35.8 35.9 35.9 35.9
mean value (eV)
35.7
term
configuration
LA1 IB 2 tA 1 IB 2 IA 2 IA t IB~ IA l tA 2
10al llla~ l 9a t 12b / 1 8b 2 19b z- i 9a I t 10b2 i lb i I I0b2 lb/i2b/ 8a i- l10b2 i 7b~ I I0b~ i 7b~ 12bi i
a See Fig. 2. b These values are the calculated values + 1.1 eV (see text).
scanning the voltages applied to the plates of the electric sector. The energetics of reaction (1) can be represented by Ep - E , ( C 6 H s F ) = D I E ( C 6 H s F ) - E ( A + --* A - ),
(2) in w h i c h Ep is the translational energy of A + (kept constant at 3 keV during the experiment), En(C6HsF) is the translational energy of A - ions generated in the reaction, DIE(C6HsF) is the double-ionization energy of C6HsF and E(A+--* A - ) is the energy released in converting A + to A - . The populating of different electronic states of C6HsF 2+ clearly must give rise to different values of En(C6HsF) which result in the peaks in the DCT spectrum of A - ion current against En(C6HsF). To convert the positions of the peaks into double-ionization energies, the translational-energy scale had to be converted into a double-ionization-energy scale. This was done by obtaining a DCT spectrum with xenon since the double-ionization energies to Xe 2+ states are known [7]. The relevant energy equation is Ep - En(Xe ) = DIE(Xe) - E ( A + ~ A - )
(3)
and it follows that E . ( C 6 H s F ) - En(Xe ) = DIE(Xe) - DIE(C6HsF ),
(4) from which DIE(C6HsF) can be determined. Thus, by measuring the E,(C6HsF) positions of the peaks
in the DCT spectrum, the double-ionization energies to various electronic states were determinable. If the spin-conservation rule applies to the double-electron-capture reactions with C6HsF, it follows that, since H - is detected in its l s 2 bound singlet electronic state, the doubly-charged ions are produced in the same spin-state as C6HsF, i.e. in singlet states. The stable O H - ion detected also has a singlet configuration ( ' E + ) , so triplet states of C6HsF 2+ should be populated when using the OH + projectile ion as it has a 3E- ground state.
3. Computational The multiple-scattering X a (MSXa) method [8] has been used in a semi-empirical form [9] to calculate double-ionization energies to electronically excited states of C6HsF 2+. In an approximate singleconfiguration picture of the double-ionization process, when two electrons are removed from spin orbitals a and b of the molecule, the double-ionization energy may be regarded as the sum of the single-ionization energies IE, and IE b for each spin orbital, plus the interaction energy V~b between the two positive holes created, i.e. DIE~b = IE~ + IF b +
V~b,
(5)
Values of gab w e r e calculated as described previously [9], while the values of single-ionization energies required in Eq. (5) were obtained from a photo-
360
S.E. Silcocks et a l . / Chemical Physics Letters 262 (1996) 355-361
electron-spectroscopic study of C6HsF [10], thus making the theoretical method semi-empirical.
pectation values of two-hole configurations, there provide only a rough indication of the positions of the peaks observed in the DCT spectra.
4. Results and discussion
4.2. Double ionization to singlet states of C 6 H5 F2 +
4.1. Double ionization to triplet states of
C6HsF 2 +
A typical O H + / X e DCT spectrum has been published in a previous Letter [11]. It consists of two peaks, the mid-position of which corresponds to DIE(Xe) = 33.94 eV. A typical O H ÷ / C 6 H s F spectrum is shown in Fig. 1. Eight peaks are clearly evident, with some evidence of much weaker peaks at lower translational energies. Twenty such spectra were recorded over a two-day period, each spectrum having associated with it a xenon calibration spectrum. The double-ionization energies determined from the peak positions have been calculated for each spectrum; these have been averaged, and are presented in Table 1 together with the standard deviations in the mean values. Also tabulated are the previously measured values and the values calculated in the present investigation. To aid comparison with the experimental data, all the calculated double-ionization energies have been increased by 0.8 eV, this change making the lowest calculated value the same as that measured. Such a procedure has proved to be a general requirement in the comparison of theoretical and experimental double-ionization energies. The advantage of using a spectrometer of higher resolving power is clearly evident from the experimental data shown in Table 1. In the present investigation, eight peaks are observed in the 25-34 eV range, whereas three were observed previously [1]. If close-lying calculated data are grouped together as shown in the table, it can be seen that good agreement is evident between the resulting mean values and those determined from the positions of peaks A-F. The calculated data, therefore, indicate which electronic transitions give rise to those peaks. Above 31 eV, the calculated density of states becomes high, and it is much less straightforward to group the calculated double-ionization energies together in a logical fashion to correspond to measured values. At these higher energies, configuration interaction and satellite excitations should both be significant and the calculated double-ionization energies, being ex-
A DCT spectrum obtained when H ÷ ions undergo double-electron-capture reactions with xenon has within it one strong peak (see, for example, Ref. [2]). Its position corresponds to En(Xe) and has associated with it a double-ionization energy of 35.447 eV, i.e. to the tD 2 state of Xe 2+. Such a spectrum was obtained in the present investigation after each C6HsF spectrum to calibrate the double-ionizationenergy scale. Twenty spectra were recorded over two non-consecutive days of investigation. A typical spectrum is shown in Fig. 2. Double-ionization energies determined from the corresponding peaks A - J in the spectra have been averaged; the results are shown in Table 2 together with the standard deviations in the mean values. Also shown in the table are the calculated energies. These are the original data to which 1.1 eW has been added in order to make the mean of the two lowest energies (which are separated by only 0.3 eV) the same as the lowest measured value of 25.9 eV. It can be seen that the mean values obtained when groups of calculated data are established, as shown in Table 2, are in good agreement with those measured. However, the number of electronic transitions corresponding to the last five peaks is quite E
100-
B
"~
D
F
l
50-
.E
2990
2980
29~70
F~(CdlsF) (eV) Fig. 2. A typical H + DCT spectrum for C6HsF.
S.E. Silcocks et al. / Chemical Physics Letters 262 (1996) 355-361
large. On the other hand, for the first five peaks, the transitions are clearly identifiable. Above 31 eV, as for the triplet excitations, the predicted density of singlet dicationic states becomes large, and the correlation between individual calculated and observed energies much weaker.
5. Conclusions The calculated data show that the density of singlet and triplet electronic states of C6HsF 2+ increases markedly as the energy increases. However, the first six peaks in the OH + DCT spectrum, and the first five peaks in the H + DCT spectrum, closely match the energies of predicted electronic transitions, thereby verifying the association of these peaks with double ionization to individual states of C6HsF 2+.
Acknowledgement SES and NJ are grateful to the Engineering and Physical Sciences Research Council for financial support. FMH and DEP thank the British Mass Spectrometry Society for a grant which allowed them
361
to purchase the computer used in the calculation of the double-ionization energies.
References [1] W.J. Griffiths, M.L. Langford and F.M. Harris, J. Am. Soc. Mass Spectrom. 4 (1993) 513. [2] J. Appell, in: Collision spectroscopy, ed. R.G. Cooks (Elsevier, Amsterdam, 1978) p. 244. [3] F.M. Harris, in: Springer series in chemical physics, Vol. 54. Physics of ion impact phenomena, ed. D. Mathur (Springer, Berlin, 1991)p. 199. [4] F.M. Harris, Intern. J. Mass Spectrom. Ion Processes 120 (1992) 1. [5] W.J. Griffiths and F.M. Hams, Chem. Phys. 157 (1991) 249. [6] S.R. Andrews and D.E. Parry, Rapid Commun. Mass Spectrom. 7 (1993) 548. [7] C. Moore, Atomic energy levels, NSRDS-NBS Circular No. 467 (US GPO, Washington, 1949). [8] M. Cook and D.A. Case, XASW, QCPE program No. 465 Quantum Chemistry Program Exchange, Indiana University, Bloomington, IN. [9] S.R. Andrews and D.E. Parry, Chem. Phys. Lett. 196 (1992) 630. [10] D.W. Turner, C. Baker, A.D. Baker and C.R. Brundle, Molecular photoelectron spectroscopy (Wiley-lnterscience, London, 1970). [ll] S.R. Andrews and F.M. Hams, Chem. Phys. Lett. 253 (1996) 403.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 262 (1996) 355-361
Double ionization of fluorobenzene to singlet and triplet states of its dication S.E. Silcocks a, N. Jeffreys a, F.M. Harris ~, S.R. Andrews b, D.E. Parry a Mass Spectrometry Research Unit, University of Wales Swansea, Singleton Park, Swansea SA2 8PP. UK b Department o f Chemistry, University of Wales Swansea, Singleton Park, Swansea SA2 8PP. UK Received 20 August 1996
Abstract Double-ionization energies of the fluorobenzene molecule to singlet and triplet electronic states of its dication have been measured using double-charge-transfer spectroscopy. The values obtained are compared with those calculated using a semi-empirical form of the multiple-scattering X a computational method. The calculated data show that the density of electronic states of C6HsF 2+ increases markedly as the energy increases. However, the first six measured energies to triplet states, and the first five to singlet states, closely match the energies of predicted electronic transitions, thereby verifying the association of these measured energies with double ionization to specific states of C6HsF 2÷.
1. Introduction In a previous experimental study [1] of fluorinated benzene molecules, double-ionization energies to electronic states of their dications were measured. Double-charge-transfer (DCT) spectroscopy was used, and it was assumed that triplet states of the dications were populated since OH + and F + were chosen as the projectile ions. This assumption is justified since many double-electron-capture reactions, which are central to DCT spectroscopy, satisfy spin conservation [2-4]. Further, comparison of experimental results with theoretical predictions suggests that triplet states of C6H62+ were populated in a DCT spectroscopy study of benzene using these projectile ions [5]. The spectra obtained for C 6 H s F [1] showed five fairly broad peaks corresponding to double-ionization energies between 25.2 + 0.5 eV and 38.4 + 1.0 eV. Calculated values were not available at that time, so it was not possible to interpret the energies in
terms of electronic transitions. Since that time, however, a theoretical study of C r H s F has been completed, the results of which are presented in this Letter. They suggest that more peaks would have been observed if in the previous experimental study a spectrometer of higher resolving power had been used. Such a spectrometer has been used in the investigation reported here to obtain more details of double-ionization energies to triplet states of C r H s F 2+. In addition, double-ionization energies to singlet states have been measured using H + as the projectile ion in DCT spectroscopy. The experimental results are interpreted in terms of the electronic transitions predicted in the theoretical part of the study.
2. Experimental The spectrometer used is a reversed-geometry double-focusing mass spectrometer (model 8230,
0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0009-2614(96)01072-X
356
S.E. Silcocks et a l . / Chemical Physics Letters 262 (1996) 355-361
Table 1 Calculated and measured double-ionization energies of C 6 H 5F to triplet electronic states of C 6 H5 F 2 + Measured values
Calculated values
present investigation
previous investigation
present investigation
Peak a
DIE (eV)
DIE (eV)
DIE b (eV)
A
25.4 + 0.2
25.2 _+ 0.5
25.4
B
27.8 + 0.4
28.0 ___0.5
mean value (eV)
term
configuration
3B 2
la~- 13b~- t
3A I
2b~-13b~- J
3A 2
10bf 13b~- I
3B2 3B j 3B 1
2b~- J l a ] l 10b~- ~la~ t 1 la~- 13bj i
3A 2
1 la I l l a ] i
3A 2
9b~- 13b/I
3B I
9b~-lla~- I
3A 2
8b~- 13b~- I
3B I 3131 3A 2
10a~- 13b~ i 8 b f Ila2 I 2b~- I lOb2 l
3A 2 3B I 3B 2 3A I 3B 1
10a~ 11a~- i l l a j 12b~- 1 lla lll0b2 i Ibm-13b~- l 9a~ 13b~- i
32.1 32.2 32.4 32.4 32.7
3B 2 3A 2 3A 2 3A i 3B I
lb~- I la 2 z 7b~- 13b~- i 9a~-Ila~l 9b 2 z 10b2 J 7b-Ila2 i
33.3 33.3 33.3 33.4
3A 2 3A I 3B 2 3B I
8b~- 12b~- I 8b~- Jl0b~ -I 9b~- l 1 la~- 2 8a~- )3b~- i
33.8 33.8 34.0 34.0
3B I 3B 2 3B 2 3A 2
10a~- 12b~- l 10a~- J 10b~- i 8b~ 11 la~- i 8a( ~la2-I
34.6 34.6 34.8 34.8
3A 2 3A I 3A I 3B I
lb~- 110b2 10a~- i 1 la~- i lb l- 12b / i 9a 7 '2b~- i
27.8 27.8 27.8
C
28.5 + 0.3
D
29.5 _+ 0.3
28.4 28.5 28.7
28.5
29.2 29.3 29.4 30.1 E
30.2
30.2 5:0.2 30.3
F
31.2 + 0.2
31.2 __. 0.5
30.8 30.8 30.8
30.8
31.4 31.6 31.7 31.7 31.9 G
H
31.9 + 0.3
32.1
33.4 + 0.2
33.6
S.E. Silcocks et a l . / Chemical Physics Letters 262 (1996) 355-361
357
Table 1 (continued) Measured values
Calculated values
present investigation
previous investigation
present investigation
Peak a
DIE (eV)
DIE b (eV)
DIE (eV)
mean value (eV)
term
configuration
3Ai 3B2 3A 1 3A 2
3BI
8b 2 9a t 7b~ 7b, Ib t
35.4 35.7 35.9 36.2 36.2 36.5 36.5 36.5 36.7
3B2 ~A I 3B2 3A2 3B2 3B I 3B 2 3B 2 3A L
10ai 19b ? i 9a L II la~ i 7b 2 i 1 la / i lb I 19b 2 t 10a/18b 2 i 8a~ 12b~ t 8a i II0b2 t 9a] t9b_, 1 7b 2 19b2 I
37.2 37.3 37.3
3A I 3B 2 3A:
3BI
8a/]lla/j 9a~ t8b2 i lb / 18b 2 I Ibt ]10a/I
3AI 3B 2 3B 2 3Al
9a i 8a i 7b~ 7b 2
34.9 34.9 35.2 35.2
35.3
35.1 + 0.8
19b, 1 l l0b ' t 110b_, i 12b i i ~1 la / L
35.5
37.5 38.4 4- 1.0
37.7 37.8 38.1 38.1 38.1
tl0a~ 1 19b2 ] II0ai i 18b 2 k
a See Fig. 1. b These values are the calculated values +0.8 eV (see text).
manufactured by Finnigan MAT, Bremen, Germany). It has been modified [6] for DCT spectroscopy and is capable of resolving peaks which are 0.7 eV apart. The projectile ions (OH + and H ÷) were generated by electron ionization of water, accelerated to 3 keV and mass-selected using the spectrometer's magnet. C6HsF molecules were introduced into a collisiongas cell located close to the intermediate focal point in the second field-free region. The ions were transmitted through the cell, some undergoing the double-electron-capture reaction represented by A + + C6HsF ~ A - + C6H~ + ,
(l)
in which A ÷ represents OH + or H ÷. The negative ions generated were transmitted to the detector by
BD
100.
C E
A
50.
H
@
) 2~
29'80
2~0
En(CdltsF) (eV) Fig. 1. A typical OH + DCT spectrum for C6HsF.
358
S.E. Silcocks et a l . / Chemical Physics Letters 262 (1996) 355-361
Table 2 Calculated and measured double-ionization energies of C 6 H sF to singlet electronic states of C 6 H sF 2÷ Measured values
Calculated values
Peak a
DIE (eV)
A
25.9 + 0.2
DIE b (eV)
mean value (eV)
term
configuration
'A
3b~- 13b~ 1
26.0
~B 2
la~-'3b~- 1
26.9
IA 1
la 2 ~la 2
~A i
2b~- J3b~- I
~A2
10b~ J3b I 1
'B 2 ~B ~B~
2 b . I la 2 l 10b2, la 2 i lla~- 13b~- '
~A 2
lla~- I l a ] I
30.1
IA 2
9b~- '3b~- '
31.2 5:0.4
30.9 31.0 31.4 31.4 31.5 31.5 31.6
31.3
~B~ IA 2 'A~ 'B, IA2 ~B 1 ~A 1
9b 2 i la 2- i 8b~- 13b~- l 2b~- 12b~- J 10a~ 13b~- i 2bj- I10b~- J 8b 2 i la ~- I 10b~- 110b~- i
32.5
IA 2 ' B, IB 2 IB I IA 1
10a~- 'la~- 1
32.3 + 0.3
32.2 32.3 32.4 32.6 32.7
9a~- J3b~- i lb~- 13b~- I
33.2
IA 2 ZAz 'B 2 IA 2 IA I 'B, IA I
7b 2 13b ~- I 9b 2 J2b~- I lbl- I la 2 i 9a~ I la~- i 9b 2 J 10b 2 i 7b~ ~la~- ' l l a ~ - ' l l a ~ -I
34.4
IA tA, 'B2 tB t iB I IB 2 IA 2 IB 2 IA 1
8b 2 J2b l- J 8b~- J l 0b 2 z 9b 2 I I la~- i 8a I- 13b ~- l 10a~- 12b~- i 10a~- I10b2 J 8a~ l l a ] t 8b~- I I la 7 i 9b 2- 19b 2 i
25.7
B
26.6 + 0.3
C
28.0 + 0.3
25.9
28.4 28.5 28.5
D
29.0 + 0.2
E
30.1 + 0.4
29.1 29.2 29.5
29.3
30.0
F
G
H
I
30.0
33.2 ___0.3
33.0 33.1 33.2 33.2 33.3 33.5 33.5
34.1 _+ 0.3
33.9 34.0 34.1 34.1 34.4 34.6 34.7 34.8 35.0
1 la~ 12b~- i 1 la~ 110b~ 1
S.E. Silcocks et a l . / Chemical Physics Letters 262 (1996) 355-361
359
Table 2 (continued) Measured values Peak a
J
Calculated values DIE (eV)
DIE b (eV)
35.2 +_ 0.3
35.4 35.5 35.5 35.6 35.6 35.8 35.9 35.9 35.9
mean value (eV)
35.7
term
configuration
LA1 IB 2 tA 1 IB 2 IA 2 IA t IB~ IA l tA 2
10al llla~ l 9a t 12b / 1 8b 2 19b z- i 9a I t 10b2 i lb i I I0b2 lb/i2b/ 8a i- l10b2 i 7b~ I I0b~ i 7b~ 12bi i
a See Fig. 2. b These values are the calculated values + 1.1 eV (see text).
scanning the voltages applied to the plates of the electric sector. The energetics of reaction (1) can be represented by Ep - E , ( C 6 H s F ) = D I E ( C 6 H s F ) - E ( A + --* A - ),
(2) in w h i c h Ep is the translational energy of A + (kept constant at 3 keV during the experiment), En(C6HsF) is the translational energy of A - ions generated in the reaction, DIE(C6HsF) is the double-ionization energy of C6HsF and E(A+--* A - ) is the energy released in converting A + to A - . The populating of different electronic states of C6HsF 2+ clearly must give rise to different values of En(C6HsF) which result in the peaks in the DCT spectrum of A - ion current against En(C6HsF). To convert the positions of the peaks into double-ionization energies, the translational-energy scale had to be converted into a double-ionization-energy scale. This was done by obtaining a DCT spectrum with xenon since the double-ionization energies to Xe 2+ states are known [7]. The relevant energy equation is Ep - En(Xe ) = DIE(Xe) - E ( A + ~ A - )
(3)
and it follows that E . ( C 6 H s F ) - En(Xe ) = DIE(Xe) - DIE(C6HsF ),
(4) from which DIE(C6HsF) can be determined. Thus, by measuring the E,(C6HsF) positions of the peaks
in the DCT spectrum, the double-ionization energies to various electronic states were determinable. If the spin-conservation rule applies to the double-electron-capture reactions with C6HsF, it follows that, since H - is detected in its l s 2 bound singlet electronic state, the doubly-charged ions are produced in the same spin-state as C6HsF, i.e. in singlet states. The stable O H - ion detected also has a singlet configuration ( ' E + ) , so triplet states of C6HsF 2+ should be populated when using the OH + projectile ion as it has a 3E- ground state.
3. Computational The multiple-scattering X a (MSXa) method [8] has been used in a semi-empirical form [9] to calculate double-ionization energies to electronically excited states of C6HsF 2+. In an approximate singleconfiguration picture of the double-ionization process, when two electrons are removed from spin orbitals a and b of the molecule, the double-ionization energy may be regarded as the sum of the single-ionization energies IE, and IE b for each spin orbital, plus the interaction energy V~b between the two positive holes created, i.e. DIE~b = IE~ + IF b +
V~b,
(5)
Values of gab w e r e calculated as described previously [9], while the values of single-ionization energies required in Eq. (5) were obtained from a photo-
360
S.E. Silcocks et a l . / Chemical Physics Letters 262 (1996) 355-361
electron-spectroscopic study of C6HsF [10], thus making the theoretical method semi-empirical.
pectation values of two-hole configurations, there provide only a rough indication of the positions of the peaks observed in the DCT spectra.
4. Results and discussion
4.2. Double ionization to singlet states of C 6 H5 F2 +
4.1. Double ionization to triplet states of
C6HsF 2 +
A typical O H + / X e DCT spectrum has been published in a previous Letter [11]. It consists of two peaks, the mid-position of which corresponds to DIE(Xe) = 33.94 eV. A typical O H ÷ / C 6 H s F spectrum is shown in Fig. 1. Eight peaks are clearly evident, with some evidence of much weaker peaks at lower translational energies. Twenty such spectra were recorded over a two-day period, each spectrum having associated with it a xenon calibration spectrum. The double-ionization energies determined from the peak positions have been calculated for each spectrum; these have been averaged, and are presented in Table 1 together with the standard deviations in the mean values. Also tabulated are the previously measured values and the values calculated in the present investigation. To aid comparison with the experimental data, all the calculated double-ionization energies have been increased by 0.8 eV, this change making the lowest calculated value the same as that measured. Such a procedure has proved to be a general requirement in the comparison of theoretical and experimental double-ionization energies. The advantage of using a spectrometer of higher resolving power is clearly evident from the experimental data shown in Table 1. In the present investigation, eight peaks are observed in the 25-34 eV range, whereas three were observed previously [1]. If close-lying calculated data are grouped together as shown in the table, it can be seen that good agreement is evident between the resulting mean values and those determined from the positions of peaks A-F. The calculated data, therefore, indicate which electronic transitions give rise to those peaks. Above 31 eV, the calculated density of states becomes high, and it is much less straightforward to group the calculated double-ionization energies together in a logical fashion to correspond to measured values. At these higher energies, configuration interaction and satellite excitations should both be significant and the calculated double-ionization energies, being ex-
A DCT spectrum obtained when H ÷ ions undergo double-electron-capture reactions with xenon has within it one strong peak (see, for example, Ref. [2]). Its position corresponds to En(Xe) and has associated with it a double-ionization energy of 35.447 eV, i.e. to the tD 2 state of Xe 2+. Such a spectrum was obtained in the present investigation after each C6HsF spectrum to calibrate the double-ionizationenergy scale. Twenty spectra were recorded over two non-consecutive days of investigation. A typical spectrum is shown in Fig. 2. Double-ionization energies determined from the corresponding peaks A - J in the spectra have been averaged; the results are shown in Table 2 together with the standard deviations in the mean values. Also shown in the table are the calculated energies. These are the original data to which 1.1 eW has been added in order to make the mean of the two lowest energies (which are separated by only 0.3 eV) the same as the lowest measured value of 25.9 eV. It can be seen that the mean values obtained when groups of calculated data are established, as shown in Table 2, are in good agreement with those measured. However, the number of electronic transitions corresponding to the last five peaks is quite E
100-
B
"~
D
F
l
50-
.E
2990
2980
29~70
F~(CdlsF) (eV) Fig. 2. A typical H + DCT spectrum for C6HsF.
S.E. Silcocks et al. / Chemical Physics Letters 262 (1996) 355-361
large. On the other hand, for the first five peaks, the transitions are clearly identifiable. Above 31 eV, as for the triplet excitations, the predicted density of singlet dicationic states becomes large, and the correlation between individual calculated and observed energies much weaker.
5. Conclusions The calculated data show that the density of singlet and triplet electronic states of C6HsF 2+ increases markedly as the energy increases. However, the first six peaks in the OH + DCT spectrum, and the first five peaks in the H + DCT spectrum, closely match the energies of predicted electronic transitions, thereby verifying the association of these peaks with double ionization to individual states of C6HsF 2+.
Acknowledgement SES and NJ are grateful to the Engineering and Physical Sciences Research Council for financial support. FMH and DEP thank the British Mass Spectrometry Society for a grant which allowed them
361
to purchase the computer used in the calculation of the double-ionization energies.
References [1] W.J. Griffiths, M.L. Langford and F.M. Harris, J. Am. Soc. Mass Spectrom. 4 (1993) 513. [2] J. Appell, in: Collision spectroscopy, ed. R.G. Cooks (Elsevier, Amsterdam, 1978) p. 244. [3] F.M. Harris, in: Springer series in chemical physics, Vol. 54. Physics of ion impact phenomena, ed. D. Mathur (Springer, Berlin, 1991)p. 199. [4] F.M. Harris, Intern. J. Mass Spectrom. Ion Processes 120 (1992) 1. [5] W.J. Griffiths and F.M. Hams, Chem. Phys. 157 (1991) 249. [6] S.R. Andrews and D.E. Parry, Rapid Commun. Mass Spectrom. 7 (1993) 548. [7] C. Moore, Atomic energy levels, NSRDS-NBS Circular No. 467 (US GPO, Washington, 1949). [8] M. Cook and D.A. Case, XASW, QCPE program No. 465 Quantum Chemistry Program Exchange, Indiana University, Bloomington, IN. [9] S.R. Andrews and D.E. Parry, Chem. Phys. Lett. 196 (1992) 630. [10] D.W. Turner, C. Baker, A.D. Baker and C.R. Brundle, Molecular photoelectron spectroscopy (Wiley-lnterscience, London, 1970). [ll] S.R. Andrews and F.M. Hams, Chem. Phys. Lett. 253 (1996) 403.