- Email: [email protected]
Double-ionization energies to singlet and triplet electronic states of the acetaldehyde dication measured by double-charge-transfer spectroscopy
and 10”Processes
ELSEVIER
International
Journal
of Mass Spectrometry
and Ion Processes
139 (1994) 163-168
~-
Double-ionization energies to singlet and triplet electronic states of the acetaldehyde dication measured by double-charge-transfer spectroscopy J.C. Severs, F.M. Harris*, Mass Spectrometry
W.J. Griffiths’
Research Unit, University College of Swansea. Singleton Park, Swansea SA2 8PP, UK Received
27 July 1994; accepted
19 August
1994
Abstract Double-charge-transfer spectroscopy has been used to measure the double-ionization energies of acetaldehyde to singlet and triplet electronic states of the dication. The values, measured within the range 30642eV, are in good agreement with previously calculated data thus allowing the characteristics of the spectra to be interpreted in terms of specific electronic transitions. Keywords:
Double-charge-transfer
spectroscopy;
Double-ionization
1. Introduction In a previous, detailed investigation [l], the doubly ionized states in formaldehyde, acetaldehyde, acetone and formamide were studied by X-ray excited core-valence-valence Auger electron (AE) spectroscopy. Assignments of the peaks in the spectra were made using ab initio Hartree-Fock, Green’s function and configuration interaction (CI) calculations. The contributions from transitions to triplet final states were illustrated graphically in the paper, but it was deduced that these were very weak, and thus they were not discussed. *Corresponding
author. address: Department of Medical Biochemistry and Biophysics, Karolinska Institutet, S171 77 Stockholm, Sweden.
’Present
0168-l 176/94/$07.00 0 1994 Elsevier SSDZ 0168-1176(94)04065-6
Science
B.V. All rights
energies
The positions of peaks observed in the AE spectra gave values of double-ionization energies of the molecules which were interpreted in terms of calculated energies to singlet states of the dications. As part of a programme of research in this Laboratory to provide the complementary experimental information on double-ionization energies to triplet states of the dications, double-charge-transfer (DCT) spectroscopy had been used to study doubleionization of acetone and acetaldehyde. DCT spectroscopy is based on double-electroncapture (DEC) reactions which take place between a fast-moving projectile ion and the molecule under investigation. Ample evidence exists [2,3] to show that spin is conserved in such reactions, and that, by appropriately selecting the projectile ion, the multiplicity of reserved
164
J.C. Severs et al./International Journal of Mass Spectrometry and Ion Processes 139 (1994) 163-168
the resulting doubly charged ion is known. For example, for ground-state molecules which are closed-shell, and thus in singlet states, the use of H+ as the projectile ion populates singlet states of the molecular dications. This occurs because H+ and H- have electronic spin equal to zero, so the doubly charged ion will have the same spin state as the neutral molecule. If, on the other hand, a projectile such as OH+ is used, this having a 3Cground state, double ionization to triplet states occurs because the OH- ions generated in the DEC reaction must be in the ‘LX+state, this being the only one with a sufficiently long lifetime to ensure the ions survive the journey between the reaction region and the detector. The results of a DCT spectroscopy study of acetone using OH+ as the projectile ion have been published [4]; fifteen double-ionization energies were measured, these correlating well with calculated individual energies or groups of energies identifiable with triplet states of the dication. The results of a similar study of acetaldehyde are reported here. In addition, a DCT spectroscopy study was carried out using H+ to obtain double-ionization energies to singlet states of CH3CH02+. In the previous AE spectroscopy study of acetaldehyde, the first doubly ionized state appeared as a shoulder at about 30.3eV on a more intense structure centred at 31.6 eV. It was considered likely that a H+ DCT spectroscopy study would enable the lowest singlet-state energy to be measured more precisely, and could also possibly give more detailed information about the singletstate configuration, particularly up to about 40 eV.
2. Experimental procedure If A+ represents in general a projectile ion (OH+ or H+ in the present investigation), then
the DEC reaction A+ + CH3CH0
of interest
here is
+ A- + CH3CH02+
(1)
The underlying arrows indicate that A+ and A- have high velocities; in this investigation, both projectile ions had a translational energy of 3 keV. The acetaldehyde molecules were introduced into a cell through which the projectile ions passed; in considering the energetics of reaction (1) the molecules can be regarded as at rest. The difference in the translational energies of A+ and A- is the endoergicity of reaction (1) which is given by AE = IE,(CH,CHO)
- E(A+ + A-)
(2)
where IE2(CH3CHO) is the double-ionization energy of the molecule, and E(A+ -+ A-) is the energy released in the charge inversion of A’ to A-. Strictly, a term should be included in Eq. (2) for the translational energy acquired by the CH3CH02+ ions generated in the reaction. It is readily shown, however, that this is sufficiently small to be neglected in comparison with the other energy terms. If different electronic states of CH3CH02+ are populated, having associated with them double-ionization energies of IEi(CH3CHO), IEi(CH,CHO) etc., then AE will have values LIE’, AE”, etc. With the translational energy of A+ kept constant, a scan of the translational energies of A- will give rise to a spectrum with peaks appearing at positions corresponding to different AE values. These positions are used to determine the double-ionization energies once the energy-loss scale has been calibrated using a DCT spectrum obtained for a gas such as xenon, for which the double-ionization energies are known accurately [5]. The experiments were carried out using a reversed-geometry Finnigan 8230 mass spectrometer [6] which has been modified for DCT spectroscopy [7]. The projectile ions OH+ and H+ were generated by dissociative electron ionization of water molecules, and
J.C. Severs et al./International Journal of Mass Spectrometry and Ion Processes 139 (1994) 163-168
were accelerated to 3 keV translational energy. After mass selection, the ions interacted with CH3CH0 molecules contained in a collisiongas cell located in the spectometer’s second field-free region. The DCT spectrum was obtained by scanning the voltages applied to the electric sector which had been set to transmit negative ions. The CHsCHO molecules were replaced by xenon atoms to obtain a calibration spectrum. Numerous pairs of sample/calibration spectra were obtained during the investigation; average values of the double-ionization energies were obtained from the individual energies derived from each pair of spectra.
3. Results 3.1. Double-ionization of CH3CH02+
energies to triplet states
Twenty-eight pairs of sample/calibrant spectra were recorded over three non-consecutive days of investigation. A typical OH+ DCT spectrum obtained with CHsCHO is shown in Fig. 1. The peaks marked A-H were clearly observed in over 70% of the other spectra recorded. Double-ionization energies were determined from the peak positions and the corresponding information from the calibration spectra xenon which corresponds to known double-ionization energies of xenon. The procedure is outlined in detail elsewhere [8]. Average values of the these energies are presented in Table 1, the uncertainties being standard deviations in these values. Also listed in the table are double-ionization energies to triplet states of the acetaldehyde dication which were calculated in a previous investigation [l]. A one-particle model was used which is based on the simple expression for the double-ionization energies Ep4 = Ip + I&&
(3)
165
where Ip and rq denote single-ionization energies for states with holes in the orbitals p and q. Siq is a hole-hole interaction term which is dependent on the spin coupling of the two-hole state. The double-hole ionization potentials are then simply given by sums of two such energies corrected for by the hole-hole interaction parameter. This is evaluated over the canonical Hartree-Fock orbitals of the ground state. Computational models such as the one just described are often successful in predicting differences in state energies but can give erroneous absolute values. It is usual, therefore, when comparing calculated and measured state energies, to ‘normalize’ the calculated values by shifting them appropriately. The values calculated previously [l] have been adjusted by the subtraction of 4.56eV, this having been chosen to provide the best correlation of calculated to experimental values. It can be seen from Table 1 that, except for the highest energy, all the calculated values agree with those measured to within the experimental uncertainty. When two calculated state energies lie close to one another, they have been grouped together and averaged to indicate the one double-ionization energy which would be expected to be measurable because of experimental limitations in resolving power. The results shown in Table 1 demonstrate that most of the OH’ DCT spectrum can be explained in terms of the electron losses indicated in the fourth column of the table. The exception is peak D corresponding to a double-ionization energy of 35.5 eV; it appears that the theory fails to predict a state of the dication having this energy. As will be seen below, a similar situation exists in the singlet-state data. 3.2. Double-ionization
to singlet states of
CH, CH02+
A typical Hf DCT spectrum obtained with
166
J.C. Severs et al./International Journal of Mass Spectrometry and Ion Processes 139 (1994) 163-168
Translational
energy of OH-ions
Fig. 1. A typical OH+ DCT spectrum
CH3CH0 is shown in Fig. 2. This is one of thirty-six spectra recorded over two nonconsecutive days. The peaks marked A-K were clearly evident in over 75% of the spectra. Calibration of the energy-loss scale was carried out by obtaining a Hf DCT spectrum of xenon following each CHsCHO spectrum. As previously [8], the strong peak due to the populating of the ‘D2 state of Xe2+ was used in Table 1 Measured double-ionization values [l]
energies
Peak
Measured
A
31.2f0.2
B C D
32.1 f 0.3 34.1 f 0.4 35.5 f 0.4
E
36.8 * 0.4
F G H
38.1 f 0.3 39.2 f 0.4 40.5 It 0.4
a Values obtained
by shifting
(in electronvolts)
all the calculated
31.9 34.1 37.0 37.3 > 37.2 38.1 39.5 41.2
of acetaldehyde.
the calibration since the double-ionization energy to that state is known to be 35.447eV [51. The average double-ionization energies determined from the position of peaks A-K are listed in Table 2 together with standard deviations. Also shown in the table are the double-ionization energies determined by AE spectroscopy [l], and Hartree-Fock
to triplet electronic
Calculateda
(eV)
states of CH3CH02+
together
with previously
calculated
State configuration (9a))‘(lOa)-’ (la)-‘(lOa)-’ (9a)-‘(2a))’ (2a)-‘(lOa)-’ _ (la)-‘(2a)-’ (8a))‘(9a)-’ (8a))‘(lOa))’ (8a))‘(la)-’ (la)-‘(9a)-’
values by -4.56 eV (see text), and then rounding
off to the first decimal
place
J.C. Severs et aLlInternational
Journal of Mass Spectrometry
3015
Fig. 2. A typical
double-ionization values [l]
Peak
A B C
energies (in electronvolts)
to singlet electronic
(eV)
of acetaldehyde.
states of CHsCHO*+
together
with previously
Measured (present)
Measured (AE spectroscopy) [I]
Calculated”
Ul
State configuration
30.4 f 0.2 31.3 f 0.3 32.3 i 0.4
30.3 31.2 31.7
_ 34.2 i 0.3
33.0 34.1
30.2 31.2 31.8 > 32.0 32.1 _
(9a)-‘(lOa))’ (la))‘(lOa))’ (lOa))* (9a))‘(2a)-’ (2a)-‘(lOa)-’ _
35.2 f 0.3 36.1 f 0.3
35.4 _
37.1 f 0.3
37.3
38.2 f 0.3 39.2 i 0.4
a Values obtained
energy of H-ions
H+ DCT spectrum
40.3 i 0.4
40.2
41.2 f 0.3
_
by shifting
all the calculated
167
2985
2995
3005
Translational
Table 2 Measured calculated
and Ion Processes 139 (1994) 163-168
35.2 36.4 37.1 37.3 37.5 i 37.6 37.6 31.7 38.6 39.0 39.0 > 39.2 39.8 40.2 > 40.6 41.7
31.9
37.5
39.1
40.2
values by -5.16 eV (see text) and then rounding
measured
(2a)-* (la)-‘(2a))’ (8a))‘(9a))’ (9a))* @a)-‘(lOa)-’ @-‘(la)-’ (la)-‘(9a)-’ @a)-‘(2a))’ (7a))‘(2a)-’ (7a)-‘(2a)-’ (7a)-‘(9a)-’ (7a)-‘(la)-’ (la)-2 (6a)-‘(lOa)-’ (6a)-‘(2a)-’ @a)-* off to the first decimal
place.
and
168
J.C. Severs et al.lInternational
Journal of Mass Spectrometry
calculated values [l]. As for the tripletstate data, the calculated values of doubleionization energies to singlet states have adjusted to provide maximum overlap with the values measured in the present investigation. The lowest value measured in the H+ DCT spectroscopy study is 30.4 f 0.2 eV which agrees with the value of 30.3eV deduced from the position of a weak feature in the AE spectrum [l]. Looking down the columns in Table 2, it can be seen that peaks A, B and C have equivalent peaks in the AE spectrum corresponding to similar double-ionization energies. However, a peak at 33.0eV seen in the AE spectrum is not observed in the DCT spectrum. Interestingly, this is the one peak seen in the methyl C-Auger spectrum; all the other AE spectroscopy values in Table 2 derive from the O-Auger spectrum. The double-ionization energy of 34.2* 0.3 eV derived from peak D is close to that of 34.7eV derived from one of the peaks of the AE spectrum [l]. However, they appear not to correlate with any of the calculated energies. A similar situation existed for the triplet-state information in that a double-ionization energy of 35.5eV had no equivalent calculated energy. Looking further down the columns of Table 2, it can be seen that four DCT-spectrum peaks (F, H, I and K) have no AE spectrum equivalents; they correlate well, however, with either single calculated energies or, in the case of peak I, the average of three close-lying energies.
4. Conclusions The OH+ DCT spectroscopy study has provided double-ionization energies to triplet states of CHsCHO*+, within the range 3041 eV, which had not been available previously. The H+ DCT spectroscopy study of
and Ion Processes 139 (1994) 163-168
acetaldehyde has confirmed in the range 3042 eV most of the double-ionization energies to singlet states of the dication measured previously [I] by AE spectroscopy, as well as providing new data for several more. In the main, the experimental data to singlet and triplet states correlate well with previously calculated values [l]. The DCT spectra are thus readily interpreted in terms of the electronic transitions identifiable with the calculated data. Two exceptions, one in the singlet-state data and the other in the triplet-state data, exist in that experimental peaks are observed which cannot be explained by theory. It would be interesting, therefore, if a more sophisticated theory, based say on the Green’s function method, were applied to acetaldehyde to see if electronic transitions associated with these peaks could be identified. Acknowledgements
J.C.S. thanks Zeneca Pharmaceutical and Agrochemical Divisions for financial support. We are grateful to Dr. N. Correia for providing us with the calculated energies to triplet states of CH3CH02+ which were previously published in a graphical form. References [ll N. Correia,
A. Naves de Brito, M.P. Keane, L. Karlsson, S. Svensson, C.-M. Liegner, A. Cesar and H. Agren, J. Chem. Phys., 95 (1991) 5187. PI J. Appell, in R.G. Cooks (Ed.), Collision Spectroscopy, Elsevier, Amsterdam, 1978, p. 244. Ion Processes, 120 131 F.M. Harris, Int. J. Mass Spectrom. (1992) 1. [41 W.J. Griffiths, A. Naves de Brito, N. Correia, J.C. Severs and F.M. Harris, Int. J. Mass Spectrom. Ion Processes, 134 (1994) 197. Bureau of [51 C.E. Moore, Atomic Energy Levels, National Standards, Washington, D.C., 1949. [61 Finnigan MAT, 2800 Bremen 14, Germany. Mass [71 S.R. Andrews and F.M. Harris, Rapid Commun. Spectrom., 7 (1993) 548. [81 S.R. Andrews, F.M. Harris and D.E. Parry, Chem. Phys., submitted.
ELSEVIER
International
Journal
of Mass Spectrometry
and Ion Processes
139 (1994) 163-168
~-
Double-ionization energies to singlet and triplet electronic states of the acetaldehyde dication measured by double-charge-transfer spectroscopy J.C. Severs, F.M. Harris*, Mass Spectrometry
W.J. Griffiths’
Research Unit, University College of Swansea. Singleton Park, Swansea SA2 8PP, UK Received
27 July 1994; accepted
19 August
1994
Abstract Double-charge-transfer spectroscopy has been used to measure the double-ionization energies of acetaldehyde to singlet and triplet electronic states of the dication. The values, measured within the range 30642eV, are in good agreement with previously calculated data thus allowing the characteristics of the spectra to be interpreted in terms of specific electronic transitions. Keywords:
Double-charge-transfer
spectroscopy;
Double-ionization
1. Introduction In a previous, detailed investigation [l], the doubly ionized states in formaldehyde, acetaldehyde, acetone and formamide were studied by X-ray excited core-valence-valence Auger electron (AE) spectroscopy. Assignments of the peaks in the spectra were made using ab initio Hartree-Fock, Green’s function and configuration interaction (CI) calculations. The contributions from transitions to triplet final states were illustrated graphically in the paper, but it was deduced that these were very weak, and thus they were not discussed. *Corresponding
author. address: Department of Medical Biochemistry and Biophysics, Karolinska Institutet, S171 77 Stockholm, Sweden.
’Present
0168-l 176/94/$07.00 0 1994 Elsevier SSDZ 0168-1176(94)04065-6
Science
B.V. All rights
energies
The positions of peaks observed in the AE spectra gave values of double-ionization energies of the molecules which were interpreted in terms of calculated energies to singlet states of the dications. As part of a programme of research in this Laboratory to provide the complementary experimental information on double-ionization energies to triplet states of the dications, double-charge-transfer (DCT) spectroscopy had been used to study doubleionization of acetone and acetaldehyde. DCT spectroscopy is based on double-electroncapture (DEC) reactions which take place between a fast-moving projectile ion and the molecule under investigation. Ample evidence exists [2,3] to show that spin is conserved in such reactions, and that, by appropriately selecting the projectile ion, the multiplicity of reserved
164
J.C. Severs et al./International Journal of Mass Spectrometry and Ion Processes 139 (1994) 163-168
the resulting doubly charged ion is known. For example, for ground-state molecules which are closed-shell, and thus in singlet states, the use of H+ as the projectile ion populates singlet states of the molecular dications. This occurs because H+ and H- have electronic spin equal to zero, so the doubly charged ion will have the same spin state as the neutral molecule. If, on the other hand, a projectile such as OH+ is used, this having a 3Cground state, double ionization to triplet states occurs because the OH- ions generated in the DEC reaction must be in the ‘LX+state, this being the only one with a sufficiently long lifetime to ensure the ions survive the journey between the reaction region and the detector. The results of a DCT spectroscopy study of acetone using OH+ as the projectile ion have been published [4]; fifteen double-ionization energies were measured, these correlating well with calculated individual energies or groups of energies identifiable with triplet states of the dication. The results of a similar study of acetaldehyde are reported here. In addition, a DCT spectroscopy study was carried out using H+ to obtain double-ionization energies to singlet states of CH3CH02+. In the previous AE spectroscopy study of acetaldehyde, the first doubly ionized state appeared as a shoulder at about 30.3eV on a more intense structure centred at 31.6 eV. It was considered likely that a H+ DCT spectroscopy study would enable the lowest singlet-state energy to be measured more precisely, and could also possibly give more detailed information about the singletstate configuration, particularly up to about 40 eV.
2. Experimental procedure If A+ represents in general a projectile ion (OH+ or H+ in the present investigation), then
the DEC reaction A+ + CH3CH0
of interest
here is
+ A- + CH3CH02+
(1)
The underlying arrows indicate that A+ and A- have high velocities; in this investigation, both projectile ions had a translational energy of 3 keV. The acetaldehyde molecules were introduced into a cell through which the projectile ions passed; in considering the energetics of reaction (1) the molecules can be regarded as at rest. The difference in the translational energies of A+ and A- is the endoergicity of reaction (1) which is given by AE = IE,(CH,CHO)
- E(A+ + A-)
(2)
where IE2(CH3CHO) is the double-ionization energy of the molecule, and E(A+ -+ A-) is the energy released in the charge inversion of A’ to A-. Strictly, a term should be included in Eq. (2) for the translational energy acquired by the CH3CH02+ ions generated in the reaction. It is readily shown, however, that this is sufficiently small to be neglected in comparison with the other energy terms. If different electronic states of CH3CH02+ are populated, having associated with them double-ionization energies of IEi(CH3CHO), IEi(CH,CHO) etc., then AE will have values LIE’, AE”, etc. With the translational energy of A+ kept constant, a scan of the translational energies of A- will give rise to a spectrum with peaks appearing at positions corresponding to different AE values. These positions are used to determine the double-ionization energies once the energy-loss scale has been calibrated using a DCT spectrum obtained for a gas such as xenon, for which the double-ionization energies are known accurately [5]. The experiments were carried out using a reversed-geometry Finnigan 8230 mass spectrometer [6] which has been modified for DCT spectroscopy [7]. The projectile ions OH+ and H+ were generated by dissociative electron ionization of water molecules, and
J.C. Severs et al./International Journal of Mass Spectrometry and Ion Processes 139 (1994) 163-168
were accelerated to 3 keV translational energy. After mass selection, the ions interacted with CH3CH0 molecules contained in a collisiongas cell located in the spectometer’s second field-free region. The DCT spectrum was obtained by scanning the voltages applied to the electric sector which had been set to transmit negative ions. The CHsCHO molecules were replaced by xenon atoms to obtain a calibration spectrum. Numerous pairs of sample/calibration spectra were obtained during the investigation; average values of the double-ionization energies were obtained from the individual energies derived from each pair of spectra.
3. Results 3.1. Double-ionization of CH3CH02+
energies to triplet states
Twenty-eight pairs of sample/calibrant spectra were recorded over three non-consecutive days of investigation. A typical OH+ DCT spectrum obtained with CHsCHO is shown in Fig. 1. The peaks marked A-H were clearly observed in over 70% of the other spectra recorded. Double-ionization energies were determined from the peak positions and the corresponding information from the calibration spectra xenon which corresponds to known double-ionization energies of xenon. The procedure is outlined in detail elsewhere [8]. Average values of the these energies are presented in Table 1, the uncertainties being standard deviations in these values. Also listed in the table are double-ionization energies to triplet states of the acetaldehyde dication which were calculated in a previous investigation [l]. A one-particle model was used which is based on the simple expression for the double-ionization energies Ep4 = Ip + I&&
(3)
165
where Ip and rq denote single-ionization energies for states with holes in the orbitals p and q. Siq is a hole-hole interaction term which is dependent on the spin coupling of the two-hole state. The double-hole ionization potentials are then simply given by sums of two such energies corrected for by the hole-hole interaction parameter. This is evaluated over the canonical Hartree-Fock orbitals of the ground state. Computational models such as the one just described are often successful in predicting differences in state energies but can give erroneous absolute values. It is usual, therefore, when comparing calculated and measured state energies, to ‘normalize’ the calculated values by shifting them appropriately. The values calculated previously [l] have been adjusted by the subtraction of 4.56eV, this having been chosen to provide the best correlation of calculated to experimental values. It can be seen from Table 1 that, except for the highest energy, all the calculated values agree with those measured to within the experimental uncertainty. When two calculated state energies lie close to one another, they have been grouped together and averaged to indicate the one double-ionization energy which would be expected to be measurable because of experimental limitations in resolving power. The results shown in Table 1 demonstrate that most of the OH’ DCT spectrum can be explained in terms of the electron losses indicated in the fourth column of the table. The exception is peak D corresponding to a double-ionization energy of 35.5 eV; it appears that the theory fails to predict a state of the dication having this energy. As will be seen below, a similar situation exists in the singlet-state data. 3.2. Double-ionization
to singlet states of
CH, CH02+
A typical Hf DCT spectrum obtained with
166
J.C. Severs et al./International Journal of Mass Spectrometry and Ion Processes 139 (1994) 163-168
Translational
energy of OH-ions
Fig. 1. A typical OH+ DCT spectrum
CH3CH0 is shown in Fig. 2. This is one of thirty-six spectra recorded over two nonconsecutive days. The peaks marked A-K were clearly evident in over 75% of the spectra. Calibration of the energy-loss scale was carried out by obtaining a Hf DCT spectrum of xenon following each CHsCHO spectrum. As previously [8], the strong peak due to the populating of the ‘D2 state of Xe2+ was used in Table 1 Measured double-ionization values [l]
energies
Peak
Measured
A
31.2f0.2
B C D
32.1 f 0.3 34.1 f 0.4 35.5 f 0.4
E
36.8 * 0.4
F G H
38.1 f 0.3 39.2 f 0.4 40.5 It 0.4
a Values obtained
by shifting
(in electronvolts)
all the calculated
31.9 34.1 37.0 37.3 > 37.2 38.1 39.5 41.2
of acetaldehyde.
the calibration since the double-ionization energy to that state is known to be 35.447eV [51. The average double-ionization energies determined from the position of peaks A-K are listed in Table 2 together with standard deviations. Also shown in the table are the double-ionization energies determined by AE spectroscopy [l], and Hartree-Fock
to triplet electronic
Calculateda
(eV)
states of CH3CH02+
together
with previously
calculated
State configuration (9a))‘(lOa)-’ (la)-‘(lOa)-’ (9a)-‘(2a))’ (2a)-‘(lOa)-’ _ (la)-‘(2a)-’ (8a))‘(9a)-’ (8a))‘(lOa))’ (8a))‘(la)-’ (la)-‘(9a)-’
values by -4.56 eV (see text), and then rounding
off to the first decimal
place
J.C. Severs et aLlInternational
Journal of Mass Spectrometry
3015
Fig. 2. A typical
double-ionization values [l]
Peak
A B C
energies (in electronvolts)
to singlet electronic
(eV)
of acetaldehyde.
states of CHsCHO*+
together
with previously
Measured (present)
Measured (AE spectroscopy) [I]
Calculated”
Ul
State configuration
30.4 f 0.2 31.3 f 0.3 32.3 i 0.4
30.3 31.2 31.7
_ 34.2 i 0.3
33.0 34.1
30.2 31.2 31.8 > 32.0 32.1 _
(9a)-‘(lOa))’ (la))‘(lOa))’ (lOa))* (9a))‘(2a)-’ (2a)-‘(lOa)-’ _
35.2 f 0.3 36.1 f 0.3
35.4 _
37.1 f 0.3
37.3
38.2 f 0.3 39.2 i 0.4
a Values obtained
energy of H-ions
H+ DCT spectrum
40.3 i 0.4
40.2
41.2 f 0.3
_
by shifting
all the calculated
167
2985
2995
3005
Translational
Table 2 Measured calculated
and Ion Processes 139 (1994) 163-168
35.2 36.4 37.1 37.3 37.5 i 37.6 37.6 31.7 38.6 39.0 39.0 > 39.2 39.8 40.2 > 40.6 41.7
31.9
37.5
39.1
40.2
values by -5.16 eV (see text) and then rounding
measured
(2a)-* (la)-‘(2a))’ (8a))‘(9a))’ (9a))* @a)-‘(lOa)-’ @-‘(la)-’ (la)-‘(9a)-’ @a)-‘(2a))’ (7a))‘(2a)-’ (7a)-‘(2a)-’ (7a)-‘(9a)-’ (7a)-‘(la)-’ (la)-2 (6a)-‘(lOa)-’ (6a)-‘(2a)-’ @a)-* off to the first decimal
place.
and
168
J.C. Severs et al.lInternational
Journal of Mass Spectrometry
calculated values [l]. As for the tripletstate data, the calculated values of doubleionization energies to singlet states have adjusted to provide maximum overlap with the values measured in the present investigation. The lowest value measured in the H+ DCT spectroscopy study is 30.4 f 0.2 eV which agrees with the value of 30.3eV deduced from the position of a weak feature in the AE spectrum [l]. Looking down the columns in Table 2, it can be seen that peaks A, B and C have equivalent peaks in the AE spectrum corresponding to similar double-ionization energies. However, a peak at 33.0eV seen in the AE spectrum is not observed in the DCT spectrum. Interestingly, this is the one peak seen in the methyl C-Auger spectrum; all the other AE spectroscopy values in Table 2 derive from the O-Auger spectrum. The double-ionization energy of 34.2* 0.3 eV derived from peak D is close to that of 34.7eV derived from one of the peaks of the AE spectrum [l]. However, they appear not to correlate with any of the calculated energies. A similar situation existed for the triplet-state information in that a double-ionization energy of 35.5eV had no equivalent calculated energy. Looking further down the columns of Table 2, it can be seen that four DCT-spectrum peaks (F, H, I and K) have no AE spectrum equivalents; they correlate well, however, with either single calculated energies or, in the case of peak I, the average of three close-lying energies.
4. Conclusions The OH+ DCT spectroscopy study has provided double-ionization energies to triplet states of CHsCHO*+, within the range 3041 eV, which had not been available previously. The H+ DCT spectroscopy study of
and Ion Processes 139 (1994) 163-168
acetaldehyde has confirmed in the range 3042 eV most of the double-ionization energies to singlet states of the dication measured previously [I] by AE spectroscopy, as well as providing new data for several more. In the main, the experimental data to singlet and triplet states correlate well with previously calculated values [l]. The DCT spectra are thus readily interpreted in terms of the electronic transitions identifiable with the calculated data. Two exceptions, one in the singlet-state data and the other in the triplet-state data, exist in that experimental peaks are observed which cannot be explained by theory. It would be interesting, therefore, if a more sophisticated theory, based say on the Green’s function method, were applied to acetaldehyde to see if electronic transitions associated with these peaks could be identified. Acknowledgements
J.C.S. thanks Zeneca Pharmaceutical and Agrochemical Divisions for financial support. We are grateful to Dr. N. Correia for providing us with the calculated energies to triplet states of CH3CH02+ which were previously published in a graphical form. References [ll N. Correia,
A. Naves de Brito, M.P. Keane, L. Karlsson, S. Svensson, C.-M. Liegner, A. Cesar and H. Agren, J. Chem. Phys., 95 (1991) 5187. PI J. Appell, in R.G. Cooks (Ed.), Collision Spectroscopy, Elsevier, Amsterdam, 1978, p. 244. Ion Processes, 120 131 F.M. Harris, Int. J. Mass Spectrom. (1992) 1. [41 W.J. Griffiths, A. Naves de Brito, N. Correia, J.C. Severs and F.M. Harris, Int. J. Mass Spectrom. Ion Processes, 134 (1994) 197. Bureau of [51 C.E. Moore, Atomic Energy Levels, National Standards, Washington, D.C., 1949. [61 Finnigan MAT, 2800 Bremen 14, Germany. Mass [71 S.R. Andrews and F.M. Harris, Rapid Commun. Spectrom., 7 (1993) 548. [81 S.R. Andrews, F.M. Harris and D.E. Parry, Chem. Phys., submitted.