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Low-energy electron impact excitation of 1,3,5-trans-hexatriene
Volume
22,
CHEMICAL PHYSICS LETTEKS
number 2
LOW-ENERGY
ELECTRON
IMPACT EXCITATION
1 October
1973
OF 1,3,5- 7’RANSHEXATRIENE
F.W.E. KNOOP FOM-Imtitute
for Atomic
and Molecular Physics.
Amsterdam,
The Nrtherlands
and L.J. OOSTERHOFF Department
of Theoretical
Organic Chemistry,
Univrrsiry
of Leiden,
The Netherlands
Received 5 July 1973
Electronic excitation by low energy electron impact has been \ludied weak absorption is observed. On the basis of the results of semi-empirical to the second triplet state.
Electron-impact spectroscopy enables the determination of many excited states which have not been observed before by other methods. It appears that the selection rules for the excitation of atoms and molecules depend strongly on the energy of the incident electron. While at high-energy electron impact the selection rules approach the optical ones, at lowenergy electron impact practically all electronic transitions are allowed. Transitions to triplet states are manifest just above their threshold of excitation. Threshold excitation processes can be studied by means of the trapped electron (TE) method [I]. The apparatus used is described in previous work [2,3]. Calibration of the excitation spectra is effected by introduction of a mixture of helium and the gas under investigation using as reference the 2 3S excitation value of helium (19.82 eV). The accuracy of the data is estimated to be better than 0.1 eV. This study deals with results obtained for 1,3,5@Qns-hexatriene. Threshold excitation spectra of this compound are presented in fig. 1 for two excess energies (Mq = 0.05 eV; A,? = 0.5 eV) of the inelastically scattered electrons. A rather intensive absorption is found at 2.6 eV which can be identified as the first triplet state of frQns-hexatriene [4, 51. In both spectra a shoulder appears at = 4.2 eV. Up to now no transition of trans-hexatriene has been observed at this energy.
for 1,3,5-trarfs-hexatriene. calculations this absorption
1
I
At = 4.2 cV. 3 can be assigned
I
,
1 I
1.3.5
Trans-hexatrwne
Electron I
c_
energy I
A (nm) LOO) 300
loss
E kA’)
-
1
1
200
150
I ip. 1, Trapped electron \psctra ot 1,3.5-trawls-heatriene mea\ured 31 C\CC\S cncrFie\ f AL7 of 0.05 e\’ Jnd 0.5 eV (dotted line).
For reasons of identification it might be helpful to compare our measured data with theoretical values ‘47
Volume
22. number
CHEMICAL
2
1.3,5-frans-hexatriene 1 heoretical
PHYSICS
1 October
LETTERS
Table 1 (excitation
energies
in eV)
ExperImental
values a)
1973
data
.~ Excited levels
complete CI
CI of singly and doubly states
CI of singly states
electron impact, this research
4.85 5.26 5.80
4.76 5.29 5.59
6.00 4.66 6.90
5.1 5.7 (?J
5.13 b)
2.58 3 97 4.84
2.21
2.6
2.6 b> c)
3.65 4.52
= 4.2
photon
impact
singlet ‘Ag
’h, ’13, triplet
3Ag 3BLl
2.40 3.87 4.76
h, ref. 141;
c) ref. IS].
3k,
3) Ref. [3];
f
-
resulting from quantum-chemical semi-empirical calculations. In a study of Knoop [3] the amount of configuration interaction (CI) was varied from including only singly excited configurations, singly and doubly excited configurations up to inclusion of all configurations (complete CI). In every stage of CI the parametrization has been optimized with respect to the known energy levels of benzene. In table 1, the calculated excitation energies for tvuns-hexatriene, as abstracted from this work [3], are presented to be compared with the experimental data. It appears that an assignment as the second triplet state 3A, looks very probable for the 4.2 eV absorption. The most intensive peak in our spectra, located at 5.1 eV, is probably characterized as the optically allowed singlet transition 1B,. This state has been observed by photon-impact at 5.13 eV [4]. A very weak shoulder at - 5.7 eV (TE spectrum, U = 0.05 eV) might correspond to the third singlet state calculated at 5.8 eV (complete CI). For polyenes the existence of an excited singlet state at lower energy than the lowest optically allowed state has been suggested by Hudson and Kohler [6]. No indication for the presence of such a state has been found in absorption spectroscopic investigations of cis- and trans-hexatriene [7]. However, semi-empirical calculations with extensive CI [3, S] support this sug gestion. Inclusion of doubly excited configurations is
248
essential to shift the optically forbidden * A, level below the optically allowed state 1B,, at least in the case of trans-butadiene and trans-hexatriene [3]. Inclusion of higher excited configurations will not change the order (see also table 1). In our spectrum (Al? = 0.05 eV) a possible shoulder might be distinguished just below the 5.1 eV absorption, but it may also be explained as due to noise fluctuations. The continued interest of Professor J. Kistemaker is gratefully acknowledged. Thanks are due to Miss J. Duyndam who prepared 1,3,5-tr~rlshexatriene with high purity (98%). This work is sponsored by F.O.M. with financial support of Z.W.O.
References 1] G.J. Schulz, l’hl‘;. Rev. 112 (1958) 150. 31 F.W.E. Knoop. H.H. Brongersma and 4.J.H. Boerboom, C‘hem. Phys. I.etters 5 (1970) 450. 31 I .W.E.Knoop. Thesis, Leiden (1972). 11 N.G. Mmndard. Thesis. Loiden (1970). 5 ) D.1.. Evans. J. Chem. Sot. (1960) 1735. h ] B.S. Iludwn and B.L. Kohler. (‘hem. Phqs. I.ti’rter\ 1-i t 1972) 299. 71 R.M. Gavin Jr., S. Rwmhrrg and S.A. R~cc. J. Chum. Phya. 58 (1973) 3160. 81 K. Schulten and M. Karplus, Chcm. Phy,. Letters 14 (1972) 305.
22,
CHEMICAL PHYSICS LETTEKS
number 2
LOW-ENERGY
ELECTRON
IMPACT EXCITATION
1 October
1973
OF 1,3,5- 7’RANSHEXATRIENE
F.W.E. KNOOP FOM-Imtitute
for Atomic
and Molecular Physics.
Amsterdam,
The Nrtherlands
and L.J. OOSTERHOFF Department
of Theoretical
Organic Chemistry,
Univrrsiry
of Leiden,
The Netherlands
Received 5 July 1973
Electronic excitation by low energy electron impact has been \ludied weak absorption is observed. On the basis of the results of semi-empirical to the second triplet state.
Electron-impact spectroscopy enables the determination of many excited states which have not been observed before by other methods. It appears that the selection rules for the excitation of atoms and molecules depend strongly on the energy of the incident electron. While at high-energy electron impact the selection rules approach the optical ones, at lowenergy electron impact practically all electronic transitions are allowed. Transitions to triplet states are manifest just above their threshold of excitation. Threshold excitation processes can be studied by means of the trapped electron (TE) method [I]. The apparatus used is described in previous work [2,3]. Calibration of the excitation spectra is effected by introduction of a mixture of helium and the gas under investigation using as reference the 2 3S excitation value of helium (19.82 eV). The accuracy of the data is estimated to be better than 0.1 eV. This study deals with results obtained for 1,3,5@Qns-hexatriene. Threshold excitation spectra of this compound are presented in fig. 1 for two excess energies (Mq = 0.05 eV; A,? = 0.5 eV) of the inelastically scattered electrons. A rather intensive absorption is found at 2.6 eV which can be identified as the first triplet state of frQns-hexatriene [4, 51. In both spectra a shoulder appears at = 4.2 eV. Up to now no transition of trans-hexatriene has been observed at this energy.
for 1,3,5-trarfs-hexatriene. calculations this absorption
1
I
At = 4.2 cV. 3 can be assigned
I
,
1 I
1.3.5
Trans-hexatrwne
Electron I
c_
energy I
A (nm) LOO) 300
loss
E kA’)
-
1
1
200
150
I ip. 1, Trapped electron \psctra ot 1,3.5-trawls-heatriene mea\ured 31 C\CC\S cncrFie\ f AL7 of 0.05 e\’ Jnd 0.5 eV (dotted line).
For reasons of identification it might be helpful to compare our measured data with theoretical values ‘47
Volume
22. number
CHEMICAL
2
1.3,5-frans-hexatriene 1 heoretical
PHYSICS
1 October
LETTERS
Table 1 (excitation
energies
in eV)
ExperImental
values a)
1973
data
.~ Excited levels
complete CI
CI of singly and doubly states
CI of singly states
electron impact, this research
4.85 5.26 5.80
4.76 5.29 5.59
6.00 4.66 6.90
5.1 5.7 (?J
5.13 b)
2.58 3 97 4.84
2.21
2.6
2.6 b> c)
3.65 4.52
= 4.2
photon
impact
singlet ‘Ag
’h, ’13, triplet
3Ag 3BLl
2.40 3.87 4.76
h, ref. 141;
c) ref. IS].
3k,
3) Ref. [3];
f
-
resulting from quantum-chemical semi-empirical calculations. In a study of Knoop [3] the amount of configuration interaction (CI) was varied from including only singly excited configurations, singly and doubly excited configurations up to inclusion of all configurations (complete CI). In every stage of CI the parametrization has been optimized with respect to the known energy levels of benzene. In table 1, the calculated excitation energies for tvuns-hexatriene, as abstracted from this work [3], are presented to be compared with the experimental data. It appears that an assignment as the second triplet state 3A, looks very probable for the 4.2 eV absorption. The most intensive peak in our spectra, located at 5.1 eV, is probably characterized as the optically allowed singlet transition 1B,. This state has been observed by photon-impact at 5.13 eV [4]. A very weak shoulder at - 5.7 eV (TE spectrum, U = 0.05 eV) might correspond to the third singlet state calculated at 5.8 eV (complete CI). For polyenes the existence of an excited singlet state at lower energy than the lowest optically allowed state has been suggested by Hudson and Kohler [6]. No indication for the presence of such a state has been found in absorption spectroscopic investigations of cis- and trans-hexatriene [7]. However, semi-empirical calculations with extensive CI [3, S] support this sug gestion. Inclusion of doubly excited configurations is
248
essential to shift the optically forbidden * A, level below the optically allowed state 1B,, at least in the case of trans-butadiene and trans-hexatriene [3]. Inclusion of higher excited configurations will not change the order (see also table 1). In our spectrum (Al? = 0.05 eV) a possible shoulder might be distinguished just below the 5.1 eV absorption, but it may also be explained as due to noise fluctuations. The continued interest of Professor J. Kistemaker is gratefully acknowledged. Thanks are due to Miss J. Duyndam who prepared 1,3,5-tr~rlshexatriene with high purity (98%). This work is sponsored by F.O.M. with financial support of Z.W.O.
References 1] G.J. Schulz, l’hl‘;. Rev. 112 (1958) 150. 31 F.W.E. Knoop. H.H. Brongersma and 4.J.H. Boerboom, C‘hem. Phys. I.etters 5 (1970) 450. 31 I .W.E.Knoop. Thesis, Leiden (1972). 11 N.G. Mmndard. Thesis. Loiden (1970). 5 ) D.1.. Evans. J. Chem. Sot. (1960) 1735. h ] B.S. Iludwn and B.L. Kohler. (‘hem. Phqs. I.ti’rter\ 1-i t 1972) 299. 71 R.M. Gavin Jr., S. Rwmhrrg and S.A. R~cc. J. Chum. Phya. 58 (1973) 3160. 81 K. Schulten and M. Karplus, Chcm. Phy,. Letters 14 (1972) 305.