7.2.2 UV Spectrum of Carbon Dioxide
The UV absorption spectrum of carbon dioxide has been widely studied both
experimentally [35] and theoretically [36] for identifying the excited states involved
in the electronic transitions. To this end, electron impact spectroscopy has proved to
be an additional useful diagnostic tool [37]. Due to the fact that it is transparent in
the visible and near-to-middle ultraviolet regions, at least down to 210 nm (hm <
5.91 eV), both in the liquid phase [38] and in the gas phase [23a], the UV spectrum
has scarce application in synthetic chemistry as diagnostic tool. Only weak
absorptions can be observed below 11 eV. CO 2 shows three maxima in the vacuum
ultraviolet region at 147.5 (8.41 eV), 133.2 (9.31 eV), and 112.1 nm (11.08 eV),
respectively. The bands corresponding to the maxima at 147.5 and 133.2 nm
exhibit a vibrational structure, which is poor, irregular, and apparently complex for
the maximum at 147.5 nm, while is sharper and more regular for that at 133.2 nm.
The assignment of these absorption maxima has been controversial for long time.
UV spectroscopy is more useful for understanding the photochemistry of CO 2 and
its molecular dissociation into CO and [O] at 157 nm (Eq. 7.6).
CO 2 ! CO þ O
ð7:6Þ
Notably, the dissociation of CO 2 to CO(
1
R) + O(
3 P) violates the spin conservation rule, as the spin of CO 2 in the ground state (
1
R
þ
g ) is zero, whereas the total
spin of CO(
1
R) + O(
3 P) is one and, therefore, such dissociation is a spin-forbidden
process. Attempts have been done to explain such situation and the photolysis of
CO 2 has been carried out under different conditions, using an Hg lamp. The dissociation has been explained on the basis of an electronic transition of the CO 2
molecule from the singlet ground state (
1
R
þ
g ) to the upper (bound) singlet state
1 B 2,
which is below the asymptote of the state
1 B 2 , but above the crossover zone with
the state
3 B 2 . Dissociation can occur if the molecule can reach the crossover region
(by collisions, for instance) and undergo a transition to the triplet state
3 B 2 .
Experiments carried out with labeled C- and O-atoms (
13 C and
17 O) have shown
an isotopic enrichment observed in CO and O 2 products. Therefore, it has been
proposed that the nuclear spin of
13 C (I = 1/2) and
17 O (I = 5/2) may introduce
additional coupling by hyperfine interaction, which may increase the dissociation
rate of the iso-topologues containing
13 C and
17 O. Interestingly, the
17 O enrichment
is 2.2 ± 0.2 times higher than that of
13 C, a value very close to the value of 2.7,
which gives the µ(
13 C)/µ(
17 O) magnetic moment ratio for the two nuclei.
The influence of nuclear spin in photodissociation of molecules is a novel effect
which may have important implications and potential applications in the study of
terrestrial and planetary atmospheres [39].
7.2 Spectroscopic Techniques Applied to the CO 2 States
113
The UV absorption spectrum of carbon dioxide has been widely studied both
experimentally [35] and theoretically [36] for identifying the excited states involved
in the electronic transitions. To this end, electron impact spectroscopy has proved to
be an additional useful diagnostic tool [37]. Due to the fact that it is transparent in
the visible and near-to-middle ultraviolet regions, at least down to 210 nm (hm <
5.91 eV), both in the liquid phase [38] and in the gas phase [23a], the UV spectrum
has scarce application in synthetic chemistry as diagnostic tool. Only weak
absorptions can be observed below 11 eV. CO 2 shows three maxima in the vacuum
ultraviolet region at 147.5 (8.41 eV), 133.2 (9.31 eV), and 112.1 nm (11.08 eV),
respectively. The bands corresponding to the maxima at 147.5 and 133.2 nm
exhibit a vibrational structure, which is poor, irregular, and apparently complex for
the maximum at 147.5 nm, while is sharper and more regular for that at 133.2 nm.
The assignment of these absorption maxima has been controversial for long time.
UV spectroscopy is more useful for understanding the photochemistry of CO 2 and
its molecular dissociation into CO and [O] at 157 nm (Eq. 7.6).
CO 2 ! CO þ O
ð7:6Þ
Notably, the dissociation of CO 2 to CO(
1
R) + O(
3 P) violates the spin conservation rule, as the spin of CO 2 in the ground state (
1
R
þ
g ) is zero, whereas the total
spin of CO(
1
R) + O(
3 P) is one and, therefore, such dissociation is a spin-forbidden
process. Attempts have been done to explain such situation and the photolysis of
CO 2 has been carried out under different conditions, using an Hg lamp. The dissociation has been explained on the basis of an electronic transition of the CO 2
molecule from the singlet ground state (
1
R
þ
g ) to the upper (bound) singlet state
1 B 2,
which is below the asymptote of the state
1 B 2 , but above the crossover zone with
the state
3 B 2 . Dissociation can occur if the molecule can reach the crossover region
(by collisions, for instance) and undergo a transition to the triplet state
3 B 2 .
Experiments carried out with labeled C- and O-atoms (
13 C and
17 O) have shown
an isotopic enrichment observed in CO and O 2 products. Therefore, it has been
proposed that the nuclear spin of
13 C (I = 1/2) and
17 O (I = 5/2) may introduce
additional coupling by hyperfine interaction, which may increase the dissociation
rate of the iso-topologues containing
13 C and
17 O. Interestingly, the
17 O enrichment
is 2.2 ± 0.2 times higher than that of
13 C, a value very close to the value of 2.7,
which gives the µ(
13 C)/µ(
17 O) magnetic moment ratio for the two nuclei.
The influence of nuclear spin in photodissociation of molecules is a novel effect
which may have important implications and potential applications in the study of
terrestrial and planetary atmospheres [39].
7.2 Spectroscopic Techniques Applied to the CO 2 States
113