744
Photoionization dynamics in CS fragmented from
CS2 studied by high-resolution photoelectron
spectroscopy1
Anouk M. Rijs, Ellen H.G. Backus, and Cornelis A. de Lange
Abstract: The photoionization dynamics of CS have been studied using high-resolution laser photoelectron spectroscopy. The photodissociation of CS2 at ~308 nm results in highly rotationally excited CS in its X1Σ+ singlet ground
state, as well as in rotationally cold CS in the excited a3Π triplet state. The ground-state CS fragments are formed together with sulfur in its 3P, 1D, and 1S electronic states; triplet CS is produced in coincidence with ground-state sulfur
(3P). In both channels the photoelectron spectra are dominated by ∆v = 0 propensity, but transitions involving ∆v = 1
and 2 are also observed.
Key words: photoelectron spectroscopy, photoionization, photodissociation, excited states, reactive intermediates.
Résumé : On a étudié la dynamique de photoionisation du CS en faisant appel à la spectroscopie photoélectronique laser à haute résolution. La photodissociation du CS2 à environ 308 nm conduit à la formation de CS fortement excité
d’un point de vue rotationnel, dans son état fondamental singulet X1Σ+, ainsi que de CS rotationnellement froid dans
un état triplet excité a3Π. Les fragments CS dans l’état fondamental se forment en même temps que du soufre dans les
états électroniques 3P, 1D et 1S; les quantités de CS à l’état triplet se forment en coïncidence avec le soufre à l’état
fondamental (3P). Dans les deux voies, les spectres photoélectroniques sont dominés par la propensité ∆v = 0, mais on
observe aussi des transitions de ∆v = 1 et 2.
Mots clés : spectroscopie photoélectronique, photoionisation, photodissociation, états excités, intermédiaires réactifs.
[Traduit par la Rédaction]
Rijs et al.
749
Introduction
Resonance-enhanced multiphoton ionization, in combination with kinetic energy resolved photoelectron spectroscopy
(REMPI-PES), has proven to be an excellent and powerful
method to study rotationally resolved photoionization and
photodissociation dynamics of rotationally hot photofragments
(1–3). In particular, with REMPI-PES, detailed information
about the spectroscopy and photoionization dynamics of the
intermediate state can be obtained. Fragments formed in a
photofragmentation process are often generated with an appreciable degree of electronic, vibrational, and rotational
excitation. With sufficient rotational excitation, even for
fragments with small rotational constants, the spacings between excited rotational levels in the ion may exceed the energy resolution of the magnetic bottle electron spectrometer
used, and rotationally resolved photoelectron spectra can be
obtained. These rotationally resolved photoelectron spectra
provide information on the photoionization dynamics.
In the present work the photodissociation of CS2, resulting
in CS photofragments, has been studied. The ionization energy of CS from its neutral ground state to the lowest X2Σ+
ionic state amounts to 11.33 eV (4). The rotational constant
of CS+ in its ionic ground state is only 0.857662 cm–1 (5).
However, when the CS photofragments are formed with sufficient rotational excitation, rotational levels in the ion with
high J+ are accessed, and rotationally resolved PES can be
observed.
Carbon disulphide (CS2) has a structure and a photodissociation behavior similar to that of the previously studied N2O and OCS (1–3). These molecules all possess a
linear ground-state geometry with a 16 valence electron configuration. The absorption spectrum of CS2 shows many
absorption bands between ~100 and 400 nm (6). The photo-
Received 7 June 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 25 June 2004.
A.M. Rijs. Laboratory for Physical Chemistry, University of Amsterdam, Nieuwe Achtergracht 127–129, 1018 WS Amsterdam, The
Netherlands and Laser Centre and Department of Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The
Netherlands.
E.H.G. Backus. Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, P.O. Box 9502, 2300 RA Leiden, The
Netherlands.
C.A. de Lange.2 Laser Centre and Department of Physics, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The
Netherlands and Department of Chemistry, The University, Southampton SO17 1BJ, UK.
1
2
This article is part of a Special Issue dedicated to the memory of Professor Gerhard Herzberg.
Corresponding author (e-mail: delange@science.uva.nl).
Can. J. Chem. 82: 744–749 (2004)
doi: 10.1139/V04-015
© 2004 NRC Canada
Rijs et al.
chemistry of CS2 is strongly dependent on the wavelength of
photolysis. Absorption of one photon of 193 nm results in
the production of singlet CS (X1Σ+) fragments and sulfur atoms in the singlet or triplet state (1D or 3P) (7, 8). For
photolysis wavelengths towards the ultraviolet, the production of triplet CS (a3Π) becomes more important, and between 140 and 125 nm, the CS fragments are almost
completely produced in the triplet state (9). CS2 shows a
strong absorption band around 340–290 nm. This band has
~
~
~
been assigned to the 1B2 ← X1Σ +g transition. This 1B2 upper
state is bent with an SCS angle of 131° and correlates to the
1
∆u valence state in the linear configuration (10). Onephoton excitation of CS2 at ~308 nm (4.03 eV) correlates to
~
the transition to the 1B2 state. CS photofragments are formed
by the sequential (1+1) absorption of two photons at
~
~308 nm (total energy 8.06 eV) via the 1B2 intermediate
state to a predissociative excited state lying near 150 nm (6).
This state predissociates, yielding CS photofragments in
combination with S atoms. There are several dissociation
pathways in our energy region that can be accessed, producing singlet and triplet CS fragments (see Table 1), as follows:
[1]
CS2 → CS (X1Σ+) + S (3PJ)
[2]
CS2 → CS (X1Σ+) + S (1D2)
[3]
CS2 → CS (X1Σ+) + S (1S0)
[4]
CS2 → CS (a3Π) + S (3PJ)
Dissociation pathways [2], [3], and [4] are spin-allowed
processes from the optically prepared state. The first dissociation pathway [1] is a spin-forbidden process, which
becomes weakly allowed by the spin–orbit interaction associated with the presence of the two sulfur atoms in CS2.
The present paper is concerned with rotationally and
vibrationally resolved laser photoelectron spectroscopy of
CS fragments. At about 308 nm, the CS photofragments can
be produced either in the singlet or the triplet state. The CS
fragments formed in the X1Σ+ ground state are rotationally
hot and are studied via a (2+1) ionization scheme using the
intermediate B1Σ+ Rydberg state as a stepping-stone. The intermediate B1Σ+ Rydberg state represents the lowest member
of a Rydberg series converging upon the X2Σ+ ionic ground
state of CS+ and is located at about 8.04 eV above the CS
(X1Σ+) ground state (11). Together with the CS fragments
formed in their singlet state, S atoms are also produced.
These sulfur atoms can be formed via dissociation pathways
[1], [2], and [3], resulting in ground- (3PJ) and excited-state
(1D2, 1S0) S atoms. Various (2+1) ionization transitions of S
(3PJ, 1S0) are used for calibration.
In addition to ground-state CS (X1Σ+) fragments, excited
CS fragments can also be formed in the triplet state (a3Π)
via the dissociation route [4]. The electronically excited triplet CS (a3Π) fragments, which possess low rotational excitation, are studied employing a (1+1) excitation step via the
intermediate CS (3Σ+) state, resulting in CS+ (X2Σ+) ionic
fragments. With our photon energies, the CS (a3Π) fragments can only be formed together with ground-state S (3PJ)
atoms. The appearance energy of the other dissociation
channels of triplet CS (a3Π) + S (1D2, 1S0) lies above our
two-photon dissociation energy.
745
Table 1. Threshold energies of the product states in the
photodissociation of CS2.
Pathway
CS channel
[1]
[2]
[3]
[4]
CS
CS
CS
CS
(X1Σ+)
(X1Σ+)
(X1Σ+)
(a3Π)
S channel
S
S
S
S
(3PJ)
(1D2)
(1S0)
(3PJ)
Energies (eV)
4.46
5.61
7.21
7.88
Experimental
The experimental setup has been reported in great detail
previously (1). Briefly, the laser system consists of an XeCl
excimer laser (Lumonics HyperEx-460), producing pulsed
radiation with a fixed wavelength of 308 nm and a pulse duration of ~10 ns. The laser pulses are generated with a repetition rate of 30 Hz and have an energy of 200 mJ pulse–1.
The excimer laser output is used to pump a dye laser
(Lumonics HyperDye 500), operating on Rhodamine (bandwidth ~0.08 cm–1). The dye laser output (about 615 nm) is
frequency-doubled in a Lumonics HyperTrak 1000, using a
KD*P crystal, resulting in 10-ns pulses with a maximum energy of about 15 mJ and a bandwidth of ~0.2 cm–1. The laser
light is focused into the ionization region of a magnetic bottle spectrometer by a quartz lens with a focal length of
25 mm. The laser pulse intensity is kept low enough to avoid
nonresonant ionization of the parent compound CS2. In the
present experiments, the magnetic bottle spectrometer is
used in the electron detection mode for both wavelength
scans and photoelectron spectra and operates with a collection efficiency of ~50%. Since a time-of-flight method is
used, time windows can be selected to achieve optimum resolution. Signal intensities are essentally independent of kinetic energies (1).
The CS photofragments are generated in situ by twophoton dissociation of CS2 (Sigma-Aldrich, HPLC grade).
The CS2 gas sample is effusively introduced into the ionization chamber through a 13 mm outer diameter Pyrex flow
tube, leading to a pressure in the ionization region of typically 10–3 mbar (1 bar = 100 kPa). The photodissociation of
CS2 and the subsequent ionization of the CS fragments are
performed with the same laser photons of about 308 nm. For
calibration of both the wavelength and the photoelectron kinetic energies, well-known resonances of sulfur atoms at
two-photon energies of 64 889.0 and 66 741.6 cm–1 are used
(12, 13).
Results and discussion
At 308 nm, the dissociation of CS2 produces CS in the
X1Σ+ ground state or in the a3Π triplet state. Figure 1 shows
a two-photon excitation spectrum of CS fragments in the
two-photon energy range 65 000 – 65 170 cm–1. The spectrum was obtained by monitoring electrons with a kinetic energy of about 0.75 eV (flight time range of 2840–3000 ns).
These photoelectrons arise from a (2+1) ionization process,
starting from the X1Σ+ (v′′ = 1; N′′) electronic ground state
via the intermediate B1Σ+ (v′ = 1; N′) Rydberg state to the
X2Σ+ (v+ = 1; N+) ionic ground state. The rotationally resolved excitation spectrum shows the rotational distribution
© 2004 NRC Canada
746
Can. J. Chem. Vol. 82, 2004
Fig. 1. Rotationally resolved (2+1) REMPI excitation spectrum
with electron detection of the Q branch of the B1Σ+ ← ← X1Σ+
transition of hot CS fragments in the two-photon energy range
65 000 – 65 170 cm–1 (vacuum energies).
of the Q branch (∆N = 0) of CS in the B1Σ+ (v′ = 1) state,
extending from N ′ = 35 up to N ′ = 71. The comb above the
spectrum in Fig. 1 indicates the assignment of the N ′ rotational levels.
In the absorption step, only the transition from the v′′ = 1
level of X1Σ+ to the v′ = 1 level of B1Σ+ is observed. No transitions arising from X1Σ+ for v′′ ≠ 1 were observed. This
unusual situation has to do with the properties of the vibrational levels associated with the B1Σ+ state. It is known from
the literature (14) that the B1Σ+ Rydberg state predissociates,
especially from the v′ = 0 and the v′ = 2 levels. Moreover,
Sapers and Donaldson (15), who studied the rovibrational
states of the CS X1Σ+ fragments formed in the CS2 308-nm
two-photon photolysis with laser-induced fluorescence detection, observed that CS was formed in its electronic
ground state with a bimodal vibrational distribution that
peaked at v′′ = 1 with a secondary maximum at v′′ = 5. In
view of the fact that the CS X1Σ+ fragments are predominantly formed in their v′′ = 1 vibrational state, and considering the significant predissociation of the v′ = 0 and v′ = 2
levels of the B1Σ+ Rydberg state, it is not surprising that we
observe the CS X1Σ+ fragments only in their v′′ = 1 vibrational state. The O and S branches associated with the
B1Σ+ (v′ = 1) ← X1Σ+ (v′′ = 1) transition are not observed
because of the great difference in line strengths between the
Q and the O and S branches (16).
The assignment of the peaks in the excitation spectrum of
Fig. 1 and the rotational analysis of the Q branch were performed by assigning the measured rotational lines in an iterative fashion. By using the published ground-state rotational
constants (17), the B1Σ+ excited-state rotational constants B′
were determined by fitting all the rotational energies of the
Q branch to the following expression:
[5]
Q(N ′′) = v11 + (B′1 – B1′′ )N ′′(N ′′ + 1)
– (D ′1 – D 1′′ )N ′′ 2(N ′′ + 1)2
Fig. 2. (a) Rotational fit of the measured line positions for the
B1Σ+ (v′ = 1) Rydberg state, based on the rotational constants of
the X1Σ+ ground state of CS (v′′ = 1) (17). Results are given in
Table 2; (b) Residual plot, the difference between the experimental and calculated line positions.
Table 2. Rotational constants (in cm–1) for the B1Σ+ (v′ = 1)
Rydberg state of CS (left column), and a comparison with the
literature (14) (central column).
v11
B1
D1
B1Σ+ (v′ = 1)
B1Σ+
X1Σ+
64 952.60(36)
0.85188(24)
1.3802(38)×10–6
—
0.8520
1.56×10–6
—
0.811163722
1.337566×10–6
Note: The ground-state rotational constants of CS (X1Σ+) (refs. 17 and
18) are presented in the right column.
In our excitation spectrum of rotationally excited CS, we
measured only high rotational levels, from N ′ = 35 up to
N ′ = 71. Because we observe these high rotational levels, the
centrifugal distortion constant D1 of the B1Σ+ (v′ = 1) state
can be determined. The results of the rotational fit are shown
in Fig. 2a, and the difference plot between the experimental
and calculated line positions is presented in Fig. 2b. The improved rotational constants for the CS B1Σ+ (v′ = 1) state are
listed in Table 2 together with the CS X1Σ+ ground-state
constants of refs. 17 and 18.
The two-photon dissociation of CS2 at about 308 nm produces, in addition to CS fragments, S atoms in various states
(see dissociation pathways [1], [2], and [3]). Several transitions of the S (3PJ) and S (1S0) fragments are observed following (2+1) ionization. These well-known absorption lines
are used for wavelength and kinetic-energy calibration. Dissociation resulting in S (1D2) (route [2]) is not observed because no transitions can be excited via a (2+1) ionization
scheme with 308-nm photons. To observe S (1D2) atoms, the
photon energy must be shifted to higher energies.
Photoelectron kinetic energy scans were obtained by onephoton ionization from the intermediate CS B1Σ+ (v′ = 1; N ′)
state into the continua of the ionic ground state X2Σ+ (v+ = 1;
© 2004 NRC Canada
Rijs et al.
Fig. 3. Photoelectron spectrum of rotationally hot CS, produced
in the two-photon dissociation of CS2. The spectrum is obtained
by (2+1) ionization, by employing the Q(55) transition via the
v′ = 1 level of the intermediate B1Σ+ Rydberg state. (a) In the
spectrum, transitions to X2Σ+ (v+ = 1, 2, 3) are shown; (b) more
detailed scan for ∆v = 0 (v+ = 1), showing some rotational structure.
N+). Figure 3a presents the photoelectron spectra associated
with the Q(55) rotational transition at 65 077.6 cm–1 via v′ =
1. The dominant peak at about 0.74 eV arises from the ∆v =
0 transition. The smaller peaks can be assigned to the ∆v =
(v+ – v′) = +1, +2 transitions. The vibrational ∆v = 0 propensity observed for this X2Σ+ ← B1Σ+ step is expected for onephoton ionization of a Rydberg state.
Despite the high rotational excitation of the CS fragment,
the ionic spacings are not large enough to measure goodquality rotationally resolved photoelectron spectra. However,
if we make a more detailed scan for the ∆v = 0 transition at
about 0.74 eV, some partially resolved rotational structure is
747
Fig. 4. (1+1) REMPI excitation spectrum with electron detection
of the 3Σ+ ← a3Π transition of CS photofragments in the onephoton energy range 33 075 – 33 400 cm–1 (vacuum energies).
observed (see Fig. 3b). Besides the Q(55) (∆N = 0) feature,
peaks are also observed at about 20–25 meV from the Q(55)
peak. These peaks can be assigned to ∆N = ±2 transitions.
The rotational spacings between the ∆N = 0 peak and the
∆N = +2 and –2 peaks are expected to be B+(4N+ + 6)
(24 meV) and B+(4N+ – 2) (23 meV), respectively. Here
B+ = 0.857662 cm–1 (5) is the ionic rotational constant, and
N+ is the rotational quantum number of CS+. Higher order
rotational contributions are neglected in this estimate. The
rotational structure in the photoelectron spectra of CS is subject to selection rules. For one-photon ionization from the
B1Σ+ (v′) state, the selection rule ∆N + ᐉ = odd applies,
where ∆N = N+ – N ′ and ᐉ is a partial wave component of
the photoelectron (19, 20). In the present case, the CS
Rydberg electron occupies a 3sσ molecular orbital. In an
atomic-like picture, ionization of this 3sσ Rydberg electron
is expected to lead to p (ᐉ = 1) partial waves, and, on this basis, strong ∆N = even transitions would be expected; ∆N =
±1 transitions are forbidden on the basis of these selection
rules. However, with the present quality of the rotationally
resolved photoelectron spectra, no firm conclusions can be
drawn.
Following two-photon dissociation of CS2 at 308 nm, the
CS photofragments can be formed in the triplet state, resulting in CS (a3Π) + S (3PJ). Figure 4 shows the one-photon excitation spectrum in the energy range 33 075 – 33 500 cm–1,
corresponding to the CS fragment produced in its a3Π state.
The features observed belong to the 3Σ+ (v′ = 0) ← a3Π (v′′ =
2) transition (21, 22). The origin (v00) of this transition lies
at 282 nm, whereas we excite with ~300 nm. Ono and
Hardwick (21) observed three absorption bands at 282, 291,
and 300 nm. These bands are assigned to the 3Σ+ (v′ = 0) ←
a3Π (v′′ = 0, 1, 2) transitions, respectively. In our experiment, we have studied only the 3Σ+ (v′ = 0) ← a3Π (v′′ = 2)
transition. Since the same photons are used for the photodissociation of CS2 and for the spectroscopy of CS, the other
transitions could not easily be accessed in a one-color experiment. Our rotational assignment is based on the results of
© 2004 NRC Canada
748
Fig. 5. Photoelectron spectrum of CS (a3Π), resulting from the
(1+1) photoionization via the 3Σ+ Rydberg state.
Ono and Hardwick (21). The complexity of the rotational
structure that is evident from Fig. 4 is typical for a 3Σ+ ← 3Π
transition between the two states described in Hund’s case
(b) (23). Because of the high threshold energy of 7.88 eV for
the formation of CS (a3Π) (see Table 1) and the vibrational
excitation (v′′= 2), less energy is available for excitation of
the internal degrees of freedom, resulting in rotationally cold
(N′′= 4–22) CS (a3Π) (21). This situation is in sharp contrast
to the formation of CS (X1Σ+) with significant rotational excitation. Since CS (X1Σ+) is produced in a (2+1) and CS
(a3Π) in a (1+1) REMPI experiment, the very different conditions do not allow any reliable conclusions about branching ratios between both channels.
Figure 5 presents the experimental photoelectron spectrum
obtained for (1+1) photoionization of CS (a3Π, v′′ = 2) via
the CS (3Σ+, v′ = 0) Rydberg state at 33 327.8 cm–1, producing the X2Σ+ (v+) ionic ground state of CS+. The dominant
peak in the photoelectron spectrum at about 0.60 eV is the
∆v = 0 transition. The neighboring peaks at the left-hand
side of the spectrum arise from ∆v = +1, +2 transitions. No
rotationally resolved features are observed in the photoelectron spectrum of CS (a3Π, v′′ = 2) because of the low rotational excitation and the small rotational B+ constant. The
shoulder on the right-hand side of the ∆v = 0 peak is the result of nonresonant multiphoton ionization of 1S sulfur atoms.
In summary, the photodissociation of CS2 produces CS
molecules both in the singlet ground state and in a triplet excited state. The ground state (X1Σ+, v′′ = 1) formed is highly
rotationally excited, while the triplet state (a3Π, v′′ = 2) generated is rotationally cold. Comparing the results of the
photodissociation of CS2 with previous studies of the photodissociation of the similar molecules N2O (2) and OCS (3),
the photodissociation of these molecules produced rotationally hot N2 and CO photofragments and O and S atoms in
their 1D2 singlet excited states. In the photodissociation of
CS2 we observe both similarities and differences. In the case
of photodissociation of N2O and OCS, the N2 and CO photofragments are only formed in their electronic ground states,
Can. J. Chem. Vol. 82, 2004
and nearly all them are formed in their ground vibrational
states (2, 3). The photodissociation of CS2 produces both
ground-state and excited-state CS fragments and results in
vibrationally excited CS species, both in the singlet electronic X1Σ+ ground state and in the electronically excited
triplet a3Π state. In the photodissociation process of OCS
and N2O, the C—O and the N—N fragments appear to act
solely as spectators, while in CS2 the nonbonding electrons
of both S atoms contribute to the absorption and both C—S
bonds are excited and stretched (24). Ultimately, if one C—S
bond breaks, the other is strongly vibrationally excited.
The photoelectron spectra of ground-state CS photofragments are dominated by ∆v = 0 propensity, as was the
case with the photoelectron spectra of ground-state N2 and
CO. However, the photoelectron spectra of N2 and CO could
be measured with excellent rotational resolution, whereas in
the case of CS it appears more difficult to obtain rotationally
resolved photoelectron spectra. This is mostly because the
rotational constant of the CS+ ion is approximately half that
of N2+ or CO+.
Finally, the fact that our experiments on CS, both in its
singlet and triplet electronic states, have been performed in
the ~1 T field of a magnetic bottle spectrometer should not
affect the observed dynamics. Systematic studies have
shown that possible magnetic field effects in this type of
spectrometer may only be significant for high Rydberg states
close to the ionization threshold (25–27).
Conclusions
The photodissociation dynamics of CS2 at about 308 nm
encompasses a singlet and a triplet CS dissociation channel.
The photodissociation via the singlet channel produces
rotationally hot CS (X1Σ+) fragments in their first
vibrationally excited level. A rotational distribution of CS
extending from N ′ = 35 up to N ′ = 71 (with a maximum at
N ′ = 58) has been observed and assigned. For the first time,
a (2+1) high-resolution partly rotationally resolved excitation spectrum of CS (X1Σ+) via the Rydberg level B1Σ+ has
been recorded. Improved spectroscopic rotational parameters
have been obtained for the CS (B1Σ+) state. The other dissociation channel of CS2 at 308 nm results in the production
of CS (a3Π) in its v′′ = 2 vibrational level, with low rotational excitation. In both cases the photoelectron spectra
were dominated by ∆v = 0 propensity, with much weaker
transitions involving ∆v = 1 and 2.
Acknowledgements
AMR acknowledges the Holland Research School of Molecular Chemistry for a Ph.D. fellowship. CAdL acknowledges very useful discussions with Professor J.M. Dyke and
members of his group during a stay in Southampton in the
context of the EC Network on Reactive Intermediates.
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