Rotationally resolved photoionization dynamics of hot CO fragmented from OCS

Article abstract of DOI:10.1063/1.1434993

Hot rotational levels of CO, photodissociated from OCS, were studied using (2+1) REMPI via the B¹Σ⁺ Rydberg state leading to the X²Σ⁺ state of the ion. Improved spectroscopic rotational parameters were obtained

Full text of DOI:10.1063/1.1434993

Rotationally resolved photoionization dynamics of hot CO fragmented from
OCS
Anouk M. Rijs, Ellen H. G. Backus, Cornelis A. de Lange, Maurice H. M. Janssen, Nicholas P. C. Westwood et
al.
Citation: J. Chem. Phys. 116, 2776 (2002); doi: 10.1063/1.1434993
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 116, NUMBER 7
15 FEBRUARY 2002
Rotationally resolved photoionization dynamics
of hot CO fragmented from OCS
Anouk 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
Ellen H. G. Backus
Laboratory for Physical Chemistry, University of Amsterdam, Nieuwe Achtergracht 127-129,
1018 WS Amsterdam, the Netherlands
Cornelis A. de Lange
Laboratory for Physical Chemistry, University of Amsterdam, Nieuwe Achtergracht 127-129,
1018 WS Amsterdam, the Netherlands, and Department of Chemistry, The University, Southampton SO17 1BJ,
United Kingdom
Maurice H. M. Janssen
Laser Centre and Department of Chemistry, Vrije Universiteit,
DeBoelelaan 1083, HV Amsterdam, the Netherlands
Nicholas P. C. Westwood
Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry,
University of Guelph, Guelph, Ontario, N1G 2W1, Canada
Kwanghsi Wang and Vincent McKoy
Arthur Amos Noyes Laboratory for Chemical Physics, California Institute of Technology, Pasadena,
California 91125
͑
Received 2 April 2001; accepted 21 November 2001͒
The photoionization dynamics of rotationally hot CO, photodissociated from OCS, have been
1
ϩ
studied using laser photoelectron spectroscopy via the intermediate B ⌺ Rydberg state leading to
2
ϩ
the X ⌺ of the ion. The photodissociation of OCS near 230 nm produces rotationally hot, but
1
ϩ
1
vibrationally cold CO (X ⌺ ,NЉ,vЉϭ0,1) fragments along with S ( D) atoms. These high
rotational levels show photoelectron spectra with a very strong ⌬Nϭ0 transition and weaker ⌬N
ϭϮ1, Ϯ2, and Ϯ3 transitions. Agreement between measured and calculated spectra is good and
suggests that there is significant angular momentum coupling in the photoelectron orbital. In the
ionization step not only ⌬vϭ0, but also off-diagonal, non-Franck–Condon (⌬vÞ0) transitions are
observed. The intensities of these transitions vary strongly within the region studied and can be
2
explained by the excitation of superexcited Rydberg states with an A ⌸ core. © 2002 American
Institute of Physics. ͓DOI: 10.1063/1.1434993͔
I. INTRODUCTION
out by probing the photofragment product channels, either by
2
,3
laser-induced fluorescence, by ion imaging combined with
OCS belongs to the linear triatomic 16 valence electron
systems such as CO , CS , and N O. Like all these 16
valence electron systems, OCS is linear (Cϱv) in the ground
4–7
(
2ϩ1) REMPI,
or by REMPI in combination with pho-
2
2
2
8–10
toelectron spectroscopy.
shows a bimodal structure,
The CO rotational distribution
2–5,8–10
with maxima at NЉϭ50
state, and either linear or bent in the excited states. Electronic
and NЉϭ63. This bimodal distribution is mainly due to two
1
ϩ
1
Ϫ
1
˜
transitions from the ground state (X ⌺ ) to ⌺ and ⌬
excited states are forbidden in the linear geometry due to the
selection rules. However, these transitions are weakly al-
1
1
different processes on the 2 AЈ ( ⌬) surface, namely direct
dissociation and surface crossing to the ground state.5
Resonance enhanced multiphoton ionization in combina-
tion with kinetic energy resolved photoelectron spectroscopy
͑REMPI-PES͒ is the method of choice to obtain detailed in-
formation about the spectroscopy and the photoionization
dynamics of the intermediate state. With a ‘‘magnetic bottle’’
high-resolution electron spectrometer, rotationally resolved
photoelectron spectra and ion rotational branching ratios can
be obtained. These rotationally resolved photoelectron spec-
tra provide information on the photoionization dynamics and
on the exchange of angular momentum between the ion and
lowed in the bent configuration (C ). At around 230 nm two
dissociation pathways of OCS can be followed via the ⌬
s
1
1
Ϫ
and the ⌺ excited states (C ), which correspond to the
ϱv
1
1
1
1
1
Ϫ
2
AЈ, 2 AЉ( ⌬), and 1 AЉ( ⌺ ) states in the bent geom-
1
etry (C ). The photodissociation of OCS near 230 nm oc-
curs mainly via the 2 AЈ state producing rotationally hot,
but vibrationally cold CO (X ⌺ ) fragments along with S
D) atoms. As the fragments separate, a strong torque is
s
1
1
ϩ
1
(
exerted on the CO fragments, leading to rotationally hot CO.
Several photodissociation studies of OCS have been carried
0021-9606/2002/116(7)/2776/7/$19.00
2776
© 2002 American Institute of Physics
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J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Hot CO fragmented from OCS
2777
the leaving photoelectron. To obtain rotationally resolved
photoelectron spectra, molecules with sizable rotational con-
stants are very suitable, providing ionic rotational spacings
that are large compared to the resolution of the electron spec-
trometer. A well-studied class of molecules with large rota-
tional constants are the diatomic hydride radicals such as
explanation, namely the excitation of appropriate ‘‘superex-
2
ϩ
cited’’ states in the X ⌺ ionization continuum.
A brief description of the experimental setup is given in
Sec. II, while the theoretical formulation employed is out-
lined in Sec. III. The experimental results and a discussion
and comparison with the ab initio quantum-chemical calcu-
lations are presented in Sec. IV. In Sec. V the conclusions are
summarized.
11
12,13
14,15
OH, NH,
and SH.
In this work, rotationally resolved photoelectron spec-
troscopy is carried out on hot CO photofragments produced
in the photodissociation of OCS. The rotational constant of
II. EXPERIMENT
ϩ
2
ϩ
Ϫ1 16
CO in the X ⌺ ionic ground state is only 1.9677 cm
.
The experimental setup has been described in great de-
Nevertheless, the high rotational excitation of the CO frag-
ment in the present experiment leads to large ionic spacings,
and thus allows the measurement of photoelectron kinetic
energies with rotational resolution. Rotationally resolved la-
ser photoelectron spectroscopy was recently applied to the
10
tail previously. Therefore, we shall give only a brief de-
scription here.
The laser system consists of a XeCl excimer laser ͑Lu-
monics HyperEx 460͒ producing pulsed radiation with a
fixed wavelength of 308 nm and a pulse duration of ϳ10 ns.
The light pulses are generated with a repetition rate of 30 Hz
and have an energy of 200 mJ/pulse. The output of the exci-
mer laser is used to pump a Lumonics HyperDye 500 dye
comparable isoelectronic N photofragments produced in the
2
1
7
photodissociation of N O at ϳ203 nm. By combining our
2
PES results with the results of ab initio quantum-chemical
1
8,19
calculations,
detailed information was obtained on the
Ϫ1
laser ͑bandwidth 0.08 cm ͒, operating on Coumarine 460.
dynamics of the photoionization process.
The dye laser output ͑about 460 nm͒ is frequency doubled in
a Lumonics HyperTrack 1000 using a BBO crystal resulting
in 10 ns pulses with a maximum energy of about 15 mJ and
In this work the rotationally resolved laser photoelectron
spectroscopy of hot CO fragments is studied via a two-step
1
ϩ
ionization process using the B ⌺ 3s␴ ͑6␴͒ Rydberg state
Ϫ1
a bandwidth of ϳ0.2 cm . During the wavelength scans the
1
ϩ
as a stepping-stone. The intermediate B ⌺ Rydberg state
power of the 230 nm laser beam is monitored with a photo-
diode. 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. In the present experiments, both
wavelength scans and photoelectron spectra are obtained by
using the ‘‘magnetic bottle’’ spectrometer in the electron de-
tection mode. In this case, the spectrometer operates with a
collection efficiency of ϳ50% and an energy resolution of
about 10 meV.
The rotationally hot CO photofragments are generated in
situ by one-photon dissociation of OCS ͑Matheson͒. The
OCS gas sample is effusively introduced into the ionization
chamber of the ‘‘magnetic bottle’’ spectrometer. The photo-
dissociation of OCS and the subsequent (2ϩ1) REMPI of
the rotationally excited CO fragments are performed with the
same laser photons of 230 nm. For wavelength and photo-
represents the lowest member of a Rydberg series converging
2
ϩ
ϩ
upon the X ⌺ ionic ground state of CO , and is located
1
ϩ
ϳ10.7 eV above the electronic ground state of CO (X ⌺ ).
Several groups have measured CO produced in the photodis-
1
ϩ
sociation of OCS using (2ϩ1) REMPI via the B ⌺ Ryd-
berg state in the wavelength range near 230 nm.4
–10
The B
ϪX transition in CO has also been studied employing high-
resolution vacuum ultraviolet ͑VUV͒ emission and absorp-
tion spectroscopy. Accurate vibrational and rotational con-
1
ϩ
stants of the B ⌺ state, based on the observation of
rotational levels NЈϭ0–63 ͑R branch͒ and NЈϭ0–33 ͑P
branch͒, were obtained.20
The one-photon ionization step from the intermediate
B ⌺ Rydberg state to the X ⌺ ionic ground state of CO
1
ϩ
2
ϩ
ϩ
is expected to be highly Franck–Condon allowed, implying a
strong ⌬vϭ0 propensity. However, in a previous ͑REMPI-
PES͒ study of CO via the B ⌺ Rydberg state Sha et al.
electron kinetic energy calibration well-known resonances of
1
ϩ
21
Ϫ1 24
xenon atoms at a two-photon energy of 88 379.6 cm
of sulfur atoms at a two-photon energy of 86 921.4 cm
are used.
and
Ϫ1 25
observed significant non-Franck–Condon behavior in the
photoionization process. This is very surprising given the
2
0
strong similarity between the intermediate ͑R ϭ1.1197 Å ͒
e
2
ϩ
22
and the ionic ground state ͑X ⌺ , R ϭ1.1151 Å ͒ and the
small difference in vibrational spacings of CO (B ⌺ ) and
CO (X ⌺ ). A Rydberg–valence interaction of the B ⌺
III. THEORETICAL FORMULATION
e
1
ϩ
The general theory of molecular REMPI processes used
in the present study has been described previously.19 Here we
present just a brief outline of some essential features as it is
used to obtain rotationally resolved photoelectron spectra. In
the present case, ionization originating from each of the
ϩ
2
ϩ
1
ϩ
Rydberg state, which can cause the non-Franck–Condon
transitions in the photoelectron spectra, does not seem to be
2
1
operational. For the B state, neither indications for predis-
sociation nor perturbations arising from closely neighboring
1
ϩ
(
2Jϩ1) magnetic sublevels of the B ⌺ state of CO forms
2
3
21
electronic states have been observed. Sha et al. reported a
connection between the non-Franck–Condon behavior and
the observed angular distribution of the photoelectrons, but
the essence of the mechanism responsible for the non-
Franck–Condon behavior in the photoionization step re-
mains in the dark. We have explored this non-diagonal be-
havior in more detail, and we shall present a very likely
an independent channel. The total cross section ␴ for ioniza-
ϩ
tion of a J level of the intermediate state leading to a J
level of the ion can be written as
␴
ϰ
␳
͒
Jϩ͉C ͑M MJϩ ͉
2,
͑1͒
͚
lm
M M
lm
J
J
M M
Jϩ
J
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2778
J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Rijs et al.
where ␳
is the population of a specific M level of the intermediate state. The coefficients C (M MJϩ) of Eq. ͑1͒ are
J lm J
M M
Jϩ
J
related to the probability for photoionization of the M level of the intermediate state leading to the M level of the ionic
Jϩ
J
state. For a Hund’s case ͑b͒ coupling scheme, the C (M M ) coefficients have the form
Jϩ
lm
J
4
␲
N
S
J
ϩ
ϩ
1/2
P
C ͑M ,M ͒ϭ
Jϩ
ͱ
͓͑2N ϩ1͒͑2Nϩ1͒͑2Jϩ1͒͑2J ϩ1͒͑2Sϩ1͔͒
͑Ϫ1͒ ͑2N ϩ1͒
ͩ
ͪ
lm
J
͚
t
3
M
Ms ϪMJ
ϩ
ϩ
ϩ
ϩ
ϩ
ϩ
N
S
J
S
1/2
S
N
N
Nt
Nt
1
l
ϫ
ͩ
ͪͩ
ͪͩ
ͪͩ
ͪ
ϩ
M
Ms
ϩ
ϪMJ
ϩ
Ms
ϩ
M␴ ϪMs ϪM
M
mt Ϫmt ␮0 Ϫm
ϩ
N
N
Nt
N
1
l
͑Ϫ1͒Ϫ⌳ϩϪ␮
I
t
,
͑2͒
˜
ϫ
ͩ
ͪͩ
ͪ
͚
l␭␮
ϩ
␮
␭
Ϫ⌳
⌳
␭t Ϫ␭t
␮
Ϫ␭
t
with
IV. RESULTS AND DISCUSSION
The photodissociation of OCS at 230 nm produces rota-
tionally excited CO in the electronic ground state, mainly in
its vibrational ground state, but also in the first vibrationally
excited state. Figure 1 presents the excitation spectrum of
ϩ
ϩ
1
2
PϭM ϩMJ
ϩ
ϪMϩ␮ ϪNϪN ϩSϩ ,
͑3͒
0
where ␭ and m are projections of the photoelectron’s angular
momentum l in the molecular and laboratory frames, respec-
tively, N is the electronic plus rotational angular momentum
of the intermediate state, N is the angular momentum trans-
fer, ⌳ is the projection of electronic orbital angular momen-
CO photofragments in the two-photon energy range 86 850–
Ϫ1
8
7 100 cm . CO is photoionized in a two-step process,
1
ϩ
t
starting from the X ⌺ (vЉϭ0,1;NЉ) ground state via the
1
ϩ
B ⌺ (vЈϭ0,1;NЈ) intermediate Rydberg state to the
˜
2
ϩ
ϩ
ϩ
tum along the internuclear axis, S is the total spin. I
vibrationally averaged photoelectron matrix element between
the resonant state and the photoelectron continuum wave-
is the
X ⌺ (v ϭ0,1;N ) lowest ionic state. The spectra were
obtained by monitoring photoelectrons with a kinetic energy
of about 2.15 eV. This excitation scan shows a well-resolved
bimodal distribution of the Q branch of CO, extending from
NЈϭ37 up to NЈϭ87 with two maxima at NЈϭ50 and NЈ
l␭␮
function, and ␮ and ␮ are the light polarization indices in
0
the molecular and laboratory frames, respectively. The sym-
bols with superscript ϩ are related to these same quantities
in the ion. Equation ͑2͒ provides a selection rule of the form
Ϫ1
ϭ63. At ϳ86 917 cm CO in low rotational states near the
origin is detected. At lower energies, the first hot band (vЈ
ϭ1;NЈ� vЉϭ1;NЉ) is observed, also with a bimodal rota-
Ϫ1
tional distribution, and at 86 856 cm the origin of this hot
⌬
Nϩlϭodd,
͑4͒
band is located. Simultaneously, in another kinetic energy
1
window at about 1.6 eV, ionization of S( D ) atoms via the
2
for photoionization. Since the 3s␴ ͑6␴͒ orbital has strong s
character ͑ϳ74% s, 24% p, and 2% d at R ϭ2.1322a ͒, the
2
o
S „( D )7f … states is observed. These resonances are used
e
0
p-wave component of the photoelectron matrix element is
expected to be dominant with some weaker contributions
from even partial waves. Both even and odd ⌬N transitions
are hence expected.
1
ϩ
For the wave function of the B ⌺ resonant state, we
use the improved virtual orbital ͑IVO͒ method26 in which the
2
ϩ
core orbitals are taken to be those of the fully relaxed X ⌺
ion and the 6␴ resonant orbital is obtained as an eigenfunc-
tion of the static-exchange potential of the core. The orbital
basis functions used in these calculations are the same as in
Ref. 27. With this basis and choice of wave functions, we
obtain a selfconsistent-field energy of 108.605 407 a.u. at
1
ϩ
R ϭ2.1322a for the B ⌺ state.
e
0
For the final state we assume a frozen-core Hartree–
Fock model in which the core orbitals are taken to be those
2
ϩ
of the ⌺ ion and the photoelectron orbital is a solution of
the one-electron Schr o¨ dinger equation. To obtain the photo-
electron orbitals, we used an iterative procedure, based on
the Schwinger variational principle,18 to solve the
Lippmann–Schwinger equation associated with the one-
electron Schr o¨ dinger equation.
FIG. 1. Rotationally resolved (2ϩ1) REMPI excitation spectrum with elec-
1
ϩ
1
ϩ
tron detection of the Q branch of the B ⌺ � � X ⌺ transition of rota-
tionally hot CO (vЈϭ0,1;NЈ) photofragments in the energy window
Ϫ1
86 840–87 075 cm ͑vacuum energies͒.
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J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Hot CO fragmented from OCS
2779
TABLE I. Rotational constants ͑in cmϪ1͒ for the B ⌺ (vЈϭ0) Rydberg
1
ϩ
state of CO ͑left column͒, and a comparison with the literature ͑Ref. 20͒
1
ϩ
͑
central column͒. The ground state rotational constants of CO X ⌺ ͑Ref.
28͒ are presented in the right column.
1
ϩ
Present work
Ref. values
X ⌺ constants
͑Ref. 28͒
Ϫ1
͑Ref. 20͒ ͑cmϪ1͒
Constants
͑cm ͒
86 916.18a
1.948 186a
6.6355͑12͒
Ϫ2.161͑19͒
86 916.18͑3͒
1.948 186͑72͒
6.784͑21͒
-
-
v00
B0
1.922 629
6.1212
0.5290
6
D •10
0
11
H •10
0
a
Note: fixed at reference values by Eidelsberg et al. ͑Ref. 20͒, see the center
column.
1
ϩ
constants.28 With these improved rotational constants, the O
and S branches are recalculated. The residual plots ͑not
shown͒ are virtually identical to the Q branch plot.
FIG. 2. ͑a͒ Rotational fit of the measured line positions for the B ⌺ (vЈ
1
ϩ
ϭ0)� � X ⌺ (vЉϭ0) transition in CO, based on the known rotational
1
ϩ
constants of the vibrationless ground state X ⌺ (vЉϭ0) ͑Ref. 28͒ and the
first-order rotational constant of the B state ͑Ref. 20͒. Results are given in
Table I. ͑b͒ Residual plot, the difference between experimental and calcu-
lated line positions.
Photoelectron kinetic energy scans were obtained by
1
ϩ
one-photon ionization from the intermediate CO B ⌺ (vЈ
ϭ0,1;NЈ) state into the continua of the ionic ground state
2
ϩ
ϩ
ϩ
X ⌺ (v ϭ0,1;N ). Because of the high rotational exci-
tation of the CO fragment, the ionic spacings are large
enough to measure rotationally resolved photoelectron spec-
tra. Hence, instead of one broad unresolved peak, a rotation-
ally resolved feature is observed. Figure 3 shows the experi-
mental ͑left͒ and calculated ͑right͒ photoelectron spectra of
the vibrationless Q(66) rotational transition at 87 016.7
for wavelength calibration. In addition to our earlier
1
0
results, the much weaker O and S branches are observed
Ϫ1
Ϫ1
around 86 500–86 650 cm and 87 250–87 550 cm , re-
spectively. These O and S branches show also a bimodal
distribution, with maxima occurring at the same rotational
levels NЈϭ50 and 63 as in the Q branch ͑not shown͒.
The assignment of the spectra of Fig. 1 and the rotational
fit of the Q branch were performed in an iterative fashion.
Ϫ1
cm . The dominant peak in the experimental photoelectron
ϩ
spectrum at 2.15 eV arises from the ⌬vϭ0 and ⌬NϭN
1
ϩ
ϪNЈϭ0 transition. The small neighboring peaks arise from
The B ⌺ excited state rotational constants were deter-
mined by fitting all the rotational energies of the Q branch to
the following expression:
⌬
NϭϮ1, Ϯ2, or Ϯ3 transitions. These peaks are separated
from the main feature by ϳ32 meV, equivalent to approxi-
ϩ
ϩ
ϩ
Ϫ1
mately 2B N . Here B ϭ1.9677 cm
͑Ref. 16͒ is the
⌬
E͑NЈ͒ϭ␯ ϩ͑BЈϪBЉ͒NЈ͑NЈϩ1͒
ϩ
0
0
0
0
ionic rotational constant, and N signifies the end-over-end
rotation.
2
2
Ϫ͑DЈϪDЉ͒NЈ ͑NЈϩ1͒
0
0
The rotational structure in the photoelectron spectra of
CO is subject to selection rules. For one-photon ionization of
3
3
ϩ͑HЈϪHЉ͒NЈ ͑NЈϩ1͒ .
͑5͒
0
0
1
ϩ
the B ⌺ (vЈ) state, the selection rule of Eq. ͑4͒, i.e., ⌬N
In our excitation spectrum of rotationally excited CO, we
measured only high rotational levels from NЈϭ34 up to NЈ
ϭ87. These high rotational levels mainly determine the
higher-order rotational constants D and H . On the other
0
0
hand, the first-order rotational constant B predominantly de-
0
pends on the position of low rotational levels. In order to
perform the best rotational analysis possible, we use the very
1
ϩ
accurate rotational constant BЈ for the B ⌺ (vЈϭ0) Ryd-
0
berg state, as well as the value for v00 , of Eidelsberg et al. 20
With our rotational fit we improve the DЈ and HЈ rotational
0
0
1
ϩ
constants of the CO B ⌺ (vЈϭ0) state. These excited state
rotational constants are obtained by using the well-known
1
ϩ
rotational constants ͑BЉ , DЉ , and HЉ͒ of the CO (X ⌺ )
0
0
0
2
8
ground state. Because of the high rotational excitation ob-
served in the present study, addition of the third-order rota-
tional constant HЈ provides a significant improvement of the
0
rotational fit. The results of this rotational fit are shown in
Fig. 2͑a͒, and the difference plot between the experimental
and calculated line positions is presented in Fig. 2͑b͒. The
FIG. 3. Experimental ͑left͒ and calculated ͑right͒ rotationally resolved pho-
toelectron spectra of rotationally hot CO, produced in the photodissociation
of OCS. The experimental photoelectron spectrum is obtained by (2ϩ1)
photoionization via the Q(66) transition to the intermediate B ⌺ (vЈ
ϭ0) Rydberg state. The calculated spectrum is representative for all photo-
electron spectra from vЈϭ0 and high NЈ.
1
ϩ
improved rotational constants for the CO B ⌺ (vЈϭ0)
state are listed in Table I, together with the BЈ and v values
1
ϩ
0
00
of Eidelsberg et al.,20 and the CO (X ⌺ ) ground state
1
ϩ
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2780
J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Rijs et al.
ϩ
ϩlϭodd applies, where ⌬NϭN ϪNЈ and l is a partial wave
component of the photoelectron.2
9,30
In the present case the
CO Rydberg electron occupies a 3s␴ ͑6␴͒ molecular orbital.
In an atomic-like picture, ionization of this 6␴ (nϭ3) Ryd-
berg electron is expected to lead to p (lϭ1) partial waves,
and, on this basis, strong ⌬Nϭeven transitions would be
expected. As seen in Fig. 3, in addition to these even ⌬N
ϭ0, Ϯ2 peaks, ⌬NϭϮ1, Ϯ3 transitions are also observed.
These ⌬Nϭodd transitions are due to even photoelectron
partial waves which, in turn, may arise from p or f compo-
1
ϩ
nents of the 3s␴ orbital in the B ⌺ Rydberg state ͑an
initial-state effect͒ or from angular momentum coupling in
the photoelectron wave function ͑a final-state effect͒. Indeed
1
ϩ
at R ϭ2.1322a , the 3s␴ ͑6␴͒ orbital of the B ⌺ state has
e
0
74% s, 24% p, and 2% d character. Furthermore, at a kinetic
energy of 2.15 eV, the magnitudes of the partial-wave com-
ponents of the dipole matrix element for ionization out of the
3
s␴ orbital are 1.016, 1.568, 1.194, 0.282, and 0.154 ͑atomic
units͒ for the lϭ0, 1, 2, 3, 4 channels of the ␴ continuum and
.245, 0.244, 0.258, and 0.110 for the lϭ1, 2, 3, 4 channels
FIG. 4. Photoelectron spectra of CO, resulting from (2ϩ1) photoionization
1
ϩ
via the B ⌺ (vЈϭ0) Rydberg state for ͑a͒ the origin, and ͑b͒ NЈϭ36. In
2
ϩ
ϩ
both spectra transitions to X ⌺ (v ϭ0,1,2,3,4) are shown.
1
of the ␲ continuum, respectively. The magnitudes of the s
and d wave components of the photoelectron matrix element
͑1.016 and 1.194, respectively͒ relative to the p-wave com-
ponent ͑1.568͒ in the ␴ channel are much larger than the
ratio of the p ͑24%͒ to s ͑74%͒ contributions to the 3s␴
orbital, and a likely explanation is that there is significant
angular momentum coupling in the photoelectron orbital. On
the other hand, the magnitude of the d-wave component
REMPI-PES experiment we can employ every populated ro-
1
ϩ
tational level associated with the B ⌺ Rydberg state of the
rotationally hot CO fragment as a stepping stone. Hence, we
can approximate the (2ϩ1Ј) experiment by ‘‘tuning’’ the
three-photon energy in the continuum in small steps, deter-
mined by the rotational spacing in the B state. At every en-
ergy thus reached in the continuum, the corresponding PE
spectra can be measured. Our results show that for NЈ
Ռ48 the expected ⌬vϭ0 behavior is indeed observed. How-
ever, in the narrow energy region where the three-photon
energy lies between 16.16–16.18 eV, strong deviations from
the expected ⌬vϭ0 propensity occur. In Fig. 4͑a͒ we show a
PE spectrum obtained via rotational levels of the B state
close to the band head (NЈϭ0). At this three-photon energy
͑0.244͒ of the photoelectron matrix element in the ␲-channel
relative to the p-wave component ͑1.24͒ would be consistent
with an initial-state effect arising from the 24% character of
the 3s␴ orbital.
The measured ͑left͒ and calculated ͑right͒ photoelectron
spectra of Fig. 3 show the same features: a dominant peak
resulting from the ⌬Nϭ0 transition, and the neighboring
smaller peaks from the ⌬NϭϮ1, Ϯ2, Ϯ3 transitions. Agree-
ment between the measured and calculated spectra is good in
the normal ⌬vϭ0 energy region, but the calculated spectra
show stronger ⌬NϭϮ1 and weaker ⌬NϭϮ2 and Ϯ3 tran-
sitions. Further examination shows a p-wave contribution of
Ϫ1
ϳ130 376 cm , corresponding to 16.16 eV, the strongest
deviation from diagonal Frank–Condon behavior, with vi-
ϩ
brational excitation up to v ϭ4 is observed. Figure 4͑b͒
9
9% to the ⌬Nϭ0 peak, s and d wave contributions of 55%
shows similar, although less pronounced, ⌬vÞ0 behavior at
Ϫ1
and 45%, respectively, to the ⌬Nϭ1 peak, and p and f con-
tributions of 59% and 41%, respectively, to the ⌬Nϭ2 peak.
It is interesting to note that even though the s- and d-wave
contributions to the photoelectron matrix element in the ␴
channel are of the same magnitude as that of the p wave, the
a three-photon energy of 130 424.4 cm , corresponding to
1
ϩ
16.17 eV, where the B ⌺ (vЈϭ0,NЈϭ36) level of rota-
tionally excited CO is employed as a stepping stone. Above
Ϫ1
ϳ130 460 cm (NЈϷ48) these nondiagonal transitions are
not observed anymore. Clearly, the appearance of these sig-
nificant non-Frank–Condon transitions depends strongly on
the laser wavelength, and takes place in a narrow three-
⌬
⌬
NϭϮ1 transitions are much weaker ͑ϳ20%͒ than that of
Nϭ0. To some extent, this may be due to interference in
Ϫ1
the d-wave components of the ␴ and ␲ channels.
photon energy region of less than 100 cm ͑12 meV͒ only.
So far we have only considered the rotational structure
in our photoelectron kinetic energy measurements. We now
focus our attention on the vibrational behavior during the
A previous REMPI-PES study on CO via the B state of Sha
et al.,21 taken at a single wavelength in the same energy re-
gion, also showed comparably strong non-Franck–Condon
behavior. In a sense the significant rotational excitation with
which the CO molecules are formed allows us to explore the
ionization continua in rather small energy steps. This brings
to light variations in vibrational branching ratios that would
be hard to observe otherwise.
1
ϩ
ionization step. Based on the similarity between the B ⌺
2
ϩ
Rydberg state and the X ⌺ ionic state potentials, only
vϭ0 transitions would be expected. In order to explore the
⌬
2
ϩ
vibrational branching in the X ⌺ ionic continuum, a two-
color (2ϩ1Ј) REMPI-PES experiment would be ideal, be-
cause it would allow us to probe the energy in the continuum
in a continuous fashion. In our current one-color (2ϩ1)
In our experiments, an energy region of a few eV above
the first ionic limit is accessed. It is well known that in laser
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J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Hot CO fragmented from OCS
2781
photoelectron spectroscopy an abundance of neutral states
located above the lowest ionic limit and belonging to
Rydberg series converging upon higher ionic limits, often
termed superexcited states, can play a key role in
Condon diagonal transitions in terms of excitation and sub-
sequent autoionization from different superexcited states, de-
pending on the rotational level involved. The non-Franck–
Condon peaks resulting from the rotationless excitation may
arise from the autoionization from the 5‘d’␦ (vϭ1)/‘5s
ϩ4d’␴ (vϭ3) superexcited states, which feature in the
76.67–76.69 nm region in the one-photon VUV absorption
spectrum.34 For higher rotational levels, for example NЉ
ϭ45, the 3‘d’␦ (vϭ7) state may be excited in a three-
photon process, and thus lead to non-Franck–Condon behav-
ior.
1
5,31,32
photoionization.
More often than not there is a compe-
tition between direct ionization and excitation of such super-
excited states, which can subsequently undergo a variety of
decay processes. These decay processes may include auto-
ionization, ͑pre͒dissociation, or absorption of additional pho-
2
ϩ
ϩ
tons. The ground ionic threshold X ⌺ of CO lies at
1
6
2
1
4.0136 eV. The first excited ionic state A ⌸ has an ion-
ization energy of 16.536 eV. Since our experiments are
3
3
The question remains whether these superexcited states
1
ϩ
carried out at a three-photon energy ϳ16.2 eV, we access an
can be accessed in a one-photon transition from the B ⌺
2
1
ϩ
energy region only ϳ0.4 eV below A ⌸, where a plethora
state, or in a three-photon transition from the X ⌺ ground
state. The one-photon excitation of the required superexcited
states via the B state is formally forbidden, but may gain
some intensity from effects of electron correlation, while the
of superexcited states belonging to series converging upon
2
A ⌸ can be expected. The energy region above the lowest
ionic threshold of CO has been studied in some detail in a
number of previous studies.16,34–37 We shall focus on a VUV
one-photon absorption study of Huffman et al.34 and a vibra-
tionally resolved photoionization cross section study of Leyh
1
ϩ
three-photon excitation from the X ⌺ ground state can be
quasi-resonant with and can borrow some intensity from the
B state. The fact that vibrationally excited levels of these
various superexcited 3‘d’␦ (vϭ7), 4‘d’␦ (vϭ3) and
5‘d’␦ (vϭ1)/‘5sϩ4d’␴ (vϭ3) states can be accessed at
all may arise from the rather large difference in equilibrium
3
6
et al. They show in the wavelength region 70–80 nm im-
2
portant Rydberg series converging upon the A ⌸ ionic state.
The strongest deviation of the Franck–Condon behavior,
ϩ
1
ϩ
22
with v levels populated up to 4, is observed for resonance
geometries between the X ⌺ ͑R ϭ1.1283 Å ͒ and the
e
1
ϩ
1
ϩ
20
enhancement near the electronic origin of the B ⌺ state,
where low rotational levels are accessed. The three-photon
energy of 16.17 eV, at which this strong non-Franck–Condon
behavior is observed, corresponds rather closely to a feature
at 76.67–76.69 nm in the one-photon absorption spectrum
of Fig. 4 of Ref. 34. This feature has been assigned by
B ⌺ Rydberg state ͑R ϭ1.1197 Å ͒ on the one hand and
the superexcited states possessing an A ⌸ ionic core ͑R
e
2
e
2
2
ϭ1.2436 Å ͒ on the other. Also, in a recent pulsed field
ionization photoelectron ͑PFI-PE͒ study,16 deviations in the
rotationally resolved PFI-PE spectra were observed for the
production of v ϭ6–9 in CO (X ⌺ ). This region was
accessed in one-photon excitation with narrow band synchro-
tron radiation at energies from 16–16.8 eV ͑see Figs. 1–3 in
Ref. 16͒. These deviations were attributed to perturbations by
near-resonant Rydberg levels. Summarizing, there is exten-
sive discussion and evidence in the literature that in the re-
ϩ
ϩ
2
ϩ
3
6
Leyh et al. as composed of the 5‘d’␦ (vϭ1) and the
5sϩ4d’␴ (vϭ3) members of a P(3) Rydberg pro-
‘
gression ͓note that we use the labeling as employed before
in the literature, P(3) does not indicate a ⌬NϭϪ1 transi-
tion starting from Nϭ3͔. Non-Franck–Condon transi-
tions are also observed, when rotational levels in the B state
up to NЈϭ48 are employed as stepping-stones. In a one- or
two-photon absorption process rotational quantum numbers
are not expected to change by much. However, when we
consider a transition from a high rotational level in the
ground state via the B state to a superexcited state with
2
gion ϳ1 eV below the A ⌸ ionic limit autoionization plays
a significant role. Therefore, we attribute the non-Franck–
Condon behavior observed in our experiments on CO as aris-
ing from excitation to and subsequent autoionization from
the 5‘d’␦ (vϭ1) and ‘5sϩ4d’␴ (vϭ3) states at low ro-
tational levels and 3‘d’␦ (vϭ7) and 4‘d’␦ (vϭ3) for
higher rotational levels.
2
an A ⌸ core, the differences in rotational constants for
Ϫ1
the ground state (B Љϭ1.922 629 cm ), the B-state (B0Ј
In the narrow energy region where the above superex-
cited states occur, they also affect the rotationally resolved
photoelectron spectra. The rotational structure in the photo-
electron spectra in the region where large deviations from
⌬vϭ0 propensity are observed shows striking asymmetries
as exemplified in the photoelectron spectrum associated with
ionization out of Q(45). This spectrum is depicted in Fig. 5.
Clearly, the ⌬NϭϪ1 peak is missing, while the other tran-
sitions (⌬Nϭ0,ϩ1,Ϯ2) remain visible. Of course the cal-
culated rotationally resolved photoelectron spectra, which
would be symmetrical in the limit of classical rotation, do
not predict these observations because they do not include
the role of the superexcited states. Quantum interference be-
tween two isoenergetic ionization pathways, viz. direct ion-
ization via excitation into the ionization continuum on the
one hand, and excitation of a superexcited state followed by
autoionization on the other, may lead to Fano-type line
0
Ϫ1
ϭ1.948 186 cm ) and a typical superexcited state (B0*
Ϸ1.58 cm ) will involve very similar amounts of rotational
Ϫ1
energy in X and B states, but a significantly smaller amount
of rotational energy in the superexcited state. For example,
for a three-photon excitation from NЉϭ45 to a superexcited
state with N*ϭ45, the rotational energy in the ground state
Ϫ1
is E Љϭ3980 cm , while the rotational energy in the su-
rot
Ϫ1
perexcited state is E *Ϸ3270 cm . Upon excitation from
NЉϭ45 with three photons of 43 483.5 cm , a superexcited
rot
Ϫ1
state with N*ϭ45 is reached with an energy of 134 430.5
Ϫ1
1
ϩ
cm above the ground state (X ⌺ ;vЉϭ0,NЉϭ0). The
origin of this superexcited state is then estimated to lie at
Ϫ1
1
31 160.5 cm ͑76.24 nm͒, and this closely corresponds to
the peak measured at 76.31 nm in Ref. 34. This feature has
been assigned by Leyh et al.36 as the 3‘d’␦ (vϭ7) state.
Therefore, we can interpret our observed non-Franck–
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2782
J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
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1
11
12
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absorption cross section may also result, as observed in our
13
1
1
4
5
photoelectron spectra. Fano-type phenomena are a common
occurrence involving superexcited states in atoms,3
8,39
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1
1
6
7
V. CONCLUSIONS
1
1
2
8
9
0
Hot rotational levels of CO, photodissociated from OCS,
1
ϩ
have been studied using (2ϩ1) REMPI via the B ⌺
2
ϩ
Rydberg state leading to the X ⌺ state of the ion. Im-
2
2
1
2
proved spectroscopic rotational parameters have been ob-
1
ϩ
tained for the B ⌺ state for NЈ up to 87. These high rota-
tional levels show photoelectron spectra with a very strong
⌬
Nϭ0 transition and weaker ⌬NϭϮ1, Ϯ2, and Ϯ3 transi-
tions. The agreement between measured and calculated spec-
tra is good, indicating that there is significant angular mo-
mentum coupling in the photoelectron orbital. In the narrow
energy region where the competition between direct ioniza-
tion and the excitation of superexcited states is apparent,
our vibrationally and rotationally resolved photoelectron
spectra reveal strong non-Franck-Condon behavior and show
indications for quantum interference between isoenergetic
pathways.
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ACKNOWLEDGMENTS
A.M.R. acknowledges the Holland Research School of
Molecular Chemistry for a Ph.D. fellowship. C.A.dL. ac-
knowledges very useful discussions with Professor J. M.
Dyke and members of his group during a stay in Southamp-
ton in the context of the EC Network on Reactive Interme-
diates. C.A.dL. and A.M.R. acknowledge helpful discussions
at the Lisbon meeting of the EC Network on Reactive Inter-
mediates ͑April 2001͒, and thank Ing. D. Bebelaar for tech-
nical support. This work was supported by a grant from the
National Science Foundation.
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