Predissociation of the Rydberg states of CO: State specific predissociation to the triplet channel

Article abstract of DOI:10.1063/1.475553

Photofragment yield spectra of the 3dπL′ ¹Π(v = 1) and 4pπL ¹Π(v = 0) Rydberg states have been observed by monitoring the fragment atoms generated by the predissociation. The Rydberg states of CO were prepared by two-color double res

Full text of DOI:10.1063/1.475553

Predissociation of the Rydberg states of CO: State specific predissociation to the
triplet channel
Akihiro Okazaki, Takayuki Ebata, Takumichi Sutani, and Naohiko Mikami
Citation: The Journal of Chemical Physics 108, 1765 (1998); doi: 10.1063/1.475553
View online: http://dx.doi.org/10.1063/1.475553
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Predissociation of the Rydberg states of CO: State specific predissociation
to the triplet channel
Akihiro Okazaki, Takayuki Ebata, Takumichi Sutani, and Naohiko Mikami
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-77, Japan
͑
Received 7 November 1997; accepted 21 November 1997͒
1
1
Photofragment yield spectra of the 3d␲LЈ ⌸(vϭ1) and 4p␲L ⌸(vϭ0) Rydberg states have
been observed by monitoring the fragment atoms generated by the predissociation. The Rydberg
1
ϩ
states of CO were prepared by two-color double resonant excitation through the 3s␴B ⌺ state,
3
1
3
and both C͑ P and D) and O͑ P) atom fragments were observed by two-photon resonant three
1
photon ͑2ϩ1͒ ionization. In particular, it has been found that the C( D) fragmentation takes place
in these Rydberg states, which is the first evidence of the predissociation to the triplet channel.
Predissociation rates to the triplet channel were estimated, indicating that the rates are comparable
to those to the singlet channel. © 1998 American Institute of Physics. ͓S0021-9606͑98͒03805-7͔
only 60 cmϪ1, they show quite different behavior in the
predissociation rate. According to the rotational linewidth
I. INTRODUCTION
Rydberg states of CO has been extensively studied both
7
4–6
measurements by Drabbels et al. and Ubachs et al.,
the
1
–12
13–17
1
experimentally
and theoretically.
Letzelter et al. ob-
1
Ϫ1
linewidth of the 3d␲LЈ ⌸(vϭ1) state is 1.22 cm inde-
served the absorption spectrum in the region of 90–115 nm
and obtained the photoabsorption and photodissociation
cross sections. They concluded that the photodissociation of
CO takes place after the absorption to the discrete states,
most of which are considered to be Rydberg states.
Eidelsberg and co-workers2 extensively investigated the
one-photon absorption spectrum of the Rydberg states and
reported the molecular constants of these states. Ubachs and
1
pendent of J, while that of the 4p␲L ⌸(vϭ0) state de-
pends not only J but also the parity of the rotational level. In
addition, two dissociation channels are energetically possible
1
1
for the 3d␲LЈ ⌸(vϭ1) and 4p␲L ⌸(vϭ0) states; a sin-
3
3
glet channel leading to C( P)ϩO( P) and a triplet channel
,3
1
3
3
1
to C( D)ϩO( P). The detection of the C( P) and C( D)
fragments is essential to investigate the potential crossings
between the Rydberg and the corresponding manifold of the
repulsive states of CO. In the present work, the generation of
4
–6
7
co-workers and Drabbels et al. reported the predissocia-
tion rates of the Rydberg states (nϭ3–6) by measuring line-
1
the C( D) fragments was directly confirmed and it was
widths with
co-workers
a
narrow bandwidth laser. Ebata and
found that the predissociation to the triplet states are compa-
rable to that to the singlet states.
8
–11
also reported a spectroscopic study of the
Rydberg states (nϭ4–7 for vЈϭ0, nϭ4–10 for vЈϭ1) of
CO by using the ion-dip method with triple resonant excita-
tion, and determined the potential curve of the repulsive
II. EXPERIMENT
DЈ ⌺ state. Very recently, Mellinger et al.18,19 investi-
gated the triplet Rydberg states, and analyzed the rotational
structure by using laser-reduced fluorescence spectroscopy.
Contrary to the extensive spectroscopic studies of the
Rydberg states of CO, very few experiments have been done
1
ϩ
Figure 1 shows the excitation scheme and experimental
setup of the photofragment yield measurement. Jet-cooled
1
ϩ
CO was excited to the 3s␴B ⌺ (vϭ0) state by two-
photon absorption of ␯ and further excited to the specific
1
Rydberg state by successive excitation with second laser ␯2 .
The fragment atoms generated by the predissociation of the
Rydberg state were monitored by two-photon resonant three
on the measurement of the photofragment atoms produced
by the predissociation process. Forch and Merrow2
0,21
ob-
served the atomic oxygen generated after the two-photon ab-
sorption of CO at 193 nm. Hill et al.22 detected an emission
of the atomic carbon produced by the multiphoton dissocia-
tion of CO. However, none of those studies have specified
the states from which the predissociation takes place. It is
fundamentally important to select the initial state to investi-
gate the state-to-state dynamics of the predissociation, and to
determine the potential curves of the repulsive states. In our
previous work, we reported the photofragment yield spectra
photon ionization ͑2ϩ1͒ REMPI with ␯ . The transitions
3
3
3
used for the detection of the atomic states were D ← P
J
J
Ј
Љ
3
1
1
1
3
3
3
for C( P), P ← D for C( D) and P ← P for O͑ P).
1
2
J
J
Ј
Љ
Two dye lasers ͑Lambda Physik FL3002 and Molectron
DL14͒ were simultaneously pumped by a XeCl excimer laser
͑Lambda Physik LPX 100iMC͒. A frequency doubled output
of FL3002 and a fundamental output of DL14 were used as
␯ and ␯ , respectively. The laser beams of ␯ and ␯ were
1
2
1
2
introduced into a vacuum chamber in a mutually counter-
propagated geometry, being focused by lenses (fϭ250 mm͒.
For ␯3 a second harmonic of Nd:YAG laser-pumped dye
laser ͑Quanta-Ray GCR-230-10/Continuum ND6000͒ was
used, which was coaxially introduced with ␯2 by using a
beam combiner. All the laser beams were linearly polarized.
The delay time between ␯ ͑or ␯ ͒ and ␯ was set to 20 ns by
3
by monitoring the C( P) atom and directly confirmed that
the dissociation occurs from the Rydberg states.12 In the
present study, we concentrated our study on the detec-
tion of the fragments generated by the predissociation
1
1
of the 3d␲LЈ ⌸(vϭ1) and 4p␲L ⌸(vϭ0) states at
Ϫ1
ϳ103 220 cm . Though the two states are separated by
1
2
3
J. Chem. Phys. 108 (5), 1 February 1998
0021-9606/98/108(5)/1765/4/$15.00
© 1998 American Institute of Physics
1765
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1766
Communications
FIG. 2. Photofragment yield spectra of the 3d␲LЈ 1⌸(vϭ1)
FIG. 1. ͑Upper͒ Energy levels and the diagram of the excitation of CO and
the detection of the C( P), C( D), and O( P) fragments. ͑Lower͒ Sche-
matic diagram of the experimental setup.
3
1
3
1
3
and the 4p␲L ⌸(vϭ0) states observed by monitoring ͑a͒ C( P),
1
3
1
1
͑b͒ C( D), and O( P) fragments. ͑d͒ Simulated LЈ ⌸(vϭ1), L ⌸(v
1
ϩ
ϭ0)←3s␴B ⌺ (vϭ0) absorption spectra by assuming Trotϭ15 K.
using the digital delay generator ͑SRS Model DG535͒. Pure
CO was expanded into a vacuum chamber through a pulsed
nozzle having an orifice of 400 ␮m and the three laser beams
crossed at 15 mm downstream of the nozzle. The ions were
separated by a time-of-flight ͑TOF͒ tube and were detected
by an electron multiplier ͑Murata Ceratron͒. The signal was
amplified by an amplifier ͑NF Model BX-31͒ and was inte-
grated by a boxcar integrator ͑PAR Model 4420͒ connected
with a personal computer.
7
4–6
Drabbels et al. and the group of Ubachs. In this simula-
tion, we assumed the Boltzmann rotational distribution for
1
ϩ
the 3s␴B ⌺ (vϭ0) state to be T ϭ15 K, which is
rot
thought to be equal to the rotational temperature of the jet.
First of all, it should be noted that the present work has
presented the first state-specific observation of the C( D)
fragment generated by the predissociation of the Rydberg
states of CO. The result is the direct evidence for predisso-
ciation involving the triplet channel. As seen in the figures,
all the photofragment yield spectra of the 3d␲LЈ ⌸(v
ϭ1) state are similar with each other and are in good agree-
1
1
III. RESULTS AND DISCUSSION
1
ϩ
1
ϩ
In the 3s␴B ⌺ (vϭ0)←X ⌺ (vϭ0) two-photon
ment with the simulated spectrum ͑d͒. On the other hand, the
photofragment yield spectra of the 4p␲L ⌸(vϭ0) are
1
transition by using the linearly polarized ␯ , only an intense
1
bandhead of the Q branch was observed, indicating several
rotational transitions are overlapped. This is because the ro-
tational constants of both states are similar and the Q branch
has strong intensity with the linearly polarized light. Thus
quite different from each other, and do not fit with the simu-
lated spectrum ͑d͒ except for ͑c͒. As seen in the figures, the
Q branch is weak compared with the P and R branches when
3
the C( P) atom is monitored ͓Fig. 2͑a͔͒, while the Q branch
␯1
pumps CO to several rotational levels of the
is much stronger than the P and R branches and the intensi-
ties of the R and P branches monotonously decrease with J
1
ϩ
3
s␴B ⌺ (vϭ0) state with the rotational distribution simi-
1
ϩ
1
lar to that of the X ⌺ (vϭ0) state in the jet.
Figures 2͑a͒, 2͑b͒, and 2͑c͒ show the photofragment yield
spectrum of the 3d␲LЈ ⌸(vϭ1) and 4p␲L ⌸(vϭ0)
when the C( D) atom is monitored ͓Fig. 2͑b͔͒. Since the
3
O( P) atom is the common product for the singlet and triplet
1
1
3
dissociation channels, the O( P) yield spectrum agrees very
1
ϩ
←
3s␴B ⌺ (vϭ0) transitions obtained by monitoring ͑a͒
well with the simulated spectrum ͓Fig. 2͑c͔͒. Among the
3
1
3
3
C( P), ͑b͒ C( D), and ͑c͒ O( P) atoms, respectively. Fig-
photofragment yield spectra, the O( P) yield spectrum is
1
3
ure 2͑d͒ shows simulated 3d␲LЈ ⌸(vϭ1) and
broadened due to saturation of ␯ . This is because the O( P)
2
1
1
ϩ
3
1
4
p␲L ⌸(vϭ0)←3s␴B ⌺ (vϭ0) spectra by using the
ion signal was weaker than those of C( P) and C( D), and
rotational constants and the rotational linewidths reported by
higher ␯ power ͑200 ␮J͒ was necessary to obtain the spec-
2
J. Chem. Phys., Vol. 108, No. 5, 1 February 1998
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Communications
1767
trum in good signal-to-noise ratio. For other fragment mea-
surements, the ␯ power was reduced to 50 ␮J.
2
According to the linewidth measurements of the
1
1
ϩ
4
p␲L ⌸(vϭ0)←3s␴B ⌺ (vϭ0) transition reported by
7 4–6
Drabbels et al. and by the group of Ubachs, the linewidth
of the Q branch (f-symmetry component͒ is independent of
J, while those of the P and R branches (e-symmetry com-
ponent͒ increase proportionally with J(Jϩ1). The J depen-
dence suggests that the e-symmetry component of the
1
1
ϩ
4
p␲L ⌸(vϭ0) state predissociates to the repulsive ⌺
1
ϩ
potential surface, that is, DЈ ⌺ state, and that the predis-
sociation of the f-symmetry component of the 4p␲L ⌸(v
ϭ0) state dissociates to the repulsive ⌸ state, that is the
1
1
1
13,17
2
⌸ state.
By assuming that the radiative decay rates of
the Rydberg states are much smaller than the predissociation
rates, the total quantum yield to the singlet and triplet disso-
ciation channels is considered to be unity. As a consequence,
the change of the predissociation rate to the singlet channel
leads to the change in the quantum yield of the triplet chan-
FIG. 3. Photofragment yield and simulated spectra of 4p␲L 1⌸(v
1
ϩ
3
1
ϭ0)←3s␴B ⌺ (vϭ0) transition of CO. ͑a͒ C( P) and ͑b͒ C( D) frag-
ment monitored spectra.
1
cluded that the predissociation of the 4p␲L ⌸(vϭ0) state
1
to the triplet channel competes with that to the singlet chan-
nel with comparable rate.
nel product, that is C( D). Thus the observed dependence
3
1
1
for the C( P) and C( D) products for the 4p␲L ⌸(v
ϭ0) state can be explained by that the predissociation pro-
cesses to the singlet and triplet channels are competing with
the similar rate and the rotational dependence is due to that
of the predissociation rate to the singlet channel.
Furthermore, it is suggested that the predissociation rate
1
of the 3d␲LЈ ⌸(vϭ1) to the triplet channel is also com-
1
parable to that to the singlet channel because the C( D) sig-
1
nal intensity of 3d␲LЈ ⌸(vϭ1) is similar to that of
1
4
3
ϳ10
p␲L ⌸(vϭ0). Estimated predissociation rate of the
We estimated the predissociation rates to the singlet and
triplet channels from the observed photofragment yield spec-
tra and from the reported total decay rate, k, of each rota-
tional levels obtained from the linewidth measurement. Here
1
d␲LЈ ⌸(vϭ1) state to the triplet channel is kT
11
Ϫ1
s .
Shafer III et al. calculated several valence states for the
triplet repulsive states to which the Rydberg states predisso-
the total decay rates are the sum of the singlet (k ) and triplet
s
3
ϩ
3
Ϫ
3
ciate. According to their calculation, ⌺ , ⌺ , and ⌸
states are known to cross the potential curves of the Rydberg
states at relatively low energy region. At this stage, we can
not predict which state is mainly responsible for the predis-
sociation. More detailed analysis is in progress, which in-
cludes the rotational and vibrational dependencies of the pre-
dissociation rates.
(
kT) dissociation rates. For the e-symmetry component, the
predissociation rate to the singlet manifold is expressed as
the sum of the J independent and dependent terms,
e
S
k ϭk ϩk J͑Jϩ1͒.
͑1͒
0
J
For the f-symmetry component, on the other hand, the rate
includes only J independent term,
f
ACKNOWLEDGMENTS
k ϭk .
͑2͒
S
0
The authors would like to thank Dr. Asuka Fujii and Dr.
Haruki Ishikawa for their helpful discussions and T. Ebata
would like to thank Professor Robert W. Field for his interest
of this work and valuable suggestions.
For the triplet channel, we assumed that k is independent of
T
J, since the spin–orbit coupling does not include J. Though
this assumption is rather crude, the simulated photofragment
yield spectrum well reproduced the observed spectra. The
observed signal intensities are given by
1
C. Letzelter, M. Eidelsberg, F. Rostas, J. Breton, and B. Thieblemont,
kT
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͑3͒
͑4͒
J J
Ј Љ
kSϩkT
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3
I͑ P͒ϭS
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Ј Љ
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ϩ
where S
is the 4p␲L ⌸(vЈϭ0,JЈ)←B ⌺ (vЉϭ0,JЉ)
J J
Ј Љ
transition intensity. From the linewidth measurements by
Drabbels et al., kJϭ1.2ϫ109
7
s
Ϫ1
,
and k ϩk ϭ1.9
0 T
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T
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