Photodissociation spectroscopy of CICN in the vacuum ultraviolet region

Article abstract of DOI:10.1016/S0301-0104(00)00064-1

The quantitative photofragment fluorescence spectroscopy, using the synchrotron radiation as an exciting light source, was applied to study the Rydberg and high-lying valence states of ClCN observed as congested structures in the vacuum ultraviolet region. The absolute cross-section and quantum yield for the CN(B²Σ⁺-X ²Σ⁺) emission produced in the photodissociation process of ClCN were determined in the wavelength range 105-145 nm (69 000-95 200 cm^-1). The quantum yield for the CN(B²Σ⁺) production takes a maximum value of ?0.13 at ?84 000 cm^-1. The emission of CN(B ²Σ⁺-X²Σ⁺) transition was found to be partially polarized with respect to the direction of the electric vector of the excitation synchrotron radiation. The polarization anisotropy of this emission, which depends on the symmetry of absorption transitions into the photodissociative states of ClCN was measured as a function of exciting wavelength. The relative cross-section for the production of CN(A²Π(i)-X²Σ⁺) emission was also determined. Based on the measured photochemical properties of the high-energy electronic states, the observed bands of the Rydberg and intravalence transitions are assigned. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S0301-0104(00)00064-1

Chemical Physics 255 (2000) 369±378
www.elsevier.nl/locate/chemphys
Photodissociation spectroscopy of ClCN in the vacuum
ultraviolet region
Kazuhiro Kanda a,*, Mitsuhiko Kono b,1, Takashi Nagata , Atsunari Hiraya
b,2
b,3
,
b,4
Kiyohiko Tabayashi , Kosuke Shobatake
b,5
a
Laboratory of Advanced Science and Technology for Industry, Himeji Institute of Technology, Kamigori, Hyogo 678-1205, Japan
b
Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan
Received 21 February 2000
Abstract
The quantitative photofragment ¯uorescence spectroscopy, using the synchrotron radiation as an exciting light
source, was applied to study the Rydberg and high-lying valence states of ClCN observed as congested structures in the
2
‡
2
‡
vacuum ultraviolet region. The absolute cross-section and quantum yield for the CN(B R ±X R ) emission produced
in the photodissociation process of ClCN were determined in the wavelength range 105±145 nm (69 000±95 200 cm ).
�
1
2
‡
� 1
The quantum yield for the CN(B R ) production takes a maximum value of ꢀ0.13 at ꢀ84 000 cm . The emission of
2
‡
2
‡
CN(B R ±X R ) transition was found to be partially polarized with respect to the direction of the electric vector of the
excitation synchrotron radiation. The polarization anisotropy of this emission, which depends on the symmetry of
absorption transitions into the photodissociative states of ClCN was measured as a function of exciting wavelength.
2
2
‡
i
The relative cross-section for the production of CN(A P ±X R ) emission was also determined. Based on the measured
photochemical properties of the high-energy electronic states, the observed bands of the Rydberg and intravalence
transitions are assigned. Ó 2000 Elsevier Science B.V. All rights reserved.
1
. Introduction
*
In the vacuum UV (VUV) photodissociation
process of ClCN, the electronically excited radicals
Corresponding author.
Present address: Solar-Terrestrial Environment Laboratory,
1
2
‡
2
Nagoya University, Toyokawa, Aichi 442-8507, Japan
2
Present address: Department of Basic Science, Graduate
of CN(B R ) and CN(A P
i
) are produced as the
2
‡
photofragments and the subsequent CN(B R ±
School of Arts and Sciences, Meguro-ku, Tokyo 153-8902,
Japan.
2
‡
2
2
‡
i
X R ) and CN(A P ±X R ) emissions have been
3
observed in the UV and visible region and in the
near infrared region, respectively. The knowledge
of the high-lying electronic states is important to
discuss the photodissociation dynamics of ClCN in
the VUV region. However, the congested features
of the VUV absorption spectrum of ClCN pre-
vented us from the fully understanding the prop-
erty of the precursor states, (ClCN)*, through
Present address: Department of Material Science, Faculty of
Science, Hiroshima University, Higashi-Hiroshima, Hiroshima
7
39-8526, Japan.
4
Present address: Department of Chemistry, Graduate
School of Science, Hiroshima University, Higashi-Hiroshima,
Hiroshima 739-8526, Japan.
5
Present address: Department of Materials Chemistry,
School of Engineering, Nagoya University, Chikusa-ku, Nag-
oya 464-8603, Japan.
0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 0 1 0 4 ( 0 0 ) 0 0 0 6 4 - 1

3
70
K. Kanda et al. / Chemical Physics 255 (2000) 369±378
which fragmentation proceeds. Although elec-
tronic excited states of ClCN have been investi-
gated by several workers, their assignments are not
all consistent and the peaks in the k < 120 nm
have not been assigned [1±7].
2. Experimental
The experimental apparatus and procedures
employed in the present study have been described
in the previous paper [8,9,11]. The measurement
was performed at the BL2A stage of UVSOR in
the Institute for Molecular Science. The synchro-
tron radiation provided by the 0.75-GeV electron
storage ring was dispersed by a 1-m Seya-Namioka
monochromator (Hitachi SNM-2) and was intro-
duced into the 12.3-cm-length reaction ¯ow cell
through the LiF window. Slit widths of the
monochromator were 100 lm for the measurement
of the absorption and ¯uorescence excitation
spectra, and 300 lm for the measurement of the
polarization anisotropy of CN(B±X) emission.
These resolutions of exciting light wavelength were
about 0.1 and 0.3 nm, respectively, which were
estimated from the spectral width of the atomic
absorption line of Kr at 123.6 nm and those of Xe
at 119.20 and 146.96 nm. A wavelength reading of
the monochromator was calibrated against these
atomic absorption lines. The uncertainty in
the calibrated wavelength was estimated to be
Æ0.03 nm.
Through systematic work, we have investigated
the highly excited states of the cyanogen halide
leading to the production of CN(A) and CN(B)
radicals. In our previous works, the transition
peaks observed in the VUV absorption spectra of
BrCN and ICN have been assigned to the Rydberg
series and the intravalence transitions [8,9]. The
same transitions are expected to appear in the
photoabsorption spectrum of ClCN, because ClCN
has the chemical properties similar to these mole-
cules. On the other hand, the spectral positions
must be shifted because the ionization potentials of
ClCN are higher than those of BrCN and ICN.
In the present study, the absolute cross-section
and quantum yield for the production of CN(B) in
the VUV photodissociation process of ClCN were
determined as a function of excitation wavelength
ranging from 105 to 145 nm. In addition to these
quantitative measurements, the polarization an-
isotropy of the resulting CN(B R ±X R ) emis-
sion was measured by taking advantage of the
linear polarization of the synchrotron radiation.
The polarization anisotropy of the fragment
emission with respect to the direction of the elec-
tric vector of incident radiation gave us an infor-
mation on the symmetry of the initial electronic
transition into the dissociative state. Measure-
ments for the polarization anisotropy of CN(B±X)
observed in the VUV photodissociation process of
ClCN have been reported by Simons and co-
workers at several excitation wavelengths in the
2
‡
2
‡
The ClCN sample was prepared by the reaction
of NaCN with Cl
2
in a CCl
4
medium at )10°C
[12]. The pressure of sample in the reaction ¯ow
cell monitored with a capacitance manometer
(Baratron Type 315) was typically 15±45 mTorr
(1 Torr ˆ 133.322 Pa). The VUV photons passing
through the cell were detected by a Hamamatsu
R585 photomultiplier after conversion to visible
¯uorescence by sodium salicylate coated on the
outside of the exit LiF window. The output signal
from the photomultiplier tube was fed into a pi-
coammeter (TDA AM-271A). The photoabsorp-
tion cross-section, r, was determined from the
ratio of the VUV photon ¯uxes through the ¯ow
cell measured with and without sample gas. The
uncertainty of the absolute absorption cross-
section was estimated to be ꢀ10% due to the un-
certainty of the sample gas pressure.
126±154 nm region [3,4] and by Zare and co-
workers at 157.6 nm [10]. In the present experi-
ment, the polarization anisotropy of CN(B±X) was
measured as a function of wavelength ranging
from 105 to 145 nm at intervals of 0.02 nm. Our
`
`close'' measurement distinguished the tiny struc-
tures in the excitation function from the underly-
ing di€use bands and made the spectral
assignments of the VUV absorption bands of
ClCN reliable. The relative cross-section for the
CN(A) production in the VUV photodissociation
process of ClCN was also measured.
The fragment emissions were collimated
through a quartz lens attached on the side wall of
the reaction ¯ow cell and focused through another
quartz lens to a detector. The CN(B±X) emission
in the UV and visible region was isolated by

K. Kanda et al. / Chemical Physics 255 (2000) 369±378
371
a band-pass ®lter (Toshiba C-39A) and detected by
a photomultiplier (Hamamatsu R585). The emis-
sion of CN(A±X) transition observed in the near
infrared region was monitored with a combination
of a sharp-cut ®lter (Toshiba R-65) and a photo-
multiplier (Hamamatsu R955) equipped with a
photomultiplier cooler (Hamamatsu C659). The
emission signal was acquired using an Ortec pho-
ton counting system. The emission cross-section
determined in the present study corresponds to the
determined by using a photoelastic modulator
(PEM, Hinds International Inc. PEM-80) and a
multichannel scaler (MCS) at several excitation
wavelengths, in order to eliminate the factors giv-
ing rise to the uneven signal levels of I
and I . The
?
details in the PEM±MCS technique have been
k
described in Ref. [11]. The statistical errors in the
ratio gave rise to the
measurement of the I
k
=I
?
uncertainty of ꢂ0.01 of R.
B
cross-section for the production of CN(B), r ,
because the radiative decay can be regarded as the
dominant deexcitation process for the CN formed
3
. Results and discussion
in the B state. The absolute value of r was scaled
B
3
.1. Electronic structure of cyanogen chloride
by a comparison of the intensity of the CN(B±X)
emission produced in the photodissociation of
HCN, for which the absolute emission cross-
section has been reported [13]. The details about
the scaling procedure to obtain the absolute
emission cross-section from the emission intensity
and relative optical responses of the detection
systems were described in Ref. [9]. The relative
The molecular orbital (MO) con®guration of
ClCN in the ground state is given in the following
[14±16]:
2
2
2
4
2
4
1
‡
ꢁ
1r† ꢁ2r† ꢁ3r† ꢁ1p† ꢁ4r† ꢁ2p† ;
R
ꢁ2†
The ionization potentials, IP, the frequencies of
the vibrational modes, m , m , and m and spin±orbit
uncertainty of r was estimated to be ꢀ10%. On
B
1
2
3
the other hand, the absolute values of the cross-
splittings, SO, are listed in Table 1. The three band
systems are observed in the photoelectron spec-
trum of cyanogen chloride. The cyanogen chloride
cations in these three lowest energy ionic states
and the cyanogen chloride in the ground state have
been reported to be linear [14±17].
A
section for the production of CN(A), r , could not
be determined, because the detection system for
the near infrared region did not cover all the
spectral region of CN(A±X) transitions, as dis-
cussed in Ref. [9]. The quantum yield for the
�
1
production of CN(B), C , was calculated as the
The ®rst band system (IP ˆ 99770 cm ) in the
photoelectron spectrum are due to the removal of
an electron from the 2p MO, which originates
from the combination of the 3p orbital of the
chlorine atom and the p bonding orbital of the CN
group out of phase. The splitting of this band
system by spin±orbit coupling, i.e. the splitting
B
ratio of the emission cross-section to the absorp-
tion cross-section. The relative uncertainty of C
was estimated to be ꢀ20%.
B
The polarization anisotropy, R, is de®ned as
R ˆ ꢁI
k
=I
?
� 1†=ꢁI
k
=I
?
‡ 2†;
ꢁ1†
‡
2
‡
2
between ClCN (X P3=2) and ClCN (X P1=2), is
small in comparison with BrCN and ICN. The
where I
k
and I are the intensities of the emission
?
2
with the light component parallel and perpendi-
cular to the direction of the electric vector of the
incident radiation, respectively. In order to mea-
sure the polarization anisotropy of CN(B±X)
emission produced in the photodissociation pro-
cess of ClCN, two identical sets of a band-pass
photoelectron intensity of the X P3=2 component
is much smaller than that of the X P1 compo-
2
=2
�
1
nent. The second band system (IP ˆ 111300 cm )
is due to ionization of cyanogen chloride to the
‡
2
R state, which corresponds to the removal of an
electron from the 4r MO. The 4r orbital is re-
garded primarily as the lone pair localized on the
nitrogen atom. The third band system (IP ˆ
®
lter and a Glan-Taylor prism polarizer (Karl±
Lambrecht MGTYA15) were mounted behind the
collimating quartz lenses [8]. The absolute values
�
1
124000 cm ) arises from the removal of an elec-
of the I =I ratio for the CN(B±X) emission were
tron from the 1p MO. This MO is the combination
k
?

3
72
K. Kanda et al. / Chemical Physics 255 (2000) 369±378
Table 1
Ionization potentials, vibrational frequencies and spin±orbit splittings (cm
�
1
)
a
State
IP
Vibrational frequencies
SO
m
1
m
2
m
3
ClCN
1
‡
b
b
b
X R
0
714.02
378.62
2215.24
‡
ClCN
�
1
2
c
d
d
d
(
2p) , X P3=2
99 770
827
376
1915
d
d
2
3
76
�
1
2
d
d
(
(
(
2p) , X P1=2
829
774
531.8
1914
�
1
2
‡
c
c
d
d
4r) , A R
111 300
124 000
545
303.1
�
1
2
d
d
d
1p) , B P3=2
2128.5
68
�
1
2
d
(
1p) , B P1=2
529.7
a
b
c
1 2 3
m : C±N stretching, m : bending, m : C±Cl stretching.
Ref. [17].
Ref. [14].
Ref. [20].
d
of the 3p orbital of the chlorine atom and the p
bonding orbital of the CN group in phase.
plotted against the excitation wave number, m~ , in
�
1
the range 69 000±95 200 cm . The excitation
spectrum of CN(B±X) emission formed from the
VUV photodissociation of ClCN has been re-
ported previously by Macpherson and Simons
3.2. Absorption and excitation spectra
�
1
Fig. 1 shows the VUV absorption spectrum of
�
[3,4] in the region m~ < 81 900 cm (122 nm) while
the absolute cross-section of the CN(B±X) emis-
sion has not been determined in their studies. The
dominant photodissociation channels in the
1
ClCN in the 69 000±95 200 cm
region. The
spectral features and positions of the absorption
bands observed in the present study are in good
agreement with the photoabsorption spectrum of
ClCN, which has been previously reported by
Simons et al. [3,4] and Felps et al. [5,7].
�
1
69 000±95 200 cm region lead to the production
of CN fragments in the X and/or A states, because
the observed maximum value of C
at ꢀ84 000 cm .
B
is only ꢀ0.13
�
1
In Fig. 2(a), the absolute cross-section and
quantum yield for the CN(B) production are
Fig. 2(b) depicts the polarization anisotropy, R,
of the subsequent CN(B±X) emission as a function
of the excitation wave number. The values of R
expected in the photodissociation of cyanogen ha-
lide have been discussed in Ref. [8]. By considering
the fact that ClCN is a linear molecule and that the
transition dipole moment responsible for the frag-
2
‡
2
‡
ment CN(B R ±X R ) emission is ®xed along the
molecular axis of the CN fragment, the limiting
value of R merely depends on the type of absorp-
tion transitions relevant to the photodissociation of
ClCN. In the case of parallel-type transition, when
the transition dipole moment for the absorption of
ClCN, labs, is parallel with the molecular axis, i.e.,
l
abs is perpendicular to the angular momentum, J,
theoretical limit of R value is calculated to be 0.1.
On the other hand, when labs is perpendicular to
Fig. 1. Photoabsorption cross-section of ClCN in the 69 000±
�
1
9
5 200 cm range. The spectral resolution is ꢀ0.1 nm.

K. Kanda et al. / Chemical Physics 255 (2000) 369±378
373
2
‡
� 1
Fig. 2. (a) Absolute cross-section (±±) and quantum yield (±±) for the production of CN(B R ) from ClCN in the 69 000±94 000 cm
range. (b) Polarization anisotropies of CN(B R ±X R ) emission.
2
‡
2
‡
the molecular axis (perpendicular-type transition),
i.e. labskJ, the R value will be 0.1 and )0.2 in the P
and R branches and in the Q branch, respectively.
In a predissociation process, the observed magni-
tude of R is reduced to be less than these theoretical
limit by the rotational motions of the dissociating
Rydberg states. The transition frequency, m~ , of a
Rydberg series obeys the formula
R
1
m~ ˆ IP �
;
ꢁ3†
2
ꢁ
n � d†
represents the Rydberg constant (10 973
where R
1
[
ClCN*] molecule during its lifetime. Simons et al.
�
1
cm ) and n is the principal quantum number. The
quantum defect, d, is characteristic of a given
Rydberg series, however, no regular Rydberg se-
ries are expected to appear in the photoabsorption
spectra of cyanogen halide due to strong interac-
tion between intravalence and Rydberg states [8,9].
In the following sections, we discuss the assign-
ments of the photoabsorption peaks and photo-
chemical properties of the excited states on the
basis of the measurements of the excitation spectra
and polarization anisotropy.
have measured the I =I ratio of the CN(B±X)
k
?
emission from ClCN at several excitation wave-
lengths [3,4]. The R values measured in the present
study at the corresponding wavelengths agree well
with those previously reported values [3,4] within
the experimental accuracy.
Fig. 3 shows the excitation spectrum and rela-
2
tive quantum yield for the CN(A P ) production
i
�
1
in the wave number region 69 000±95 200 cm .
The wave number dependence of the cross-section
for the CN(A) production resembles that for the
CN(B) production in this region.
3
.3.1. 2p ! nsr Rydberg transition
The absorption bands observed in the 71 000±
�
1
3.3. Spectral assignments
78 000 cm region have been assigned to the
leading members of the 2p ! nsr Rydberg series
by Felps and co-workers [5±7]. The vibrational
structures are characteristic of a linear±linear
transition of triatomic molecule, i.e. the vibrational
The photoabsorption peaks observed in the
present measurement were interpreted in terms of
excitations to the valence excited states and the

3
74
K. Kanda et al. / Chemical Physics 255 (2000) 369±378
2
� 1
Fig. 3. Relative cross-section (±±) and quantum yield (±±) for the production of CN(A P
range.
i
) from ClCN in the 69 000±94 000 cm
band associated with the m mode (bending vibra-
that the threshold energy for the Cl ‡ CNꢁB†
� 1
2
tion) was not observed except for very weak 71 705
channel is predicted to locate at 110 140 cm [7].
The vibrational analysis of this band system is
listed in Table 2.
�
1
cm band. The fragment CN(B±X) emission in-
trinsically processes negative polarization because
a sharp dip is observed at the center of the intense
absorption peaks in this region of the R± m~ curve.
This indicates that the photoexcitation at the band
center results in a negative R value of the CN(B±
X) emission. The dependence of polarization on m~
is characteristic of the Q branch excitation of a
perpendicular-type transition of ClCN [18]. The
�
1
The peak at 85 822 cm (f) is accompanied with
and m vibrational bands at 87 750 (g) and
m
3
1
‡ m
3
�
1
88 449 cm (h). These structures of vibrational
progression resemble those of peaks (A), (C) and
(D). Dips in the R± m~ curve show that perpendi-
cular-type transition is responsible for this
band system. Therefore, this band system is as-
�
1
1
quantum defect for band origin at 71 398 cm
with respect to the ®rst ionization potential is
calculated to be 1.03.
signable to the 2p ! 4srꢁ P† Rydberg transi-
1
tion. The band origin of the 2p ! 5srꢁ P†
Rydberg transition is assigned to the peak at
�
1
The baseline of the excitation spectra is not
zero, which is ascribable to the contribution of an
underlying continuum. The R value takes ꢀ0.09 in
the excitation energy region, where the underlying
continuum makes a dominant contribution to the
production of CN(B). The positive R value indi-
cates that the photoexcitation responsible for the
CN(B) production occurs through a parallel-type
transition of ClCN [18]. This underlying continu-
um is assignable to the high-energy tail of the a
continuum (the 2p ! 3p intravalence excitation),
92 268 cm (s).
3.3.2. 2p ! 6r intravalence transition
�
1
In the energy range 74 000±77 000 cm , an-
other vibrational progressions (E, H and I) appear
in the photoexcitation spectrum of ClCN. Because
these bandwidths are clearly wider than near
Rydberg bands i.e. 2p ! 3sr, the dissociation
dynamics through these electronic states should be
di€erent from the Rydberg states. Felps et al. have
�
1
1
which extends over 62 000±70 600 cm [7]. The
large magnitude of R, which is close to the limiting
value, 0.1, is also consistent with the operation
occurrence of direct photodissociation via the a
assigned these peaks to the 2p ! 6rꢁ P† intrava-
lence transition [7]. These peaks are considered to
arise from a perpendicular-type transition because
the R± m~ curve shows a minimum at the center of
these peaks. Our experimental result reinforces
their assignments. The present vibrational analysis
of this band system is shown in Table 3.
�
B
above 71 000 cm
1
continuum. A non-zero C
obviously con¯icts with the semiquantitative pre-
diction based on a simple one-electron MO model

K. Kanda et al. / Chemical Physics 255 (2000) 369±378
375
Table 2
Wave numbers/wavelengths and assignments of Rydberg transitions in the vacuum UV absorption spectrum of ClCN
a
� 1
b
c
� 1
d
e
Peak
m~ (cm
)
k (nm)
D m~ (cm
)
n
Assignment
d
1
2
p ! nsrꢁ P†
0
0
A
B
C
D
F
G
f
71 398
71 705
73 346
73 975
75 233
75 884
85 822
87 750
88 449
90 367
92 268
92 937
94 091
140.06
139.46
136.38
135.18
132.94
131.78
116.52
113.96
113.06
110.66
108.38
107.60
106.28
3
1.03
0
1
307
1948
629
2
1
3
1 3
1
1
2
2
1887
651
3
1 3
1
0
0
1
4
5
0
1.20
1.18
g
1928
699
3
1 3
1
1
2
h
m
s
1
1918
1 3
0
0
0
1
t
669
1823
1
1
y
3
1
‡
2
p ! nppꢁ R †
0
0
P
W
b
j
79 745
81 673
83 556
88 984
89 445
90 204
91 458
92 166
93 985
93 738
94 357
125.40
122.44
119.68
112.38
111.80
110.86
109.34
108.50
106.40
106.68
106.28
3
0.66
0
1
1928
1883
3
2
3
0
0
0
0
4
4
0
(triplet)
0.74
k
l
0
1
759
2013
708
1
1
p
r
3
1 3
1
1
2
1
x
w
z
1819
1 3
0
5
4
0
0.74
0.44
0
1
619
1
1
f
2
4
4
p ! ndrꢁ P†
0
0
o
q
u
91 125
91 878
93 127
109.74
108.84
107.38
0
1
753
2002
1
1
3
1
‡
r ! nsrꢁ R †
0
0
c
d
e
84 076
84 846
85 587
118.94
117.86
116.84
3
3
0.99
0.79
0
1
770
741
1
2
1
1
‡
r ! nprꢁ R †
0
0
i
88 842
90 761
112.56
110.18
0
1
n
1919
3
a
b
c
The symbols correspond to those in Figs. 2 and 3.
� 1
Errors in m~ are estimated to be Æ25 cm
.
Errors in k are estimated to be Æ0.03 nm.
d
Frequency interval for the vibrational progressions. These values are comparable to the wave numbers of the corresponding m
1
, m
2
and
‡
m
3
modes of ClCN (see Table 1).
Quantum defect.
e
3
1
3
.3.3. 2p ! 4p intravalence transition
these bands to the P and P manifolds arising
from the 2p ! 3pr Rydberg transition [3]. Felps
et al. have mentioned this band system only due to
Rydberg transitions [7]. The positive R, ꢀ0.09,
which is close to the theoretical limit for direct
�
1
The band structures observed at ꢀ78 000 cm
in the photoexcitation spectrum are too di€use to
identify their band origins and vibrational pro-
gressions. Macpherson and Simons have assigned

3
76
K. Kanda et al. / Chemical Physics 255 (2000) 369±378
Table 3
Wave numbers/wavelengths and assignments of the intrava-
lence transitions in the vacuum UV absorption spectrum of
parallel-type transition is responsible for this peak.
The R± m~ curve also shows humps at 81 673 (W)
�
1
a
and 83 556 (b) cm peaks, which are assignable to
the vibrational members of the m progressions.
Based on these ®ndings, this band system is asso-
ClCN
�
1
3
Peak
m~ (cm
)
k (nm)
Dm~
Assignment
�
1
(cm )
1
‡
1
ciated with the 2p ! 3ppꢁ R † transition. Our
present result supports the assignment by Mac-
pherson and Simons [3]. The n ˆ 4 and n ˆ 5
members of this transition are observed at 89 445
2
p ! 6rꢁ P†
0
0
0
E
H
I
74 471
76 301
76 959
134.30
131.06
129.94
1
1830
658
3
1 3
1
1
�
1
� 1
1
‡
b
cm (k) and 93 738 cm (w), respectively, as
listed in Table 2.
2
p ! 4pꢁ R †
0
0
0
J
77 459
77 761
78 064
78 395
78 715
79 340
79 962
129.10
128.60
128.10
127.56
127.04
126.04
125.06
1
K
L
302
303
331
320
625
622
1
2
1
3
.3.5. 1p ! 5r intravalence transition
In the energy range 80 200±81 300 cm , several
3
M
N
O
Q
1
� 1
4
1
6
peaks appear in the photoabsorption spectrum of
ClCN. Macpherson et al. and Felps et al. have
assigned these peaks to the 4r ! 3sr and
2p ! 3pp Rydberg transitions, respectively [3,7].
However, dips at the center of the absorption
peaks are observed in the R± m~ curve. The dips are
1
8
1
1
1
p ! 5rꢁ P†
0
0
0
R
S
80 218
124.66
124.26
123.98
123.28
123.02
121.76
121.42
121.14
120.74
1
80 476
80 658
81 116
81 288
82 129
82 359
82 549
82 823
258
182
640
172
1911
230
190
694
2
2
T
U
V
X
Y
Z
a
2
1 2
1
1
2
� 1
also observed at ꢀ82 100 cm , corresponding to
1
1 2
1
3
the m excitation. Felps et al. have estimated the
3
2 3
1
1
1
1
transition energy of the 1p ! 5r intravalence
� 1
2
2 3
transition to be ꢀ80 460 cm [7]. Based on these
1
1 3
observations, this band system is assigned to the
1
a
See legend of Table 2.
Tentative assignments (see text).
1p ! 5rꢁ P† intravalence transition. The molec-
b
ular structure of this excited state is presumably
bent, because the m2 (bending vibration) mode
progressions were observed. The molecular bond
angle could be changed from 180° by the strong
dissociation values, indicates that a parallel-type
transition makes a dominant contribution to these
band structures. It is concluded from this obser-
vation that the relevant upper state is dissociative
and that its lifetime is much shorter than the ro-
tational period of ClCN. This band system is
1
Renner±Teller e€ect due to the P type intrava-
lence electronic state. The spectral position of band
origin is not obvious. The tentative vibrational
analysis of this band system is listed in Table 3.
1
‡
tentatively assigned to the 2p ! 4pꢁ R † intrava-
lence transition, which was also observed in the
photoabsorption spectrum of ICN as discussed in
Ref. [9]. The vibrational analysis of this band
system is listed in Table 3.
3
.3.6. 4r ! nsr Rydberg transition
The band at 84 076 cm (c) is accompanied
with m and 2m vibrational bands at 84 846 (d) and
5 587 cm (e), respectively. The positive R values
�
1
1
1
�
1
8
indicate that parallel-type transition makes a
dominant contribution to this band system. The
�
1
3
.3.4. 2p ! npp Rydberg transition
quantum defect for the band origin at 84 076 cm
with respect to the second ionization potential
The most intense absorption band peaking at
�
1
� 1
7
9 745 cm has been assigned to the 2p ! 3pp
(113 000 cm [14]) is calculated to be 0.99; an
Rydberg transition by Macpherson and Simons
3]. As shown in Fig. 2(b), the R± m~ curve shows a
maximum at the center of the intense absorption
electron promotion to an s-type Rydberg orbital is
involved in the transition [19]. Based on these
considerations, this band system is assigned to the
[
�
1
1
‡
peak located at 79 745 cm (P). This implies that a
4r ! 3srꢁ R † Rydberg transition (see Table 2).

K. Kanda et al. / Chemical Physics 255 (2000) 369±378
377
This band has the highest quantum yield for the
production of CN(B) in the present observed re-
gion. The promotion of the 4r electron preferen-
tially leads to the production of CN(B), from the
molecular orbital correlation between parent
ClCN and CN(B) fragment.
photoabsorption of BrCN and ICN, however, the
3
P manifold was not observed.
Rydberg transitions observed in the present
study are converging to the ®rst or second IPs of
ClCN. These IPs exhibit in the shorter wavelength
region than the present measurement range. We
have measured the photoexcited spectra of cya-
nogen halides in the 30±105 nm by using the gas-
®lter apparatus. We will report the assignments
and the chemical properties of the high-energy
excited states of cyanogen halides in the forth-
coming publication.
3
.3.7. 4r ! npr Rydberg transition
�
1
The peak located at 88 842 cm (i) is assigned
1
‡
to the band origin of the 4r ! 3prꢁ R † Rydberg
transition because the quantum defect for the or-
igin with respect to the second ionization potential
is calculated to be 0.79, which accords with an
electron promotion to a p-type Rydberg orbital
[
19]. The R± m~ curve shows a small hump at
Acknowledgements
�
1
90 761 cm (n) peak, which is assignable to the
progression.
vibration member of the m
3
This work was supported by the Joint Studies
Program of the Institute for Molecular Science.
K.K. thanks Professor S. Matsui and Dr. Y.
Haruyama for their interest and encouragement in
the present study.
3
.3.8. 2p ! ndr Rydberg transition
The type of absorption transition responsible
�
1
for the 91 125 cm peak (o) cannot be determined
de®nitely from R value, due to the intense neigh-
�
1
boring 90 761 cm peak (n), which is assigned to
1
the 3 band of the 4r ! 3pr transition. This band
References
1
is assignable to the 2p ! 4drꢁ P† transition from
a quantum defect d ꢀ 0:44. The 91 878 (q) and
[
1] G.W. King, A.W. Richardson, J. Mol. Spectrosc. 21 (1966)
39.
[2] G.W. King, A.W. Richardson, J. Mol. Spectrosc. 21 (1966)
53.
�
1
93 127 (u) cm peaks are assignable to the vibra-
3
tional members of the m
pectively (see Table 2).
1
3
and m progression, res-
3
[3] M.T. Macpherson, J.P. Simons, J. Chem. Soc. Faraday
Trans. II 75 (1979) 1572.
[4] M.N.R. Ashfold, A.S. Georgiou, A.M. Quinton,
J.P. Simons, J. Chem. Soc. Faraday Trans. II 77 (1981)
4. Conclusions
259.
[
[
[
[
5] W.S. Felps, S.P. McGlynn, G.L. Findley, J. Mol. Spec-
trosc. 86 (1981) 71.
The high-energy lying states of ClCN was in-
vestigated by the measurement of the excitation
spectra and the polarization anisotropy of relevant
6] S.P. McGlynn, W.S. Felps, G.L. Findley, Chem. Phys.
Lett. 78 (1981) 89.
¯
uorescence by taking advantage of the linear
7] W.S. Felps, K. Rupnik, S.P. McGlynn, J. Phys. Chem. 95
(1991) 639.
polarization of synchrotron radiation. Almost
dominant absorption bands in the wavelength
range 105±145 nm were assigned on the basis of
the symmetry of the upper electronic state. The
8] K. Kanda, S. Katsumata, T. Nagata, Y. Ozaki,
T. Kondow, K. Kuchitsu, A. Hiraya, K. Shobatake,
Chem. Phys. 175 (1993) 399.
[
9] K. Kanda, S. Katsumata, T. Nagata, T. Kondow,
A. Hiraya, K. Tabayashi, K. Shobatake, Chem. Phys.
218 (1997) 199.
2
4
p ! 4p; 1p ! 5r intravalence transitions and
r ! nsr; 4r ! npr and 2p ! ndr Rydberg
[
10] J.A. Guest, M.A. O'Halloran, R.N. Zare, Chem. Phys.
Lett. 103 (1984) 261.
series were assigned ®rst in the present study.
Higher members of the 2p ! nsr; 2p ! npp
Rydberg series could be assigned up. The transi-
tions observed in the photoabsorption spectrum of
ClCN are consistent with those observed in the
[
[
11] M. Kono, K. Shobatake, J. Chem. Phys. 102 (1995) 5966.
12] W.C. Fernelius, Inorganic Syntheses, vol. 2, McGraw-Hill,
New York, 1946.
[13] L.C. Lee, Chem. Phys. 72 (1980) 6414.

3
78
K. Kanda et al. / Chemical Physics 255 (2000) 369±378
[
[
14] E. Heilbronner, V. Hornung, K.A. Muszkat, Helv. Chim.
Acta 53 (1970) 347.
15] R.F. Lake, H. Thompson, Proc. Roy. Soc. A 317 (1970)
[18] M.T. Macpherson, J.P. Simons, R.N. Zare, Mol. Phys. 38
(1979) 2049.
[19] A.B.F. Duncan, Rydberg Series in Atoms and Molecules,
Academic Press, New York, 1971.
1
87.
[
[
16] J.M. Hollas, T.A. Sutherley, Mol. Phys. 22 (1971) 213.
17] D.H. Whi€en, Spectrochim. Acta 34A (1978) 1173.
[20] F.G. Celii, J. Fulara, J.P. Maier, M. R o sslein, Chem. Phys.
Lett. 131 (1986) 325.