Laser induced fluorescence spectroscopy of AlNC/AlCN in supersonic free expansions

Article abstract of DOI:10.1016/S0009-2614(97)01408-5

We have generated AlNC/AlCN in supersonic free expansions and measured the laser induced fluorescence (LIF) spectra. Prior to the experiments, ab initio calculations were carried out to obtain information necessary for the LIF spectroscopic searching. The

Full text of DOI:10.1016/S0009-2614(97)01408-5

13 February 1998
Ž
.
Chemical Physics Letters 283 1998 337–344
Laser induced fluorescence spectroscopy of AlNCrAlCN in
supersonic free expansions
Masaru Fukushima 1
AdÕanced Technology Research Center, Mitsubishi HeaÕy Industries, LTD., Sachiura, Kanazawa, Yokohama 236, Japan
Received 24 October 1997; in final form 3 December 1997
Abstract
We have generated AlNCrAlCN in supersonic free expansions and measured the laser induced fluorescence LIF
Ž
.
spectra. Prior to the experiments, ab initio calculations were carried out to obtain information necessary for the LIF
spectroscopic searching. The dispersed fluorescence spectra from the single vibronic level show two kinds of vibronic bands;
isolated sharp bands and congested bands. It is concluded that the sharp features are attributed to the progressions localized
in the vibrational normal coordinates of AlNC, while the congested structures are ascribed to vibronic transitions terminating
in the vibrational levels on the isomerization coordinate of AlNC l AlCN. q 1998 Elsevier Science B.V.
1. Introduction
AlNCrAlCN system is quite small; the molecules
have never been observed experimentally by any
w x
Much attention has been devoted to the AlNC and
AlCN molecules from both astronomical and reac-
tion dynamical points of view. Astronomically,
AlNCrAlCN is considered as one of the most proba-
ble candidates for the carrier of unidentified mi-
crowave molecular lines observed toward the cir-
cumstellar envelope of the carbon-rich mass-losing
spectroscopic method 3 and a few theoretical stud-
ies have been recently reported for the ground states
w
x
1,2,4,5 In the present Letter, we report the spectro-
scopic observation of the AlNC and AlCN molecules
and some ab initio results for both the ground and
excited states, which were performed to predict spec-
troscopic information for the spectral search.
w x
star IRCq10216 1 . With regard to reaction dy-
namics, calculations suggest that the AlNCrAlCN
w x
system has a relatively low isomerization barrier 2
2. Experiment and computation
and the system is thought to be a suitable example to
explore the isomerization reaction of A–BC l
BC–A type, i.e the bond rearrangement reaction
along the ground state potential surface In spite of
the interest, the number of investigations of the
Most of the experimental apparatus employed in
this study was the same as that reported previously
w
x
6–8 . The only difference is the generation method
of the transient species; laser ablation was used to
generate AlNCrAlCN, while ArF laser photolysis or
pulsed discharge was used in previous work. We
1 E-mail: fukushim@atrc.mhi.co.jp.
w x
used a laser vaporization fixture of the usual type 9 .
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
Ž
.
PII S0009-2614 97 01408-5

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)
338
M. FukushimarChemical Physics Letters 283 1998 337–344
The fixture with a gas flow channel, 6= 13 mm,
was mounted on the orifice flange of a pulsed valve
and a 6 mm diameter Al rod was rotated at 5 mm
downstream under the valve orifice in the fixture.
The Ar gas containing CH3CN at the saturated vapor
pressure at room temperature was expanded from the
valve orifice, diameter 800 mm, into a vacuum
chamber through the channel at 10 Hz and synchro-
nized with the gas flow pulse, the second harmonic
10000rXL, Silicon Graphics, with sixteen R10000
.
CPUs belonging to the ATRC computational physics
group.
3. Computational results
1
q
˜
Only the ground state X S had been computed
w
x
in previous works on AlCNrAlNC 1,2,4,5 and
information on the excited state was scarcely avail-
able. Prior to the experiments, some ab initio calcula-
tions were carried out utilizing the GAUSSIAN 94
of a pulsed Nd3q YAG laser Spectron, SL284G of
about 40 mJ was used to irradiate the metallic Al
surface. The molecules were generated in an Ar
plasma produced by the laser ablation of Al. The
fragments from CH3CN in the Ar plasma were used
as CN sources and Al was supplied from the laser
vaporization of the Al surface. At about 40 mm
downstream under the end surface of the fixture, the
Ž
.
w
x
program package 10 , to obtain some information
for the optical spectrum search. The calculations
were carried out at four levels of theory; coupled
Ž
.
cluster CC methods using split valence polarized
Ž .
and triple zeta polarized basis sets, CCDr6-31G d
Ž .
and CCDr6-311G d , respectively and quadratic
Ž
.
Ž
.
laser induced fluorescence LIF was observed by
exciting with the second harmonic of the output from
a pulsed Nd3q YAG laser, a pumped dye laser
configuration interaction QCI methods using a triple
zeta polarized and diffuse basis set, QCISDr6-311
Ž
.
Ž .
Ž
.
qG 3fd and QCISD T r6-311qG 3fd . The com-
puted state energies and molecular geometries are
listed in Tables 1 and 2, respectively and the ener-
gies are schematically shown in Fig. 1 with the
Ž
Continuum, NY-82 and Lambda Physik,
.
Ž
FL3002CSE . A 0.5 m monochromator Spex, M-
500 was used to disperse the LIF. The time profiles
.
w x
of the LIF were observed and recorded by a digital
isomerization potential barrier of the ground state 2 .
Ž
.
oscilloscope Tektronix, TDS744 . Acetonitrile
The results for the ground state are consistent with
1
q
Ž
.
w
x
Wako Pure Chemical, Special Grade and the Ar
the previous reports 1,2,4,5 ; S states are calcu-
lated as the electronic ground state for both AlNC
and AlCN and linear geometries are found to be the
equilibrium nuclear configurations of both species in
Ž
.
carrier gas Nippon Sanso, Research Purity were
used without further purification. An aluminum rod
for industrial use was used. Computations were per-
1
q
˜
Ž
formed on
a
workstation Power Challenge
the X S states. AlNC is the lowest energy species
Table 1
The computed total and relative energies for AlNC and AlCN
1
3
X
1A
q
˜
Calculation level
X S
P
AlNC
AlCN
AlNC
AlCN
AlNCrAlCN
Ž .
CCDr6-31G d
y334.535530
y334.528402
1564.5
y334.433136
y334.442252
20472.4
y2000.8
y334.490895
20768.9
y1665.6
y
y334.399840
29780.7
7307.5
y334.448081
30165.5
7370.9
a
b
c
a
b
c
a
b
a
b
0
22473.2
y
y
0
Ž .
CCDr6-311G d
y334.585525
y334.577181
1831.2
y334.483306
22434.6
0
y
y
0
y
y
y
y
Ž
.
QCISDr6-311qG 3df
y334.647638
y334.637442
2237.6
y334.513772
29380.3
y
0
y
Ž .
Ž
.
QCISD T r6-311qG 3df
y334.663303
y334.654281
1980.0
y
y
0
y
a The computed total energies in hartree.
b
1
Relative energies measured in cmy1 from the lowest state X S of AlNC.
q
˜
c Relative energies measured in cmy1 from the lowest triplet state P of AlCN.
3

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M. FukushimarChemical Physics Letters 283 1998 337–344
339
Table 2
The optimized ab initio geometries for AlNCrAlCN
a
rAl – N
rAl – C
rC – N
/C–Al–N
rAl – x
/Al–x–N
1
q
˜
the X S state of AlNC
1.8639
3.0516
1.1877
0.0
2.4120
0.0
b
b
1
q
˜
the X S state of AlCN
3.1927
2.0215
1.1712
0.0
2.2599
2.116
180.0
102.9
0.0
the transition state on the isomerization coordinate, AlNC l AlCN
w x
2.307
2.060
1.175
30.6
Ref. 2
the 3P state of AlNC
1.8197
3.0071
1.9414
1.1874
1.1576
1.2238
0.0
2.3676
2.2564
1.8511
c
c
d
the 3P state of AlCN
3.0991
0.0
180.0
86.2
X
the 1A state of AlNCrAlCN
1.8946
2.0108
36.3
a rAl – N : the Al-N bond length, rAl – C : the Al–C bond length, rC – N : the C–N bond length, /C–Al–N: the C–Al–N bond angle, rAl – x: the
Ž
.
distance between Al and the center-of-mass of the CN part x , /Al–x–N: the angle between the CN bond axis and the Al-x line.
b
Ž .
Ž
.
QCISD T r6-311qG 3df level.
c
Ž .
CCDr6-311G d level.
d
Ž
.
QCISDr6-311Gq 3df level.
and the isomer AlCN is predicted to lie about 2000
cmy1 above AlNC. The vibrational frequencies for
the three vibrational normal modes of the C–N
stretching, n1 Al–NC, or Al–CN stretching n2 and
Al–N–C or Al–C–N bending vibrations n3, in the
using a numerical procedure. The normal modes are
illustrated graphically in Fig. 2. The calculated fre-
quencies are consistent with previous calculations;
similar frequencies of the three normal modes for
AlNC and AlCN are computed and similar trends,
AlCN ) AlNC for the n1 and n2 modes and AlCN
- AlNC for the n3 mode, are also reproduced. In
addition to the ground state, we have obtained pre-
dictions of the excited states. For the excited state
computation, an electronic configuration resulting
from single excitation to the lowest unoccupied
1
q
˜
X S states are calculated as shown in Table 3,
Ž
.
molecular orbital LUMO from the highest occupied
Ž
.
molecular orbital HOMO was used as the initial
configuration, where the LUMO and HOMO had s
X
XX
Ž .
Ž
.
A and p A symmetries in the ground configura-
tion, respectively and thus the lowest electronic state
XX
A
Ž
.
with P
symmetry was taken as the excited
state. The computed results for the excited states are
3
also listed in Tables 1 and 2. The P states, lying
1
about 22500 cmy1 above the X S states, are
q
˜
1
3
1 X
q
˜
Fig. 1. The schematic energy diagram of the X S , P and A
states of AlNCrAlCN computed in this work and of the isomer-
predicted as the lowest triplet states of both AlNC
and AlCN. states, linear geometries are suggested as
w x
ization potential barrier of the ground state 2 .

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340
M. FukushimarChemical Physics Letters 283 1998 337–344
the equilibrium conformations. However, different
from the ground states, AlCN is suggested as the
lower species in the triplet states with AlNC about
1650 cmy1 above AlCN. The triplet state computa-
Ž .
tion was restricted to the CCDr6-31G d level, be-
cause of accessible machine time. The excited singlet
1
X
state is predicted to be the A, lying about 7300
3
cmy1 above the P states and about 29800 cmy1
1
q
˜
˜
above the X S states. Different from the linear X
1
3
Sq and P states, the prediction indicates a signifi-
cantly bent geometry of the A state and suggests
that the A state corresponds to a common excited
singlet state both for the X S states of AlNC and
1
X
1
X
1
q
˜
AlCN From the Mulliken population analysis, the
excited geometry seems to be T-shaped with an ionic
chemical bond between Alq and the triple bond of
the CNy moiety, while the transition state geometry
on the isomerization path in the ground state is a
bent with a terminal N atom and with a chemical
Fig. 2. The computed vibrational normal modes of AlNCrAlCN
1
1
X
1
q
q
˜
˜
in the X S and A states. The modes of AlCN in the X S
state are identical to the illustrations of AlNC after substitution of
w x
bond between Al and C 2 . The T-shaped geometry
C and N. x denotes the center-of-mass of the CN part and the
is one of the most noticeable characteristics of
AlNCrAlCN, since linear geometries of related
molecules, such as MgNCrMgCN, have been pre-
1
X
dashed lines in the A state denote the axis both through Al and
x.
w
x
dicted in the ground and excited states 11 . The
excited state computation was confined to the QCI
calculation including single and double excitations
QCISD, while triple excitations were included for
easy to recognize in Fig.2. According to the blend-
ing, it is noticed that Jacobi internal coordinates are
better for describing the vibrational motion of
AlNCrAlCN, rather than Cartesian internal coordi-
nates. The blending furthermore indicates diversified
vibronic structure in the electronic spectrum.
Ž .
the ground state computation QCISD T . The com-
puted vibrational frequencies in the lowest singlet
excited state 1A, are listed in Table 3, and the normal
X
modes are illustrated in Fig. 2. As expected from the
difference of the molecular geometries between the
1
1 X
q
˜
X S and excited A states, strong blending of the
normal modes between the states has been suggested
by normal mode analysis; the blending between the
nX2X and nX3X modes corresponding to nX2 and nX3 is
4. Results from experiment
As a result of several attempts to search the
spectrum utilizing the predicted information, LIF
signals were observed in the UV region. Fig. 3
shows the major part of the observed LIF spectrum.
The notation of the vibronic bands in Fig. 3 indicates
the energy spacing measured from the origin band,
i.e.the lowest and strongest band at 28752.7 cmy1 The
electronic. transition energy observed is close to the
predictions in Table 1. The LIF signals were only
observed when the Al surface was ablated by the
second harmonic of the Nd3q YAG laser, but not the
Table 3
y1
Ž
.
for AlNC and AlCN
The calculated vibrational frequecies cm
State
Species
v1
v2
v3
1
q
˜
X S
AlNC
AlCN
2105.2
2222.9
1667.2
554.2
469.9
549.5
111.9
168.3
421.2
X
1A
AlNCrAlCN
n1: C–N stretching.
n2: Al–NC or Al–CN stretching.
n3: Al–N–C or Al–C–N bending.
QCISDr6-311qG 3df level.
2
q
˜
˜
stainless steel surface. The LIF of the B S – X
2Sq transition of the CN radical was also observed
in both the Al and steel cases, though the steel gave
a
Ž
.

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M. FukushimarChemical Physics Letters 283 1998 337–344
341
cannot identify the spectral carrier from the rota-
tional structure. The time profiles of the LIF signals
follow that of the excitation laser pulse, about 10 ns
width at FWHM and the signals are too short to
evaluate the fluorescence lifetime under our experi-
mental conditions. The order of the fluorescence
lifetime is estimated to be less than a ns.
To identify the spectral species, we measured the
Ž
.
dispersed fluorescence DF spectra from the single
Ž
.
vibronic level SVL by fixing the excitation laser
frequency at a certain vibronic band in the excitation
Ž .
spectrum. Fig. 4 a shows the SVL DF spectrum
obtained by the excitation of the origin band. We
find that there are two kinds of vibronic bands with
respect to their band shape in the spectrum; one is
sharp and the other congested. The vibrational struc-
ture in the sharp bands is easily interpreted in three
parts; the first part, which is the major structure, is
dominated by a progression with a frequency of
about 520 cmy1, the second is a minor progression
with the same 520 cmy1 interval starting from the
1974 cmy1 band and the last is 126 and 250 cmy1
bands associated with the major 520 cmy1 progres-
sion. The vibrational structure constituting the sharp
Fig. 3. The LIF excitation spectrum of AlNCrAlCN observed in
the supersonic free expansions of Ar. The numbers on the vibronic
bands denote the energy spacing measured from the lowest and
strongest vibronic bands at 28753.7 cmy1
.
more intense and higher temperature CN signals than
Al. The LIF signals of the Al dimer are weakly
observed in our spectrum, but our signals do not
w
x
appear in the Al2 spectrum 12,13 , where no trace
of CH3CN was used and where the fundamental of a
Nd3q YAG laser was used as the vaporization laser.
Unfortunately, we could not examine the effect of
CH3CN on the LIF signals by ourselves, because
stable Ar plasma produced by the laser ablation on
the Al surface could not be obtained under our
vaporization laser power condition, when pure Ar
was used instead of our gas sample, i.e.Ar with
saturated CH3CN vapor. Based on the appearance
conditions of our signals, it is chemically concluded
that the carrier of the signals is likely to be any
species containing Al and the fragments from
CH3CN. The vibronic structure of the spectrum is
too complicated to find clear regularity. However, it
is clearly recognized that the rotational contours of
the vibronic bands are all shading to the blue. The
rotational contours indicate a larger B rotational
constant in the upper state than the lower, which is
consistent with our theoretical predictions, i.e.. the
upper state with T-shaped geometry and the lower
with linear. The contour is partly reproduced by that
simulated using the predicted rotational constants.
The rotational structure of the vibronic bands is not
resolved even at the excitation laser resolution 0.04
cmy1, of our preliminary measurements, thus we
w
x
bands was analyzed using the general formula 14 ;
G0 Õ ,Õ ,Õ
s
.
v0 Õ q
x0 Õ Õ
Ž
Ý
Ý Ý
i kGi
1
2
3
i
i
ik i k
i
q
g0 l l .
Ý Ý
kGi
ik i k
i
The vibrational constants obtained from the analysis
are listed in Table 4. Due to the lack of assigned
vibronic bands, the constants obtained are restricted.
The constants of the three normal vibrational modes
show good agreement with those of AlNC predicted
by the ab initio computations listed in Table 2. The
nX2X mode with the constant v20 of 523.5 cmy1, of
which the major progression is composed, is as-
signed to the stretching vibrational mode between Al
and the CN group. The n1XX mode with a constant v10
of 1974.5 cmy1, which appears as the beginning of
the minor nX2X progressions, is assigned to the C–N
stretching vibrational mode and the nX3X mode with a
low frequency v30 of 131.9 cmy1, which constructs
the progression associated with the major nX2X pro-
gression, is interpreted as the Al–N–C bending vi-
brational mode. Enhanced intensity of the even over-
tones of the nX3X progression concurs with the assign-

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M. FukushimarChemical Physics Letters 283 1998 337–344
by the origin band excitation, appear in the spectra
obtained by the excitation of the q107.4, q391.0
and q643.9 cmy1 bands. Not only the band posi-
tions but also the band shapes of the vibronic bands
observed in these SVL DF spectra agree with those
in the origin band spectrum, though the intensity
distributions drastically depend on the fluorescent
levels. The vibrational structures of the spectra are
analyzed using a similar procedure to the origin band
spectrum described above. In the SVL DF spectrum
obtained by the excitation of the q954.7 cmy1 band
Ž .
as shown in Fig. 4 b , the low frequency bending
mode progressions with the sharp feature show much
stronger intensities than those in the origin spectrum.
Ž .
Fig. 4 a and appear as the major progression, but a
short progression consisting of, at most, four quanta
is observed. In the SVL DF spectrum, four vibronic
bands with moderate intensity in the lower energy
region are only attributed to the sharp band group
only. The assigned vibronic bands categorized to the
sharp band group are all included in estimating the
vibrational constants listed in Table 4. The other
noticeable features found in the analysis of the other
SVL DF spectra measured are:
2
1. the two Al atomic lines assigned to an S – 2 P
transition are observed in the dispersed fluorescence
spectra obtained by the excitation at q1879.5,
q2305.0 and q2857.5 cmy1. The existence of dis-
sociation levels correlating to the excited Al atom
2 S, around the T-shaped excited state of AlNCrAlCN
is understandable, because the atomic transitions are
25374.69 and 25235.65 cmy1. The existence is pre-
Fig. 4. The SVL dispersed fluorescence spectra obtained by the
excitation of the origin
and q945.7 cmy1 bands b . The
Ž . Ž .
a
abscissa represents the energy measured from the each excitation
wavenumber, 28752.7 and 29698.4 cmy1, respectively. The as-
signment solid lines indicate the vibronic bands categorized to the
sharp band group and assigned to the transitions terminating to a
specific vibronic level of AlNC. The assignment dashed lines
signify the progressions of the vibronic bands grouped to the
congested band.
Table 4
The vibrational constants cm
1
y1
q
˜
Ž
.
of the AlNC X S state
ment of nX3X to the degenerate bending vibrational
mode. On the basis of this, we therefore identify the
species observed in the LIF excitation spectra as to
AlNCrAlCN. In contrast with the sharp vibronic
bands, the analysis of the structure in the congested
vibronic bands is not straightforward and only the
Al–NC stretching progressions starting from the 1360
and 2710 cmy1 bands have been recognized.
a
b
Constants
This work
Ab initio
v10
v20
v30
x202
x303
x102
x203
1974.5 1.5
2105.2
554.2
111.9
Ž
.
.
.
.
.
.
.
Ž
523.5 0.7
Ž
131.9 1.3
Ž
y2.1 0.1
Ž
y2.2 0.4
Ž
0.5 0.6
Ž
6.5 0.6
n1: C–N stretching.
n2: Al–NC stretching.
n3: Al–N–C bending.
The two different types of the vibronic bands, i.e.
the sharp and congested bands, also appeared in the
other SVL DF spectra. Most of the vibronic bands
a Values in parentheses denote one standard deviation
b
Ž .
Ž
.
observed in the SVL DF spectrum Fig. 4 a , obtained
QCISDr6-311qG 3df level.

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)
M. FukushimarChemical Physics Letters 283 1998 337–344
343
tional frequencies predicted for the nX2 and nX3 modes
of the A state are much larger than those deter-
w
x
dicted in the high level calculation under way 15 .
Thus, this finding indicates the opening of the pre-
dissociation channel in the excited state.
1
X
1
mined for the bending mode nX3X of the X S state
q
˜
and these two modes, nX2 and nX3, are considered to
2. The vibrational structure attributed to fluores-
cence to the ground electronic state of AlCN is
observed in some of the SVL DF spectra obtained by
the excitation of the vibronic bands with energy
higher than 2300 cmy1 from the origin band. This
observation indicates that the T-shaped excited state
has an electronic transition moment to the ground
states both of AlNC and AlCN, but the lower portion
of the vibrational levels of the state has more prefer-
able Franck-Condon factors to AlNC than to AlCN.
The other local minimum of the excited state, which
has favorable Franck-Condon activity to AlCN, may
be implied.
1
q
˜
share most of the projection of the X S state
1
X
bending mode in the A state. The second reason is
acceptable, when we consider the bending coordinate
of the A state as the linear transformation from the
1
X
1
q
˜
mode of the X S state, which is not actually a
1
X
normal mode in the A stateactually; if we take the
Jacobi internal coordinate for AlNCrAlCN, the dras-
tic shift is more easily realized, because the angle
Al–x–N, i.e. the CN orientation to the axis both
through Al and the center-of-mass of CN, has a
difference of almost pr2 between the X S and 1A
states as seen in Table 2. The fairly long and highly
harmonic progressions of the Al–NC stretching mode
and of the mode combined with the C–N stretching
can be seen in the vibronic structures of the SVL DF
spectra. The energies of the highest vibrational levels
of the observed progressions are more than the calcu-
lated energy gap 2000 cmy1, between the isomers,
AlNC and AlCN and besides the energy of the latter
combination progression is comparable with the pre-
1
X
q
˜
5. Discussion
From the predicted geometries of AlNC and AlCN
in the X S states listed in Table 2, a large contri-
1
q
˜
bution of both the bending and the Al–NC stretching
normal modes to the isomerization reaction coordi-
nate of AlNC l AlCN would be expected, because
the isomerization needs the intramolecular rotation
of CN associated with the rAl – x stretching. The
necessity of the stretching is concluded from the
dicted isomerization barrier, 4600 cmy1 2 . The two
w x
normal modes are therefore considered to be rela-
tively isolated from the isomerization reaction coor-
dinate, unless the two modes combine with the bend-
ing nX2X mode.
˜
relatively large differences among the rAl – x of the X
1
Sq states of AlNC and AlCN and of the transition
state as listed in Table 2, while the necessity of the
CN rotation is not in question. The fairly large and
positive value of x23 indicates strong coupling be-
tween nX2X and nX3X.
The appearance of the congested band with the
Al–NC stretching mode progressions in the SVL DF
spectra is one of the most remarkable results in this
work. For the source of the congested bands, we
have two interpretations; one taking the interaction
within the vibrational normal coordinates of AlNC
into account, and the other including the considera-
tion of the isomerization coordinate, AlNC l AlCN,
in addition to that of the normal coordinates. The
first interpretation is that the congestion is because
of anharmonic resonance among the vibrational lev-
els within the normal modes of AlNC. The basis of
The minor structures of the Al–N–C bending
progression in the observed SVL DF spectra of the
bent-linear transition are understandable, though gen-
erally bent-linear transitions show major bending
progressions. We think that the weakness of the
bending mode comes from the weak Franck-Condon
activity due to the strong blending of the normal
1
1 X
q
˜
modes between the X S and excited A states as
mentioned above. The concrete reasons are the fol-
the interpretation is that the frequency the Al–NC
y1
Ž .
Ž
.
lowing; 1 the large difference in the spread of the
stretching n2 mode 523.5 cm , is close to four
1
X
vibrational wavefunctions between the X S and 1A
q
˜
Ž
quanta of the Al–N–C bending n3 mode 131.9
cmy1 . Therefore, anharmonic resonance among the
Ž .
.
states and 2 the drastic phase shift of the wavefunc-
Ž
.
tions, in other words, the drastic shift of the origin,
0,m,Py4m levels with ms0,1,... Pr4, where
Ps4Õ2 qÕ3, can be expected. For example, in the
1
X
of the bending coordinate between the X S and 1A
q
˜
states. The first reason is expected, since the vibra-
SVL DF spectrum obtained by the excitation of the

(
)
344
M. FukushimarChemical Physics Letters 283 1998 337–344
Ž .
origin band as show in Fig. 4 a , the lowest con-
reaction, AlNC l AlCN. To confirm our under-
standing, we are now undertaking further experi-
ments, such as measurements of the high-resolution
LIF excitation spectrum and of rotationally resolved
spectra of the SVL DF spectra using stimulated
emission pumping.
gested vibronic band at 1364 cmy1 is attributed to
Ž
.
Ž
.
Ž
.
0,0,10 l 0,1,6 l 0,2,2 with Ps10 and the
next band at 1890 cmy1 is attributed to 0,0,14 l
Ž
.
Ž
.
Ž
.
Ž
.
0,1,10 l 0,2,6 l 0,3,2 with Ps14 and so
Ž
.
on. Since the 0,m,Py4m levels with Ps6, which
is the vibrational level just lower than that with
Ž
.
Ps10, consists of only the two levels, 0,0,6
l
Acknowledgements
Ž
.
0,1,2 , no observation of the levels as the congested
band, but as the sharp band, can be understood by
the some number of the levels necessary to induce
the resonance. But we are not able to answer the
MF greatly thanks M. Soda, T. Wada and M.
Kato for providing the favorable research environ-
ment at ATRC and the members of the computa-
tional physics group of ATRC especially K. Satake
and Y. Inoue, for helpful support operating the com-
puter. This research has been supported as part of the
research program, R100-108100, of the ATRC.
Ž
selectivity of the resonance, i.e. why the 0,m,Py
.
4m levels with Ps4nq2, where n is an integer,
appear specially as congested bands, while other
Ž
.
series characterized by 0,m,Py4m do not appear;
for example, the Al–CN stretching mode n2 pro-
gression should show congestion, since the progres-
Ž
.
sion can be expressed as 0,m,Py4m with Ps4n,
where n is an integer. The second explanation is that
the congestion comes from not only the resonance
within the AlNC levels, but also the level mixing
with the isomer AlCN levels. Since it is expected
that the relation between the bending coordinates and
the isomerization reaction coordinate is relatively
strong, it is considered that, when the level energy is
close to the isomerization barrier, the bending vibra-
tional level is strongly perturbed with the levels of
the isomer, AlCN. It is then understood that the
congested bands do not exist in the energy region
lower than the predicted isomer level, about 2000
cmy1, but appear at energies close to that level.
Since the excited state geometry is close to the
transition state geometry of the isomerization reac-
tion, AlNC l AlCN, rather than to the ground state
linear geometries of AlNC and AlCN, favorable
Franck-Condon factors and significant transition mo-
ments would be expected for the transition terminat-
ing to the levels on the reaction coordinate. We
therefore think that the advantageous conditions make
it possible to observe the curious vibronic levels on
the reaction coordinate using the LIF technique. Why
the congested bands show the n2 progression with
high harmonicity is still under question.
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w x
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.
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Ž
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