2
Electronic absorption spectra of C2
4 and C6 chains in neon matrices
Patrick Freivogel, Michel Grutter, Daniel Forney, and John P. Maier
Institut für Physikalische Chemie der Universität Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland
~Received 10 March 1997; accepted 27 March 1997!
2
2 1
2
The absorption spectra of the electronic transitions A 2 S 1
g ←X P g , B S u ←X P g ,
2
1
2
2
2
2
2
2
2
2
(2) P u ←X P g , and (3) P u ←X P g of C4 , as well as A S g ←X P u , (2) P g ←X P u ,
and (3) 2 P g ←X 2 P u of C2
6 have been obtained in neon matrices. The spectra were measured after
mass-selected deposition of the anions with excess of neon at 5 K. The assignments are based on
mass-selection, spectroscopic evidence, photobleaching behavior, and ab initio calculations.
Vibrational frequencies in the electronically excited states have been inferred. © 1997 American
Institute of Physics. @S0021-9606~97!01325-1#
I. INTRODUCTION
Neutral and ionic carbon molecules are of interest because of their importance in astrophysics, combustion processes, and soot formation.1 Especially the discovery of the
fullerenes has stimulated the research in the field of carbon
species. A number of papers appeared on the spectroscopy of
the neutral molecules.2 However, experimental data on C2
n
ions are scarce.
2
Ion mobility studies led to the conclusion that C2
5 –C9
2
2
3,4
are linear, but C10 –C30 comprise linear and cyclic forms.
Mass spectrometric investigations of laser vaporized graphite
proposed a structural transition from chain to cyclic between
2 5
6
C2
13 and C14 , consistent with reactivity studies. Consideration of the vibrational structure in the photoelectron spectra
of carbon anions showed that the most stable isomers of
C2
n are: linear chains (n55,7,9), monocyclic rings (2n
55 – 9), and bicyclic rings ~n520, 24, 28!.7,8 An excited
21
electronic state of C2
below the electron
6 lying only 43 cm
detachment threshold has been identified.9 The
2
2 1
B 2S 1
u ←X S g electronic transition of C2 in the gas phase
10
has been known for some time. The first new transition in
the gas phase, 2 P g ←X 2 P u of C2
5 , was observed by
resonance-enhanced multiphoton electron detachment.11 The
same approach detected the C 2 P←X 2 P transitions of
2
2
12,13
C2
following the report of
4 , C6 , and C8 in the gas phase,
these electronic spectra for the C2
2n (n52 – 10) chains in
neon matrices.14,15 Ab initio calculations are available on
16,17 2 18,19 2 20,21
electronically excited states of C2
C6 ,
C8 ,
and
4,
2 16
C10.
In this contribution a number of electronic absorption
2
spectra of C2
4 and C6 , trapped in solid neon have been observed and identified. The matrices were grown at 5 K by
2
codepositing mass-selected C2
4 or C6 with neon.
II. EXPERIMENT
The technique used combines mass-selection with matrix
isolation spectroscopy.22 The carbon anions are produced in
a cesium sputter source14,23 and accelerated to 50 eV kinetic
energy. The ions are selected with a quadrupole mass filter
and deposited together with excess of neon on a rhodium
coated sapphire plate held at 5 K. A mass spectrum scanned
22
J. Chem. Phys. 107 (1), 1 July 1997
prior to deposition shows beside CsC2
2 ~157 u! only pure
2
carbon anions C2
n up to the mass of C14. This allows the
resolution of the quadrupole filter to be set as 63 u, necessary to obtain sufficient ion currents which are typically 400
2
nA for C2
4 and 60 nA for C6 . A matrix of ;200 m m thickness is grown in ;5 h.
The absorption spectrum of the matrix is measured with
two techniques. In the 220–1100 nm region, the light of a
xenon arc or halogen lamp passes through a monochromator
and then traverses the 2 cm length of the matrix in a waveguide technique.24 A photomultiplier or silicon diode is used
to detect the beam. The near infrared (2000– 10 000 cm21)
is measured with a Fourier-transform spectrometer ~1 cm21
resolution!. The light is focused with a parabolic mirror onto
the matrix, reflected over a flat angle of ;15° to an elliptical
mirror, and finally led to a liquid nitrogen cooled InSb detector. Photobleaching experiments are carried out with a
high or medium pressure mercury lamp.
III. RESULTS AND DISCUSSION
2
C2
4 and C6 are mass-selected and codeposited with neon
on a 5 K substrate. The absorption spectra of these matrices
~Figs. 1 and 2! are dominated by the systems of the previously reported C 2 P←X 2 P electronic transitions of linear
C2
and C2
with origins at 456.7 and 607.6 nm,
4
6
respectively.15 Several new bands, which are specific to the
selected mass, are detected between 300 and 1300 nm. Some
of them overlap with the band systems of the
3 2
S u ←X 3 S 2
electronic transitions of linear C4
g
23
(;380 nm) and C6 (;511 nm). 14 Furthermore, bands due
2
1
to C3, C2, C1
2 , C2 , N2 , and CN2 appear in the spectra. These
derive from fragmentation processes during deposition and
from background gas.
Earlier studies with this approach showed that massselected species may undergo fragmentation but not combination reactions during the deposition process.25,26 The new
absorption systems are unique to the selected mass and bands
due to possible fragment molecules like C2n or C2n11 are
known from previous studies.15,27 Consequently, the systems
designated in Figs. 1 and 2 are attributed to molecules containing four, respectively, six carbon atoms.
0021-9606/97/107(1)/22/6/$10.00
© 1997 American Institute of Physics
2
Freivogel et al.: Absorption spectra of C2
4 and C6 chains
23
FIG. 2. Absorption spectrum of the electronic band systems of linear C2
6 .
The spectrum was recorded after mass-selected deposition with excess of
neon at 5 K.
FIG. 1. Absorption spectrum showing several electronic transitions of linear
C2
4 trapped in a 5 K neon matrix after mass-selection.
All these new bands disappear by irradiating the matrices with a medium pressure mercury lamp ~maximum energy
of ;5.4 eV!. The C 2 P←X 2 P absorption systems of C2
4
and C2
6 show the same behavior. On the other hand, the
3 2
bands due to the 3 S 2
u ←X S g transition of the neutral
chains C4 and C6 grow somewhat. No intensity changes are
observed for any of these bands after irradiation with a highpressure mercury lamp ~maximum energy ;4.9 eV!. The
electron detachment energies of these anions embedded in
solid neon derived from the photobleaching studies lie between 4.9 and 5.4 eV. In comparison the gas phase electron
affinity is 3.882 eV for C4 and 4.185 eV for C6. 28 Solvation
is known to increase the electron detachment energy of anions in a neon matrix by ;1 eV. Taking this into account the
detachment energies in the gas phase and neon-matrix are
comparable. In addition, all new systems connected with
2
2
C2
4 show a site structure similar to the C P u ←X P g electronic transition, which changes in unison on illumination
~Fig. 3!.29 Consequently, the systems are attributed to elec2
tronic transitions of linear C2
4 ~Fig. 1! and similarly for C6
~Fig. 2!.
the ground state16 ~Table I!. Hence, the dominant band at
926.9 nm ~1.34 eV! is attributed to the origin of the
2
2
B 2S 1
u ←X P g electronic transition of C4 . Built upon this
are vibrational progressions and combinations whose assign-
A. 2 S— X 2 P g transitions of C2
4
The absorption spectrum of C2
4 shows in the near infrared a complex vibrational pattern and Franck–Condon profile which can hardly arise from a single electronic transition
~Fig. 4!. An ab initio calculation predicts two electronic
2 1
states, A 2 S 1
g and B S u lying 1.072 and 1.388 eV above
FIG. 3. Site structure changes of the 0 00 band in the electronic transitions of
C2
4 in a 5 K neon matrix. The upper traces show the spectra before, the
lower after broad band uv irradiation from a high pressure mercury lamp.
J. Chem. Phys., Vol. 107, No. 1, 1 July 1997
2
Freivogel et al.: Absorption spectra of C2
4 and C6 chains
24
TABLE II. Observed absorption bands ~maxima 60.2 nm! in the
2
P u ←X 2 P g and 2 S←X 2 P g electronic transitions of linear C2
4 in a 5 K
neon matrix and the suggested assignment.
n (cm21)
l ~nm!
1205.9
1034.5
978.9
953.8
907.3
926.9
884.4
853.3
845.2
815.2
793.0
778.5
748.5
723.7
672.2
383.6
375.8
373.6
372.8
346.4
341.8
335.4
2
2 1
2
FIG. 4. Absorption spectrum of the A 2 S 1
g ←X P g and B S u ←X P g
electronic transitions of C2
in
a
5
K
neon
matrix.
Some
of
the
proposed
4
vibrational assignment is indicated; the vertical lines above the bands indicate the absorptions listed in Table II.
ments are given in Table II and outlined in Fig. 4. This yields
21
the
frequencies
n 1( s 1
n 2( s 1
g )52056(5) cm ,
g )
21
21
5930(4) cm , and n 5 ( p u )5259(2) cm in the B 2 S 1
u
state ~Table III!. These values are in good agreement with
the calculated harmonic frequencies v 1 52147 and v 2
5921 cm21. 16
0
2
The bands located to the red of the B 2 S 1
u ←X P g 0 0
transition are attributed to vibronic excitations within the
2
A 2S 1
g ←X P g system. This is dipole forbidden, but may
be allowed as a result of vibronic interactions. In particular
the 0 00 band will be absent, but transitions to vibronic states
of P u , S u , or D u symmetry will be allowed. A proposed
assignment of the absorption bands is as follows ~Table II!.
The first discernible band at 1205.9 nm is attributed to the
5 10 transition, with an upper state vibronic symmetry of
P u . The next band at 1034.5 nm is assigned to the 3 10 excitation with S 1
u vibronic symmetry. Three combination bands
due to the 3 10 5 20 , 3 10 2 10 , and 3 10 2 10 5 20 transitions are also discernible. This analysis yields the frequencies of the n 2 ( s 1
g )
5817(3) cm21 and n 5 ( p u )5275(2) cm21 modes in the
A 2S 1
g state ~Table III!. The harmonic frequency of v 2 was
calculated to be 878 cm21 in this state,16 which is in accord
2
with the assignment. The origin of the A 2 S 1
g ←X P g transition can be estimated by subtracting the frequency of the
n 5 mode (275 cm21) from the band at 1205.9 nm. This leads
to an origin lying at ;1247 nm (;1 eV), in agreement with
the ab initio calculations16,17 ~Table I!.
a
(3) 2 P u ←X 2 P g
(2) 2 P u ←X 2 P g
C 2 P u ←X 2 P g
2
B 2S 1
u ←X P g
2 1
A S g ←X 2 P g
a
Reference 16.
Reference 17.
b
T 0 /eV
neon matrix
;3.6
3.23
2.71
1.34
;1
T e /eV
calculation
2.802a
1.388a
1.072a
3.78b
2.91b
1.63b
1.24b
1
2
A S1
g ←X P g 50
1
30
3 10 5 20
3 10 2 10
3 10 2 10 5 20
0
2
B 2S 1
u ←X P g 00
5 20 (1C2)
2 10
5 40
2 10 5 20
2 20
1 10
1 10 5 20
1 10 2 10
1 20
(2) 2 P u ←X 2 P g 000
5 20
4 20
2 10
(3) 2 P u ←X 2 P g
2
a
a11374
a11923
a12191
a12729
0
518
930
1043
1478
1821
2056
2571
3029
4088
0
541
698
755
B. 2 P u — X 2 P g transitions of C2
4
The spectrum between 400 and 500 nm is dominated by
the C 2 P u ←X 2 P g electronic transition of linear C2
4 ~origin
at 456.7 nm! ~Fig. 1!.15 Several bands located below 390 nm
belong also to this anion, but cannot be part of the latter
transition. The origin of those bands is located at 383.6 nm
~3.23 eV! as can be seen in the bottom trace of Fig. 5. In the
middle trace the spectrum observed after removal of anions
by photobleaching the matrix with a medium pressure mercury lamp is displayed. It consists mainly of the
23
3 2
S u ←X 3 S 2
The trace at the
g band system of linear C4.
TABLE III. Comparison of experimental ~neon matrix! and calculated vibrational frequencies (cm21) of linear C2
4 in different electronic states.
X Pg
Transition
Assignment
a
Shift from the origin.
2
TABLE I. Comparison of the experimental excitation energies T 0 of linear
C2
4 with ab initio calculations T e .
8293
9667
10 216
10 484
11 022
10 789
11 307
11 719
11 832
12 267
12 610
12 845
13 360
13 818
14 877
26 069
26 610
26 767
26 824
28 868
29 257
29 815
Dn (cm21)
A 2S 1
g
B 2S 1
u
C 2P u
(2) 2 P u
a
Expt.
Calc.b
Calc.c
Expt.d
Calc.c
Expt.d
Calc.c
Expt.e
Calc.c
Expt.d
n 1( s 1
g )
n 2( s 1
g )
2047~20!
2084
2083
936~20!
893
911
817~3!
878
930~4!
921
759~5!
777
755~20!
a
Reference 29.
Reference 13.
c
Reference 16.
d
This work.
e
Reference 15.
b
J. Chem. Phys., Vol. 107, No. 1, 1 July 1997
2112
2056~5!
2147
1913
n 3( s 1
u )
n 4( p g)
n 5( p u)
1884
396~20!
505
240
275~2!
259~2!
349~10!
271~10!
2
Freivogel et al.: Absorption spectra of C2
4 and C6 chains
25
TABLE V. Observed absorption bands ~maxima 60.2 nm! in the
2
2
2
2
A 2S 1
g ←X P u and P g ←X P u electronic transitions of linear C6 in a
5 K neon matrix and the suggested assignment.
l ~nm!
FIG. 5. The top trace is the absorption spectrum measured after codeposition of mass-selected C2
4 with neon at 5 K. In the middle is the spectrum
after irradiation with a medium pressure mercury lamp. At the bottom a
subtraction of the middle spectrum from the top one is shown. The bands
1
connected with C2
4 and N2 point upwards, the C4 system downwards.
1069.3
1040.8
1017.6
999.7
887.8
872.4
862.2
852.7
840.8
838.6
835.1
826.0
759.8
748.5
737.6
498.4
491.0
486.9
484.0
478.5
470.4
444.1
438.0
432.1
417.8
410.2
401.2
n (cm21)
9 352
9 608
9 827
10 003
11 264
11 463
11 598
11 727
11 893
11 924
11 975
12 107
13 161
13 360
13 557
20 064
20 367
20 538
20 661
20 899
21 259
22 517
22 831
23 143
23 935
24 378
24 925
Dn (cm21)
0
256
475
651
1912
2111
2246
2375
2541
2572
2623
2755
3809
4008
4205
0
303
474
597
835
1195
0
314
626
1418
1861
2408
Assignment
0
2
A S1
g ←X P u 00
2
90
7 20 (1C2)
3 10
2 10
1 10
2
2 10 7 20
2 10 3 10
1 10 7 20
1 10 3 10
2 20
1 10 2 10
1 20
(2) 2 P g ←X 2 P u 000
9 20
7 20
3 10
3 10 9 20
3 20
(3) 2 P g ←X 2 P u 000
9 20
3 10
2 10
bottom is a subtraction of the C4 spectrum from the top one,
1
so that only the C2
4 and N2 bands remain pointing upwards.
An ab initio calculation predicts one allowed transition
in this energy range.17 It is the (2) 2 P u ←X 2 P g electronic
2
FIG. 6. Absorption spectrum of the A 2 S 1
g ←X P u electronic transition of
C2
6 measured after mass-selected deposition with excess of neon at 5 K.
Some of the proposed vibrational assignment is indicated; the vertical lines
above the bands indicate the absorptions listed in Table V.
TABLE IV. Comparison of experimental and calculated excitation energies
T 0 and T e of linear C2
6 .
Transition
(3) 2 P g ←X 2 P u
(2) 2 P g ←X 2 P u
C 2 P g ←X 2 P u
2
B 2S 1
u ←X P u
2 1
A S g ←X 2 P u
a
Reference 18.
Reference 19.
b
T 0 /eV
neon matrix
2.79
2.49
2.04
1.16
T e /eV
calculation
2.120a
1.353a
1.313a
3.68b
3.06b
2.62b
2.20b
2.15b
FIG. 7. The absorption spectrum after deposition of mass-selected C2
6 with
neon is reproduced at the top. The middle trace is observed after photobleaching with a medium pressure mercury lamp. The spectrum at the
bottom is a subtraction of the middle from the top one.
J. Chem. Phys., Vol. 107, No. 1, 1 July 1997
2
Freivogel et al.: Absorption spectra of C2
4 and C6 chains
26
TABLE VI. Comparison of experimental ~neon matrix! vibrational frequencies (cm21) of linear C2
6 in different electronic states with calculations.
X Pu
2
A 2S 1
g
C 2P g
(2) 2 P g
(3) 2 P g
a
a
Calc.
Calc.b
Expt.c
Calc.a
Expt.d
Calc.a
Expt.c
Expt.c
n 1( s 1
g )
n 2( s 1
g )
n 3( s 1
g )
2145
2171
2111~4!
2179
2064~5!
2189
1804
1823
1912~4!
1948
1817~5!
1805
637
628
651~3!
624
607~5!
599
597~12!
626~15!
1861~17!
n 4( s 1
u )
n 5( s 1
u )
n 6( p g)
n 7( p g)
n 8( p u)
n 9( p u)
1943
1167
540
261
238~2!
437
119
128~2!
237~6!
c
Reference 18.
Reference 13.
b
152~6!
157~7!
This work.
Reference 14.
d
transition with an excitation energy of 3.78 eV. Thus, the
absorption system near 380 nm is attributed to this electronic
transition. A suggested assignment of the vibrational structure is given in Table II. The inferred frequencies of the
modes excited are given in Table III.
Another set of bands, also associated with C2
4 , appears
in the 330–350 nm region ~Fig. 5!. These are likely to be
part of another electronic transition, which we label as
(3) 2 P u ←X 2 P g .
2
2
C. A 2 S 1
g — X P u transition of C6
After mass-selection and deposition of C2
6 a new band
system is detected between 700 and 1100 nm ~Fig. 6!. In
contrast to C2
4 the bands can be assigned to a single electronic transition. Calculations predict one allowed electronic
2
transition, A 2 S 1
g ←X P u , in this energy range at 1.313 eV
18,19
Consequently, the observed system is attrib~Table IV!.
uted to this electronic transition. A proposed assignment of
the vibrational structure is given in Table V and outlined in
Fig. 6. Three symmetric stretching frequencies inferred for
1
1
the A 2 S 1
g state, n 1 ( s g )52111(4), n 2 ( s g )51912(4), and
1
21
n 3 ( s g )5651(3) cm , agree with the calculated harmonic
values v 1 52179, v 2 51948, and v 3 5624 cm21 ~Table
VI!.18 Moreover, the assignment yields two bending frequencies, n 7 ( p g )5238(2) cm21 and n 9 ( p u )5128(2) cm21.
D. 2 P g — X 2 P u transitions of C2
6
In the visible range of the spectrum the previously identified C 2 P g ←X 2 P u electronic transition of C2
6 is seen
~Fig. 2!.14 The bands apparent below 500 nm are not part of
this band system because they do not fit the vibrational pattern neither in spacings nor in intensities. The top trace of
Fig. 7 shows this part of the measured spectrum in detail,
whereas the middle spectrum is observed after removal of
the anions in the matrix by photobleaching. It consists of the
14
3 2
S u ←X 3 S 2
The botg electronic transition of linear C6.
tom trace is a subtraction of the C6 spectrum from the spectrum at the top by using an appropriate scaling factor.
The first prominent band in the spectrum, at 498.4 nm
~2.49 eV!, is the origin of a new band system. This is presumably the (2) 2 P g ←X 2 P u transition which an ab initio
calculation predicts at 3.06 eV ~Table IV!.19 A proposed vibrational assignment of the bands is given in Table V. Three
vibrational modes n 3 ( s 1
g )5597(12), n 7 ( p g )5237(6), and
n 9 ( p u )5152(6) cm21 in the excited electronic state are inferred from the spectrum. The band at 501.4 nm is probably
the 2 20 transition of the C 2 P g ←X 2 P u system. It has been
observed in the gas phase at 501.50 nm in a multiphoton
13
detachment spectrum of C2
as have two bands that are
6,
0
1
now assigned as 0 0 and 3 0 transitions ~Fig. 7!.
A further electronic band system is apparent below 450
nm, with origin band at 444.1 nm ~2.79 eV!. This band is
also seen in the gas phase spectrum at 445.87 nm.13 An ab
initio calculation predicts the (3) 2 P g excited state to lie
0.62 eV above the (2) 2 P g state ~Table IV!.19 Thus, the
band system is attributed to the (3) 2 P g ←X 2 P u transition
and a vibrational assignment is given in Table V. The vibrational structure corresponds to the excitation of n 2 ( s 1
g )
21
51861(17), n 3 ( s 1
)5626(15),
and
n
(
p
)5157(7)
cm
9
u
g
modes.
IV. CONCLUSIONS
2
Several electronic band systems of C2
4 and C6 have been
detected in the 300–1300 nm range using the approach of
mass-selection in combination with neon matrix absorption
spectroscopy. The electronic transitions of these anions have
been assigned and vibrational frequencies of several modes
in the electronically excited states inferred. With these data
in hand, gas-phase measurements of these transitions should
follow.
ACKNOWLEDGMENT
This work is part of project No. 20-41768.94 of the
Swiss National Science Foundation.
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