electronic spectroscopy of long carbon chains hc 2n h (n= 8-13) in the gas phase
DESCRIPTION
+. -. HC. C. C. CH. -. C. HC. C. C. HC. C. C. CH. -. C. C. C. HC. -. +. CH. HC. C. C. Electronic Spectroscopy of long Carbon Chains HC 2n H (n= 8-13) in the Gas Phase. Felix Güthe*, Hongbin Ding, Thomas Pino and John P. Maier - PowerPoint PPT PresentationTRANSCRIPT
2 8 0 2 9 0 3 0 0 3 1 0 3 2 0 3 3 0 3 4 0 3 5 0 3 6 0W a v e l e n g t h ( n m )
H C2 6
H
H C1 8
H
H C2 0
H
H C2 2
H
H C2 4
H
H C1 6
H
Electronic Spectroscopy of long Carbon Chains HCElectronic Spectroscopy of long Carbon Chains HC2n2nH (n= 8-13) in the Gas PhaseH (n= 8-13) in the Gas PhaseElectronic Spectroscopy of long Carbon Chains HCElectronic Spectroscopy of long Carbon Chains HC2n2nH (n= 8-13) in the Gas PhaseH (n= 8-13) in the Gas Phase
Felix Güthe*, Hongbin Ding, Thomas Pino and John P. Maier
Institut für Physikalische Chemie, Universität Basel, Klingelbergstrasse 80, CH-4056 Basel, Schweiz*email: [email protected]
Mass spectum of diacetylene discharge recorded at 157 nm CnHm from discharge source to record REMPI spectra
A new apparatus for the investigation of neutral CnHm
molecules has been built in to perform resonance enhanced
multi photon ionization (REMPI) studies. They are
produced in an pulsed electrical discharge source from a
mixture of Diacetylene (0,2 – 0,5 %) in Argon 1,2 and are
cooled by supersonic expansion. The apparatus couples the
discharge source with a REMPI detection in a linear time of
flight (TOF) mass-analyzer (R50%= ~900 ), enabling to
combine the information on the masses and the electronic
spectra. 200 or more species can be recorded
simultaneously.
After deflection of charged species formed in the source the
neutral molecules are ionized by lasers. For the ionization
either two photons of the same wavelength (R2PI) or two
photons of different wavelength (R2C2PI) can be used. For
the second colour an F2 excimer laser operating in the VUV
(157nm – 7.9 eV) is used.
References 1) F. Güthe; H. Ding; T. Pino; J. P. Maier, Chemical Physics, 269, 347 (2001).2) T. Pino; H. Ding; F. Güthe; J. P. Maier, Journal of Chemical Physics, 2208, 114, (2001).3) Kirkwood, D. A.; Tulej, M.; Pachkov, M. V.; Schnaiter, M.; Güthe, F.; Grutter, M.; Wyss, M.; Maier, J. P.; Fischer, G. J. Chem. Phys.1999, 111, 9280 .
AcknowledgmentThe authors would like to thank Georg Holderied and Dieter Wild and the mechanical workshop for their technical assistance. Tomasz
Motylewski and Danielle Furio are also kindly thanked for their help with software development. for the experiment.
Gas phase electronic spectra of the HC2nH species for n=8-13 in the UV
obtained using a R2C2PI scheme.They are assigned to the strong
electronic transition known for the polyacetylenic
chains HC2nH. All bands are lifetime broadened (independent of the laser
pulse energy) due to internal conversion to lower electronic states. The
VUV ionisation from the vibrationally excited ground state is favoured
due to because of good Franck-Condon factors.
A very large red-shift relative to the gas phase position, of -3700 cm-1 in
methanol and -1280 cm-1 in the neon matrix, is observed for n=8-12
without a significant dependence on the size of the chain.
2 f ix e d
1 s c a n n e d
C n
*
C n
C + en
+ -
U0=500-1000VWidth=10-200sI=0.2-5 A
p = ~ 1 0 b a r
1
+-
C Hn M
+
C H in A r 4 2
ex p an s io nch am b er
C Hn m
e in ze llen s
M C P -d e tec to r
T O F -ch am b e r
U 2
•Spectra of the much weaker A-system
•Work in the visible
Future work
The CnHm species are important intermediates in
combustion processes and interstellar chemistry.
Although these species are known from mass spectra
their electronic absorption spectra are not well
known.
Direct absorption methods yield spectra, but the
assignment to their carriers is difficult without
information about their mass.
Their electronic spectra are important for their
identification and could be related to the diffuse
interstellar band (interstellar absorptions in the
visible range) problem. 120 240 360 480 600 720 840 960 1080 1200 1320
C10
C20
C30
C40
C50
C60
C70
C80
C90
C100
C110
C4H
2
C114
C44
C60
Masse / amu
C24
C25
C24
Hm
0 1 2 3 4 5 6 7 8 9 101,20
1,22
1,24
1,26
1,28
1,30
1,32
1,34
1,36
1,38
1,40
Ethine
Ethene
Allene
Bond length alternation in HC10
H and HC10
-
"triple"
"single"
Bon
d le
ngth
(Å)
C-C bond
HC10
H B3LYP
HC10
- MP2
0
2
4
6
8
10
12
14 [1+1'] 2 colour scheme for REMPI on polyacetylenes
UV Excitation
IP
VUV Ionisation
A
IC < nsIC < ps
X (2u)
cation
B(1+
u)
X(1+
g)E
nerg
ie (
eV)
Franck-Condon coordinate
11 ug
X
Bond length Alternation and electron delocalisation
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28100
150
200
250
300
350
400
450
500
550
600
Neutral
HC2n
H : 1+
u1+
gAnions
Ac. HC2n
- : 1+1+
HC2n
H
CNH-
Wav
elen
gth
(nm
)
Number of carbon atoms
HC2nH HC2n-
CHC C CH
+CHC C CH
-
+CHC C CH
-
CHC C C
-
CHC C C
-
Electronis spectroscopy and intramolecular dynamics
28400 28600 28800 29000 29200 29400 29600
26300 26500 26700 26900 27100 27300 27500
~ 344 nm
wavenumber cm-1
~ 363 nm
~ 371 nm
wavenumber cm-1
C10
H2
1u,1-
u1+
g ?
Resonance structures
Nonlinear /N-plotFor the neutral polyacetylenes!