Diagnosis of a benzene discharge with a mass-selective spectroscopic technique
1
Diagnosis of a benzene discharge with a Diagnosis of a benzene discharge with a mass-selective spectroscopic technique mass-selective spectroscopic technique Felix Güthe, Hongbin Ding, Thomas Pino and John P. Maier Institute of Physical Chemistry, University of Basel , Klingelbergstrasse 80, CH-4056 Basel, Switzerland. Mass spectrometry with different laser excitation schemes Diagnosis of a plasma with REMPI detection Identification of molecules by R2PI spectroscopy in the UV range Referen ces A new experiment has been built in the group to perform resonance enhanced multi photon ionization (REMPI) studies of the neutral C n H m clusters. They are produced in a plasma formed by a pulsed discharge source 1,2 and are quenched by cooling in a supersonic expansion. The apparatus couples the discharge source with a REMPI detection in a linear time of flight (TOF) mass- analyzer (R 50% = ~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 the neutral molecules are ionized by lasers. For the ionization either one two photons of the same wavelength (R2PI) or two photons of different wavelength (R2C2PI) can be used. 1) F. Güthe; H. Ding; T. Pino; J. P. Maier, Chemical Physics, accepted. 2) T. Pino; H. Ding; F. Güthe; J. P. Maier, Journal of Chemical Physics, 2208, 114, (2001). 3) http://www.chemie.unibas.ch/~linnartz/ 4) W. Jenneskens, M. Sarobe, Polycyclic Aromatic Compounds,. Acknowledgme nt Mass spectra of the neutral clusters produced with an benzene discharge recorded with different ionization wavelengths: A VUV-laser in one case (F 2 - excimer at 157 nm; 7.59 eV) and a spectrum taken during a scan from 278-290 nm (~4.5 eV). The arrows point to the masses 128 and 178, which are coinciding with the masses of the first members of the polycyclic aromatic hydrocarbons (PAH). But only the electronic spectra can reveal the identity of the carriers of this mass peaks ! The authors would like to thanks Georg Holderied and Dieter Wild and the mechanical workshop for their technical assistance. Tomasz Motylewski and Danielle Furio (LPPM, Orsay France) are also kindly thanked for their help in developing the software of the experiment. For the benzene discharge we recorded R2PI spectra between 320 and 280 nm. Electronic spectra of more 30 different molecules have been obtained. In the graphs the spectra of phenylacetylene (C 8 H 6 ), styrene (C 8 H 8 ), indene (C 9 H 8 ), methylstyrene (C 9 H 10 ), fluorene (C 13 H 10 ), tolane (C 14 H 10 ) as well as the of molecule C 10 H 8 are shown. From these 6 molecules could be identified by there spectra from literature. Note that the spectrum of . C 14 H 10 is not that of anthracene or the phenanthrene, the compact all 6-ring PAHs, but that of the tolane molecule. The strong S 0 -S 2 phenantrene transition is clearly absent. The spectrum of the C 10 H 8 molecule is not that of the bicyclic naphthalene, but probably that of a monocycclic substituted benzene. 2 fix ed 1 scanned C n * C n C + e n + - 96 108 120 132 144 156 168 180 F 2 -L aser:157nm M ass (am u) C 8 C 9 C 10 C 11 C 12 C 13 C 14 C 15 C 14 H 10 C 10 H 8 R 2PI:278-290nm U 0 =500-1000V Width=10-200s I=0.2-5 A C 8 C 9 C 10 C 11 M ass S pectra w ith F 2 -L aser ionisation G asoline 95 O ktane 96 102 108 114 120 126 132 138 M ass (a.m .u.) C 6 H 6 D ischarge Intensity G asoline 95 O ktane D ischarge 0 0 0 a 1 a 1 b 2 a 1 a 1 a 1 a 1 a 1 a 1 1 2 3 4 N apht.4 0 1 N apht.7 0 1 8 N apht.8 0 1 7 0 1 8 0 1 279 281 283 285 287 289 291 293 295 297 299 301 0 0 0 29 1 0 28 1 0 42 2 0 25 1 0 24 1 0 19 1 0 17 0 1 33000 33400 33800 34200 34600 35000 35400 35800 m /z = 166 w avenum ber /cm -1 m /z = 128 CH 3 w avelength /nm m /z = 118 34600 34800 35000 35200 35400 35600 35800 36000 S 2 S 0 :phen a(B 2u ) A (B 1u ) b(B 2u ) B (B 1u ) 0 0 0 30 0 1 29 0 1 28 0 1 43 0 1 42 0 1 26 0 1 25 0 1 23 0 1 21 0 1 0 0 0 41 2 0 29 1 0 28 1 0 27 1 0 26 1 0 25 1 0 24 1 0 17 1 0 18 1 0 278 279 280 281 282 283 284 285 286 287 288 0 0 0 a b m /z = 178 w avenum ber /cm -1 m /z = 116 m /z = 104 w avelength /nm m /z = 102 Picture of the plasma from a slit nozzle discharge on C 2 H 2 taken from H. Linnartz 3 . In this work a nozzle of circular geometry with benzene as precursor gas was used but could not be photographed. p = ~ 1 0 bar 1 + - C H n M + C H in Ar 6 6 ex p an sio n cham ber C H n m ein z ellen s M C P -detector T O F-cham ber U 2 The species identified in this work are known as intermediates in several chemical models for combustion. The ethynyl-PAHs (E-PAH) and cyclopentafused PAH (CP-PAH) have been proposed to be intermediates in fullerene formation and in the build up of larger carbon species. Thus the direct sampling of the benzene discharge in the molecular beam reflects the pyrolysis process in an early stage, where the most stable isomers have not yet been formed. Their formation might occur at later stage at higher temperature 4 . The formation of the neutral species seen in the spectra can be assumed to occur by two stages in analogy to cation chemistry known from electron impact work in high pressure sources 5 : CH EI CH n,m 6 CH CH C H C H E PA H CP PA H ? supersonic cooling 6 6 n m ,., 6 6 n m ,., n+6 m +6 ,., n+6 m +6 ,., • Combination of a plasma discharge source with spectroscopic characterisation by REMPI. •-A model system for hydrocarbon flames?? Chemistry in the discharge plasma: Conclusion •-Identification of other species (C 6 D 6 as precursor ...) to gain deeper understanding of the chemistry in plasmas •-Characterisation of other mixtures •-Work in the visible. Future work The produced C n H m species are important intermediates in combustion processes and interstellar chemistry. Their electronic spectra are important for their identification and could be related to the diffuse interstellar band problem.
description
Diagnosis of a benzene discharge with a mass-selective spectroscopic technique. Felix Güthe, Hongbin Ding, Thomas Pino and John P. Maier Institute of Physical Chemistry, University of Basel , Klingelbergstrasse 80, CH-4056 Basel, Switzerland. Diagnosis of a plasma with R EMPI detection. - PowerPoint PPT Presentation
Transcript of Diagnosis of a benzene discharge with a mass-selective spectroscopic technique
Diagnosis of a benzene discharge with a Diagnosis of a benzene discharge with a
mass-selective spectroscopic techniquemass-selective spectroscopic technique
Diagnosis of a benzene discharge with a Diagnosis of a benzene discharge with a
mass-selective spectroscopic techniquemass-selective spectroscopic technique
Felix Güthe, Hongbin Ding, Thomas Pino and John P. Maier
Institute of Physical Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland.
Mass spectrometry with different laser excitation schemes
Diagnosis of a plasma with REMPI detection
Identification of molecules by R2PI spectroscopy in the UV range
References
A new experiment has been built in the group to perform resonance
enhanced multi photon ionization (REMPI) studies of the neutral CnHm
clusters. They are produced in a plasma formed by a pulsed discharge
source1,2 and are quenched by cooling in a 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 the neutral molecules are ionized by
lasers. For the ionization either one two photons of the same wavelength
(R2PI) or two photons of different wavelength (R2C2PI) can be used.
1) F. Güthe; H. Ding; T. Pino; J. P. Maier, Chemical Physics, accepted. 2) T. Pino; H. Ding; F. Güthe; J. P. Maier, Journal of Chemical Physics, 2208, 114, (2001).3) http://www.chemie.unibas.ch/~linnartz/ 4) W. Jenneskens, M. Sarobe, Polycyclic Aromatic Compounds,. 14/15, 169 (1999). 5) C. Lifshitz, G. Reuben, J. Chem. Phys. 50, 951 (1969).
Acknowledgment
Mass spectra of the neutral clusters produced with an benzene
discharge recorded with different ionization wavelengths: A
VUV-laser in one case (F2-excimer at 157 nm; 7.59 eV) and a
spectrum taken during a scan from 278-290 nm (~4.5 eV).
The arrows point to the masses 128 and 178, which are
coinciding with the masses of the first members of the
polycyclic aromatic hydrocarbons (PAH). But only the
electronic spectra can reveal the identity of the carriers of this
mass peaks !
The authors would like to thanks Georg Holderied and Dieter Wild and the mechanical
workshop for their technical assistance. Tomasz Motylewski and Danielle Furio
(LPPM, Orsay France) are also kindly thanked for their help in developing the
software of the experiment.
For the benzene discharge we recorded R2PI spectra between 320 and 280 nm. Electronic
spectra of more 30 different molecules have been obtained. In the graphs the spectra of
phenylacetylene (C8H6), styrene (C8H8), indene (C9H8), methylstyrene (C9H10), fluorene
(C13H10), tolane (C14H10) as well as the of molecule C10H8 are shown. From these 6
molecules could be identified by there spectra from literature. Note that the spectrum of .
C14H10 is not that of anthracene or the phenanthrene, the compact all 6-ring PAHs, but
that of the tolane molecule. The strong S0-S2 phenantrene transition is clearly absent. The
spectrum of the C10H8 molecule is not that of the bicyclic naphthalene, but probably that
of a monocycclic substituted benzene.
2 f ix e d
1 s c a n n e d
C n
*
C n
C + en
+ -
96 108 120 132 144 156 168 180
F2-Laser: 157nm
Mass (amu)
C8
C9
C10
C11
C12
C13
C14
C15
C14
H10
C10
H8
R2PI: 278-290nm
U0=500-1000VWidth=10-200sI=0.2-5 A
C8
C9
C10
C11
Mass Spectra with F2-Laser ionisation
Gasoline 95 Oktane
96 102 108 114 120 126 132 138
Mass (a.m.u.)
C6H
6 Discharge
Int
ensi
ty
Gasoline 95 Oktane Discharge
00
0
a1
a1
b2
a1 a
1
a1
a1 a
1
a1
1 23
4
Nap
ht. 0
0 0
IP(N
apht
. )/2
Nap
ht. 4
0 1
Nap
ht. 7
0 1 8
Nap
ht. 8
0 1 70 1 8
0 1
279281283285287289291293295297299301
00
0
291
0
281
0
422
0
251
0
241
0
191
0
170
1
33000 33400 33800 34200 34600 35000 35400 35800
m/z = 166
wavenumber / cm-1
m/z = 128
CH3
wavelength / nm
m/z = 118
34600 34800 35000 35200 35400 35600 35800 36000
S 2S 0: p
hen
a(B2u
)
A(B1u
)b(B
2u)
B(B1u
)
00
0
300
1
290
1
280
1 430
1 420
1
260
1
250
1 230
1210
1
00
0
412
0
291
0281
0
271
0
261
0
251
0
241
0
171
0181
0
278279280281282283284285286287288
00
0a
b
m/z = 178
wavenumber / cm-1
m/z = 116
m/z = 104
wavelength / nm
m/z = 102
Picture of the plasma from a slit nozzle discharge on
C2H2 taken from H. Linnartz3. In this work a nozzle of
circular geometry with benzene as precursor gas was
used but could not be photographed.
p = ~ 1 0 b a r
1
+-
C Hn M
+
C H in A r 6 6
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
The species identified in this work are known as intermediates in several chemical models for combustion. The ethynyl-PAHs (E-PAH) and cyclopentafused PAH (CP-PAH) have been proposed to be intermediates in fullerene formation and in the build up of larger carbon species. Thus the direct sampling of the benzene discharge in the molecular beam reflects the pyrolysis process in an early stage, where the most stable isomers have not yet been formed. Their formation might occur at later stage at higher temperature4.The formation of the neutral species seen in the spectra can be assumed to occur by two stages in analogy to cation chemistry known from electron impact work in high pressure sources5:
C H EI C H n, m 6
C H C H C H
C H E PAH CP PAH ?
supersonic cooling
6 6 n m,.,
6 6 n m,.,
n+6 m+6,.,
n+6 m+6,.,
• Combination of a plasma discharge source with spectroscopic characterisation by REMPI.•-A model system for hydrocarbon flames??
Chemistry in the discharge plasma:
Conclusion
•-Identification of other species (C6D6 as precursor ...) to gain deeper understanding of the chemistry in plasmas
•-Characterisation of other mixtures
•-Work in the visible.
Future work
The produced CnHm species are
important intermediates in combustion
processes and interstellar chemistry.
Their electronic spectra are important
for their identification and could be
related to the diffuse interstellar band
problem.
