Electronic Spectroscopy of long Carbon Chains HC 2n H (n= 8-13) in the Gas Phase

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!