Photodetachment spectrum of l-C3H2-: The role of

dipole bound states for electron attachment in interstellar clouds F. Güthe*1,3, M. Tulej2,3, and J. P. Maier3

1 Alstom Schweiz, Baden, Switzerland 2 Paul Scherrer Institut, Villigen, Switzerland3 Institut für Physikalische Chemie der Universität Basel, Basel, Switzerland

*email: felix.guethe@power.alstom.com

Electronic spectrum of l-C3H2-

The electronic spectrum of the linear propadienylidene anion l-C3H2- has been recorded at 50 K by

resonance enhanced two photon detachment spectroscopy. This resembles the absorption spectrum to a dipole bound state involving bound-bound and bound-free transitions as well as to Feshbach states.

References:1. Tulej, M.; Güthe, F.; Schnaiter, M.; Pachkov, M. V.; Kirkwood, D. A.; Maier, J. P.; Fischer, G. J. Phys. Chem. A

1999, 103, 9712.

2. Güthe, F.; Tulej, M.; Pachkov, M. P.; Maier, J. P. Astrophys. J , 555, (2001)

3. Yokoyama, K., Leach, G. W., Kim, J. B., & Lineberger, W. C., 1996, J. Chem. Phys., 105, 10696 and 10706

C n

C n

C nU ~ 5 0 0 V

t ~ 1 0 s

p ~ 1 0 b a r

1 % C 2H 2 in A r

U

C n

1 2

2 f ix ed

1 sc an n e d

C n

C n

C n + e

Experimental Setup1 of the R2CPD gas phase experiment:

00

0: 01

62

0: 01

EA

670672674676678680682684686688690692694696698700702704706

14200 14300 14400 14500 14600 14700 14800 14900

wavelength / nm

two colour

one colour ; >5 mJ/ cm2

one colour ; <2 mJ/ cm2

l-C3H

2

-

wavenumber / cm-1

relaxation h

vibrationalFeshbachresonances

EA

Ebind

1A12A

1

2B2

e-

l-C3H

2

Neutral

l-C3H

2

-

Anion

18000 19000 20000 21000 22000

B1

B2

B3

B4

B5

B6

B7

B8

B9

B10

B11

B12

C1

C3

C4

C5

C6

C7 C

8

B1

B2B

3

B4

B5

B6

B8

B9

B10

B11

C1

C2C3

C4C

5C

6C

7

b) l-C3D

2

-

a) l-C3H

2

-

wavenumber / cm-1

Bound states (below the detachment threshold) can only be seen if a second photon is absorbed! -> Two colour two photon detachment (R2CPD)

Dipole bound state Electronic Feshbach state

A

N=

k e

k B k k I

-att

-

col det rec+

k kk

k k katt fr

DA AD r

In interstellar conditions only the K =10 components should be visible in absorption

To explain the abundance of this carrier in interstellar clouds the ratio of anions to neutrals is estimated for small polar molecules with high electron affinities using chemical considerations.

An appreciable amount of anions can result in diffuse clouds because the electron attachment to polar molecules is enhanced via dipole bound states.

CnH Picsexperimental : 3 cases 1. slide 2. C4H to C8H3. C4H to C4D HIRES4. Feshbach things5. general DBS things polar +e- denstiy

14000 16000 18000 20000 22000 24000 26000 28000 cm-1

A2A1 X2 B

1

B2A" X2 B1

C2A1 X2 B

1D2B

1 X2 B

1

~~~~

~ ~

~~

H2CCC-

14100 14400 14700 15000 15300 15600 15900 16200 cm-1

EA

A1

A2

A3

A4

A5

B1

B2 B

3

B4 B

5

B6 B

7

B8

B9 B

10

B11

B12

C1

C3

C4

C5

C6

C7 C

8

A1

A2

A3

A4

A5

610620630640650660670680690700 nm

A2A1 X2 B

1 a) H2CCC-

b) D2CCC-

~ ~

Autodetachingexcited states

Dipole boundstates

Bound excitedvalence states

Groundstate

Electron affinity(EA)

Probe photon

28640 28650 28660 28670 28680 28690 28700

Fit

A = 30.5 cm-1

hotband -

-

T =100 K ?? new fit

T =320 K

linear -bent:

B'' = 0.1553 cm-1

origin = 28645.91 cm-1

A' = 30.0, B' = 0.15945, C'= 0.157867

wavenumers /cm-1