Gas phase electronic spectra of linear carbon chains: HCn 1H, HCnH , HCn 1 , HCn

1
Gas phase electronic spectra of linear carbon
chains HCn1H, HCnH , HCn1 , HCn
Felix Güthe1, Hongbin Ding, Thomas Pino3, Tim
W. Schmidt4, Andrei Boguslavskiy John
Maier Institut für Physikalische Chemie der
Universität Basel, Basel, Switzerland 1 abcd
Switzerland Ltd., Baden, Switzerland 2
Institut für Physikalische Chemie der Universität
Basel, Basel, Switzerland 3 Laboratoire de
Photophysique Moleculaire, Universite Paris-Sud,
Orsay, France 4 Sydney University, Sydney,
Australia Bunsentagung , Dresden 2004
2
Nanowires
hypothetical new allotrope molecular
wire precursor nano tubes, fullerenes
etc. interstellar molecules
hypothetical new allotrope diamond
sp3 graphite sp2 polyyne sp
optical properties band gaps absorption in the
ISM -gtspectroscopy
optical properties transition n-gt 88 band gap
bulk behaviour
3
Interstellar molecules
taken from http//cfa-www.harvard.edu/cfa/mmw/mmw
lab/ismmolecules_organic.html
4
Flames
K.-H. Homman, Angew. Chem. 1998, 110, 2572
Angew. Chem. Int. Ed. Engl. 1998, 37, 2435
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Pulsed Electrical Discharge
Picture H. Linnartz
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Experiment
l2 157 nm, 189nm, 212nm
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Mass spectrum
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electronic transitions- HC2nH
excitations 1Sg ?? (n)pu/ (n-1)pg ??
(n)pg/u A1Du (n)pu/ (n-1)pg ?? (n)pg/u
B1Su (n)pu/ (n-1)pg ?? (4n)sg/u
1Pu (4n-1)sg/u ?? (n)pg/u 1Pu
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R2CPI-spectra of acetylenic chains
HC2nH(n8-13) 1Sg ??1Su
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D states HC6H, HC8H, HC10H, HC12H, HC14H

• HC2nH(n3-7)1Sg??1Du, 1S-u dipole-forbidden (np
  bending)

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Observed and Calculated Values
strong B-transiton!
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electronic transitions- HC2n1H
excitations 3S-g ?? (n-1)pg/ (n-1)pu ??
(n)pu/ (n-1)pg a3S-u (n)pu/ (n-1)pg ??
(n)pg/u b3S-u (n)pu/ (n-1)pg ?? (4n4)sg/u
C3Pu
mixing of degenerate a(3S-u) and b(3S-u)
yields A(3S-u ) ab/sqrt(2) B(3S-u
) a-b/sqrt(2)

• Dewar-Longuet-Higgins (1954, Proc. Phys. Soc.) on
  odd alternant hydrocarbons
• A occurs at longer wavelength and is weaker than
  B
• B must be the strongest transition

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The HC2n1H Series HC7H, HC9H, HC11H, HC13H

• HC2n1H(n3-6) X 3S-g ?? A 3S-u,

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HC13H ... HC19H X 3S-g ?? B 3S-u,

• strong B-transiton
• HC19H is weak in mass spectrum, but still visible

MRCI Mühlhäuser, Peyerimhoff et al. (2002)
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HC13H ... HC19H X 3S-g ?? B 3S-u,

• as predicited in 1954 !

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extrapolation to C
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isoelectronic HCn- system
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Solvent and endgroup effect
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Conclusions

• for odd and even chains strong B-states
• fNc
• position in the visible
• broad peaks
• in the ISM
• similar for kation (HC2n1H, HC2n1H-), anion
• sp allotrope bandgap in UV/visible
• matrix shifts
• bondlength alternation

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HC2n1H anion - neutral- cation

• ground state (n-1)p4(n)p1,2,3 (n1)p0
• (n-1)p4(n)p1,2,3 ?? (n-1)p3(n)p2,3,4 a3S-u
• (n)p1,2,3 (n1)p0 ??(n)p0, 1,2 (n1)p1 b3S-u
• same behaviour for anions and cations
• a and b degenerate-gt mixing to yield
• weak A and
• strong B transition

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Bond length alternation Acetylenic vs cumulenic
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Bond length alternation even and odd
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Bond length alternation neutral and anionic
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backup longchains

• additional material

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Spectroscopic techniques

• Spectral range UV/visible for DIBs
• Direct absorption
• I/I0
• sensitivity and selectivity
• multiple passes and Cavity Ring Down Spectroscopy
• or
• Laser induced Fluorescence
• excited state lifetime, fluorescence quantum
  yield
• Mass selective techniques
• Resonance Enhanced Multi Photon Ionisation (and
  related - R2ColourPhotoDetachment)
• change in the m/z ratio (anion ? neutral
  neutral ? cation , cation ? Fragment)
• sensitivity for ion detection is high!
• additional molecular information mass
• physics of the ionisation/detachment process is
  important

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REMPI scheme
Cn
Cn Cn-m Cm
IonD0
Cn Cn-m Cm
IP
UV
exit channels?
S1
IP/2
near UV
near UV
S1
vis
NeutralS0
common example
uncommon ExampleCn
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Excitation scheme
Franck-Condon factors
even
odd
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strong solvent shift 4000 cm-1 to the red
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-
HC
C
-
HC
C
HC2n-

-
HC
CH
HC
CH

-
HC
CH
C
HC2nH
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R2CPI-spectra of acetylenic chains
1Sg ??1Su
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Even D HC6H, HC8H, HC10H, HC12H, HC14H

• HC2nH(n3-7) 1Sg ??1Du

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The HC2n1H Series HC7H, HC9H, HC11H, HC13H ...
HC19H
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end polayacetylenes

• additional material

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Gas phase electronic spectra of linear carbon
chains HCn1H, HCnH , HCn1 , HCn
Felix Güthe1, Hongbin Ding, Thomas Pino3, Tim
W. Schmidt4, Andrei Boguslavskiy John
Maier Institut für Physikalische Chemie der
Universität Basel, Basel, Switzerland 1 abcd
Switzerland Ltd., Baden, Switzerland 2
Institut für Physikalische Chemie der Universität
Basel, Basel, Switzerland 3 Laboratoire de
Photophysique Moleculaire, Universite Paris-Sud,
Orsay, France 4 Sydney University, Sydney,
Australia Bunsentagung , Dresden 2004
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C3H- identified in the ISM by microwave
spectroscopy!

• spectrum in the visible detected via R2CPI with
  F2 laser in the VUV !!

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C3H

• complicated spectrum!
• Renner-Teller (4 atoms) distorted
• more than one electronic state

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C3H

• ground state 2P
• linear-bend transition
• 3 electronic states contribute to spectrum
• complicated Renner-Teller distorted spectrum!
• individual lines to weak to be detected in the
  ISM by vis-absorption

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electronic transitions- C2nH
excitations 2P ?? (4n1)s ?? (n)p 2S weak,
IR (n-1)p ?? (n)p 2P strong, vis (n)p ??
(n1)p 2P weak UV excitations 2S ??
(n)p ?? (4n1)s 2P (4n1)s ?? (n1)p
2P (n)p ?? (n1)p 2S
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The C2n1H Series C3H,C5H,C7H,C9H
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electronic transitions- C2n1H
excitations 2P ?? (4n3)s ?? (n)p 2D, 2S-,
2Svis (n-1)p ?? (n)p 2P vis (n)p ?? (n1)p
2P
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The C2n1H Series C3H,C5H,C7H,C9H

• 2P ?? 2D, 2S-, 2S, 2P -
• 3- 4 different electronic states!

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Extrapolation
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end longchains

• additional material

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C7H7- Tropyl vs. Benzyl

• 7 ring / 6 ring from stable C7H7 ion!

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C7H7- Tropyl vs. Benzyl

• C6H5CH2C ??X
• tropyl radical
• complex spectrum
• Jahn-Teller distorted
• D7h

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C7H3- identification of the structure!
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variety of candidates

• calculation
• energies
• rotational constants

• geometries DFT B3LLYP/6_31G

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C7H3- rotational K-structure!

• rotational structure!
• down selection -gt 3 member ring
• spin statistics -gt isomer 2

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C7H3- structure identified!

• no methyl group!
• unlike C9H3 ,C11H3 ,... (Schmidt et al. IJMS 2003)

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REMPI aromatics

• additional material

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R2PI-Spectra from Benzene-Discharge
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R2PI-Spectra from Benzene-Discharge
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end REMPI
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• AB hn -gt A B , A detected
• AB from source, hn scanned for resonance

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Fragmentation spectroscopy

• for van der Waals clusters
• MArn hn -gt M MArn-1 Ar, M HC4H

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• extrapolation of band origins

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end Fragmentation

• additional material