laser photodissociation spectra of the aniline-argon cationic cluster in the near infrared t. pino,...
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LASER PHOTODISSOCIATION SPECTRA OF THE ANILINE-ARGON CATIONIC CLUSTER
IN THE NEAR INFRARED
T. PINO, S. DOUIN, Ph. BRECHIGNAC
Laboratoire de Photophysique Moléculaire, CNRSUniversité de Paris-Sud, ORSAY (France)
- Spectroscopy of polyatomic cations requires special care
- Jet-cooling for best resolution
- Direct absorption from ions is difficult to detect Argon tagging
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The main Franck-Condon active modes in aniline
C6H5-NH2
S0 526 812 v=2 423 Kydd (1977)S1 492 798 v=2 761 Sinclair (1996)D0 521 814 v=2 655 Lin (2001)Dn ? ? ?
PES : large geometry change between S1 and D1
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S0
S1
h1
h2
h3
T.O.F.MS
MCP-Detector
6.2 6.4 6.6 6.8 7.0 7.2
Other Fragments
C10H8+
Delayed
C10H8+ C10H8
+-Ar
TOF (s)
Ion
curre
nt (a
.u.)
D0
How the experiment
works
5,4 5,6 5,8 6,0 6,2 6,4 6,6 6,8
5,4 5,6 5,8 6,0 6,2 6,4 6,6 6,8
PAH+-Ar
PAH+
Time of flight (microseconds)
Ion
sig
na
l (
u.
a.)
An+An+-Ar
R2PI ion preparation
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The D1 (2A2)D0 (2B1) transition in the Aniline+-argon cation photodepletion spectrum
Long vibrational progression : which mode ?
The only transition from 7000 to 22500 cm-1
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Effect of deuteration
NH2 6a = 520 cm-1 16a2 = 674 cm-1 1 = 887 cm-1 ND2 6a = 512 cm-1 16a2 = 654 cm-1 1 1000 cm-1
46 cm-1 blue shift
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Lorentzian Fit of data : frequencies, widths
assignments
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The two ionic states are well described by a single configuration : …(2b1)2(1a2)2(3b1)1 for the ground stateand …(2b1)2(1a2)1(3b1)2 for the excited state. The electronic transition corresponds to the electron promotion 1a2 3b1 .
The adiabatic excitation energy is found to be 1.6 eV for the cation and 4.58 eV for the neutral.
These are higher by 0.28 eV and 0.36 eV than the experimental values respectively .
Calculated oscillator strengths 1.5 10-3 for the D1 D0 (compared to 1.2 10-3 ) and 0.02 (compared to 0.021) for the S1 S0 electronic transitions.
These calculations are found in agreement with experiment
CASSCF calculations (6-311G**)
in C2v symmetry
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Evolution of bond lengths
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Lorentzian Fit of data : frequencies, widths
assignments
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Non radiative rates : fast internal conversion
Steady raise with vibrational energy
Similarity with -OH+
(D2 D0)
IVR assisted IC
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Conclusion
Spectra characterized by a long vibrational progression involving the 6a mode. On the basis of CASSCF calculations, a large change of geometry along the normal coordinate associated to this mode is found, while the amino group is found in the ring plane. Therefore a change of the conjugation of the ring rather than a charge transfer is inferred. This is thought to be the origin of the extent of the progression.
Ultrafast internal conversion was observed through the broadening of the vibronic bands, and found dependent on the vibrational state in the upper D1 electronic state.