LASER PHOTODISSOCIATION SPECTRA OF THE ANILINE-ARGON CATIONIC CLUSTER IN THE NEAR INFRARED T. PINO, S. DOUIN, Ph. BRECHIGNAC Laboratoire de Photophysique Moléculaire, CNRS Université 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

The main Franck-Condon active modes in aniline C 6 H 5 -NH 2 S v=2423Kydd (1977) S v=2761Sinclair (1996) D v=2655Lin (2001) D n ? ?? PES : large geometry change between S 1 and D 1

How the experiment works An + An + -Ar R2PI ion preparation

The D 1 ( 2 A 2 )  D 0 ( 2 B 1 ) transition in the Aniline + -argon cation photodepletion spectrum Long vibrational progression : which mode ? The only transition from 7000 to cm-1

Effect of deuteration NH 2 6a = 520 cm -1 16a 2 = 674 cm -1 1 = 887 cm -1 ND 2 6a = 512 cm -1 16a 2 = 654 cm -1 1  1000 cm cm -1 blue shift

Lorentzian Fit of data : frequencies, widths assignments

The two ionic states are well described by a single configuration : …(2b1)2(1a2)2(3b1)1 for the ground state and …(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 for the D 1  D 0 (compared to ) and 0.02 (compared to 0.021) for the S 1  S 0 electronic transitions. These calculations are found in agreement with experiment CASSCF calculations (6- 311G**) in C 2v symmetry

Evolution of bond lengths

Lorentzian Fit of data : frequencies, widths assignments

Non radiative rates : fast internal conversion Steady raise with vibrational energy Similarity with  -OH + (D 2  D 0 ) IVR assisted IC

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 D 1 electronic state.