Transcript TX07.ppt
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 C6H5-NH2 S0 S1 D0 Dn 526 812 v=2 423 Kydd (1977) 492 798 v=2 761 Sinclair (1996) 521 814 v=2 655 Lin (2001) ? ? ? PES : large geometry change between S1 and D1 Ion signal (u. a.) 5,4 PAH 5,8 6,0 6,2 6,4 6,6 6,8 How the experiment works + An+ Ion current (a.u.) MCP-Detector 5,6 +-Ar -Ar AnPAH + Other Fragments Delayed + + C10H8 -Ar C10H8 5,4 + C10H8 6.2 5,6 5,8 6,0 6,2 6,4 6,6 Time of flight (microseconds) 6.4 T.O.F.MS 6.6 6.8 TOF (s) 7.0 7.2 n ove h1 h2 D0 h3 S1 S0 R2PI ion preparation 6,8 The D1 (2A2)D0 (2B1) transition in the Aniline+-argon cation photodepletion spectrum The only transition from 7000 to 22500 cm-1 Long vibrational progression : which mode ? Effect of deuteration 46 cm-1 blue shift NH2 ND2 6a = 520 cm-1 6a = 512 cm-1 16a2 = 674 cm-1 16a2 = 654 cm-1 1 = 887 cm-1 1 1000 cm-1 Lorentzian Fit of data : frequencies, widths assignments CASSCF calculations (6311G**) in C2v symmetry 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 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 Evolution of bond lengths Lorentzian Fit of data : frequencies, widths assignments Non radiative rates : fast internal conversion Steady raise with vibrational energy Similarity with F-OH+ (D2 D0) 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 D1 electronic state.