Resonant-enhanced two photon ionization and mass-analyzed threshold ionization spectroscopy of jet-cooled 2-aminopyridines (2AP-NH2,-NHD,-NDH,-ND2)

Abstract

Resonantly-enhanced two photon ionization and mass-analyzed threshold ionization (MATI) spectra of 2-aminopyridine (2AP-NH2) and its deuterated analogs have been obtained using two-photon (1+1') excitation process via S-1 intermediate states for ionization. Ionization energies of 2AP-NH2 and 2AP-ND2 are both precisely and accurately determined to be 8.1086+/-0.0005 and 8.1027+/-0.0005 eV, respectively. Two geometrical isomers, 2AP-NHD or 2AP-NDH, of which H or D is hydrogen-bonded to the nitrogen atom on the pyridine ring, respectively, are spectroscopically well isolated using the hole-burning spectroscopy in the S-1 states. Corresponding ionization energies are thus separately determined to be 8.1067+/-0.0005 or 8.1048+/-0.0005 eV for 2AP-NHD or 2AP-NDH, respectively. Vibrational bands of 2-aminopyridine ions associated with various aromatic ring-skeletal modes are identified in the MATI spectra and appropriately assigned with the aid of ab initio calculation. All of the ring-skeletal vibrational frequencies observed in this work become slightly higher than those in the S-1 states when the molecules are ionized, consistent with the fact that the S-1-S-0 excitation is due to pi*-pi transition. According to ab initio calculation, the amino group is in the molecular plane both in the S-1 and D-0 states, while it is slightly distorted in the ground state of 2-aminopyridine. Inversion modes in 2AP-ND2 and 2AP-NDH in S-1 states are split into two bands due to their strong coupling with the other mode, which is most probably due to torsional motion of the amino group. Strong mode couplings are clearly manifested in interferencelike patterns observed in vibrational band structures of MATI spectra taken via those two bands in S-1 states as intermediate states. A new spectroscopic scheme, in which MATI signals are used for obtaining mode-resolved spectra for the intermediate state is introduced. The vibrational band at 911 cm(-1) from the S-1-S-0 origin that has been previously assigned as the inversion mode of 2AP-NH2 is found to actually consist of two closely-spaced different modes giving two clearly-resolved different Franck-Condon active modes in corresponding MATI spectra. (C) 2002 American Institute of Physics.