The high-resolution ionization spectroscopy of DNA bases is reported for the first time. Vacuum-UV mass-analyzed threshold ionization (VUV-MATI) spectrum of jet-cooled thymine provides not only the most precise ionization potential but also its vibrational structure in the ground cationic state.

Accurate and precise ionization energies of methylamines (CH 3 NH 2 and CH 3 ND 2 ) are determined to be 9.0422Ϯ0.0012 and 9.0532Ϯ0.0012 eV, respectively, by ͑1ϩ1Ј͒ two-photon mass-analyzed threshold ionization ͑MATI͒ spectroscopy. From selective ionizations from specified intermediate quantum states, fundamental frequencies of amino-wagging and CH 3 -rocking modes of CH 3 NH 2 ϩ (CH 3 ND 2 ϩ ) in D 0 states are determined to be 738 ͑573͒ and 1013 ͑1024͒ cm Ϫ1 , respectively. The frequency of the amino wag is largely blueshifted from that of the neutral S 1 state, while the CH 3 -rocking frequency is little shifted from that of S 1 . Internal rotational constants associated with the nearly free internal rotation of the top ͑amino group͒ with respect to the frame ͑methyl group͒ about the C-N axis are accurately determined, from which the geometries of methylamine ions are revealed. Barrier heights for torsional motion in CH 3 ND 2 ϩ are determined to be 25Ϯ5 and 34Ϯ5 cm Ϫ1 at the origin and first ND 2 -wagging bands, respectively. Mode-resolved spectroscopy using the MATI signal for resolving overlapped spectral features in the intermediate state is employed for clarifying the vibrational assignment of the intermediate state. An ab initio calculation at the QCISD level is carried out, giving good agreement with the experiment.

Mass-resolved two-photon (1+1) resonance-enhanced multiphoton ionization spectra of the ~A-X transitions of various methylamine isotopomers (CH(3)NHD, CD(3)NH(2), CD(3)NHD, and CD(3)ND(2)) cooled in the supersonic jet expansion have been measured and analyzed. The band analysis using the Hamiltonian for the internal and overall rotational motions provides the accurate vibrational band positions, allowing for unambiguous assignments for all observed vibrational bands of methylamine isotopomers in the ~A states. Amino wagging (nu(9)) and methyl rocking (nu(7)) modes are found to be Franck-Condon active, and associated anharmonicity constants are precisely determined to give the detailed shape of the potential energy surface in the vicinity of the minimum electronic molecular structure. The barrier height for the nearly free internal rotation about the C-N bond in the ~A state is calculated to be strongly dependent on the excitation of the other higher-frequency vibrational modes, and it is found that the trend is consistent with the experiment. Experimentally measured spectroscopic constants are compared with ab initio calculations, confirming all vibronic assignments. Experimental and theoretical results on all possible HD isotopomers of methylamine in this work, with the earlier report on CH(3)NH(2) and CH(3)ND(2) Baek et al., [J. Chem. Phys. 118, 11026 (2003)], provide the complete spectroscopic characterization of the A state of methylamine.

The one-photon mass-analyzed threshold ionization (MATI) spectroscopy of bis(η6-benzene)chromium is reported. The adiabatic ionization energy is accurately measured to be 44 087 ± 5 cm-1 (5.4661 ± 0.0006 eV). The Cr−benzene stretching vibrational mode in the ion with a fundamental frequency of 264 cm-1 is found to be optically active. The MATI spectra of bis(η6-benzene)chromium·Ar and bis(η6-benzene)chromium·benzene clusters are also reported to give ionization potentials of 43 941 ± 5 cm-1 (5.4480 ± 0.0006 eV) and 42 576 ± 5 cm-1 (5.2788 ± 0.0006 eV), respectively. A van der Waals mode with the fundamental frequency of 65 cm-1 is optically active in the MATI spectrum of the bis(η6-benzene)chromium·benzene cluster, suggesting an intermolecular structural change upon ionization. The larger bis(η6-benzene)chromium·(benzene) n (n = 2−9) clusters are found to be generated in the molecular beam. Their ionization potentials, from photoionization efficiency spectra, show the gradual decrease as the cluster size gets larger, down to the value of 5.02 eV for n = 9.

Resonantly-enhanced one-color two-photon ͑1ϩ1͒ ionization spectra of jet-cooled methylamines (CH 3 NH 2 and CH 3 ND 2) reveal the vibrational structures of these molecules in predissociative à states. Rotational fine structure is clearly resolved for CH 3 ND 2 at the origin and first wagging vibrational level in the excited state. The spectral linewidth becomes homogeneously broadened to give only vibrationally resolved spectral features for the higher vibrational energy levels of CH 3 ND 2 (Ã). From the spectral analysis of the Ã-X transition of CH 3 ND 2 , it is found that the methyl moiety rotates nearly freely about the C-N axis with respect to the amino group in the à state, indicating that the removal of an electron from the nonbonding orbital of N is responsible for the free internal rotation. Vibrational levels are only barely resolved in the Ã-X excitation spectrum of CH 3 NH 2 due to severe homogeneous line-broadening, indicating ultrashort lifetimes of ϳ0.4 ps for predissociating CH 3 NH 2 molecules in the à state. Spectral interpretation of the Ã-X excitation spectrum of CH 3 NH 2 is carried out by the comparison with that of CH 3 ND 2 , giving the confirmative vibrational assignment of methylamines in à states for the first time. The dramatic difference of CH 3 NH 2 and CH 3 ND 2 in their lifetimes in à states suggests that the major dissociation channel of the excited methylamine may be the N-H ͑or D͒ bond dissociation.