We have observed the IR spectra of the melamine molecule and its deuteriated counterpart in the gas phase at ca. 150 ¡C and in a solid argon-matrix at 10 K. The assignment of the vibrations of melamine has been facilitated by the calculated thirty nine normal modes using several ab initio and density functional methods. By scaling the calculated vibrational frequencies, the theoretical computations have been demonstrated to be in good agreement with the experimental observations. The optimized equilibrium structure of melamine has been shown to be a planar but distorted-hexagonal triazine ring with three pyramidal amino groups, which result in di †erent conformers. This has been supported by the comparison between the observed and the calculated spectra for non-planar conformers 1 and 2 vs. the planar structure 3. In view of the small energy di †erences D 3h between the calculated conformers 1 and 2 and the " transition state Ï 3 (corresponding to a third-order saddle point on the potential-energy hypersurface), the melamine molecule has a Ñat potential-energy hypersurface near the equilibrium structures and the conformers can rapidly rearrange.
We have studied the fragmentation of the melamine (2,4,6-triamino-s-triazine) molecule and its deuterated counterparts via electron impact ionization (EI), laser desorption ionization (LDI), and collision-induced dissociation (CID). Our EI and LDI measurements show that the dissociation of melamine is different from the concerted triple dissociation pathway of s-triazine. In EI experiments, the protonated and parent melamine ion (m/z ) 127 (C 3 N 6 H 7 + ) and 126 (C 3 N 6 H 6 + )) were formed initially with 20 and 70 eV electron bombardment. Other fragment ions, such as m/z ) 43 (CN 2 H 3 + ), 53 (C 2 N 2 H + ), 56 (CN 3 H 2 + ), 68 (C 2 N 3 H 2 + ), 83 (C 2 N 4 H 3 + ), 85 (C 2 N 4 H 5 + ), 99 (C 2 N 5 H 5 + ), 110 (C 3 N 5 H 4 + ), etc., were subsequently formed from the decomposition of metastable melamine ions. This speculation was supported by our additional CID measurements. On the other hand, in the LDI experiments the melamine molecule was pumped to 1 1 A′′ and 2 1 A′ excited electronic states, respectively, with 266 and 193 nm lasers. In view of the same fragment ions (m/z ) 43, 45 (CN 2 H 5 + ), 60 (CN 3 H 6 + ), 85, and 127) resulting from the different excited 1 1 A′′ and 2 1 A′ states, we conclude that the fragmentation of melamine in LDI proceeds via internal conversion to its ground potential energy surface (1 1 A′) prior to dissociation. The decomposition mechanism in the ground electronic state has been investigated using the density functional B3LYP/6-31G* and B3LYP/cc-pVTZ methods. All the molecular ions observed in EI experiments can be produced from major and minor neutral fragments of melamine dissociation. The calculations demonstrate the reaction pathways leading to these fragments and predict the corresponding activation energies. The dissociation mechanism of melamine is shown to be distinct from that of s-triazine, because of the presence of mobile hydrogen atoms in the amino groups.
The object of this study is to quantitatively elucidate the laser-power dependence in transient degenerate four-wave mixing ͑DFWM͒ with an emphasis on the high laser-pump intensity region. We discuss our investigation on the power dependence of transient DFWM by taking gas-phase iodine (I 2 ) molecules as a testing example. The distinct physical feature is that in the high-power laser pump, where both laser-pulse duration and the inverse of pump rate are much shorter than the molecular population relaxation time, the steady-state DFWM theory of Abrams and Lind ͓Optical
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