lower O+ component are allowed and result in type C bands whereas those that terminate on the upper inversion component 0-give rise to forbidden Herzberg-Teller bands of AB mixed band type. A similar situation exists for the transitions to the syn G and G* isomers. As the O+-O-inversion splitting is small for both anti and syn isomers (less than 1 cm-I), the major bands in the spectrum are expected to be of mixed A, B, and C character.
ConclusionsThe vapor-phase absorption spectrum of formic acid which arises from a n -a* electron promotion consists of a complex of bands that are built onto the side of a strong absorption continuum. The fluorescence excitation spectrum of formic acid in this region is greatly simplified and consists of a group of bands that display a well-defined rotational fine structure. A comparison of the deuterium-hydrogen frequency shifts among the four isotopomers of formic acid allowed the origin to be assigned to the band at 37431.5 cm-I. The modes u3(C=O), u,(O-C=O), u8(CH), and u,(OH) were observed to form intervals in the spectrum. The activity of the ug torsional mode is of particular interest, as it must result from a SI equilibrium configurationwhere the O H group is twisted from the 0-C=O frame of the molecule. The activity of v8 mode demonstrates that the CH bond is also displaced from the molecular plane. Both the aldehyde and the hydroxy groups would be distorted from the 0-C=O molecular frame, which would result in a complex equilibrium structure for SI formic acid.The equilibrium structure and the dynamics of the So and TI states of formic acid were evaluated by ab initio SCF theory using a 6-31G* basis set. The calculations correctly showed that the anti was more stable than the syn conformer in the lower electronic state, although the calculated 2 142.9-cm-I energy difference was substantially larger than the experimental value of 1365 cm-'. The structures calculated for the the TI state were more complex. The stable equilibrium configuration was calculated to have the O H and CH bonds twisted from the 0-C=O frame by 67.99' and 45.87', respectively. The calculations predicted a second conformation with torsional and wagging angles of -58.29' and 40.71'. This form is less stable by 464.1 cm-I. Acknowledgment. F.I. and D.C.M. thank the Natural Sciences and Engineering Research Council of Canada for continuing financial support. We express our gratitude to Dr. J. D. Goddard and R. B. Ogawa for their assistance in the ab initio calculations and Dr. J. Karolczak for his help with the experiments. This paper presents extensive investigations of state-selective vibrational excitation induced by an IR picosecond laser pulse, including the first simulation of such processes by means of fast Fourier transform propagation of a molecular wave packet e(t). For simplicity, we consider a model Morse oscillator with potential V, and semiclassical dipole interaction -pLE(f).A selective resonant laser field E(t) with sin2 shape is chosen and compared with several alternatives, including much...
Using 550 previously calculated vibrational energy levels and dipole moments we performed simulations of the HCN-->HNC isomerization dynamics induced by sub-one-cycle and few-cycle IR pulses, which we represent as Gaussian pulses with 0.25-2 optical cycles in the pulse width. Starting from vibrationally pre-excited states, isomerization probabilities of up to 50% are obtained for optimized pulses. With decreasing number of optical cycles a strong dependence on the carrier-envelope phase (CEP) emerges. Although the optimized pulse parameters change significantly with the number of optical cycles, the distortion by the Gaussian envelope produces nearly equal fields, with a positive lobe followed by a negative one. The positions and areas of the lobes are also almost unchanged, irrespective of the number of cycles in the half-width. Isomerization proceeds via a pump-dumplike mechanism induced by the sequential lobes. The first lobe prepares a wave packet incorporating many delocalized states above the barrier. It is the motion of this wave packet across the barrier, which determines the timing of the pump and dump lobes. The role of the pulse parameters, and in particular of the CEP, is to produce the correct lobe sequence, size and timing within a continuous pulse.
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