Intensity distribution in rotational line spectra

Completely resolved Doppler-free rotational line spectra of six vibronic two-photon bands in benzene C6 H6 and C6 D6 are presented. The excited final states possess different excess energies in S1 (1567 to 2727 cm−1 ) and are embedded in dense manifolds of background states with differing densities of states (1<rho<60 1/cm−1 ). The bands are analyzed by a statistical procedure. The intensity distribution of several hundreds of lines of each band is investigated. It is found that all weakly perturbed bands display a similar, peaked intensity distribution while in strongly perturbed bands the number of lines decreases monotonically with increasing intensity. The origin of this difference is discussed in terms of coupling to the many background states. The Journal of Chemical Physics is copyrighted by The American Institute of Physics

Emission spectra from single rovibronic quantum states in S1 benzene after Doppler-free two-photon excitation

Dispersed emission from single rovibronic quantum states in S1 benzene is measured after Doppler-free two-photon excitation under low pressure conditions (0.3 Torr). This was made possible by a long-term stabilization of the single-mode dye laser yielding a stability of better than 1 MHz/h. The emission spectra of unperturbed rotational levels in the 141 and the 14111 vibronic states reveal a great number of detailed results on Duschinsky rotation and long-range Fermi resonances in the electronic ground state. By contrast, it is seen that the emission spectra from perturbed rovibronic states are contaminated by additional bands. The analysis of these bands leads in most cases to an identification of the coupled dark background state and the responsible rotation–vibration coupling process (H42 resonances). The emission spectra clearly demonstrate that even for a density of states of 60 1/cm−1, coupling in S1 benzene is still selective and far from the statistical limit. It is further demonstrated that the dark and the light states are more efficiently mixed by short-range couplings with coupling matrix elements of some GHz than by long-range Fermi resonances. The Journal of Chemical Physics is copyrighted by The American Institute of Physics

Rotationally resolved fluorescence-dip and ion-dip spectra of single rovibronic states of benzene

We report fluorescence-dip as well as ion-dip spectra of single rovibronic one-photon states of benzene with a linewidth as narrow as 0.14 cm&apos;1. The selective excitation of the rovibronic states was achieved through the combination of a frequency-doubled pulsed amplified cw dye laser (Avuv 100 MHz) and a collimated molecu-lar beam. The detailed analysis of the dip spectra shows that the observed spectral features correspond to single rovibronic transitions if suitable states are excited. From the spectra, precise harmonic frequencies and anharmonic constants for the So state are determined. A hitherto unknown Darling-Dennison resonance of the overtones of v, with the 52 state is found. 1

High Resolution Sub-Doppler Experiments on Benzene

It is shown that sub-Doppler spectroscopy enables one to resolve individual rotational states in the S^ manifold of polyatomic molecules. This i s an essential to the understanding of the primary photophysics within the molecule. Spectra of benzene are found to undergo substantial changes as the vibrational energy i s raised within S^. Due to the increased density of vibrational states, Coriolis coupling, which is already seen at low energies, can lead to effective IVR above 3000 cm""1 excess energy. This onset of IVR may be responsible for the onset of "Channel Three" in benzene and probably produces gross changes in the photophysical behavior of any polyatomic molecule

Sub-Doppler High-Resolution Spectra of C6H6

We have measured the Doppler-free rotationally resolved two-photon spectrum of two vibronic bands of C6H6, 14; 1; and 14; 1:. Compared to the 14; 1; band (E,,,, = 2492 cm-’) a drastically reduced number of sharp lines is observed in the 14; 1: band (EexCe=88 3 412 cm-’1. The K = 0 rotational lines are still seen, while K # 0 lines disappear. This can be understood in terms of Coriolis coupling between the 14l l2 state and other vibrational states within Sl and a subsequent nonradiative process. This coupling might be the explanation for the drastic onset of line broadening in C6H6 at an excess energy of 3000 cm-l, “channel three”. The line broadening is then due to a different process than that responsible for the population lifetime of S1

Sub-Doppler Spectroscopy of Benzene in the "Channel-three" Region

In addition to an increased accuracy in the determination of structural parameters sub- Doppler spectroscopy of large molecules is shown to render valuable information about intramolecular processes and relaxation mechanisms. As an example we have measured the Doppler-free rotationally resolved two-photon spectrum of two vibronic bands of CsHs, 1461; and 14i1;. Compared with the 14i1: band (EcxE :,:=css 2492 cm-') a drastically reduced number of sharp lines is observed in the 14i1; band (Eence=ss 3 412 cm-I). The X = 0 rotational lines are still seen, while K # 0 lines disappear. This can be understood in terms of Coriolis coupling between the 14'1' state and other vibrational states within S1 and a subsequent om-radiative process. This coupling might be the explanation for the drastic onset of line broadening in C6H6 at an excess energy of 3000 cm-', " channel three." The line broadening is then due to a different process from that responsible for the population lifetime of s1