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

Pathways for Intramolecular Relaxation in S1 Benzene

Sub-Doppler spectra of various one- and two-photon vibronic bands of benzene are discussed and analysed to determine the pathways of intramolecular relaxation for S1 benzene. New results are presented for the 14011011622 band of C6H6 and the 1401102 band of 13C6H6. The decay behaviour depends strongly on the excess energy and the rotational quantum numbers rather than on the vibrational character and symmetry of the excited state. At low vibrational excess energy the pathway for intramolecular relaxation is a coupling in the strong limit between pairs of states in S1 leading to shifts of lines, whereas at intermediate excess energy coupling in the weak limit to background states in S1 is present. These background states are strongly broadened due to a fast electronic non-radiative process. The intramolecular relaxation is found to be initiated by the coupling to the broadened S1 background states and energy can flow via these states to the T1 or S0 state. The rotationally selective disappearance of lines is believed to be due to an intricate interplay of the rotational dependence of the coupling matrix elements and accidental resonances, which lead to interference of possible decay channels

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

Sub-Doppler UV spectroscopy by resonance-enhanced two-photon ionization: the structure of the benzene20,22Ne cluster

In this work it is demonstrated that mass selected multiphoton ionization is a powerful technique for high resolution spectroscopy, isotope cluster separation and investigation of the structure of van der Waals clusters. The rotationally resolved UV spectra of the benzene-20Ne and benzene-22Ne clusters are selectively measured in a natural isotopic mixture of benzene and benzene-Ne clusters in a cooled supersonic jet. The analysis of these spectra yields accurate values for the rotational constants of both species. From this data it is found that the Ne atom is located on the C6 rotational axis of the benzene ring at an average distance of 3.46 Å with a slight difference for the two isotopes. This distance decreases by 40 mÅ when benzene is electronically excited. The influence of the large amplitude van der Waals vibrations on the average bond length is discussed