Heterogeneous perturbations in the Doppler-free S1 ← S0 two-photon spectrum of benzene: Evidence for intrastate coupling

Rotational perturbations are identified in Doppler-free two-photon spectra of the 1410 and 1410110 vibronic bands in C6H6. Evidence is found that Coriolis coupling between some of the rotational levels of two distinct vibrational states within S1 is the mechanism responsible. This coupling mechanism is thought to be responsible for irreversible intramolecular relaxation at higher excess energies and higher vibrational state densities

Sub-doppler two-photon spectrum of asymmetric rotor molecules

The Doppler-free two-photon excitation spectrum of the qqQ branch of the 1410 vibrational band of the S1(1B2u) ← S0(1A1g) transition of benzene-d1 has been recorded using a cw single-mode dye laser coupled to an external concentric resonator. The spectrum has been analysed using a non-rigid Watson Hamiltonian. More than 200 lines with J up to 20 have been assigned and the rotational constants which best reproduce the spectrum are A1v = 0.181435, B1v = 0.169990, C1v = 0.089055 cm−1. The Ka = odd lines of the qqQ5(J) subbranch show small and quite regular perturbations of 60 ± 5 MHz which are probably due to a coupling to another vibrational state of the S1 manifold

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

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 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

Frequency shifting of pulsed narrow-band laser light in a multipass Raman cell

A multipass cell is described which allows efficient stimulated Raman frequency shifting for low pump laser intensities and low gas pressures. The latter is important for Raman shifting of narrow-band Fourier-transform limited light pulses (Δv=75 MHz). It is shown that frequency broadening of the Raman shifted light can be largely avoided in the Dicke narrowing regime at low pressures. For 75 MHz pump pulses and an H2 density of 2.5 amagat we found a negligible broadening to 90 MHz of the stimulated Stokes light. This is far below the value of 250 MHz expected from spontaneous emission. The narrow-band Stokes pulses achieved in CO2 enabled us to measure the pressure shift coefficient (-0.71×10-2 cm-1/amagat) of this gas. It is demonstrated, for the example of benzene, that our technique provides a very practical light source for high resolution molecular spectroscopy