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I. DOPPLER-FREE TWO-PHOTON EXCITATION SPECTOSCOPY OF BENZENE

Doppler-free high-resolution spectroscopic technique is powerful tool for studying the structure and dynamics of excited polyatomic molecules in detail and unambiguously. Single-mode auto-scan laser systems which work in UV-Visible-NearIR region, the absolute wavenumber measurement system, and several Doppler-free high-resolution spectroscopic measurement systems have been constructed to investigate the excited molecules. High-resolution and high-accuracy of the spectral lines enable to observe rotational resolved electronic transition and to find out the excited state dynamics through the fairly deviation of the spectral position, intensity anomaly and the spectral linewidth. High-resolution laser spectroscopy is very useful to accurately study the molecular structure and dynamical processes in the electronic excited such as internal conversion (IC), intersystem crossing (ISC), and intramolecular vibrational redistribution (IVR). Benzene is a prototype aromatic molecule, and its dynamics in excited states have been studied extensively. Electronic relaxation from the S₁ state can occur via three pathways: radiative transition to the S₀ state, nonradiative transition to isoenergetic levels of vibrationally highly excited levels of the S₀ state (IC), and the one of a triplet state (ISC). The fluorescence quantum yield of the S₁ state of an isolated benzene was observed to be small, and nonradiative decays excited to low vibrational levels of the state were attributed to ISC. Extremely high-resolution can be achieved by Doppler-free two-photon excitation spectroscopy, and Zeeman spectra combined with high-resolution spectroscopies are available to identify the coupling between singlet and triplet states.

Figure 1. Doppler-free two-photon excitation spectra of benzene at the magnetic field H = 1.2 T (upper trace) and H = 0 T (lower trace). Assignments of the Q(K)Q(J) lines are indicated as JK. Remarkable Zeeman splittings (broadenings) are observed for K=J lines. The blue and red lines shows the perturbed and perterbing levels for 2521 and 2623, respectively. Arrows are indicated the calculated (unperturbed) line positions.

Figure 2. (left) K-dependence of the observed Zeeman splittings of the Q(K)Q(40) lines at H=1.2T. (right) J-dependence of the observed Zeeman splittings of the Q(K=J)Q(J) lines at H=1.2T.

II. DOPPLER-FREE POLARIZATION LABELING SPECTROSCOPY OF NAPHTHALENE

The level density of the rovibronic states of polyatomic molecule makes difficult to assign the rotational resolved high-resolution spectrum. Optical-optical double resonance (OODR) technique is very useful to simplify and assign for such a complicated spectrum. Doppler-free polarization labeling (DFPL) spectroscopy is a very sensitive technique, and it is easily accessible Doppler-free OODR polarization labeling (DFOOPL) spectroscopy which is a powerful tool for the unambiguous assignment for the complicated spectrum even for the perturbed line. Naphthalene is a prototype aromatic molecule next to benzene, and a number of studies have been reported on the electronically excited states and radiationless transitions. The linewidth of the several vibronic bands of the S₁←S₀ transition in a supersonic jet was reported to increase as the vibrational energy in the S₁ state increases, and this line broadening was assigned to intramolecular vibrational redistribution (IVR). High resolution spectroscopic techniques are useful to observe such line broadening for resolved single rotational lines. DFPL and DFOOPL spectroscopies are applied to measure the high resolution spectra and confirm the unambiguous rotational assignment for the interacting region.

Figure 3. The difference between the observed transition energy Eobs and the energy Ecal calculated from the obtained molecular constants is plotted for the rPKa(J) line (Ka=0-9).

III. HIGH RESOLUTION SPECTROSCOPY OF DIBENZOFURAN

Toxic dioxins are serious environmental problems. The three prototype molecules are dibenzo-p-dioxin, dibenzofuran, and biphenyl, and their chlorinated and brominated compounds are highly toxic. It is great important to investigate the molecular structure and the excited state dynamics. High-resolution fluorescence excitation spectrum of dibenzo-p-dioxin in a collimated molecular beam has been studied and found that this molecule is bent out-of-plane and predissociating in the S₁¹B_1u state. This spectroscopic method also applied to dibenzofuran molecule, and fully rotational resolved high-resolution spectrum has been observed for the S₁ ← S₀ transition.

Figure 4. Ultrahigh-resolution fluorescence excitation spectra of the a) 000 and b) 000 + 443cm-1 bands of the S1 ← S0 transition of dibenzofuran in a collimated molecular beam.

For these molecules, benzene, naphthalene, dibenzofuran, the Zeeman splittings were observed !