Horn-Bond Lin

Low quantum-yield molecular fluorescence: excitation energy dependence and fluorescence polarisation in xanthene dyes

Abstract Consecutive two-photon laser excitation has been used to populate elevated singlet states of the xanthene dyes rhodamine 6G and rhodamine B. Fluorescence has been observed from several states in each molecule, terminating in the ground state, in the range 240-400 nm. It is demonstrated that the shape of the spectrum depends on both excitation energy and polarisation, and the hydrocarbon rubrene is used as an example of the excitation intensity dependence of the weak emission spectrum from a fluid solution.

Low quantum-yield molecular fluorescence. Aromatic hydrocarbons in solution at 300 K

Abstract Low quantum-yield fluorescence spectra are presented for a number of polycyclic aromatic hydrocarbons in solution at 300 K in the region 240–370 nm. This emission corresponds to fluorescence from highly excited singlet states directly to the ground state. Strong evidence is presented for emission from vibrationally unrelaxed states.

Excited singlet state kinetics on a subnanosecond time-scale

Abstract It is possible to study directly the absorption time-profiles of short-lived excited singlet states by sampling the transient excitation spectra for fluorescence from upper states. This approach has many advantages over direct absorption measurements, since the effects of triplet state absorption can be suppressed and the population time-profiles of excited singlet states can be studied in detail. The first direct measurements are reported of singlet state absorption time profiles in a number of aromatic aza- and carbonyl compounds.

Fluorescence from subpicosecond states of 3,4,9,10-dibenzpyrene

Abstract A systematic study has been made of the fluorescence from higher excited singlet states of 3,4,9,10-dibenzpyrene. Particular attention has been paid to the dependence on the excitation conditions of the intensity, structure and overall shape of the fluorescence spectra. It is confirmed that the upper states relax electronically before significant vibrational relaxation occurs: in fact excess vibrational energy is found to increase the rate of internal conversion, consistently with observations made on isolated small molecules. Calculations have been made of the relaxation times of several electronic states, all in the subpicosecond region.

Optical hole-burning in the ultraviolet spectrum of 3,4,9,10-dibenzpyrene at room temperature

Ultraviolet fluorescence spectra are presented for 3,4-benzpyrene, 1,12-benzeperylene and 3,4,9,10-dibenzpyrene, which were obtained by single-laser, consecutive two-proton excitation. Experiments using two-laser excitation have permitted the study of fluorescence line-narrowing in the spectrum of dibenzpyrene at 300 K in hexane solution. These unique observations are interpreted in terms of optical site-selection.

Spectroscopic measurements of biphenylene singlet states

Abstract We report the observation of fluorescence from higher singlet levels of biphenylene. The molecule was excited by either two- or three-photon absorption and the resulting fluorescence to the ground state was observed in a broad band centered near 265 nm. We have measured the decay time of the lowest excited singlet state to be 240 ± 20 ps in hexane at 300 K.

Evidence for vibrational mode-selectivity in the radiationless relaxation of a large molecule

Abstract Resonance fluorescence effects are absent in the two-photon-excited upper state fluorescence spectra of a centrosymmetric aromatic hydrocarbon. Spectra of states populated by radiationless relaxation are structured and sensitive to the excitation conditions. The results are interpreted in terms of vibration mode-selectivity of the internal conversion process between two highly excited singlet states.

Measurement of Singlet Lifetimes by Upper State Fluorescence Excitation

Increasingly sophisticated analysis of chemical and related phenomena in the subnanosecond time-range has imposed increasingly stringent requirements on the accuracy and reproducibility of experimental results obtained through fast laser experiments. Foremost among the problems being studied in this fashion are relaxation processes involving excited singlet states of small and medium-sized aromatic molecules, particularly in liquid environments. Much current attention is being turned towards weakly or effectively non-fluorescent states since their relaxation is dominated by strong, non-radiative interactions. Where the lowest states are fluorescent, there are several ways to time-resolve the transient populations. However, where the fluorescence is weak, two-photon gating techniques including transient absorption and fluorescence excitation spectroscopy have much to offer.

Fluorescence Spectroscopy of Subpicosecond States in Liquids

We have recently shown [1,2] that it is possible to make direct observations of phenomena originating from short-lived upper states of aromatic molecules in solution. The method which has proved to be the most successful in observing such upper states uses consecutive two-photon pulsed laser excitation, followed by fluorescence detection.