Poor detection limits of two-photon excited fluorescence in cylindrical capillaries are attributed to photothermal expansion and beam astigmatism. Photothermal expansion is demonstrated for excitation in a 1 cm cell and is inferred for the larger diameter capillaries. Astigmatism is caused by focal differences between rays in a plane longitudinal to the capillary and rays in a plane transverse to the capillary. Data were obtained by integrating the fluorescence from variously sized cylindrical and square capillaries, and by photographing the fluorescence within a 1 cm cell.
Several common aminopeptidase probes are examined for the strength of two-photon excited fluorescence. The fluorophores 4-methoxy-β-naphthylamine and 7-amino-4-methylcoumarin have limits of detection of 930 pM and 200 pM, respectively. The coumarin derivative is shown to have a two-photon cross section at 590 nm, which is exceptionally large for an aromatic amine. Spectral data for this compound also indicate that excitation near 690 nm should permit quantitation at concentrations as low as 40 pM.
Dispersed fluorescence ͑DF͒ spectroscopy is used to explore the rovibrational structure of highly excited S 0 formaldehyde ͑H 2 CO͒. A narrowband laser excites formaldehyde molecules to a single S 1 rovibronic quantum state, and the resulting fluorescence is dispersed with a monochromator. DF spectra of ten vibrational levels with excitation in 2 , the carbon-oxygen stretch, and 4 , the out-of-plane bend, have been recorded, and the effective A, B, and C rotational constants are extracted. Five of the effective A rotational constants and seven of the effective B and C rotational constants are new to the literature. The dependence of these effective rotational constants on vibrational state are both calculated and discussed with regard to both the present and previous experiments. Particular attention is given to the manner in which that the effective A rotational constant depends on increasing excitation in 4 due to the strong A-axis 4 / 6 Coriolis interaction. For states where v 2 is less than two, quantitatively accurate values for the nonlinear dependence of the A rotational constant on quanta in 2 and 4 is achieved by a simultaneous consideration of the strong A-axis 4 / 6 Coriolis interaction and the 1 1 ↔4 2 , 1 1 ↔6 2 , and 5 1 ↔2 1 6 1 Fermi interactions.
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