## LSU Historical Dissertations and Theses

1987

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Abstract

The first experimental observation of laser induced fluorescence from the furan cation, C$\sb4$H$\sb4$O$\sp+$, is reported. The ion is formed in a supersonic free jet expansion via a two photon photoionization involving the 193 nm output of an ArF excimer laser. The B $\to$ X electronic transition is probed and vibrational assignments are presented for both states. Comparisons to the infrared, raman, multiphoton ionization, and photoelectron spectrum of the parent molecule are made. The term value, T$\sb{00}$, for the B $\to$ X electronic transition is located at 24,675 cm$\sp{-1}$, which differs drastically with previously reported values from both experimental and theoretical investigations. These differences are discussed and probable explanations proposed. Our rotational analysis bears out the expectation that the furan cation is a near oblate top. Rotational constants and assignments are discussed. The lifetime of the vibrationless level of the B state is measured to be 330 ns. We also report the observation of C$\sb2$ Swan band emission (d$\sp3\Pi\sb{\rm g}$ $\to$ ${\rm a}\sp3\Pi\sb{\rm u}$, 170 ns lifetime) in a supersonic free jet using a pulsed laser/vaporization source with a graphite target. In helium, Swan band emission is observed at distances of up to 15 nozzle diameters from the orifice and as long as 6 $\mu$s after the laser pulse, which immediately establishes the existence of some mechanism for the continuous localized formation of excited C$\sb2$ molecules at this point in the free jet. The emission data are reported as a function of laser intensity, backing pressure, downstream distance, and delay times. Evidence is presented supporting the continuous localized formation of excited C$\sb2$ molecules in the free jet via dissociative processes involving larger neutral and charged carbon clusters. The bearing of these results on the interpretation of cluster stability data is discussed.

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