Excited state interactions of isolated molecules

The spectral properties of photoexcited polyatomic molecules under isolated expansion conditions have been investigated by both direct and indirect spectroscopic methods. A novel spectrometer was constructed in order to perform excited state spectroscopy and dynamics on a number of case-study molecules by the complimentary techniques of laser-induced fluorescence spectroscopy (LIF) and laser-induced phosphorescence (LIP) spectroscopy (LIP). Chapter III includes the first case-study molecule to be examined. Using the techniques of laser-induced fluorescence (LIF) and dispersed fluorescence (DF), the spectroscopy of the S1(A2') - S0(A1') electronic spectrum of jet-cooled 1,3,5-trifluorobenzene (TFB) is reported. To aid analysis of the observed spectra, theoretical calculations were performed, providing estimates for equilibrium geometries and vibrational frequencies relating to the ground and first excited electronic states. Assignments are reported for 14 of the 20 S1 excited state vibrations, and 10 for the first time. The emission spectra from almost all S1 levels was observed to be complex, and in many cases very congested. To explore whether the congestion is due to intramolecular vibrational redistribution (time-dependent) or state mixing (time-independent), the technique of picosecond photoelectron imaging was employed in Chapter IV. In Chapter V, the technique of LIF and DF spectroscopy is applied to the S2(A') -S0(A') electronic spectrum of jet-cooled isoquinoline (IQ). The vibrational assignments and interpretation of previous studies is confirmed, and thirteen previously unassigned S2 frequencies were identified. Chapter VI reports upon the simultaneous measurement of the LIF and LIP excitation spectra for jet-cooled samples of biacetyl. The biacetyl experiments reported previously have served to establish the technique of dual fluorescence and sensitized phosphorescence measurement of jet-cooled samples new spectrometer. Chapter VII finally reports upon the simultaneous measurement of the LIF and LIP excitation spectroscopy for the S1(1B2u) - S0(1A1g) of jet-cooled benzene for the first time. The LIP excitation spectrum of benzene was recorded using a crystalline biacetyl matrix as sensitizer. The excitation spectra reported in this work correspond to transitions within the vicinity of the Channel three energy region. Analysis of the simultaneously recorded LIF and LIP excitation spectra has revealed quantum yields and decay rates of ISC and IC for 24 single vibrational levels of the S1(1B2u) electronic state