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A frequency doubled, tunable dye laser has been used to excite fluorescence from the first excited singlet state of SO2 (2600–3250 A). Two different groups of rovibronic states, with different collision‐free lifetimes and bimolecular quenching rates, are observed throughout the region. The major group of states has exceptionally long collision‐free lifetimes (80–600 μsec), which are up to a factor of 20 longer than previously reported. The minor group, which has not been observed by previous workers, has ∼50 μsec lifetimes and a bimolecular quenching rate that is an order of magnitude above the gas kinetic rate. No evidence for collisional equilibration between the groups is found. The exceptionally long lifetimes invalidate the results of previous experiments, from which it was concluded that the first excited singlet state undergoes unimolecular radiationless transitions. The major group is discussed in terms of extensive Renner rovibronic perturbation between an excited, quasilinear 1B1 state and the g...

A pulsed, frequency doubled dye laser has been used to study time and wavelength resolved emission from low pressure quinoxaline, quinazoline, and cinnoline vapors. The S1, 1(n−π*) states of quinoxaline and quinazoline undergo statistical limit intersystem crossing, while the S2 (π−π*) states undergo S2−S1 and singlet‐triplet intermediate strong coupling. Excitation into S2 yields a two‐component ``fluorescence'' having an S1‐type emission spectrum, and a longer component lifetime 102–103 longer than predicted by the S0−S2 f number. This anomalous emission is discussed principally in terms of a sequential decay theory of Tric and co‐workers. Intermediate strong coupling appears to evolve into statistical limit coupling as a function of excitation energy. The 3B2 quinoxaline collision free phophorescence lifetime is within a factor of 2 of its value in condensed phases at low temperature. Thus the existance of rotational degrees of freedom in the gas does not significantly enhance the T1→S0 intersystem cro...

The fluorescence decay of NO3 excited to vibronic levels between the origin at 661.9 and ∼606 nm is measured and found to be nonexponential. An exponential fit to the long‐lived portion of the decay gives an apparent, collision‐free lifetime of 340±20 μs. Electronic quenching rates have been measured for a variety of collision partners including He, N2, O2, C3H8, and HNO3. Our results are interpreted in terms of the Douglas effect, i.e., an extensive coupling of excited electronic levels with nonemissive vibronic levels of the ground electronic state. This coupling is likely responsible for the anomalously long lifetime and the apparent diffuseness of the absorption spectrum.

The technique of laser induced fluorescence saturation spectroscopy is evaluated for determination of absolute atomic and molecular concentrations in flame sources. The combustor is an aspirating slot burner using premixed acetylene and air to create a stable homogeneous flame source. Concentration measurements of atomic sodium and MgO (X 1Σ+ and A 1Π) are made by saturation spectroscopy and laser induced fluorescence techniques. Independent atomic and molecular absorption measurements are made for Na and MgO (X 1Σ+) which are in agreement with saturation spectroscopy concentration determinations. The effect of using various laser beam profiles (rectangular, Gaussian, and truncated Gaussian) are evaluated. It is found that MgO exists in the flame in a significant inversion in the (A 1π) state and possible production mechanisms for MgO in the flame are considered.

The excited state dynamics of sym ‐tetrazine are investigated following dye laser excitation into single vibronic levels of the 1B3u and 3B3u states. The lifetime of the 1B3u state, τf ≤ 100 nsec, is probably controlled by unimolecular internal conversion into the 1Ag ground state. Intersystem crossing is not observed at low pressure and is not induced by hard sphere collisions. The 3B3u → 1Ag phosphorescence, τp = 72 ± 2 μsec, is reported for the first time. Theory suggests that this lifetime is not controlled by radiationless intersystem crossing, and it may be that the 3B3u lifetime is controlled by a radiative transition to a possible lower lying 3B1g or 3B2g state. Energy transfer experiments from triplet p ‐benzoquinone to 3B3u sym ‐tetrazine confirm the lifetime and spin‐forbidden character of the phosphorescence emission.

Time‐resolved emission from low pressure p‐benzoquinone‐h4 (and ‐d4) is observed following single vibronic excitation of the 1B1g and 3Au states with a tunable dye laser. Evidence is presented showing that 1B1g levels undergo an intermediate strong coupling with vibronic levels of lower excited electronic states. These hybrid states, which have collision‐free emissive liftimes of 7–15 μsec, are collisisonally relaxed at a near gas kinetic rate, with at least partial conversion to the vibrationally equilibrated 3Au state. A simple three level relaxation model is investigated and deemed to be inadequate. The 3Au phosphorescence liftime is 28.4 μsec (37.6 μsec for benzoquinone‐d4,) independent of pressure for added argon and nitrogen and benzoquinone, itself. Comparison of the 3Au phosphorescence lifetime with 1Ag→3Au f number indicates that the 3Au lifetime is controlled by a unimolecular process other than 3Au→ Ag phosphorescence. We prospose that the unimolecular process competing with the 3Au→1Ag phospho...

The reaction rate of C2(a3Πu)+CH4 is reported for temperatures of 337 to 605 K. The C2(a3Πu) is produced by multiphoton 193 nm laser photolysis of hexafluorobutyne‐2 and monitored with a tunable dye laser. The data are fit to an Arrhenius equation of the form kII(T)?0.20)×10−11 exp {[−5.57±0.11 kcal/mol]/RT} cm3 molecule−1 s−1.