A. Hamdan

Unexpectedly rich vibronic structure in supersonic jet spectra of sulfur dioxide between 360 and 308 nm

Abstract An extensive vibrational resolution study of SO2 between 27780 and 32500 cm−1, spanning absorption to the 3B1, 1A2, and 1B1 electronic states, has been carried out via fluorescence excitation in a molecular beam, achieving substantial improvements in sensitivity and rotational cooling over previous studies. Characterization of the dependence of transition intensities on molecular beam conditions has been used to identify and separate the contributions of cold and hot transitions in the spectra. Vibrational-mode-selective, carrier-gas-dependent cooling is observed and is instrumental in the identification of a group of vibrational levels exhibiting novel Franck-Condon patterns and little or no cold absorption. At least 135 vibronic levels are identified in the energy range studied, adding 71 to the previous total reported from our own and other laboratories. Eight levels below 29000 cm−1 are assigned to the 3B1 electronic state, supported by a set of anharmonic constants to approximate the observed vibrational structure. In the higher energy region, the accepted two-singlet-electronic-state model is compared to the experimental results and shown to be inadequate to account for the large number of observed levels.

Vibrational structure in the low energy region of the 1A2 state of SO2

Abstract Molecular beam fluorescence excitation spectra of sulphur dioxide in the wavelength range 348—334 nm are presented. Identification of cold transitions is accomplished by controlled variation of vibrational temperature, and narrow rotational contours permit accurate determination of the corresponding vibrational band origins. These data conform well with the proposed assignment of 1 A 2 — 1 A 1 vibrational progressions of Hamada and Merer, although different progressions are found to display markedly different intervals. The energy below which bands of the 3 B 1 — 1 A 1 transition exceed those of the 1 A 2 — 1 A 1 system in intensity is found near 347 nm.

Laser-induced fluorescence of selectively excited bands located within the E band's spectral range of SO2

Abstract An attempt to investigate the vibrational bands near the origin of the E band of SO 2 has been made in a supersonic jet experiment. Several cold vibrational bands have been identified within the 32850–32880 cm −1 region and their fluorescence spectra have been selectively excited and measured. A hot band activity at ≈ 32872 cm −1 was found to persist at low rotational temperatures, which caused the Frank-Condon intensity pattern of the E bands fluorescence spectrum to vary when measured under slightly different cooling conditions. The bands upper levels were not all found to have the same vibronic symmetry as that of the E band.

Zero-order 1B1(n,0,0) vibrational levels of sulfur dioxide

More than 100 single vibronic level fluorescence spectra of SO2 throughout the 30 400–34 230 cm−1 spectral region were selectively excited and recorded in crossed supersonic jet and laser beams under conditions of cold rotational temperatures. It was found that the vibronic origins can be grouped into 5 overlapping regions based on identical Franck–Condon patterns of the X 1A1(n,0,0) progressions. The spacing between the centers of these regions was found to range between 572 cm−1 and 647 cm−1 as opposed to the previously estimated value of 690 cm−1. The onset of the 1B1 state in the vibronic coupling was found to be at the 30 553 cm−1 band, which is below the previously reported threshold energy by 220 cm−1. The results provide qualitative verification of the multilevel coupling mechanism proposed by Kullmer and Demtroder [Chem. Phys. 92, 423 (1985)].

Relative mixing coefficients of zero-order 1B1(n,0,0) levels in the 1B1–1A2 hybrid wave functions of the vibronic bands of SO2 in the 32 184–34 040 cm−1 region

The relative mixing coefficients of the zero-order 1B1(n,0,0) vibrational levels present in the hybrid 1A2–1B1 wave functions are estimated according to the observed Franck–Condon patterns in dispersed fluorescence spectra. Non-Condon effects and spin–orbit coupling have been ignored in the treatment. The data cover nearly 50 cold vibronic bands in the 32 895–34 040 cm−1 region in addition to 9 vibronic bands in the 32 185–32 559 cm−1 region.

Identification of the SO2 cold vibronic bands in the 32 200–33 300 cm−1 region

SummaryThe fluorescence excitation spectrum of SO2 in the 32 200–33 300 cm−1 energy region was recorded, at very low rotational temperature, in a pulsed supersonic jet. The band centres and relative intensities of about 60 well-resolved vibronic bands were determined in this region, which extends the previously available data by 800 cm−1. Single vibronic level fluorescence spectra of two neighbouring vibronic bands in theD-band region along with a few filtered excitation spectra are also presented as an example of the resolution for the listed bands.

Phosphorescence from indirectly excited 3B1 vibrational levels of jet-cooled SO2

Abstract A supersonic jet of SO2, seeded in Ar, was crossed with a 301.9–304.3 nm tilted laser beam and the resulting luminescence spectrum from the collisionally populated low SO2 vibronic levels was measured under different molecular beam parameters. The spectrum was found to originate from two distinct sets of upper levels which were described in terms of the 1A2 and 3B1 vibrational levels. Collisions of SO2∗Ar type were found to favor 1A2 → 3B1 intersystem crossing and appeared to dominate at high rovibronic temperatures, while SO2∗SO2 collisions favored the reverse intersystem crossing 1 A 2 ← 3 B 1 . The results also suggest that the reason why the earlier value of the 3 B 1 → 1 A 1 phosphorescence quantum yield appeared to increase at low SO2 pressure could be attributed to some collision-induced vibrational transitions that lie in the same spectral region.

Identification of the S O vibronic bands in the 31120-32160 cm- region 34 16 2 1

Single vibronic level fluorescence spectra of bands in the 31120-32160 cm−1 region of SO2 were measured under low rotational temperature conditions in an expanded supersonic jet. Eight weak bands were confirmed as belonging to the 34S16O2 system : their resonance vibrational transitions were found to match the term energies of this isotope. The correlating bands in the 32S16O2 system were identified by noting that the Franck-Condon patterns of their dispersed fluorescence spectra were similar to those of the 34S16O2 bands. The analysis of the spectra also concludes that some of the weak bands in the above region can be due to 32S16O2 bands whose upper levels have Bev 2 = Be 2 × av 1 vibronic symmetry.