T. Winkler

Gateway-type intramolecular collisional transfer in 15NO(a4π, b4Σ− → B2π)

Abstract Excitation of 15NO molecules into the B 2π state by collisional intramolecular energy transfer from the metastable a 4π state was studied under single-collision conditions. Passing a beam of 15NO (a4π) through a cell filled with argon at a few mTorr, B 2π → × 2π emission was observed. It originated from only three specific B state levels, namely 2π1/2, J = 21·5 and 2π3/2, J = 21·5 in v = 2 and 2π1/2, J = 10·5 in v = 3. This selectivity is characteristic of a gateway-type process mediated by spectroscopic perturbations. The analysis showed that in the last case there is a direct spin-orbit interaction of the emitting B state level with a well-specified level of NO(a). The other two perturbations are caused by interactions of the respective NO(B) levels with uniquely identifiable levels of the short-lived NO(b4Σ−) state, which in turn are collisionally populated from a broad range of NO(a 4π) levels. This latter mechanism is thus a novel type of gateway process. The results explain the anomalous sp...

Laser-induced phosphorescence of jet-cooled pyrimidine

Laser-induced phosphorescence excitation spectra of pyrimidine molecules have been observed under molecular beam conditions. The origin of the lowest triplet state T1(3B1) was measured to be at 28534.0 cm−1. More than 30 vibrational bands of the T1←So transition were observed in the region E = 0–1300 cm−1 from the origin. High-resolution (0.03 cm−1) laser scans of the most intense bands show parallel type contours. Lifetimes of selected T1 state vibrational bands were measured to be ⩾ 400 μs for the origin, decreasing to 55 μs at E = 1261.0 cm−1.

Gateway coupling between quartet and doublet states of 15N18O studied in molecular beams

Abstract Collision-induced intramolecular electronic energy transfer in NO was studied using a beam of metastable 15 N 18 O(a 4 Π) molecules colliding with Ar atoms in a cell. As in our earlier work on 14 N 16 O and 15 N 16 O, selective excitation of certain NO(B 2 Π) levels was observed by means of their β-band emission. In the B, ν = 2 level three emitting levels could be uniquely identified in terms of rotational, fine structure and Λ component quantum numbers. They are gateway-coupled to likewise fully characterized b-state levels, which are collisionally populated from the a state. Weaker and less specific emission from the B, ν = 3 level was also observed, but not rotationally assigned.

Molecular beam experiments clarifying conflicting reports on magnetic field effects in NO(B) afterglows

Abstract A search for NO(BX) β -band emission from the impact of metastable Ar on N 2 O and metastable N 2 on NO was conducted under single-collision conditions. For the cross sections to produce NO(B), upper limits of 0.001 and 0.1 A 2 respectively, were established. The β -band emission reported from afterglow studies of these two reactions was therefore probably due to a secondary process. Most likely NO(B 2 Π) was formed via NO(a 4 Π) as a precursor. This would explain the reported magnetic-field induced quenching of the β -band intensity, by the mechanism studied by us in a beam of NO(a 4 Π). On the other hand, we detected β -band emission from the interaction of a beam containing metastable N atoms (as well as molecules) with N 2 O. This agrees with corresponding afterglow studies, where no magnetic field effect on this emission was observed. NO(B) being the primary product in this case, magnetic quenching would not be expected.

Observation of the electronic energy transfer reaction CO(a3Π) + NO(X2Π) → CO(X1Σ+) + NO(a4Π) in molecular beams

Abstract Metastable CO(a 3 Π) molecules in a molecular beam were allowed to interact with ground-state NO molecules under single collision conditions. Optical emission from the collision region was spectrally analyzed at a resolution of 1.5 A FWHM. In the wavelength range 290–400 nm, NO β bands having an anomalous contour were observed. Superimposed on a broad intensity distribution were pairs of narrow “spikes”. The broadband emission is due to NO(B) formed by energy transfer from CO(a) to NO(X). The line emission is evidence of another reaction branch, giving NO(a) products. Only those NO(a) molecules can emit which originate in specific “gateway” levels, exhibiting some NO(B) character as a result of S/O perturbation. The gateway emission was filtered out of the broadband background utilizing magnetic field quenching.

Spectrally Resolved Phosphorescence and Slow Fluorescence from Selected Vibronic Levels of Jet-Cooled Glyoxal

Trans-glyoxal molecules in a seeded pulsed jet were laser-excited into selected vibrational levels of the T1 and S1 states. In the latter case, those specific rovibrational levels were chosen which exhibit S1/T1 gateway coupling. In each case, the long-lived emission into the S0 state was observed employing a spectrograph/CCD combination, covering 5000 cm-1 at 60 cm-1 FWHM resolution. A model Franck–Condon calculation of the relative ν5 band intensities demonstrates the structural similarities of the S1 and T1 states. However, there are some distinct differences, despite the almost identical S1 and T1 vibrational frequencies. The S1← S0 excitation distorts the molecule significantly more in the ν5 dimension than does T1← S0.