| Popis: |
Tunable dye laser excitation of CN near 6000 A in an atmospheric pressure CH4/N2O flame has resulted in the production of B 2Σ+−X 2Σ+ system fluorescence, detected in the Δv=0 region (≈3860 A). Analysis shows that four different mechanisms contribute to the spectrum in overlapping laser wavelength regions: (1) In the range from 5890–6016 A, the spectrum is dominated by transitions in the B 2Σ+−A 2Π (0, 0) band. This technique can be used with care as a quantitative CN A 2Π diagnostic in flames. (2) In the range 6016–6180 A the excitation spectrum is positively identified as that of the A 2Π−X 2Σ+ (8, 3) and (9, 4) bands. However, fluorescence occurs from the B 2Σ+ state which can be populated only by collisional uptransfer. Part of the A 2Π−X 2Σ+ (10, 5) band near 6300 A has also been observed via the same mechanism; however, because A 2Π, v=10 is nearly isoenergetic with B 2Σ+, v=0 endoergic collisions are not necessary to promote the intersystem crossing. (3) In the range 6180–6298 A, the spectrum consists mainly of lines produced by a collision-assisted single-color optical—optical double resonance (OODR). The first photon of the OODR is absorbed in the A 2Π−X 2Σ+ (4, 0) band, the second in the B 2Σ+−A 2Π (3, 4) band. The density of lines in the region is possible only if collisional rotational relaxation occurs in the intermediate A 2Π, v=4 level during the laser pulse. (4) In the same wavelength region (6180–6298 A) a second, independent process occurs: near-resonant two-photon excitation (NR2PE) in the B 2Σ+−X 2Σ+ (3, 0) band by means of intermediate A 2Π, v=4 rotational levels. The only B 2Σ+−X 2Σ+ (3, 0) NR2PE transitions observed are those for which the laser is detuned from a resonance in A 2Π−X 2Σ+ (4, 0) by less than 20 cm−1. This two-photon absorption process is independent of collisional conditions. The observed spectra illustrate some of the unexpected effects of collisions and laser intensity on absorption and fluorescence in this molecule, characterized by multiple low-lying electronic states. |
