High resolution lifetime measurements of the perturbed J′=0 levels of the 1B3u state of pyrazine

Abstract

The lowest excited singlet 1 B 3u state of pyrazine is known to be coupled to a number of triplet 3 B states. Using a strongly collimated molecular beam and a single frequency laser it is sh~~ that the J I = 0 of the og transition contains at least 36 states. We have individually excited eight ofthese states and studied its decay. The lifetimes found (typically 450 ns) ~o.not scale with the intensities of the excitation spectrum. This deviation is caused by a nonradlatlve decay of the zero order 3 B 3" states. With a simple model it was possible to reconstruct the absorption spectrum, the energies of the zero order states and its coupling strengths. The zero order decay rates of the singlet and triplet s~ates have been determined. The value obtained for the zero order singlet state is 5 MHz; the values for the triplet states range from 0.6 to about 5 MHz.

Magnetic field dependence of rotationally resolved excitation spectra of the 1B3u O00 transition of jet-cooled pyrazine

“…Summarising, it has been shown that (a) the strength of the singlet-triplet coupling in the B 3u states is not dependent on a magnetic field, (b) the zeroorder width of the triplet states is due to purely intramolecular relaxation and does not arise solely from collisions, and (c) the zero-order width γ of the triplet states consists both of a magnetic field sensitive part (with a typical value of about 0.3 MHz at zero field) and a magnetic field insensitive part (with a typical value of about 0.7 MHz). For low J´ this width is not the dominating part of the total width for those MEs which are responsible for the vast majority of the fluorescence intensity [8]. However, in a magnetic field this may not be the case.…”

“…We then obtain 2γ 1 (H = 0) ≈ 2Γ/(3N); i.e., γ 1 (H = 0) ≈ 0.3 MHz, where we have used γ 1 (H) ≈ 3γ 1 (H = 0). Using a typical value for γ of about 1 MHz [8], we find for γ 2 about 0.7 MHz. Note that for high J´, where [6,7] Γ ME =Γ/N+γ+Γ c ≈ γ+Γ c ,…”

“…Apparently the rotational branches do not broaden in a magnetic field. This is easily understood: in a magnetic field the set of zero-field triplet states [1,2,8] shifts away, but a new set of triplet states becomes resonant. Provided the singlet-triplet interaction does not increase in a magnetic field, the overall linewidth of the rotationally resolved branches does not broaden.…”

“…Evidently, this lifetime is collisionally shortened (the lifetime should be at least 400 ns [2,8,14] ). We could hold collisions responsible for the threefold decrease of the emission intensity in the following way.…”

“…An interpretation of the behaviour of the A + /Aratio was given by Felker et al [3], who mentioned that in a magnetic field the selection rule for singlettriplet coupling is ∆J = 0, ±1 [6,7], thus allowing for a threefold increase in the number of coupled triplets (we recall that the 1 B 3u v´ = 0, J´ = 0 state of pyrazine is known to couple to about 13 triplet states in zero field [2,8]). …”