Nuclear hyperfine structure in the electronic spectrum of propynal

摘要

The laserhyphen;induced fluorescence spectrum of propynal (S1larr;S0) (Atilde;thinsp;1Alsquo;larr;Xtilde;thinsp;1Arsquo;) has been investigated and a series of perturbations due to singletndash;triplet coupling resolved. The singletndash;triplet interaction matrix elements are of the order of 280 MHz. Fluorescence decays for both the mixed singletndash;triplet eigenstates have been measured and are shown to contain quantum beats due to coherent excitation of nuclear hyperfine levels. This Dopplerhyphen;free measurement in the time domain leads to an accurate determination of the nuclear hyperfine splittings produced by the two nonequivalent hydrogen atoms. The hyperfine structure has been analyzed for two rotational levels in the 91vibrational state and gives the following values for combinations of the Fermi contact constants and dipolendash;dipole constants of the two protons in the pure triplet state:T1(atilde;thinsp;3Alsquo;):A(1)FC+ 1/2 (T(1)xxminus;T(1)yy) sim;46 MHz;A(2)FC+ 1/2 (T(2)xxminus;T(2)yy) minus;4 MHz;T(1)zzsim;T(2)zzsim;0 MHz. A simple model for the spin distribution in propynal predicts that the Fermi contact constant should be positive for the aldehydic proton and smaller and negative for the acetylenic proton and suggests that the Fermi contact constants make the dominant contribution to the effective hyperfine constants.