N. Hemmi et Ta. Cool, HYPERFINE QUANTUM BEATS IN THE 209 NM FLUORESCENCE EXCITATION SPECTRUM OF CYANOGEN, The Journal of chemical physics, 105(18), 1996, pp. 7964-7975
Singlet-singlet, singlet-triplet, and polarization hyperfine fluoresce
nce quantum beats are observed following 209 nm excitation of the 1(1)
4(1) vibronic level of the (A) over tilde((1) Sigma(u)(-)) S-1 singlet
state. Singlet-singlet quantum beats arise from S-1-S-0 vibronic coup
ling to states of the S-0 manifold lying above the dissociation limit,
but isolated from the dissociative continuum by a high barrier to pre
dissociation. Measurements of fluorescence lifetimes and quantum beat
spectral widths set an upper bound on this predissociation rate of app
roximately 10(6) s(-1). The singlet-singlet quantum beats for the N=9
rotational state provide the first example of quantum beats arising fr
om superposition states from two separate (I=0 and I=2) nuclear spin s
tates. Singlet-triplet quantum beats for N=12 exhibit magnetically ind
uced modulations that provide further support for a theoretical model
previously used to describe similar modulations for the N=8, 12, and 1
8 rotational states of the 4(1) vibronic level excited at 219 nm. Anal
ysis of hyperfine polarization beats, associated with the reversible i
nterchange of molecular polarization and nuclear spin polarization, yi
elds results in satisfactory agreement with the predictions of the per
turbation theory formulation of Fano and Macek. (C) 1996 American Inst
itute of Physics.