Hyperfine quantum beats in the 209 nm fluorescence excitation spectrum of cyanogen

Abstract

Singlet–singlet, singlet–triplet, and polarization hyperfine fluorescence quantum beats are observed following 209 nm excitation of the 1141 vibronic level of the Ã(1Σ−u) S1 singlet state. Singlet–singlet quantum beats arise from S1–S0 vibronic coupling to states of the S0 manifold lying above the dissociation limit, but isolated from the dissociative continuum by a high barrier to predissociation. Measurements of fluorescence lifetimes and quantum beat spectral widths set an upper bound on this predissociation rate of approximately 106 s−1. The singlet–singlet quantum beats for the N=9 rotational state provide the first example of quantum beats arising from superposition states from two separate (I=0 and I=2) nuclear spin states. Singlet–triplet quantum beats for N=12 exhibit magnetically induced modulations that provide further support for a theoretical model previously used to describe similar modulations for the N=8, 12, and 18 rotational states of the 41 vibronic level excited at 219 nm. Analysis of hyperfine polarization beats, associated with the reversible interchange of molecular polarization and nuclear spin polarization, yields results in satisfactory agreement with the predictions of the perturbation theory formulation of Fano and Macek.