Time-resolved fluorescence using synchrotron radiation excitation: A powered fourth-harmonic cavity improves pulse stability

The pulsed nature or "time structure" of synchrotron radiation from electron storage rings is used to measure the kinetics of the decay of electronically excited states and is particularly useful because the wavelength of excitation can be chosen at will. However, changes in the length of the pulses of radiation from a storage ring resulting from the gradual decrease of current circulating in the ring during the course of a "fill" limit the duration of data collection, and hence photometric sensitivity. A fourth-harmonic cavity that was recently added to the vacuum ultraviolet (VUV) storage ring at the National Synchrotron Light Source slows the loss of current during a fill, and thus increases the total number of photons produced. When operated in a passive (unpowered) mode, however, the fourth-harmonic cavity increases both the average width of the photon pulses and the changes in width that occur during a fill, thus reducing the usefulness of the VUV ring in timing experiments. We demonstrate that operating the fourth-harmonic cavity in an active (powered) mode, while further increasing pulse duration, can stabilize pulse width, thereby restoring timing capabilities.