Carol Cheng

Doppler-free two-photon spectroscopy in the vacuum ultraviolet: helium 1 1S 2 1S transition

We describe techniques for laser spectroscopy in the vacuum-UV (VUV) spectral region that combine high spectral resolution with high absolute accuracy. A nearly transform-limited nanosecond laser source at 120 nm is constructed with difference-frequency mixing. This source is used to perform the first, to our knowledge, Doppler-free VUV measurement. We measure the inherently narrow 11S–21S two-photon transition in atomic helium with a spectral resolution of 7 parts in 108 (180 MHz), the narrowest line width so far observed at such short wavelengths. Careful measurements of optical phase perturbations allow us to determine the absolute frequency of the line center to a fractional uncertainty of 1 part in 108. Improvements now in progress should reduce this uncertainty to 2 parts in 109.

Determinations of EF {sup 1}{sigma}{sub g}{sup +}(leftarrow)X{sup 1}{sigma}{sub g}{sup +} transition frequencies in H{sub 2}, D{sub 2}, and HD

We have measured the energies of several rotational branches of the (0,0) band of the EF {sup 1}{sigma}{sub g}{sup +}(leftarrow)X{sup 1}{sigma}{sub g}{sup +} transition in molecular hydrogen by use of Doppler-free two-photon laser excitation at 202 nm. The accuracy for all three stable isotopic variations is 8 parts in 10{sup 9}, nominally a fourfold improvement over previously available results, but some of the transition energies differ from previous work by as much as three standard deviations. The improved accuracy derives principally from our ability to measure the optical phase evolution of nanosecond laser pulses, then to predict quantitatively the line shapes of multiphoton transitions excited by them. The results allow comparably accurate determinations of the EF state term energies, which in turn help to calibrate the entire excited-state spectrum of this fundamental molecular system. These improved calibrations are particularly useful to recent and ongoing efforts to determine the dissociation energy and ionization potential with improved accuracy.

Spectroscopy and ionization dynamics of a dense ultracold Rydberg atom gas

Summary form only given. We describe measurements of high-resolution spectra and ionization dynamics in a strongly interacting gas of cold Rydberg atoms. Ground state /sup 85/Rb atoms are cooled to a few hundred /spl mu/K in a magneto-optical trap (MOT), and then excited to high Rydberg states by use of a pulse-amplified laser operating near 300 nm. The Rydberg atom density can be higher than 10/sup 10/ cm/sup -3/, depending on the laser power. The laser system produces nearly transform-limited pulses of 8 nsec duration, allowing spectral resolution of about 80 MHz.