CRDS study of auto-ionizing Rydberg states of argon in a microwave discharge

Optical transitions from two microwave discharge excited states of argon have been observed using cavity ring-down spectroscopy. These transitions originate on the high-lying levels, 3d[1/2]degrees(1) and 3d[3/2]degrees(2), and terminate on the nf'[5/2] Rydberg (n = 8 to 22) levels, which, except for n = 8, lie between the P-2(3/2) and P-2(1/2) ionization thresholds. In total, 24 such spectral lines have been observed. The quantum defect for the f' series has been measured and is compared to previously measured values. We observe a nearly threefold jump in line width in going from n = 8 to n = 9, below and above the P-2(3/2) threshold, respectively. The line widths are broad and increase monotonically with n (above 9), in contrast to the narrowing of line widths usually observed. We cannot attribute this to a single source but conclude that collisional, quasielastic l-mixing of the nf'[5/2] Rydberg states plays a significant role.