Photoionization study of multiphoton-excited SF_6 in a molecular beam

We have used a molecular-beam photoionization technique to probe the vibrational-energy distribution of SF(6) molecules excited by an intense CO(2) laser at energy fluences close to and below the dissociation threshold. We observe a significant bottleneck at low laser intensities. Even at laser fluences close to the dissociation threshold, most of the molecules remain in the discrete levels, and only a small fraction (approximately 10-25%) are excited into the quasi-continuum.