Unpublished work by Robert Wu et al. has shown that some these data were obtained with a gas sample contaminated with acetone. The major consequences are extraneous peaks at 190 nm and 195 nm, but there may be other effects.
Our records show that the sample used for the work from 174 nm to 201 nm [from Scott Specialty Gases, Inc.] was a 10% C2H2, 90% He mixture that should have had 10 ppm "total impurities". We recently [June 2000] had the gas retested by Scott, who found 1400 (+/-250) ppm of acetone! This value is qualitatively consistent with the differences pointed out by Wu et al. between our published values and their preliminary results.
Our work in the 137 nm - 175 nm region was done with a different sample, which has not been retested. The `bump' at 149.5 nm in our data does not appear in the work of Wu et al.; it is possible that it too is due to acetone.