Infrared spectra of isotopic species of water were widely studied TABLE 1
mainly to identify their contributions to the atmospheric spectra of trace Abundance of Isotopic Species in the HD 18 O gases (1). Such works also involved the investigation of spectroscopic and D 2 18 O Infrared Measurements properties of strong pumped infrared laser lines (2 -4). However, it appears that high resolution studies on the two isotopic species HD 18 O and D 2 18 O remain limited. Toth (5, 6) has measured the line positions and linestrengths in the n 2 bands of HD 18 O and D 2 18 O at a resolution of 0.005 cm 01 . Di Lonardo and Fusina (7) have studied the n 2 band of D 2
18 O in a range from 950 to 1450 cm 01 at a resolution of 0.008 cm 01 . In their work, a total of 607 transitions were identified and fitted providing accurate spectroscopic constants for the £ 2 Å 1 state of D 2 18 O. Extensive pure rotational data were recorded by Johns (8) for the (000) -(000) bands of HD 18 O and D 2 18 O. Nevertheless, to our knowledge, rovibra-Intensity analysis shows that the B-type component is about half time tional constants for the n 2 band of HD 18 O, including high order centrifustronger than the A-type counterpart. The rotational frequency analysis gal terms and band center, have not been reported yet. Thus, it is our for this band is rather straightforward since most of the line assignments purpose in this work to produce a set of highly precise such constants have been worked out (5). Present work was to confirm each available by fitting a high resolution spectrum of the n 2 band of HD 18 O in a assignment which could also be identified in our spectrum and meanrange from 1120 to 1780 cm 01 . Moreover, for the n 2 band of D 2 18 O, while to introduce more lines of HD 18 O from a wider frequency range available pure D 2 18 O sample enabled us to identify 62 more infrared into the determination of spectroscopic parameters. The nonlinear leasttransitions. These newly assigned transitions are also reported here.
squares fitting for the line positions was performed. Combination dif-The Fourier transform absorption spectra of the n 2 band of HD 18 O ferences were, as usual, computed in order to optimize the ground state were recorded using our Bomem DA3.002 spectrometer (9) at an unconstants and on the other hand, help assign the infrared transitions of apodized resolution of 0.0036 cm 01 . The absorption gas cell was 20 the n 2 band of HD 18 O. Microwave transitions of HD 18 O and the farcm long, fitted with BaF 2 windows. The Globar source and the KBr IR measurements (8) were combined with the currently obtained 664 beamsplitter were used. The ambient temperature was about 296 K. combination differences from the n 2 band to derive a set of highly Pure sample D 2 18 O (98% 18 O) was obtained from Cambridge Isotope accurate ground state parameters. These constants were then fixed in Laboratories, Inc. Residues of H 2 16 O in the gas cell were allowed to the fitting of the spectrum to determine the upper state rovibrational combine with D 2 18 O to form HD 18 O. A total of 100 scans would yield a satisfactory signal-to-noise level spectrum using a high sensitivity parameters. During the fitting, each datum was weighted by its inverse liquid-nitrogen-cooled Hg -Cd -Te detector. The gas cell was pumped of the squared uncertainty. The uncertainties for infrared lines were down to a pressure of about 10 04 mbar before D 2 18 O vapor was introestimated to be 0.0004 cm 01 while for far-IR and microwave transitions duced. Several spectra were recorded with a pressure range from 0.4 the uncertainties reported in Ref. (8) were used. Because of the very to 0.7 mbar. The final spectrum selected for our rotational analysis high resolution used, nearly all the strong lines could be identified, consists of sufficient HD 18 O and D 2 18 O constituents, i.e., 24.0% and allowing us to include an extensive number of lines in our fitting. The 31.6% respectively. The partial vapor pressures for all the existing transitions involved in our work cover a range of J from 1 to 15 and isotopic species were determined by comparing the present observed K from 0 to 7. A total of 209 A-type transitions and 348 B-type transilinestrengths with the previously known such values (5,6, 10,11). The tions have been fitted. The obtained spectroscopic constants for the derived abundance of isotopic species is given in Table 1. The measured ground state and the upper £ 2 Å 1 state are summarized in Table 2. The lines were calibrated by reference to the standard H 2 16 O data (12). The present ground state constants, with improved standard errors for the rms deviation between the observed line positions and the calibrated lower order terms, are in general agreement with those from pure rotavalues is 0.00035 cm 01 . tional work (8). The band center for the n 2 band of HD 18 O was found The angle bend n 2 band of HD 18 O is found to be of A/B hybrid type. to be 1396.2670 { 0.0001 cm 01 . The rms value for the whole line position fitting is 0.00057 cm 01 which is actually close to our estimated uncertainty. The high order constant p K of the upper state was con-1 To whom correspondence should be addressed.