SPECTROCHIMICA ACTA

PART A

E L S E V I E R Spectrochimica Acta Part A 52 (1996) 1315 1317

R e s e a r c h N o t e

High resolution FTIR spectrum of the v 6 band of HNO3

T.L. Tan, W.F. Wang, E.C. Looi*, P.P. O n g

Department a[ Physics, Faculty ~?[ Science, National Unit'ersity o[' Singapore, Lawer Kent Ridge Road, Singapore 119260, Singapore

Received 14 December 1995: accepted 21 February 1996

Keywords: FTIR Spectroscopy: HNO3: Infrared spectra: Rovibrationat constants

In order to compile an accurate atlas of line positions and intensities necessary for the moni- toring of HNO 3 in the upper atmosphere, several high-resolution infrared studies of HNO3 have been made recently [1 4]. The v6 band of HNO3 in the 647 cm-~ region is particularly useful for this purpose because the spectrum of H20 is less prominent in this frequency region. Therefore, an accurate measurement and analysis of the v6 band would be worthwhile for a reliable atmospheric field work.

The present work gives improved spectroscopic constants for the v6 band of HNO3 using the high-resolution FTIR technique. In comparison to the work of Maki and Olson [4], the present measurements of v6 cover twice as many transi- tions with higher J and K values in a range from 629 to 669 cm ~. This present study is also a continuation of our effort in improving the accu- racy of the measurements and analysis of the bands of HNO3 [5,6].

The infrared measurements were performed with a modified [7] Bomem DA8 Fourier trans- form spectrometer at Steacie Institute for Molecu- lar Sciences, National Research Council of Canada, Ottawa. An unapodized resolution of

* Corresponding author.

0.0014 cm ~ was used for the measurements. The vapor pressure of 0.15 Torr was measured using a corrosion-resistant Baratron gauge. A liquid-he- lium-cooled Cu:Ge detector and KBr beamsplitter were used. Pure HNO3 sample was placed in a 30-cm multipass cell set for 28 transits to give a total absorption pathlength of about 840 cm at an ambient temperature of 296 K. In order to obtain a reasonable signal-to-noise ratio level, 56 scans were co-added to give the final sample spectrum.

The spectrum was calibrated using the line fre- quencies of the v 2 band of CO2 [8] which were measured with an absolute accuracy of _+ 0.0001 cm-~ The CO: spectrum was measured with the HNO3 spectrum. A correction factor of 1.0000017, by which the observed wavenumbers should be multiplied, is needed to bring them into agreement with the calibration frequencies. It is estimated that the measured wavenumber values are accurate to _+ 0.00034 c m - t .

The v6 band of HNO3 has been assigned as A-type inplane vibration [9]. In the rotational analysis, the ground state constants were fixed at values determined accurately by Maki and Olson [4]. The assigned infrared transitions of v6 were fit to obtain the upper state @6 = 1) constants using Watson's A-reduction Hamiltonian assuming I r representation. Since the rotational structure of v,,

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1316 T.L. Tan et al. / Spectrochimica Acta Part A 52 (1996) 1315 1317

Table l

Rovibrational constants (in cm ~) for HNO 3 as determined for an A-reduction of the I ' representation

Ground state ~' v 6 (this work) v6 (Ref. [4])

A 0.433999899(44) b 0.433840169(67) 0.433840185(78) B 0.403609999(29) 0.402195148(52) 0.402195141(62) C 0.208832419(37) 0.209556287(32) 0.209556289(35) A J x 106 0.2970970(170) 0.328008(47) 0.327999(60) A, K x 106 - 0.1516232(532) - 0.261002(162) - 0.260999(209) A,v × 106 0.2465743(722) 0.317216(165) 0.317254(177) (Sj × 106 0.1262656(27) 0.126696(24) 0.126690(30) ~x x 106 0.2493989(152) 0.261634(60) 0.261644(67) Hj x l0 t`, -0.016985(294) [-0.016985] ~ [-0.016985] HjK x l0 t 2 0.9420(158) 4.835(61 ) 4.836(75) Hxj x 1012 - 3.6892(700) [ - 3.6892] [ - 3.6892] H a. x 1012 4.1786(908) - 5.004(200) - 4.992(246) hj x 1012 [0.00] - 0.4696(42) - 0.4701 (45) ]tjK X 1012 [0.00] 3.648(56) 3.654(67) h,v x 1012 1.7925(331 ) [1.7925] [1.7925] LjA, KX x 1016 0.795(288) [0.795] [0.795] L x x 1016 - 1.271(384) [ - 1.271] [ - 1.271] % 646.826262(18) d 646.826234(33) Number of rotational transitions ~ 188 188 RMS deviation (MHz) 0.108 0.108 Number of infrared transitions 1640 844 RMS deviation (cm t) 0.00028 0.00038

The ground-state constants were taken from Ref [4]. b The uncertainty in the last digits (twice the estimated standard error) is given in parentheses. ~ The values given in square brackets were fixed at the ground-state values, a The uncertainty given for the band centers do not include the absolute calibration uncertainty which is about _+ 0.00034 cm ~. e The rotational transitions were taken from Ref. [10].

has been studied [4] and is perturbation-free, the present analysis was much simplified. Some rovi- brational constants which were not well-deter- mined by the data were fixed at the values for the ground state. A total of 1640 A-type infrared transitions from the present measurements and 188 rotational transitions from the millimeter- and submillimeter-wave measurements of Booker et al. [10] were included in the final fit. The infrared transitions used in the fit cover levels with quan- tum numbers up to J" = 50 and K~ = 35. Some of Booker et al.'s [10] rotational transitions involve levels with even higher quantum numbers. Transi- tions from both P- and R- branches were equally well represented in the fit. Furthermore, 150 un- blended Q-branch transitions were carefully as- signed and also included in the final analysis.

The rovibrational constants for the v6 = 1 state and the ground state are given in Table 1. The results of Ref. [4] are also included in the table for comparison. The constants for the v6 = I state,

which represent the latest improved values for v6, allow one to calculate the infrared line positions with an rms standard deviation of 0.00028 cm ~. The band center for v6 is found to be 646.826262(18) c m - ~, which is more accurate than the value given in Ref. [4]. It can be observed from Table 1 that there is an overall improvement in the accuracy of the rovibrational constants deter- mined from the present measurements. These val- ues are also in good agreement, within the standard error, with those of Maki and Olson [4].

A complete listing of the fitted results, observed transitions and their deviations from the calcu- lated transitions has been deposited with the British Library at Boston Spa, Wetherby, West Yorks, U.K., as Supplementary Publication No. SUP 13116 (21 pages). Persons wishing to obtain copies of deposited material should write, citing the accession number, directly to Service En- quiries, British Lending Library, Boston Spa, Wetherby, West Yorks, LS23 7BQ, England.

T.L. Tan et al. / Spectrochimica Acta Part A 52 (1996) 1315 1317 1317

Acknowledgements

The authors express their gratitude to Dr. J.W.C. Johns for the use of the modified Bomem DA8 spectrometer in his laboratory. The assis- tance of Dr. Z.F. Lu in recording the FTIR spectra is gratefully acknowledged.

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