The infrared emission spectra of LiF and HF

Transcription

1 The infrared emission spectra of LiF and HF H. G. HEDDERICH,' C. I. FRUM, R. ENGLEMAN, JR., AND P. F. BERNATH' Department of Chemistry, The University of Arizona, Tucson, AZ 85721, U.S.A. Received March 20, 1991 This paper is dedicated to Dr. Richard Nomn Jones Can. J. Chem. Downloaded from by on 03/10/18 H. G. HEDDERICH, C. I. FRUM, R. ENGLEMAN, JR., and P. F. BERNATH. Can. J. Chem. 69, 1659 (1991). The high resolution infrared spectrum of LiF has been measured in emission with the McMath Fourier transform interferometer at Kitt Peak. A total of 800 lines with v = 1 --, 0 to v = of the main isotopomer, 7LiF, and 250 lines with v = to v = 3 --, 2 of the minor isotopomer, 6Li~, were observed. These ro-vibrational transitions and the pure rotational transitions from the literature were fit to a set of Dunham coefficients YU and a set of mass-reduced Dunham coefficients UU. The same spectrum shows 13 pure rotational emission transitions of HF in the vibrational ground state with J = to J = These transitions were used to determine an improved set of rotational constants for HF. Key words: infrared spectra, LiF, HF. H. G. HEDDERICH, C. I. FRuM, R. ENGLEMAN, JR. et P. F. BERNATH. Can. J. Chem. 69, 1659 (1991). ; On a mesurt les spectres &emission infrarouge i haute rtsolution du LiF B l'aide d'un interferomktre B transformation de McMath Fourier au niveau Kitt Peak. On a observt 800 raies avec des valeurs de v allant de v = B v = 8 --, 7 pour I'isotopomSre principal, 7LiF, et on a observe 250 raies avec des valeurs de u allant de u = 1 --, 0 B v = 8 --, 7 pour I'isotopomSre minoritaire. On a adapt6 ces transitions de vibration-ro et les transitions de rotation pure ripertoriees dans la litteratwe 21 un ensemble de coefficients de Dunham Y, et B un ensemble de coefficients UU de masse rkduite Dunham. Ces mcmes spectres montrent 13 transitions d'tmission de rotation pure du HF dans 1'Ctat fondamental de vibration avec J = J = On a utilist ces transitions pour determiner un ensemble amcliorc de constantes de rotation pour le HF. Mots elks : spectres infrarouge, LiF, HF. [Traduit par la redaction] I. Introduction The infrared spectra of alkali halides have been of interest to spectroscopists for a long time (l,2). However, high-resolution infrared data for these molecules have only become available after the introduction of diode laser and Fourier transform infrared spectrometers. Nevertheless, the first infrared data on LiF was obtained by Vidale (3) as early as 1959 with a Perkin- Elmer grating spectrometer. He observed the spectrum of LiF both in absorption and emission. However, the resolution was not high enough to obtain an unambiguous rotational assignment and, therefore, the correct band center. Vidale gave three possible assignments. Only after the microwave measurements on 6 ~ by i Wharton ~ et al. (4) did the correct assignment become apparent. Several additional papers report the infrared spectrum of LiF trapped in low temperature matrices (5-10). The first high-resolution infrared data on LiF was published by Maki (11). He used a tunable diode laser system for the observation of ro-vibrational transitions with v = to v = for 'LiF and 6 ~ i Diode ~. lasers display many gaps in their spectral coverage. Therefore, only a few lines of each vibrational band were accessible. In this paper the complete emission spectra of 'L~F and 6 ~ between i ~ 630 and 1030 cm-i are published (Figs. 1, 2). HF is an important gas for use as an absolute wavenumber standard. In 1987 Jennings et al. (12) published rotational constants for HF calculated from measurements in four different laboratories. Highly accurate data in the far infrared (J = 1 t- 0 to J = ) were obtained with a tunable far-infrared spectrometer at the National Bureau of Standards laboratory in Boulder. Transitions with J = to J = were measured with a commercial high-resolution Fourier transform 'Also affiliated with Centre for Molecular Beams and Laser Chemistry, Department of Chemistry, The University of Waterloo, Waterloo, Ont., Canada N2L 3G1. Printed in Canada I lmprimc au Canada spectrometer at the National Research Council, Ottawa, Canada. Lines with J = 17 to J = 21 were observed with a vacuum grating spectrograph (13) and lines with J = 26 to J = 32 with a tunable diode laser spectrometer at the University of Lille, France. In 1988 Jennings and Wells (14) remeasured the J = and J = transitions by using a tunable diode laser heterodyne spectrometer (15). Except for these two lines, the mid-infrared transitions are relatively inaccurate. Unfortunately the uncertainties in the molecular constants in this publication seem to be a factor of 10 too small. As a byproduct of our work with LiF, we publish thirteen rotational transitions of HF in the range 500 to 900 cm-'. A new fit including the far-infrared data and the diode laser measurements provide an improved set of rotational constants for HF. 11. Experimental The high resolution infrared emission spectra of 6Li~, 7LiF, and HF between 500 and 1400 cm-' were observed with the McMath Fourier transform spectrometer at Kitt Peak. The unapodized resolution was cm-' with a liquid helium cooled As:Si detector and a KC1 beamsplitter. The upper wavenumber limit was set by a wedged InSb filter while the lower limit was given by the transmission of the KC1 beamsplitter and by the detector response. The spectrum of LiF was taken accidentally during an attempt to observe the infrared spectrum of MgF2. Solid MgF2 was heated to about 1500 K in an alumina tube furnace. The apparatus used for the experiment was described in detail earlier (16). The heating rate was about 5 Klmin. The temperature of the furnace was measured with a chromel-alumel thermocouple placed between the heating elements and the alumina tube. Deposition of solid material onto the windows was avoided by pressurizing the cell with 5 Torr of argon. A series of spectra were taken as the furnace heated up and then cooled down. Initially a globar was placed behind the tube furnace and its image was focused on the 8 mm aperture of the Fourier transform spectrometer. No absorption spectra were observed, but when the globar was shut off (at a temperature of about 1300 K) an emission spectrum was monitored. The inten-

2 CAN. J. CHEM. VOL. 69, 1991 TABLE 1. Ro-vibrational transitions of 7LiF and 6 ~ i ~ J" Obs./cm-' cm-' J" 0bs./cm-' cm-'

3 HEDDERICH ET AL J" O~S./C~-' /lo-' c~-' J" O~S./C~-' /lo-' c~-'

4 1662 CAN. 1. CHEM. VOL. 69, 1991 J" Obs./cm-' cm-i J" Obs./cm-' cm-i

5 HEDDEMCH ET AL Obs./cm-' cm-' J" Obs./cm-I / cm-'

6 CAN. J. CHEM. VOL. 69, 1991 J" Obs. /cmp1 /lof5 cm-' J" ~bs./cm-i /lop5 cm-'

7 HEDDERICH ET AL J" 0bs./cm-' / cm-' -- J" 0bs./cm-' cm-'

8 1666 CAN. J. CHEM. VOL. 69, 1991 J" Obs./cm-' cm-i J" ~bs./cm-' cm-i

9 HEDDERICH ET AL J" O~S./C~-I /lo-s C ~ - ] J" O~S./C~-~ C ~ - I Can. J. Chem. Downloaded from by on 03/10/18 P (50) -45 R (50) - 8 P (50) - 60 R (50) -9 P (50) - 10 R (50) -6 P (50) -13 R (50) 15 P (50) - 6 R l(50) 29 P (50) 9 R (50) -5 P (50) - 19 R (50) -11 P (50) - 37 R (50) 12 P (50) 8 R (50) - 1 P (150) R (50) - 5 P (50) 13 R l(50) - 30 P (50) - 15 R (50) -5 P (50) - 37 R (50) 1 P (50) - 16 R (50) 27 P (50) 3 R (50) -16 P (50) 3 R (50) - 24 P (50) 4 R (50) -23 P (50) 20 R (150) P (50) 0 R (150) 105 P (200) -185 R (50) -47 P (50) -5 R (50) - 80 P O 1304(50) 5 u= 3+2 P (100) 99 P (50) 13 P (50) - 22 P (50) 28 P (50) 14 P (50) -2 P l(50) - 13 P (50) - 17 P (50) -33 P (50) 6 P (50) - 25 P (50) -4 P (50) - 52 P (50) 16 P (250) -214 P l(50) -9 P (250) P (50) 0 P (50) -25 P (50) - 7 P (50) 1 P (50) 37 P (50) -31 P (50) 0 P (50) 50 P (50) -18 P (300) 289 P (50) 17 P (50) 2 P (100) -66 P (50) -7 P (50) - 18 P (50) 45 R (50) -45 P (50) 19 R (100) 83 P (50) 22 R (50) 29 P (50) 7 R OO(50) -36 P (50) 11 R (50) - 1 P (50) 13 R (50) - 2 P (50) 11 R (50) 7 P (50) 19 R (50) -9 P (50) 55 R (50) -6 P (50) - 15 R (50) -2 P (50) 19 R (50) 8 P (50) -2 R (50) -3 P (50) 1 R (50) 21 P (50) 3 R (50) 44 P (50) 1 R (50) -13 P (50) 0 R (50) 15 P (150) 136 R (50) 6 P (200) 184 R (50) 8 R (50) 5 R2O (50) 13 R (50) - 24 R (50) 29 R (50) 14 R (50) 11 R (50) -8 R (50) - 2 R (50) - 5 R (50) - 14 R (150) -121 R (50) - 6 R (50) 19 R (50) - 2 R (50) 7 R (50) 39 R (50) 11

10 1668 CAN. J. CHEM. VOL. 69, 1991 TABLE 1 (concluded) J" obs./crn-l /lo-' c ~ I - J" obs.1~~- I /lo-' C ~ - I Can. J. Chem. Downloaded from by on 03/10/18 "Numbers in parentheses are estimated uncertainties. sities of the emission lines increased with higher temperature. The maximum temperature which is accessible with our current system is about 1500 K and it was at this temperature that the best emission spectrum was obtained. All emission signals decreased rapidly as the furnace cooled and disappeared at about 1200 K Results The emission features were observed in the region where the infrared spectrum of MgF2 was expected (Figs. 1, 2). However, the high resolution spectrum showed a pattern typical of a diatomic molecule. After some thought and a literature search we found that the emission spectrum belonged to LiF (11). The presence of Li in our system was puzzling but in a previous experiment the alumina tube was used to vaporize LiNC. Despite careful cleaning of the cell, some Li remained on the walls of the alumina tube. At higher temperatures the MgF2 could react with water (which is always present in the system) and form HF. The HF can react with the Li impurities and produce LiF. Direct reduction of MgF2 with Li is also possible but MgF was not detected. An additional reaction of HF with the alumina tube took place to form AIF. The emission of A1F was observed in the spectrum (17). The spectral analysis program PC-DECOMP, developed by J. W. Brault, was used for data analysis. The rotational line profiles were fit to Voigt lineshape functions. The strong lines show a "ringing" caused by the (sin x/x) lineshape function of the Fourier transform spectrometer. The ringing was eliminated by using the "filter fitting" routine available in PC- DECOMP. The signal-to-noise ratio for the strong lines was better than 1000 and the resulting resolution-enhanced line width was cm-'. The HF lines were used for absolute calibration (+/ cm-') of the spectrum. For this calibration the HF absorption lines, taken in a spectrum at lower temperature, were calibrated against C02 (18). The assignment of the vibrational bands of the main isotopomer 7LiF was straightforward (Table 1). Data reduction was made by using Dunham's equation (19) TABLE 2. Dunham constants for LiF (in cm-i)" Value Constant 7LiF 6 ~ i ~ OEach isotopomer was fit independently. One standard deviation in parentheses. were included in the fit. The Dunham coefficients obtained from this fit are shown in Table 2. The assignment of the vibrational bands of the minor isotopomer, 6 ~ i was ~, carried out by comparing predicted line positions with the unassigned lines in the spectrum. The line positions for 6 ~ are i also ~ shown in Table 1. The mass-dependent Dunham coefficients Yi,. are listed in Table 2. Finally, all infrared and microwave data for LiF was combined and fit to the mass-reduced Dunham expression including Watson's correction due to the breakdown of the Born- Oppenheimer approximation (21, 22) Pure rotational transitions reported by Pearson and Gordy (20) where p is the reduced mass, Uij are the mass-independent parameters, mli is the atomic mass of Li (23), me is the electron mass, and Aii is the mass scaling factor for the Li atom. The

11 HEDDERICH ET AL Can. J. Chem. Downloaded from by on 03/10/18 I W avenumber FIG. 1. Infrared spectrum of 7LiF. I.O s Wavenumber FIG. 2. Infrared spectrum of 6~iF.

12 1670 CAN. J. CHEM. VOL. 69, 1991 TABLE 3. Mass-reduced Dunharn constants for LF (in cm-i)" Constant Our work TABLE 5. Rotational constants of ground state HFa Our work Jennings et al. (12) Constant /cm-' /cm-' Can. J. Chem. Downloaded from by on 03/10/18 "One standard deviation in parentheses. TABLE 4. Pure rotational transitions of HF Uncertainty (la) J ' J" Observed/cm-' cm- /lo-' cm- "Tunable far infrared spectroscopy (ref. 12). bfourier transform spectroscopy (ref. 12). 'Fourier transform spectroscopy (our work). ddiode laser heterodyne spectroscopy (ref. 14). 'Diode laser spectroscopy (ref. 12). Bo (8) (25) DO (36)E (14)E-3 HO (116)~ (50)E-6 LO (132)E (66)E- 10 MO (50)E (27)E- 15 "Two standard deviations in parentheses. data required two A's, one for Ulo ("a,") and one for Uol ("Bew). The final mass-independent coefficients and the A's are listed in Table 3. The energy levels of HF were fit to the expression (24) The fit includes all lines listed in Table 4 and Table 5 shows the determined constants. IV. Conclusion The high-resolution infrared emission spectra of LiF and HF were observed with a Fourier transform instrument. The ro-vibrational transitions of the fundamental band and of seven hot bands for 7 ~ and i of ~ three bands for 6 ~ were i ~ rotationally analyzed. Pure rotational transitions for HF were also observed and enabled us to calculate an improved set of molecular constants. The accuracy of the line positions allows the use of HF as an absolute wavenumber standard. Acknowledgements The National Solar Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under the contract with the National Science Foundation. We would like to thank J. W. Brault and J. Wagner for assistance in recording the spectra. This work was supported by the Astronautics Laboratory, Edwards Air Force Base, CA. Acknowledgement is made to the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. One of us (H.G.H.) thanks the Deutsche Forschungsgemeinschaft for a postdoctoral scholarship. 1. S. E. VEAZEY and W. GORDY. Phys. Rev. A, 138, 1303 (1965). 2. A. J. HEBERT, F. J. LOVAS, C. A. MELENDRES, C. D. HOLLOWELL, T. L. STORY, JR., and K. STREET, JR. J. Chem. Phys. 48, 2824 (1968). 3. G. L. VIDALE. J. Phys. Chem. 64, 314 (1960). 4. L. WHARTON, W. KLEMPERER, L. P. GOLD, R. STRAUCH, J. J. GALLAGHER, and V. E. DERR. J. Chem. Phys. 38, 1203 (1963). 5. M. J. LINEVSKY. J. Chem. Phys. 34, 587 (1961). 6. M. J. LINEVSKY. J. Chem. Phys. 38, 658 (1963). 7. A. SNELSON and K. S. PITZER. J. Phys. Chem. 67, 882 (1963). 8. S. SCHLICK and 0. SCHNEPP. J. Chem. Phys. 41, 463 (1964). 9. A. SNELSON. J. Chem. Phys. 46, 3652 (1967). 10. S. ABRAMOWITZ, N. AcQUISTA, and I. W. LEVIN. J. Res. Nat. Bur. Stand. Sect. A, 72, 487 (1968). 11. A. MAKI. J. Mol. Spectrosc. 102, 361 (1983). 12. D. A. JENNINGS, K. M. EVENSON, L. R. ZINK, C. DEMUYNCK, J. L. DESTOMBES, B. LEMOINE, and J. W. C. JOHNS. J. Mol. Spectrosc. 122, 477 (1987).

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