r. s. ram et al- fourier transform infrared emission spectroscopy of nacl and kcl

8/3/2019 R. S. Ram et al- Fourier Transform Infrared Emission Spectroscopy of NaCl and KCl

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OURNAL OF MOLECULAR SPECTROSCOPY 183, 360373 (1997)

RTICLE NO. MS977292

Fourier Transform Infrared Emission Spectroscopy of NaCl and KCl

R. S. Ram,* M. Dulick, , B. Guo, K.-Q. Zhang, and P. F. Bernath* ,

*Department of Chemistry, University of Arizona, Tucson, Arizona 85721; National Solar Observatory, National Optical Astronomy Observatories,

Tucson, Arizona 85726; and Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Received November 25, 1996; in revised form March 3, 1997

The infrared emission spectra of NaCl and KCl have been recorded at high resolution with a Fourier transformspectrometer. A total of 929 lines belonging to 8 vibrational bands, 10 to 87, for Na 35Cl, 252 lines of 10, 21,and 32 bands of Na 37Cl, and 355 lines of 10, 21, 32, and 43 bands of K 35Cl have been measured and combinedwith the existing microwave and millimeter-wave data to obtain a set of refined molecular constants. The data forNa35Cl and Na 37Cl have also been fitted to determine the Dunham Yij constants and mass-reduced Dunham constantsUij . In one fit all Uij s were treated as adjustable parameters while in a second fit only Ui 0s and Ui 1s were allowed tovary freely with the remaining Uij constants determined by the constraints imposed by the Dunham model. In addition,the internuclear potential energy parameters were determined by fitting the entire NaCl data set to the eigenvalues ofthe Schrodinger equation containing a parameterized potential energy function. 1997 Academic Press

INTRODUCTION fit using the Dunham equations. In a separate study, Ueharaet al. (13 ) investigated the infrared emission spectra of KCl

at a resolution of 0.1 cm01 and provided a set of DunhamDiatomic alkali halides are high temperature species ofcoefficients for the ground electronic state.undamental importance. Their low vapor pressures and

Some matrix isolation studies of NaCl and KCl are alsoighly ionic nature present challenges for gas phase experi-available. Martin and Schaber (8 ) observed the infraredmental studies (1 ). There are many studies of NaCl ( 2 9 )spectra of NaCl while Ismail et al. (14 ) observed both NaClnd KCl ( 24, 1014) in the near-ultraviolet and micro-and KCl in a solid argon matrix.wave regions. NaCl ( 2, 3 ) and KCl ( 1012 ) have a long

Extensive microwave data for NaCl and KCl have beeneries of unresolved bands in the near ultraviolet which havepublished (5, 9 ). In the initial work of Honig et al. (5 )een called fluctuation bands (15 ). These bands arise

several alkali halide molecules were investigated and theyrom transitions between individual vibrational levels of onereported rotational constants and electric dipole momentslectronic state and a rather flat portion of the potential curveIn another study, Clouser and Gordy ( 9 ) measured the milli-f the other state. The ground state of these molecules aremeter-wave molecular beam absorption spectra of NaClelatively well characterized experimentally by infrared andKCl, and several other alkali chlorides. In this work theymicrowave studies.measured a number of pure rotational transitions in severalThe NaCl and KCl molecules are important species in thevibrational levels of the ground state and reported improvedombustion of coal. Coal contains substantial amounts ofvalues for Be , ae and ge for most of the molecules studiednorganic material that are responsible for slagging and foul-They also provided indirect estimates for the vibrational con-ng of coal-fired power plants. NaCl and KCl can, in princi-stants.le, be monitored by the absorption or emission using the

The alkali halides have dissociative UV absorption spec-ibrationrotation bands.

tra. Silver et al. (16) measured the absolute UV dissociationThe low resolution infrared spectra of NaCl and KCl were cross sections for gas phase NaCl and Novick et al. (17)irst observed in absorption by Rice and Klemperer (4 ),measured the photodetachment spectra of the alkali halidewho determined the ground state vibrational constants venegative ions NaCl0 , NaBr0 , and NaI0 .nd vexe from band head positions in conjunction with the

The ionic nature of the alkali halide ground states has ledround state B values, previously reported by Honig et al.to the development of simple potential energy functions such5 ) in a microwave study. The high-resolution infrared vi-as the Rittner model ( 1820 ). There are a number of theo-rationrotation spectra of Na 35Cl and Na 37Cl have beenretical calculations of electronic structure and other proper-tudied by Horiai et al. (6) and Uehara et al. (7) by infraredties available for NaCl (2129 ) and KCl ( 3035 ). Boundsiode laser spectroscopy. These measurements were used toand Hinchliffe ( 23 ) performed ab initio calculations of theetermine a set of Dunham parameters Yij for the groundSCF pair potential and polarizability of NaCl and Leasuretate of NaCl. The potential constants for the ground state

were also calculated by Uehara et al. (7) in a least squares et al. (26) calculated the electronic structure. Swaminathan

360022-2852/97 $25.00

opyright 1997 by Academic Press

ll rights of reproduction in any form reserved.


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INFRARED EMISSION SPECTROSCOPY OF NaCl AND KCl 361

TABLE 1

The R Head Positions (in cm01 ) of Vibration

Rotation Bands in the 10 Sequence of K 35Cl

FIG. 1. A compressed portion of infrared bands of KCl marking the

and heads in the 10 sequence of K 35Cl.

t al. (28 ) calculated the potential energy of NaCl with large

tomic basis sets and extensive configuration interaction.

Bounds and Hinchliffe (32 ) studied the polarizability of KCl

nd Zeiri and Balint-Kurti ( 34 ) studied the potential energy

urves and transition dipole moments of NaCl, KCl, and

everal other alkali halides.

Some of the alkali halides are also of astrophysical impor-

ance. Recently the presence of NaCl and KCl, along with

AlCl and AlF, has been established in the carbon star IRC particularly important since high-quality spectroscopic pa-

rameters are necessary for infrared searches for these mole-/ 10216 by millimeter-wave astronomers ( 36). Normally

metals are depleted onto grains in the interstellar medium cules.

In this paper we report on the observation of vibrationut this discovery indicates that metal-containing molecules

re more abundant in cool circumstellar envelopes. Unfortu- rotation bands of Na35Cl, Na 37Cl, and K 35Cl. Eight bands

1 0, 21, 3 2, 43, 5 4, 65, 7 6, and 87, for Na 35Clately, the infrared spectra of many of the metal halide di-tomic molecules have not been investigated at high resolu- three bands, 10, 2 1, and 32, for Na37Cl, and four bands,

10, 21, 32, and 43, for K 35Cl have been observed aton and their vibrational constants are not precisely known.

The high-resolution studies of these molecules are, therefore, a resolution of 0.01 cm01 . Analysis of the data has provided

improved molecular constants and the Dunham coefficientsYij for these species. The Na

35Cl and Na 37Cl data sets have

also been fitted together to obtain mass-reduced Dunham

constants Uij and the constants of a parameterized potential

energy function.

EXPERIMENTAL

The high-resolution emission spectra of Na 35Cl, Na 37Cl

and K 35Cl were recorded with a Bruker IFS 120HR Fourier

transform spectrometer at the University of Waterloo. The

molecules were vaporized in an alumina tube furnace by

heating NaCl or KCl gradually until the operating tempera-

ture of 1000C was achieved. A 3.5 mm mylar beam splitter

and a Si:B detector (NaCl) or a 4 K bolometer (KCl) were

used to record the spectra. The cell was pressurized with

about 5 Torr of argon to avoid the deposition of the solid

material on the KRS-5 windows. The resolution was 0.01FIG. 2. An expanded portion of the 10 band of Na 35Cl near the Read. cm01 for both molecules. The final interferograms for NaCl

Copyright 1997 by Academic Press


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RAM ET AL.62

TABLE 2

Observed Rotational Lines (in cm01) in the VibrationRotation Bands of Na 35Cl and Na 37Cl


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TABLE 2 Continued



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RAM ET AL.64

TABLE 2 Continued



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TABLE 2 Continued



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RAM ET AL.66

TABLE 2 Continued



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TABLE 2 Continued



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RAM ET AL.68

TABLE 3

Observed Rotational Lines (in cm01) in the VibrationRotation Bands of K 35Cl


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TABLE 3 Continued

nd KCl involved coaddition of 100 and 400 scans, respec- The measurements of strong and unblended Na35Cl lines are

expected to have a precision of about {0.003 cm01 . Thevely.

The spectra were measured using the PC-DECOMP, a uncertainty in the measurements of Na 37Cl lines is expected

to be somewhat worse because of the weaker intensity androgram developed by J. Brault at the National Solar Obser-

atory. This program determines the position of individual overlapping from the major isotopomer. The lines of KC

are much weaker in intensity than NaCl, and K 37Cl linesnes by fitting a Voigt lineshape function to each line. The

pectra were calibrated using the measurements of H2O lines could not be identified in our spectra. The K35Cl lines are

expected to be accurate to, at best, {0.005 cm01 .37) which were also present in our spectra as an impurity.



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RAM ET AL.70

TABLE 4 350 to 387 cm01 . The intensity of the higher vibrationalRotational Constants (in cm01) of the X1S/ State of NaCl bands decreases slowly with increasing vibration and it be-

comes difficult to identify the bands with 8 because of

overlapping and the decline in intensity. In contrast to the

previous infrared observations of Horiai et al. (6) and Ueh-

ara et al. (7), we have identified a number of P lines. The

intensity of the Na 37Cl isotopomer is about 33% of the inten-

sify of Na 35Cl as expected. The rotational lines only in the

10, 21, and 32 bands of Na

37

Cl were identified andincorporated in the fit. A part of the high-resolution spectrum

of the 10 band of NaCl near the R head is provided in Fig

1 where some R lines of Na 35Cl have been marked.

The rotational lines belonging to the 10, 21, 32, and

43 bands of K 35Cl were identified in our high-resolution

spectra although band heads up to 1211 can be clearly

seen. A part of the compressed spectrum of KCl showing

the band heads is presented in Fig. 2 and a list of observed

band heads is provided in Table 1. For technical reasons

(the use of a bolometer) the KCl data are relatively poor

compared to NaCl and a much less extensive analysis was

carried out.As expected for a 1S/ 1S/ transition, each band consists

of one R and one P branch. The wavenumbers of the ob-

served transitions of NaCl and KCl are provided in Tables

2 and 3 respectively. The wavenumbers of NaCl and KCl

were fit to the customary energy level expression:

F

(J) T/ B

J(J/ 1) 0 D

[J(J/ 1)] 2

RESULTS AND DISCUSSION/ H

[J(J/ 1)] 3 .

[1]

The vibration rotation bands of NaCl and KCl are located

n the 340390 and 240300 cm01 regions, respectively. In the final least-squares fit, approximate weights for the

individual rotational lines were chosen on the basis of signal-The bands form R heads and the rotational structure of bands

with high values is heavily overlapped. Our LoomisWood to-noise ratio as well as freedom from blending. We have

also included the previous microwave measurements ofrogram was very helpful in identifying the connectingR and

lines in each band, particularly in the severely overlapped Honig et al. (5 ) and Clouser and Gordy ( 7), with appro-

priate weights in our final fit. The spectroscopic constantsegions. For KCl essentially all of the lines are blended.

The observed spectrum of NaCl consists of eight vibra- for the X1S/ state of NaCl and KCl obtained from the fit

of the combined infrared and microwave data are providedonrotation bands, 10 to 87, of Na 35Cl spread from

TABLE 5

Rotational Constants (in cm01 ) of the X 1S/ State of KCl



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TABLE 7n Tables 4 and 5, respectively. The molecular constantsDunham Constants (in cm01 ) of the X1S/ State of KClbtained in this study are in excellent agreement with values

eported earlier from infrared studies (5, 7) but are more

recise by an order of magnitude because of the inclusion

f extensive data with high J and values along with pure

otational lines. The rotational lines of Na 35Cl, Na 37Cl, and

K35Cl were fit to the Dunham energy level expression ( 38 )

E(

, J)

Yij( /

1/2)

i

[J(J/

1)]

j

. [2]

The Dunham constants (Yij) for the X1S/ state of NaCl and

KCl obtained from these fits are provided in Tables 6 and

, respectively.

In another fit the combined Na 35Cl and Na 37Cl data sets

were fit to the following expression to determine a set of

mass-reduced Dunham Uij coefficients and Born Oppenhei-

mer breakdown constants Dij (39, 40 ):

E(, J) Uijm0 ( i/2 j ) / 2 ( / 1/2) i[J(J/ 1)] j

[3]

1 1 / me

MADAij / me

MBDBij .

me is electron mass, MA and MB are masses of two atomicall of the parameters for a power series potential are uniquelyenters A and B, respectively, and DAij and D

Bij are Born

determined by the Ui 0 and Ui 1 constants (41, 42). ThisOppenheimer breakdown constants for atoms A and B. Itmeans that Uijs with j 2 can be expressed in terms ofurns out that no D parameters were necessary in our fits.Ui 0s and Ui 1s. In the second fit, therefore, Ui 0s and Ui 1sTwo fits of the combined NaCl data were obtained. In thewere treated as adjustable parameters with the remainingirst fit all Uij parameters were allowed to vary. The constantsUijs fixed to the values satisfying these constraints. Thebtained from this fit are reported in Table 8 under theconstants obtained from this fit are also reported in Table 8olumn heading unconstrained. These unconstrained Uijunder the column heading constrained.

nd Yij values of Tables 6 and 7 are thus empirical coeffi- There is no doubt that the Dunham constants reproduceients of polynomial fits. According to the Dunham modelthe transition wavenumbers over the range of and J values

observed. It is well known, however, that this model is inade-

quate for extrapolating beyond the range of experimentalTABLE 6measurements. The full advantage of high-quality, high-res-Dunham Constants (in cm01 ) of the X 1S/ State of NaClolution spectra is achieved when the extracted spectroscopic

information can also be applied in predicting the spectra

well beyond the range of experimental measurements. The

inherent failure of the Dunham model has led to the develop-

ment of a more sophisticated approach of fitting observed

measurements directly to the eigenvalues of the Schrodinger

equation containing a parameterized potential energy func-tion (4346). The BornOppenheimer potential UBO is rep-

resented by

UBO De {1 0 exp[0b(R)]}2/{1 0 exp[0b()]} 2 ,

[4]

where

b(R ) z bizi , [5]



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RAM ET AL.72

TABLE 9b() bi , [6]Internuclear Potential Energy Parameters (in cm01)

for the X1S/ State of NaClnd

z (R 0 Re )/(R / Re ). [7]

We call this approach to data reduction the parameterized

otential model and the method is described in more detail

n several previous publications ( 45, 46). This fit providesset ofbi parameters which describe (Eq. [4]) the in-

ernuclear potential function ( 45, 46). These parameters

or NaCl are provided in Table 9. Only statistically deter-

mined parameters are listed in this table along with 1s

ncertainties.

Our measurements of NaCl rotational lines are in good

greement with the measurements of Horiai et al. ( 6) and

Uehara et al. ( 7) where they overlap. The present set of

Dunham constants for Na 35Cl and Na 37Cl (Table 6) are

lso in excellent agreement with the previous values ( 6,

TABLE 8 7) . Because of our more extensive data set includingMass Reduced Dunham Constants (in cm01 ) bands up to 8 7, however, the present constants for

for the X 1S/ State of NaCl Na 35Cl are more precise than the previous values ( 5, 7)

A comparison of present band head positions of KCl with

those reported by Uehara et al. ( 13 ) indicates that on

average their values are about 0.07 cm01 higher than the

present measurements. At this point it is worth mentioning

that Uehara et al.s spectra were recorded at a resolution

of 0.1 cm01 whereas the present spectra were recorded at

0.01 cm

01

resolution. Thus the present molecular con-stants of KCl in Tables 5 and 7 are much more precise

than previous values of Uehara et al. ( 13 ). For example

the present values of Y10 , Y20 , and Y01 constants are

280.0764(49),01.3133( 34) , and 0.12863213( 33) cm01 ,

respectively, compared to the corresponding values of

279.963(22), 01.2306(83), and 0.128631(52) cm01 , re-

spectively, obtained by Uehara et al. ( 13 ) .

CONCLUSION

The infrared emission spectra of NaCl and KCl have beenobserved using a Fourier transform spectrometer. The vibra-

tionrotation spectra of a number of bands of Na35Cl

Na37Cl, and K 35Cl have been measured and the line positions

fitted to extract molecular constants for individual vibra-

tional levels and Dunham coefficients. The lines of Na 35Cl

and Na 37Cl were combined in another fit to determine the

mass-reduced Dunham parameters Uij . A second set of mass-

reduced constants was also obtained using the same data set

but with only the Ui 0s and Ui 1s treated as variables and

the rest of the Uijs were fixed by constraints imposed by

the Dunham model. The lines of NaCl have also been fit



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r. s. ram et al- fourier transform infrared emission spectroscopy of nacl and kcl

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