Linewidths of HCl broadened by CO2 and N2 and CO broadened by CO2

JOURNALOFMOLECULARSPECTROSCOEY

Linewidths

49, lo&i05 (1974)

of HCI Broadened by CO, and N, and CO Broadened ROBERT

Jet Propulsion Laboratory,

A.

by CO,

TOTH

4800 Oak Grove Urine, Pasadena,

Ca&fornia

91103

AND LANE

Department

of Physics, Untie&y

A.

DAKNTON

oj Calijornia,

Santa

Barbara,

California

93106

High resolution measurement of the linewidths of HCI broadened by CO% and Nt and CO broadened by CO% have been performed in both the 1-O and 2-O bands of HCl and the 2%) band of CO. The data were analyzed by the direct and the peak absorption methods. Values of the linewidths obtained by the two methods are in good agreement. For 1% / _<3, for the case of HCl + CO*, the agreement is good for the values obtained in both bands of HCI. However for (m 1 > 3, the HC1 + CO2 linewidths in the 1-O band are smaller than the corresponding lines in the 2-O band~by as much as 11% for (m 1 = 9. Lines ( 1% 1 5 3) of the 1-O and 2-O bands of HC1 broadened by CO2 were also analyzed in terms of the superLorentzian line profile proposed by Varanasi, S. K. Sarangi, and G. D. T. Tejaani(J. Quan. Spectr. Radiative Transfer 12, 8.57 (1972)) and the Lorentzian profile. The results indicate that near the line center (within 3r), the shape of HC1 + COP lines are Lorentzian.

1. INTRODUCTION

The work presented in this paper involved the linewidth determinations of HCl broadened by CO, and Nz and CO broadened by CO,. The foreign-gas broadener, COz, was chosen because this molecule is the dominant broadening agent in the Cytherean atmosphere in which trace amounts of HCl and CO have been observed from their spectral absorptions (I, 2). The HCl observations in the Venus spectra were in the 2-O band and in this work both the 1-O and 2-O bands of HCl were studied in terms of CO, broadening. The CO + CO* linewidths were measured in the 2-O band of CO while the NP broadening of HCl was studied in the 1-O band of HCl. Values of the linewidths were determined by the direct method and from peak absorption measurements. The spectral resolutions were : 0.07 cm-’ for measurements in the 1-O band of HCl, 0.045 cm-’ in the 2-O band of CO, and 0.055 cm-’ in the 2-O band of HCl. Previous reports on the values of HCl + COz linewidths have been given by Varanasi et al. (3) and Babrov et ~2. (4). Both of these investigations were performed for the 1-O band of HCl. Linewidth studies of CO + COz have been reported by Varanasi (.5), Tubbs and Williams (6), Crane-Robinson and Thompson (7) and Bouanich and Brodbeck (8). The work of Varanasi (5) encompassed both the fundamental and first overtone bands whereas the work of Tubbs and Williams (6) and that of Varanasi (5) covered

Copyright @ 1974 by Academic Press, Inc. AU rights of reproductionin any form reserved.

LINEWIDTH

STUDIES

OF HCl .4ND

101

CO

TABLE1

ExperimentalConditions

Absorber

Broadener

Pressure Range Of Absorber (Ton)

Pressure Range of BRX.dWIer (&I)

Path (4

1.1 - 20

1.5 - 2

.419

1 - 11

1.5 - 2

.419

l-9

HCl

CQ, L

1-O

HCl

Cch

2-o

30 - 140

co

CG

2-O

1 - 13

.92 - .9s

0

Range

of IIn]Value

1 - 21

i

the fundamental Eand. The work of Bouanich and Brodbeck (8) covered the 2-O band of CO. Nz broadening of HCl in the 1-O band has been reported by Babrov et al. (4), Benedict et al. (9) and Rank et al. (IO). 2. IiXPERIMENTAL

The instrument used in this study was a Jarrel-Ash 1.8 m grating spectrometer equipped with a 300 line/mm Bausch and Lomb replica grating. The latter has a usable width of 200 mm and was blazed at 5.7 pm in first order. The grating was single passed with observations in the first, second, and third orders for the 1-O band of HCl, 2-O band of CO, and 2-O band of HCl, respectively. A 42 cm long absorption cell was used for the HCl measurements and a White-type multiple pass absorption cell with a base length of 2 m was used for the CO measurements. Several different combinations of absorber and broader pressures were used in order that the maximum absorption should be kept between 20 and 80%. The path lengths and range of pressures used in this work are listed in Table 1. A capacitance pressure sensor was used to measure the HCl and CO pressures to an accuracy of 1%. The total pressures were measured to an accuracy of 1% or better with a Wallace and Tiernan pressure gauge and a Baratron pressure gauge. The signal-to-noise ratio of the spectra was 100 to 1 or better with a recording time constant of 3 sec. The HCl dispersion was determined from the H35C1 and H3’C1 line positions given by Rank et al. (II). The CO dispersion was determined from the CO positions given by Plyler et al. (12). Sample temperatures were measured to 1°K with a thermocouple and all data were obtained at room temperature (-296°K). 3. DATA

REDUCTION

The direct method and the peak absorption method for determining the widths from the observed spectra are identical to those discussed previously (23). The slit corrections ranged from 1 to 12% for the data in the 1-O band of HCl and from 0.3 to 8% for the measurements in the 2-O band of HCl. The range of slit corrections for the CO data were from 5 to 1.5%. Linewidths obtained by the peak absorption method are related to the values of the maximum absorption coe4icient kd,, the line strength So, the absorber partial pressure PA, and the path length I, by y = S”PAI,/T~~~~. For a given line, the slit corrections to hi and the measured linewidth were approximately equal in magnitude.

102

TOTH

AND DAKNTON TABLE 2

Observed Line Widths Y"i,l in cm" Broadened by C&I and L

x 10' for the 1-O and 2-O Bands of HCI

at&

and for the 2-O Band of CO Broadened by C&

at 8 Temperature of M°K

HCl

+ C$

HCl

lml 1 2

+

cq

HCl + &

co + CC?

1-O Band

2-O l&d

1-O Band

2-o B¶n

Direct

Peak

Direct

Peak

Direct

Peak

Direct

1630 1400 logO 935

1610 1450 1040 962 775 647

l589 1413 1144 E

1623 1426 1134 932 799 714

gz

looo 972 tz;

1210 1080 1050 1030

778

651 605 558 521 459 399

~ ;z

519 449 404

iit; 613 591

z

2;; 568

642 543

650 569

412 :;; 204 172

418 323 253 218

I

1170 1070 1050 1000 960

;z: 905 849 a25 771 752 7% 727 725 680 I 717 701

;;:

I

180

I.2 l3 14 15 16

%z 812 754 $Z 695 669 676 2: 637 625

2;: 640

01

0

I

I

I

I

2

4

6

8

I

I

IO

12

,

14

/

16

I

I

I

18

20

22

Iml-----, FIG. 1. A comparison of the values obtained with the measurements of Varanasi (squares), and Bouanich and Brodbeck (dark triangles)

in this work (dark circles) for the linewidths of CO + CO2 Tubbs and Williams (triangles), Crane-Robinson (circles) cited in Refs. 5, 6, 7, and 8, respectively.

LINEWIDTH

STUDIES

OF HCI AND

HCI

0.12 t

FIG. 2. A comparison of the values for HCl + 1\;2 linewidths those of Babrov el al. (dark triangles), Benedict et al. (triangles) J, 9, and 10, respectively.

The effects of self-broadening

TAoPA

+

+ NE

given in this study (dark circles) with and Rank el uJ. (circles) cited in Refs.

and foreign broadening y =

103

CO

yB”PB

were separated

by the relation (1)

where y is the measured linewidth as corrected for a finite slit width. ~AOand ys” are the self and foreign broadening coefficients per atmosphere, respectively. Values of the self-broadening linewidths of CO given by Hunt et al. (IS) and the self-broadened linewidths of KC1 given by Toth et al. (14) were used in the calculations. For linewidth determinations using the peak absorption method, values of the line strength of HCl given by Toth et al. (14) and those given for CO by Sorb et al. (1.5) were used. 4. RESULTS

AND

DISCUSSION

The values of the linewidths for all cases studied are presented in Table 2. The agreement between the values obtained by the direct method and those by the peak absorption method are quite good and within the 4% error assigned to the measurements. It is interesting to note that, for values of j m I< 3, the HCl + CO2 linewidth determined in the 1-O band are in good agreement with those for the 2-O band. However for /ml > 3, the linewidths in the 1-O band are smaller than those of the 2-O band by as much as -11% for /ml = 9 which indicates a slight vibrational dependence on the HCl + CO* linewidth values. Figure 1 is a plot comparing the values of linewidths of CO + CO, obtained in this study with those of Varanasi (5), Tubbs and Williams (6) Crane-Robinson and Thompson (7) and Bouanich and Brodbeck (8). The values shown for the work of Crane-

104

TOTH AND DAKNTON 0.;

I

I

1

I

/

HCI t CO2 A .

0.

. 0

1

.

E

z

q

.I- 0

'E

.

0

A

cl

x

0 .

B

B 0.

2

I

I

I

I

4

6

8

10

lmll IJIG. 3. A comparison of the values for HCl + CO? linewidths given in this study (dark circles) with those of Varanasi el al. (squares) and Babrov et al. (triangles) cited in Refs. 3 and 4, respectively.

Robinson and Thompson (7) appear to be too small in comparison with the results of this study and of (6, 8). This also appears to be the situation for the values for the lines, j m j > 10, given by Varanasi (5). Figure 2 is a plot of linewidth versus 1m / for the case of HCl + Nz for values given in this work and those of Babrov et al. (4), Benedict et ~2. (9), and Rank et al. (10). Figure 3 is a similar plot for the case of KC1 + COZ. The values shown representing this work are for the 1-O band of HCl as is the case in the two other investigations shown in Fig. 3 Varanasi (3), and Babrov et al. (4). The value of 0.20 cm-‘/atm given for / m 1= 1 by Varanasi et al. (4) for HCl + COZ is much larger than the value obtained in this study and the value given by Babrov et al. (4). Varanasi et al. (3) determined that the line shape for HCl lines broadened by CO2 is super-Lorentzian in the wings of the lines. They obtained an espression for the superLorentzian line shape, given as (3) :

(2)

1,INEU’lI)‘I’H

STUDIES

01’ HCI AiSU C’O

105

with yc = 1.75. For the Lorentzian case, r~ = 2. In Eq. (2), v0 represents the line center frequency. From Eq. (2), the line profile at the center of the line for the super-Lorentzian and Lorentzian

cases, respectively,

are :

sx 11 -sin ” k.,[ = k,, = Try ( 2 ?I> where k,, = 0.845SX/r~,

11= 1.75,

and k,, = SX/?ry,

I2 = 2.

(3)

The linewidth data obtained in this study and presented in Table 2 indicate that the lines of HCl + COZ as well as those for HCl + N, and CO + COz are Lorentzian in shape within ) v - v. 1 = y by comparing the results from the direct method with those of the peak absorption measurements where the Lorentzian profile was used in the peak absorption analysis. Further measurements were made, to determine the shape factor at distances of j v - voj = 2y and 37 for HCl lines broadened by CO,. These measurements were made in both the 1-O and 2-O bands of lines where the slit corrections were 1% or less ( jrnl< 3). The results were obtained in terms of KSY/yIKAv (hereafter called T?y) and Ks~/KM (hereafter called Tar). For the Lorentzian case, TBy and Tay are equal to 0.2 and 0.1, respectively. For the super-Lorentzian profile, Tzy and Tsy are 0.196 and 0.109, respectively. The absorption coefficient, K,, for the Lorentz case equals that for the super-Lorentzian shape when j v - VO!* 2.17~. The values obtained in this study for Tz, and Tsy are 0.201 f 0.006 and 0.102 f 0.007, respectively. These values show that there is no evidence to distinguish between the two line shapes or to claim that the lines are other than Lorentzian for 27 5 ; v - v,, / 537. ACKNOWLEDGMENT This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract Number NAS7-100, sponsored by the National Aeronautics and Space Administration.

RECEIVEIJ: June 4, 1973 KEFERENCES 1. P. CONNES,J. CONNES,L. D. KAPLAN, ANDW. S. BENEDICT,Asirapilys. J. 152, 731 (1968). 2. P. CONNES,J. CONNES,W. S. BENEDICT,AND L. D. KAPLAN, Astrophys. J. 147, 1230 (1967). 3. P. VARANASI,S. K. SARANGI,AND G. D. T. TEJWANI, J. Quant. Spectr. Rndiatiwe Transfer 12, 857 (1972). 4. 5. 6. 7. 8. 9. IO. II. 12.

H. BABBOV, G. AMEER, ANDW. BENESCH,J. Chem. Phys. 33, 145 (1960). P. VARANASI,J. Quant. Spectr. Radiati~ Transfer 11, 249 (1971). L. D. TUBBS AND D. WILLIAMS,J. Opt. Sot. Amer. 62, 423 (1972). C. CRANE-ROBINSON ANDH. W. THOMPSON,Proc. Roy. SOC. (London), Ser. A 272,453 (1963). J. P. BOUANICHANDC. BRODRECK,J. Quanf. Spectr. Radiative Transfer 13, 1 (1973). W. S. BENEDICT,R. HEKMAN,G. E. MOORE, AND S. SILVERMAN,Can. _I. Phys. 34, 850 (1956). D. H. RANK, D. P. EASTMAN,B. S. RAO, AND T. A. WIGGINS, 3. Mol. Spec. 10, 34 (1963). D. H. RANK, D. P. EASTMAN,B. S. RAO, AND T. A. WIGGINS, J. Opt. Sot. Amer. 52, 1 (1962). E. K. PLNER, H. C. ALLEN, JR., AND E. D. TIDWELL, J. Research Nat. Bureau Standards Sect. B 61, 53 (1958).

13. R. H. HUNT, R. A. TOTH, AND E. K. PLYLER, J. Chem. Phys. 49,3909 (1968). 14. R. A. TOTH, R. H. HUNT, AND E. K. PLYLER, J. Mol. Speclrosc. 35, 110 (1970). 1.5. C. L. KORB, R. H. HUNT, AND E. K. PLYLER, J. Chem. Phys. 48, 4252 (1968).