Strengths and lorentz broadening coefficients for spectral lines in the ν3 and ν2 + ν4 bands of 12CH4 and 13CH4

JOURNAL OF MOLECULAR SPECTROSCOPY

97, 333-342 (I 983)

Strengths and Lorentz Broadening Coefficients for Spectral Lines in the v3 and v2 + v4 Bands of 12CH4 and 13CH4 V.

MALATHY

DEVI, B. FRIDOVICH,

G. D. JONES, AND D.

G. S.

SNYDER

National Oceanic and Atmospheric Administration, National Earth Satellite Service, Washington, D. C. 20233 Line strengthsand self- and nitrogen-broadened half-widths were measured for spectral lines in the yj and Ye+ q bands of ‘%I& and ‘%X., from 2870-2883 cm-’ using a tunable diode

laser spectrometer. From measurements made over a temperature range from 2 I5 to 297 K, on samples of ‘%I& broadened with Ni?, we deduced that the average temperature coefficients n, defined as g(T) = !& TJo(/T&“, of the Lorentz broadening coefficients for the us and vz + V, bands of ‘%I& were 0.97 + 0.03 and 0.89 + 0.04, respectively. A smaller increase is observed in line half-width with increasing pressure for E-species fines, for both self- and nitrogen-broadening, than for other symmetry species lines over the range of pressures measured, 70 to 100 TOIT. I. INTRODUCHON

Equivalent widths and self- and nitrogen-broadened half-widths have been measured for spectral lines in the v3 and v2 + v4 bands of i2CI& and 13CI-&.In addition, for 12CH4 we have measured nitrogen broadened half-widths as a function of temperature from 215 to 297 K. For the measurements on 13CI&, we have used a 99 percent enriched sample. From the measurements we have determined the line strengths S, the Lorentz broadening coefficients for self- and nitrogen-broadening, &‘(CH4-CH4) and x(CH4-N2), and the temperature coefficient of x(‘2C&-N2), 12, expressed as b”L< T) = b~(30@0(/300)-“. A summary of measurements is given in Table I. We have observed a smaller increase in line half-width with increasing pressure for E-species lines, for both self- and nitrogen-broadening, than for other symmetry species lines. Works on the u3 CH.+ fundamental band, both experimental and theoretical, have been reported in the literature (for example, see Ref. (I) and works cited therein); most have been concerned with line assignments and intensities. Several studies on line half-width have been reported: measurements at low resolution by Varanasi (2, 3) and Varanasi et al. (4); calculations by Tejwani and Varanasi (5), Tejwani and Fox (6), Tejwani, Varanasi, and Fox (7), and Yamamato and Hirono (8); temperature dependence of half-width at 2947.88 cm-’ by McMahon et al. (9) using a He-Ne laser. Recently, very high-resolution measurements have been used to determine positions and intensities (I, 10, II) and self-broadening of low J lines in the P and R branches (12). Brown et al. (13) have recently published new tables of line assignments, position, and strength for the v2 + v4band of “CI-L,, these values are essentially those incorporated into the AFGL list (14). Because of the dearth of information on 333

0022-2852/83/020333-10$03.00/O Copyright0 1983 by AcademicRess, Inc. All rightsof reproductionin any form reserved

334

MALATHY

DEVI ET AL.

TABLE I Parameters Studied LINES MOLSCULB

%iq

BAND

PARA”ETERS

mAsIJRBD

S

v3

9

b&q-CXi4),

v2+v4

b&iq-N2)

3

S

24

$(CHq-CHq) , g,(Cli4-N2) y3

‘3cnq

6

”

2

S

10

9 miq-ai4

)

g(CfIq-N2)

3 7

s

Yz+%

5

n

3

temperature dependence of line half-width, determining this parameter was our main objective. Intensities were measured to confirm values available in the literature. II. EXPERIMENTAL

DETAILS

The diode laser spectrometer used for the measurements has been previously described (IS). While several major changes in optics and optical layout have been made they did not change the essential features reported earlier. Table II summarizes details of parameters at which various measurements were made. Ultrahigh-purity methane (99.97 percent) and research-grade nitrogen were obtained from the Matheson Company, and a 99 percent ‘3CH4-enriched gas was obtained from the Mound Laboratory, Monsanto Chemical Company. The gas handling system is almost entirely made of Pyrex, including valves. The lOO-cm stainless-steel temperature-controlled cell has been described in detail in Ref. (16). All cells were fitted with calcium TABLE II Sample Characteristics

PARAMETER

PATH (cm)

S b&H,-N2

1

bOL(CH4-CH4 ) n

0.5,

PRESSURE (Torr)

50,

100

50,

100

2.5, 100

5,

0.3-3.5

IO

295-298

70-100

295-298

70-100

295-298

70-100

297,

263,

215

STRENGTHS AND WIDTHS FOR %-I,

335

AND “CH,

fluoride windows. Cell lengths listed are nominal, the actual lengths were measured with machinists height gauges and are accurate to +0.07 mm. Sample pressures were measured with an MKS Baratron gauge. Ambient temperatures were measured either with a nearby mercury in glass thermometer or with a YSI Telethermometer whose thermistor bead was affixed to the cell wall. For the temperature-controlled cell, sample temperatures were measured with several internal iron-constantan thermocouples read out on a Fluke Model 2 190A digital indicator. III. LINE STRENGTHS

Line strengths were determined from spectral scans made at the lowest pressure consistent with cell length and adequate minimum transmission. Each line was scanned at three or more pressures with several repetitive scans made at each pressure. Equivalent widths measured on the strip chart records were used to find the line strengths using the tables of Jansson and Korb (I 7). The results of these measurements are presented in Tables III, IV, and V. Line positions and assignments are those of the 1980 AFGL compilation (14). We have listed transitions by branch, and total angular momentum (J); and within each of these by upper and lower symmetry states C

TABLE III Line Strengths (cm-’ atm-‘) of Some vj P( 14) Transitions of ‘%I%, near 3.3 pm (296 K) NO.

*cm-’ 1

TRANSITIONS

PRESENT

2873.726C

Fl(4) +F2(4)

2873.768

E(3)

2873.830

F22(3) +F1(3)

2874.532

H(3)

STDDY

AFGL

GRAY'

o.o4992(111)d

0.04550

0.04728

0.03330(631

0.03058

0.03152

0.0497?(45)

0.04550

0.04728

0.05193(21)

0.04942

0.04585

2874.736

0.03530(14)

0.02987

0.03009

28?4.823=

0.04880(202)

0.04477

0.04514

2874.973

0.08430(120

0.07608

0.07666

2875.394

0.04900~102)

0.04550

0.04585

0.0472

0.04550

0.04657

0.0472

2875.442

fE(3)

+F2(3)

7J,

GmrussIb

* D.L. Gray, A.G. mbiette and A.S. Pine, J. Mol. Spectroac., 440-456 (1979) z,t (294+OK. b S. Gherissi, A. Henry, Bl. LOate and A. Valontin, J. Mol. Spactrosc., a6, 344-356 (Igal). = Tranmltione belonging to 13C"4 are well reeolved in the present study and are masured using 13C enriched sample. d Uncertainties guotsd are MB standard deviation in the Last digit(s). e R(7) lines of y+u4.

336

MALATHY DEVI ET AL. TABLE IV Line Strengths (cme2 atm-‘) of Some v2 + v4 Transitions of ‘%H, near 3.3 pm (296 K)

NO.

V(cIc’

I

TRANSITION

SAUD

PRSSENT

1

2873.594

2

WRSNGTS ST”DY AFGL/O.Oll

P(l3)

F2(4)+Fl(4)

0.09884(11)’

2873.63ob

P(l3)

B(2)

0.06435(19)

v3

CE(2)

3

2873.7o6b

P(l3)

F2(3)+F1(3)

O.l0116(S5)

4

2074.226

P(l3)

Al(l)tAZ(l)

0.15931(159)

0.15985

5

2874.352

P(l3)

Fl(2)+F2(2)

0.09640(167)

0.10480

6

2874.453

P(l3)

F2(3)+Pl(3)

0.09272(145)

0.07794

7

2S74.So9b

P(13)

AZ(l)

0.15950(220)

S

2874.887=

P(l3)

F2(2)+F1(2)

0.10194(168)

P(l3)

E(l)

0.06935(14)

9

2S74.9lob

10

2075.387=

11

2075.542

+E(l)

UJNIDEHTIFIED)

[

P(l3) P(l))

Fl(l)+F2(1) F2(l)+Fl(O

0.06436(88)

1

0.22028(505)

0.18187 0.06583

12

2877.208

R(2)

Fl(l)

+ F2(1)

0.05736(157)

13

2877.320

R(2)

E(l)

+ E(l)

0.03842

14

2879.291

R(4)

E(l)

+ E(1)

0.02600

‘2+“4

a Uncertainties quoted within parentheses are one standard deviation in last digit(s). b Resolved from nearby '2cn4 transitions. Wavenumbers estimated from the 12CA4 line using the etalon fringes spacing. c Unlisted lines in 1980 version of the AFGL tape.

described by the representations Al, A2, E, Fl, F2, of the symmetry group Td; and (N) which refers to the number of states with the same C and J. Wherever available, other S values have been listed for comparison, with units converted as necessary. Some lines considered blended in the AFGL list (14), which are essentially the same as those of Toth et al. (I8), have been fully resolved. The positions of these lines were estimated by measuring their spacing from the nearest well-resolved “CH4 line using the etalon fringes to determine the dispersion. The few lines still unresolved are bracketed. Figure 1 shows the method used to identify previously unresolved lines. Once it was clearly established which line was a 12CH4or 13CH4line, the scan taken with two cells in series in the beam was used to determine the spacing. Line number 8 in Table IV was located as described and has been assigned as shown. Line number 10 was observed and located, but not assigned. For Table V, we adjusted the AFGL S values to our units and also scaled them by the concentration of 13CH4 in a natural methane sample.

337

STRENGTHS AND WIDTHS FOR ‘%H, AND 13CH, TABLE V Line Strengths (cm-* atm-‘) for Some v3 and u2 t v,,Transitions of 13CH((296 K)

NO.

V(cd

I

TRANSITION

8TRRNGTS

PRESENT

1

2 3

2073.422 2873.538 2873.594 2873.63@ 2874.107

Q(6) R(7) R(7)

F2(4) Al(l) Fl(0

+ Fl(1) +A2(1) CF2(0

1

CP(l3) P2(4)+Fl(41b Q(9) Fl(l) + F2(2) Q(7) F2(2) +Fl(2)

4 5 6 7 a 9 10 11

2874.513 2874.890a 2874.906 2874.931 2875.274 2875.654

Q(9) Q(7) Q(7) Q(7) Q(lO) R(3)

12 13 14 15 16 17

2876.981 2077.172 2877.480 2877.550 2878.820 2878.871

Qil;, Ai(ibA2il; R(3) Fl(2) + F2(1) Q(ll) FZ(l)+Fl(3)

18

2878.958

19

2879.004

Q(8) B(2) R(8) E(l) R(8) F2(1)

20 21 22 23 24

2879.961

Q(7)

2880.287 2880.313 2860.423 2880.516

Q(7) Q(8) Q(7) Q(8)

Al(l) Fl(2) At(l) Fl(2) E(l) F2(1)

+-U(l) +F2(2) +A2(lf -+FZ(l) +E(2) + PI(l)

Q(l2) F(l) + F2(3) Q(8) Fl(2) + F2(1) Q(8) E(2) +E(2) r R(8) Fl(l) +FZfl) Q(l2) A23(l)+Al(2

1

XXX

0.00243(O)' 0.00381(V) 0.00287(9) 0.01474(16) 0.00498(7) 0.02824(60) 0.00602(l) 0.01443(39)

AFGL

0.00250 0.00418 0.00185

0.00450 0.02368

0.00312(71 0.00609(80) 0.03922(37) 0.00556(2) 0.04417(80) 0.00646(11) 0.00320(9) 0.00610(12~ 0.00445(10~

0.00092 0.00582 0.03523 0.00548 0.04746 0.00609 0.00294 0.00800 0.00499

0.00816(24)

0.00910

+E(l) 1 +E(l) cFl(2)

0.00104(1)

0.00133

Al(Z)

+A2(1)

0.00836(14)

Fl(4) F2(3) l3(3) A2(1)

+F2Z(21 +Fl(l) +S(l) +-Al(l)

0.00949 0.00413 0.00140 0.00158 0.00546

1

a Uncertainties quoted (ve one standard deviation b u3 13 cl?4 line. ' Resolved from the adjacent 13cH4 line.. d Resolved from the 2874.906 line.

0.00369(18) 0.00174(12) 0.00157(0 0.00525(27)

in the

last

di9it(s).

IV. LINEWIDTHS

Line half-widths were measured from the strip chart records for a variety of pressures of C& or C&-N2 mixtures (see Table II). While it would have been desirable to go to higher pressures, blending of the lines prevented it. The method of data reduction has been detailed in Refs. (IS) and (19) and is briefly repeated here. Halfwidths were measured at the square root of the minimum transmission. Measurement at low pressures revealed an almost Doppler effective half-width, bb, typically three percent larger than the computed Doppler width bn. Using the method of Ref. (20), we estimated the instrumental half-width bi to be 0.0003 cm-‘. At higher pressures (70-100 Torr), half-widths ranged from 0.006 to 0.01 cm-’ and were assumed to be those of Voigt lines, bv. Values of b’, and bv were used in the expression of Oliver0 and Longbothum (21) to derive the Lorentz half-width, h, and the broadening coefficient, R = brjp(atm). For weak lines of the v2 + v4 band, where the partial pressure of methane was three Torr or more, the broadening coefficients were derived using the relation br = &‘(CH4-CH,&(C&) + @(CH,-N2)p(N2). The results of these mea-

MALATHY DEVI ET AL.

338

2374.310

.063

5.0 Torr

a10

I/

j

%I4

--IF-

Both

cells

in series

2374.733

.323

.090

.331

.373

FIG. 1. Method of identifyingand locatingspectrallines near 2875 cm-‘.

surements are presented in Tables VI and VII. A large source of error in this procedure is caused by uncertainty in locating the 100 percent transmission line, which directly affects T(O). We have estimated that an error of 0.5 percent in T(O) can produce a 1.5 percent error in bv which produces a 3-4 percent error in @. Another serious problem which may undermine, in some cases, the data reduction is shown in Fig. 2. Here we see two traces taken at low and high pressure. The central line, an E transition, remained narrow when pressure broadened, but exhibits obvious Lorentz wings. We have observed this phenomena in both self- and nitrogen-broadened E lines. Pine (22) has seen something similar in a v3 R(0) A-type transition. He showed that a self-broadened line at 100 Torr may be fit closely with a Voigt function, but that an air-broadened line at 176 Torr is Lorentzian. He ascribed this behavior to more effective dilfusional narrowing of the Doppler protile by Nz than by CI&. Tejwani and Varanasi (5) and Tejwani and Fox (6) have reported small half-widths for E-type transitions at low J values. This smaller colhsional broadening allows us to see the Doppler narrowing-an effect reported by Eng et al. (22) for water vapor. Tables VI and VII both show E-transition lines with low relative values for R derived assuming &, constant. Note in particular line number two in Table VI. This problem requires further work.

STRENGTHS

339

AND WIDTHS FOR “CH, AND 13CH4 TABLE VI

Broadening Coefficients (cm atm)-’ for Some u3 and v2 + v., Lines of ‘%X-I, (296 K)

NO.

qcm-’

1

TRANSITIO”

g(cHq:aiql

@(CH~-NZI

0.0507(20)’

0.034312)

1

2873.594

F2(4)+F1(4)

2

2S73.630b

E(2)

+ E(2)C

o.ol33d

0.003(l)

3

2873.7063)

F(2)

+ Fl(3)

0.0530(15)

0.0389(3)

4

2874.226

Al(l)+A2(1)

0.0458(9)

5

2874.352

Fl(Z)+

F2(2)

0.0422(S)

6

2874.453

F2(3)+

Fl(3)

0.0419(12)

7

2874.809

A2(1)+

Al(l)

0.0458(7)

f: Uncertainties Transitions with respect

quoted represent one standard resolved from neighboring ‘2cS4 to 12~4.

= Doppler narrowed d one masurament.

deviation line,

in

the

wavenumbers

laet

digit(e).

estimated

line.

TABLE VII Broadening Coefficients (cm atm)-’ for Some P( 13) Y, Lines of 13CI&(296 K)

NO.

1

y (cm-‘)

SAND

2874.736

TRANSITIONS

“3 [

2

2874.823

~(7) P(l4)

R(I) + E(2)+

g(l)b B(2)

R(7)

F2(2)+

Fl(ljb

P(l4)

F2(2)c

PI(Z)

1

~tctI4-ca,,

g(CR4-N2)

0.0515(7)’

0.0346(5)

0.0658(S)

0.0477(2)

3

2674.973

P(14)

AZ(l)+Al(l)

0.0580(6)

0.0337(17)

4

2875.394

P(14)

F2(1)+

Flfl)

0.0650(16)

0.0384(S)

5

2075.442

P(l4)

Fl(2)c

F2(2)

0.0632(S)

0.0328(S)

6

2875.274

Q(lO)

‘2+‘4

2075.654

R(3)

2876.981

Q(l2)

E(l)

+ E(2)

F2(1)

+ Fl(1)

0.0252(6)

Al(lkA2LZ(l)

0.0616(3)

0.0494(6)

2877.172

R(3)

0.0792(9)

0.0666(10)

Q(ll)

F2(1)+

Fl(3)

0.0616(7)

0.0444(3)

2877.550

Q(l2)

FlCl)+

F2(3)

0.0583(2)

0.0448(201

last digit(s). b u2+v4 lines.

within

parentheses

+ F2(1)

0.0621(10)

2877.480

a Uncertainties quoted

Fl(2)

0.0794(W)

represent

one

standard

deviation

in

the

340

MALATHY DEVI ET AL.

1.00

Meter

80.60 Torr N2 + CBq 1.00

Meter

2673.726 ml-1

2073.786 cm-'

2973.830 cm-'

F1(4)+P2(4)

E(3)+E(3)

F2(3)-F1(3)

FIG. 2. Line narrowing effect observed in an E-symmetry transition of the P( 14) manifold in the ‘%I-& v3 band.

V. TEMPERATURE

DEPENDENCE

OF %I%, HALF-WIDTHS

The temperature dependence of half-width has been measured for three lines in the y3 and two lines in the v2 + v4 bands of ‘*C&, using methods described in Ref. (19). A sample of data taken for several lines is shown in Table VIII. The cell is fdled at room temperature and then cooled. The partial pressures at room temperature are shown as measured. At other temperatures, the total pressure was measured and the partial pressures calculated, assuming the ratio PdPN2 remains constant. The measured pressure differs from that which would be calculated for the cold cell alone using the perfect gas law, because a length of tubing with an unknown temperature gradient distribution extends from the cell to the gauge, which is external to the cold cell. At the pressures used transpiration is insignificant, and so the measured pressure is valid for the cold cell. The quantity &((T) is calculated as [b(T) + b#‘*, where b, is the instrumental half-width. This expression, assuming bi is Gaussian, is a firstorder one, as explained in Ref. (20), and introduces an error in b’, of only 1.5 percent.

341

STRENGTHS AND WIDTHS FOR ‘%ZH,AND 13C& TABLE VIII Temperature Variation of Line Half-Widths

T

(cm-0 2874.973

2875.394

2875.442

2875.654

2877.172

(Xl

+4 (Torr)

b2 (Tore)

297.6

1.15

96.89

4 x

b;T=&,‘“’

103

Cd')

9. x 102 (cm-1 atIn-

4.430

6.975

3.38

263.1

1.08

85.16

4.174

6.931

3.84

224.0

0.98

77.78

3.852

6.840

4.46

296.2

1.11

89.61

4.539

7.410

3.83

263.5

1.04

84.35

4.292

7.373

4.29

215.5

0.93

75.35

3.929

7.358

5.20

296.2

6.911

3.29

1.11

89.61

4.490

263.5

1.04

84.35

4.241

6.817

3.67

215.5

0.93

75.35

3.872

6.735

4.44

9.911

6.21

296.3

1.13

95.63

4.430

263.2

1.06

89.90

4.175

9.964

6.86

215.5

0.95

80.33

3.775

10.185

8.23

296.2

1.01

78.07

4.780

9.358

6.61

263.1

0.95

73.47

4.545

9.398

7.32

215.5

0.85

65.55

4.180

9.468

a.71

The column labeled bv lists the average of measured half-widths of several repetitive scans made with the same gas sample. The last column lists the derived values for @CH4-N2). Table IX lists the derived temperature coefficients for the five lines TABLE IX Temperature Coefficient of N2-Broadened ‘*I-L+Transitions TIMES v(crr')

2074.973

BAND

"3

IaAsuwED

4

n

0.995+0.017

2875.394

7

0.975+0.032

2875.442

7

0.944fio.030

2875.654 2877.172

'2+'4

6

0.917+0.043

5

0.863~0.036

342

MALATHY DEVI ET AL.

measured. The last column shows the number of independent times each measurement was performed. For each band we assign a mean value of n, 0.97 k 0.03 for the u3 band and 0.89 + 0.04 for the v2 + v4 band. Values of n close to unity for the u3 band have been previously reported by Varanasi (3), and McMahon et ul. (9). RECEIVED:

August 12, 1982 REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. II. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

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