High-Resolution measurements of absolute line intensities in the Q and R branch of the ν4 fundamental of 12CH4

CIIEJfICAL

I_ Introduction fn reoznt years interest in the spectrd prtrrtmetrtrs of the Pi band of methane, generated by its use for the remore sensing of the thermal structure and corn-position of planetary imnospheres, has stimultted extended investi@on on this subject. In this context 8 controversy on the strength of the hand and si&icant discrepancies between hboratory measurements and line-by-line computations at p&rticultir wavelengths have arisen [l-9] _ In the present work the absolute line intensi:ies of sevett transitions in the Q braitch and three tram&ions in the R branch of the vs band have been mawed at high resolution using tuu&le diode &rs_ The resuhs for the absolute line intensities are compared with the theoreticai cafc&tions of iefk f2,6,fO] and for the rektive Iiue intensities with those ofref_-[I I J. Using the formalism developed in ref_ f I2f from the measured absolute line intensities a vaiue has been derived for the vibration transition moment (c(ot>_ This vaiue has been used to calculate absolute Iine intensi:ies for comparison with experiment31 data measured at high and moderate resolution_

454

PfIYSIC5

3 _.

LErr1:KS

E~perimentat

IS Ocrober

192-P

detaik

%fsthrme of99.95% ntinirnunl purity X\iis introduced in Z_I 30 cm loug stain&s steel cell kept st a temperature of296 + f K, at preuures muginS from O_O.oS to O-5 Torr_ Sample pressures were determined to a~ accuracy of about 3% with an RlKS model Baratran pressure meter. Direct absorption tneasurements were performed using semiconductor tunable diode Iasers and the experimrntaf assembly zdready described 1131. The relative frequency cahbration was established front the interference fringe pattern, obtained from diverting part of the laser beam through a 1.001 inch (2540 cm) germanium etalon, so aHowing simukmeous scanning with the absorption spectrum. Care tws taken to avoid effects of angular misalignment of the etalon on the free spectr3i range_ In spite of the use of a 93ting monochrornator with a band-pass of 1 cm-i, it was found often difficult to isohte a singie mode because of diode laser emission of several simultaneous dose Iying modes. This fact, which can be responsibk for erroneous results in the intensity measurement, was Iimitiug the useful waveIength region with respect to the overall tuning range of the Iaser. The linewidths observed in the spectra were about 10% larger than the expected Doppler broadenbIg; for

Voiumr

66. numba

this reason

3

of the line

rite d~terntinrttion

xwfs performed

from

using the refstionship

the

CHEMICAL

trtlnsitions

rite nttssttred of ref.

anaiy7ed.

t I-&] _ For

LETTERS

IS October

1979

the parameter C,, which accounts for intensity perturbations in the P and R brancbes [9,1 S]. from the equation

intensities

equivalent

;L rniniinunt

PHYSICS

widths

e.tcb one of

of ten indir i-

nteasuretnents \\ere prrhrtned at different gas pressuresThe standard deviation in each sf;‘t ofintensity me.wrrements of a g&en indi\iduat line is of the or&r of ~5% or better_ T&s an be t&x also 3s the axurxy of the measurement \~hicit resufts from errors in the pressure vaIue (rt3%). relative frequency caiibr&ion (free spectral range equaI to 0.04853 2 0.0007 cm-‘) .md line integr~tiort. dwl

3. Resstks and discunion T.tbIe I sI~otvs thr measured absohtrc inrensities compared to caiculated values taken from refs. [2,6_IO]; al1 the data are at 296 K except for those of ret [ IOj which are calculated at 300 K. The vibration-rotation transition moments of the Q tr.msitlons the value for the cibration transition moment (pal) hds been derived. The numerical average is (0.0537? i: 0.0007S) X IO-” esu_ This vafue has led to the cslcuktion of

Assiinmmt rcf_ f6f

Frequent> (cm-‘) ret1 [16g

Absdute

where J’ md J are the rotational angular momentum quantum numbers of tile f&al and initial states and m is equal to -J. 0,J f I for the P, Q and R branches respectReIy. The value of C3 obrained from the above equation is -0_0095 I 0.0016The values of the vibration transition moment and of C4 are in good agreement with those recently determined by Fos et al. [9] f(O.05OS k 0.0015) X 10-‘s esu and -0.0093 2 0.0014)] from higbresoiution I?le3surell~e~t~. Absofute line intensiries have been caicuiared using the formafism of ref. [Y] and the v&es of (irr,,) and C? determined in this work: the values have been compared with experimental measurements taken from literature_ Tile cafcufated data, show not only a very good agreement with aff the line intensities measxxred in the present investigation, but are also consistent with tfle high-resolution data of ref. [9] which are about 10% lok\er. The ccmparison with several absoIute line (or multiplet) intensities measured in ref. [4] at moderate resolution, show that tbe caIc&ted values are lo-15% lower in the Q and R

Ime mtensiry (cm-”

QfFZ(I)FI

(2)

31 296 K)

(I+

Relative intensity

=>

ref. IllI c&c_

(lo-=

ref. [IO1 c;rfr. bf

ref. 161 cafe.

ref. f’f G&T_

-_ QZFI(I)FZ(l) Q5F2(2)F1(2) Q4FI(I)FZ(I) QlZF2(3)Fr(3) QSFI(Z)F~(~)

arm-t this v.or?c

this work

exp_

CAC.

1305.709 1305.907 I30.5.991 t306.036 I306.068

0.939 I.129 I-IS9 0.106 0.608

1.051 I.255 I.326 0.114 0665

0.755 0.843 O-903 1.030 0.954 1.070 o.os3 C) 0.098 0.478 d) 0.536

0.857 I-023 1.080 0_09-t 0.5442

atis %%OrI:

exp_ 0.120 0.178 O-161 0269 0.221

1226 15-39 1625 1.43 s.20

12.86 15.71 16-32 1.50 833

1306.X 13

0.803

0.853

0.634

0.733

O-719

0.208

10.87

1 I.04

Q3AI(I)AI(I)

1306.148

1.887

2.110

I-516

1,645

1.717

0.112

25.84

25.10

R3AI(I)QZ(I)

1327-063

a)

2.752

R3Fi(l)FI(l) R3F2(1)Fl(l)

1327.244 1327.396

3 a)

1.651 1.651

1.981 I.185 t.188

2.058 I.235 1.265

2.075 I.245 I.245

0.161 0.161 0.161

33.09 19.86 19.87

31.39 18.83 19.29

a) Ref. (171. bi Calculated at 300 I;. C)The transition assignment ofref_ [ZI is AlZF2(L)Fl(tf. d, The transition nssigoment of ref_ [3] is QSFl(I)FZ(l).

4.55

Vokuc

66. numbur 3

CIIIXICAL

brrutches but are within 5% for transitions branch_ The compariso~i of the measures he

rIIi-s1cs

in ihe P

as indicated

iu ret

[9]_

to mcxsured absoiute intcn~iries

The ratio compiled

is in fact of the order of 0.9 for the data of ref. [Z J ;tqinsr

about

I.2 for the data ofref_

[6J (see

rsble I)_ In the previous work frequency

[ 16 J_ rnzinIy de\-orcd to Iinr

determintrtion

in the Q branch of the I’~

bznd. reIati\e line intensities hve The efforts devoted for ziII the sources

been measured_

in the present study to accouut

of error

pointed out the rfkct

in the mezksurements hue

of multimode

hoer to generate erroneous

emission of the

Iine intensity vtlIues_ This

for the laser used in the study of rcf_ [ 161 and its effect is considered to be responsible for severrrlinaccurcrcies in refative fine intensity Jeterminawas the exe

tions Y shown by a compxison the present work_ Therefore

with the results of

the data of ref_ [ 161

by the corresponding vahes shown in the Iast column of tabIe I normaItied to the QZFICI) F2( I) value t&en from ref_ [ 1 I] _ It appears worthwhile to point out the very good agreement of aI these data with the calculations of ref_ [I 1j _ are superseded

Acknowledgement The authqrs are gra:efuI

456

to Professor

preprint of ref_ [?J _ Tktnks C_ Ikrtolini for support.

15 October 1979

are zIso due to Professor

intcnsitics

with tlic compiled tIiCoreficzI vahrcs of refs. [Lb J show a better rrgeernent with those of ref_ [2 J corrected by the factor 296/2X

I_I.TI-ERS

I-C_Fos

for a

References P. V~r;lnxi ;md G D-T_ Tej\\ani. J_ Qwmt. Spectrb _ R.tdhtikeTr;mst>r II (1971) S-t9_ K_ IL\. An&&o!Vlbratron-Rotdtlon Spectra of Mrrhznc. _AE=CRL-TR-73-0738 (1974). F_\\‘_Ta> lor and AD. Jones. Icxus 29 (1976) 199. I‘K 60 and P. Vamnasi. J. C&ant_ Spectr& _Radiatnc -I-rzln\ferIS (1977) 11.5. G.S. Orton. Icxu~ 32 (1977) 4 I. A. Chcxlin. N. fluswn. N.A. Scott and D_ Gautier, J. Mol. Spectry-. 7 I (1976) 313. AD. Jonex. F.W. Ta)Ior and G S. Orton. J. Quzmr. Spectrr _ Rndhtirr Trrnz.t>r. to ILLpubIlhcd_ B-L. Lutz. P_M_ Sd\zggio and R-l\‘_ Boewr. to be pub&hCd.

[9l K. f-o\. NJ. ReisfeId and R.S. XfcDo\ielI. J. Chem. Ph? s. (1979). to be pubEsBed_ [IO] P. Ptnwn and J_ Dupre-%faquaire. J. blo1. Spectry.. to k publish~a!_ [ I 11 D-L_ Gray and A-G_ Robiettc. ?sfol_Phes_ 32 (1976) 1609. Ii. f=o\. Ph) +_ Rev_ AS (1973) 658. F_Cappelkmi and G. ResteUi. J. 3foL Specrv-. 75 (1979). to be publishedCD. Rodserr and A.P_ WIIiams. J. Quant. Spear) _ Radhtire Tr.tns!>r 14 (197-l) 319. R. IIernmn zmd R-F_ N’sIIis, J. Chem. Phks- 23 (195.5) 637_ G. ResteIIi and F- CzppeILmi, J. MoI. Spectry. 77 (1979). to be pubIished_ J. Botincau. J_ BfoI_Sptxtr>_41 (1972) 181.