Fine structure in the ν1 band of CH3Cl near 2970 cm−1

Reprinted

from JOURNAL OF MOLECULAR SPECTROSCOPT,Volume

Copyright

@ 1969 hy Academic

JOURNAL

OF MOLECULAR

Press, Inc.

~\~ORILLOS-CHAPEY

d’lnfrarouge,’

in

V.S.A.

31, 155-169 (1969)

in the ~1 Band hl.

I,aboratoire

Printed

SPECTROSCOPY

Fine Structure

31, iYo. 1, July 1969

of Ct-i,CI Near AXI)

(2.

Chimie Physique, Facdld HBlimenl 350 (9f) Orsajl

A S.I.S.A.M. Spectrometer with a resolution used to study the fine structure of the Y, parallel Several hundred lines have been assigned and puted for bot.h chlorine isotopes. Since the band ture, we have also discussed the assumption of Fermi resonance exists between ~1 and 2v:,

2970

cm-’

GRASER

ties Sciencen

de Paris,

limit of 0.031 cm-1 has been hand of CII&l near 2970 cm-l. all molecrllar con&ants comexhibited no anomalous strucAdel and Barker that a strong

The v1 band of CH&l has been studied with a S.I.S.A.JI.-type spectrometer (Interferometric Spectrometer with Selection by Amplitude of ,IIodulation), the principle of which has been entirely explained in 1’. Connes’s thesis (1). Since Connes’ prototype, numerous modifications have been made in the ne\v apparatuses now working i.e. one at “Laborntoire Aim6 Cotton” (2) and two at this laboratory : Of the two at this laboratory, the first one, usable between 1.2 and 2.3 p, has been previously described by one of us (G.G. j (3) : In this reference the reader will find the details of the modifications made on our spectrometers with regwd to mechanical parts, grating rotation and the oscillation system for the compensating plate. The second one has been built and adapted to work between 2 and 7..? p by one of the authors (ALlI.) and has been used for the present study; a descrip tion of its characteristics and performance has been given elsewhere (4) (5 ). Characteristics

of

the b-7.5 JoS.I.S.A.M.

The spectrometer is equipped with two 110 X 206 mm, 7X3 lines/mm gmtings; their blaze angle is 63”26’, corresponding to a wavelength of ‘24.4 12in the first order. Since the present work was done near X.4 p, we used the seventh order of the gratings. Therefore the theoretical resolving power is 105 000 and we can expect (4) (5) an actual resolution limit of 0.031 cm-’ (this value corresponds to the width at half-height of simple lines in a very low pressure spectrum. Two 1Equipe de recherche associee au C.N.H.S. 155

156

MORILLON-CHAPEY

AND

GRANER

lines can be separated even if they are less than 0.031 cm-’ apart). We used a 6-mm diameter entrance stop, which gives the optimal value for the product Luminosity X Resolution in this spectral region. Figure 1 represents the optical set-up. The source is a Globar (Silicon Carbide) in a water cooled housing, connected to a direct-current line. This is an essential condition to avoid the spurious 100 Hz source modulation which would not be completely eliminated by the amplifier, whose filter is centered at 125 Hz. For similar reasons, we had to place between the source and the entrance diaphragm a small grating premonochromator. Let us call D the change in path per unit time (6 = vt) when the compensating plate oscillates. It is easy to see (1) that each wavenumber is modulated with a specific modulation frequency N = v/x = v(r When the apparatus is set for a wavenumber go, the speed vo is chosen so that vo = NO/Q, No being the center frequency of the tuned amplifier passband. Adjacent orders of the grating, with a wavenumber (TO + Au will be modulated with a frequency No + AN where AN = VOAU. In most SISAM, this AN is sufficient to obtain an elimination of spurious orders by the electric filter. It is not true for the present spectrometer: Au = 410 cm-l, and No = 125 Hz give near u. = 3000 cm-’ a value of 17 Hz for AN. The cor-

FIG. 1. Optical set-up of the spectrometer. M: monochromator; T: tank containing the SISAM; R: gratings; S: separating plate; C: compensating plate; G: absorption cell; D: detector.

~1

157

BAND OF CIl,Cl

responding frequencies of 108 and 142 Hz arc too close to be eliminated b!- the electric filter, hence the need for a premonochromator. Upon exit from the SISAlI, the light enters the absorption cell (which is preferably put at the exit to prevent any distortion of the beam entering t.he interferometer) and is detected by a detector: For the present study, we tried both :I. photoconductive PbS detector’ at dry-ice temperature and a photovolt& InSb detector’ at liquid nitrogen temperature; ,sincc both gave comparable re sulk, we mainly used the I&b detector for the sake of convenience. We had at our disposal four grating rotation speeds, plus a rapid one: The one n-e mainly used for recording the present spectra is about 0.007 cml/sec, permitting the use of a O.&see time-constant on the amplifier. In these conditions, the noise present on the spectra is very small, and OIW can easily work even with very small pressures (0.6 Torr for path lengths around 1 m) : We feel it is a valuable advantage of our interferometer to be able t,o give spectra jvith a resolving po\vei equal t,o the best spectrometers working in this frequcnc\. range but with a signalinoise ratio much higher than the classical slit spectrometers.

21 photodiode gives a mechanical marking lvith a stroke at every motor turn, i.e. every 0.04 cm-l, or 7.5 mm on the chart. The whole spectrometer is in an evacuated tank, in a room maintained at

In a one meter cell filled to a pressure of 3 Torr, \ve recorded the l-0 band ot CO from P (30) to R (30), i.e., from 2013 to 3244 cm-l, the frequencies of these transitions have been given by Rao et al. (6). This recording was obtained in t.he fifth order of the grating, with the same limit of resolution (see above), which is almost constant throughout the whole spectral range where our spectrometer in usable.

This

spectrum

cm -I in the seventh

provides

order. Then,

reference

wavelengths

between

with the help of a computer,

2X0

and 8140

a nonlinear

inter-

polation between these reference wavelengths gives the corresponding to each marker stroke. To achieve an accurate

wavenumber value calibration, we then

record

without

rotafinn

plate

for each CH,Cl of

spectrum

one or several

the gratings: One has only to increase and turn the premonochromator

oscillation

CO lines,

stoppirq

the

the speed of the compensating to its new position to switch

from seventh to fifth order; these CO lines, recorded with the spectrum may have slightly shifted relatively to the stroke scale, when compared to the reference CO spectrum:

This

small

correction

will be applied

This method of calibration gives a good reproducibility different spectra, of the order of 0.005 cm-‘.

throughout

the spectrum.

of the frequencies

’ Kindly provided by the Soci&tB Anonyme des TBlCcommunications.

for

MORILLON-CHAPEY

1%

BND

GRANER

Using the SISAJl described above, we entered upon a general study of CH&l absorption bands in the 3000 cm-’ region. The resolving power of our instrument permits a considerable improvement over previous results: on the one hand, the identification of lines was much more elaborate, owing to the J and K rotational structure, which is very clear on our spectra and which had not been generally analysed before; on the other hand, we have been able to revise the theoretical interpretation of bands by explaining the perturbating resonances. At the present time, three bands have been recorded in this region: 2~6 parallel, vi parallel, and vi perpendicular, whose band centers are located near, 2380, The work on yLhas been previously published 2970, and 3040 cm-r, respectively. by the authors (8, 9), the one on 2~~ is in progress (14) and the present paper deals with VI. The last work concerning vr and 2vg was done in 1953 (7) with a low resolution spectrometer allowing no K-structure to be seen for QRK(J) and QPK (J) lines. Since the pioneer work of Bennett and l\leyer (10) and its interpretation b:, Adel and Barker (IS), all authors have accepted the idea of a strong Fermi resonance between VI and 2~ although IIO convincing proof has ever been given. We therefore intend to study this question more carefully.

Description

oj” the A’pecfmnl

All our spectra concerning vr were taken with a one-meter cell; several pressures were used: 10 Torr for the high J region , 2 and 03 Torr for the central region. 35Cl species Q-brand/es. K-numbering of these branches, till K = 12 was quite easy since they look like strong, well separated and fairly sharp “lines” and the ternary alternation of intensity is very clear. On the other hand, no J structure cm bc seen (Table I). P and R branches. We used several methods to assign the ‘PK (J ) and ‘RK (J ) lines. First, the rule J > K often makes it possible to find the start of a series when the corresponding “Q is known, since the first line in a QPK or ‘RK series is found at a distance of about (K + 1) (B’ + B” ) from the ‘0 branch. Secondly, TABLE 0 BR.\NCH LINE K 1 2 3 1 5 (i

FREQUENCIES

OF THE CH,

QQK 29ti7.717 2967.550 2967.281 2966.890 2966.385 2965.783

I Vl

~1 FUND.ZMENTAL

h7 8 9 10 11 12

(cm-l

in. I-cwcru)

QQK 2965.063 2964.234 2963.315 2962.274 2961.132 2959.870

~1 BAND

159

OF CHIC1

we completed and confirmed our assignments with the help of Blass and Edwards’ combination formulas (11 ) which only require knowledge of the ground state constants, B”, DJ” and D:k . In this first stage of the work, we used the microwave values of these constants (In). The advantage of these formulas is that the?. are still valid when the excited level is strongly perturbed. The series thus assigned have the following dispositions: K = 2 and K = 6 are practically on top of each other from the lowest J to J > ;iO (for instance ‘P2 (J) on @R6(J + 2) and @P?(J) on ‘1’6 (J - 2) ) ; their frequencies must be very close since the observed lines are quite sharp; moreover, their predicted intensities are similar so one cannot be hiding the other. The same situation exists for the K = 3 and 5 series except for the fact the J shift is only one instead of 2. These two coincidences are fortuitous for low J but their lasting so long is of significance, meaning for instance that D.,’ is very close to D,T”. ,411the aforemeiltioned lines being accurately measurable, their frequencies have been used in our computations. On the other hand, the K = 0 and 1 series have a very small separation, but since the K = 4 series (with a J shift of one unit) agglomerates with them, the result, is often a quite large “hump” so that we did not use their frequencies in our computations. They are nevertheless well reproduced in our final synthetic spectrum (Fig. 2). Among all these identified lines, we then chose the -IO0 sharpest ones tAJ cornpute the band constants. For a first trial, \ve took the unperturbed formula for the frequencies : 1’ = v,l + B’(J + (/I’ -

+ AJ) (J + A” -

1 + AJ) -

B”J(J

B’ + B”)K” + D,r”J’(J

D; (.I + AJ)“(J

+ 1) + 1)2

(1)

+ 1 + AJ)’

+ [D” JKJ(J+l)-D’,,(J+AJ)(J+1+AJ)]K2+

(D,“-D,‘)K”

and ran a least squares computation on an UNIVAC 110s computer. We obtained satisfactory values for the constants with a standard deviation of 0.009 cm’ on the frequencies. The constants we obtained are given in Table II. We can only provide values ford’ - d”andD K’ - D,” since only these differences may be obtained as can be seen from the above formula. Instead of using microwave values for D:k and D,“) we recomputed them from our experimental frequencies by a least squares computation on 1.52 differences QRK (J - 1) - QPK(J + 1) (see Ref. (11) ). On the contrary, the B” value used was taken from Ref (12). “7Cl Species A good number of lines still to be identified, we attributed to the 37C1species. They were less intense than the neighbouring ?l lines and could be arranged in

FIG. 2a, b, c. The ~1 absorption band of CIT&l. Path length: 1 ~1. Pressures 10, 2, 0.8 and 10 Torr successively. In each part of the figure, the observed spectrum is above and t’he computed one below. Note that both CH3Wl and CHSW~ lines are assigned. In the low frequency region (a), lines marked with a X belong to 2~5 and have not been included in the comput,ation. In the high frequency region (c), both observed and computed spectra include a part of the perpendicular ~4 band: to show its influence, the pP4- and pP3. .p I.-..,. 1-n,., mnrl,oA hT, y gnd -C hPt,ween 2997 and 3004 cm-l. Further, most lines belong to ~4 .

VI BAND

OF CH,CI

162

MORILLON-CHAPEY

AND

GRANER

~1 BAND

lli:$

OF CH&l

TABLE

II

M~LEC~JL.WCONST.LNTS FOR ~1 BAND OF CH&I Y1 Species 2967.777 f 0.002 0. 443402a 0.443459 * 0.000003 -0.05538 zk 0.00008 (5.99 f 0.04) x IO-76 (5.939 * 0.013) x 10-7 (6.0 f 0.5) X 1O-6b (5.71 k 0.06) x 10-e (2.6 f 0.7) X 1OP

_

(ALL V.~LT_TES IN cm-l) 3’C1 Species 2967.745 f 0.003 0. 436573a 0.436623 f 0.000005 -0.05474 f 0.00013 5.99 x 10-7r (5.93 f 0.02) X 10-7 6.0 x lo-EC (5.50 + 0.13) x 10 F 2.6 X lo-“<

:I 11 unr.erlainlic3.s given are slandurd deviations. ” Microwave value (1B). n In a first step, only D”J and D”JK were determined by a least squares computation on 152 experiment’al differences QRK(J - 1) - QPK(J + 1). The obtained values differ slightly from the microwave values of 6.04 X 1O-7 and 6.6 X lo-“, respectively (12). III the final complltation, D“J and D”,,R were fixed to their previorlsly comput,ed values. ’ These valrles were assumed to be the same as for the main isotopic species. very similar to the 35C1ones. E’ollowing these series down to low J values (for QR its well as for ‘P), we noticed that each of them was coming closer and closer to a “‘Cl series until they merged completely for the first J values. Assigriing the same K and J values to these merging series, we could, by the same token explain the apparent absence of QQ branches due to CH3”‘Cl: They are in nearcoincidence with those due to CH,35C1. We checked and completed our assignments by Blass’ combination formulas. Among the identified lines, we chose 170 sufficiently sharp and isolated to give a good accuracy and ran them by a least squares method to obtain the band constants given in Table II: the stand& deviation on frequencies was 0.011 cm-‘. Csing these computed constants, n-e recomputed a synthetic spectrum, superposing the bands for both isotopic species, with the help of the SYMTOP program written by one of us (G.G.). We had to perfect this reconstitution in the higher frequency region where the VI and vL bands overlap; we used there the FERCOR program (Fermi and Coriolis resonance between v4 and 3~~) (8, 9) with appropriate intensity coefficients for both bands: A ratio (E~~~/xz~~ )’ of =O seems to be reasonable. As one can judge from Fig. 2, the agreement between the experimental and computed spectra is quite satisfactory. series

DISCUSSION

Adel and Barker (13), and other authors after them, declared that the according to them abnormal intensity of the parallel component of 2vs was explairled by a strong Fermi resonance of the 2~5level with ~1 .

164

AND GRANER

MORILLON-CHAPEY

_

I

-

-

I

K=

Calc.

3

ObS.

1

3 ab?..

166

MORILLON-CHAPEY

AND

r

GRANER

-

I

-

7 7

7 7

7 7 7 7 7 8 8

R 8 8 8

16S

MORILLON-CHAPEY

AND

GRANER

A work now in progress (14) does indeed show that the parallel part of the 2~6 band has strong anomalies in its rotational structure. The tentative fitting of the line frequencies with the formula (1) yields an unsatisfactory high standard deviation of 0.055 cm-’ (as compared to 0.009 here). The rotational constants obtained in Ref. (14)-which are therefore to be considered as provisional-are the following: v. = 2579.28 cm-l

B’ -

II,’ = 1.48 X 1oP

B” = 0.0045 D:,

= - 2.14 X 1O-4

A’ - A” = -0.0732 D,”

-

D,’

= 2.09 x

1O-4

Noteworthy are the abnormally high values of the centrifugal distorsion constants and of B’ - BN (from the B5 value given in (Is), one would expect B’ - B” = 0.0032). On the contrary, the present work shows that v1 has no visible rotational anomaly and that its centrifugal distortion constants are reasonable: D,‘, Dg’ and D& , in the excited state differ only by 1, 2, and 5 %, respectively from their ground state value. Consequently, we are left with the choice between two hypotheses: (a) Either the coupling between the v1 and 2~ states is negligible and the anomalies found in 2vgare entirely caused by the interaction of the 2~ level with a third level. This hypothesis seems compatible with the intensity of 2~~ ) which, being four to seven times weaker than the intensity of ~1, is not necessarily too high. (b) Or the coupling between the VIand 2~ states is strong and has the effect of separating considerably these levels. The 2~ level is thus brought near a third level with which it interacts weakly, this interaction being sufficient to strongly modify its rotational structure. The authors rather favor the first interpretation, mainly because of the values of IQ’s distorsion constants but this problem cannot find its final solution before a complete study of 2~ has been carried out. ACKNOWLEDGMENTS The authors would like to thank Professor P. Barchewib for his kind interest work and Professor G. Amat for fruitful discussions concerning Fermi resonance. RECEIVED

in their

October 29, 196s REFERENCES

1. P. CONNES, These Paris 1957; Rev. Opt. 33, 157-201 (1959); Ibid. 38, 416-446 Ibid. 39, 402-436 (1960). 2. J. VERGES, J. Phys., Colloque C2, 28, C2-176 (1967). b. G. GRANER, These Paris 1965; J. Phys. 26, 222A-228A (1965). 4. M. MORILLON-CHAPEY AND P. CONNES, 6. R. Acad. Sci. Paris 262I3, 803-806 5. M. MORILLON, J. Phys., Colloque C2, 28, C2-181 (1967).

(1959);

(1966).

YLBAND OF CHIC1

160

6. K. NARAHARI RAO, C. J. HUMPHREYS, AND D. H. RAXH, “Wavelength

starldnrds in New York (1966). 7. J.PICKTVORTHAND H.W.THOMPSON, Trans.Faraday Soc.60,218-226 (1954). 8. C. BETRENCOIJRT,C. JOFFRIN,M. MORILLOS, AND C. ALAMICHEL, C. K. .lcarl. Sci, Puris 264B, 14581462 (1967). 9. 31. ~\~ORILLON, G. CRANER, .&NDC. ALAMICHP:L, f?. R. dead. &i. Paris 266B, 24(-244 the infrared,”

Academic

Press,

(1968). 10. W. 11. BIGXNETTAND C. F. MEYER, Phys. Rev. 32, 888 (1928). 11. W. E. BLASS AND T. H. ED~AR~S, J. Mol. Spectry. 24,111-115 (1967). 12. (a) J. KEAITCHMAN AND B. P. DAILEY,J. Chena. Phys. 22,1477 (1954); (b) W. J. ORVILLRTHOMAS, J. T. Cox, AND W. GORDY, J. Chem. Phys. 22, 1718 (1954). 13. A. ADRL AND E. F. BARIZR, J. Chem. Phys. 2, 627 (1934). 14. C. ALAMICIIEL, C. BETRENCOURT AND M. MORILLON, C. R. Acad. Sci. Paris, 267B, 205 (1968). 15. J. P. DC JONGH AND H. A. DIJKERMAN, J. Mol. Spectry. 26, 129 (1968).