Assignments and predictions of far-infrared laser lines in methyl alcohol

Volume 80, number 5,6

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15 January 1991

Full length article

Assignments and predictions of far-infrared laser lines in methyl alcohol I. M u k h o p a d h y a y l Physics Department, Universityof British Columbia, Vancouver,B.C., V6T 2A6, Canada

R.M. Lees Centre of Excellence in Molecular and Interfacial Dynamics (CEMAID) Physics Department, University of New Brunswick, Frederieton, N.B., E3B 5A3, Canada

and J.W.C. Johns Herzberg Institute ofAstrophysics, National Research Council of Canada, Ottawa, Ont., KIA OR6, Canada

Received 23 February 1990

In this work, the results of detailed spectroscopic analyses of the high-resolution Fourier transform infrared (IR) C-O stretch and far-infrared (FIR) torsion-rotation bands of methyl alcohol are applied to assign and predict FIR laser lines opticallypumped by a CO2 laser in the dense Q-branch region of the C-O stretch band. The assignments are supported by means of closed combination loops of accurately measured transition frequencies. For the predicted FIR lines, the frequencies are deduced with an accuracy of +_0.001 cm-~, which is at least an order of magnitude better than can be obtained from direct wavelength measurements. The IR and FIR spectroscopic data relevant to the assignments are included.

1. Introduction Methyl alcohol has been the subject o f great spectroscopic interest in the F I R a n d I R regions, in recent years, because o f the wide F I R laser emission spectrum p r o d u c e d when CH3OH is optically p u m p e d by a CO2 laser [ l - 1 0 ] . M o s t o f the observed F I R laser lines are transitions within the excited m e t h a n o l C - O stretching state, hence the highresolution spectral study o f the C - O stretch b a n d is an i m p o r t a n t tool in assigning these lines together with their p a r e n t I R p u m p absorptions [ 1 l - 13 ]. It is found that a substantial n u m b e r o f CO2 p u m p lines fall within the very dense Q - b r a n c h region o f the C - O stretch band. Then, because the intensities o f the Q-branch lines decrease r a p i d l y with increasPresent address: Laser Programme, Centre for Advanced Technology, Rajendranager, Indore 452012, India.

ing J rotational q u a n t u m number, the identification o f p u m p transitions o f high or m e d i u m J can be extremely difficult. W h e n a p u m p e d Q-branch transition is too weak to be clearly resolved and detected in the conventional linear absorption spectrum, one has to rely either on n o n l i n e a r saturation spectroscopy [6 ] o r on calculated frequencies o b t a i n e d by interpolation from observed P a n d R branch lines in the linear spectrum. In the present report, the results o f detailed I R a n d F I R spectroscopic analyses are a p p l i e d to d e t e r m i n e assignments for four different energy level systems in which the CO2 laser p u m p coincides with a Qbranch C - O stretch transition. The identification o f the F I R laser lines a n d the parent I R p u m p absorption could be tested in each case by the frequency closure constraint afforded by closed loops o f transitions. The c o m b i n a t i o n differences thus o b t a i n e d yield frequencies for the observed F I R lines with an

0030-40 ! 8/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)

42 5

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approximate accuracy of + 0.001 cm-1, which is an order of magnitude better than that generally obtained from direct wavelength measurements. Lastly, accurate frequencies have been deduced for a number of predicted FIR laser lines which should be pumped by seven different lines of the isotopic ~3C~802 and ~3COz laser bands.

2. Methanol energy levels The energy levels of methanol are conveniently represented by the set of five quantum numbers (nzK, j ) v, where K is the axial component of the rotational angular m o m e n t u m J, n is the torsional quantum number, and v represents the vibrational quantum number [ 1,2,8,9,13]. We will denote the C - O stretch and the CH3-rock states as v= CO and v= CH3-rock, respectively. The torsional symmetry label z can have values 1, 2 and 3 associated with the threefold symmetry of the torsional potential. A detailed description of the model and the selection rules can be found in Lees and Baker [ 14].

3. Experimental aspects The IR spectra for the C - O stretch band were recorded on two different occasions using the BOMEM Fourier transform spectrometer at the Herzberg Institute of Astrophysics. In the first run, the spectrum was obtained at a resolution of 0.004 cm -1. The analysis of this spectrum for the ground torsional state and most of the first excited torsional state appeared recently in the literature [3,8,9,13]. Since then, the spectrometer at the Herzberg Institute has been modified to achieve a resolution of 0.002 c m - 1, which is almost at the Doppler limit for methanol at 10 lam. In the present work, we recorded the C - O stretch spectrum again with an improved sensitivity at the enhanced resolution. In this new spectrum, many additional weak lines are resolved and measurable, a number of which h a v e been identified as arising from torsionally and vibrationally hot bands [ 15 ]. The spectrum was obtained at room temperature at a pressure of 0.1 Torr, with an effective absorption path length of 2 m in a White cell. The final interferogram was obtained by co-adding 75 scans. 426

15 January 1991

The list of resolved and measured peaks in this new spectrum in the range 930-1100 cm-~ contains about 8200 frequencies, compared to 2900 peaks from 9601080 c m - J in the previous spectrum at 0.004 c m - l resolution. The FIR torsion-rotation band was recorded at a resolution of 0.004 c m - l at room temperature and 2 Torr pressure in the range from 79 to 211 cm -~ and at 3 Tort pressure from 199 to 350 cm -1 in a short cell of 15 cm length. The detailed assignment of these spectral regions forms a collaborative project with Drs. G. Moruzzi and F. Strumia and their coworkers in the research group at the University of Pisa.

4. FIR laser line assignments

9P(36) C02 pump Recently, Tang et al. [ 10 ] reported the emission of a single FIR laser line at 155.29 cm-1 in CH3OH pumped by the 9P(36) CO2 line at an offset of + 184 MHz. We discovered from our FTIR analysis that the calculated frequency for the torsionally hot line Q(234, 21) of the C - O stretch band is 1031.4822 cm-1, which is in close coincidence with the reported pump frequency of 1031.4836 c m - 1. With this identification, the FIR emission line can then be assigned to the b-type torsional transition (234, 21)c°~(113, 21) c°. The net change in J value in going from the lower pump level to the lower lasing level is zero, implying a parallel relative polarization for the emitted line as observed [ 10]. The energy level scheme for this system is shown in fig. 1, and the assignments are listed in table 1. The assignments can be confirmed by forming closed combination loops of IR and FIR transitions with accurately measured frequencies, which are collected in table 2. For the ground state, the torsional branches corresponding to (n~K) = (234) o ~ ( 113 ) o were identified in the FIR absorption spectrum. The R, Q and P branches could be followed up to Jvalues of about 27, and are presented in table 3. From fig. 1, numerous independent transition loops can be constructed, with four of these containing the observed FIR laser line La as one of the members. From the self-consistency of the residual

Volume 80, number 5,6

OPTICS COMMUNICATIONS [Lb]

Vc°=1

.....!...;I [Ld!....~

A,

~'

21 20

*

(nrK) = (026) Q(026, 20) =R(026, 1 9 ) - v[ (026, 20),--(035, 19) + v[(026, 19),-(035, 19)]

.," • " L aj ,. . '

22 21

A A ,, ; • • . . . .

#~...,,

[Lc]

C:B:A:

#'."" ~,

2o i i i l l

23

',

= 1063.34270- 77.31495 +45.10220 = 1031.12995 cm - t , and

H ! G ! F! E{ D! :

p 9P(36) + 184 MHz

', : ', ,, : ,, ,, ,, ; ,

j

15 January 1991

',

:

,~

;

,'~

21

~.'::'i.:i-?..w ...." ...... g ,.;'..,:::'

20

',

h ,'",'"'

i: i: i: i: i: lille

22

..':;";"

Q(026, 20) = P(026, 21 ) + v[ (026, 21 ),-- (035, 20) - v[ (026, 20),-- (035, 20) ] = 997.31272 + 78.91291 - 45.09509

...... .."~."~,','.'c:::"

"

..'.',',;'d'.::;',."

,::."

e,,' ..." .." '.'..

= 1031.13054 cm -~ .

22

i*"

21

f ,.;?",'"

The average frequency is then 1031.13024 c m - 1, in agreement with the pump frequency of 1031.1293

20

,"

cm-1.

(113)

(n r K)

Vco = 0

(234) (nrK) = (144)

Fig. 1. Energy level scheme for the 9P(36) CO2 pump line in CH3OH. Transition frequencies are listed in tables 1 and 2.

Q(114, 1 4 ) = R ( 1 1 4 , 1 3 ) - v [ ( 1 1 4 , 14),--(114, 13) = 1053.65060-22.52093

defects remaining after adding up the frequencies around the closed loops, it can be determined that line La should be corrected by - 0.003 c m - i to give a revised frequency of 155.2870(10) cm-~. As an extra benefit of the combination loop technique, we can predict other as yet unobserved FIR laser frequencies to the same order of accuracy. Five such predicted lines are presented in table 1 for this pump system.

13-9R(18) pump With this pump line, four FIR emission lines were observed by Petersen and Duxbury [ 11 ]. They assigned the IR absorption as the C - O stretch transition Q ( l 14, 14), and thereby identified three of the FIR lines. From our spectroscopic study, we discovered that the Q-branch line Q(026, 20) should also be coincident with the CO2 pump line [ 16 ]. In the spectrum, the two absorption lines overlap and appear at 1031.13032 cm -1. The FTIR spectrum around the 13-9R(18) laser line is shown in fig. 2. When we use the observed P and R branch transitions with observed [ 17 ] and calculated ground-state frequencies, the following results are obtained:

= 1031.12967 cm -~ . and Q(l14,14)=e(l14,15)+v[(l14,15),--(114,14)] = 1007.00193 + 24.12863 = 1031.13056 cm -~ The average frequency is here 1031.13011 cm -~, again in very good agreement with the pump frequency. With these identifications, the FIR line at 47.13 cm-~ can be assigned as the b-type transition (026, 20)c°~ (035, 20d) c°, as shown in fig. 3. Here, the terminating laser level is the lower ("down") component of a Fermi-hybridized doublet [ 16 ]. More detailed discussion of this perturbation and further assignments of FIR laser lines from these Fermi-perturbed states will be communicated elsewhere. Our assignment scheme is supported by the consistency of the frequency residuals of the following three independent combination loops in fig. 3: c~ = P + d - I - L b =

- 0 . 1 9 7 8 8 cm -~ ,

52 = A + c - I - L b =

--0.19825 cm -~ , 427

Volume 80, number 5,6

OPTICS COMMUNICATIONS

15 January1991

Table 1. Assignments of FIR Laser Lines in CH3OH CO2 Pump + Offset [Freq in cm -1]

IR Absorption Q(nrK,J) [Freq in cm -1]

RR Laser Assignment (nTK,J) a

9P(36) + 184 MHz

Q(234,21)

[1031.4836]

[1031.4822] [calc]

(234,21) --, (113,21) --- (113,22) -, (113,20) --, (125,22) --, (125,21) (234,20)

13-9R(18) + 30 MHz [1031.13030]

9P(38) TEA [1029.4421]

9P(34) + 185 MHz

Q(026,20) [1031.13032]

PQ(016,10) x

O(118,9)

Line Label

Observed Frequency (cm-1)

La [Ld] [Lc]

155.29

[Lb]

Calculated Frequency b (cm-1)

M _L [111 [_L] [_L] [±] [±]

_L It _L

Lb Lc Ld Le

74.4 16.01460 34.22999 50.33616

74.7850(10) 16.015(1) 34.2302(10) 50.3366(10)

(025,9) x -, (034,9) co

La

74.3

74.7158(10)

(118,9) --, (039,9)

[ 1033.4942]

129.3

[81

46.9327(10) 78.8253(10) [42.6155(10)] [74.5662(10)] [13.4578(10)] [9.0710(20)] [31.9583(1 0)]

(025,10) x -~ (034,10) co -. (025,9) x --* (025,10) u -, (025,9) u

[Ld]

II [Z] [.L] [±] [11] [±]

[Lc]

47.13 78.99

[Refl

155.2870(10) [120.3076(10)] [188.6806(10)] [101.3012(10)] [136.1971(10)] [33.2764(10)]

(026,20) --, (035,20) d --, (035,19) d -, (035,20) u -- (035,19) u -. (035,21)d -. (035,21) u -, (026,19)

Lb La c

Rel Polzn

[13]

[16]

129.0620(10) g [Tentative Assignment]

[81

a Superscript x denotes CH3-rocking state; d and u are lower and upper components of mixed doublet. b Frequencies in brackets are calculated from frequency combination loop differences. c Transition reassigned from original reference [13].

&3= B + g - H - L b =

- 0 . 1 9 5 6 6 cm -~ .

The loop-corrected frequency of the FIR line is then 46.9327(10) cm -1. Our results also suggest a revised assignment for the FIR line at 78.99 cm -~ with polarization perpendicular to the pump observed by Petersen and Duxhury [ 11 ]. This line was originally identified by them as the (114, 14)-* ( 123, 13) transition. How428

ever, when the measured frequencies for the IR and FIR transitions are used to calculate the frequency of this transition, a value of 78.4630(10) cm -~ is obtained, differing from the observed FIR laser frequency by 0.53 cm-~. In contrast, the frequency in table 1 obtained from our combination loops for the transition La[(026, 2 0 ) - , ( 0 3 5 , 19d)] in fig. 3 is 78.8235(10) c m - [ This differs by only - 0 . 1 7 cm - l from the observed FIR line, a correction quite con-

Volume 80, number 5,6

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15 January 1991

Table 2. Infrared and Far-Infrared Spectroscopic Data for FIR Laser Assignments in CH3OH CO2 Pump + Offset

Line Label

IR Transitions Transition a Frequency (cm-1)

Line Label

FIR Transitions Transition

Frequency (cm-1)

9P(36) + 184 MHz

P A B C D E F G H

Q(234,21) P(234,21) R(234,20) P(234,22) P(113,21) R(113,20) P(113,22) P(113,23) R(113,21)

[1031.48221 b 998.20359 1065.17946 996.18634 994.09461 1061.23964 992.13351 990.15545 1062.46753

a b c d e f g h

(234,20) (234,21) (234,20) (234,21) (234,22) (234,21) (234,22) (234,22)

,,,,,,,,-

(113,20) (113,20) (113,21) (113,21) (113,21) (113,22) (113,22) (113,23)

151.34502 185.04471 117.59317 151.29314 186.59138 115.93761 151.23672 114.27874

13-9R(18) [+ 30 MHz] c

P A B C D E F G H I

Q(026,20) R(026,19) P(026,21) R(026,18) P(026,20) R(035,18) d P(035,20) d R(035,20)d P(035,21)d R(035,19) d

1031.13032 1063.34270 997.31272 1061.98595 d 999.17207 d 1060.23453 997.39909 1062.76773 995.46515 1061.51315

a b C d e f g

(026,18) ,(026,19) ,(026,19) *(026,20) ,(026,20) .(026,21) ,(026,21) ,-

(035,18) (035,18) (035,19) (035,19) (035,20) (035,20) (035,21)

45.10886 75.71497 45.10220 77.31495 45.09509 78.91291 45.08677

P A B C D E

PQ(016,10) x R(025,9) u P(025,10) u Q(034,9) R(034,8) R(034,9)

[1029.44211 1050.12255 1017.88967 1033.61938 1048.13607 1049.56458

a b c d e

(016,10) ~- (025, 9) (016,10) ,- (025,10) (025, 9),-(034, 8) (025, 9),-(034, 8) (025,10) ,- (034, 9)

54.91073 38.78420 54.51413 39.99610 56.12404

9P(38) TEA

a Superscript x denotes CH3-rocking state; d and u are lower and upper components of mixed doublet. b Frequency in brackets is calculated from frequency loop combination differences. c Predicted offset,

d Blended line.

sistent with the value of - 0 . 2 0 cm -~ determined above for line Lb in this system. Thus, it appears highly likely that the 78.99 c m - ~line should be reassigned as shown in table 1 and fig. 3. The revised assignment is supported by the consistency of the following three combination loops in fig. 3: ~4 = P + e -

F - L a = - 0 . 1 6 3 6 8 em -~

~5 = A + b - E - L a =

- 0 . 1 6 6 8 6 cm -1

<~6= B + f - F - L a =

- 0 . 1 6 3 4 6 cm -~

In table 1, we also include calculated frequencies for five other potential FIR laser transitions in this pump system.

9P(38) pump Two FIR lines were produced at 74.38 cm-~ and 74.34 c m - t with this pump line of the transversely excited CO2 laser [ 18 ]. Previously, Henningsen [ 1 ] identified the 9P (38) line, in cw mode, as the pump for the perpendicular Q-branch transition (025, 10)x~ (016, 10)% where x represents the CH3-rocking mode initially called the x-state. With this pump 429

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15 January 1991

13-9R(18) |

•

1031.O6

1031.1

,

+

1031.12

,

•

r

I

'

'

I

'

'

'

1031.16

1031.14

i

103118

Frequency in cm "1

Fig. 2. Spectrum of CH3OH around the 9R ( 18 ) ~3CO2laser line.

Table 3. Observed Branches of the (n'rK) = (234) ~- (113) Torsional FIR Transition in CH3OH J

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

R(J) (cm-1) 158.20444 159.80578 161.40350 162.99969 164.59318 166.18446 167.77332 169.35988 170.94294 172.52410 174.10198 175.67652 177.24758 178.81431 180.37862 181.93843 183.49362 185,04471 186.59138 188.13258 189.66882 191.19881 *

* Blended line.

430

O(J) (cmol)

P(J) (cm-1)

Vco =1 21 21d I

= F

* * * * * *

*

143.72415 * 142.10588 140.48609 * 138.86586 * 137.24168 135,61 707 133.98964 132.36147 130.73039 129.09715 127.46192 125.82372 124,18361 122.54040 120.89423 119.24561 117.59317 115.93761 114.27874 112.61588 110.95093

?ii

! i ! ! ,+ i

*

.

20 a ; ', • • '9'

151.76859 151.75903 151.74952 151,73698 151.72153 151,70494 151.68590 151.66417 151.64113 151.61455 151.58549 151.55333 151.51811 151.48001 151.43892 151.39353 151.34502 151.29314 151.23672 151.17529 151.11023 151.03951

,,' I,

I

i

i

i

D::C::B::A::

!

:

: : :

i

! ! i

i i

g .... i .... :-::" i

liHiGiFiEi

i ! i J 21 20

: " " : ' : : i . - " " . . " ' e +.. " " ; ; ; - ' " f...+" . . ; : . : .:..~-.-- ..-. c..~v i ," d..-"..-:.:-" . . - . - ... ...

19

, ,:"L-":--'a

!

18

..~"--"

+,

'

' i

2o

P 13-9R(18)

21

i .

20

19 '

18 Vco = 0

~ '+',,

(035)

(n'r K)

(026)

Fig. 3. Energy level scheme for the 9R(18) =3CO2pump line in CH3OH. Transition frequencies are listed in tables l and 2. a s s i g n m e n t , the t r i a d o f laser lines Lc, L d a n d Le shown in fig. 4 was then i d e n t i f i e d a n d s u b s e q u e n t l y precisely m e a s u r e d by h e t e r o d y n e t e c h n i q u e s [ 1,2 ]. T h e present w o r k suggests that the close p a i r o f F I R

Volume 80 number 5,6

OPTICS COMMUNICATIONS Lc

E ol c~:

i

i

9

10

...-" ........ ..""""

,-........... c (034)

,o

9P(34) pump

..:

..........d. ........ :::::::::'"

a'~"....

S

pendent combination loops for (a) and (b) above shows that the IR and FIR absorption transitions are correctly assigned, while the near-zero values for (c) and (d) confirm Henningsen's assignments [ l ]. The loop-corrected frequencies of the FIR laser lines La and Lb are 74.7158 and 74.7850 cm-1, respectively. The estimated accuracy of +0.001 cm -~ of these values is in this case three orders of magnitude better than the experimental error associated with the pulsed laser transients.

10

i

i

15 January 1991

(nrK) (025)

(016)

Fig. 4. Energy level scheme for the 9P(38) TEA pump line in CH3OH. Transition frequencies are listed in tables 1 and 2. lines generated with the TEA laser pumping can be identified as the Q-branch transitions La and Lb in fig. 4, in which the lower lasing levels belong to the (034) states of the C - O stretch. We can support this proposal by forming combination loops from the measured transition frequencies listed in table 2 to give the following frequency defects: (a) Loops with the lines La=74.3 cm -1 and L c = 16.01460 cm -1 81 = P + a + d - C - L c - L a = 0 . 4 1 4 9 5

cm -1 ,

82 = P + b + e - C - L c - L a = 0 . 4 1 6 3 6

cm -1 ,

83 = P + a + c - D - L c - L a = 0 . 4 1 6 2 9

cm -1 .

Recently, this pump line was reported to produce two FIR laser lines at an offset of + 185 MHz [ 10]. We propose a tentative assignment for the IR pump as Q ( l l 8 , 9), with the 129.3 cm -1 FIR line then being identified as the C - O stretch transition ( 118, 9)c°~(039, 9) c°. The frequency for this transition can be obtained more precisely from combination relations as 129.062(1) cm -1. This assignment scheme is proposed solely from the agreement between the predicted and observed frequency and polarization of the FIR laser line.

5. Prediction of FIR laser lines In table 4, we present frequencies obtained from I R / F I R combination differences for 40 potential but as yet unobserved FIR laser lines associated with seven CO2 pump systems. Two of these merit specific comments as follows.

I3/18-9R(8) pump (b) Loops with the line L b = 7 4 . 4 cm -1 &4= p + a + d - E - L b = 0 . 3 8 4 3 5

cm - I ,

85 = P + b + e - E - L b = 0 . 3 8 5 7 6

cm -1 .

(c) Loop with the line Ld=34.22999 cm -1 86 = P + a - A - L d = 0 . 0 0 0 2 4

cm-' .

(d) Loop with the line Le=50.33616 cm -1 87 = P + b -

B - L e = 0 . 0 0 0 4 2 cm -1

The self-consistency in the residuals of these inde-

This pump line is situated in the vicinity of two Qbranch transitions, Q (0310, 13 ) and Q (02 l, 8 ), and is predicted in this work to generate the six FIR transitions listed in table 4. Previously, it was suggested [ 12 ] that this pump line would be in coincidence with the Q(024, 8) absorption, with three different FIR lines being predicted. However, in our FTIR spectrum, the Q(024, 8) transition appears at 1032.88787 c m - ~ about 670 MHz below the 13/189R (8) CO2 line. This large offset makes the IR line inaccessible to normal cw laser pumping, and explains the failure to observe any of the three predicted lines [ 12 ]. 431

t~

4~

Q(0310,13) [1032.91311]

Q(021,8) [1032.90979]

P(134,17) b [998.62485]

P(012,20) b [998.62485]

13/18-9R(8) [- 16 MHz]

13118-9P(36)

13/18-9P(36) [* 40 MHz]

(012,19) -~ (012,18) (021,19) -~ (021,18) 30.4210(20) 2.07(1) 32.39(1)

25.5870(10) 56.2360(10) 81.6971(10) 9.66(1) 36.6217(10) 293.5630(20) 319.0711(20) 230.5006(10) 233.3795(10) 256.2053(10) 258.8675(10)

12.7698(10) 5.3928(10) 18.1382(10)

(021,8) -* (021,7) -* (030,8) (030,7) (134,16) (134,15) (113,16) -~ (113,15) (125,17) -~ (125,16) (013,16) (013,15) (025,16) u -~ (025,16)d (025,15) u -~ (025,15) d

20.7101(10) 61.7832(10) 82.5060(10)

(0310,12) -~ (0310,12) -. (0i9,13) -* (019,12)

II JJl

II ± II U ± _L ~ ± ± II I

± II ±

¢ ~1 J_

Pred. Freq. Pol. (crn-1) Pred.

13-9P(16) [+ 37 MHz]

13-9P(12) - 2 MHz [1027.15959]

13-9P(26) + 13.4MHz [1036.01748]

CO2 Pump [Offset] a

FIR Laser Line (rrrK,J)

16.0158(10) 249.8264(10) 267.3195(10) 283.2330(10) 240.3261(10) 257.8929(10) 273.8605(10)

(112,7) -, (112,6) -~ (021,8) -. (021,7) (021,6) -* (033,8) (033,7) (033,6) (111,10) (111,9) (020,11 ) (020,10) -* (020,9) -~ (032,11) (032,10) (032,9)

Q(112,7) [1027.15962]

P(111,10) [1004.27864]

b Overlapping transitions.

11.2545(1 0) 244.5334(10) 257.3020(10) 268.4756(10) 225.2809(10) 238.0447(10) 249.2116(10)

(225,8) -. (134,9) (134,8) (134,7) -* (116,9) (116,8) (116,7)

265.0755(10) 279.4590(10) 292.2414(10) 267.57(2) 281.85(2) 294.55(2)

stretching stats.

I

/

I I

/

I I

±

±

±

m

± ±

I

±

Pred. Freq. Pol. (cm-1) Pred.

Q(225,8) [1036.01693]

13CO2 Pump Lines

IR Absorption [Freq in cm-1 ]

c Superscripts d and u denote lower and upper components of Coriolis-hybridized doublet. All other levels are within the C-O

a Offset values in brackets are calculated. Others are known from laser Stark spectroscopy [9].

[+ 40 MHz]

FIR Laser Une (rrrK,J) c

13C1802 Pump Lines

IR Absorption [Freq in cm"1]

13/18-9R(8) [+80MHz]

CO2 Pump [Offset] a

Table 4. Predicted FIR Laser Lines in CH3OH Optically Pumped by 13C1802 and 13CO2 Lasers

¢o

Z

Z

3

c~

r~

3~

FF

T

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13/18-9P(36) pump This pump line is coincident with two CH3OH transitions, P(134, 17) and P(012, 20), which appear in our spectrum overlapped at 998.62485 cm-1. For the torsionally hot n = 1 absorption, numerous FIR laser transitions are possible with both n = 0 and n = 1 transitions in the C-O stretch state. Eleven such transitions are predicted in table 4 for this pump, while three FIR transitions can be predicted for the other n = 0 pump. It may be noted that this CO2 pump was observed by Petersen and Duxbury [ 12 ] to produce two FIR laser lines at 25.63 and 36.63 cm-1. These frequencies are persuasively close to the values of 25.5870 and 36.6217 cm -1 given in table 4 for the (134, 1 6 ) ~ ( 1 3 4 , 15) and (134, 16)-,(125, 16) transitions, respectively. However, the predicted polarizations are opposite to those reported.

6. C o n c l u s i o n s In conclusion, the results of detailed spectroscopic studies of high-resolution IR and FIR spectra of methanol have permitted assignments of FIR laser lines optically pumped by four different lines of the CO2 laser. All of the systems involve Q-branch IR pump transitions in the dense central region of the C - O stretch band of CHaOH. Frequencies are deduced for observed and predicted FIR laser transitions with an estimated precision of _+0.001 cm-1, at least one order of magnitude better than generally obtainable from direct wavelength measurements. For the 9R(18) pump line of the 1aco2 laser, a revised assignment is put forward for the 78.99 c m - i FIR laser line in which it is now proposed to terminate on the lower component of the {(035)c°/ (335)°} Fermi-mixed doublet. All proposed assignments are supported by combination relations among accurately measured IR and FIR absorption frequencies. The spectroscopic data have also been applied to calculate the frequencies of 40 potential FIR laser transitions in systems which are predicted to be optically pumped by seven lines of the isotopic 13CO2 and 13C1SO2lasers. For the 13/18-9P (36) pump, the frequencies of two of the predicted FIR laser lines

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are in very good agreement with observed values, but a discrepancy exists in that the expected polarizations are opposite to those reported [ 12 ]. In the course of the work, a new IR spectrum of the C - O stretch fundamental band was recorded at an improved resolution of 0.002 cm - I and with greater sensitivity. In this spectrum, there remain many unidentified weak lines, which are believed to originate in large part from torsional and vibrational hot bands. The presence of such transitions greatly furthers the chance of coincidence between CO2 pump lines and methanol absorptions.

Acknowledgements

This work was partially supported by the Natural Sciences and Engineering Research Council of Canada and the University of New Brunswick Research Fund. It also comes under the auspices of CEMAID (Centre of Excellence in Molecular and Interfacial Dynamics), a member of the recent Canadian federal NCE program of Networks of Centres of Excellence. Valuable discussions and private communication with Dr. G. Moruzzi and Dr. F. Strumia are gratefully acknowledged.

References

[ 1] J.O. Henningsen, Molecular spectroscopyby far infrared laser emission, Universityof Copenhagen (1984), ISBN8788318-06-0, and referencestherein. [ 2 ] M. Inguscio, F. Strumia and J.O. Henningsen, Rev. Infrared and Millimeter Wave, Vol. 2, eds. K.J. Button, M. lnguscio and F. Strumia (Plenum 1984) pp. 105-150.

[3] G. Moruzziand F. Strumia, Infrared Phys. 24 (1984) 257. [4] R.M. Lees, I. Mukhopadhyay,and J.W.C. Johns, Optics Comm. 55 (1985) 127. [ 5 ] J.O. Henningsen, Int. J. InfraredMillimeter Waves7 (1986) 1605. [6 ] S. Petersen and J.O. Henningsen, Infrared Phys. 26 (1986) 55. [7] R.M. Lees, Far Infrared Science and Technology, ed. J.R. Izatt, Proc. SPIE666 (1986) 158. [8] I. Mukhopadhyay,R.M. Lees and J.W.C. Johns, IEEE J. Quantum Electron. QE-23 (1987) 1378. [9] I. Mukhopadhyay, R.M. Lees and J.W.C. Johns, Int. J. Infrared Millimeter Waves 8 (1987) 1471. [ 10 ] F. Tang, A. Olaffson and J.O. Henningsen, Appl. Phys. B 47 (1988) 47.

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[ 11 ] J.C. Petersen and G. Duxbury, Appl. Phys. B 27 (1982) 19. [ 12] J.C. Petersen and G. Duxbury, Appl. Phys. B 34 (1984) 17. [13] G. Moruzzi, F. Strumia, P. Carnesecchi, R.M. Lees, I. Mukhopadhyay and J.W.C. Johns, Infrared Phys. 29 ( 1989 ) 583. [ 14] R.M: Lees and J.G. Baker, J. Chem. Phys. 48 (1968) 5299. [ 15 ] I. Mukhopadhyay, R.M. Lees and K.V.L.N. Sastry, Infrared Phys. 30 (1990) 291.

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[ 16] R.M. Lees, I. Mukhopadhyay and J.W.C. Johns, 12th Int. Conf. Infrared and Millimeter Waves, Lake Buena Vista, 1987, Conf. Digest, pp. 312,313. [ 17 ] G. Moruzzi, private communication. [ 18 ] P. Bernard and J.R. Izatt, Int. J. Infrared Millimeter Waves 4 (1983) 21.