International Journal o f lnfrared and Millimeter Waves, Vol. 12, No. 6, 1991

MEASUREMENTS AND ASSIGNMENTS OF NEW LARGE OFFSET CD3OH FIR LASER LINES

G. Carelli, N. Ioli, A. Moretti, D. Pereira,' and F. Strumia

Dipartimento di F/s/ca and CNR Piazza Torricelli 2, 1-56126 Pisa, Italy

Received March 25, 1991

ABSTRACT

By using an acoustooptic modulator we extend the 300 MHz tunability of a

waveguide CO 2 laser to 480 MHz. The CD3OH was optically pumped by

the 10R(32), 10R(34), and 10R(36) CO 2 laser lines, and 17 new FIR laser

lines were discovered. The Stark effect on previously known FIR laser

lines was investigated, and some tentative FIR laser lines assignments are

suggested.

Key words: Waveguide CO 2 lasers, new FIR laser lines, CD3OH

1_ Permanent address: Inst. de F/s/ca, UNICAMP, 13081 Campinas, Brasil

557

0195-9271/91/0600-0557506.50/0 �9 1991 Plenum Publishing Corporation

558 Care|ii et al.

INTRODUCTION

The introduction of the optically pumped molecular laser by Chang and Bridge in 1970 [1] has provided a new versatile and powerful tool to molecular spectroscopy in the far infrared (FIR). In fact, the optically pumped laser is not only an interesting source of coherent radiation but

also a powerful means for obtaining physical information on the lasing molecule itself. Unfortunately the method can be applied only to molecules with a permanent dipole moment, and strongly relies on the accidental coincidences with available pump lines. The possibility to investigate transitions within excited states is the most relevant advantage of the method. In addition several laser emissions may correspond to a single absorption frequency, thus making the assignments easier. Methanol, with its isotopic species, provides more than 40% of the known FIR laser lines. The CH3OH alone giving more than 500 different frequencies, 190 of which correspond to assigned transitions [2, 3,4]. This line profusion is due to the wide overlapping of the C-O stretch absorption band of those molecules with the CO 2 laser emissions. The hindered intemal rotation of the hydroxyl group with respect to the methyl group

provides a large number of energy levels. The presence of a dipole moment not parallel to the quasi-symmetry axis of the molecule increases the

number of the allowed transitions starting from the same level. The same complexity that makes the molecule such a prolific FIR source makes it a non-trivial subject of a spectroscopic analysis. The richness and efficiency of the CH3OH laser emissions in the region between 1 and 6 THz is a strong stimulus to systematic searches for new laser lines emitted by this molecule and its isotopic species.

In this work we report 17 new FIR laser lines from CD3OH pumped by CO2 laser lines in coincidence with absorbing transitions of the Q-branch of the C-O stretching mode. For all lines we have measured the wavelength, the relative polarization, the intensity, and the pump offset,

by using the transferred Lamb-dip technique (qT_~D) [5]. We have also investigated the effect of an electric field on the power output of several laser lines. Finally we present a few tentative laser line assignments.

Assignments of Laser Lines 559

EXPERIMENTAL APPARATUS

The experimental apparatus is described in previous works [5, 6, 7, 8],

and consists of a waveguide CO 2 laser [6], with a tunability of +150 MHz

around each CO 2 line. The output frequency is adjusted by varying the

cavity length. The laser operates either CW or in long pulse regime. The

frequency tunability is further extended by passing the laser beam through

an acoustooptic modulator [4] driven at 90.0 MHz. Because of the

amplitude modulation, the output of the CO 2 laser can be either up-shifted

or down-shifted by 90.0 MHz with an efficiency larger than 50%. This

tunability extension allows the excitation of several new absorption lines,

with an offset larger than 150 MHz.

Laser emission by CD3OH is obtained from two FIR laser cavities. The

first is a 100 cm long open resonator, the second is a 150 cm long hybrid

metal-dielectric WG cavity, which allows to observe the Stark effect on

laser emission. In both lasers the emitted wavelength can be measured by

using the cavity itself as a scanning Fabry-Perot interferometer. The

wavelength calibration is made by comparison to FIR laser lines of known

frequency [9]. The CD3OH sample used was 99% enriched in deuterium.

EXPERIMENTAL RESULTS

Dyubko et al. first reported FIR laser action in CD3OH in 1975 [I0]. Since

then many new laser lines have been discovered. Recently, we have

observed 108 new lines excited by a WG CO 2 laser [7], thus bringing the

total number of known CD3OH FIR laser lines to about 340. This molecule

is now, with CH3OH, the most prolific laser source. An important

characteristic of the lines reported in [7] was that 24 new laser lines (about

25%) are obtained by using only three CO 2 laser lines: the 10R(32),

10R(34) and 10R(36). This result follows from the coincidence of these

CO 2 lines with the very dense Q-branch of the C-O stretch mode of

CD3OH, that covers the energy interval between 982.5 cm" 1 and 986.0 cm -1. Another common feature for these pump lines was a broad and

strong optoacoustic absorption spectrum over the entire 300 MHz tuning

range of the CO 2 laser, thus showing the possibility to reach new pump

560 Carei|i et al.

lines of even larger offset. Previous data of diode laser absorption spectroscopy in this region [11] confirm this possibility. As an example figure 1 shows part of the rich absorption spectra of the CD3OH C-O stretching obtained with a high resolution FTS around the 10R(32) CO 2 and the 10R(34) CO2transition lines respectively. We can see that in coincidence with the 10R(34) we have a broad absorption that begins at -250 MHz and finishes at +200

MHz of the central CO 2 laser frequencies.

10R(32) [

i i i T i

..... ' [ ' '98;.25 98;.30 983.35 983.20

10Rq I 34)

984~.30 ' '98~.35' 98~.40 '98~.45 . . . . . 98~.50

Fig.l- Fourier transform absorption spectrum around the IOR(34) CO a laser line. The spacing between the etalon modes is 0.01631 cm -t.

This expectation was confirmed when we increased the frequency tuning from 300 MHz to 480 MHz by using the acoustooptic modulator. We observed 17 new FIR laser lines, ranging from 21.7 to 635.7 p.m, in correspondence of the 10R(32), 10R(34) and 10R(36) CO 2 pump lines, as

shown in Table I.

Assignments of Laser Lines 561

Table I - CD3OH FIR laser fines: Stark measurements and new lines

Pump FIR Rel. Offs. Rel. Press. Stark Splitt. Lme ~m Pol. MHz Pow. Pa Pow. MHz.

cm&V

Comm.

10R36 575.7+0.6 l -165 W 7 579.3+0.6 i -165 W 7 635.7+0.6 // -154 W 7 253.8+0.2 l 18 S 7 PE 30

69.2_+0.1 42 W 13 PE 54 95.9+0.1 l 152 S 11

112.9+0.1 l 229 W 13 632.3+0.6 l 229 W 7

10R34 112.0+0.1 / -160 S 13 117.7+0.1 1 -160 S 8 138.4+0.1 -67 W 13 PE 29

42.5_+0.1 // -23 M 13 PE - 181.2_+0.1 / -23 M 13 PE 57 129.0!0.1 64 W 13 PE 32

66.4_+0.1 112 W 11 PE 50 76.9_+0.1 l 112 W 20 PE 46 21.7+0.7 / 199 W 13

10R32 47.5+0.1 // -200 W 7 52.0+0.1 l -200 W 7

312.0+0.3 // -200 W 9 317.8+0.3 // -200 W 7 579.5+0.6 l -200 W 9 166.8+0.1 l -108 M 11 174.0!0.2 i -108 M 11 417.0!0.3 // -108 M 9

83.9_+0.1 // -43 S 21 PD 49.1_+0.1 80 W 13 PE 45 44.6_+0.1 88 W 11 PD

139.4+0.1 l 206 S 11 244.5+0.2 // 206 M 11 387.6+0.4 / 206 M 8

10R24 50.3_+0.1 81 M 20 PD 61.7_+0.1 # 81 S 37 PD

10R22 76.0!0.1 -145 W 24 PD 10R20 1290 //

NEW NEW NEW

NEW NEW NEW NEW NEW

NEW NEW NEW NEW NEW NEW

NEW NEW NEW

S62

49.8__-t-0.1 68 W 12 PE 52.0-20.1 68 W 12 55.3_+0.1 68 W 12 PD

10R18 43.697 // 0 VS 10 PD 10R16 81.6_+0.1 // 27 M 21 PD 10R14 68.7_+0.1 -51 M 15 PD

83.7_-/-0.1 45 W 7 PD 10R12 123.6+0.1 // 130 VS 11 PD 10R08 117.6_+0.1 // -95 S 16 PD

141.0-L-_0.1 _1_ -95 S 11 PD 71.2_+0.1 // -13 S 24 PD 45.0-20.1 // 88 M 16 PD 71.3_+0.1 145 W 13 PD

10P08 45.7__+0.1 // 116 S 20 PD 10P12 172.6+0.1 // -109 S 16 PD

147.6+0.1 // 133 S 13 PD

Carelli et aL

24

The following comments may be done:

10R(32): The eleven new FIR laser lines, reported in [7] and assigned to

seven different pump offsets from the TLD signals (fig.2a), increased the

number of known FIR laser lines to 15. We have observed a new

optoacoustic absorption at - 200 MHz by using the down-shifted pump

frequency. Associated to this absorption, five new lines were discovered

(fig.2b).

Another optoacoustic absorption was observed at + 206 MHz and three

new FIR lines discovered (fig.2c), with the up-shifted pump frequency.

The total number of FIR laser lines for this pump is now of 23.

10R(34): This is the most interesting pump line for CD3OH, since it is

known to excite 27 FIR laser lines up to now, including 9 lines given in

[7]. From the TLD signal we were can assign this 9 lines plus 4 old lines to

seven different offsets as shown in fig.3a.

In the down-shift pump frequency we have observed a new optoacoustic

absorption at - 160 MHz (fig.3b) with two new FIR laser lines. A new

optoacoustic absorption was also observed at + 199 MHz (fig.3c), with

Assignments of Laser Lines

one new FIR laser line, by using the up-shift pump frequency.

We have now 30 FIR laser lines associated to the 10R(34) CO 2 pump.

563

i) ii) iii)

Fig.2i-OA spectrum and laser lines in correspondence of CO 2 10R(32)

line, frequency tuning 300 MHz: a) CO 2 output power b) OA

absorption spectrum c) FIR laser emission at 174.0 gm, d) at 83.9

gm, e) at 93.88 gm, f) at 131.6 gm, h) at 44.55 gin, i) at 421 p~m,

from [7]. 2ii) as 2a but with CO 2 frequency up-shifted by 90 MHz:

c) FIR laser emission at 387.6 gm. 2iii) as 2i but with CO 2

frequency down-shifted by 90 MHz: c) FIR laser emission at 597.5

~m, d) at 174.0 gm

10R(36): Eleven FIR laser lines associated to four different pump offsets

were already known [7] (fig.4a). Two new optoacoustic absorptions were

observed with the down-shift pump frequency, and three new FIR lines

discovered, one centered at - 154 MHz and two at - 165 MHz (fig.4b).

Also with the up-shift pump frequency we observed two new optoacoustic

absorptions and three new FIR lines, one at + 152 MHz and two at + 229

MHz (fig.4c). Thus the number of known FIR lines for this CO 2 pump

line increases to 17.

5 6 4 Carel | i et a i .

I

+90 MHz

b

-90 MHz

Fig.3i-OA spectrum and laser lines in co/respondence of CO a IOR(34) line, tuning 300 MHz: a) CO 2 output power b) OA absorption spectrum c) FIR laser power at 119.9 p-m, d) at 138.4 p.m, e) at 181.2 p-m, f) at 297.6 p-m, g) at 128.96 p-m, h) at 41.46 ~tm, i) at 435.3 gm [7]. 3ii) as 3i but with the pump up-shifted by 90 MHz: c) FIR laser power at 21.7 p.m. 3iii)-as 3a but with the pump down- shifted by 90 Mttz: c) FIR laser power at 112.0 gin.

+90 MHz

0 R 36 d ~

f J ~

MHz

Fig.4i-OA spectrum and laser lines in correspondence of CO 2 10R(36) line, frequency tuning 300 MHz: a) CO 2 output power b) OA absorption spectrum c) FIR laser power at 254,26 gin, d) at 235.8 I.tm, e) at 69.18 p-m, f) at 453.1 p.m, from [7]. 4ii)-as 4a with CO 2 frequency up-shifted by 90 MHz: c) FIR laser power at 632.3 p.m, d) at 95.87 gm. 4iii)-as 4a with CO 2 frequency down-shifted by 90 MHz: c) FIR laser power at 635.7 p-m, d) at 579.3 p-re.

Assignments of Laser Lines 565

The effect of an electric field was investigated on two lines of the 10R(36),

six of the 10R(34), three of the 10R(32) pump lines, and on few other

known FIR laser lines. The polarization of the CO 2 radiation in the FIR

laser cavity was oriented orthogonal to the direction of the static field, the

polarization of the FIR radiation was also forced to be orthogonal because

of the cavity losses. Thus the selection rule was AM----&_I for both pump

absorption and FIR emission. A power enhancement (PE) was observed

for 9 FIR lines as a consequence of a favorable condition for the non linear

Hanle effect [12]. In particular for all the 6 lines pumped by the 10R(34)

we have observed a PE as shown in fig.5.

5-

4 - d

100 200 300 400

E L E C T R I C F I E L D V/cm

Fig.5-Power enhancement effect observed for six lines pumped by

10R(34) CO 2 laser line: a) 42.5 gm, b) 66.4 gm, c) 76.9 gm, d)

128.96 gm, e) 138.4 p.m, f) 181.2 I.tm. The relative FIR powers are

normalized to the zero Stark field intensity

Another interesting feature of the methyl alcohol spectrum is the property

of many laser lines to split into only two Stark components, symmetric around the unperturbed frequency, because of the AJr and AKe0

selection rules. Usually the splitting is linear with the applied field

566 Carelli et aL

intensity, thus giving a simple and easy method of frequency tuning and modulation of the laser frequency [13]. This effect was found for all the laser lines that showed also a PE. Fig. 6 shows the Stark effect splitting for the lines pumped by 10R(34).

All these results are given in table I, arranged by the CO 2 pump lines. For each FIR laser line the wavelength, the polarization relative to that of the pump line (_k for perpendicular,//for parallel), the offset relative to the line

center, the intensity, the optimum pressure of the lasing gas, the Stark behavior, the linear splitting and a comment when the line is a new one are

given.

N

r~

.=.

20

15

10

0 0

f , , '~ /~n ~ )1 138.4 lsm

o

~ ' / ~ , ~ " . m 66.4 urn M [] 76.9 gm

100 200 300 400 500

Electric Field (V/cm)

Fig.6-Stark splitting, vs. electric field strength, measured for five FIR lines

pumped by the 10R(34) CO 2 laser line.

ASSIGNMENTS OF FIR LASER LINES

Assignments of Laser Lines 567

Henningsen [14] and Danielewicz et at. [15] reported the first assignments

of FIR laser lines from optically pumped methyl alcohol molecule in 1977.

The search for new assignments is a very interesting and open branch of

molecular spectroscopy.

Today we have more than 190 FIR laser lines assigned for CH3OH [3].

This partial success is a consequence of a somewhat precise theoretical

model for the calculation of the vibration-overall rotation-internal rotation

energy level structure of the ground vibrational and C-O stretching modes

[16], and the assignment of the high resolution spectra in IR [17] and FIR

[18] regions.

Less extensive experiments for the determination of the molecular

parameters were done [16,19] for the CD3OH, and only a few assignments

associated to the rotation-internal rotation were found [20,21,22,23],

despite the great number of FIR laser lines. A second reason is the smaller

number of coincidences between the CO 2 laser and the CD3OH P and R

branch lines of the n=0 series, as pointed out in [21].

We present here a few tentative assignments for the FIR laser transitions

presented both in ref.7 and in this work.

The CD3OH level energy was calculated by diagonalizing a Hamiltonian

essentially based on the Kwan and Dennison [16] model. The levels are

labelled following the usual notation A(n,K,J), EI(n,K,J ) or E2(n,K,J),

where J is the total angular momentum and K its projection on the quasi

symmetry methyl axis respectively, while n=0,1, 2 . . . . and A, E 1 or E 2

define the internal rotation states classified according to the irreducible

representation of the point contact group C 3. The A states are split into

asymmetry doublets for low K (K_<5) values, the two components being

labeled by a + superscript whereas the degeneracy between EI(n,K,J ) and

E2(n,-K,J) is not removed. Within each internal rotation symmetry type the selection rules for the transitions are AJ=0,+I, AK=0,__+I and An arbitrary,

whereas transitions arriong states of different symmetry are strictly

forbidden. The transitions between A states follow the additional selection rule:

+ ( - ) ---> + ( - ) for AI+An odd and - ( + ) ---> + ( - ) for AJ+An even.

568 Carelli et al.

o

i

5

0

I

o E

<

<

N

$~??$????$? ~?$

0

~ 0 ~ t '~ t ~ 0 ~ ' , t-.- ~1 ',~ ~ ' , ~ 0 o~ ~ ~ t '~ , . . . . . 0 �9 �9 �9 �9 �9 0 �9 �9 0 �9

t ~ ~ ~ t".- ~I~ ~ t"-I t ~ ~ t ~ ~r ~ e,,I 0 C',I 0 ~'~

I I I I I

o ~ ~o ~o ~o o~ ~o

Assignments of Laser Lines 569

The molecular parameters used in the calculation were the ones given by

Weber and Maker [19]. Following the assignment procedure proposed by

Henningsen [24], we can assign tentatively some FIR laser transitions as

shown in Table II. The assignments of the pump lines 10P(12), 10R(36),

and 10R(24) were confirmed recently by Mukhopadhyay et al. [23], in a work reporting data of high resolution Fourier Transform Spectroscopy.

The following comments may be done for the 10R(34), and the 10R(20)

pump lines

10R(34): Only one laser line was discovered for this pump at an offset of-

67 MHz. Here we propose a tentative assignment as (1,7,14) ---> (2,6,13).

The observed Stark effect supports this assignment. In fact the selection

rules are AJ = AK = - 1 and K --- J/2. A P E effect is expected and the line is

predicted to split into two components as found experimentally. We

calculate a Stark splitting of about 31.5 MHz cm/kV, by assuming 0.9

Debye for the dipole moment of CD3OH ( against the 0.889 D for CH3OH )

in both the upper and the lower laser level. This is in good agreement with the experimentally observed value of 29 MHz cm/kV.

10R(20): At +68 MHz we have observed [7] a new line of 49.8 ~tm

together to three old lines, the 52.0 ~tm, the 55.4 I.tm and the 1290.0 ~tm.

We present tentative assignments for these lines, with the common upper

level, labeled (1,3,6) and of E a symmetry. The lower level of the 49.8 I.tm

line is assigned as (0,2,5), while those of the 55.4 I.tm and 1290.0 ~tm

lines are (0,2,7) and (1,3,5), respectively. The proposed assignment for

the 55.4 ~tm line agrees with the observed PD Stark behavior.

CONCLUSION

In this work we confirm the potentiality of the tunability extension of a

waveguide CO a laser using an efficient acoustooptic modulator for the excitation of new FIR laser lines. In fact, we could observe 17 new FIR

laser lines with pump offsets outside the + 150 MHz tunability of our CO 2

laser, all of them excited by only 3 CO 2 laser lines. The effect of an electric

field on the power output of several laser lines was also investigated.

570 Carelli eta[.

Finally, we propose some tentative assignments for FIR laser lines of CD3OH.

REFERENCES

1- T.Y. Chang and T.J. Bridge: Opt. Comm. 1_., 423-6 (1970) 2- K.J. Button, M. Inguscio and F. Strumia eds.: "Optically Pumped FIR

Lasers" -Plenum Press, N.Y. (1984) and reference therein 3- G. Moruzzi, F. Strumia, R.M. Lees and I. Mukhopadhyay: Infrared

Phys.,in press

4- N. Ioli, A. Moretti, D. Pereira and F. Strumia: Conf. Digest "11 th Int. Conf. Infr. mm Waves" Pisa (1986), pp. 466-8, and in press

5- M. Inguscio, A. Moretti and F. Strumia: Opt. Comm. 30, 355-60 (1979)

6- N. Ioli, G. Moruzzi and F. Stumia: Lett. al Nuovo Cimento 2_~8, 257-64 (1980) N. Ioli, V. Pachenco, M. Pellegrino and F. Strumia: Appl. Phys. 23-30 (1985) F. Strumia and N. Ioli: "High power tunable wg CO 2 laser" in "Physic of New Laser Sources" N.B. Abraham, F.T. Arecchi, A. Mooradian, A. Sona eds., Plenum Press N.Y. (1985), pp.189-199

7- G. Carelli, N. Ioli, A. Moretti, D. Pereira and F. Strumia: Appl. Phys. !344, 111-117 (1987)

8- N. Ioli, A. Moretti, and F. Strumia: Appl. Phys. B48, 305-309 (1989) 9- M. Inguscio, G. Moruzzi, K.M. Evenson and D.A. Jennings: J. Appl.

Phys. 60, R161-R192 (1980) 10-S.F. Dyubko, V.A. Svich and L.D. Fesenko: Izv. Vyssh. Uchebn.

Zavied., Radiofiz. 18,1434 (1975) l l -D. Pereira and A. Scalabrin: Private communication. The FTS is

reproduced by courtesy of Dr. G. Moruzzi and Dr.B.P. Winnewisser 12-M. Inguscio, A. Moretti and F. Strumia: IEEE J. Quantum Electron.

Q.E.16, 955-64 (1980) M. Inguscio, A. Moretti and F. Strumia: in "Laser Spectroscopy V" A.R. Mc Keller, T. Oka and B.P. Stoicheff eds. -Springer (1981), pp. 255-9 F. Strumia: J.Phys. (Paris) 44C7, 117-26 (1983)

Assignments of Laser Lines 571

G. Moruzzi, F. Strumia and N. Beverini: "The Nonlinear Hanle effect and its application to laser physics" in "Hanle effect and level crossing spectroscopy" G. Moruzzi and F. Strumia eds. -Plenum Press N.Y. (1991), pp.123-235

13-F. Strumia and M. Inguscio :"Stark spettroscopy and frequency tuning in optically pumped far-infrared molecular lasers" in "Infrared and Submillimeters Wave" Vol. 5 K.J. Button ed., Academic Press N.Y. (1982), pp.129-213

14-J.O. Henningsen: IEEE J. Quantum Electron OE13, 435-41 (1977) 15-E.J. Danielewicz and P.D. Coleman: IEEE J. Quantum Electron QE13,

488-490 (1977) 16- R.M. Lees and J.G. Baker: J.Chem. Phys. 4__8_8, 5299-5318 (1968)

Y.Y. Kwann and D.M. Dennison: J. Mol. Spectrosc. 4__33, 291-319 (1972)

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18-G. Moruzzi, F. Strumia, P. Carnesecchi, B. Carli and M. Carlotti: Infrared Phys. 2947-86 (1989) G.Moruzzi, P. Riminucci,F. Strumia, B. Carli, M. Carlotti, R.M. Lees, I. Mukhopadhyay, J.W.C. Johns, B.P. Winnewisser and M. Winnewisser: J. Mol. Spectrosc. 14.__A4, 139-200(1990)

19-W.H. Weber and P.D. Maker: J. Mol. Spectrosc. 93, 131-53 (1982)

20- T. Kachi and S. Kon: Int. J. Infrared and Millimeter Waves 4, 767-777 (1983)

21-D. Pereira and A. Scalabrin: Conf. Digest "11 th Int. Conf. Infr. mm

Waves", G.Moruzzi Ed., E.T.S.Pisa 1986, pp. 108-110. 22-N.I oli, A. Moretti, G. Moruzzi, D. Pereira, F. Strumia: and G.

Carelli: Proc. 4 th Intern. Conf. on Infrared Phys., R.Kesselring and F.K.Kneubuhl Eds., Zurich 1988, pp.376-378.

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