Physics and spectroscopic applications of carbon monoxide lasers, a review

InI?ared Phys. Technol. Vol. 36, No. I, pp. 465 473, 1995

Pergamon

Copyright ~' 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1350-4495/95 $9.50 -r 0.00

1350-4495(94)tl0085-9

PHYSICS

AND

CARBON

SPECTROSCOPIC MONOXIDE

APPLICATIONS

LASERS,

OF

A REVIEW

WOLFGANG URBAN Institut f/Jr Angewandte Physik der Universit/it Bonn, Wegelerstr. 8, D-53115 Bonn, Germany

(Received 30 May 1994) Abstract--The gain media in the CO 2 laser and the CO laser are compared and the emission structure is discussed on the basis of Patel's small-signal gain formula. Some features of a liquid-N-cooled CO laser plasma are discussed with respect to optimization for different wavelength regions. The corresponding resonator conditions are explained. The considerable number of lines available and their distribution over a wide wavelength region make the CO laser an interesting tool for molecular spectroscopy. Examples in the field of laser magnetic resonance (LMR), the possibilities of generating a secondary frequency standard at 5 # m and the potential of the CO laser in photoacoustic spectroscopy (PAS) are discussed.

1. I N T R O D U C T I O N : B U I L D U P OF GAIN IN CO The best known, and most widely used molecular gas laser is the C 0 2 laser, operating at 9.6 and 10.6/am on both P- and R-branch transitions between different vibrational modes of the linear triatomic molecule O - - C - - O . The discrete structure of the three vibrational modes v~, v2, v3 of the CO2-molecule and the accidental coincidence of the vibrational frequency in the N2-molecule with the highest eigenfrequency v3 of CO2, makes it easy to produce inversion in the CO~ molecular system. The energy of vibrationally excited N~ cannot be de-excited by radiation, however, it matches perfectly with the v3-mode in CO2 that is fairly isolated from the other vibrational modes of CO2. Via collisional transfer the v3-mode of CO2 is strongly populated by excited N2 resulting in a strong degree of inversion of this vibrational mode v3 compared to lower lying vibrational states of other modes, e.g. 2v 2 and v~, into which vib-rot transitions are allowed in that particular case. The first CO2-1aser was operated by Patel in 1964. "1 A typical spectrum of a CO2-1aser is shown in Fig. 1. The CO laser is also a molecular gas laser developed first in the early days of the laser, however, it never reached the popularity of the CO2 laser. Names associated with its development are again Patel ~2~and also Legay and coworkers. °~ As a diatomic molecule with a very deep and only slightly anharmonic potential (Fig. 2) the vibrational spectrum of CO has an almost constant density of states of only one type. The pumping mechanism is quite different in CO as compared to CO2. It has first been described theoretically by Treanor et al) 41 and is therefore often referred to as "Treanor pumping" or "anharmonic VV-pumping". The vibrational degree of freedom is heated up by this pumping but the rotational distribution is kept at low temperature. An experimentally determined distribution is given in Fig. 3.~5"6~The essential result is, that N , + t I N , < 1, however it comes close to i between 10 ~< v <~ 30. The distribution shows a strong disequilibrium between the various degrees of freedom of the CO molecule, due to very different exchange rates between translational (T), rotational (R) and vibrational (V) excitations. The rotational distribution equilibrates almost at each gas kinetic collision (RT), vibrational excitations exchange within ,--50 collisions (VV) but the equilibration of vibrational energy into translation (VT) needs more than 105 collisions. Energy is fed into the tnv

~

I

e~

465

466

WOLFGANGURBAN

"Z~I,--2

r./)

"

c ~Z

I----I ~

~

l-r-

9.0

9,2

9.4

9.6

9.El

11~. I~

[.-,Jevel engLh

10.2 [um]

10.4

11~.6

10.8

11.e

Fig, 1. The emission spectrum of a room-temperature flowing gas CO 2 laser. Both P- and R-branches are showing the two bands. The emission is starting from (0@' 1), the level is in resonance with the N, r = 1 --, 0 transition. 100

t

80

1016 60

1015

"7 40

20

1014 ,

Nv

5

1013

10

20

1012

i 0 II

1.0

1.5

2.0

2.5

Internuclear distance (,~) Fig. 2. The very deep and slightly anharmonic potential of the .,Y~£+ ground state of CO. Laser emission can be observed starting from t, = I ---, 0 up the vibrational ladder to t~ = 37--* 36.

0

1

I

I

I

I0

20

30

40

Vibrational quantum number, v Fig. 3. Population distribution for CO in a liquid-N-cooled plasma. (6~ The points correspond to experimental data, the line is according to Treanor-type modelling.

Physics and spectroscopic applications of carbon monoxide lasers

467

vibrational degree of freedom by electron impact in a low-pressure discharge; this heats the vibrational excitation, going up the ladder beyond v = 40. Each vibrational state, however, is thermalized rotationally to the bath temperature of the discharge. In our case it can be kept as low as 120 K by immersing the plasma tube into liquid N. As for the pumping process, the CO2 laser is an N2-CO2 laser, whereas the CO laser is pumped by CO collisions and thus is a CO C O laser. For lasing on lower vibrational bands (v < 10) the role of N~ is to increase the energy loading. Helium is always needed to sustain the discharge and acts as a thermal conductor. In a global and simplified description of a gain medium we need inversion for the laser transition. However, for a molecule we have to take into account that there is the rotational splitting and there are degeneracies as well as selection rules leading to P- and R-branch transitions. According to a gain formula valuable both for CO2 and CO lasers given already by Patel ~7~we can calculate for Nupper/N1 . . . . = 3 that there is gain for both P- and R-branch transitions between the corresponding states. The results of such calculations are plotted in Fig. 4. Even if it is not to scale, it more or less corresponds to the observed distribution of the CO,-laser lines in one band (see Fig. 1). If we would change the distribution to Nopp,/N~ .... = 1, in the simplified global description we would expect a "'transparent" medium. Patel's formula, however, gives the results of Fig. 5. absorption for the R-branch and gain for the P-branch. Thus we can progress even further to a lower ratio Nupp~/N~.... < 1. Figures 6 and 7 represent the results for 0.9 and 0.8, and we see that crossover from gain to absorption shifts to higher J-values within the P-branch. The evaluation of experimental results show data of T~o, and N,+ ~/N, in the Treanor plateau region that coincide with the data in Fig. 6 for A t ' = I and for A t = 2 overtone transitions in Fig. 7 since (N,. + 2/N,.) ~ (N,.+ i/N,) 2.

Patel' s Formula: T= 120K, N ' / N ' '=3.0

0.~

.,=

0.~

0.,~

0.~

0.£

R

20 18 16 14 12 10 8

6

4

2

0

1

3 j,,

5

7

9 11 13 15 17 19 21 23 25

p

Fig. 4. Gain distribution, calculated with Patel's formula, °~ valid for both CO_, and CO laser transitions. For this plot, there is "total inversion" according to N, + t/N, = 3. The molecular parameters are chosen according to the CO molecule and Tro, = 120 K, nevertheless this inversion corresponds to the situation in the CO 2 laser ( N ' = N~pper, N" = Nl,,~er, T = Try,, ).

468

WOLVGANGUkBAN

Patel's Formula: T=120K, N'/N"=I.0

0.2

0.O

eto

-0.2

-0.4

R

20 18 16 14 12 10 8 6 4 2 0

1 3 5 7 9 11 13 15 17 19 21 23 25

p

J"

Fig. 5. Evaluating Patel's formula (7) for equal population of two adjacent vibrational states (Nup_ per/Nt .... = 1) and Trot = 120 K. All transitions in the R-branch are absorbing, all those in the P-branch produce gain.

Patel's Formula: T=120K, N'/N"=0.9 0.2-

-0.0

m

e~o -0.2

-0.4

-0.6

R

20 18 16 14 12 10 8 6

4

2

0

1

3 5 j,)

7

9 11 13 15 17 19 21 23 25

Fig. 6. In spite of "'positive vibrational temperature" (Nupper/N I. . . . = 0.9) there is gain on most of the P-branch transitions. This situation is referred to as "partial inversion". It corresponds to the actual data in a liquid-N2-cooled CO laser on At, = I.

469

Physics and spectroscopic applications of carbon monoxide lasers

o2]

Patel's Formula: T=120K, N'/N"=0.8

0.0

"~ -0.2

-0.4

R

20 18 16 14 12 10 8

6

4

2

0

1

3

j,,

5

7 9 11 13 15 17 19 21 23 25

P

Fig. 7. If the population ratio is even less favorable than in Fig. 6 (Nupm~/N L..... = 0.8), we still get gain for the rotational quantum number J > 5. In reality this corresponds to the population difference we get ~N for an overtone transition At' = 2, since N, , 1,;N, ~ (N,~ ~/N, )z

Nupperlow= er

II.

THE CO LASER

PLASMA

T h e e x p e r i m e n t a l p l a s m a c o n d i t i o n s h a v e b e e n d e s c r i b e d earlier in detail mg~ a n d are a p p r o x i m a t e l y the f o l l o w i n g for a l i q u i d - N - c o o l e d t w o - b r a n c h d i s c h a r g e o f 1 m c o l d length: d i s c h a r g e c u r r e n t per b r a n c h is --~4-10 m A , t o t a l p r e s s u r e ~ 10 m b a r w i t h gas c o m p o n e n t s in the f o l l o w i n g ratios: H e : N 2 : C O : a i r -,~ 10: 2 : ( 0 . 1 - 2 ) : 0 . 2 . L o w c u r r e n t a n d low C O - c o n t e n t f a v o u r s low t e m p e r a ture a n d l o w v i b r a t i o n a l t r a n s i t i o n s , w h e r e a s high v i b r a t i o n a l b a n d s n e e d h i g h C O - c o n t e n t a n d also

_ A A A A /X J

Gratingdrive

~

~

High voltage

I

power supply

IA A A A A A

I

~

ction grating

Curvde end mirror

F

AirCON~ He Fig, 8. Block-diagram of a liquid-N-cooled CO laser. The conditions of the plasma are indicated in the text. In this case, the output coupling is achieved via the zeroth order refraction of the reflection grating.

WOLFGANGURBAN

470

r - " l i.n -

~Xc~ E t...z ®

4.)f~ CO

I~evel engt, h

Eum~

Fig. 9. Carbon monoxide laser spectrum taken under conditions optimized for the medium-to-lower vibrational transition At: = 1. Adjacent vibrational bands completely overlap. The J-manifold is largest in the centre region and comprises J = 5 to J = 16 for the r, = 8--* 7 up to v = 13---, 12 bands. There are various lines missing, mostly due to water absorption in the resonator, but in some cases due to accidental coincidences with an R-branch transition of a neighbouring band. By changing the discharge conditions one can shift the spectrum to lower or to higher vibrational bands.

F-7

E~ L.d

c0~"

c

~;~

~ 2 . 7

,

, 2.

,

,

,

,

,

,

,,

, 3,1

,

,

,

,

,,

,

,i 3 . 3

,

,

,

,

,

,

,

,

,i 3 . 5

[-,Jove I en g Lh

,

,

,

,

,

,

,

,

, i 3 . 7

. . . . . . . . .

i 3 . 9

,,

,

,

,

,

,

,~..i

Eum]

Fig. 10. The CO-overtone spectrum A~, = 1 taken for one plasma condition. The spectrum starts with the v = 13---* 11 band and goes up to t, = 37--~ 35. There is no overlap between adjacent bands, due to the two-fold bigger anharmonicity shift for the overtone. Also the J-manifold is reduced according to the smaller N, pp~r/N~o..,,e,ratio.

1

Physics and spectroscopic applications of carbon monoxide lasers

471

/ rnW 222018. 16. 14 12 10 8 6 4 2 0

i

f

7

8

N 1 ,

9

l0

11

v

12

i

13

i

14

15

16

j"

Fig. 11. Distribution of the maximum achievable output powers after individual optimization of the discharge parameters."7~ In general the fundamental band laser can bear only small amounts of CO in the active discharge. A big "'anharmonicity defect" is simulated by adding N2 to the discharge to a higher concentration than usual.

higher currents. The role of oxygen, c o n t a i n e d in the " a i r " is very crucial, as m e n t i o n e d in earlier publications. "H2~ It is needed to prevent fast dissociation of C O into its c o m p o n e n t s a n d it is also beneficial to b r i n g the electron t e m p e r a t u r e down, however, at the same time it acts as a very strong VT-relaxation c o m p o u n d which would prevent the C O from reaching high v i b r a t i o n a l states, if there was too much present. The internal processes inside the C O plasma, particularly at low temperatures, are far from being well u n d e r s t o o d a n d obviously very long t i m e - c o n s t a n t equilibration processes take place. Otherwise it c a n n o t be explained why the a d d i t i o n of CO to get lasing above v = 30 needs to be d o n e slowly a n d not in one step.*

III. THE SINGLE LINE TUNABLE

RESONATOR

The setup of a l i n e - t u n a b l e C O laser is as straightforward as for the CO2 laser. T u n a b i l i t y is achieved by a reflection grating in L i t t r o w - m o u n t a n d thus is a s t a n d a r d technique (Fig. 8). The ruling technique has been perfected d u r i n g the last decade and n o w a d a y s grating efficiencies as high as 9 7 - 9 9 % are available, This is essential for the lasing region far off the gain m a x i m u m , e.g. for very low v ° 2 ) a n d particularly for the overtone laser Av = 2. " ° ' ~ F o r the long wavelength region (high v) we used 200 l i n e s / m m grating, for the shorter 300 a n d for the f u n d a m e n t a l b a n d laser (v = 1 --, 0) up to 350 lines/mm. The overtone laser Av = 2 needs up to 450 lines/mm. The other laser m i r r o r has a 5, 10 or 30 m curvature. The total length of the resonator is on the order of 1.5 m and the l o n g i t u d i n a l modes are 100 M H z apart, which compares to the gain linewidth.

*I feel obliged to add one comment on the ozone, that caused a very bad reputation for the liquid-N-cooledCO laser. When air is added at too high partial pressure (which can easily occur via a leak in the gas inlet system) ozone is produced in the discharge tube and sooner or later will appear as a dark-blue liquid at the bottom of the tube, Wherever this should happen, one should not panic, but rather try to take out the air and the CO from the gas; if it is a leak, switch offthe discharge, and wait, but never switch offthe vacuum pump of the gas flow system. I have tried to trace those events in other groups where explosion of discharge tubes has been reported, and it always seemed to be caused by switching off everything, including the pump. During 20 yr of operation of the liquid-N-cooled CO laser, we have never had an accident of this type in our laboratory.

472

WOLFGANGURBAN

Figure 9 shows the emission spectrum of our CO laser Av = I. The rotational structure of adjacent bands is strongly overlapping. Figure 10 shows the overtone laser spectrum Av = 2 and here the vibrational bands do not overlap. There are two reasons for this: one is the bigger shift between adjacent bands, since the anharmonicity enters twice into the frequency. The other reason is that due to N~p~/N~.... ~ 0.8 instead of 0.9 and thus we only have a narrower J-manifold that can be brought to lasing. Further details are described in Refs (10, 11). The fundamental band CO laser needs a somewhat different approach in detail, which mainly concerns the plasma. Since VV-pumping is not efficient for depleting the v = 0 state of CO, the anharmonicity defect is simulated by the difference in vibrational frequency between CO and N 2, thus one can call this laser an N2-CO laser. Further details can be found in recent publications, (~2"t4) although the first operations date back to 1973 (iS) and 1976/6) The maximum intensities, achieved with individual optimization, are plotted in Fig. 11. ()7)

IV. S P E C T R O S C O P I C A P P L I C A T I O N S

IV.A. Laser magnetic resonance (LMR) For almost two decades, the CO laser has been used by other groups in LMR, ¢18'2°)however, this method was the stimulus for our group to improve the performance both with respect to beam quality and tunability. (2° The whole region of the Av = 1 laser is fruitful for free radical vibration-rotation spectroscopy, and the CO-overtone laser Av = 2 has opened up another, most interesting range. L M R in the M I R has gained strong impact by introducing the Faraday-effect as a polarization-type detection method/2z'23) By this method we have produced a considerable amount of spectroscopic data. Here we only want to refer to a small selection of papers in that field.(24 30)

IV.B. High-precision saturation spectroscopy The CO2 laser transitions can be stabilized to high precision on Lamb-dips of CO2-absorptions, thus forming a grid of secondary frequency standards. Such a standard of comparable quality had been missing in the 5 / t m region. The obvious thing to do was to use CO laser lines to produce CO absorption Lamb-dips and for this a fundamental band laser was needed. Thus we got a series of CO laser transitions stabilized to better than 20 kHz or 3 × 10-~0 6v/v,(3~) but there are also some accidental coincidences with the OCS-molecule m) another standard reference absorber in the MIR.(33) The CO laser is easily stabilized on a saturation Lamb-dip, provided the coincidence falls within the Doppler-width. The sum frequency of two standard CO2 lasers and the CO laser is then directly read from an MIM-diode into a microwave frequency analyzer. (3)) We can increase the coincidence range by adding microwave sidebands to the CO laser. The principle of operation was first used in connection with a CO2 laser by Magerl (34) and applied to CO-frequencies by Schwendeman and coworkers. (35)Our setup has been developed in close collaboration with Glorieux and Legrand from Lille, (36) and is now starting to produce results that are also in the I0 ~0 quality range. (37"38)

IV.C. Photoacoustic detection of trace gases As has been verified by the co-organizer of this conference, the CO laser is very suitable for photoacoustic spectroscopy ( P A S ) . (39) In collaboration with the group of R e u s s (4°) we have set up a PAS detection system and extended it not only to the CO laser but also for the CO overtone laser. Here we get into the absorption range of CH, O H and N H groups and can thus expect the extension to very interesting molecules. HCI, formaldehyde and methane are some examples, the limits for the first go below the ppb, the latter down to 20 ppt in the volume. Further work is in progress.(4z.42)

Physics and spectroscopic applications of carbon monoxide lasers

473

Acknowledgements--lt is obvious that the results presented here are not all obtained by the author himself. Many generations of coworkers have contributed and their work has been referred to in the text, nevertheless their enthusiastic cooperation should be acknowledged explicitly. Considerable benefit has also come from our workshops. Here 1 want to mention explicitly our glass blowing artist Hans Kath and the leading persons for the mechanical engineering and constructions of all laser equipment, Josef Lfitz and Rainer Langen. Professor Jerry A. Weiss from Worcester Polytechnic Institute, Wayland, MA, has made me think about the physics of CO lasers from a different point of view, which has brought new light onto old ideas. I very much appreciate the comments of Dr J. S. Wells from NIST Boulder, CO to the manuscript.

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8. 9.

10. 11. 12. 13. 14. 15. 16~ 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.

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