Vibrational relaxation measurements in carbon dioxide using induced infrared fluorescence techniqu

Flynn, G. W.

Hocker, L. O.

Javan, A.

Kovacs, M. A.

Rhodes, C. K.

English

NASA Technical Reports Server

2 
Using an Induced Fluorescence Techniques 
Vibrational Relaxation Measurements i n  CO 
L. 0. Hocker, M. A. Kovacs, C. K. Rhodes, G. W. Flynn and A. Javan 
Physics Department, Massachusetts Institute of Technology 
GPO PRICE s 
CFSTI PRICEtS) S 
s Work supported by National Aeronautics and Space Administration and 
A i r  Force Cambridge Reseamh Laboratories. 
n . 
I UCCESSlON NUMBERS ITHRUS 
II 
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(NASA CR OR TMX OR AD NUMBER) ICATFQORI) 
https://ntrs.nasa.gov/search.jsp?R=19660022081 2020-03-16T20:40:45+00:00Z
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This letter is a summary of preliminary experimental studies of 
a number of relaxation processes involving excited vibrational levels of 
the C02 molecule. These studies involve the use of a new technique in 
which infrared fluorescence induced by the application of intense pulses 
of Q-switched 1 0 . 6 ~  laser 
measure of the rate of volume quenching of the first excited asymmetrical 
stretching mode, (Odl), of the C 0 2  molecule. 
a number of widely spaced vibrational levels of this molecule a r e  highly 
coupled through collisions; 
these levels is coupled to the C .  W. power output of the 1 0 . 6 ~  laser os- 
cillation (but not necessarily to the Q-switch power output). The 10. 6p 
oscillation occurs on a transition involving only one pair  of these 
collisionally coupled vibrational levels. The time constant involved in this 
collisional coupling is such that for a few microseconds after application 
of Q-switched radiation, an inversion of population may be obtained 
among these vibrational levels. 
Q-switched laser oscillation at 4. 3p. 
both in  the presence of a D. C. discharge current and without a discharge 
where the steady state level populations are at room temperature. 
is used to obtain an accurate 
W e  have also shown that 
and because of this, the energy stored in 
This has enabled attainment of a new 
The relaxation studies are done 
In this experiment, a short sample tube is placed within the resona- 
tor of a Brewster angle laser system with infrared transmitting windows. 
An additional infrared window is provided on the side of the sample tube, 
which is connected to a gas handling system that allows varying amounts of 
C 0 2  together with other gases to be introduced into the tube. . The sample 
C' 
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tube is equipped with a pair of electrodes which allows D. C. discharge 
excitation when desired. The laser discharge tube is much longer i n  length 
than the sample tube, apd in this way, the gain or attenuation of the sample 
tube at the laser frequency is kept a t  a level considerably lower than the 
gain of the laser tube. 
small  perturbation on the performance of the laser system. 
Accordingly, the sample tube introduces only a 
Consider the laser operating in the Q-switch mode on the ( l O o O ) c  
The 0 (00 1) rotation-vibration transition of COz at 10. 6p, (see Fig. 1). 
high intensity Q- switch beam introduces rapid radiative transitions between 
the corresponding levels of the C 0 2  molecules in the sample tube. This 
occurs during a very short time corresponding to the pulse-length of each 
Q-switch pulse and results in sudden changes in level populations of (10OOj  
and (00'1) states. 
their steady state values. 
tored by viewing the infrared spontaneous emission from COz levels 
through the side window of the sample tube. 
infrared monochromator is placed in front of the side window, and the 
output of the monochromator is detected by a fast response infrared 
detector. 
After the pulse, the level populations slowly relax to 
The change in level populations may be moni- 
For  this purpose, a grating 
In th i s  experiment, w e  have studied the fluorescence of nearly 
overlapping spontaneous emission bands arising from transitions within 
the asymmetrical stretch vibrational levels, (00'~- 1) +- (OOOv), (see Fig. 1). 
These bands fall a t  about 2300 cm-l(4. 5p). - The monochromator may be 
tuned in this region to select the emission originating primarily from 
individual ( OOOv) levels. 
+ -  
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The rate of volume quenching and diffusion of the ( O O o l )  levels was  
studied with the sample tube at room temperature and without a discharge. 
In this case, the steady state level populations are in thermal equilibrium 
at room temperature. The 4 - s w i t c h  laser pulfie suddenly increases the 
0 population of the (00 1) level and results in  a rapid rise in the spontaneous 
emission from this band. The decay of this emission signal back to its 
steady state value is found to be a pure exponential. 
measured rate  of decay of this signal as a function of C 0 2  pressure in  
pure C 0 2 .  
sample tube is responsible for departure from linear dependence at low 
pressure.  (The average diameter of the sample tube w a s  about 1 inch). 
Fig. 2 gives the 
The diffusion of excitation of the (00'1) levels to the w a l l s  of the 
Analysis of the curve in  Fig. 2 gives a rate of volume quenching for the 
( O O o l )  level due to collisions with the ground state C02 molecules. 
is 385/sec/mm corresponding to a cross section of 
The analysis of this curve at  low pressure gives a c ross  section for the 
self diffusion coefficient for diffusion of excitation of the ( O O o l )  state to the 
wall. This is 0. 50cm /sec normalized to one atmosphere. In this latter,  
This 
2 
= (3 .  3+. - 3) x . 
2 
estimate, we have not included the possibility that a collision with the w a l l  
may not necessarily lead to an immediate decay of the excitation of the 
molecular vibration. This effect may readily be studied by repeating the 
experiment with different types of walls for the sample tube. 
the non-cylindrical geometry of the tube, caused by the presence of the 
Furthermore, 
side window, also introduces an uncertainty in the exact value of the diffu- 
sion length. A s  a result, the measured value of the diffusion coefficient is 
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correct to within a factor of two. 
when extended, does not go through the origin. 
radiative decay of the (00'1) level. Wowever, to deduce from this an exact 
value of this radiative lifetime, the effect of self trapping of resonance 
radiation must be included. 
The linear portion of the curve in Fig. 2, 
This is caused by the 
In the above experiment, the decay of the fluorescence signal is 
observed by direct display of the detector output on an oscilloscope. 
signal to noise may be improved considerably by using standard electronic 
circuitry to achieve long integration times. 
possible observations at very low pressures of C02 and detailed studies 
of the effects of various buffer gases on the vibrational relaxation. 
The 
This is expected to make 
Let us now consider relaxation and collisional coupling between various 
levels in  the presence of a D. C. discharge in  the sample tube. 
the densities of the excited vibrational levels are so large that collisions 
between molecules in excited vibrational levels play important roles. 
Because of these collisions, the relaxation of a level depends on the exact 
densities of a number of collisionally coupled levels and hence is a complex 
function of the current through the discharge tube. The collisional coupling 
between the (OOOv) levels may be studied by observing the spontaneous 
emission side light originating from these levels a s  the sample tube is 
subjected to C. W. o r  Q-switch pulses of 10.6~ laser radiation. 
very narrow Q-switch pulse, one finds that, as the population of the (OOol) 
is suddenly changed, those of the higher (OOOv) levels also suffer an 
appreciable change, but with a time delay. 
In this case, 
Using a 
The change of the spontaneous 
L 
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emission signal due to population changes in the (OOOv) levels is roughly 
half as large a s  the steady state spontaneous emission signal in the absence 
of 10. 6p laser radiation. The time deiay in  the spread of excitation among 
these levels is less than ten microseconds. This was measured in  a discharge 
consisting of a mixture of He-N2-C02 and under conditions of near optimum 
gain for the 10.6~ transition. ' This indicates the presence of a large rate 
of upward excitation among the (00'~) levels. 
between adja ,ent (OOOv) vibrational levels is about 2300 cm-l) .  
(Note that the energy spacing 
Such an 
excitation may be accounted for by considering collisions between two 
vibrationally excited molecules causing further excitation of one and de- 
excitation of the other. 
molecules or possiblga C02 and a N2 molecule. 
a collision is small  and the corresponding cross section is expected to be 
large. 
1 
This may occur in  collisions between two C 0 2  
The energy defect in such 
In order to inspect the degree of importance and the consequences of 
the above collisional couplings, it is important to know the relative popula- 
tions of adjacent (OOOv) vibrational levels in the discharge tube. 
this, the intensity profile of the nearly overlapping emission band of the 
(OOOv- 1) C- (OOOv) transition w a s  analyzed in detail. 
the sample discharge tube subjected to 10.6~ C. W. laser  radiation as w e l l  
a s  without the laser  irradiation. 
of adjacent (OOOv) vibrational levels remained the same in both cases. 
To establish 
_I 
This was studied both with 
It was found that the ratio of the population 
Furthermore, thw'ratio was found to be close to unity for a number of low 
I 
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lying (OOOv) levels corresponding to relatively high positive temperature 
among them. This w a s  found to hold for levels up to and including v = 4. 
For la rger  v, the level populations fall drastically. 
ments, it was important to allow for the effect of self trapping of the 
(OOOO) C (OOol) transition). 
presence of tight collisional couplings between the (OOOv) levels have a 
number of important consequences: 
(In these measure- 
This observation, together with that of the 
1. While the unsaturated gain of the 10. 6p transition is determined 
only by the populations of the (10'0) and (OOol) levels, the laser output 
power, which correspmds to saturated gain at the laser transition, is 
determined by the populations ef at least three additional (OOOv) levels. 
Accordingly, the energy stored in the populations of at least four of 
the low lying (OOOv) levels is coupled to the C. W. 1 0 . 6 ~  laser output, 
and each of these levels contributes more or less by the same amount to 
the total 10. 6p laser power. 
2. 
are non-inverted. However, immediately after the application of a 
Q-switch 10.6~ laser pulse, where the population of the (OOO1) level 
is suddenly decreased, an inverted population may be obtained among 
the (OOOv) levels. 
microseconds determined by the relaxation time constant discussed 
above. 
sizable gain at a different frequency. 
laser oscillations induced by Q-switching of the 1 0 . 6 ~  transition. 
The steady state distribution of population among the (OOOv) levels 
This w i l l  occur in a time of the order of a few 
Accordingly, the 10. 6p Q-switch laser pulse "switches" on a 
A search was  made for new 
A 
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strong pulse was  detected at 4. 3p in a He-C02-N2 discharge. 
molecular rotation vibration parameters of the excited (OOOv) levels 
are not accurately known, but the best fit of the measured wavelengths 
to the known molecular parameters suggests a tentative ass ignmnt  
The 
to P( 31) transition between levels ( 00°2), (00'1). 
of t h e  4. 3p pulse follows the mechanism described above. 
i f  the pulse length of the 10.6~ laser oscillation is increased to a value 
in  excess of a few microseconds, the 4. 3p pulse occurs only at its 
onset and its pulse length cannot be extended by any means. 
should be pointed out that the conditions for obtaining optimum output 
power coupling for the 10. 6p pulse a r e  appreciably different than those 
for the 4. 3p pulse. 
output power of the 4. 3p pulse. 
3. 
allowed to oscillate, the population of the higher (OOov) levels a r e  no 
longer coupled to the laser field. 
pulse is generally less than the time constant of the collisional coupling 
between adjacent (OOOv) levels. 
It should be noted that other excited vibrational levels of the C02 
The overall behavior 
For instance, 
Finally, it 
This should be allowed for in obtaining optimum 
In a Q-switch system where the 1 0 . 6 ~  laser transition alone is 
This is because the duration of a 
molecules may also be tightly coupled to the laser  levels. 
studied by observing the influence of 1 0 . 6 ~  Q-switch laser  radiation on the 
infrared emission originating from excited bending modes o r  the symmetrical 
stretching mode. 
These may be 
The induced fluorescence technique used in the above experiments is 
- . 
P I -  - 8 -  
I 
an  extension of that technique applied earlier in studies of the thermalization 
of a number of closely spaced levels in excited atomic Ne(3). The relaxation 
studies described above give information related to the behavior of individual 
vibrational levels which is not readily obtainable from shock tube studies. 
Furthermore, in a gas discharge, the behavior of the induced fluorescence 
signal may be interpreted, in a manner which is free from additional com- 
plexities generally encountered in  other experimental methods. 
because , in the induced fluorescence techniques, the presence of excitation 
processes which a r e  unaffected by the laser  field do not appear in the final 
measurements. However, i n  an experimental method such a s  afterglow 
emission studies of pulsed discharges, the decay of other sources of excitation 
of molecular levels, for example, cascade transitions from a long lived 
excited electronic state, may lead to ambiguities in the interpretation of the 
observed behavior. , 
This is 
W e  acknowledge with thanks the expert technical assistance of 
Mr. E. T. Leonard and Mr. R. W. Solomon. 
0 
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References 
1 .  M. A. Kovacs, G. W. Flynn, and A. Javan, Applied Physics Letters, 
- 8 ,  No. 3, p. 62 (1966). 
2. G. W. Flynn, M. A. Kovacs. C. K.  Rhodes, and A. Javan, Applied 
Physics Letters, - 8, No. 3, p. 63-65 (1966) 
3. J. H. Parks and A. Javan, Phys. Rev. , 139, No. SA, A 1  351 -1 358 -
( 196 5) .  
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Vibrational relaxation measurements in CO2 USING an induced fluorescence technique

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