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Submitted on 1 Jan 1987
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HIGH RESOLUTION STIMULATED RAMAN SPECTROSCOPY WITH A 3 MHz ACCURACY
WAVEMETER
G. Millot, B. Lavorel, R. Chaux, R. Saint-Loup, M. Terki-Hassaine, J. Santos, G. Pierre, H. Berger
To cite this version:
G. Millot, B. Lavorel, R. Chaux, R. Saint-Loup, M. Terki-Hassaine, et al.. HIGH RESOLUTION
STIMULATED RAMAN SPECTROSCOPY WITH A 3 MHz ACCURACY WAVEMETER. Journal
de Physique Colloques, 1987, 48 (C7), pp.C7-763-C7-764. �10.1051/jphyscol:19877188�. �jpa-00227013�
JOURNAL DE PHYSIQUE
Colloque C7, suppl6ment au n012, Tome 48, dkcembre 1987
HIGH RESOLUTION STIMULATED
RAMANSPECTROSCOPY WITH A 3 MHz RCCURACY WAVEMETER
G.
MILLOT, B. LAVOREL, R. CHAUX, R. SAINT-LOUP, M. TERKI-HASSAINE, J. SANTOS, G. PIERRE and H. BERGER
Laboratoire de Spectronomie MolBculaire. et Instrumentation Laser, Universite de Bourgogne, 6 , Bd Gabriel, F-21000 Dijon, France
Stimulated Raman Spectroscopy (SRS) has demonstrated remarkably high sensitivitgr for high resolution applications.
The best compromise for obtaining a high signal to noise ratio in SRS is to use a pulsed pump laser and a CW probe laser. In our apparatus the beam of a single mode dye ring laser is passed through 4 stages of an amplifier system pumped by a frequency doubled Nd-YAG laser. We obtain pulses of more than 1 MW peak power and 12ns duration.
The Fourier limited linewidth which is about 2.10-~ cm-I determines the resol- ving power of the spectrometer.
The probe laser is a 514,5nm single mode argon ion laser which is gate-modu- lated to form 40ps pulses to avoid saturation of the photodiode. The laser is acti- vely stabilized with a Fabry-Pgrot interferometer andtherefore the linewidth is reduced to 1 MHz. Moreover, its frequency is locked to a Doppler free saturated absorption line of Ig. The dye and argon laser beams are crossed at a small angle or superposed in a colinear way in the sample. The probe laser is detected by a photodiode and a high frequency amplifier isolates the Raman loss dip in the probe laser signal. The Raman signal is averaged in a home-made data acquisition system of boxcar type.
We report here new improvements in c a m a t i o n of Raman lines and laser frequency measurements. Our wavementer is a Michelson interferometer whose corner cube is travelling vertically in an evacuated cylinder on a distance of about 1,2 meter'. The moving corner cube, controlled by a micropmessor, is accelerated and retarded in the extreme phases of movements. In these two phases,measurements are memorized and a polynomial interpolation permits to determine the fractional fringe order.
Spurious modulations of the laser beam such as bubbles in the dye circulation can be responsible for incorrect fringe order determination; so, in order to avoid these wrong measurements, we have realized a differential set-up for every beam.
For each frequency determination we repeat several consecutive measurements to increase the precision; the standard deviation is about 3 MHz. The reference frequency is that of an iodine stabilized He-N, laser.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877188
C7-764 JOURNAL DE PHYSIQUE
The absolute acctlracy-ofthe wavemeter a d i t s c o r r e c t o p e r a t i o n are v e r i f i e d &ring an experiment by measuring t h e frequency o f t h e ~ r + probe l a s e r , whose a b s o l u t e frequency i s known w i t h a p r e c i s i o n of about 1 MHz.
I n p r a c t i c e t h e c a l i b r a t i o n i s o b t a i n e d i n t h e following way: during every scanning of about 1 . 2 cm -1
,
t h e i n i t i a l and f i n a l wavenumbers a r e determined byu s i n g t h e wavemeter, t h e r e s t being i n t e r p o l a t e d from t r a n s m i s s i o n f r i n g e s o f a temperature s t a b i l i z e d c o n f o c a l Fabry-PBrot e t a l o n having a f r e e s p e c t r a l range o f 300.96 MHz.
The accuracy of Raman f r e q u e n c i e s depends on t h e q u a l i t y of t h e l i n e s : a r e c e n t t e s t on t h e Q-branch o f CO l i n e s l e d t o a s t a n d a r d d e v i a t i o n of 1 0 MHz with r e s p e c t t o f r e q u e n c i e s deduced from F o u r i e r transform spectroscopy ( 1 ) .
The performance opens t h e way f o r new h i g h r e s o l u t i o n i n v e s t i g a t i o n s ; f o r exam- p l e , t h e d e n s i t y frequency s h i f t measured on t h e N2 molecule ( 2 ) .
Moreover, o u r d a t a e x t r a p o l a t e d t o z e r o d e n s i t y allowed new r e f i n e m e n t s o f t h e N molecular c o n s t a n t s ( 2 ) : v - 2329.91165 (17)cm-I
,
B1-Bo = 0.017 3714 (22) cm-I2
2;
and D, I -Do u = 7.6 2 5.0 x cm
.
Another s t u d y a t high r e s o l u t i o n o f 1 3 ~ ~ 4 i l l u s t r a t e s t h e p o t e n t i a l o f o u r s t i m u l a t e d Raman s p e c t r o m e t e r .
A wide frequency range h a s been i n v e s t i g a t e d covering 45 cm-' i n c l u d i n g more t h a n 300 l i n e s vl, v3, 2 v2, 2 v 4 and v2
+
v4 bands. It i s noteworthy t h a t t h e overtones 2 v2 and 2 v4 a r e observed f o r t h e f i r s t time i n a methane l i k e molecule by a c o h e r e n t Raman p r o c e s s . The i n t e n s i t y o f t h e s e harnonic bands i s enhanced by t h e Fermi i n t e r a c t i o n with v1'
The spectrum h a s been r e c o r d e d a t 4 KPa p r e s s u r e , except 23 where t h e p r e s s u r e 2
was about 20 KPa. The c o l l i s i o n a l width (HWHM) extractedfrom t h e Raman p r o f i l e s h a s been c a l c u l a t e d t o 84.10-3 cm-'/MPa. The Raman l i n e s measured with a n accuracy a t l e a s t e q u a l t o 0.001 cm-I a r e combined with i n f r a r e d t r a n s i t i o n s ( r e c o r d e d by HENRY and VALENTIN on t h e F o u r i e r t r a n s f o r m s p e c t r o m e t e r o f t h e " L a b o r a t o i r e d e Spectro- nomiegg i n P a r i s ) i n a simultaneous a n a l y s i s o f f i v e i n t e r a c t i n g l e v e l s . The observed Raman t r a n s i t i o n s a r e reproduced with an o v e r a l l weighted s t a n d a r d d e v i a t i o n o f
1.8 x cm-I.
REFERENCES
(1) G. GUELACHVILI, J . of Mol. S p e c t r o s c . ,
2
(1979) 251.( 2 ) B. LAVOREL, R . CHAUX, R . SAINT-LOUP and W. BERGER, O p t i c s Comm.
