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MEASUREMENT OF THE LOCAL ELECTRIC FIELD IN DISCHARGES USING LASER STARK
SPECTROSCOPY OF THE NaK MOLECULE
J. Derouard, H. Debontride, N. Sadeghi
To cite this version:
J. Derouard, H. Debontride, N. Sadeghi. MEASUREMENT OF THE LOCAL ELECTRIC FIELD IN
DISCHARGES USING LASER STARK SPECTROSCOPY OF THE NaK MOLECULE. Journal de
Physique Colloques, 1987, 48 (C7), pp.C7-725-C7-727. �10.1051/jphyscol:19877178�. �jpa-00227002�
JOURNAL D E PHYSIQUE
Colloque C7, supplement au n012, Tome 48, dkcembre 1987
MEASUREMENT OF THE LOCAL ELECTRIC FIELD I N DISCHARGES USING LASER STARK SPECTROSCOPY OF THE NaK MOLECULE
J. DEROUARD, H. DEBONTRIDE and N. SADEGHI
Laboratoire de Spectrometric Physique (CNRS UA-08 and Greco Celphyra), Universit6 Scientifique, Technologique et MBdicale de Grenoble, BP 87, F-38402 Saint-Martin-d'Heres Cedex, France A heteronuclear diatomic molecule has a permanent electric dipole and consequently can interact strongly with an electric field.In some favorable cases this results in spectacular modifications in the laser induced fluorescence spectrum.
Q N~K(B'UV=S
J=4) Fig. 1. Effect of electric Rfield on the laser induced fluorescence signal for NaK excited in the (B1 lT;v=5, j=4e) level.
Fig.1 shows the fluorescence spectrum of the NaK molecule, excited in the e X-doublet component of the B1 Tl, v ' -5, j ' =4 level by mean of a C.W. single mode laser tuned to the (B1ll;v1=5,j'=4-XlZi;v"=0,j"=5) transition at 17286.58cm-1
.
I n the absence of an electric field, the fluorescence spectrum to the ground X I I + state is composed of a series of "P-R" doublets,one of which is shown on Fig.1. However an external electric field couples the e to the f A-doublet component 111, and consequently the fluorescence spectrum becomes a series of "P-Q-R"triplets. The ratio of Q to R (or P ) line intensities can be directly related [1,2] to the amplitude and direction of the electric field, relative to the polarization of the exciting laser beam and of the observed fluorescence spectrum (Fig.2). It can thus be used to measure the electric field in low pressure discharges containing a small amount of NaK [ 3 ] .
Previously, Gottscho and coworkers 141 have studied discharges in electronegative gases using the same molecular laser Stark spectroscopy technique with BC1 instead of NaK as the spectroscopic probe.
2 Fig. 2. Ratio of Q/R line intensities for NaK(B1 lT;v=5, j) -
In each case the laser is tuned @ to a P or R , transition, n
exciting only the e A-doublet 0 \
component.
0:experiment. -:theory.
0 100 200 300 400 500
Electric F ~ e l d (V/rrn)
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877178
JOURNAL D E PHYSIQUE
As shown on Fig.2, the sensitivity can be adjusted by choosing the rotational level j' excited by the laser. It goes from a few V/cm for j 1 = 4 up to 1000 V/cm for j 1 = 2 4 . Higher fields could be measured as well using larger j'.
On Fig.3 is shown a series of laser induced fluorescence spectra of NaK(B-X) recorded at different positions in a discharge established in a mixing of Ar+K+Na. The fall of the electric field is clearly apparent. The spatial resolution of about 0.2 to 0.3 mm is mostly determined by the slit width of the monochromator and could be improved by focusing the laser beam.
Fig.3. Laser induced fluorescence spectra of NaK(B111;v=5,j=10e) present as traces in a mixing of Ar+K+Na.
a) without discharge. b) with a D.C. discharge,as a function of the distance to the cathodetmm)
On Fig.4 we show some results obtained with the present technique concerning the distribution of the electric field in discharges in a Ar(98%)+K(2%)+Na(OS04%) mixing (0.4 Torr 483OK) for different values of the current density. This kind of measurements is presently used to test the validity of different discharge models.
Fig. 4.Distribution of the electric field, measured using the Laser Stark Spectroscopy in NaK, in discharges in a mixing of Ar+K+Na.
DISTANCE TO CATHODE ( m m )
Finally Fig.5 demonstrates the ability of the method to be used to do time resolved measurements of electric fields in non stationnary (RF or pulsed) discharges. Represented is the predicted response of the NaK molecule to steps of electric field of various amplitudes applied at t=O. The response is defined as the ratio of integrated fluorescence intensities. One sees that the stationnary value is reached within about 5ns.
I ' I ' I ' I '
E = 1000V/crn- Fig. 5. Calculation of the o-zc-
response of the NaK molecule excited in the Bl n;v=7, j=18e
level, vs time, to steps of bOOV/crn
electric fields applied at t=O. Oa2'- -
The response is defined as the
300V/cm.
0. ,100V/cm-
0 10 20 30 4 0 50
TEMPS 0' INTEGRATION ( n o )
In conclusion the present laser Stark spectroscopy technique has many advantages to probe the electric field in low pressure discharges. It is nonintrusive,local,fast and sensitive. However the use of NaK as the gpectroscopic probe requires the presence of Na and K as impurities in the gas studied. It i s thus restricted to the study of electropositive gases heated at 200 to 250 OC. This is still interesting for improving our knowledge on discharges physics and may be of relevance for optimizing alkali vapor lighting devices.
Work supported in part by D.R.E.T. under contract no 85-071 1. J. Derouard and N. Sadeghi Opt. Comm. 57 (1986) 239
2. J. Derouard and M.H. Alexander J. Chem. Phys. 85 (1986) 134 3. J. Derouard and N. Sadeghi IEEE Trans. Plasma. Sc. PSl4 (1986) 51 4. C.A. Moore,G.P. Davis and R.A. Gottscho Phys. Rev. Lett. 52 (1984) 538;M.L. Mandich,C.E. Gaebe and R.A. Gottscho J. Chem. Phys. 83 (1985) 3349;R.A. Gottscho Phys. Rev. A (1987) (to be published)
