ATOM DENSITIES OF THE FIRST EXCITED STATE IN LOW PRESSURE NEON DISCHARGES

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ATOM DENSITIES OF THE FIRST EXCITED STATE IN LOW PRESSURE NEON DISCHARGES

N. van Schaik, L. Steenhuijsen, P. van Bommel, J.C.A.M. van de Nieuwenhuyzen

To cite this version:

N. van Schaik, L. Steenhuijsen, P. van Bommel, J.C.A.M. van de Nieuwenhuyzen. ATOM DENSI-

TIES OF THE FIRST EXCITED STATE IN LOW PRESSURE NEON DISCHARGES. Journal de

Physique Colloques, 1979, 40 (C7), pp.C7-27-C7-28. �10.1051/jphyscol:1979713�. �jpa-00219102�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppzdment au n07, Tome 40, J u i Z t e t 1979, vage C7- 27

ATOM DENSITIES OF THE FIRST EXCITED STATE IN LOW PRESSURE NEON DISCHARGES

N. Van Schaik, L.W.G. Steenhuijsen, P.J.M. Van Bommel, J.C.A.M. Van de Nieuwenhuyzen.

Eindhoven University o f TechnoZogy, Eindhoven, The Netherlands.

Introduction. The relative and absolute densities of the various neon Is levels were studied, by means of optical fluorescence, in cilindrical discharge tubes (d = 31 mm) 7). A multi-mode CW dye laser (Spectra Physics model 370) was used as excitation source.

The relative densities and their ratio's were measured in the active discharge and in the afterglow of a neon discharge.

The absolute densities were measured in the positive

the absorption profile; this is justified because the width of the laser spectrum is about 15 times broader than the width of the absorption profile g.(v). Moreover we assume that the laser mode configuration does not change substantially with a change in the laser wavelength. The slight differ- ences between the various functions gi(v) are ne- glected because of the small energy gap between the

Is levels. The ratio between the fluorescence radiation when ls5 and Is4 are irradiated is:

column of the neon discharge by means of a modifiei

+M =

u~~

n, A,~/A* : ~ ~ i v ~ ) g ~ ( u ~ ) = JM(tot*) fi n

absorption method; the absorption of the laserlight

-

^{M2 H}

in the plasma was derivea from the measured at- ^{( V K )}

tot)

aRZnli

tenuation of the fluorescence signal along the path (2)

of the laser beam. In this case only that part of the laserlight spectrum that can be absorbed con- tributes to the measuring signal; disturbing effects at the entrance and the exit of the tube are eliminated; the accuracy is enhanced because the absorption can be measured for a large number of different path lengths in the same tube.

The relative densities. The fluorescence radiation Qi originating from the 2p2-Is2 transition

(Paschen notation) after optical pumping of a level 1s. to 2p. is given by:

$i = k ni(vk)~i2nigi(~K)~22/~X

(1) i= M, R, S, T for respectively the Is5, Is4, Isg and Is level. Biz is the in stein coefficient '*233*2); ni is the density of level isi; AZ2 is the spontaneous transition probability for the 2p2- Is transition; A~ is the total %:ransition pro-

2

bability of the level 2p2; gi(vK) is the value at frequency

vK

of the normalised absorption profile of level Isi and J.(vK) the radiative energy density in laser mode k when level Is. is irradiated.

The summation is made over all laser modes. We assume that Ji(vK) is constant for all

vK

within

JM(tot), J (tot) are the values of the leser beam R

power when level ls5 respectively Is4 is irradiate?.

Corresponding relations with (2) can be derived for other Isi combinations. From (2) follows that nM/n, can be determined from the fluorescence radiation when all other parameters of the experimental set up are kept constant. In this way we have measured for a few values of pressure and discharge current the relative densities of the ls5, 1s4 and Is 3 levels. This was done in the active discharge as well as in the afterglow. Figure 1 gives the ratios of the relative Is densities in a 5 torr tube as function of the discharge current. At 30 ULA we fin<

%/nR = 1.6 which is in reasonable agreement with the value 1.9 given by Soldatev 6 , for a corresponding situation. Figure 2 gives the values of

%/nR in the afterglow for pressures 0.5, 1 , 2 and 5 torr and a discharge current of 22 mA.

It must be remarked that important failures may arise if the laser mode configuration is not stable or if the laser beam power is too large. The latter condition may be checked carefully. In a number of cases we have checked the mode configuration by analysing the laser line shape with the help of a scanning Fabry-Perot interferometer.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979713

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The absolute densities. We have measured the absolute densities with the experimental set up given in figure 3. The laser beam enters the discharge tube at point 0 and may be adjusted parallel with the tube axis. Detection system DSI is fixed, but DSII can be moved parallel to the tube axis. The distance from 0 along the tube axis is denoted by x.

We will derive a relation for the ratio of the fluorescence radiation @(x2)/@(x1). $(x2) and @(xl) are emitted simultaneously by P2 (at x=x ) and P I

2 (at x=xl).

The laser energy density per mode and per unit volume JM(vK,x) is a function of x according:

with k(vo) is the absorption coefficient for the central laser frequency v ' k(vo) = '3(vo)nM with

0'

a(vo) is the absorption cross section at v=vo3);

gX(v,)= g (vK/g(v0).

When JM(vK,x) is substituted in relation (1) and using vK%vo we find:

with G(x) = 2 ~ g ~ ( v ~ ) e x ~ (k(vo)x( 1-gx(vK))).

K

Figure 4 gives the calculated values of

ln(@(x2)/@(xl)) versus (x2-xi) for xl = 0.04 m.

Relation (4) may be approximated within 5% for k2(vo)x < 2.44 by:

From the slope of In (@(x2)/+(xl)) at x2 = x l , we can determine the value k(vo) and from this the 1s ₅ density. Figure 5 gives a few examples of our experimental results. When the measured absorption curve is not a straight one we have k(v ) determined by curve fitting. The inaccuracy is better than 10%.

It can be remarked that k(vo) depends on the used spectral transition. So it is possible to measure in the same discharge a different absorption behaviour by using different spectral transitions. Excitation of the Is5 level with 1 = 588.2 nm resuits in k(vo)

= 21.7, n, = 3.52 l0l7d3; with 1- 597.5 nm results 17 -3

in k(vo) = 7.14, nM = 3.44 10 m

.

Figure 5 gives the absolute density with P = 1 torr

at various currents. They are in good agreement with measurements from Ricard 5).

References :

1) Lochte-Holtgreve, Plasma diagnostics, North- Holland Publ. Co. (1968).

2) Mitchell and Zemanskjr, Resonance radiation and excited atoms, Cambridge Univ. Press (1934,1971).

3) A. Comey, Atomic and Laser spectra, Clarendon Press, Oxford (1977).

4) S. Inatsugu et al. Phys. Rev. A1 1,1,26 (1975).

5) A. Ricard, Le Journal de PFysique 28, 523 (1975).

6) A.M. Soldatev, Opt. Spect. 34,6 (1973).

7) L.W.G. Steenhuijsen, thesis EUT (1979).

'Figure I : Measured r a t i o s o f the d e n s i t i e s of the 1s5,1s4,1s3 ZeveZs i n a 5 t o m a s c h q e . The c m s s e s (zl were obtained ;fmm Ricard

').

Figure 3: ExperimentaZ s e t up f o r the absorp- t i o n measurements.

Tigure 4 : LaLcuLated absorption curves f o r various

-.

vaZues o f k i v o l .

Figure 5: Measured absorption curves for p=l t o m and k588.2 nm a t oarious currents.