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Resonant three-photon ionization and two-photon laser-induced fluorescence of atomic oxygen in a discharge and post-discharge
Volume 175, number I,2
CHEMICAL PHYSICS LETTERS
30 November1990
Resonant three-photon ionization and two-photon laser-induced fluorescence of atomic oxygen in a discharge and post-discharge G. Sultan, G. Baravian and J. Jolly Laboratoirede Physiquedes Gazet des Plasmas (Associb CNRS), Universith de ParisSud, 91405 Orsay,France Received 11 July 1990; in final form I 1September 1990
A method is presented which would allow the determination of the absolute number density of 0 atoms in discharge and postdischarge by simultaneous measurements of the number of photoions created from 2 + 1resonant-enhanced multiphotoionization (REMPI) of atomic oxygen and the LIF (laser-induced fluorescence) from two-photon excitation. The REMPI technique is used in post-discharge, leading to the determination of the number density of0 atoms in depletion condition in the interaction volume. The LIF techniqueis used in the same discharge conditions, allowing then the calibration which would be used to deduce the absolute number density of 0 atoms in a discharge.
1. Introduction Since UV tunable lasers have been available, several experiments are nowadays devoted to the determination of O-atom number density in postdischarges, in plasmas and essentially in flames. Laserinduced fluorescence (LIF) is the main non-intrusive diagnostic technique used principally in flames [ 1 1, and, more rarely, the MPI technique, the socalled optogalvanic detection [ 2 1. The LIF technique used alone leads only to relative information about the concentration of atoms. In addition, a complementary experiment is necessary to reach, by calibration, the absolute concentration. For example, the calibration for two-photon excited fluorescence signals in flames was made by using measurements of known concentrations of atoms generated in a discharge system where the authors use gas-phase titration [ 31, by photolysis of 0, via the Herzberg continuum absorption [4] or by electron-paramagnetic-resonance measurements of 0 and 0, concentrations downstream from the discharge [ 51. In this paper, we present a different calibration technique spatially resolved for the determination of O-atom density by LIF in a discharge. This calibration uses the detection of the ionization
signal in post-discharge near saturation conditions. In section 2, we describe the experimental procedure which was composed of two parts: the first part is concerned with the study of a post-discharge in a flowing afterglow where the techniques of REMPI and LIF were simultaneously used; and in the second part we study the dc discharge where only the LIF technique can be used. Finally, we show how the calibration used in the first experiment would lead to the absolute value of the 0 concentration in the discharge.
2. Experiment For the part corresponding to the detection of 0 atoms by REMPI, the experimental arrangement was approximately the same as described in a previous paper [ 6 ] for the detection of H atoms. The arrangement for the LIF measurement is given in fig. 1; Briefly, the laser used was a Quantel Datachrom5000 model. The emitted UV photons around 226 nm were generated by mixing the residual 1064 nm radiation from the Nd: YAG laser and the doubling of the dye-laser output around 574 nm generatedwith a mixture of rhodamines R590 and R6 10. The laser
0009-26 14/90/$ 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland )
37
CHEMICAL PHYSICS LElTERS
Volume 175, number 1,2
COMPUTER
BOXCAR
b
.
X-Y
f
AVERAOER
p
OIQITAL
PLOTTER
4
30 November 1990
OSCILLOSCOPE4
-,
z ’
4,
TAIQGER
JOULEMETER
574 LASER
SYSTEM
nm
-
207
nm
Y
LOAD
Fig. 1. Schematic diagram of the experimental apparatus.
provided radiation pulses of 2.5 mJ of maximum energy and 5 ns duration (fwhm). Typical conditions of discharge were a flow of 0.1 !2/mn (NTP) and a pressure of about 1 Torr. The current intensity in the discharge was in the range 5-50 mA. In the first part of the experiment, the 0 atoms were created in a dc discharge between two electrodes and were carried along by the gas flow and detected downstream in a steel vessel where the laser beam was focused. The Of ions resulting from the interaction with the laser were collected by a dc electric field. Moreover, the fluorescence at 844 nm was simultaneously observed. The resulting signals (photoions and fluorescence) were then analyzed using a 9400 Lecroy digital-storage oscilloscope and a 4420 EG&G boxcar averager. In the second part of the experiment, the discharge was created in the interaction volume and the laser beam was focused directly into the plasma.
3. Results and discussion In discharge as well as in post-discharge condi38
tions, the laser interaction with a gas containing 0 atoms and O2 molecules leads to the following reactions. The first step is the two-photon excitation of the ground level 2p 3P of atomic oxygen, 0(2p3P)+2hvi,-t0(3p3P), which can be followed by the subsequent reactions: 0(3p3P)+hz++O+te
forREMP1
or 0(3p3P)+0(3s3So)+hz+
forLIF
or
and
for the quenching followed by LIF, where n,=cl Y,= 226 nm is the incident laser radiation, AI =c/ Y , = 844 nm is the LIF from the 3p 3P level and AZ = cl Q= 777 nm is the LIF from the 3p 5P level.
Volume 175, number I,2
0+
+
CHEMICAL PHYSICS LETTERS
e _,
3p5P
Fig. 2. Energy levels of 0 atoms involved in the excitation, the ionization and the quenching and detection scheme.
The reactions are schematically represented in fig. 2. 3. I. Post-discharge 3. I. 1. REMPI When the laser beam is tuned in the range 224227 nm, the 3p 3P level is populated by a two-photon process and the ionization takes place with a third photon. A typical resonance-enhanced ionization profile is shown in fig. 3a. The ionization signal is almost similar to that observed in an atmosphericpressure stoichiometric hydrogen/oxygen/argon flame [ 2 1. There are three peaks due to the tinestructure splitting of the 2p ‘P state. 3.1.2. LIF The curve in fig. 3b shows the fluorescence signal in the post-discharge; scarcely observed in such conditions, it is similar to those obtained in rf discharge in a mixture 02+CF4+Ar [4]. 3.2. Discharge The REMPI technique is not simply and directly transposable in discharge because, Iirsdy, there are many more ions in the initial plasma than produced by multiphoton ionization so the distinction is difficdt; and, secondly, the collection electric field necessary in REMPI diagnostic technique may change
30 November 1990
the plasma parameters, and the measurement becomes intrusive. Therefore, the LIF technique was chosen exclusively in this case. Fig 3c shows the experimental results. Depletion effect. The study of the ion yield versus the energy of the laser beam shows that the slope in log-log coordinates (see fig. 4) is equal to 2.6 +O. 1 for low laser energies and to 1 f 0.1 for higher laser energies. These values are similar to those given by Bischel et al. [ 8 1, who find a slope equal to 2.6, and to that obtained by Walkup et al. [4] for highest energies (slope equal to 1); Alden et al. [ 71 find a slope equal to 2. The change of the slope at about I mJ corresponds to the depletion effect; roughly speaking, we can say that the whole focal volume is depleted for this energy value and all the 0 atoms contained in this volume before interaction are ionized or transferred by LIF or by two-photon excitation followed by collisional de-excitation (quenching) to another excited level. Fig. 3d shows the LIF obtained for the 177 nm transition in the postdischarge. 3.3. Number of ions The circuit detection for ions is the same as that used in ref. [ 61. The leading-edge voltage I’is proportional to the number N of ions detected through the relation N= CV/e, where C is the total parallel capacitive value of the circuit equal in this case to 50 pF and e is the electron charge. At the depletion value of about 1 mJ of the laser energy, a voltage of 800 mV is obtained for the ionization of the level ‘Pz. This voltage leads to an N value equal to 2.5 x 1OS ions. The total number of ions is obtained by measuring the ions issued from the three levels of the ground state. We find N= 3.6 x 108. 3.4. Number of photons When the 0 atoms are submitted to the laser flux, there first occurs a two-photon excitation to the level 3P, part of these excited atoms are ionized with a third photon and the result is the production of ions, the others are de-excited towards the levels ‘S or %, which can occur by radiative processes or collisional transfer followed by radiation in the latter case resulting in the emission of photons, In this discussion, 39
Volume 175, number 1,2
CHEMICAL PHYSICS LETTERS
1
REMPI POST
J = 1
P
5-
3
DISCHARGE
FIG. 3a
:“r
L ii-
FIG. 3d
k
I
LIF
1:
v)
POST
DISCHARGE
4m
a0
WAVELENGTH
FIG.
POST
( 777 NM 1
ma
WAVELENGTH
LIF
30 November 1990
3b
( 844 NM 1
FIG.
3c
4
I II
DISCHARGE
LIF
( 844 MI 1
LGCHARGE 6
WAVELENGTH
1
Y
WAVELENGTH
Fig. 3. Laser excitation scans across the 226 nm region (indicated by the term wavelength on the x-axis) detecting: (a) resonant 2+ I ionization ion signal in the post-discharge; (b) fluorescence induced by laser from the 3p ‘P level at 844 nm in the post-discharge; (c) FIL at 844 nm in the discharge; (d) FIL from the 3p ‘P level at 777 nm in the post-discharge.
we do not take into account the stimulated emission which is important only for relative large pressures (> 15 Torr) [9]. The population of the resonant level n2 can be written as follows in the post-discharge: dn2/dr=al~Zn,-a2~n~-(1/rtko[02])n2, (1) 40
where gI is the cross-section for the two-photon excitation of the ground state whose population is n,, 13,is the cross-section for the ionization of the excited state, @ is the laser flux, z is the radiative lifetime of the ‘P level, kQ is the quenching term of this last level with the molecules of oxygen whose population density is represented by [O,] .
CHEMICALPHYSICSLETTERS
Volume 175, number 1,2
30 November 1990
al. [ 131. It seems reasonable to take a mean value between the two above-cited values, i.e. /co= (7.5 & 1.2) x 10e10cm3 s-r. For a pressure equal to 1 Tort, l/r+b[O,] = 5.4x 107 s-‘, and the two terms (radiative and quenching) are almost identical. The ionization term can be calculated for a laser energy equal to 1 mJ, which corresponds to the saturation conditions with a focal length of 36 cm for 226 nm. For the divergence of the laser beam, 8, equal to lO-3 rad, Cpis equal to 2 x IO*W/cm2 and s@= 10.2x 10’s_‘; the number of ions is about two times larger than the deexcitation terms. With the experimental values, the relative importance of each of the three above-mentioned processes is, respectively, in the proportion 10.2:2.8:2.6. As the interaction volume is fully depopulated of 0 atoms in the ground state, these atoms are transformed either to ions (65%), give LIF at 844 nm ( 18%), or transfer their excitation by collision ( 17%); the total number of 0 atoms in the ground state, which were present in the interaction volume before the interaction, is then equal to ( 15.6/ 10.2)N=5.5x lo*. In a first approximation, we can consider that these atoms were present in a spherical volume v whose radius is equal to jj9, and then ~~2.44~ 10S5 cm3. The O-atom density at the focus point located 40 cm downstream from the discharge is then equal to 2.25x lOI cmP3. 3.6. Absolute density of 0 atoms in the focus point into the discharge Fig. 4. Ion yield versus laser energy at the wavelength corresponding to the resonance for the most intense component for REMPI.
3.5. Absolute dhnsity in the post-discharge
of0
atoms in the focus point
Comparison between the ionization and the de-excitation. We have to compare the relative impor-
tance of the terms a# and l/r-t/co [O,] with our experimental conditions. The cross-section for the photoionization of the n2 can be taken equal to about 1OP2d3/n5 cm* [ 10 1, where n is the considered level and A the wavelength of the light in cm; for A=226 nm, ~2=5x lo-l9 cm2, l/r=2.8~10’ s-’ (ref. [ 1111, the values given for /co are equal to 6.3 x 10P’O by Bittner et al. [ 121 and 8.7~ IO-‘O by Bamford et
The previous determination allows the calibration of the setup for the LIF. The respective numbers of photoions and photons at 844 nm in the post-discharge are in the proportion 65% and 18%, i.e. when 100 photoions are produced, 28 photons are emitted by LIF at 844 nm. The photons are collected through a Jobin-Yvon spectrometer and detected by a photomultiplier (Hamamatsu R928S) and within a solid angle equal to about 1O-’ sterad. The LIF signal obtained for the strongest transition is 20 mV, which corresponds to 800 mV for the ion signal and to a neutral-atom density equal to 2.25 X 10”. Under the same conditions, we obtained 1012for hydrogen [ 61. When the laser beam is focused into the discharge, it is not yet possible to measure the number of photoions but only the fluorescence signal, which is found 41
Volume 175, number I ,2
CHEMICAL PHYSICS LETTERS
to be about 80 mV at the same laser energy. The equation governing the population of the excited level is the same as eq. ( 1) plus a stationary term for n2 due to the plasma, which leads to a continuous signal for the 844 nm transition. We can then conclude that the 0 atom concentration into the discharge at a distance from the cathode equal to about 1 cm is 9 x 10’’ cme3. The ratio O/O2 is equal to 6.4~ 10m4in the post-discharge and to 2.5 x 10e3 in the discharge. These values are of the same order of magnitude as those given by Granier et al. [ 141, who find for 1 Torr, concentrations of 0 atoms of about 10” cme3 in the discharge and 6 x lOI cmS3 at 40 cm from the limit of the discharge. Moreover, the ratio values between the densities of 0 atoms in the discharge and in the post-discharge are similar to that found by Dimauro et al. [ 5 ] by using the electron-paramagneticresonance technique. The values here given are obviously subject to uncertainties due to the fact that, firstly, the interaction volume is not accurately defined and, secondly, it is necessary to take into account the spatio-temporal distribution of the laser flux, which is not trivial to measure accurately in this volume. One can consider that these determinations are, at the present time, correct to one order of magnitude but they can be improved. Moreover, for different plasma parameters, such as the initial pressure of the gas, it is necessary to proceed to a new calibration of the detection system. One should note that the density of 0 atoms can be determined for values as low as 4~ 10” cmS3 in post-discharge and 8~ 10” cmS3 in discharge because, in this latter case, the LIF signal is to be extracted from the stationary emission of the plasma. This corroborates the assertion of Dimauro et al. [ 51, who think that densities as low as 10” cmm3 might be measured.
42
30 November 1990
Acknowledgement This work was supported in part by Direction des Recherches, Etudes et Techniques, Paris.
References [ 11 W.K. Bischel, B.E. Perry and D.R. Crosley, Chem. Phys. Letters 82 (1981) 85; M.J. Dyer and D.R. Crosley, Opt. Letters I4 ( 1989) 12. [2] J.E.M. Goldsmith, J. Chem. Phys. 78 (1983) 1610. [3] U. Meier, K. Kohse-Hiiinghaus and T. Just, Chem. Phys. Letters 126 (1986) 567. [ 41 R.E. Walkup, K.L. Saenger and G.S. Selwyn, I. Chem. Phys. 84 (I 986) 2668. [ 51L.F. DiMauro, R.A. Gottscho and T.A. Miller, J. Appl. Phys. 56 (1984) 2007. [ 61 G. Baravian, J. Jolly, P. Persuy and G. Sultan, Chem. Phys. Letters 159 (1989) 361. [7] M. AldCn, H. Edner, P. Grafstr6m and S. Svanberg, Opt. Commun. 42 (1982) 244. [8] W.K. Bischel, B.E. Perry and D.R. Crosley, Appl. Opt. 21 (1982) 1419; I.J. Wyson, J.B. Jeffries and D.R. Crosley, Opt. Letters 14 (1989) 767. [ 9 ] M. AldCn, U. Westblom and J.E.M. Goldsmith, Opt. Letters 14 (1989) 305. [ lo] V.S. Letokhov, Laser photoionization spectroscopy (Academic Press, New York, 1987): D-L. Book, Plasma Formulary (Naval Research Laboratory, Washington, 1980). [ 111 W.L. Wiese, M.W. Smith and B.M. Glennon, Atomic transition probabilities, Vol. I, NSRDS-NBS Circular No. 4 (US GPO, Washington, 1966). [ 121 J. Bittner, K Kohse-Hoinghaus, H. Meier and T. Just, Chem. Phys. Letters 143 (1988) 571. [ 131 D.J. Bamford, L.E. Jusinski and W.K. Bischel, Phys. Rev. A 34 (1986) 185. [ 141 A. Granier, S. Pasquiers, C. Boisse-Laporte, R. Darchicourt, P. Leprince and J. Marec, J. Phys. D 22 (1989) 1487.
CHEMICAL PHYSICS LETTERS
30 November1990
Resonant three-photon ionization and two-photon laser-induced fluorescence of atomic oxygen in a discharge and post-discharge G. Sultan, G. Baravian and J. Jolly Laboratoirede Physiquedes Gazet des Plasmas (Associb CNRS), Universith de ParisSud, 91405 Orsay,France Received 11 July 1990; in final form I 1September 1990
A method is presented which would allow the determination of the absolute number density of 0 atoms in discharge and postdischarge by simultaneous measurements of the number of photoions created from 2 + 1resonant-enhanced multiphotoionization (REMPI) of atomic oxygen and the LIF (laser-induced fluorescence) from two-photon excitation. The REMPI technique is used in post-discharge, leading to the determination of the number density of0 atoms in depletion condition in the interaction volume. The LIF techniqueis used in the same discharge conditions, allowing then the calibration which would be used to deduce the absolute number density of 0 atoms in a discharge.
1. Introduction Since UV tunable lasers have been available, several experiments are nowadays devoted to the determination of O-atom number density in postdischarges, in plasmas and essentially in flames. Laserinduced fluorescence (LIF) is the main non-intrusive diagnostic technique used principally in flames [ 1 1, and, more rarely, the MPI technique, the socalled optogalvanic detection [ 2 1. The LIF technique used alone leads only to relative information about the concentration of atoms. In addition, a complementary experiment is necessary to reach, by calibration, the absolute concentration. For example, the calibration for two-photon excited fluorescence signals in flames was made by using measurements of known concentrations of atoms generated in a discharge system where the authors use gas-phase titration [ 31, by photolysis of 0, via the Herzberg continuum absorption [4] or by electron-paramagnetic-resonance measurements of 0 and 0, concentrations downstream from the discharge [ 51. In this paper, we present a different calibration technique spatially resolved for the determination of O-atom density by LIF in a discharge. This calibration uses the detection of the ionization
signal in post-discharge near saturation conditions. In section 2, we describe the experimental procedure which was composed of two parts: the first part is concerned with the study of a post-discharge in a flowing afterglow where the techniques of REMPI and LIF were simultaneously used; and in the second part we study the dc discharge where only the LIF technique can be used. Finally, we show how the calibration used in the first experiment would lead to the absolute value of the 0 concentration in the discharge.
2. Experiment For the part corresponding to the detection of 0 atoms by REMPI, the experimental arrangement was approximately the same as described in a previous paper [ 6 ] for the detection of H atoms. The arrangement for the LIF measurement is given in fig. 1; Briefly, the laser used was a Quantel Datachrom5000 model. The emitted UV photons around 226 nm were generated by mixing the residual 1064 nm radiation from the Nd: YAG laser and the doubling of the dye-laser output around 574 nm generatedwith a mixture of rhodamines R590 and R6 10. The laser
0009-26 14/90/$ 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland )
37
CHEMICAL PHYSICS LElTERS
Volume 175, number 1,2
COMPUTER
BOXCAR
b
.
X-Y
f
AVERAOER
p
OIQITAL
PLOTTER
4
30 November 1990
OSCILLOSCOPE4
-,
z ’
4,
TAIQGER
JOULEMETER
574 LASER
SYSTEM
nm
-
207
nm
Y
LOAD
Fig. 1. Schematic diagram of the experimental apparatus.
provided radiation pulses of 2.5 mJ of maximum energy and 5 ns duration (fwhm). Typical conditions of discharge were a flow of 0.1 !2/mn (NTP) and a pressure of about 1 Torr. The current intensity in the discharge was in the range 5-50 mA. In the first part of the experiment, the 0 atoms were created in a dc discharge between two electrodes and were carried along by the gas flow and detected downstream in a steel vessel where the laser beam was focused. The Of ions resulting from the interaction with the laser were collected by a dc electric field. Moreover, the fluorescence at 844 nm was simultaneously observed. The resulting signals (photoions and fluorescence) were then analyzed using a 9400 Lecroy digital-storage oscilloscope and a 4420 EG&G boxcar averager. In the second part of the experiment, the discharge was created in the interaction volume and the laser beam was focused directly into the plasma.
3. Results and discussion In discharge as well as in post-discharge condi38
tions, the laser interaction with a gas containing 0 atoms and O2 molecules leads to the following reactions. The first step is the two-photon excitation of the ground level 2p 3P of atomic oxygen, 0(2p3P)+2hvi,-t0(3p3P), which can be followed by the subsequent reactions: 0(3p3P)+hz++O+te
forREMP1
or 0(3p3P)+0(3s3So)+hz+
forLIF
or
and
for the quenching followed by LIF, where n,=cl Y,= 226 nm is the incident laser radiation, AI =c/ Y , = 844 nm is the LIF from the 3p 3P level and AZ = cl Q= 777 nm is the LIF from the 3p 5P level.
Volume 175, number I,2
0+
+
CHEMICAL PHYSICS LETTERS
e _,
3p5P
Fig. 2. Energy levels of 0 atoms involved in the excitation, the ionization and the quenching and detection scheme.
The reactions are schematically represented in fig. 2. 3. I. Post-discharge 3. I. 1. REMPI When the laser beam is tuned in the range 224227 nm, the 3p 3P level is populated by a two-photon process and the ionization takes place with a third photon. A typical resonance-enhanced ionization profile is shown in fig. 3a. The ionization signal is almost similar to that observed in an atmosphericpressure stoichiometric hydrogen/oxygen/argon flame [ 2 1. There are three peaks due to the tinestructure splitting of the 2p ‘P state. 3.1.2. LIF The curve in fig. 3b shows the fluorescence signal in the post-discharge; scarcely observed in such conditions, it is similar to those obtained in rf discharge in a mixture 02+CF4+Ar [4]. 3.2. Discharge The REMPI technique is not simply and directly transposable in discharge because, Iirsdy, there are many more ions in the initial plasma than produced by multiphoton ionization so the distinction is difficdt; and, secondly, the collection electric field necessary in REMPI diagnostic technique may change
30 November 1990
the plasma parameters, and the measurement becomes intrusive. Therefore, the LIF technique was chosen exclusively in this case. Fig 3c shows the experimental results. Depletion effect. The study of the ion yield versus the energy of the laser beam shows that the slope in log-log coordinates (see fig. 4) is equal to 2.6 +O. 1 for low laser energies and to 1 f 0.1 for higher laser energies. These values are similar to those given by Bischel et al. [ 8 1, who find a slope equal to 2.6, and to that obtained by Walkup et al. [4] for highest energies (slope equal to 1); Alden et al. [ 71 find a slope equal to 2. The change of the slope at about I mJ corresponds to the depletion effect; roughly speaking, we can say that the whole focal volume is depleted for this energy value and all the 0 atoms contained in this volume before interaction are ionized or transferred by LIF or by two-photon excitation followed by collisional de-excitation (quenching) to another excited level. Fig. 3d shows the LIF obtained for the 177 nm transition in the postdischarge. 3.3. Number of ions The circuit detection for ions is the same as that used in ref. [ 61. The leading-edge voltage I’is proportional to the number N of ions detected through the relation N= CV/e, where C is the total parallel capacitive value of the circuit equal in this case to 50 pF and e is the electron charge. At the depletion value of about 1 mJ of the laser energy, a voltage of 800 mV is obtained for the ionization of the level ‘Pz. This voltage leads to an N value equal to 2.5 x 1OS ions. The total number of ions is obtained by measuring the ions issued from the three levels of the ground state. We find N= 3.6 x 108. 3.4. Number of photons When the 0 atoms are submitted to the laser flux, there first occurs a two-photon excitation to the level 3P, part of these excited atoms are ionized with a third photon and the result is the production of ions, the others are de-excited towards the levels ‘S or %, which can occur by radiative processes or collisional transfer followed by radiation in the latter case resulting in the emission of photons, In this discussion, 39
Volume 175, number 1,2
CHEMICAL PHYSICS LETTERS
1
REMPI POST
J = 1
P
5-
3
DISCHARGE
FIG. 3a
:“r
L ii-
FIG. 3d
k
I
LIF
1:
v)
POST
DISCHARGE
4m
a0
WAVELENGTH
FIG.
POST
( 777 NM 1
ma
WAVELENGTH
LIF
30 November 1990
3b
( 844 NM 1
FIG.
3c
4
I II
DISCHARGE
LIF
( 844 MI 1
LGCHARGE 6
WAVELENGTH
1
Y
WAVELENGTH
Fig. 3. Laser excitation scans across the 226 nm region (indicated by the term wavelength on the x-axis) detecting: (a) resonant 2+ I ionization ion signal in the post-discharge; (b) fluorescence induced by laser from the 3p ‘P level at 844 nm in the post-discharge; (c) FIL at 844 nm in the discharge; (d) FIL from the 3p ‘P level at 777 nm in the post-discharge.
we do not take into account the stimulated emission which is important only for relative large pressures (> 15 Torr) [9]. The population of the resonant level n2 can be written as follows in the post-discharge: dn2/dr=al~Zn,-a2~n~-(1/rtko[02])n2, (1) 40
where gI is the cross-section for the two-photon excitation of the ground state whose population is n,, 13,is the cross-section for the ionization of the excited state, @ is the laser flux, z is the radiative lifetime of the ‘P level, kQ is the quenching term of this last level with the molecules of oxygen whose population density is represented by [O,] .
CHEMICALPHYSICSLETTERS
Volume 175, number 1,2
30 November 1990
al. [ 131. It seems reasonable to take a mean value between the two above-cited values, i.e. /co= (7.5 & 1.2) x 10e10cm3 s-r. For a pressure equal to 1 Tort, l/r+b[O,] = 5.4x 107 s-‘, and the two terms (radiative and quenching) are almost identical. The ionization term can be calculated for a laser energy equal to 1 mJ, which corresponds to the saturation conditions with a focal length of 36 cm for 226 nm. For the divergence of the laser beam, 8, equal to lO-3 rad, Cpis equal to 2 x IO*W/cm2 and s@= 10.2x 10’s_‘; the number of ions is about two times larger than the deexcitation terms. With the experimental values, the relative importance of each of the three above-mentioned processes is, respectively, in the proportion 10.2:2.8:2.6. As the interaction volume is fully depopulated of 0 atoms in the ground state, these atoms are transformed either to ions (65%), give LIF at 844 nm ( 18%), or transfer their excitation by collision ( 17%); the total number of 0 atoms in the ground state, which were present in the interaction volume before the interaction, is then equal to ( 15.6/ 10.2)N=5.5x lo*. In a first approximation, we can consider that these atoms were present in a spherical volume v whose radius is equal to jj9, and then ~~2.44~ 10S5 cm3. The O-atom density at the focus point located 40 cm downstream from the discharge is then equal to 2.25x lOI cmP3. 3.6. Absolute density of 0 atoms in the focus point into the discharge Fig. 4. Ion yield versus laser energy at the wavelength corresponding to the resonance for the most intense component for REMPI.
3.5. Absolute dhnsity in the post-discharge
of0
atoms in the focus point
Comparison between the ionization and the de-excitation. We have to compare the relative impor-
tance of the terms a# and l/r-t/co [O,] with our experimental conditions. The cross-section for the photoionization of the n2 can be taken equal to about 1OP2d3/n5 cm* [ 10 1, where n is the considered level and A the wavelength of the light in cm; for A=226 nm, ~2=5x lo-l9 cm2, l/r=2.8~10’ s-’ (ref. [ 1111, the values given for /co are equal to 6.3 x 10P’O by Bittner et al. [ 121 and 8.7~ IO-‘O by Bamford et
The previous determination allows the calibration of the setup for the LIF. The respective numbers of photoions and photons at 844 nm in the post-discharge are in the proportion 65% and 18%, i.e. when 100 photoions are produced, 28 photons are emitted by LIF at 844 nm. The photons are collected through a Jobin-Yvon spectrometer and detected by a photomultiplier (Hamamatsu R928S) and within a solid angle equal to about 1O-’ sterad. The LIF signal obtained for the strongest transition is 20 mV, which corresponds to 800 mV for the ion signal and to a neutral-atom density equal to 2.25 X 10”. Under the same conditions, we obtained 1012for hydrogen [ 61. When the laser beam is focused into the discharge, it is not yet possible to measure the number of photoions but only the fluorescence signal, which is found 41
Volume 175, number I ,2
CHEMICAL PHYSICS LETTERS
to be about 80 mV at the same laser energy. The equation governing the population of the excited level is the same as eq. ( 1) plus a stationary term for n2 due to the plasma, which leads to a continuous signal for the 844 nm transition. We can then conclude that the 0 atom concentration into the discharge at a distance from the cathode equal to about 1 cm is 9 x 10’’ cme3. The ratio O/O2 is equal to 6.4~ 10m4in the post-discharge and to 2.5 x 10e3 in the discharge. These values are of the same order of magnitude as those given by Granier et al. [ 141, who find for 1 Torr, concentrations of 0 atoms of about 10” cme3 in the discharge and 6 x lOI cmS3 at 40 cm from the limit of the discharge. Moreover, the ratio values between the densities of 0 atoms in the discharge and in the post-discharge are similar to that found by Dimauro et al. [ 5 ] by using the electron-paramagneticresonance technique. The values here given are obviously subject to uncertainties due to the fact that, firstly, the interaction volume is not accurately defined and, secondly, it is necessary to take into account the spatio-temporal distribution of the laser flux, which is not trivial to measure accurately in this volume. One can consider that these determinations are, at the present time, correct to one order of magnitude but they can be improved. Moreover, for different plasma parameters, such as the initial pressure of the gas, it is necessary to proceed to a new calibration of the detection system. One should note that the density of 0 atoms can be determined for values as low as 4~ 10” cmS3 in post-discharge and 8~ 10” cmS3 in discharge because, in this latter case, the LIF signal is to be extracted from the stationary emission of the plasma. This corroborates the assertion of Dimauro et al. [ 51, who think that densities as low as 10” cmm3 might be measured.
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30 November 1990
Acknowledgement This work was supported in part by Direction des Recherches, Etudes et Techniques, Paris.
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