MOLECULAR ION DETECTION BY A LASER INDUCED CHANGE IN MOBILITY

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MOLECULAR ION DETECTION BY A LASER INDUCED CHANGE IN MOBILITY

R. Walkup, R. Dreyfus, Ph. Avouris

To cite this version:

R. Walkup, R. Dreyfus, Ph. Avouris. MOLECULAR ION DETECTION BY A LASER IN- DUCED CHANGE IN MOBILITY. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-441-C7-446.

�10.1051/jphyscol:1983742�. �jpa-00223299�

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Colloque C7, suppl6ment au n o l l , Tome 44, novembre 1983 page C7-441

MOLECULAR I O N D E T E C T I O N B Y A LASER INDUCED CHANGE I N M O B I L I T Y

R. Walkup, R.W. D r e y f u s and Ph. A v o u r i s

IBM Thornas J . Watson Research Center, Yorktown Heights, New York 10598, U.S.A.

R6sum6 - Nous prgsentons une dgtection optogalvanique d f i o n s moleculaires (N;, CO+) par changement de mobilitB des ions i n d u i t par l a s e r . Cette technique l i e e 5 l a d i f - ference de dgplacement dfi aux c o l l i s i o n s e n t r e des ions dans un B t a t e x c i t 6 e t des ions dans un & t a t fondamental, f o u r n i t une m6thode t r b s sensible de d6tection des ions au voisinage de l a cathode dans des d6charges lmineuses.

Abstract - We report the optogalvanic detection of molecular ions ( N ~ , c o + ) via a laser induced change in ion mobility. The technique relies on a difference in collision limited transport of excited vs. ground state ions, and provides a uniquely sensitive probe of ions in the cathode sheath region of glow discharges.

Positive ions in plasmas are of interest for a variety of reasons including their fundamental role in plasma-surface interacti0ns.l In this article we discuss optogalvanic (O.G.) detec- tion of molecular ions ( N ~ , c o + ) via a laser induced change in ion mobility.2 The mobility based mechanism is supported by analysis of the spatial, temporal, and pressure depend- ence of the O.G. signal. We show that O.G. spectroscopy provides a uniquely sensitive probe of ions in the cathode sheath region.

The experimental apparatus is rather simple. A N2-laser-pumped tunable dye laser is used to probe DC glow discharges containing molecular species such as N:, and CO. The short pulse duration of the laser (-5nsec) allows analysis of the temporal behavior of the O.G.

signal. The discharge is an abnormal glow with plane parallel electrodes (25mm dia., 12mm apart) and gas pressures in the 1 torr range. The discharge is in series with a 10KQ load resistor and the DC current is typically 7mA. The voltage across the discharge is monitored through a DC blocking capacitor and processed with a boxcar averager. The discharge system is shielded by a grounded enclosure to minimize RF interference from the N2 laser.

Figure l shows an O.G. spectrum of N; obtained from a discharge in 1.0 torr of pure nitrogen. About 30 rotational transitions are observed as the laser is tuned across the (0,O) band of the X~Z:-.B~Z; transition. The observed 2 to 1 intensity a l t e r a t i o n is a consequence of nuclear spin degeneracy and the symmetry requirements of homonuclear diatomic molecules, and shows that the O.G. signal is linear in the number of N; molecules excited by the laser. Spin-rotation splitting (50.3cm-l)3 is not clearly resolved owing to the laser linewidth (-0.3cm-l).

Figure 2 shows the amplitude of the O.G. signal vs. distance between the laser beam and the cathode surface (the laser beam is parallel to the electrode surfaces). O.G. signals are obtained only for laser excitation within the cathode sheath region of the discharge (the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983742

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25,550 25,650 25,750

LASER FREQUENCY (cm-')

Figure 1 A laser optogalvanic (L.O.G.) spectrum of N; is shown. The laser excites individual rotational lines in the (0,0) band of the x2~;+B22: transition.

cathode dark space). This contrasts sharply with the spatial dependence of laser induced fluorescence (LIF) obtained by monitoring B22:(v = O ) + x 2 ~ i ( v = 1) emission with the laser tuned to the (0,O) band head. The LIF signal is approximately proportional to ground state N; density and shows that the N; density is much larger (factor of -50) in the negative glow region than in the cathode sheath. The spatial sensitivity of O.G. detection thus complements that of LIF and optical emission. In fact we obtain much better absolute sensitivity to ions in the cathode sheath with O.G. detection than with LIF.

-CATHODE ANODE-

- -

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^-

I

I ^\ \

\

-

0 2 4 6 8 1 0

DISTANCE (mm)

Figure 2 The spatial dependences of laser optogalvanic (L.O.G.) signal and laser induced fluorescence (L.I.F.) are shown. The L.O.G. effect is localized to the cathode dark space region 54mm from the cathode for this 1.0 torr nitrogen discharge.

With short pulse laser excitation, the O.G. signal for the X ~ X ; - . B ~ Z : transition of N;

appears as a temporary increase in discharge current (a voltage decrease) that occurs promptly after the laser pulse. The observed temporal behavior is independent of the distance between the laser beam and the cathode and is relatively insensitive to discharge pressure. The width of the current pulse is -180nsec FWHM for N2 pressures below -1 torr and decreases slowly with increasing N2 pressure (-100nsec at 8 torr). The O.G.

current pulse contains a charge equivalent to a substantial fraction (-50%) of the number of ions excited by the laser. In order to make this comparison, the N; density ( - 5 x 1 0 ~ c m - ~ ) was estimated from the measured current density and known drift velocity4, and the frac ion of ions excited by the laser was estimated from known partition functions and observe

d

We have also obtained O.G. spectra due to laser excitation of CO+ in carbon monoxide discharges. In these experiments, the pulsed dye laser was tuned across the (0,O) band of the x * z + + A ~ I I trdnsition of C O + . A temporary ( s l y s e c ) increase in discharge current occurs promptly after the laser pulse, and the effect is again localized to laser excitation within the cathode sheath. In contrast with N;, the radiative lifetime of excited CO+

( - 2 , ~ s e c ) ~ is greater than (or comparable to) the time for ion transit across the sheath. We observed O.G. signal levels for COf comparable to those for N;, however, a thorough investigation was hampered by excessive discharge noise that was associated with the formation of carbon deposits on the cathode surface.

To understand the mechanism by which ion excitation produces an O.G. signal, consider the basic physical processes occurring in the cathode sheath region where the O.G. effect is observed. The cathode sheath is a region of positive space charge, and positive ions are the principal current carriers. The ion mobility is limited by symmetric charge exchange collisions. For ground electronic state N; with kinetic energy -1?V, the cross section for symmetric charge exchange5 with ground state neutral N2 is - 5 0 ~ ~ . The mean free path (-6x 1 0 - ~ c m at 1 torr) is much smaller than the cathode sheath thickness (-0.3cm at 1 torr). Thus there are many (-50) charge exchange collisions during the transit of N;

across the cathode sheath.

Excited state ions are expected to have a smaller charge exchange cross section than ground state ions for collisions with ground state neutrals because the transfer of a single valence electron from the neutral molecule to the excited ion would produce an excited neutral molecule and a ground state ion. Such a reaction is endothermic by -3eV due to the difference in electronic energies of excited neutral vs. ionic molecules. There is always an energy resonant channel, but this involves one electron transfer plus additional electron- ic excitation/de-excitation. Excited ions therefore have an increased mobility, consistent with our observation that laser excitation of the ions leads to an increase in discharge current (or equivalently a decrease in voltage). The specificity of the O.G. signal to the cathode sheath region follows from the fact that positive ions are significant current carriers only in this portion of the discharge.

As an additional test of the mechanism of O.G. signal generation, we examined the pressure dependence of the N; O.G. signal obtained in discharges of pure nitrogen and 5% N2 in He. The signal dramatically declines for partial pressures of N2 b'elow -100mtorr. This corresponds to the point where the mean time between charge exchange collisions of ground state ions is larger than the radiative lifetime of excited ions ( 6 0 n ~ e c ) . ~ For lower pressures the excited ions decay in a time much less than the time between ground state collisions, and this precludes an effective change in mobility.

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The mobility based mechanism discussed here differs qualitatively from the more commonly mentioned mechanism for O.G. signal generation based on enhanced ionization of the excited state species. This ionization mechanism can easily be ruled out because (1) the short radiative lifetime (60nsec) of excited ions severely restricts the probability of electron impact ionization (prob. 5 1 0 - ~ ) , (2) an ionization type mechanism would appear princi- pally in the negative glow region where the electron and ion densities are large rather than in the cathode sheath region as observed, and (3) the difference in ionization potentials of excited (24eV) vs. ground state (27.leV) N; is relatively small.

The laser induced current change (for times small compared to an ion collection time) can be written as

where nr is the density of excited state ions produced by the laser, (v*-v:) is the differ- ence in velocity of excited vs. ground state ions, and the remaining factors are geometrical in origin (the cross section area for current flow A, and the fraction of the sheath volume illuminated by the laser). Our experimental observations for N; agree quantitatively with both the absolute amplitude and the time dependence of the current change predicted by this expression. This is illustrated in Fig. 3 which shows the observed signal and the current change calculated according to the expression above assuming that the excited ions accelerate freely in the local electric field. Note that for N; the excited state lifetime is small compared to the ion collection time (-lpsec) and thus t t e time dependence of the O.G. signal is determined by the excited state lifetime (via n i ) and by acceleration of excited ions (via v; ). The increase in excited ion velocity with time increases the duration of the O.G. curreni pulse to several radiative lifetimes. For a longer lived excited ion such as CO+, the duration of the O.G. current pulse should be limited by the ion transit time.

have not noise.

Figure 3 The time dependence of the optogalvanic signal for N; in a 1 torr nitrogen discharge is shown. Experimental values (dots) are compared with the calculated current change (solid line). The theoretical curve was scaled by a numerical factor of order unity (-0.7) in order to match the peak experimental current change.

In general, ion detection by a laser induced change in mobility should be effective provided that (1) ion motion in the sheath is collisionally limited, (2) there is a difference in transport cross sections for excited vs. ground state ions, and (3) the time between collisions involving ground state ions is less than the excited state lifetime.

Our experimental sensitivity to ions in the sheath is S/N-l for -105 ions excited by the laser (averaging -100 laser shots). This could be substantially improved in principle since the discharge current noise typically exceeded the shot noise limit by at least one order of magnitude.

We have used O.G. detection as a plasma diagnostic to obtain ion kinetic energies from Doppler broadening, and internal energy distributions from rotational line intensities.

Doppler broadening is apparent in O.G. line shapes taken with the laser propagating parallel to the DC electric field which is present in the cathode sheath. This is illustrated in Fig. 4. The observed broadening corresponds to a distribution of ion kinetic energies with an average value of x2eV for a 1 torr N2 discharge. This is in approximate agreement with the expected energy gained by acceleration in the local electric field for one mean free path for charge exchange. The rotational energy distribution is Boltzmann with T,,,=840f 20K for the spectrum shown in Fig. 1. This temperature is considerably hotter than the ambient gas kinetic temperature ( ~ 3 1 0 K ) and indicates that the ions are rotation- ally heated by their transit through the sheath. The O.G. spectra show no evidence of vibrationally excited ions, indicating that the vibrational temperature is TVib< 1000K.

LASER FREQUENCY (cm-' )

Figure 4 Line shapes obtained with the laser propagating parallel (upper curve) and perpendicular (lower curve) to the DC electric field in a 1.0 t o g qitrogen discharge are shown. There is additional broadening (Doppler effect) for k l ( E due to ion motion towards the cathode.

In summary, we have reported the first spectroscopic study of molecular positive ions by a laser optogalvanic technique. The detection mechanism involves a difference in collision limited transport of laser excited vs. ground state ions. This difference results in a current pulse when ions in the cathode sheath are excited. The specificity to the cathode sheath region follows from the fact that ions are the principal current carriers in only this region of the discharge. Optogalvanic spectroscopy is a useful diagnostic tool in the study of complex processes occurring in the cathode sheath and can provide information essential to the understanding of plasma-surface interactions.

Acknowledgements: We thank B. Nikolaus, J. Jasinski, R. Hodgson and J. Wynne for helpful discussions.

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References

1. Chapman B., Glow Discharge Processes, (1980) John Wiley and Sons Inc.

2. Walkup R., Dreyfus R. W., Avouris Ph., Phys. Rev. Lett. 50 (1983) 1856.

3. Huber K. P. and Herzberg G., Constants of Diatomic Molecules, (1979) Van Nostrand Reinhold Co.

4. Varney R. N., Phys. Rev. 89 (1953) 708.

5 . Kobayashi N., J. Phys. Soc. Jap. 38 (1975) 519.