Anomalous behavior of optogalvanic signal in a miniature neon discharge lamp

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Optics Communications 281 (2008) 129–134 www.elsevier.com/locate/optcom

Anomalous behavior of optogalvanic signal in a miniature neon discharge lamp V.K. Saini *, V.K. Shrivastava, R. Khare Laser Systems Engineering Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India Received 23 February 2007; received in revised form 26 August 2007; accepted 30 August 2007

Abstract A strong optogalvanic effect has been observed in a negative glow of a miniature neon discharge lamp using tunable pulse dye laser pumped by a copper vapor laser. A comparative study on temporal evolution of optogalvanic signal in a positive and negative dynamic resistance region of the discharge is described. Dye laser beam was tuned to various neon transitions 1si ! 2pj (Paschen notations) within 570–617 nm wavelength range. Anomalous behavior of optogalvanic signal was observed at 588.2 nm for (1s5 ! 2p2) neon transition at low discharge current (<220 lA). This anomalous behavior is the attributes of damped oscillations of optogalvanic signal that correlate with negative dynamic resistance (dV/di < 0) of the discharge. Penning ionization at low discharge current and small energy mismatch is assumed to be the main cause of the negative dynamic resistance. Penning ionization process has been explained by resonantly ionizing energy transfer via collisions between neon buffer gas atoms in the lowest metastable state (1s5) and electrode sputtered atoms in ground state using their partial energy level diagram. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.62.Fi; 82.80.Kq Keywords: Anomalous; Optogalvanic; Penning ionization; Dye laser; Sputtering

1. Introduction The optogalvanic (OG) effect discovered by Penning [1] has been used as a powerful spectroscopic tool to investigate the characteristics of atoms and molecules particularly in discharge environments. Traditionally, miniature neon lamps have been widely used as a circuit components and indicator lamps in electrical circuits, however we have used this lamp as a miniature discharge geometry for OG measurements. Such lamps are constructed by mounting two parallel electrodes in a glass enclosure filled with inert gas at reduced pressure. Construction wise indicator lamps are very rugged and free from any mechanical shocks and vibrations. These lamps are smaller in size, lighter in *

Corresponding author. Fax: +91 731 2442400. E-mail addresses: [email protected], [email protected] (V.K. Saini). 0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.08.066

weight and cost-effective as compared to any commercially available hollow cathode lamps. The OG spectra of neon and argon in such a miniature glow discharge lamps have been studied for wavelength calibration of lasers [2]. The characteristics of OG signal depend upon various parameters of the discharge, such as electrode’s material, geometry, applied discharge voltage, current, pressure of the filled gas and illuminated discharge region. Optogalvanic mechanism has been studied well in discharge tubes and hollow cathode lamps [3–12]. However, OG effect is not restricted to hollow cathode discharge environment, it has been observed in helium–neon laser, CO2 laser and copper vapor laser [13–15]. Also, we have observed OG effect in a miniature neon indicator lamp [21]. Pavicic et al. have reported a study on optovoltaic effect in the similar lamp [16]. Some interesting observation on optogalvanic signal has also been reported and attributed to anomalous behavior. These studies were mainly carried

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out in hollow cathode lamps [17–19]. To the best of our knowledge there is no report on anomalous behavior of optogalvanic signal in a miniature neon discharge lamp. Here, we report the role of Penning ionization in the laser induced anomalous behavior of OG signal in a negative glow discharge produced at low discharge current (<220 lA) in a miniature neon discharge lamp. A comparative study on temporal evolution of OG signals in a positive and negative dynamic resistance region is also presented. 2. Experimental details A schematic of the experimental setup is shown in Fig. 1. Inset shows a real photograph of commercially available miniature neon lamp (N). Two straight electrodes made of alloy (Fe, Ni, Cu) with typical length 4.5 mm, diameter 0.5 mm and separation of 1.4 mm were enclosed in a neon filled glass envelope. A fairly good negative glow discharge just adjacent to cathode was observed. Also a large Faraday dark space region was formed between negative glow and anode. No positive column or anode glow was observed due to miniature geometry of the lamp [20]. The lamp was powered by a regulated dc power supply (Aplab, India) through a current limiting ballast resistance. When polarity of power supply was changed, position of the negative glow and dark space regions with respect to electrodes get altered, this insures that the discharge glow just adjacent to cathode was negative glow. A specific OG circuit along with ballast resistance RB (0.6 MX) was used for signal detection [21]. A pulsed tunable dye laser (Rhodamine 6G, 30 ns, 6.9 kHz, 570–617 nm, 200 mW average power) pumped by copper vapor laser was used. The dye laser beam was focused by a lens (L) of focal length 50 cm and passed through the negative glow discharge region without interacting by electrodes. A beam splitter (BS) was used to pass the laser beam in monochromator [0.5 m, MP 1021, PACIF Precision Instruments]. The variation in discharge impedance due to resonant absorption was measured using digital oscilloscope (LeCroy LP-142, 100 MHz) via 300 pF coupling capacitor. A current–voltage characteristic of miniature neon discharge lamp was measured using a Protek 506 Multimeter.

3. Results and discussion The low discharge current operation is suitable for analyzing the galvanic reactions in gaseous discharge under the influence of weak perturbations (electromagnetic radiation, electrical and magnetic perturbation) [22]. At low discharge current of few hundreds of micro-amperes, we observed a laser induced time dependent normal and anomalous OG response. In spite of miniature discharge geometry, strong OG signals with high signal to noise ratio and precision were achieved. Frequent on/off operation of lamp did not affect the performance of OG signals and that all confirms the lamp’s high optogalvanic sensitivity and reliability. This feature of miniature neon lamp has been used for the calibration and frequency-locking of the lasers [2,36]. To produce a proper discharge medium, a dc voltage applied to the lamp was increased up to 180 V, where a discharge breakdown occurred and lamp starts glowing. After the discharge breakdown, voltage was reduced down to 140 V and below this voltage lamp did not maintain the discharge glow and extinguished abruptly. At 140 V, it was observed that discharge varied spatially over the electrode in the negative glow region, very similar observation was also reported in discharge tube by Druyvesteyn et al. [23]. Such a constricted discharge conventionally known by ‘‘subnormal discharge’’ was observed at a low discharge current of a few hundreds of micro-amperes in the lamp. To extract the useful current and voltage discharge operation region of gaseous discharge, I–V characteristic of the lamp was measured for cathode–anode voltage vs. discharge current (Fig. 2). A positive and negative dynamic resistance was observed above and below the 220 lA discharge current that corresponds to P–B (dV/di > 0) and P–A (dV/di < 0) region respectively.

170

Cathode - Anode Voltage (V)

130

Neon miniature lamp

165

160

155

A

150 0

P 200

B 400

600

800

Discharge current (µA) Fig. 1. Optogalvanic experimental setup.

Fig. 2. I–V Characteristic of miniature neon discharge lamp.

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3.1. Positive dynamic resistance region (dV/di > 0) Initially, the lamp was operated in the positive dynamic resistance region and irradiated by a pulsed dye laser beam within 570–617 nm tuning range. Three strong neon OG signals at 588.2, 594.5, 597.6 nm and few weak signals at 603.0, 607.4, 609.6, 614.3, 616.4 nm were observed. The lowest excited state configuration of neon (1s22s22p53s) gives four terms 3P2,3P1,3P0, and 1P1 that are related to Paschen notations 1s5,1s4,1s3 and 1s2 respectively [24]. The states 1s5 and 1s3 are metastable with radiative life time of several seconds to the ground state [25], however resonance levels (1s4, 1s2) have very short life time of 32 ns and 2 ns respectively [26]. The strong OG peaks have been assigned to 1s5 ! 2p2 (588.2 nm), 1s5 ! 2p4 (594.5 nm) and 1s5 ! 2p5 (597.6 nm) transitions. Transitions originated from metastable state (1s5) were large in amplitude and this indicates 1s5 state is the most populated neon lowest metastable state. When dye laser beam excites the neon atoms from lower energy levels 1si (i = 2–5) to higher energy levels 2pj (j = 1–10) the energy gap between the higher excited and ionization state of neon limits in the range of 2.5–3 eV. As a result, probability of collisional ionization of excited atoms increases. A large fraction of discharge electrons having sufficient required energy, ionize the 2pj excited neon atoms. At this moment due to the change in effective rate of ionization, discharge impedance and hence a voltage drop across the discharge decreases abruptly. This accounts for the first negative peak of the OG signal (Fig. 3). The effect of laser power and discharge current was also noticed on OG signal. Initially, OG signal increases exponentially with the increase of dye laser power and get saturated on 100 mW power at 2 mA current. Keeping the laser power fixed near to 50 mW, as the discharge current increases, OG pulse becomes narrow and achieves its steady state rapidly and this refers to the reduced relaxation times of the excited atoms. The OG signal (Fig. 3) observed at 588.2 nm (1s5 ! 2p2) was initially

negative for 2.5 ls, followed by a broad positive exponential part of 66 ls and finally reached a steady state value within 80 ls. In the similar way, other OG signals observed at 594.5, 597.6, 603.0, 607.4, 609.6, 614.3 and 616.4 nm, initially peaks within 2.0–2.5 ls, thereafter follow a broad positive exponential part and finally attain steady state with different relaxation times, within 40–70 ls. The overall temporal evolution of OG signal has been formulated by rate equations based on four excited states model. This model includes three (2pj, 1s2, 1s4) excited levels and one (1s5, 1s3) level with similar characteristics. The OG signal is fitted to the following equation [3–5] (Fig. 3) DV ðtÞ ¼ A½a1 expðt=s1 Þ þ a2 expðt=s2 Þ þ a3 expðt=sÞ; where s1, s2 are the relaxation times and a1, a2 are the coefficients related to the ionization rate of the respective states. Third term that includes parameters a3 and s are related to the response function of instrumentation. The time constant s is the response time of monitoring circuit that also includes resistance and capacitance of discharge lamp. The time constant (s  0.076 ls) is much smaller than relaxation times s1 and s2 as obtained from the curve fitting results (Fig. 3). 3.2. Negative dynamic resistance region (dV/di < 0) As the discharge current is reduced down to 220 lA to the point of inflexion (P) on I–V curve (Fig. 2), the dynamic response of OG signal changes dramatically (Fig. 4). The pulsed laser excitation at 588.2 nm (1s5 ! 2p2) initially produces a first negative peak of OG signal for 2.5 ls similar to normal OG signal. After a fast relaxation, OG signal having a positive pulse of width (FWHM) about 16 ls followed by another positive peak at 44 ls was observed. This signal is very different from that of normal optogalvanic neon signal (in P–B region). This is due to dominant

588.2nm 1s5 2p 2

10

OG signal (mV)

131

0

-10

Experimental data Curve fitted to

-20

-0.355t

ΔV(t) = -60 ( e

-30

0

-0.044t

- 0.425 e

30

-13.22t

-0.24 e

60

) mV 90

Time (µs) Fig. 3. Temporal evolution of OG signal at 588.2 nm for 2 mA current along with numerical fit (solid line).

Fig. 4. Temporal evolution of OG signal at 588.2 nm for 0.22 mA.

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Fig. 5. Partial energy level diagram of Fe+, Ni+, Cu+ and Ne atoms.

Fig. 6. X-ray energy dispersive diagram of miniature neon lamp electrodes material.

Penning ionization process at low discharge current [27]. This occurs due to the presence of Penning mixture of neon gas and sputtered metal atoms of the electrode that have a lower ionization potential than the first excited state (1s5) of the neon gas. Composition of electrode materials was estimated by energy dispersive X-ray analysis (EDAX) and results are shown in Fig. 6. Results show the presence of iron, nickel and copper elements in electrodes material (Fe:Ni:Cu;42.48:50.35:7.17 atomic percent). It is interest-

ing to note that the first ionization potential of Fe (7.90 eV), Ni (7.633 eV) and Cu (7.724 eV) lies well below the neon’s first excited state 1s5 (16.62 eV) [30]. Hence the above results confirm likely formation of Penning mixture of neon gas and sputtered metal atoms of electrode elements (Fe, Ni, and Cu). We consider Penning-type resonant ionizing energy transfer is one of the main reaction responsible to the negative dynamic resistance characteristics of the gaseous discharge. Similar explanation is given

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by Jung et al. [28] and Lee et al. [29] for different Penning mixtures in hollow cathode lamps. The effect of increase in conductivity of discharge has been noticed in P–A region (Fig. 2). This additional conductivity attributes to the presence of neon atoms in excited metastable state (1s5) and sputtered atoms of electrode (Fe, Ni, Cu) in ground state. This can be understood by Penning-type ionization reaction: Ne þ M ! Ne þ Mþ þ DE þ e producing additional large number of metal ions (M+) and electrons (e). Due to these additional charges a negative dynamic resistance has been observed. This negative dynamic resistance is correlated with anomalous OG signal (Fig. 7a and b). The origin of anomalous OG signal due to Penning ionization has been understood using partial

133

Table 1 Energy transfer reactions and data relevant to energy levels of Fe, Ni, Cu and Ne Cathode elements

Ionization energy (cm1)

Nearest excited ionic energy level (cm1)

Nearest excited ionic energy state

(DE) Energy mismatch (cm1)

3d6 (3G) 4p 29, 180 (J = 7/2,9/2) Ni 61 579 72 375 3d8 (3P3/2) 4p 90 Cu 62 317 71 920 3d9 (2D3/2) 4p 193 Ne(3P2) + Fe(5D4) ! Ne(1S0) + Fe+[3d6(3G)4p(J = 7/2, 9/2)] + DE + e Ne(3P2) + Ni(3F4) ! Ne(1S0) + Ni+[3d8(3P)4p(J = 3/2)] + DE + e Ne(3P2) + Cu(2S1/2) ! Ne(1S0) + Cu+[3d9 (2D)4p(J = 3/2)] + DE + e

Fe

63 700

70 315, 70 524

energy level diagram of sputtered electrode atoms and neon atoms (Fig. 5). Summary of Penning ionization reaction for sputtered electrode materials are shown in Table 1. A small energy mismatch (DE  kT) between neon metastable state (1s5) and excited ionic state of the electrode elements produces efficient Penning ionization reaction. For case study a detailed description of the Penning ionization reaction only for iron (Fe) sputtered atoms is described here. The electronic configuration of Fe (1s22s22p63s23p63d64s2) gives the ground state term 5D4 and first ionization energy 7.90 eV (63 700 cm1) [30]. The energy of excited states 1si (i = 2–5) of neon lies between 16.62 eV and 16.85 eV [24]. The ground state of Ne (1S0) and Fe (5D4) atoms have common zero energy level. The excited ionic energy levels of Fe [3d64p (3G9/2 = 70 524 cm1) and 3d64p (3G7/2 = 70 315 cm1)] with respect to Fe+ ionic (63 700 cm1) state lies just above and below to neon metastable state (1s5) [30]. In the absence of dye laser beam, the excited Ne atoms make several collisions with ground state sputtered Fe atoms. During this process, energy of the excited neon atoms is transferred to ground state sputtered iron atoms. As a result, iron atoms go to its ionic excited state and neon atoms settled down to their ground state (1S0). This process can be expressed as 3

Neð3 P2 Þ þ Feð5 D4 Þ ! Neð1 S0 Þ þ Feþ ½3d6 4pð G7=2;9=2 Þ þ DE þ e

Fig. 7. Anomalous optogalvanic signal at 588.2 nm (1s5 ! 2p2) for (a) 0.20 mA, (b) 0.15 mA.

where DE is the small energy mismatch that corresponds to 29 cm1 for J = 7/2 and 180 cm1 for J = 9/2. This DE lies with in thermal energy (kT) and attributes to the translational energy of the collisional partners. Energy transfer in such collisions is more efficient for small energy mismatch values [31]. In above reaction it is 4 and 22 meV only. When dye laser was tuned at 588.2 nm it resonantly excites the electrons from neon 3P2 (1s5) level to higher energy level 2p2. Now excited neon atoms are no longer available to ionize the sputtered metal (Fe, Ni, Cu) atoms. This results to reduce the rate of Penning ionization. Reduction of Penning ionization leads to decrease of metal ions and increase in discharge impedance. Increased discharge impedance results to an additional positive peak of OG signal after a few micro-seconds (Fig. 4). This is a very similar phenomenon that is reported in hollow cathodes lamps

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it suitable to an efficient OG device for many spectroscopic applications such as calibration and frequency-locking of the lasers [2,36]. Acknowledgements The author is thankful to Dr. B.M. Suri for fruitful technical discussion, Pragya Tiwari for X-ray energy dispersive analysis and U. Nundy for a critical reading of the manuscript. References [1] [2] [3] [4] Fig. 8. Anomalous optogalvanic signal at 597.6 nm (1s5 ! 2p5) for 0.20 mA.

[5] [6] [7] [8]

[32–35]. As the discharge current reaches to a value 0.20 mA (threshold current value) several numbers of positive and negative peaks of OG signal (damped oscillations) were observed (Fig. 7a). This abnormal variation of OG signal is referred to as anomalous behavior. First positive peak corresponds to normal OG signal of neon and other three peaks could be due to the Penning ionization of three different metals present in electrodes. These changes were even more pronounced when discharge lies entirely in negative dynamic resistance region (dV/di < 0) at 0.15 mA (Fig. 7b). This is because at low discharge current Penning ionization of minor species (Fe, Ni and Cu) by metastable neon atom plays predominant role than at high discharge current. Very similar conclusion was made by Smyth et al. [27] and that has been explained by photon induced bi-directional conductivity changes in gas discharge. The discharge shows unstable intensity in 70–150 lA current range and finally disappear after this range. Similar anomalous OG signal was also observed when laser tuned at different wavelength (597.6 nm) that corresponds to 1s5 ! 2p5 transition (Fig. 8). However, the frequency and decay time of these oscillations was different because of different excited energy levels involved.

[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

4. Conclusions [29]

In summary, we have observed strong optogalvanic effect, Penning ionization and anomalous optogalvanic behavior in a miniature neon discharge lamp at 588.2 nm. The anomalous response was interpreted in terms of negative dynamic resistance (dV/di < 0) offered by the discharge operated in a subnormal glow discharge region, for that Penning ionization is considered to be the most probable cause. The simplicity of the experiment along with high OG sensitivity of the miniature neon discharge lamp makes

[30] [31] [32] [33] [34] [35] [36]

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