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Studies on inverse optogalvanic and Penning ionization effects in ytterbium and neon transitions in Yb-Ne hollow cathode lamp
Optics Communications 313 (2014) 42–48
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Studies on inverse optogalvanic and Penning ionization effects in ytterbium and neon transitions in Yb-Ne hollow cathode lamp P. Kumar n, V.K. Saini, G.S. Purbia, O. Prakash, S.K. Dixit, S.V. Nakhe Laser System Engineering Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India
art ic l e i nf o
a b s t r a c t
Article history: Received 29 July 2013 Received in revised form 17 September 2013 Accepted 22 September 2013 Available online 7 October 2013
This paper presents novel observations on inverse optogalvanic effect in Yb transition at 679.9 nm (3P1-3S1) in contrast with the observed normal optogalvanic effect at 648.9 nm (3P0-3S1) transition and Penning ionization in Yb-Ne mixture by probing Ne transitions at 626.65 (1s3-2p5), 633.44 (1s5-2p8), 650.65 (1s4-2p8) and 659.89 nm (1s2-2p2) in Yb-Ne hollow cathode lamp. These conclusions are derived by studying the optogalvanic signals temporal profile probed by DCM dye based narrow line-width 2 GHz, short pulse 20 ns, high repetition rate 5.0 kHz tunable dye laser, as a function of discharge current. The observed inverse optogalvanic effect is attributed to the transfer of Yb population in the level 3 P0 through radiative decay from the upper level 3S1 of the transition. This proposition is confirmed by recording the emission spectra of Yb-Ne hollow cathode lamp. The Penning ionization signature in Ne optogalvanic signals is due to the quasi-resonances between Yb and Ne energy levels. Penning signature observed in optogalvanic signal of Ne transition at 650.65 nm is unique and attributed to the increase in concentration of Ne metastable level 1s5 through radiative decay from the 2p8 level. & 2013 Elsevier B.V. All rights reserved.
Keywords: Hollow cathode lamp Optogalvanic signal Inverse OG effect Penning ionization
1. Introduction Laser optogalvanic (OG) effect has been used in wide ranges of applications such as atomic and molecular spectroscopy [1], plasma diagnostics [2,3] and Penning ionization spectroscopy [4]. The OG effect is an electrical response of the discharge medium upon absorption of resonant radiation by the species (atoms, ions or molecules) present in the gaseous discharge plasma. After irradiation of discharge plasma with a short duration ( 10 ns) resonant laser pulses, temporal evolution of the OG signal containing the information of relaxation and energy transfer processes can be studied. The hollow cathode (HC) lamp is a device generally used to observe the OG effect, in which discharge is generated within a hollow cylindrical cathode made up of the element of interest. Collisions with energetic ions, electrons, and neutral atoms within the discharge populates many excited states including the states that are optically forbidden, making the HC lamp useful for a wide range of spectroscopic investigations. Pulsed laser OG effect has been studied extensively by several authors. These studies include OG signal signs and Penning type ionizing collisions. Both these facts can be extracted from the temporal evolution of the OG signals. The first result of the pulsed OG effect has been presented by Miron et al. [5] for uranium and neon in U–Ne HC lamp. Shuker
n
Corresponding author. Tel.: þ 91 731 2442455; fax: þ91 731 2442400. E-mail address: [email protected] (P. Kumar).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.09.051
et al. [6] studied the OG effect in Ne transitions showing inverse OG signals in some neon transitions. They attributed this phenomena to the population inversion created between the upper and lower level of the transition. van de weijer et al. [7] observed the inverse OG effect in low pressure mercury discharge for the transition at 546 nm. In case of mercury, the inverse OG effect was attributed to the population transfer from the upper level (through radiative decay) to the level below the lower level of the investigated transition after the pulsed laser excitation. Piracha et al. [8] studied the pulsed OG effect in Ne transitions from 1si states in the wavelength range 614–672 nm. They also observed the inverted OG signals at 659.9 nm, 667.8 nm and 671.7 nm. Recently, the inverse OG effect is also observed by authors in Yb transition at 555.648 nm (1S0-3P1) originating from ground state [9]. This was attributed to the reduction in Penning ionization contribution upon the resonant excitation at 555.648 nm. Also the Penning ionization has been studied extensively by several researchers. Shuker et al. [4,10] first observed the Penning ionization in Ca–Ne HC lamp through Ne transitions at 614.3 nm. Ben Amar et al. [10] observed the Penning ionization by probing Ne transition at 594.4 nm in Sr3Ne HC lamp. Reddy et al. [11] observed the Penning ionization in Hg3Ar discharge lamp irradiated by excimer laser pumped pulsed dye laser. They studied the Penning ionization through Ar transitions at 451.07 nm, 459.6 nm and 462.84 nm. In Cs–Ne HC lamp, the effect of Penning ionization was observed by probing the OG signal profile of Ne at 588.19 nm [12]. Saini et al. [13,14] observed the anomalous behavior of OG signal due to Penning ionization in
P. Kumar et al. / Optics Communications 313 (2014) 42–48
miniature Ne discharge plasma. Khare et al. [15] observed the Penning ionization in Zr3Ne HC lamp by probing the OG signal profile of Ne transitions at 588.2 nm, 594.5 nm, 597.6 nm and 614.3 nm originating from the common metastable level. Ytterbium is a lanthanide that is of interest since the past few decades because of its widely ranging applications in Bose–Einstein condensation [16], atomic clocks and frequency standard [17], parity nonconservation (PNC) experiments [18], and industrial, medical and nuclear applications [19]. In most of these applications, the Yb transitions at 398.8 nm (1S0-1P1), 555.6 nm (1S0-3P1), 578.4 nm (1S0-3P0, doubly forbidden), 581.1 nm (3P1-(7/2, 3/2)2), 648.9 nm (3P0-3S1) and 679.9 nm (3P1-3S1) etc. are of importance. Thus, it is of interest to study these transitions. Here we present the studies on temporal evolution of OG signals of Yb and Ne transitions in a Yb-Ne HC discharge lamp. The OG signals are studied as a function of HC discharge current. The OG signal associated with Yb transitions at 648.9 nm (3P0-3S1) and 679.9 nm (3P1-3S1) have been studied. Also the Ne transitions at 626.65 (1s3-2p5), 633.442 (1s5-2p8), 650.65 (1s4-2p8) and 659.89 nm (1s2-2p2) excited from the four 1si states are studied. An in-house developed copper vapor laser pumped pulsed dye laser of line-width 2 GHz, pulse duration 20 ns and pulse repetition frequency 5 kHz is used to excite the transitions. We report the inverse OG effect for Yb transition at 679.9 nm in contrast to normal OG signal for the transition at 648.9 nm and Penning ionization in Yb-Ne atomic mixture.
2. Experimental details The schematic of the pulsed laser OG setup is shown in Fig. 1. It consisted of a pulsed dye laser, Yb-Ne hollow cathode lamp, wavemeter, high power DC power supply and a digital oscilloscope. The copper vapor laser pumped pulsed dye laser used to excite the transitions is an indigenously developed system producing an output radiation power 450 mW and line-width of about 2 GHz. The DCM dye dissolved in dimethyl sulphoxide (DMSO) solvent is used to cover the required wavelength range of 625–680 nm. The desired wavelengths are obtained by tuning the pulsed dye laser output radiation using a pico-motor driven tuning mirror. The Yb-Ne HC lamp is a commercial see through type lamp having length 20 mm and bore diameter 2.5 mm respectively. A high voltage DC power supply (Stanford Research Systems, INC PS310/1250V-25W) is used to take discharge in the HC lamp. A ballast resistor (R¼ 15k) is used to limit the discharge current through the lamp. The dye laser output beam is passed through the axis of the HC discharge lamp. While passing through the discharge lamp, a spherical lens having focal length of 25 cm is used to focus the dye laser beam. A beam splitter (4%) sends a part of the dye laser output beam to the high precision wave-meter used to monitor the wavelength of the dye laser.
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3. Results and discussion 3.1. OG studies of ytterbium The observed profile of the OG voltage signals for Yb transitions at 648.9 nm (3P0-3S1) and 679.9 nm (3P1-3S1) are shown in Fig. 2(a and b) for different discharge current values. It is observed that, for the transition at 648.9 nm originating from the metastable state, the OG signals (initial negative sign) are as usual. In normal pulsed OG signals, the discharge impedance decreases after the excitation of the atoms into higher level upon absorption of resonant pulsed laser radiation. This decrease is due to relatively higher ionization rate of the excited level (closer to ionization level) compared to the lower level. Thus, the negative sign of OG voltage signal can be interpreted as due to decreased impedance of the discharge medium upon absorption of resonant radiation. While in the case of the transition at 679.9 nm, inverse OG effect (positive OG voltage signal) is observed for the investigated discharge current range of 5 mA–15.0 mA. The positive OG voltage signal corresponds to the increase in discharge impedances (inverse OG effect) after the absorption of resonant radiation. The inverted OG signals were observed earlier in Ne and Hg transitions. Shuker et al. [6] attributed the inverse OG effect in Ne transitions to the population inversion created between the upper and lower level of the transition. van de weijer et al. [7] attributed the inverse OG effect in Hg discharge to the population transfer from the upper level (through radiative decay) to the level below the lower level of the investigated transition after the pulsed laser excitation. Recently, the authors also observed the inverse OG effect in Yb transition at 555.648 nm (1S0-3P1) originating from the ground state [9]. This was attributed to the reduction in Penning ionization contribution upon the resonant excitation at 555.648 nm. For the present case of Yb transition at 679.9 nm, the observed inverted OG signal may be due to the radiative transfer of the population from the upper level 3S1 of the transition to the level 3P0 below the lower level 3P1 of the transition. Fig. 3 shows the energy level diagram of Yb relevant to the present situation. Upon the resonant laser pulse absorption at 679.9 nm, Yb population from the level 3P1 transfers to the excited level 3S1 having a lifetime of about 15.9 ns [20]. The excited level 3S1 of Yb is radiatively coupled to the levels 3P0, 3P1, and 3P2. And the sign of the OG signal is decided by the effective decay rates of Yb population transfer in these levels. Fig. 4 shows the selected portion of the emission spectra of Yb-Ne HC lamp at discharge current of about 10 mA. From this figure, it is clear that the emission line at 678.9 nm is stronger than the 769.9 nm line. Now the much larger strength of the Yb emission line at 648.9 nm compared to the emission line at 769.9 nm suggests that effectively more population transferred to level 3P0 compared to 3P2. Thus, we can say that upon the absorption of resonant laser pulse, the population effectively got transferred to the level 3P0 which is below the lower level 3P1 of the transition. Since the electron impact ionization probability decreases for the levels far from and far below the ionization potential. Consequently, the discharge impedance increases resulting in the positive OG voltage signal (inverse OG effect). Such a situation is not possible for the transition at 648.9 nm originating from the 3P0 level as the 3S1 is not radiatively coupled to ground state level 1S0, which is the only level below 3P0. 3.2. Penning ionization in Yb-Ne atomic mixture The ground state electronic configuration of Ne is 1s22s22p6 S0. The excited levels of Ne can be best described by the jcK-coupling scheme [21], through a simplified set of notations, called Paschen notation often used to name these levels. According
1
Fig. 1. Schematic of the experimental setup.
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P. Kumar et al. / Optics Communications 313 (2014) 42–48
Fig. 2. OG signals of Yb transitions at (a) 648.9 nm and (b) 679.9 nm.
Fig. 3. Energy level diagram of Yb.
to these notations, the first group of excited levels is named as 1si and is subdivided into four levels namely, 1s5, 1s4, 1s3, and 1s2. Here 1s5 and 1s3 represent the two metastable levels with lifetimes of the order of seconds, while 1s4 is the semi-metastable and 1s2 is the resonance level with radiative lifetimes of 16 ns and 1.2 ns, respectively [22]. The second group of excited levels is expressed by the 2pj levels and is subdivided into 10 levels namely, for j ¼1–10. These levels are radiatively connected to at least one of the 1si levels and have typical radiative lifetimes of about 19 ns [23]. The 1si–2pj yields a total of 30 allowed radiative transitions and is a system of our interest. In an HC discharge lamp, the excited states are populated by electron collisions. In discharges, the lowest metastable state plays a cardinal role in the discharge in the sense that it constitutes an effective ground state. In Yb-Ne HC discharge lamp, a major factor contributing to the discharge current is the ionization of the Ne atoms by electron impact and collisions among the excited atoms. Normally atoms in the metastable levels tend to get ionized and contribute to the discharge current. The Ne þ ions created in the
Fig. 4. Emission spectra of Yb-Ne hollow cathode lamp.
discharge are accelerated across the large electric field of the cathode fall region and impinge on the cathode surface causing ejection (sputtering) of Yb atoms. The sputtered Yb atoms, largely ground state atoms, diffuse from cathode walls into the negative
P. Kumar et al. / Optics Communications 313 (2014) 42–48
glow region of the discharge plasma. The electronic configuration of ground state Yb atom, [Xe] 4f146s2, gives the term 1S0 as ground state and has an ionization potential of 50,443 cm 1 (6.25 eV) [24]. The atoms in an HC discharge plasma can be ionized by four processes, viz. photo ionization, charge transfer ionization, electron impact ionization, and Penning ionization [25]. The ionization of the sputtered metal atoms in a Ne discharge is not dominated by photo ionization [25]. In the Yb-Ne hollow-cathode discharge, the crosssection for charge transfer is small because the ionization potentials of Ne and Yb are widely different; 21.56 and 6.25 eV, respectively. Thus, the important processes that ionize sputtered Yb atoms in YbNe HC discharge are electron impact ionization and Penning ionization. However, the Penning ionization plays significant role only when the involved levels are metastable and the energy difference between the participating element energy levels lies within the thermal limit. Fig. 5 shows a partial energy diagram of the relevant energy levels of Yb and Ne in the Penning ionization process. It is clear from this figure that Ne's lowest metastable level 1s5 at 13,4044 cm 1 and the excited state 2D3/2 of Yb þ are very close ( 239 cm 1) [24]. Also the next metastable state 1s3 of Ne at 13,4821 cm 1 lies very close to Yb þ excited states 2Do9/2,7/2,5/2. The energy difference between Ne metastable state 1s3 and these excited states of Yb þ are 160 cm 1, þ66 cm 1 and þ 255 cm 1 correspondingly. These Ne–Yb energy level differences are well within the thermal limit ( kT) of the HC lamp operating temperature (400–500 1C). These quasi-resonances leads to the ionization of the ground state Yb atoms to higher excited states of Yb þ via Penning type collisional energy transfer process by the metastable Ne atoms (Fig. 5). The Penning ionization process in Yb-Ne mixture is governed by the collisional energy transfer reactions as follows: Yb (1S0)þ Nen (1s5)-Yb þ n (2D3/2)þ Ne (1S0)þe þ ΔE (þ 239 cm 1) 1
n
þn
2
(1)
Do9/2) 1
)
(2)
Yb (1S0)þ Nen (1s3)-Yb þ n (2Do7/2) þNe (1S0)þ e þ ΔE ( þ66 cm 1)
(3)
Yb ( S0)þ Ne (1s3)-Yb ( þNe (1S0)þ e þ ΔE ( 160 cm
Fig. 5. Partial energy level diagram of Yb and Ne showing relevant energy levels involved in Penning ionization.
Yb (1S0)þNen (1s3)-Yb þ n (2Do5/2) þNe (1S0)þe þ ΔE (þ 255 cm 1)
45
(4)
The Penning effect introduces the extra features like double humped structure in the temporal profile of the OG signal [4,10– 15]. At lower discharge current values, the Penning ionization of Yb by meta-stable Ne atoms mainly controls the Yb þ density in the discharge plasma. These Yb þ further sputters Yb atoms from the cathode more efficiently than Ne þ because of the two reasons: first is larger mass difference between Ne þ and Yb; and the second is because of Ne þ 's higher probability for resonant charge transfer with neutral Ne atoms present in the discharge plasma at high density [26]. Now when the laser is tuned to the resonance of the Ne transitions, this Penning ionization contribution significantly reduces. Consequently, this reduction results in alteration of the properties of the discharge impedance. The altered discharge impedance reflects in the temporal evolution of the OG signal corresponding to the transition of interest. 3.2.1. Transitions originating from metastable states of Ne The states 1s5 and 1s3 of Ne are the metastable states having lifetimes of the order of seconds [22]. Since the maintenance of the discharge is closely tied to the Ne metastable density, by depleting the Ne metastable concentration, one can effectively perturb the discharge medium. Fig. 6 shows the temporal evolution of OG signal obtained for the Ne transitions at 626.65 (1s3-2p5) and 633.442 (1s5-2p8) originating from the metastable levels at 13,4044 and 13,4821 cm 1 respectively. The temporal evolution of OG signal in Yb-Ne HC lamp is recorded as a function of discharge current. Below a discharge current of 15 mA, the OG signal profile has a double hump structure in the positive OG signal part. The four parts of the observed OG signals shown in Fig. 6 are as follows. The resonant absorption of 626.65 nm (633.44 nm) laser radiation by Ne atoms at level 1s3 (1s5) leads to an increase in population of 2p5 (2p8) level. The 2p5 (2p8) level is much closer to the ionization level at 17,3932 cm 1 compared to level 1s3 (1s5). Therefore the increased population of the level 2p5 (2p8) leads to an enhanced ionization rate of Ne atoms resulting in decrease of the discharge impedance. Thus, the voltage drop across the discharge reduces. Consequently, the pulse shape of OG voltage signal is initially negative (a). After termination of the laser pulse ( 20 ns), the depopulation of level 2p5 (2p8) begins, which increases the impedance of the discharge and thus the negative OG signal decreases. The population of 2p5 (2p8) decays to many lower levels including the ground state. Thus the depopulation of the level 2p5 (2p8) does not lead to steady state value and the negative OG signal crosses zero and becomes positive (b). These two features are typical pure Ne OG signals, occurring in the first 2–25 ms [27] and are referred to as the “neon part.” The Penning effect follows the Ne part and causes two extra features in the OG signal. First, the relative decrease in the Ne metastable population results in a reduction in the rate of the Penning ionization of neutral Yb atoms and hence in the production of Yb þ . The resulting increase in neutral Yb atom density causes a reduction in OG voltage signal (c) as lower voltage can sustain the discharge. The Penning interaction also shortens the relaxation time of the Ne metastable level [4]. Secondly, after a diffusion time, the decrease in Yb þ density results in a drastic fall in the sputtering rate of Yb atoms. This reduction in Yb atom density results in the positive pulse (d). This part of the OG signal has a time scale of 30–40 ms and is referred to as the “Penning part”. Subsequently the population of different levels return to thermal equilibrium and the positive OG signal decays exponentially to zero (steady state value). At higher discharge current 415 mA, the OG signal profile is like pure Ne OG signal. This shows that the Penning ionization contribution is dominant in the lower discharge current range. The Penning type collisions are more efficient when the energy mismatch between the participating elements is lesser. However,
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P. Kumar et al. / Optics Communications 313 (2014) 42–48
Fig. 6. Temporal evolution of OG signals of Ne transitions at (a) 626.65 nm and (b) 633.44 nm.
other factors like mass ratios of the participating elements, fill gas pressure, galvanic detection circuitry also play a significant role in Penning ionization.
3.2.2. Transitions originating from non-metastable states of Ne The levels 1s4 and 1s2 of Ne are the non-metastable states having lifetimes of about 16 ns and 1.2 ns respectively [22]. Fig. 7a shows the OG signals of Ne transition at 650.65 nm (1s4-2p8) for various discharge current values. The 2p8 level of Ne has relatively high transition probability of decaying to the metastable level 1s5 compared to other non-metastable levels 1s2 and 1s4. The radiative decay of the laser excited 2p8 level increases the metastable level 1s5 concentration [28,29]. The effect of increased metastable concentration is twofold. First is that, the resulting OG signal has a larger negative part as can be seen clearly in Fig. 7a at a discharge current of 1 mA [28,29]. Secondly, the increase in metastable
concentration leads to higher Penning ionization rates and hence Yb þ density. Because of this increased Yb þ density at the cost of neutral Yb atoms, the negative OG signal starts decreasing. After diffusion time, these Yb þ further sputter the neutral Yb atoms from the cathode surface resulting in an increase in negative OG voltage signal. Thus the OG signal parts (c) and (d) as shown in Fig. 6 become inverted. Fig. 7b shows OG signals profile of Ne transition at 659.89 nm (1s2-2p2) for various discharge current values. Because of very short radiative lifetime ( 1.2 ns) of the Ne level 1s2, the possibility of population inversion arises [6,26]. In this case, following the laser induced emission, the density of 1s2 level increases leading to an increase in the metastable density by electron collision induced transition within 3 s manifold. These processes produce an inverted OG signal i.e. an initial positive part followed by a negative one as shown here in Fig. 7b. Upto this, the OG signal is like a pure Ne part [8,30]. The increase in metastable density in Yb-Ne discharge plasma causes higher Penning
P. Kumar et al. / Optics Communications 313 (2014) 42–48
47
Fig. 7. Temporal evolution of OG signals of Ne transitions at (a) 650.65 nm and (b) 659.89 nm.
ionization rates and consequently Yb þ density. Thus, also in this case, the OG signal parts (c) and (d) of Fig. 6 becomes inverted (Fig. 7b). We have also recorded the OG signal profile of Ne at 667.83 nm (1s2-2p4) (not shown here) that have a similar characteristic feature. Thus, the second negative peak appearing in between 20 and 40 μs in the negative part of the OG signals at 650.65 and 659.89 nm below discharge current of 15 mA confirm that Penning ionization of Yb atoms is a significant contribution at lower discharge currents (o15 mA).
for the first time. This is in contrast to the normal OG signal at 648.9 nm (3P0-3S1). The observed inverse OG effect is attributed to the transfer of Yb population in the level 3P0 residing below the lower level 3P1 via radiative decay from the upper level 3S1 of the transition. Penning type collisional energy transfer is also observed in Yb-Ne mixture by probing Ne transitions at 626.65 (1s3-2p5), 633.442 (1s5-2p8), 650.65 (1s4-2p8) and 659.89 nm (1s2-2p2). The Penning ionization is a significant contribution to the temporal evolution of OG signals below the discharge current of 15 mA resulting in the double humped structure of the OG signals.
4. Conclusion Acknowledgments In conclusion, studies on temporal evolution of OG signals in Yb-Ne HC lamp are presented. The OG signals profile of Yb and Ne transitions are studied as a function of HC discharge current. The inverse OG effect in Yb transition at 679.9 nm (3P1-3S1) is reported
The authors duly acknowledge the copper vapor laser power supply maintenance support from Shri P.K. Agarwal and Shri K. Murali Krishnan.
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Studies on inverse optogalvanic and Penning ionization effects in ytterbium and neon transitions in Yb-Ne hollow cathode lamp P. Kumar n, V.K. Saini, G.S. Purbia, O. Prakash, S.K. Dixit, S.V. Nakhe Laser System Engineering Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India
art ic l e i nf o
a b s t r a c t
Article history: Received 29 July 2013 Received in revised form 17 September 2013 Accepted 22 September 2013 Available online 7 October 2013
This paper presents novel observations on inverse optogalvanic effect in Yb transition at 679.9 nm (3P1-3S1) in contrast with the observed normal optogalvanic effect at 648.9 nm (3P0-3S1) transition and Penning ionization in Yb-Ne mixture by probing Ne transitions at 626.65 (1s3-2p5), 633.44 (1s5-2p8), 650.65 (1s4-2p8) and 659.89 nm (1s2-2p2) in Yb-Ne hollow cathode lamp. These conclusions are derived by studying the optogalvanic signals temporal profile probed by DCM dye based narrow line-width 2 GHz, short pulse 20 ns, high repetition rate 5.0 kHz tunable dye laser, as a function of discharge current. The observed inverse optogalvanic effect is attributed to the transfer of Yb population in the level 3 P0 through radiative decay from the upper level 3S1 of the transition. This proposition is confirmed by recording the emission spectra of Yb-Ne hollow cathode lamp. The Penning ionization signature in Ne optogalvanic signals is due to the quasi-resonances between Yb and Ne energy levels. Penning signature observed in optogalvanic signal of Ne transition at 650.65 nm is unique and attributed to the increase in concentration of Ne metastable level 1s5 through radiative decay from the 2p8 level. & 2013 Elsevier B.V. All rights reserved.
Keywords: Hollow cathode lamp Optogalvanic signal Inverse OG effect Penning ionization
1. Introduction Laser optogalvanic (OG) effect has been used in wide ranges of applications such as atomic and molecular spectroscopy [1], plasma diagnostics [2,3] and Penning ionization spectroscopy [4]. The OG effect is an electrical response of the discharge medium upon absorption of resonant radiation by the species (atoms, ions or molecules) present in the gaseous discharge plasma. After irradiation of discharge plasma with a short duration ( 10 ns) resonant laser pulses, temporal evolution of the OG signal containing the information of relaxation and energy transfer processes can be studied. The hollow cathode (HC) lamp is a device generally used to observe the OG effect, in which discharge is generated within a hollow cylindrical cathode made up of the element of interest. Collisions with energetic ions, electrons, and neutral atoms within the discharge populates many excited states including the states that are optically forbidden, making the HC lamp useful for a wide range of spectroscopic investigations. Pulsed laser OG effect has been studied extensively by several authors. These studies include OG signal signs and Penning type ionizing collisions. Both these facts can be extracted from the temporal evolution of the OG signals. The first result of the pulsed OG effect has been presented by Miron et al. [5] for uranium and neon in U–Ne HC lamp. Shuker
n
Corresponding author. Tel.: þ 91 731 2442455; fax: þ91 731 2442400. E-mail address: [email protected] (P. Kumar).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.09.051
et al. [6] studied the OG effect in Ne transitions showing inverse OG signals in some neon transitions. They attributed this phenomena to the population inversion created between the upper and lower level of the transition. van de weijer et al. [7] observed the inverse OG effect in low pressure mercury discharge for the transition at 546 nm. In case of mercury, the inverse OG effect was attributed to the population transfer from the upper level (through radiative decay) to the level below the lower level of the investigated transition after the pulsed laser excitation. Piracha et al. [8] studied the pulsed OG effect in Ne transitions from 1si states in the wavelength range 614–672 nm. They also observed the inverted OG signals at 659.9 nm, 667.8 nm and 671.7 nm. Recently, the inverse OG effect is also observed by authors in Yb transition at 555.648 nm (1S0-3P1) originating from ground state [9]. This was attributed to the reduction in Penning ionization contribution upon the resonant excitation at 555.648 nm. Also the Penning ionization has been studied extensively by several researchers. Shuker et al. [4,10] first observed the Penning ionization in Ca–Ne HC lamp through Ne transitions at 614.3 nm. Ben Amar et al. [10] observed the Penning ionization by probing Ne transition at 594.4 nm in Sr3Ne HC lamp. Reddy et al. [11] observed the Penning ionization in Hg3Ar discharge lamp irradiated by excimer laser pumped pulsed dye laser. They studied the Penning ionization through Ar transitions at 451.07 nm, 459.6 nm and 462.84 nm. In Cs–Ne HC lamp, the effect of Penning ionization was observed by probing the OG signal profile of Ne at 588.19 nm [12]. Saini et al. [13,14] observed the anomalous behavior of OG signal due to Penning ionization in
P. Kumar et al. / Optics Communications 313 (2014) 42–48
miniature Ne discharge plasma. Khare et al. [15] observed the Penning ionization in Zr3Ne HC lamp by probing the OG signal profile of Ne transitions at 588.2 nm, 594.5 nm, 597.6 nm and 614.3 nm originating from the common metastable level. Ytterbium is a lanthanide that is of interest since the past few decades because of its widely ranging applications in Bose–Einstein condensation [16], atomic clocks and frequency standard [17], parity nonconservation (PNC) experiments [18], and industrial, medical and nuclear applications [19]. In most of these applications, the Yb transitions at 398.8 nm (1S0-1P1), 555.6 nm (1S0-3P1), 578.4 nm (1S0-3P0, doubly forbidden), 581.1 nm (3P1-(7/2, 3/2)2), 648.9 nm (3P0-3S1) and 679.9 nm (3P1-3S1) etc. are of importance. Thus, it is of interest to study these transitions. Here we present the studies on temporal evolution of OG signals of Yb and Ne transitions in a Yb-Ne HC discharge lamp. The OG signals are studied as a function of HC discharge current. The OG signal associated with Yb transitions at 648.9 nm (3P0-3S1) and 679.9 nm (3P1-3S1) have been studied. Also the Ne transitions at 626.65 (1s3-2p5), 633.442 (1s5-2p8), 650.65 (1s4-2p8) and 659.89 nm (1s2-2p2) excited from the four 1si states are studied. An in-house developed copper vapor laser pumped pulsed dye laser of line-width 2 GHz, pulse duration 20 ns and pulse repetition frequency 5 kHz is used to excite the transitions. We report the inverse OG effect for Yb transition at 679.9 nm in contrast to normal OG signal for the transition at 648.9 nm and Penning ionization in Yb-Ne atomic mixture.
2. Experimental details The schematic of the pulsed laser OG setup is shown in Fig. 1. It consisted of a pulsed dye laser, Yb-Ne hollow cathode lamp, wavemeter, high power DC power supply and a digital oscilloscope. The copper vapor laser pumped pulsed dye laser used to excite the transitions is an indigenously developed system producing an output radiation power 450 mW and line-width of about 2 GHz. The DCM dye dissolved in dimethyl sulphoxide (DMSO) solvent is used to cover the required wavelength range of 625–680 nm. The desired wavelengths are obtained by tuning the pulsed dye laser output radiation using a pico-motor driven tuning mirror. The Yb-Ne HC lamp is a commercial see through type lamp having length 20 mm and bore diameter 2.5 mm respectively. A high voltage DC power supply (Stanford Research Systems, INC PS310/1250V-25W) is used to take discharge in the HC lamp. A ballast resistor (R¼ 15k) is used to limit the discharge current through the lamp. The dye laser output beam is passed through the axis of the HC discharge lamp. While passing through the discharge lamp, a spherical lens having focal length of 25 cm is used to focus the dye laser beam. A beam splitter (4%) sends a part of the dye laser output beam to the high precision wave-meter used to monitor the wavelength of the dye laser.
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3. Results and discussion 3.1. OG studies of ytterbium The observed profile of the OG voltage signals for Yb transitions at 648.9 nm (3P0-3S1) and 679.9 nm (3P1-3S1) are shown in Fig. 2(a and b) for different discharge current values. It is observed that, for the transition at 648.9 nm originating from the metastable state, the OG signals (initial negative sign) are as usual. In normal pulsed OG signals, the discharge impedance decreases after the excitation of the atoms into higher level upon absorption of resonant pulsed laser radiation. This decrease is due to relatively higher ionization rate of the excited level (closer to ionization level) compared to the lower level. Thus, the negative sign of OG voltage signal can be interpreted as due to decreased impedance of the discharge medium upon absorption of resonant radiation. While in the case of the transition at 679.9 nm, inverse OG effect (positive OG voltage signal) is observed for the investigated discharge current range of 5 mA–15.0 mA. The positive OG voltage signal corresponds to the increase in discharge impedances (inverse OG effect) after the absorption of resonant radiation. The inverted OG signals were observed earlier in Ne and Hg transitions. Shuker et al. [6] attributed the inverse OG effect in Ne transitions to the population inversion created between the upper and lower level of the transition. van de weijer et al. [7] attributed the inverse OG effect in Hg discharge to the population transfer from the upper level (through radiative decay) to the level below the lower level of the investigated transition after the pulsed laser excitation. Recently, the authors also observed the inverse OG effect in Yb transition at 555.648 nm (1S0-3P1) originating from the ground state [9]. This was attributed to the reduction in Penning ionization contribution upon the resonant excitation at 555.648 nm. For the present case of Yb transition at 679.9 nm, the observed inverted OG signal may be due to the radiative transfer of the population from the upper level 3S1 of the transition to the level 3P0 below the lower level 3P1 of the transition. Fig. 3 shows the energy level diagram of Yb relevant to the present situation. Upon the resonant laser pulse absorption at 679.9 nm, Yb population from the level 3P1 transfers to the excited level 3S1 having a lifetime of about 15.9 ns [20]. The excited level 3S1 of Yb is radiatively coupled to the levels 3P0, 3P1, and 3P2. And the sign of the OG signal is decided by the effective decay rates of Yb population transfer in these levels. Fig. 4 shows the selected portion of the emission spectra of Yb-Ne HC lamp at discharge current of about 10 mA. From this figure, it is clear that the emission line at 678.9 nm is stronger than the 769.9 nm line. Now the much larger strength of the Yb emission line at 648.9 nm compared to the emission line at 769.9 nm suggests that effectively more population transferred to level 3P0 compared to 3P2. Thus, we can say that upon the absorption of resonant laser pulse, the population effectively got transferred to the level 3P0 which is below the lower level 3P1 of the transition. Since the electron impact ionization probability decreases for the levels far from and far below the ionization potential. Consequently, the discharge impedance increases resulting in the positive OG voltage signal (inverse OG effect). Such a situation is not possible for the transition at 648.9 nm originating from the 3P0 level as the 3S1 is not radiatively coupled to ground state level 1S0, which is the only level below 3P0. 3.2. Penning ionization in Yb-Ne atomic mixture The ground state electronic configuration of Ne is 1s22s22p6 S0. The excited levels of Ne can be best described by the jcK-coupling scheme [21], through a simplified set of notations, called Paschen notation often used to name these levels. According
1
Fig. 1. Schematic of the experimental setup.
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P. Kumar et al. / Optics Communications 313 (2014) 42–48
Fig. 2. OG signals of Yb transitions at (a) 648.9 nm and (b) 679.9 nm.
Fig. 3. Energy level diagram of Yb.
to these notations, the first group of excited levels is named as 1si and is subdivided into four levels namely, 1s5, 1s4, 1s3, and 1s2. Here 1s5 and 1s3 represent the two metastable levels with lifetimes of the order of seconds, while 1s4 is the semi-metastable and 1s2 is the resonance level with radiative lifetimes of 16 ns and 1.2 ns, respectively [22]. The second group of excited levels is expressed by the 2pj levels and is subdivided into 10 levels namely, for j ¼1–10. These levels are radiatively connected to at least one of the 1si levels and have typical radiative lifetimes of about 19 ns [23]. The 1si–2pj yields a total of 30 allowed radiative transitions and is a system of our interest. In an HC discharge lamp, the excited states are populated by electron collisions. In discharges, the lowest metastable state plays a cardinal role in the discharge in the sense that it constitutes an effective ground state. In Yb-Ne HC discharge lamp, a major factor contributing to the discharge current is the ionization of the Ne atoms by electron impact and collisions among the excited atoms. Normally atoms in the metastable levels tend to get ionized and contribute to the discharge current. The Ne þ ions created in the
Fig. 4. Emission spectra of Yb-Ne hollow cathode lamp.
discharge are accelerated across the large electric field of the cathode fall region and impinge on the cathode surface causing ejection (sputtering) of Yb atoms. The sputtered Yb atoms, largely ground state atoms, diffuse from cathode walls into the negative
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glow region of the discharge plasma. The electronic configuration of ground state Yb atom, [Xe] 4f146s2, gives the term 1S0 as ground state and has an ionization potential of 50,443 cm 1 (6.25 eV) [24]. The atoms in an HC discharge plasma can be ionized by four processes, viz. photo ionization, charge transfer ionization, electron impact ionization, and Penning ionization [25]. The ionization of the sputtered metal atoms in a Ne discharge is not dominated by photo ionization [25]. In the Yb-Ne hollow-cathode discharge, the crosssection for charge transfer is small because the ionization potentials of Ne and Yb are widely different; 21.56 and 6.25 eV, respectively. Thus, the important processes that ionize sputtered Yb atoms in YbNe HC discharge are electron impact ionization and Penning ionization. However, the Penning ionization plays significant role only when the involved levels are metastable and the energy difference between the participating element energy levels lies within the thermal limit. Fig. 5 shows a partial energy diagram of the relevant energy levels of Yb and Ne in the Penning ionization process. It is clear from this figure that Ne's lowest metastable level 1s5 at 13,4044 cm 1 and the excited state 2D3/2 of Yb þ are very close ( 239 cm 1) [24]. Also the next metastable state 1s3 of Ne at 13,4821 cm 1 lies very close to Yb þ excited states 2Do9/2,7/2,5/2. The energy difference between Ne metastable state 1s3 and these excited states of Yb þ are 160 cm 1, þ66 cm 1 and þ 255 cm 1 correspondingly. These Ne–Yb energy level differences are well within the thermal limit ( kT) of the HC lamp operating temperature (400–500 1C). These quasi-resonances leads to the ionization of the ground state Yb atoms to higher excited states of Yb þ via Penning type collisional energy transfer process by the metastable Ne atoms (Fig. 5). The Penning ionization process in Yb-Ne mixture is governed by the collisional energy transfer reactions as follows: Yb (1S0)þ Nen (1s5)-Yb þ n (2D3/2)þ Ne (1S0)þe þ ΔE (þ 239 cm 1) 1
n
þn
2
(1)
Do9/2) 1
)
(2)
Yb (1S0)þ Nen (1s3)-Yb þ n (2Do7/2) þNe (1S0)þ e þ ΔE ( þ66 cm 1)
(3)
Yb ( S0)þ Ne (1s3)-Yb ( þNe (1S0)þ e þ ΔE ( 160 cm
Fig. 5. Partial energy level diagram of Yb and Ne showing relevant energy levels involved in Penning ionization.
Yb (1S0)þNen (1s3)-Yb þ n (2Do5/2) þNe (1S0)þe þ ΔE (þ 255 cm 1)
45
(4)
The Penning effect introduces the extra features like double humped structure in the temporal profile of the OG signal [4,10– 15]. At lower discharge current values, the Penning ionization of Yb by meta-stable Ne atoms mainly controls the Yb þ density in the discharge plasma. These Yb þ further sputters Yb atoms from the cathode more efficiently than Ne þ because of the two reasons: first is larger mass difference between Ne þ and Yb; and the second is because of Ne þ 's higher probability for resonant charge transfer with neutral Ne atoms present in the discharge plasma at high density [26]. Now when the laser is tuned to the resonance of the Ne transitions, this Penning ionization contribution significantly reduces. Consequently, this reduction results in alteration of the properties of the discharge impedance. The altered discharge impedance reflects in the temporal evolution of the OG signal corresponding to the transition of interest. 3.2.1. Transitions originating from metastable states of Ne The states 1s5 and 1s3 of Ne are the metastable states having lifetimes of the order of seconds [22]. Since the maintenance of the discharge is closely tied to the Ne metastable density, by depleting the Ne metastable concentration, one can effectively perturb the discharge medium. Fig. 6 shows the temporal evolution of OG signal obtained for the Ne transitions at 626.65 (1s3-2p5) and 633.442 (1s5-2p8) originating from the metastable levels at 13,4044 and 13,4821 cm 1 respectively. The temporal evolution of OG signal in Yb-Ne HC lamp is recorded as a function of discharge current. Below a discharge current of 15 mA, the OG signal profile has a double hump structure in the positive OG signal part. The four parts of the observed OG signals shown in Fig. 6 are as follows. The resonant absorption of 626.65 nm (633.44 nm) laser radiation by Ne atoms at level 1s3 (1s5) leads to an increase in population of 2p5 (2p8) level. The 2p5 (2p8) level is much closer to the ionization level at 17,3932 cm 1 compared to level 1s3 (1s5). Therefore the increased population of the level 2p5 (2p8) leads to an enhanced ionization rate of Ne atoms resulting in decrease of the discharge impedance. Thus, the voltage drop across the discharge reduces. Consequently, the pulse shape of OG voltage signal is initially negative (a). After termination of the laser pulse ( 20 ns), the depopulation of level 2p5 (2p8) begins, which increases the impedance of the discharge and thus the negative OG signal decreases. The population of 2p5 (2p8) decays to many lower levels including the ground state. Thus the depopulation of the level 2p5 (2p8) does not lead to steady state value and the negative OG signal crosses zero and becomes positive (b). These two features are typical pure Ne OG signals, occurring in the first 2–25 ms [27] and are referred to as the “neon part.” The Penning effect follows the Ne part and causes two extra features in the OG signal. First, the relative decrease in the Ne metastable population results in a reduction in the rate of the Penning ionization of neutral Yb atoms and hence in the production of Yb þ . The resulting increase in neutral Yb atom density causes a reduction in OG voltage signal (c) as lower voltage can sustain the discharge. The Penning interaction also shortens the relaxation time of the Ne metastable level [4]. Secondly, after a diffusion time, the decrease in Yb þ density results in a drastic fall in the sputtering rate of Yb atoms. This reduction in Yb atom density results in the positive pulse (d). This part of the OG signal has a time scale of 30–40 ms and is referred to as the “Penning part”. Subsequently the population of different levels return to thermal equilibrium and the positive OG signal decays exponentially to zero (steady state value). At higher discharge current 415 mA, the OG signal profile is like pure Ne OG signal. This shows that the Penning ionization contribution is dominant in the lower discharge current range. The Penning type collisions are more efficient when the energy mismatch between the participating elements is lesser. However,
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Fig. 6. Temporal evolution of OG signals of Ne transitions at (a) 626.65 nm and (b) 633.44 nm.
other factors like mass ratios of the participating elements, fill gas pressure, galvanic detection circuitry also play a significant role in Penning ionization.
3.2.2. Transitions originating from non-metastable states of Ne The levels 1s4 and 1s2 of Ne are the non-metastable states having lifetimes of about 16 ns and 1.2 ns respectively [22]. Fig. 7a shows the OG signals of Ne transition at 650.65 nm (1s4-2p8) for various discharge current values. The 2p8 level of Ne has relatively high transition probability of decaying to the metastable level 1s5 compared to other non-metastable levels 1s2 and 1s4. The radiative decay of the laser excited 2p8 level increases the metastable level 1s5 concentration [28,29]. The effect of increased metastable concentration is twofold. First is that, the resulting OG signal has a larger negative part as can be seen clearly in Fig. 7a at a discharge current of 1 mA [28,29]. Secondly, the increase in metastable
concentration leads to higher Penning ionization rates and hence Yb þ density. Because of this increased Yb þ density at the cost of neutral Yb atoms, the negative OG signal starts decreasing. After diffusion time, these Yb þ further sputter the neutral Yb atoms from the cathode surface resulting in an increase in negative OG voltage signal. Thus the OG signal parts (c) and (d) as shown in Fig. 6 become inverted. Fig. 7b shows OG signals profile of Ne transition at 659.89 nm (1s2-2p2) for various discharge current values. Because of very short radiative lifetime ( 1.2 ns) of the Ne level 1s2, the possibility of population inversion arises [6,26]. In this case, following the laser induced emission, the density of 1s2 level increases leading to an increase in the metastable density by electron collision induced transition within 3 s manifold. These processes produce an inverted OG signal i.e. an initial positive part followed by a negative one as shown here in Fig. 7b. Upto this, the OG signal is like a pure Ne part [8,30]. The increase in metastable density in Yb-Ne discharge plasma causes higher Penning
P. Kumar et al. / Optics Communications 313 (2014) 42–48
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Fig. 7. Temporal evolution of OG signals of Ne transitions at (a) 650.65 nm and (b) 659.89 nm.
ionization rates and consequently Yb þ density. Thus, also in this case, the OG signal parts (c) and (d) of Fig. 6 becomes inverted (Fig. 7b). We have also recorded the OG signal profile of Ne at 667.83 nm (1s2-2p4) (not shown here) that have a similar characteristic feature. Thus, the second negative peak appearing in between 20 and 40 μs in the negative part of the OG signals at 650.65 and 659.89 nm below discharge current of 15 mA confirm that Penning ionization of Yb atoms is a significant contribution at lower discharge currents (o15 mA).
for the first time. This is in contrast to the normal OG signal at 648.9 nm (3P0-3S1). The observed inverse OG effect is attributed to the transfer of Yb population in the level 3P0 residing below the lower level 3P1 via radiative decay from the upper level 3S1 of the transition. Penning type collisional energy transfer is also observed in Yb-Ne mixture by probing Ne transitions at 626.65 (1s3-2p5), 633.442 (1s5-2p8), 650.65 (1s4-2p8) and 659.89 nm (1s2-2p2). The Penning ionization is a significant contribution to the temporal evolution of OG signals below the discharge current of 15 mA resulting in the double humped structure of the OG signals.
4. Conclusion Acknowledgments In conclusion, studies on temporal evolution of OG signals in Yb-Ne HC lamp are presented. The OG signals profile of Yb and Ne transitions are studied as a function of HC discharge current. The inverse OG effect in Yb transition at 679.9 nm (3P1-3S1) is reported
The authors duly acknowledge the copper vapor laser power supply maintenance support from Shri P.K. Agarwal and Shri K. Murali Krishnan.
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