Studies on the Optogalvanic Effect and Isotope-Selective Excitation of Ytterbium in a Hollow Cathode Discharge Lamp Using a Pulsed Dye Laser

Abstract

This paper presents studies on the pulsed optogalvanic effect and isotope-selective excitation of Yb 555.648 nm (0 cm⁻¹ → 17 992.007 cm⁻¹) and 581.067 nm (17 992.007 cm⁻¹ → 35 196.98 cm⁻¹) transitions, in a Yb/Ne hollow cathode lamp. The Yb atoms were excited by narrow linewidth (500–1000 MHz) Rh110 and Rh6G dye based pulsed lasers. Optogalvanic signal inversion for ground state transition at 555.648 nm was observed beyond a hollow cathode discharge current of 8.5 mA, in contrast to normal optogalvanic signal at 581.067 nm up to maximum current of 14 mA. The isotope-selective excitation studies of Yb were carried out by recording Doppler limited optogalvanic signals as a function of dye laser wavelength. For the 581.067 nm transition, three even isotopes, ¹⁷²Yb, ¹⁷⁴Yb, and ¹⁷⁶Yb, and one odd isotope, ¹⁷¹Yb, were clearly resolved. These data were compared with selective isotope excitation by 10 MHz linewidth continuous-wave dye laser. For 555.648 nm transition, isotopes were not clearly resolved, although isotope peaks of low modulation were observed.

Keywords

Hollow cathode lamp Dynamic optogalvanic signal Isotope-selective excitation

INTRODUCTION

Hollow cathode (HC) discharges have been widely used as spectroscopic sources in a wide range of applications, including high-resolution atomic and molecular spectroscopy,¹ frequency stabilization,² isotopic analysis,³ plasma diagnostics,⁴ wavelength calibration,⁵ and Penning ionization.⁶ It is the optogalvanic (OG) effect that has made the HC discharge lamps useful in these wide ranges of applications. The OG effect is an electrical response of a discharge medium upon the absorption of resonant laser radiation by the species present in the discharge medium. In an HC discharge lamp, two types of OG signals can be detected, depending upon the type of experimental arrangement. When the HC discharge plasma is irradiated by a short-duration (tens of nanoseconds) resonant pulse, the dynamic OG signal (time resolved) can be studied. Generally, it gives information about relaxation processes occurring in the discharge plasma after the absorption of the laser pulse. For this reason, the dynamic OG signal can serve as an explicit marker for plasma diagnostics. In contrast, a time-integrated, averaged-amplitude OG signal can be obtained in pulsed laser OG setup using a boxcar averager. An averaged-amplitude OG signal also can be obtained with a lock-in amplifier system when a chopped continuous-wave (CW) laser light is used. Both of these OG responses have their own advantages in spectroscopic applications. Ytterbium is a lanthanide widely used in experiments involving Bose–Einstein condensation;⁷ atomic clocks and frequency standards;⁸ and industrial, medical, and nuclear applications.⁹ It has seven isotopes, ¹⁶⁸Yb, ¹⁷⁰Yb, ¹⁷¹Yb, ¹⁷²Yb, ¹⁷³Yb, ¹⁷⁴Yb, and ¹⁷⁶Yb, with the abundances 0.135, 3.03, 14.31, 21.82, 16.13, 31.84, and 12.73%, respectively. Out of these isotopes, ¹⁶⁸Yb and ¹⁷⁶Yb are of particular interest because of their important applications in nuclear and medical science. These applications have led to considerable interest in spectroscopic studies and isotope separation of Yb.⁹ Several species (atoms, ions, or molecules) have been studied in HC discharge lamps using both pulsed and CW dye lasers.^10–13 The first results of the pulsed OG effect were presented by Miron et al.¹¹ for U and Ne in U/Ne HC lamp. Shuker et al.¹² studied the OG effect in Ne transitions showing inverse OG signals in some Ne transitions. They attributed this phenomenon to the population inversion created between the upper and lower level of the transition. Van De Weijer et al.¹³ observed the opposite OG effect in low-pressure Hg discharge for the transition at 546 nm. In case of Hg, 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. Furthermore, using the CW laser, Jung et al.¹⁴ studied the isotopic shift studies of Yb in a Yb/Ar HC lamp at 581.067 nm. Several high-resolution spectra of Yb transitions have been measured in collimated atomic beams and by using a single-mode CW laser.^15,16 However, for efficient isotope separation processes, pulsed lasers are most commonly used. To the best of our knowledge, for Yb, no OG studies using pulsed dye lasers are available in the literature, although successful laser isotope separation has been demonstrated in Yb atomic vapor source using pulsed dye lasers.^9,17 Hence, there is a general interest for studying the pulsed OG effect and isotope-selective excitations in Yb using a pulsed dye laser in a HC lamp.

This paper presents the first time studies on pulsed OG effect and isotope-selective excitation of Yb 555.648 nm (0 cm⁻¹ → 17 992.007 cm⁻¹) and 581.067 nm (17 992.007 cm⁻¹ → 35 196.98 cm⁻¹) transitions, in a Yb/Ne HC lamp. The Yb atoms were excited by narrow linewidth (500–1000 MHz) Rh110 and Rh6G based pulsed dye lasers. The OG signal inversion was observed beyond certain discharge current value for ground state transition at 555.648 nm in contrast to normal OG signal at 581.067 nm. The isotope-selective excitation studies of Yb were carried out by recording Doppler-limited OG signals as a function of dye laser wavelength for both the 555.648 and 581.067 nm transitions. These data were compared with selective isotope excitation by 10 MHz linewidth single mode CW dye laser.

EXPERIMENTAL

The OG studies of Yb are done inside an HC lamp using both pulsed and CW dye laser systems. The OG experimental setup using a pulsed dye laser is shown in Fig. 1. it consisted of an indigenously developed pulsed dye laser system, Yb/Ne HC lamp, boxcar averager, photodiode, wave-meter, oscilloscope, and personal computer. The pulsed dye laser system used was a copper vapor laser pumped dye laser comprising double grating and a prism beam expander for narrow band operation (500– 1000 MHz), producing an average output power of 50–100 mW. The copper vapor laser system used to pump the dye laser oscillator also was a home-made laser system producing an average output power of 8 W (510.6 nm) operating at a pulse repetition frequency of 6 kHz. Tuning of the dye laser system was done through a picomotor-driven tuning mirror. For accessing the required laser wavelength around 555 and 581 nm, Rh110 and Rh6G laser dyes of 1 mM concentration in ethanol were used.

Fig. 1.

Schematic of the experimental setup.

The Yb/Ne HC lamp (Heraeus, 3QQNY/Yb) used was a commercial see-through HC lamp based on direct current (DC) discharge having a length of 20 mm and a bore diameter of 2.5 mm, drawing a maximum current of 15 mA. The Ne buffer gas in the HC lamp was filled at a pressure of about 6.6 Torr. The dye laser wavelength was monitored by a high-precision wave-meter (WS7, HighFinesse, Tübingen, Germany). A boxcar averager (SR280, Stanford Research Systems, Sunnyvale, CA) system was used to obtain the gated time integrated OG voltage signal. The Yb atomic vapors were produced in Ne gas discharge by sputtering. The dye laser output beam was directed axially through the bore of the hollow cathode lamp. While passing through the HC lamp, a spherical lens of focal length 25 cm was used to focus the dye laser beam. A small part of the dye laser beam was fed into the wavelength meter to measure the wavelength. To record the OG spectra, the pulsed dye laser was tuned near the resonance of the transitions and then scanned very smoothly up to 20 GHz. The dynamic OG voltage signal was recorded across the HC lamp using a capacitor (C = 100 pF) coupled to the digital oscilloscope (1 GHz, WaveRunner 610Zi, Teledyne LeCroy, Chestnut Ridge, NY). A high-voltage DC power supply (PS310/1250V-25W, Stanford Research Systems) was used to take discharge in the HC lamp. A ballast resistor (R = 15 kΩ) was used to limit the discharge current. The OG spectra was recorded using a data acquisition card (NI DAC, USB-4716, National Instruments, Austin, TX) and indigenously developed LabVIEW program recording simultaneously the wavelength meter reading and boxcar averager output signal.

The CW dye laser system used to verify the isotopic shift results was a commercial Nd: YAG laser pumped ring dye laser (Radiant Dyes Laser & Accessories GmbH, Wermel-skirchen, Germany) producing narrow linewidth (10 MHz) output radiation power of more than 100 mW.

RESULTS AND DISCUSSION

Inverse OG Effect in Yb Transition. Figures 2 and 3 shows the observed dynamic profile of the OG voltage signals for Yb transitions at 555.648 and 581.067 nm for different values of discharge current. It was observed that for the transition at 555.648 nm, for the HC discharge current less than 8.5 mA, the dynamic OG signals were as normally observed. However, for current exceeding 8.5 mA, an additional positive peak appears at the initial point for higher current values as shown in Figs. 2b, 2c, and 2d. The inverse (anomalous) OG effects have been observed previously in Ne and Hg transitions. Shuker et al.¹² studied the inverse OG effect in Ne transitions and attributed this phenomenon to the population inversion created between the upper and lower level of the transition. Van De Weijer et al.¹³ observed the opposite OG effect in Hg discharge. They attributed this inverse OG effect 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. However, all of the above-mentioned inverse OG effects were seen while exploring the optical excitation studies in higher-lying energy levels of the element under investigation. For the present case of Yb, this inverse OG effect is observed in lowest-lying ground state to first excited-state transition (0 cm⁻¹ → 17 992.007 cm⁻¹). This result was unexpected. In normal pulsed OG signals, the discharge impedance decreases with the excitation of the atoms into higher level upon absorption of resonant pulsed laser radiation. This decrease is attributed to the ionization rate of the excited state (closer to ionization level) usually being higher than that of 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. The positive OG voltage signal corresponds to the increase in discharge impedances (inverse OG effect) after the absorption of resonant radiation. Previously reported inverse OG effects in Ne and Hg transitions because of inverted population and population transfer mechanisms could not happen for Yb, as the lower laser level is the ground level. Therefore, both these explanations are not applicable for Yb. The observed inverse OG effect for the transition at 555.648 nm may be explained as follows. Figure 4 shows the relevant energy levels of Yb and Ne. It is clear from this figure that the Ne lowest metastable level ³P₂ at 134 044 cm⁻¹ and the excited state ²D_3/2 of Yb⁺ at 134 283 cm⁻¹ are very close (∼239 cm⁻¹).¹⁸ Also, the next metastable state ³P₀ (∼134 821 cm⁻¹) of Ne lies very close to the Yb+ excited states ²D°_9/2, ²D°_7/2, and ²D°_5/2. The energy differences between Ne metastable state ³P₀ and these excited states of Yb⁺ are −160, +66, and +255 cm⁻¹, respectively. These Ne–Yb energy level differences are well within the thermal limit (∼kT) of the HC lamp operating temperature (400–500 °C). These quasiresonances lead to the ionization of the ground state Yb atoms to higher excited states of Yb⁺ via the Penning collision process by the metastable Ne atoms (Fig. 4). The Penning ionization process governed by the collision energy transfer reaction as follows:

Fig. 2.

Dynamic OG signal for the transition at 555.648 nm for HC discharge current of (a) 7.26 mA, (b) 9.09 mA, (c) 10.32 mA, and (d) 13.42 mA.

Fig. 3.

Dynamic OG signal for the transition at 581.067 nm for HC discharge current of (a) 5.18 mA, (b) 9.43 mA, (c) 11.90 mA, and (d) 13.80 mA.

Fig. 4.

Partial energy level diagram of Yb and Ne showing relevant energy levels involved in Penning ionization.

\begin{array}{l} Yb (^{1} S_{0}) + {Ne}^{*} (^{3} P_{2}) & \to & {Yb}^{+ *} (^{2} D_{3 / 2}) + Ne (^{1} S_{0}) + e^{-} \\ + Δ E (+ 239 {cm}^{- 1}) \end{array}

(1)

\begin{array}{l} Yb (^{1} S_{0}) + {Ne}^{*} (^{3} P_{0}) & \to & {Yb}^{+ *} (^{2} D_{9 / 2}^{o}) + Ne (^{1} S_{0}) + e^{-} \\ + Δ E (+ 160 {cm}^{- 1}) \end{array}

(2)

\begin{array}{l} Yb (^{1} S_{0}) + {Ne}^{*} (^{3} P_{0}) & \to & {Yb}^{+ *} (^{2} D_{7 / 2}^{o}) + Ne (^{1} S_{0}) + e^{-} \\ + Δ E (+ 66 {cm}^{- 1}) \end{array}

(3)

\begin{array}{l} Yb (^{1} S_{0}) + {Ne}^{*} (^{3} P_{0}) & \to & {Yb}^{+ *} (^{2} D_{5 / 2}^{o}) + Ne (^{1} S_{0}) + e^{-} \\ + Δ E (+ 255 {cm}^{- 1}) \end{array}

(4)

It also is known that Penning-type collisions in a metal-Ne HC lamp are very dominant, however only below a certain value of discharge current.¹⁹ It seems, in the present study, for the discharge current <8.5 mA, the usual behavior of decrease in discharge impedance is due to increase in Yb⁺ density as a cumulative effect of Penning ionization of ground state Yb atoms, as well as the usual electron impact ionization of laser-excited Yb atoms. However, beyond a discharge current of 8.5 mA, probably due to significant reduction of Penning effect compared with the increase in electron impact, ionization leads to severely reduced Yb⁺ densities. This reduction would lead to a significant increase in discharge impedance leading to a positive OG signal (Fig. 2). This inverse OG process continues for the investigated discharge current range up to 13.42 mA, as the reduced Penning process dominates over any increase in plasma conductivity due to usual electron impact ionization of laser excited Yb atoms in the excited state ³P₁. It is worth mentioning here that the detailed explanation for the inverse OG signal demands a study that goes beyond the scope of our present work.

In contrast, the normal dynamic OG voltage signals for Yb transition at 581.067 nm for different values of discharge current up to a maximum of 14 mA were observed (Fig. 3). It is because no such effect (Penning ionization) happens in this case. The closest level to the metastable excited state (³P₂) of Ne atoms is the excited state (²F°_9/2) of Yb⁺ with respect to the excited state ³P₁ (17 992 cm⁻¹) of Yb atoms. The energy difference between Yb⁺ excited state and Ne metastable state

³P₂ is 4506 cm⁻¹, a value that is much beyond the thermal limit.¹⁸ Also, the lower level (³P₁) of investigated Yb transition at 581.067 nm is not a metastable state. Thus, the Penning type collisions could not happen in this case. Therefore, after the laser pulse absorption by the lower level (³P₁) of Yb atoms, the Yb population got transferred to the higher excited state at 35 196.98 cm⁻¹, in turn leading to increased ionization rate due to electron impact ionization. Correspondingly, the discharge impedance decreases as expected. Hence, normal OG signal is observed in this case.

Selective Excitation of Yb Isotopes. Figure 5 shows the recorded OG spectra at 581.067 nm transition using a pulsed dye laser. The OG spectra was recorded at a discharge current of about 12 mA and 50 mW average input power of the pulsed dye laser. The pulsed dye laser system was tuned through picomotor driving unit. The dye laser linewidth was 800 MHz, with frequency stability within 300 MHz. Five peaks are clearly resolved in the Doppler-limited spectrum of Yb. Of these peaks, three peaks (from left to right in Fig. 5) can be clearly assigned to the even isotopes ¹⁷⁶Yb, ¹⁷⁴Yb, and ¹⁷²Yb, respectively, whereas the remaining two peaks correspond to hyperfine components of the isotope ¹⁷¹Yb. These isotope assignments are done based on relative isotopic shifts reported in the literature.²⁰ However, the heights of the peaks do not reflect the correct abundance ratio of the corresponding isotopes because of the hyperfine structure of the odd isotopes. This study shows the possibility that the approximate isotope shifts of the second step transition of Yb, originating from highly excited state, can be readily obtained from OG spectroscopy.

Fig. 5.

Optogalvanic spectra of Yb recorded at 581.067 nm using pulsed dye laser of linewidth 800 MHz.

The OG spectra of Yb at 555.648 nm using pulsed dye laser are shown in Figs. 6a and 6b for two different laser linewidths. In this case, the OG spectra were recorded at a discharge current of about 5 mA and 50 mW average input power of the pulsed dye laser. For the laser linewidth (900 MHz), no clear isotopic resolution was observed (Fig. 6a) because of smaller isotopic shifts compared with the transition at 581.067 nm and comparable isotopic shifts with the Doppler width of the transition at 555.648 nm. For the reduced dye laser linewidth (500 MHz), at this Yb transition, three peaks appear, although not clearly resolved (Fig. 6b). Out of these peaks, only the middle peak can be loosely assigned to a particular isotope ¹⁷⁴Yb, whereas the others cannot be assigned to a single isotope as these peaks are due to mixed signal of hyperfine components of ¹⁷¹Yb and ¹⁷³Yb. However, in the reported literature, well-resolved spectra of Yb isotopes using atomic beam fluorescence spectroscopy by an ultra narrow linewidth (<10 MHz) CW dye laser are available.^21,22

Fig. 6.

Optogalvanic spectra of Yb recoded at 555.648 nm using pulsed dye laser of linewidth (a) 900 MHz and (b) 500 MHz.

Figure 7 shows the typical OG spectra obtained for the excited state transition of Yb at 581.067 nm using single-mode CW ring dye laser. All peaks obtained using pulsed dye laser OG spectroscopy are reproduced using a CW dye laser. The even mass number isotopes ¹⁷⁶Yb, ¹⁷⁴Yb, and ¹⁷²Yb can be clearly assigned to the three peaks out of five, and the remaining two peaks correspond to the hyperfine components of ¹⁷¹Yb. Table I summarizes the relative isotopic shifts observed in the present study for Yb transition at 581.067 nm using both pulsed and CW dye laser OG spectroscopy. The isotopic shifts were obtained by measuring the peak-to-peak separations. These values also are compared with the reported standard values in the literature.^20,22 The isotopic shifts estimated from pulsed laser measurements differ from the standard results. This finding is as expected as the isotopic peaks in the observed experimental OG spectrum are not very sharp due to the effect of pulsed laser linewidth and Doppler broadening of Yb atomic transition. The typical errors in estimating the peak location and hence estimation of isotopic shifts are within ±200 MHz.

TABLE I.

Relative isotopic shifts of Yb transition at 581.067 nm.

Isotope	Observed relative shifts using pulsed dye laser (MHz)	Observed relative shifts using CW dye laser (MHz)	Reported^20,22 relative isotopic shifts (MHz)
¹⁷⁶Yb–¹⁷⁴Yb	+2399	+2575	+2578
¹⁷⁴Yb–¹⁷⁴Yb	0	0	0
¹⁷²Yb–¹⁷⁴Yb	−2665	– 2752	– 2711
¹⁷¹Yb (3/2 → 5/2)–¹⁷⁴Yb	−5775	– 6038	−5973
¹⁷¹Yb (3/2 → 3/2)–¹⁷⁴Yb	−8440	−8879	−8805

Fig. 7.

Optogalvanic spectra of Yb recorded at 581.067 nm using CW dye laser of linewidth about 10 MHz.

Finally, as Doppler broadening plays an important role in limiting the isotopic resolution in the obtained spectra using pulsed dye laser, it is of interest to calculate the Doppler width of the transitions. The Doppler width observed was 622 MHz, estimated through the observed line-profile of the ¹⁷¹Yb (Fig. 6). This particular isotope was chosen for the Doppler width estimation as it was the only isolated isotope in the recorded OG spectra.

CONCLUSIONS

This paper presents studies on pulsed OG effect and isotope-selective excitation of Yb 555.648 nm (0 cm⁻¹ → 17 992.007 cm⁻¹) and 581.067 nm (17 992.007 cm⁻¹ → 35 196.98 cm⁻¹) transitions in a Yb/Ne HC lamp. The Yb atoms were excited by a narrow linewidth (500–1000 MHz) Rh110 and Rh6G based pulsed dye lasers. Inverse OG voltage signal for ground state transition at 555.648 nm was observed beyond a HC discharge current of 8.5 mA, in contrast to normal OG voltage signal at 581.067 nm up to a maximum current of 14 mA. The isotope-selective excitation studies of Yb were carried out by recording Doppler-limited, boxcar-integrated OG signals as a function of dye laser wavelength. For the 581.067 nm transition, the three even isotopes ¹⁷²Yb, ¹⁷⁴Yb, and ¹⁷⁶Yb and the odd isotope ¹⁷¹Yb were clearly resolved. These data were compared with selective isotope excitation studies by 10 MHz linewidth CW dye laser. For the transition at 555.648 nm, isotopes were not clearly resolved, although isotope peaks of low modulation are observed.

Footnotes

ACKNOWLEDGMENTS

We acknowledge the technical support from Shri G.S. Purbia during the experiment. The software support from Smt. A. Mokhariwale and Shri S.K. Agarwal are duly acknowledged.

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