- Email: [email protected]
Two-dimensional optogalvanic spectroscopy
Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 226–231
Contents lists available at SciVerse ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt
Two-dimensional optogalvanic spectroscopy Fariman Fathi, Mahmoud Tabrizchi n, Hossein Farrokhpour Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e in f o
abstract
Article history: Received 1 August 2011 Received in revised form 17 October 2011 Accepted 24 October 2011 Available online 6 November 2011
In this work, the time domain of optogalvanic signal is considered as an extra dimension for the analysis of the optogalvanic spectra. A time window was used to integrate over the different time regions of the temporal OG signals for each wavelength. The method enhanced the resolution of spectra considerably so that two closed transitions, which differ only by 4 pm, were precisely separated. In addition a new transition of Neon around 640 nm masked by a transition at 640.229 nm was observed and assigned using the new method. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Optogalvanic Atomic spectroscopy Laser spectroscopy Hollow-cathode lamp Neon
1. Introduction Optogalvanic (OG) effect is widely used as an effective and excellent spectroscopic method on species present in a discharge medium. This technique has a wide variety of uses such as wavelength calibration [1–3], atomic and molecular spectroscopy [4] and isotope enrichment [5]. It is relatively simple, and does not require complicated detectors, electronics and instrumentation. In fact, the impedance of the gas discharge itself is used as the detector. When a discharge is created in gas by an applied electric field, the excited electronic states of the atoms or molecules become populated to form a new steady state population distribution over all the states. When the discharge is illuminated by a laser beam with the wavelength in resonant with two excited states, the steady state distribution is changed. Consequently, the ionization rates and the impedance of the discharge are changed, which leads to a variation in the discharge current. In
n Corresponding author. Tel.: þ 98 311 3913272; fax: þ98 311 3912350. E-mail addresses: [email protected] (F. Fathi), [email protected] (M. Tabrizchi), [email protected] (H. Farrokhpour).
0022-4073/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2011.10.009
practice, a hollow-cathode lamp is used as the discharge source and the variation in the discharge current is measured while the laser wavelength is scanned. When the laser is pulsed, a sudden perturbation occurs in the discharge and results in a fluctuation in the current. If the laser pulse is shorter than the relaxation time of the discharge, the evolution of the perturbation can be followed. The time behavior of the fluctuation contains information about the states involved in the optogalvanic transition. A simplified theory of the OG signals in both cw and pulsed lasers was given by Erez et al. [6]. Several theoretical models have been developed and used for explaining the shape of the OG temporal signal [7–10]. Other researchers have proposed modified theoretical models to obtain quantitative information such as the collisional decay rate of the upper excited states, which has the main role in the shape of the OGs [11,12]. As the nature of the states involved in the transition forms the temporal shape of the optogalvanic signal, if two very close transitions overlap, the overall signal contains two sets of information corresponding to the two transitions. In fact, the signal is the sum of two separate signals with different time behaviors. The aim of this work is to discover this difference and refer that to different close transitions. In fact, the time evolution of
F. Fathi et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 226–231
the optogalvanic signals is used for further enhancement of the resolution of the OG signals without requiring additional experimental tools. It will be shown that two very close peaks, which are beyond the experimental resolution, could be well separated by putting a time window on a selected region of the temporal signal. By this technique we were able to separate two superimposed transitions at 667.828 and 667.832 nm, previously observed by Narayanan et al. using a different technique [13]. In addition a new one-photon transition of Neon around 640 nm, masked by a broadened transition at 640.229 nm, which has not been observed by previous researchers [13–15], will be reported.
227
the plasma species, the voltage and current across the hollow cathode lamp will vary. The change in the current was coupled through a 0.01 mF capacitor and then fed to a digital oscilloscope (Picoscope ADC212, UK). The variation in voltage as a function of time was recorded for each laser shot. The voltage–time graphs were averaged over 30 shots for each wavelength. Then, the averaged voltage time graphs were integrated over a fixed time window for all wavelengths. The OG spectrum was finally obtained by plotting the integrations versus the wavelengths. 3. Results and discussions 3.1. The method
2. The experimental setup The experimental setup is displayed in Fig. 1. The photon source is a tunable dye laser (Quantel TDL-90, France) pumped by an Nd:YAG-laser (Quantel model YG80, France) of 10 ns pulse width. The DCM dye was used to create a laser light in the range of 620 and 670 nm. The dye laser wavelength was scanned with a scan step of 0.002 nm using a commercial program provided by the manufacturer. The dye laser produced 100–200 mJ/pulse with the line width of 0.08 cm 1 corresponding to 3 pm at 600 nm. A wavelength meter (WS6-Highfinesse, Germany) was used to record the laser wavelength while recording optogalvanic signals. A commercial Ni–Ne hollow cathode lamp along with a home-made adjustable dc power supply (200–600 V) was used to create discharge in neon. The discharge current was controlled by a current limiting load resistor (100 kO). A ballast resistor (22 kO) was used to read the discharge current. The laser beam illuminates the plasma through the window on top of the lamp. The discharge current was adjusted between 1 and 10 mA. When the laser beam is resonantly absorbed by
Fig. 1. Experimental setup for recording the OG signals. Voltage of the point A is read by the probe of an oscilloscope.
The typical OG signal at 630.480 nm is demonstrated in Fig. 2a. Following the laser shot, a sudden increase in the discharge current is observed, which is followed by a sharp exponential decay to negative part and a second increase; then, the current smoothly reaches its initial value. The wavelength was simply scanned in order to obtain a series of different OG signals at each wavelength, i.e. a threedimensional plot, shown in Fig. 2b. The signal is integrated over certain time intervals (positive, negative or absolute overall) at each wavelength. This yields a peak, which corresponds to the transition between the two electronic states. The peak intensity and its sign depend on the time window for integration but its position is independent of the window width. It will be demonstrated that there is additional information in the time domain that is lost if only the overall integral is considered. Here, a method is proposed for disclosing the details of the transition by a systematic partial integration of the time-resolved OG signals. For example, the signal presented in Fig. 2 was partially integrated over a time window of 5 ms at each wavelength. The window was moved over the whole range of the time and the integration was repeated. The partial integrations along with overall integral are shown in Fig. 3. The overall integration was obtained from the absolute values of the temporal signal. It is clear that the overall integral as well as other plots have similar Gaussian shape with the same peak position. However, a very narrower peak is observed among the plots, shown as inset in Fig. 3. The width of this peak is almost half of that for the overall integral; hence, an increase occurred in resolution. Another type of time resolved optogalvanic signal at 614.308 nm is shown in Fig. 4a, where a change in the curvature of the negative part is observed around 50 ms. One may think of another transition close to the main transition. This can be examined by the step by step integration of this signal at each wavelength as shown in Fig. 4b. Here again, no difference is observed in the shape and position of the peak. In fact, the dependency of the additional structure on the wavelength is the same as that for other parts of the signal. This type of OG signals has been commonly observed and well described in literature [16,17] assuming several models. The additional feature is due to the involvement of other states in the relaxation process [17].
228
F. Fathi et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 226–231
Fig. 2. (a) Typical shape of time resolved optogalvanic signal at 630.480 nm and (b) 3d plot obtained from scanning the laser wavelength around the resonance of the transition.
Fig. 3. Integration of Fig. 2 over time window of 5 ms at different window positions as a function of wavelength. The inset shows overall integral in comparison with the narrowest peak obtained from the partial integrations.
A more complicated spectrum is shown in Fig. 5. This signal corresponds to two close transitions of 629.004 nm (not assigned) and 629.031 nm (2p3/2 3s 2[3/2]j ¼2–2p3/2 5s2[3/2]j ¼2). The partial integration method shown in Fig. 5 revealed that the relative intensities considerably changed. In some regions, one of the two peaks disappeared completely while, in another region, one peak changed its sign. This behavior is demonstrated in Fig. 6. The insets show the partial integrated spectra at the corresponding time delay from the laser shot. This phenomenon was previously observed by Thakur et al. [18] in which two-photon transitions were separated from onephoton optogalvanic transitions by a judicious choice of experimental parameters, such as the boxcar-gate delay. The spectrum with a long gate delay predominantly
exhibited two-photon transitions while that with a short delay only showed one-photon transitions. This method is useful for resolving two overlapping transitions in a way that one transition can be recorded in the absence of the other through choosing the appropriate region. A different type of time resolved optogalvanic signal that we recently observed is shown in Fig. 7. The signal is an inverted type as observed and assigned to a one photon transition at 667.828 nm (2p1/2 3s 2[1/2]j¼1–2p3/2 5s 2[3/ 2]j¼2) previously [12,13]. However, apart from the general shape, a sharp positive extra feature is observed at the beginning of the signal. The relative intensity of this feature depended on the wavelength. In addition, the overall shape of the signal changed considerably with the wavelength. This behavior suggests that two time-resolved optogalvanic signals corresponding to two different transitions may be superimposed on each other. This assumption can be verified by considering the symmetry of the overall integral. As Fig. 7 shows, the overall integral fitted well with two overlapped Gaussian peaks at 667.828 nm and 667.832 nm. The assumption of two overlapped transitions may be further confirmed by the partial step by step integration method (with a time window of 2 ms). The results are shown in Fig. 8. Clearly, unlike the other two mentioned cases, the partial integrations are totally different. At the first glance, the lower trace may seem to be two overlapped negative peaks. However, other trends show that there is a small positive and a large negative peak superimposed on each other. The small positive peak appeared as a shoulder on the negative traces while destroying the symmetry of the positive traces. The positions of the two opposite peaks are the same as that obtained from the multi-peak fittings in Fig. 7 (bottom). The two peaks differ by only 4 pm. Based on the calculated energy levels of neon reported on Nist Database, and by Saloman and Sansonetti [19,20]. The energy
F. Fathi et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 226–231
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Fig. 4. (a) Temporal OG signal of 614.308 nm and (b) integration over time window of 5 ms as a function of wavelength.
Fig. 5. (a) Temporal OG signal of 629.004 and 629.031 nm transitions and (b) integration over time window of 5 ms as a function of wavelength.
diagram of neon around this region is shown in Fig. 9. The second transition could be either a one photon transition from 2p1/2 3p 2[3/2]j¼2 to 2p3/2 5s 2[3/2]j¼ 2 or a two photon transition from 2p3/2 3s 2[1/2]j¼1 to 2p3/2 5s 2[3/ 2]j ¼2. There were no more single or multi photon allowed transitions around this wavelength region. Its narrow width, compared with the neighboring one photon peak, confirmed the two-photon nature of the transition [18]. In addition, the population of the 3p states was much lower than that of 3s. Hence, observing the
transition from 3p was unlikely. Furthermore, the difference between the two peaks (4 pm) matched better with the difference in energy levels. It is possible to choose the right window to mask one transition from another. This is demonstrated in the inset of Fig. 8 where two transitions were completely separated. These two closed transitions (667.828 and 667.832 nm) were in fact reported by Narayanan et al. [13]. Due to the lack of resolution both transitions were observed at 14,969 cm 1 equivalent to 668 nm. Although their
230
F. Fathi et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 226–231
Fig. 6. Variation of the intensities of two close transitions (629.004 and 629.031 nm) with the steps of partial integration. The insets show the integrated spectra at different steps of the time window.
Fig. 8. Integration over time window of 2 ms as a function of wavelength for transition at 667.828 nm. Inset shows the separation of the two transitions, by choosing the right integration window. 6–8 ms for the first peak (667.828 nm) and 2–4 ms for the second peak (667.832 nm).
Fig. 9. Energy schematic diagram of neon correspond to the two closed transitions observed in Fig. 8.
Fig. 7. (up) Temporal OG signal of 667.828 nm. (down) Integration over the whole range of the temporal OG signal (symbols). The solid blue line shows the fitted spectrum. Dots and dashed lines represent the fitted components related to 667.828 and 667.832 nm transitions, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
experimental resolution was not enough to resolve the two peaks, based on the change in the sign of the signal at higher laser power, they realized that there should be two transitions. Narayanan showed that in lower laser power the one photon transition is dominant while in higher powers the two-photon is the main transition. Another example is a new transition at 640.113 nm (2P1/2 3p 2[1/2]j¼1 to 2P1/2 5s 2[1/2]j¼ 1), which is masked by an intense transition at 640.229 nm (2P3/2 3s 2 [3/2]j¼ 2 to 2P3/2 5s 2[5/2]j ¼3). The temporal shape of the OG signal is shown in Fig. 10. The selected partial integrations are also shown in the figure. The overall
integral as well as the integration of the negative part do not show any sign of the small peak at 640.113, but it is about to appear in the positive integral. However, the small peak is very clear when partial step by step integration is used (trace d Fig. 10). In fact, the small peak at 640.113 is missed out because of the broadening of the intense 640.229 peak, and step by step integration extracts the small peak from the main broad peak. 4. Conclusion In this work, a new method was presented in order to increase the resolution of the optogalvanic spectrum without any need for improving the instrumentation. The method added an extra dimension to the analysis of the conventional optogalvanic signals through considering the partial integration of the time resolved signals, which considerably enhanced the resolution of optogalvanic spectroscopy. This method was demonstrated to be able to resolve two close transitions of neon at 667.828
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OG signal, one may focus on a certain transition in the absence of overlapping transitions.
Acknowledgments The authors are grateful to Sensor and Green Chemistry Center of Excellence, Isfahan University of Technology for their support. References
Fig. 10. (up) Temporal OG signal of 640.229 nm. (down ) OG Neon transition obtained from integration over (a) whole region, (b) the negative part, (c) the positive part and (d) selected window interval of 1 ms and a delay of 22 ms.
and 667.832 nm. The two transitions differed only by 4 pm, which were beyond the typical optogalvanic peak widths. Also using this technique we were able to resolve the Neon spectrum around 640 nm. The peak at 640.113 nm was hidden in the shoulder of its broadened neighbor peak at 640.229 nm. Even two fully overlapped transitions could also be separated if the two transitions differ in their time evolution behavior. In addition, it was demonstrated that the relative intensities of optogalvanic signals depended strongly on the integration region. Therefore, the relative intensities of optogalvanic signals were generally not reliable. However, by choosing the right time window on the temporal
[1] Dovichi NJ, Moore DS, Keller RA. Use of the optogalvanic effect and the uranium atlas for wavelength calibration of pulsed lasers. Applied Optics 1982;21:1468–73. [2] Hippler M, Pfab J. Optogalvanic spectroscopy of argon and wavelength calibration in the near-ultraviolet. Optics Communications 1993;97:347–51. [3] Zhu X, Nur AH, Misra P. Laser optogalvanic wavelength calibration with a commercial hollow cathode iron–neon discharge lamp. Journal of Quantitative Spectroscopy and Radiative Transfer 1994;52:167–77. [4] Sasi Kumar PR, Nampoori VPN. Vallabhan CPG.Photoemmission optogalvanic effect studies in N2, NO2, and Ar discharges under pulsed laser excitation. Journal of Physics D—Applied Physics 1993;26:1–3. [5] Kushner MJ. Optogalvanic isotope enrichment of Cu ions in Cu–Ne positive column discharges. Applied Optics 1983;22:1970–5. [6] Erez G, Lavi S, Miron E. A simplified theory of the optogalvanic effect. IEEE Journal of Quantum Electronics 1979;QE-15:1328–32. [7] Han XL, Wisehart V, Conner SE, Su MC, Monts DL. Collisional ionization of excited state neon in a gas discharge plasma. Contributions to Plasma Physics 1995;34:439–52. [8] Stewart RS, McKnight KW, Hamad KI. Theory and expriment for the optogalvanic effect in the neon positive column. Journal of Physics D—Applied Physics 1990;23:832–41. [9] Mahmood S, Anwar-ul-haq M, Riaz M, Baig MA. Temporal evolution and variation of the laser optogalvanic signals in the spectra of inert gases. Optics Communications 2004;236:411–7. [10] Janossy M, Gateva S, Andreeva Ch, Cartaleva S. Optogalvanic effect sign change in a hollow cathode discharge plasma. Vacuum 2000;58:272–9. [11] Mahmood S, Anwar-ul-haq M, Riaz M, Baig MA. The study of dominant physical processes in the time-resolved optogalvanic spectra of neon. European Physical Journal D 2005;36:1–9. [12] Piracha NK, Nesbett K, Marrotta S. The study of optogalvanic effect associated with the 1s states of neon. Journal of Molecular Structure 2008;881:1–10. [13] Narayanan K, Ullas G, Rai SB. One- and two-photon optogalvanic spectrum of Ne: 610–730 nm. Chemical Physics Letters 1989;156: 55–60. [14] Smyth KC, Keller RA, Grim FF. Photon-induced ionization changes in a neon discharge. Chemical Physics Letters 1978;55:473–7. [15] Smyth KC, Schenck PK. Opto-galvanic spectroscopy of a neon discharge: mechanism studies. Chemical Physics Letters 1978;55: 466–72. [16] Ben-Amar A, Erez G, Shuker R. Pulsed resonant optogalvanic effect in neon discharges. Journal of Applied Physics 1983;54:3688–98. [17] Han XL, Su MC, Haridass C, Misra P. Collisional dynamics of the first excited states of neon in the 590–670 nm region using laser optogalvanic spectroscopy. Journal of Molecular Structure 2004;695–696:155–62. [18] Thakur SN, Narayanan K. Rydberg series in the visible two-photon optogalvanic spectrum of neon. Optics Communications 1992;94: 59–65. [19] NIST database /www.physics.nist.govS, 2011. [20] Saloman EB, Sansonetti CJ. Wavelengths, energy level classifications, and energy levels for the spectrum of neutral neon. Journal of Physical and Chemical Reference Data 2004;33:1113–58.
Contents lists available at SciVerse ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt
Two-dimensional optogalvanic spectroscopy Fariman Fathi, Mahmoud Tabrizchi n, Hossein Farrokhpour Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e in f o
abstract
Article history: Received 1 August 2011 Received in revised form 17 October 2011 Accepted 24 October 2011 Available online 6 November 2011
In this work, the time domain of optogalvanic signal is considered as an extra dimension for the analysis of the optogalvanic spectra. A time window was used to integrate over the different time regions of the temporal OG signals for each wavelength. The method enhanced the resolution of spectra considerably so that two closed transitions, which differ only by 4 pm, were precisely separated. In addition a new transition of Neon around 640 nm masked by a transition at 640.229 nm was observed and assigned using the new method. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Optogalvanic Atomic spectroscopy Laser spectroscopy Hollow-cathode lamp Neon
1. Introduction Optogalvanic (OG) effect is widely used as an effective and excellent spectroscopic method on species present in a discharge medium. This technique has a wide variety of uses such as wavelength calibration [1–3], atomic and molecular spectroscopy [4] and isotope enrichment [5]. It is relatively simple, and does not require complicated detectors, electronics and instrumentation. In fact, the impedance of the gas discharge itself is used as the detector. When a discharge is created in gas by an applied electric field, the excited electronic states of the atoms or molecules become populated to form a new steady state population distribution over all the states. When the discharge is illuminated by a laser beam with the wavelength in resonant with two excited states, the steady state distribution is changed. Consequently, the ionization rates and the impedance of the discharge are changed, which leads to a variation in the discharge current. In
n Corresponding author. Tel.: þ 98 311 3913272; fax: þ98 311 3912350. E-mail addresses: [email protected] (F. Fathi), [email protected] (M. Tabrizchi), [email protected] (H. Farrokhpour).
0022-4073/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2011.10.009
practice, a hollow-cathode lamp is used as the discharge source and the variation in the discharge current is measured while the laser wavelength is scanned. When the laser is pulsed, a sudden perturbation occurs in the discharge and results in a fluctuation in the current. If the laser pulse is shorter than the relaxation time of the discharge, the evolution of the perturbation can be followed. The time behavior of the fluctuation contains information about the states involved in the optogalvanic transition. A simplified theory of the OG signals in both cw and pulsed lasers was given by Erez et al. [6]. Several theoretical models have been developed and used for explaining the shape of the OG temporal signal [7–10]. Other researchers have proposed modified theoretical models to obtain quantitative information such as the collisional decay rate of the upper excited states, which has the main role in the shape of the OGs [11,12]. As the nature of the states involved in the transition forms the temporal shape of the optogalvanic signal, if two very close transitions overlap, the overall signal contains two sets of information corresponding to the two transitions. In fact, the signal is the sum of two separate signals with different time behaviors. The aim of this work is to discover this difference and refer that to different close transitions. In fact, the time evolution of
F. Fathi et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 226–231
the optogalvanic signals is used for further enhancement of the resolution of the OG signals without requiring additional experimental tools. It will be shown that two very close peaks, which are beyond the experimental resolution, could be well separated by putting a time window on a selected region of the temporal signal. By this technique we were able to separate two superimposed transitions at 667.828 and 667.832 nm, previously observed by Narayanan et al. using a different technique [13]. In addition a new one-photon transition of Neon around 640 nm, masked by a broadened transition at 640.229 nm, which has not been observed by previous researchers [13–15], will be reported.
227
the plasma species, the voltage and current across the hollow cathode lamp will vary. The change in the current was coupled through a 0.01 mF capacitor and then fed to a digital oscilloscope (Picoscope ADC212, UK). The variation in voltage as a function of time was recorded for each laser shot. The voltage–time graphs were averaged over 30 shots for each wavelength. Then, the averaged voltage time graphs were integrated over a fixed time window for all wavelengths. The OG spectrum was finally obtained by plotting the integrations versus the wavelengths. 3. Results and discussions 3.1. The method
2. The experimental setup The experimental setup is displayed in Fig. 1. The photon source is a tunable dye laser (Quantel TDL-90, France) pumped by an Nd:YAG-laser (Quantel model YG80, France) of 10 ns pulse width. The DCM dye was used to create a laser light in the range of 620 and 670 nm. The dye laser wavelength was scanned with a scan step of 0.002 nm using a commercial program provided by the manufacturer. The dye laser produced 100–200 mJ/pulse with the line width of 0.08 cm 1 corresponding to 3 pm at 600 nm. A wavelength meter (WS6-Highfinesse, Germany) was used to record the laser wavelength while recording optogalvanic signals. A commercial Ni–Ne hollow cathode lamp along with a home-made adjustable dc power supply (200–600 V) was used to create discharge in neon. The discharge current was controlled by a current limiting load resistor (100 kO). A ballast resistor (22 kO) was used to read the discharge current. The laser beam illuminates the plasma through the window on top of the lamp. The discharge current was adjusted between 1 and 10 mA. When the laser beam is resonantly absorbed by
Fig. 1. Experimental setup for recording the OG signals. Voltage of the point A is read by the probe of an oscilloscope.
The typical OG signal at 630.480 nm is demonstrated in Fig. 2a. Following the laser shot, a sudden increase in the discharge current is observed, which is followed by a sharp exponential decay to negative part and a second increase; then, the current smoothly reaches its initial value. The wavelength was simply scanned in order to obtain a series of different OG signals at each wavelength, i.e. a threedimensional plot, shown in Fig. 2b. The signal is integrated over certain time intervals (positive, negative or absolute overall) at each wavelength. This yields a peak, which corresponds to the transition between the two electronic states. The peak intensity and its sign depend on the time window for integration but its position is independent of the window width. It will be demonstrated that there is additional information in the time domain that is lost if only the overall integral is considered. Here, a method is proposed for disclosing the details of the transition by a systematic partial integration of the time-resolved OG signals. For example, the signal presented in Fig. 2 was partially integrated over a time window of 5 ms at each wavelength. The window was moved over the whole range of the time and the integration was repeated. The partial integrations along with overall integral are shown in Fig. 3. The overall integration was obtained from the absolute values of the temporal signal. It is clear that the overall integral as well as other plots have similar Gaussian shape with the same peak position. However, a very narrower peak is observed among the plots, shown as inset in Fig. 3. The width of this peak is almost half of that for the overall integral; hence, an increase occurred in resolution. Another type of time resolved optogalvanic signal at 614.308 nm is shown in Fig. 4a, where a change in the curvature of the negative part is observed around 50 ms. One may think of another transition close to the main transition. This can be examined by the step by step integration of this signal at each wavelength as shown in Fig. 4b. Here again, no difference is observed in the shape and position of the peak. In fact, the dependency of the additional structure on the wavelength is the same as that for other parts of the signal. This type of OG signals has been commonly observed and well described in literature [16,17] assuming several models. The additional feature is due to the involvement of other states in the relaxation process [17].
228
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Fig. 2. (a) Typical shape of time resolved optogalvanic signal at 630.480 nm and (b) 3d plot obtained from scanning the laser wavelength around the resonance of the transition.
Fig. 3. Integration of Fig. 2 over time window of 5 ms at different window positions as a function of wavelength. The inset shows overall integral in comparison with the narrowest peak obtained from the partial integrations.
A more complicated spectrum is shown in Fig. 5. This signal corresponds to two close transitions of 629.004 nm (not assigned) and 629.031 nm (2p3/2 3s 2[3/2]j ¼2–2p3/2 5s2[3/2]j ¼2). The partial integration method shown in Fig. 5 revealed that the relative intensities considerably changed. In some regions, one of the two peaks disappeared completely while, in another region, one peak changed its sign. This behavior is demonstrated in Fig. 6. The insets show the partial integrated spectra at the corresponding time delay from the laser shot. This phenomenon was previously observed by Thakur et al. [18] in which two-photon transitions were separated from onephoton optogalvanic transitions by a judicious choice of experimental parameters, such as the boxcar-gate delay. The spectrum with a long gate delay predominantly
exhibited two-photon transitions while that with a short delay only showed one-photon transitions. This method is useful for resolving two overlapping transitions in a way that one transition can be recorded in the absence of the other through choosing the appropriate region. A different type of time resolved optogalvanic signal that we recently observed is shown in Fig. 7. The signal is an inverted type as observed and assigned to a one photon transition at 667.828 nm (2p1/2 3s 2[1/2]j¼1–2p3/2 5s 2[3/ 2]j¼2) previously [12,13]. However, apart from the general shape, a sharp positive extra feature is observed at the beginning of the signal. The relative intensity of this feature depended on the wavelength. In addition, the overall shape of the signal changed considerably with the wavelength. This behavior suggests that two time-resolved optogalvanic signals corresponding to two different transitions may be superimposed on each other. This assumption can be verified by considering the symmetry of the overall integral. As Fig. 7 shows, the overall integral fitted well with two overlapped Gaussian peaks at 667.828 nm and 667.832 nm. The assumption of two overlapped transitions may be further confirmed by the partial step by step integration method (with a time window of 2 ms). The results are shown in Fig. 8. Clearly, unlike the other two mentioned cases, the partial integrations are totally different. At the first glance, the lower trace may seem to be two overlapped negative peaks. However, other trends show that there is a small positive and a large negative peak superimposed on each other. The small positive peak appeared as a shoulder on the negative traces while destroying the symmetry of the positive traces. The positions of the two opposite peaks are the same as that obtained from the multi-peak fittings in Fig. 7 (bottom). The two peaks differ by only 4 pm. Based on the calculated energy levels of neon reported on Nist Database, and by Saloman and Sansonetti [19,20]. The energy
F. Fathi et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 226–231
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Fig. 4. (a) Temporal OG signal of 614.308 nm and (b) integration over time window of 5 ms as a function of wavelength.
Fig. 5. (a) Temporal OG signal of 629.004 and 629.031 nm transitions and (b) integration over time window of 5 ms as a function of wavelength.
diagram of neon around this region is shown in Fig. 9. The second transition could be either a one photon transition from 2p1/2 3p 2[3/2]j¼2 to 2p3/2 5s 2[3/2]j¼ 2 or a two photon transition from 2p3/2 3s 2[1/2]j¼1 to 2p3/2 5s 2[3/ 2]j ¼2. There were no more single or multi photon allowed transitions around this wavelength region. Its narrow width, compared with the neighboring one photon peak, confirmed the two-photon nature of the transition [18]. In addition, the population of the 3p states was much lower than that of 3s. Hence, observing the
transition from 3p was unlikely. Furthermore, the difference between the two peaks (4 pm) matched better with the difference in energy levels. It is possible to choose the right window to mask one transition from another. This is demonstrated in the inset of Fig. 8 where two transitions were completely separated. These two closed transitions (667.828 and 667.832 nm) were in fact reported by Narayanan et al. [13]. Due to the lack of resolution both transitions were observed at 14,969 cm 1 equivalent to 668 nm. Although their
230
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Fig. 6. Variation of the intensities of two close transitions (629.004 and 629.031 nm) with the steps of partial integration. The insets show the integrated spectra at different steps of the time window.
Fig. 8. Integration over time window of 2 ms as a function of wavelength for transition at 667.828 nm. Inset shows the separation of the two transitions, by choosing the right integration window. 6–8 ms for the first peak (667.828 nm) and 2–4 ms for the second peak (667.832 nm).
Fig. 9. Energy schematic diagram of neon correspond to the two closed transitions observed in Fig. 8.
Fig. 7. (up) Temporal OG signal of 667.828 nm. (down) Integration over the whole range of the temporal OG signal (symbols). The solid blue line shows the fitted spectrum. Dots and dashed lines represent the fitted components related to 667.828 and 667.832 nm transitions, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
experimental resolution was not enough to resolve the two peaks, based on the change in the sign of the signal at higher laser power, they realized that there should be two transitions. Narayanan showed that in lower laser power the one photon transition is dominant while in higher powers the two-photon is the main transition. Another example is a new transition at 640.113 nm (2P1/2 3p 2[1/2]j¼1 to 2P1/2 5s 2[1/2]j¼ 1), which is masked by an intense transition at 640.229 nm (2P3/2 3s 2 [3/2]j¼ 2 to 2P3/2 5s 2[5/2]j ¼3). The temporal shape of the OG signal is shown in Fig. 10. The selected partial integrations are also shown in the figure. The overall
integral as well as the integration of the negative part do not show any sign of the small peak at 640.113, but it is about to appear in the positive integral. However, the small peak is very clear when partial step by step integration is used (trace d Fig. 10). In fact, the small peak at 640.113 is missed out because of the broadening of the intense 640.229 peak, and step by step integration extracts the small peak from the main broad peak. 4. Conclusion In this work, a new method was presented in order to increase the resolution of the optogalvanic spectrum without any need for improving the instrumentation. The method added an extra dimension to the analysis of the conventional optogalvanic signals through considering the partial integration of the time resolved signals, which considerably enhanced the resolution of optogalvanic spectroscopy. This method was demonstrated to be able to resolve two close transitions of neon at 667.828
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OG signal, one may focus on a certain transition in the absence of overlapping transitions.
Acknowledgments The authors are grateful to Sensor and Green Chemistry Center of Excellence, Isfahan University of Technology for their support. References
Fig. 10. (up) Temporal OG signal of 640.229 nm. (down ) OG Neon transition obtained from integration over (a) whole region, (b) the negative part, (c) the positive part and (d) selected window interval of 1 ms and a delay of 22 ms.
and 667.832 nm. The two transitions differed only by 4 pm, which were beyond the typical optogalvanic peak widths. Also using this technique we were able to resolve the Neon spectrum around 640 nm. The peak at 640.113 nm was hidden in the shoulder of its broadened neighbor peak at 640.229 nm. Even two fully overlapped transitions could also be separated if the two transitions differ in their time evolution behavior. In addition, it was demonstrated that the relative intensities of optogalvanic signals depended strongly on the integration region. Therefore, the relative intensities of optogalvanic signals were generally not reliable. However, by choosing the right time window on the temporal
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