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Fourier transform optogalvanic spectroscopy
Volume 215, number 1,2,3
CHEMICAL PHYSICS LETTERS
26 November 1993
Fourier transform optogalvanic spectroscopy Jacob Baker ‘, V.C. Gibbons and Peter A. Hamilton Departmentof Chemistry,Queen Mary and WestfieldCollege,Mile End Road, London El 4NS UK Received 27 August 1993
High-resolution optogalvanic spectra have been observed for the first time using a Fourier transform spectrometer. Spectra from neon, argon, krypton, and xenon were observed over the range 5000 to 18000 cm-’ using a modified commercial hollow cathode lamp. Signal-to-noise ratios of several hundred to one were readily achieved on the strongest transitions but in these initial experiments the sensitivity is still substantially below that of a laser experiment. If the sensitivity can be improved by changes in lamp design then the very wide coverage scans and the ability to operate in the infrared and near-infrared regions will make this an important new spectroscopic method.
1. Introduction Optogalvanic spectra are observed by measuring the change in the impedance of a gas discharge, caused by the absorption of light, as a function of wavelength. The optogalvanic effect was first observed in 1928 using resonance lamps as intense line sources [ 111, which have high spectral brightness, to perturb the equilibrium processes occurring in the discharge. However, it was not until the introduction of the dye laser, which is tunable and also has high spectroscopic brightness, that optogalvanic spectroscopy became a practical spectroscopic method. Laser optogalvanic (LOG) spectra of both atoms and small molecules have subsequently been observed with good sensitivity in most spectral regions and in a wide variety of discharges [2,3]. The technique is most sensitive to those atomic and molecular states which couple most strongly to the ionization processes, or which effect the translational energy of the free electrons [ 4,5 ] and so it is often used for high-lying Rydberg and metastable states. In addition, for molecules, fragmentation processes will also lead to impedance changes in the discharge [ 61. While laser spectroscopic techniques demonstrate unsurpassed sensitivity and resolution they are dif’ Present address: DAMAP et URA 812 du CNRS, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France.
licult to scan over extended regions and require careful wavelength calibration. Modern Fourier transform (FT) instruments are now capable of achieving near-Doppler limited resolution from infrared to W wavelengths and the sensitivity of these systems is continually improving. FT spectrometers have excellent absolute wavelength accuracy and are able to operate across very wide wavelength regions including regions where reliable tunable lasers are not readily available. For these reasons many methods which were originally developed as laser techniques for the selective or indirect detection of trace species have been transferred to Fourier transform instruments including polarization modulation [ 7 1, circular dichroism [ 8 1, Zeeman modulation [ 91, velocity modulation [ lo], concentration modulation [ 111, time-resolved emission [ 12 1, and optoacoustic spectroscopy [ 131. In this work we demonstrate the feasibility of Fourier transform optogalvanic spectroscopy (FTOG) by recording spectra from lowpressure rare gas discharges in the visible and nearIR regions.
2. Experimental Spectra were recorded by placing commercial hollow cathode lamps at the sample focus of a Bomem DA3.02 spectrometer. The standard internal quartz
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
163
Volume 215, number 1,2,3
CHEMICAL PHYSICS LETTERS
tungsten-halogen light source with a near-IR/visible optimized beamsplitter produced about 3.5 mW of intensity at the 5 mm diameter focus, of which 50% was interference modulated. The modulated light was directed into the hollow cathode, passing through an anode ring electrode, to give the maximum illumination to the inside of the cathode. The discharges were run at currents from 0.3 to 20 mA in the series with a load resistor of 1 to 10 kR, depending on the discharge stability and gas fill pressure. The time varying signal was coupled off using a 0.1 pF capacitor, amplified using a low noise amplifier and frequency filtered, before being introduced into the standard signal channel of the spectrometer. Several commercial hollow cathode lamps (Westinghouse and S&J Juniper) were successfully used but there was a variation by several orders of magnitude in the noise level between the lamps and so some of the lamps did not produce useful spectra. In all the lamps the negative glow was entirely contained within the hollow cathodes, which were typically 4 mm in internal diameter and z 10 mm in length. One commercial lamp was modified to allow refilling with a variety of gases. Research grade gases were used for static fills without further purification. Before filling, the lamp was evacuated to better than 10m4Torr and was cleaned by running a helium discharge at high currents in both forward and reverse bias for several tills. Even after this cleaning process there was some loss of signal with time during normal operation of the refilled lamp as impurities built up over the period of a day. With the relatively low spectral brightness of the continuous source the optogalvanic signals were weak and were not observable without signal averaging. At the optimum low noise operating conditions the discharge noise was x 4.5 mV peak to peak measured across a 4.2 kQ ballast resistor without frequency filtering. By installing an optical chopper in the light path and signal averaging, the total chopped optogalvanic signal under the same conditions was found to be = 1.5 mV peak to peak for all lines. With this large noise contribution to the spectra from the discharge itself, noise from the light source and analogue to digital conversion process are negligible in comparison. Light from the continuously scanning interferometer focused on the discharge is modulated by in164
26 November 1993
terference at a frequency Fmodgiven by F ,,=2xscanspeed
(cm/s)xw
(cm-‘),
where o is the wavenumber of interest and the scan speed is the speed of the moving mirror in the interferometer. The optogalvanic signals are thus modulated over a range of frequencies rather than at a single frequency as used in LOG spectroscopy. At frequencies outside this modulation range there is no optogalvanic signal and so electrical bandpass filters can be used to eliminate the noise contribution generated by the lamp at the outlying frequencies prior to amplification. This procedure does not change the S/N at the signal frequencies but does allow a better match between the signal of interest and the analogue to digital converter (ADC) range. Even after filtering the signal remained small in comparison to the noise from the discharge, but we were able to use amplifier gains of 60 to 90 dB in our system. The frequency response of the lamps used in this study was relatively low and the signal decreased rapidly at scan speeds which gave modulation frequencies above x 6 kHz. Scan speeds of 0.07 to 0.2 cm/s were used as a compromise between signal magnitude and the length of time required per scan. Scan times varied between 5 min for low resolution scans to 15 h for the coaddition of several thousand high-resolution scans.
3. Results and discussion Typical spectra observed by FTOG for some of the rare gases are given in fig. 1. Spectra from neon, argon, krypton, and xenon were observed in this study. In common with LOG spectra, both positive and negative peaks are observed as a transition can raise or lower the impedance of the discharge depending on how it perturbs the local dynamic equilibrium. The spectra shown in fig. 1 are not fully corrected for phase errors and on the scale of this figure a number of the peaks exhibit both positive and negative components. The correct relative sense, i.e. positive or negative, for each peak is readily determined from an examination of the lineshape. Table 1 gives the discharge conditions under which spectra were typically recorded using the modified hollow cathode lamp. The observed FTOG spectral lines, for all spe-
Volume 215, number 1,2,3
CHEMICAL PHYSICS LETTERS
ioioo.
lijoo
Wavenumber (cm-‘)
4 (b)
I
13700.
15200. Wavenumber (cm-‘)
,
16700.
Fig. 1. Typical FTOG spectra of neon and xenon. These spectra are not phase corrected and so peaks may have both positive and negative components in this plot. (a) Xenon in refillable cell, 1024 scans coadded, 0.5 cm-’ resolution, (b) neon in commercial sealed lamp, 4100 scans coadded, 0.5 cm-’ resolution.
ties studied, are readily assigned to excitation transitions from the lowest-lying, and presumably most populated, ns metastable states to low-lying np states,
26 November 1993
see table 1. The strongest transitions could be observed by coadding as few as 32 scans but typically over 100 scans were used to achieve reasonable signal-to-noise ratios. The S/N ratio varied linearly with source intensity over the intensity range accessible with a tungsten lamp. As observed in LOG spectroscopy [6] the S/N varied significantly with discharge current. In our rare gas discharges the optimum S/N was obtained at low currents near threshold, where the noise went through a minimum before rising rapidly as the lamp became unstable at even lower currents. The S/N also varied with gas pressure, the optimum being fairly broad but different for each gas, as shown in table 1. Variation in the pressure, and to a lesser extent the current, changed the relative intensities of the peaks and affected the phase of the signal. This change of phase and signal amplitude is no doubt due to variations in the complex diffusion, excitation, and relaxation processes which are in dynamic equilibrium in the steady state discharge. Fig. 2 shows the effect in our spectra of improving the resolution by increasing the distance over which the moving mirror is translated (i.e. increasing the maximum optical path difference), with a constant source aperture diameter (5 mm), a constant number of coadded scans (256), and a constant scan speed. For a sparse spectrum with zero background, such as these FTGG spectra, the intensity of the interference signal does not decrease rapidly with optical path difference and so the signal-to-noise in all parts of the interferogram is nearly constant. There is thus only a small increase in the absolute noise level of the spectrum as the resolution is increased and more data is necessarily collected at large optical path
Table 1 Typical discharge conditions for optimal S/N in rare gas FTGG spectra ‘) Gas
Pressure (Torr )
Current (mA)
Region (cm-‘)
Lower levels b,
neon argon krypton xenon
3.4 2.6 1.6 0.8
0.6 0.5 1.1 0.3
13000-17000 9500-14400 5300-13500 9200-12000
2p53s, 2~~3s’ 3p54s, 3p54s’ 4p55s, 4p55s’ 5~~6s
a A commercial hollow cathode lamp modified to allow refilling with a static gas was used with a 4.7 kf2 ballast resistor for these measurements. b These are the lower levels of the observed FTOG transitions. All allowed transitions from these levels occurring in the region covered were observed.
165
Volume 215, number 1,2,3
I
CHEMICAL PHYSICS LETTERS
,,,I,,,,,,,,,,,,.,,
15500
15800 16100 Wavenumber (cm-‘)
16400
Fig. 2. The effect of resolution. All spectra were run under identical experimental conditions except for different maximum mirror displacements giving different nominal apodized resolutions. (a) 5 cm-‘; (b) 2 cm-‘; (c) 0.3 cm-‘.
differences. The signal peak height on the other hand increased dramatically as the resolution is increased to approach the intrinsic resolution of the interferometer and/or sample. The net effect is an improvement in signal-to-noise with increasing resolution as the intrinsic linewidth of the experiment is approached. In some experiments this linewidth will be limited by sample considerations such as Doppler or pressure broadening but in these experiments it is determined by the spectrometer. The intrinsic linewidth (full width at half maximum, fwhm) of an interferometer is governed by the size of the source aperture which determines the collimation of the light in the interferometer according to intrinsic resolution=&05
x D/F)’
,
where o is the wavenumber of interest, D is the di166
26 November 1993
ameter of the source aperture and F is the focal length of the mirror used for collimation of the light (FE 32.5 cm in our instrument). For the 5 mm diameter aperture used in these experiments a limiting resolution of 0.9 cm-’ fwhm is expected, while, in fact, a linewidth of 0.17 cm-’ was achieved. This apparent anomaly can be explained by noting that the optical arrangement of the DA3 spectrometer is symmetric with respect to the source and sample foci and so restricting the image size at the sample focus has the same effect on the collimation, and hence on the intrinsic resolution, as does a restriction of the source aperture size. This implies that only a very small area of the discharge, nominally = 2 mm in diameter for the 0.2 cm-’ linewidth observed, was actively contributing to the signal and the light falling on the remaining portion of the lamp was not detected. This miss-match between active sample area and source area must be a major factor limiting the sensitivity of these preliminary experiments; effectively only about z 16% of the available light is contributing to the signal, which varies linearly with light intensity. Optogalvanic signals are known to vary strongly with position in discharges [ 141, both within the cathode and between the cathode and anode glows. With the large focus of an FT spectrometer it is not possible to selectively illuminate the small region of these commercial lamps where the signal is large and so much of the source intensity is wasted. This overlap problem could presumably be substantially improved by using a more uniform discharge configuration designed to match the larger illuminated area inherent with FT spectrometers. With a low signal-to-noise ratio and sparse spectrum it is difficult to correct automatically for phase errors in the spectrum, which vary with wavenumber due to the non-constant phase shift in the amplifiers, electrical filters and in the lamp itself. This variation of phase error was relatively slow over the wavenumber regions scanned in this work and so constant phase corrections of between 0 and 7~14were used, although the resulting lineshapes were seldom symmetric throughout the entire spectrum. Phase errors do not present any particular problems in the detection of signals but it is noted that a careful correction of phase error would have to be made numerically if accurate wavenumber measurements were required.
Volume 2 15, number 1,2,3
CHEMICAL PHYSICS LETTERS
The importance of having a very low noise lamp has been greatly emphasized in the literature and there are many designs of low noise lamps available for LOG spectroscopy [ 2,15 1. With FIOG the lamp noise is certainly the limiting factor and a well designed, low noise, discharge lamp with good matching to the excitation light would substantially improve the sensitivity. A light source with higher spectral brightness would also lead to substantial improvements in S/N and would presumably drive excitation transitions in other less populated metastable levels. Light sources for absorption spectroscopy, such as the tungsten lamp used here, need to be very stable and noise free as the source noise is the major S/N limitation. In FTOG experiments the source noise and stability is relatively unimportant and so a noisier source with higher spectral brightness such as an xenon arc lamp would offer substantial improvements. Scan times for high resolution spectra were on the order of 2 to 15 h. While in absolute terms this is a substantial time, as single scan covers a very wide wavenumber region and so the information content of FTOG compares very favorably with LOG. The wavenumber region which can be scanned in an FT instrument is determined by the combination of source intensity and beamsplitter efficiency. The resources used in this experiment limited the useful scan range to x 5000 to ~~20000 cm-‘, which in comparison to any laser-based technique is very wide indeed. In particular, with different resources, FT systems can readily access the near-infrared and infrared regions where easily tunable lasers are not readily available. As in all FT experiments absolute wavenumber accuracy is very high and the scans are continuous.
4. Conclusions In this preliminary study we have shown that optogalvanic spectra can be recorded using a commer-
26 November 1993
cial FT spectrometer. Further studies to extend these observations to the infrared region, to optimize the cell design for this experiment, and to extend the observations to small molecules are planned. If through optimization of source intensity and cell design the sensitivity demonstrated in this work could be improved by a factor of ten or so, then FTOG would provide a useful new technique for optogalvanic spectroscopy of atoms and molecules.
Acknowledgement The authors wish to thank the Interdisciplinary Research Centre for Semi-Conductor Materials for financial support and Professor G. Duxbury for the loan of the visible/near-IR beamsplitter.
References [ 1 ] F.M. Penning, Physica 8 (1928) 137. [2] B. Barberi, N. Bererini and A. Sasso, Rev. Mod. Phys. 62 (1990) N3 601. [ 3 ] C. Hameau, J. Wascat, D. Dangoise and P. Glorieux, Opt. Commun. 21 (1986) 1465. [ 41 R.A. Kellar and E.F. Zalewski, Appl. Opt. 19 ( 1980) 3301. [ 51P. Labastie, F. Biraben and E. Giacobino, J. Phys. B 15 (1982) 2595. [6] Y. Kawashima, Chem. Phys. Letters 168 (1990) 20. [ 71 M. Elhanine, R. Farrenq and G. Guelachvili, Appl. Opt. 28 (1989) 4024. [ 81 L.A. Natie, M. Diem and D.W. Vidrine, J. Am. Chem. Sot. 101 (1979) 496. [ 9 ] G. Guelachvili, J. Opt. Sot. Am. B 3 ( 1986) 17 18. [IO] P.A. Martin and G. Guelachvili, Phys. Rev. Letters 65 (1990) 2535. [ 111 A. Ben&r, G. Guelachvili and P.A. Martin, Chem. Phys. Letters 177 (1991) 563. 1121 J.J. Sloan and E.J. Kruus, Time resolved spectroscopy, eds. R.J.H. Clark and R.E. Hester (Wiley, New York, 1989) p. 219. 131 J. Huang and M.W. Urban, J. Chem. Phys. 98 (1993) 5259. 141 C.R. Webster, S. McDermid and C.T. Rettner, J. Chem. Phys. 78 (1983) 646. 151 C.R. Webster, Rev. Sci. Instr. 54 (1983) 1454.
167
CHEMICAL PHYSICS LETTERS
26 November 1993
Fourier transform optogalvanic spectroscopy Jacob Baker ‘, V.C. Gibbons and Peter A. Hamilton Departmentof Chemistry,Queen Mary and WestfieldCollege,Mile End Road, London El 4NS UK Received 27 August 1993
High-resolution optogalvanic spectra have been observed for the first time using a Fourier transform spectrometer. Spectra from neon, argon, krypton, and xenon were observed over the range 5000 to 18000 cm-’ using a modified commercial hollow cathode lamp. Signal-to-noise ratios of several hundred to one were readily achieved on the strongest transitions but in these initial experiments the sensitivity is still substantially below that of a laser experiment. If the sensitivity can be improved by changes in lamp design then the very wide coverage scans and the ability to operate in the infrared and near-infrared regions will make this an important new spectroscopic method.
1. Introduction Optogalvanic spectra are observed by measuring the change in the impedance of a gas discharge, caused by the absorption of light, as a function of wavelength. The optogalvanic effect was first observed in 1928 using resonance lamps as intense line sources [ 111, which have high spectral brightness, to perturb the equilibrium processes occurring in the discharge. However, it was not until the introduction of the dye laser, which is tunable and also has high spectroscopic brightness, that optogalvanic spectroscopy became a practical spectroscopic method. Laser optogalvanic (LOG) spectra of both atoms and small molecules have subsequently been observed with good sensitivity in most spectral regions and in a wide variety of discharges [2,3]. The technique is most sensitive to those atomic and molecular states which couple most strongly to the ionization processes, or which effect the translational energy of the free electrons [ 4,5 ] and so it is often used for high-lying Rydberg and metastable states. In addition, for molecules, fragmentation processes will also lead to impedance changes in the discharge [ 61. While laser spectroscopic techniques demonstrate unsurpassed sensitivity and resolution they are dif’ Present address: DAMAP et URA 812 du CNRS, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France.
licult to scan over extended regions and require careful wavelength calibration. Modern Fourier transform (FT) instruments are now capable of achieving near-Doppler limited resolution from infrared to W wavelengths and the sensitivity of these systems is continually improving. FT spectrometers have excellent absolute wavelength accuracy and are able to operate across very wide wavelength regions including regions where reliable tunable lasers are not readily available. For these reasons many methods which were originally developed as laser techniques for the selective or indirect detection of trace species have been transferred to Fourier transform instruments including polarization modulation [ 7 1, circular dichroism [ 8 1, Zeeman modulation [ 91, velocity modulation [ lo], concentration modulation [ 111, time-resolved emission [ 12 1, and optoacoustic spectroscopy [ 131. In this work we demonstrate the feasibility of Fourier transform optogalvanic spectroscopy (FTOG) by recording spectra from lowpressure rare gas discharges in the visible and nearIR regions.
2. Experimental Spectra were recorded by placing commercial hollow cathode lamps at the sample focus of a Bomem DA3.02 spectrometer. The standard internal quartz
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
163
Volume 215, number 1,2,3
CHEMICAL PHYSICS LETTERS
tungsten-halogen light source with a near-IR/visible optimized beamsplitter produced about 3.5 mW of intensity at the 5 mm diameter focus, of which 50% was interference modulated. The modulated light was directed into the hollow cathode, passing through an anode ring electrode, to give the maximum illumination to the inside of the cathode. The discharges were run at currents from 0.3 to 20 mA in the series with a load resistor of 1 to 10 kR, depending on the discharge stability and gas fill pressure. The time varying signal was coupled off using a 0.1 pF capacitor, amplified using a low noise amplifier and frequency filtered, before being introduced into the standard signal channel of the spectrometer. Several commercial hollow cathode lamps (Westinghouse and S&J Juniper) were successfully used but there was a variation by several orders of magnitude in the noise level between the lamps and so some of the lamps did not produce useful spectra. In all the lamps the negative glow was entirely contained within the hollow cathodes, which were typically 4 mm in internal diameter and z 10 mm in length. One commercial lamp was modified to allow refilling with a variety of gases. Research grade gases were used for static fills without further purification. Before filling, the lamp was evacuated to better than 10m4Torr and was cleaned by running a helium discharge at high currents in both forward and reverse bias for several tills. Even after this cleaning process there was some loss of signal with time during normal operation of the refilled lamp as impurities built up over the period of a day. With the relatively low spectral brightness of the continuous source the optogalvanic signals were weak and were not observable without signal averaging. At the optimum low noise operating conditions the discharge noise was x 4.5 mV peak to peak measured across a 4.2 kQ ballast resistor without frequency filtering. By installing an optical chopper in the light path and signal averaging, the total chopped optogalvanic signal under the same conditions was found to be = 1.5 mV peak to peak for all lines. With this large noise contribution to the spectra from the discharge itself, noise from the light source and analogue to digital conversion process are negligible in comparison. Light from the continuously scanning interferometer focused on the discharge is modulated by in164
26 November 1993
terference at a frequency Fmodgiven by F ,,=2xscanspeed
(cm/s)xw
(cm-‘),
where o is the wavenumber of interest and the scan speed is the speed of the moving mirror in the interferometer. The optogalvanic signals are thus modulated over a range of frequencies rather than at a single frequency as used in LOG spectroscopy. At frequencies outside this modulation range there is no optogalvanic signal and so electrical bandpass filters can be used to eliminate the noise contribution generated by the lamp at the outlying frequencies prior to amplification. This procedure does not change the S/N at the signal frequencies but does allow a better match between the signal of interest and the analogue to digital converter (ADC) range. Even after filtering the signal remained small in comparison to the noise from the discharge, but we were able to use amplifier gains of 60 to 90 dB in our system. The frequency response of the lamps used in this study was relatively low and the signal decreased rapidly at scan speeds which gave modulation frequencies above x 6 kHz. Scan speeds of 0.07 to 0.2 cm/s were used as a compromise between signal magnitude and the length of time required per scan. Scan times varied between 5 min for low resolution scans to 15 h for the coaddition of several thousand high-resolution scans.
3. Results and discussion Typical spectra observed by FTOG for some of the rare gases are given in fig. 1. Spectra from neon, argon, krypton, and xenon were observed in this study. In common with LOG spectra, both positive and negative peaks are observed as a transition can raise or lower the impedance of the discharge depending on how it perturbs the local dynamic equilibrium. The spectra shown in fig. 1 are not fully corrected for phase errors and on the scale of this figure a number of the peaks exhibit both positive and negative components. The correct relative sense, i.e. positive or negative, for each peak is readily determined from an examination of the lineshape. Table 1 gives the discharge conditions under which spectra were typically recorded using the modified hollow cathode lamp. The observed FTOG spectral lines, for all spe-
Volume 215, number 1,2,3
CHEMICAL PHYSICS LETTERS
ioioo.
lijoo
Wavenumber (cm-‘)
4 (b)
I
13700.
15200. Wavenumber (cm-‘)
,
16700.
Fig. 1. Typical FTOG spectra of neon and xenon. These spectra are not phase corrected and so peaks may have both positive and negative components in this plot. (a) Xenon in refillable cell, 1024 scans coadded, 0.5 cm-’ resolution, (b) neon in commercial sealed lamp, 4100 scans coadded, 0.5 cm-’ resolution.
ties studied, are readily assigned to excitation transitions from the lowest-lying, and presumably most populated, ns metastable states to low-lying np states,
26 November 1993
see table 1. The strongest transitions could be observed by coadding as few as 32 scans but typically over 100 scans were used to achieve reasonable signal-to-noise ratios. The S/N ratio varied linearly with source intensity over the intensity range accessible with a tungsten lamp. As observed in LOG spectroscopy [6] the S/N varied significantly with discharge current. In our rare gas discharges the optimum S/N was obtained at low currents near threshold, where the noise went through a minimum before rising rapidly as the lamp became unstable at even lower currents. The S/N also varied with gas pressure, the optimum being fairly broad but different for each gas, as shown in table 1. Variation in the pressure, and to a lesser extent the current, changed the relative intensities of the peaks and affected the phase of the signal. This change of phase and signal amplitude is no doubt due to variations in the complex diffusion, excitation, and relaxation processes which are in dynamic equilibrium in the steady state discharge. Fig. 2 shows the effect in our spectra of improving the resolution by increasing the distance over which the moving mirror is translated (i.e. increasing the maximum optical path difference), with a constant source aperture diameter (5 mm), a constant number of coadded scans (256), and a constant scan speed. For a sparse spectrum with zero background, such as these FTGG spectra, the intensity of the interference signal does not decrease rapidly with optical path difference and so the signal-to-noise in all parts of the interferogram is nearly constant. There is thus only a small increase in the absolute noise level of the spectrum as the resolution is increased and more data is necessarily collected at large optical path
Table 1 Typical discharge conditions for optimal S/N in rare gas FTGG spectra ‘) Gas
Pressure (Torr )
Current (mA)
Region (cm-‘)
Lower levels b,
neon argon krypton xenon
3.4 2.6 1.6 0.8
0.6 0.5 1.1 0.3
13000-17000 9500-14400 5300-13500 9200-12000
2p53s, 2~~3s’ 3p54s, 3p54s’ 4p55s, 4p55s’ 5~~6s
a A commercial hollow cathode lamp modified to allow refilling with a static gas was used with a 4.7 kf2 ballast resistor for these measurements. b These are the lower levels of the observed FTOG transitions. All allowed transitions from these levels occurring in the region covered were observed.
165
Volume 215, number 1,2,3
I
CHEMICAL PHYSICS LETTERS
,,,I,,,,,,,,,,,,.,,
15500
15800 16100 Wavenumber (cm-‘)
16400
Fig. 2. The effect of resolution. All spectra were run under identical experimental conditions except for different maximum mirror displacements giving different nominal apodized resolutions. (a) 5 cm-‘; (b) 2 cm-‘; (c) 0.3 cm-‘.
differences. The signal peak height on the other hand increased dramatically as the resolution is increased to approach the intrinsic resolution of the interferometer and/or sample. The net effect is an improvement in signal-to-noise with increasing resolution as the intrinsic linewidth of the experiment is approached. In some experiments this linewidth will be limited by sample considerations such as Doppler or pressure broadening but in these experiments it is determined by the spectrometer. The intrinsic linewidth (full width at half maximum, fwhm) of an interferometer is governed by the size of the source aperture which determines the collimation of the light in the interferometer according to intrinsic resolution=&05
x D/F)’
,
where o is the wavenumber of interest, D is the di166
26 November 1993
ameter of the source aperture and F is the focal length of the mirror used for collimation of the light (FE 32.5 cm in our instrument). For the 5 mm diameter aperture used in these experiments a limiting resolution of 0.9 cm-’ fwhm is expected, while, in fact, a linewidth of 0.17 cm-’ was achieved. This apparent anomaly can be explained by noting that the optical arrangement of the DA3 spectrometer is symmetric with respect to the source and sample foci and so restricting the image size at the sample focus has the same effect on the collimation, and hence on the intrinsic resolution, as does a restriction of the source aperture size. This implies that only a very small area of the discharge, nominally = 2 mm in diameter for the 0.2 cm-’ linewidth observed, was actively contributing to the signal and the light falling on the remaining portion of the lamp was not detected. This miss-match between active sample area and source area must be a major factor limiting the sensitivity of these preliminary experiments; effectively only about z 16% of the available light is contributing to the signal, which varies linearly with light intensity. Optogalvanic signals are known to vary strongly with position in discharges [ 141, both within the cathode and between the cathode and anode glows. With the large focus of an FT spectrometer it is not possible to selectively illuminate the small region of these commercial lamps where the signal is large and so much of the source intensity is wasted. This overlap problem could presumably be substantially improved by using a more uniform discharge configuration designed to match the larger illuminated area inherent with FT spectrometers. With a low signal-to-noise ratio and sparse spectrum it is difficult to correct automatically for phase errors in the spectrum, which vary with wavenumber due to the non-constant phase shift in the amplifiers, electrical filters and in the lamp itself. This variation of phase error was relatively slow over the wavenumber regions scanned in this work and so constant phase corrections of between 0 and 7~14were used, although the resulting lineshapes were seldom symmetric throughout the entire spectrum. Phase errors do not present any particular problems in the detection of signals but it is noted that a careful correction of phase error would have to be made numerically if accurate wavenumber measurements were required.
Volume 2 15, number 1,2,3
CHEMICAL PHYSICS LETTERS
The importance of having a very low noise lamp has been greatly emphasized in the literature and there are many designs of low noise lamps available for LOG spectroscopy [ 2,15 1. With FIOG the lamp noise is certainly the limiting factor and a well designed, low noise, discharge lamp with good matching to the excitation light would substantially improve the sensitivity. A light source with higher spectral brightness would also lead to substantial improvements in S/N and would presumably drive excitation transitions in other less populated metastable levels. Light sources for absorption spectroscopy, such as the tungsten lamp used here, need to be very stable and noise free as the source noise is the major S/N limitation. In FTOG experiments the source noise and stability is relatively unimportant and so a noisier source with higher spectral brightness such as an xenon arc lamp would offer substantial improvements. Scan times for high resolution spectra were on the order of 2 to 15 h. While in absolute terms this is a substantial time, as single scan covers a very wide wavenumber region and so the information content of FTOG compares very favorably with LOG. The wavenumber region which can be scanned in an FT instrument is determined by the combination of source intensity and beamsplitter efficiency. The resources used in this experiment limited the useful scan range to x 5000 to ~~20000 cm-‘, which in comparison to any laser-based technique is very wide indeed. In particular, with different resources, FT systems can readily access the near-infrared and infrared regions where easily tunable lasers are not readily available. As in all FT experiments absolute wavenumber accuracy is very high and the scans are continuous.
4. Conclusions In this preliminary study we have shown that optogalvanic spectra can be recorded using a commer-
26 November 1993
cial FT spectrometer. Further studies to extend these observations to the infrared region, to optimize the cell design for this experiment, and to extend the observations to small molecules are planned. If through optimization of source intensity and cell design the sensitivity demonstrated in this work could be improved by a factor of ten or so, then FTOG would provide a useful new technique for optogalvanic spectroscopy of atoms and molecules.
Acknowledgement The authors wish to thank the Interdisciplinary Research Centre for Semi-Conductor Materials for financial support and Professor G. Duxbury for the loan of the visible/near-IR beamsplitter.
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