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Optogalvanic spectroscopy with rf discharge
Volume 38, number 5,6
OPTICS COMMUNICATIONS
1 September 1981
OPTOGALVANIC SPECTROSCOPY WITH rf DISCHARGE T. SUZUKI Institute of Physical and Chemical Research, Hirosawa, Wako, Saitama 351, Japan
Received 16 February 1981 Revised manuscript received 23 April 1981
A simple scheme for observing optogalvanic effects in the rf discharge is reported. The discharge is sustained by applying an rf field between two metal rings wound outside of the discharge tube. Changes in the discharge current are detected by a coil wound on the tube. Signals are obtained in experiments performed in the wavelength region between 580 nm and 630 nm with discharge in Ar, N2, NH3 and NO2. A new Rydberg band of the N 2 molecule (c~ 1Zu ~ a,q Z~ ) has been observed around 590 nm.
1. Introduction In this report we describe a simple scheme for observation of optogalvanic effect in the rf discharge. As the optogalvanic detection method with rf discharge needs no electrodes inside the discharge tube, it may be expected to be a useful complement to the conventional optogalvanic method with a dc discharge. The rf optogalvanic spectroscopy will be particularly useful for studying unstable molecules and to high resolution spectroscopy. Optogalvanic detection is a simple and sensitive technique for monitoring optical absorption in a d c discharge cell by observing changes in its current [1]. It has been applied to observe molecular transitions from ground and metastable states [2]. It has also been applied to Doppler-free spectroscopy [3,4]. Twophoton transitions from energy levels other than metastable or ground levels were observed [4]. Its high detectivity is remarkable and its sensitivity is limited only by shot noise in the direct current sustaining the discharge [3]. Optogalvanic spectroscopy with rf discharge (rf OGS) has two advantages over the optogalvanic spectroscopy with dc discharge (dc OGS). At first, noises caused by electrode sputtering can be avoided in rf OGS. Secondly, it is applicable for studies of unstable molecules which are reactive to metal electrodes in the discharge tube. 364
There are two detection schemes for the optogalvanic effect in the rf discharge. One is to detect impedance changes in the discharge induced by molecular transitions, for example, by monitoring the reflected rf field +. The other uses a pick-up coil of the rf current to detect changes in the rf current induced by molecular transitions. We have adopted the latter detection scheme and have developed a simple electrical network for observation of the optogalvanic effect in the rf discharge. The rf current through the discharge tube is monitored by a pick-up coil wound outside of the cell. The discharge plays the part of a transmitting antenna, and the circuit, that of a receiving antenna. We have applied this techniques as a demonstration to discharges in Ar, N2, NH 3 and NO 2. Since these were studied by dc OGS [2], the comparison between two optogalvanic methods is facilitated. In N 2 discharge, a new Rydberg band has been found in the wavelength region around 590 nm.
2. Experimental setup A schematic diagram of the apparatus is shown in * Recently, Stanciulescu et al. [5 ] have applied an optogalvanic detection through impedance changes in the rf discharges with a different method.
0 0 3 0 - 4 0 1 8 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 02.50 © North-Holland Publishing Company
Volume 38, number 5,6
OPTICS COMMUNICATIONS RF TRANSMITTER GAS ~
--
~
~D.
~ IG
PUMP
CUPPER
1 September 1981
by 5 to 10 cm as shown in fig. 1. The electrodes are cupper plates with a width of 1 cm which are wound around the tube. The excited plasma region remained between two electrodes when the following data were taken.
R[~
3. Results and discussion
Fig. 1. Experimelatal apparatus for rf optogalvanic spectroscopy.
fig. 1. A cw dye laser (Coherent, CR 590) is operated at a bandwidth of 20 GHz over the tuning range of rhodamine 6G pumped with a krypton-ion laser (CR 3000K). The spectra shown are not corrected for the varying laser output which has a maximum power of 0.25 W around 600 nm. The laser beam is mechanically chopped at a frequency of 218 Hz and directed into the discharge with a beam diameter of 5 mm. An rf electrical field at 13.56 MHz is applied to the ionization region of discharge by two cupper rings wound outside the tube. A pick-up coil (10 nil) wound around the tube and a capacitor forms a tank circuit'~vhich is adjusted to be resonant with the rf of 13.56 MHz. Signals picked up by the coil are rectified and passed through an amplifier tuned to the modulation frequency of 218 Hz. The output is monitored by an oscilloscope and measured with a lock-in amplifier at an integration time of 0.3 s. The laser wavelength is scanned at an approximate rate of 0.03 nm/s with a gear-coupled stepping motor which rotates the micrometer drive of the birefringent fdter. The wavelength of distinct lines is measured by a monochromator (spex, 1701). The gas is continuously pumped through the discharge, where the pressure measured by a pirani gauge at the outlet of the tube is between 0.01 and 0.4 Torr. The discharge condition depends on gas pressure, tube diameter, electrode spacing, rf power and molecular species. The best combination of these parameters which gives the maximum signal is different for each molecule. We have used pyrex tubes of 40 cm length with diameters between 10 and 30 mm. The discharge is produced by connecting an rf transmitter having an output power of 4 to 45 W to two electrodes separated
Fig. 2 shows a typical optogalvanic spectrum of N 2 discharge at a pressure of 0.1 Torr and an rf power of 20 W. A discharge tube of 10 mm O.D. is used. The prominent band seen in fig. 2 can be identified as one of bands detected and assigned by Ledbetter [6]. The lines of this band is assigned as shown in fig. 2 representing transitions between the Rydberg states c 4 1 II u ~- a" I y,g+ . This band was also observed by dc OGS [2] at low pressures below 1 Torr and high discharge voltages with some disturbance by interfering lines of the first positive system. In our experimental condition the first positive system expected in this wavelength region [7] does not appear. The identification of the Ledbetter system is based on shifts of the P(10) and R(8) lines caused by a perturbation in the upper state at J ' = 9. This perturbation also appears in rf
--, Q
v v v
c IIT ... a,,1F•
WAVELENGTH~=~
o:~u
ou,~lnm)
Fig. 2. Optogalvanic spectrum of N2 discharge. Assignments by Ledbetter [6] are indicated for P-and R-branches for c4 ] Flu ,-- a" 1: ~ transition. The identification of c'51E+ +- a" 1E~ is based on the comparison with the absorption data in the vacuum ultraviolet region by Carroll and Yoshino [8]. This gives the assignment shown, following the similarity of spectrum profile and the coincidence of wavelength for transitions indicated by solid lines. 365
Volume 38, number 5,6
OPTICS COMMUNICATIONS
OGS spectrum with opposite sign of P(10) and R(8) lines. The P(11), P(12), R(9) and R(10) lines in fig. 2 are seen to be also perturbed. In the shorter wavelength region than the Ledbetter band, there appear some lines which have not been reported in the literature so far. Comparison with the Rydberg system series of nitrogen in the vacuum ultraviolet region [8] shows that these lines belong to the c~ 1Eu+ ._ a" 12;~ transition. This is confirmed by the following discussions. Carroll and Yoshino [8] have observed two Rydberg series, cn 117 ~ X 1 ~ ; and C ' n l ] ~ + "~- X l y , g+ in the far ultraviolet absorption spectrum. The spectrum profile shown in fig. 2 is very similar to the photographic spectrum shown in fig. 1 of ref. [8]. This can be expected when transitions of two bands in fig. 2 originate from the ground Rydberg state (a" 1Z~- ) which has a similar electronic configuration to the molecular ground state (X 12;~-). By comparing fig. 2 with the photographic spectrum in ref. [8], P(8), P(9), R(12) and R(13) lines can be assigned as shown in fig. 2. In our data, the wavelengths of three transitions indicated by solid lines in fig. 2 are measured with a monochromator. The energy difference between X 12;~ and a" 12;~- in ref. [5], and the energy difference between c~ 12;+ and X lye; in ref. [7] are used to calculate the possible transition frequency. This gives a coincidence within -+1 cm -1 with the measured three lines when they are assigned as shown in fig. 2. The other lines may be also assigned as shown by dotted lines when compared with data in ref. [8]. Carroll and Yoshino pointed out the coincidence of energy levels of c4 117u and c~ 12;~ at J = 20 and observed P-branch for J ~< 16. This also appears in our data observed by rf OGS. Further discussions are not practical from our data obtained with a low resolution of 20 GHz. We are currently investigating these bands with a single-frequency dye laser. Fig. 3 is a typical optogalvanic spectrum of NH 3 discharge measured at a pressure of 0.1 Torr and an rf power of 13 W. A discharge tube of 30 mm O.D. is used. The spectrum shows a similar profile observed by Feldmann with dc OGS [2]. The most prominent band is identified as transitions of the 2A 1 - 2 B 1 systim of NH 2 when compared with the data obtained by absorption [9]. The strongest optogalvanic signal obtained with the Q branch of the E vibronic band has a maximum intensity which is comparable with the strongest N 2 signal obtained by the Q branch of Ledbetter band. Noises in fig. 3 is ten times larger 366
1 September 1981
(qg,0)
NH 2
- (O,qO)
0.1 Torr 13 W
P -t (0.9,0) - (0.o.o) (0,10.0)
0¢
- (O,qO)
.V. ee 0
echo
610
590
T (nm) 580
WAVELENGTH
Fig. 3. Optogalvanic spectrum of NH3 discharge. Assignments by Dressier and Ramsay [9] are shown according to their absorption data of NH2. than those shown in fig. 2. The optimum conditions for the NH 3 discharge have not been searched for. Fig. 4 shows a typical optogalvanic spectrum of argon discharge measured at a pressure of 0.4 Torr and an rf power of 10 W. A discharge tube of 30 mm O.D. is used. All atomic lines in this wavelength region listed in [10] can be observed. The strongest signal has a comparable magnitude with the strongest N 2 signal. Optogalvanic spectrum in NO 2 discharge at a pressure of 0.07 Torr and an rf power of 45 W shows structures similar to those observed by Feldmann with dc OGS [2]. The optogalvanic effect in the rf discharge with a simple pick-up circuit is found to be useful. In our experimental system, its sensitivity is limited by the dis-
Ar ~Torr
tO W
$ i
J i L,,t, 63o
620
6;0
~o
s~o 5~oc~
WAVELENGT H
Fig. 4. Optogalvanic spectrum of Ar discharge.
Volume 38, number 5,6
OPTICS COMMUNICATIONS
charge noises. For consideration of possibility for improving the sensitivity, noises in the rf discharge are analyzed with a selective level meter (Anritsu, ML11A), which has an analyzing bandwidth of 8 Hz. A typical noise power spectrum is shown in fig. 5, which is for argon discharge with a tube of 30 mm O.D. and an rf power of 4 W. The argon pressure is 0.4 Torr. Strong components at an interval of 50 Hz are due to residual line components in the dc power supply of the rf transmitter. A large signal at 1.8 kHz originates in a superposed amplitude modulation of 1 0 - 4 in depth to the driving rf field. Noises caused by the discharge have different frequency with different discharge conditions, but have similar profile shown in fig. 5. By comparing noise spectrum for various discharge conditions, we find its tendency as follows. In the low frequency region below 500 Hz, apparant noises besides line components have its power rapidly decaying with frequency. In the region between 1 and 3 kHz, there appear very large noise caused by ion instabilities. Its center frequency becomes higher as the gas pressure is decreased. In the higher frequency region, there appear harmonics of the above noise until about 5 kHz. Above this frequency, there appear no such distinct noises up to the observed 20 kHz. Therefore, modulation above 5 kHz is suitable for minimizing discharge noises. It is found that noise power at 218 Hz is typically 15 dB higher than that at 5 kHz. This implies that noise voltage at 218 Hz is about 5 times larger than that at 5 kHz. The comparison of the magnitude of discharge noise with that of amplifier noise may give an estimation to
I 0.5
1 September 1981
the desired fabrication of amplifier to be used in rf OGS. Discharge noises at the input of the lock-in amplifier can be estimated from the recorder trace to be ~100 ~V at 1 Hz bandwidth in argon experiment. The homemade tuned amplifier has a gain of 40 dB at 218 Hz and a bandwidth of 60 Hz. The intrinsic noise voltage of this amplifier can be estimated by monitoring output noise voltage when there are no rf fields. The output voltage thus obtained is 2.5/aV ~ . This value is a moderate noise voltage for an amplifier and is 40 times smaller than that of discharge noise at 218 Hz. In our experimental system, it is found that discharge noise is still one order of magnitude larger than the amplifier noise even with 5 kHz modulation.
4. Conclusion The simplicity and wide applicability of rf OGS has been shown with preliminary results. It is found that our experimental system is not fully optimized for rf OGS. Nevertheless, the signal-to-noise ratio of larger than 200 is obtained for a molecular transition with a detector bandwidth of 1Hz. Optimization of the discharge tube design and electrical circuit for optogalvanic spectroscopy will lead to improvements in the signal-to-noise ratio. We are currently investigating the nature of this problem further. We have found rf OGS is a powerful method both for studying optical transition gaseous discharges and for investigations of discharges processes. In addition, high resolution laser spectroscopy of Rydberg state of N 2 has become possible.
Acknowledgements
1
k_ 2
FREQUENCY
3 4 5 6 7 ~ 9(kHz)
Fig. 5. Noise power spectrum of discharge in argon taken with a bandwidth of 8 Hz.
The author wishes to thank Professor K. Shimoda for valuable comments. Thanks are also due to Dr. T. Kasuya, Dr. M. Kakimoto and Dr. S. Iwata for helpful discussions, and to H. Kato and Y. Nomiya for technical assistance.
References [1] R.B. Green, R.A. Keller, G.G. Luther, P.K. Schenck and J.C. Travis, Appl. Phys. Lett. 29 (1976) 727. 367
Volume 38, number 5,6
OPTICS COMMUNICATIONS
[2] D. Feldmarm, Optics Comm. 29 (1979) 67. [3] J.E. Lawler, A.I. Ferguson, J.E.M. Goldsmith, D.J. Jackson and A.L. Schawlow, Phys. Rev. Lett. 42 (1979) 1046. [4] J.E.M. Goldsmith and A.V. Smith, Optics Comm. 32 (1980) 403. [5 ] C. Stanciulescu, R.C. Bobulescu, A. Surmeian, D. Popescu, I. Popescu and C.B. Collins, Appl. Phys. Lett. 37 (1980) 888.
368
1 September 1981
[6] J.W. Ledbetter Jr., J. Mol. Spectr. 42 (1972) 100. [7] A. Lofhus and P.H. Krupenie, J. Phys. Chem. Ref. Data 6 (1977) 113. [8] P.K. CarroU and K. Yoshino, J. Phys. B5 (1972) 1614. [9] K. Dressier and D.A. Ramsay, Phil. Trans. Roy. Soc. Lond. A251 (1959) 553. [10] American Institute of Physics Handbook, ed. D.E. Gay (McGraw-Hill, 1972).
OPTICS COMMUNICATIONS
1 September 1981
OPTOGALVANIC SPECTROSCOPY WITH rf DISCHARGE T. SUZUKI Institute of Physical and Chemical Research, Hirosawa, Wako, Saitama 351, Japan
Received 16 February 1981 Revised manuscript received 23 April 1981
A simple scheme for observing optogalvanic effects in the rf discharge is reported. The discharge is sustained by applying an rf field between two metal rings wound outside of the discharge tube. Changes in the discharge current are detected by a coil wound on the tube. Signals are obtained in experiments performed in the wavelength region between 580 nm and 630 nm with discharge in Ar, N2, NH3 and NO2. A new Rydberg band of the N 2 molecule (c~ 1Zu ~ a,q Z~ ) has been observed around 590 nm.
1. Introduction In this report we describe a simple scheme for observation of optogalvanic effect in the rf discharge. As the optogalvanic detection method with rf discharge needs no electrodes inside the discharge tube, it may be expected to be a useful complement to the conventional optogalvanic method with a dc discharge. The rf optogalvanic spectroscopy will be particularly useful for studying unstable molecules and to high resolution spectroscopy. Optogalvanic detection is a simple and sensitive technique for monitoring optical absorption in a d c discharge cell by observing changes in its current [1]. It has been applied to observe molecular transitions from ground and metastable states [2]. It has also been applied to Doppler-free spectroscopy [3,4]. Twophoton transitions from energy levels other than metastable or ground levels were observed [4]. Its high detectivity is remarkable and its sensitivity is limited only by shot noise in the direct current sustaining the discharge [3]. Optogalvanic spectroscopy with rf discharge (rf OGS) has two advantages over the optogalvanic spectroscopy with dc discharge (dc OGS). At first, noises caused by electrode sputtering can be avoided in rf OGS. Secondly, it is applicable for studies of unstable molecules which are reactive to metal electrodes in the discharge tube. 364
There are two detection schemes for the optogalvanic effect in the rf discharge. One is to detect impedance changes in the discharge induced by molecular transitions, for example, by monitoring the reflected rf field +. The other uses a pick-up coil of the rf current to detect changes in the rf current induced by molecular transitions. We have adopted the latter detection scheme and have developed a simple electrical network for observation of the optogalvanic effect in the rf discharge. The rf current through the discharge tube is monitored by a pick-up coil wound outside of the cell. The discharge plays the part of a transmitting antenna, and the circuit, that of a receiving antenna. We have applied this techniques as a demonstration to discharges in Ar, N2, NH 3 and NO 2. Since these were studied by dc OGS [2], the comparison between two optogalvanic methods is facilitated. In N 2 discharge, a new Rydberg band has been found in the wavelength region around 590 nm.
2. Experimental setup A schematic diagram of the apparatus is shown in * Recently, Stanciulescu et al. [5 ] have applied an optogalvanic detection through impedance changes in the rf discharges with a different method.
0 0 3 0 - 4 0 1 8 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 02.50 © North-Holland Publishing Company
Volume 38, number 5,6
OPTICS COMMUNICATIONS RF TRANSMITTER GAS ~
--
~
~D.
~ IG
PUMP
CUPPER
1 September 1981
by 5 to 10 cm as shown in fig. 1. The electrodes are cupper plates with a width of 1 cm which are wound around the tube. The excited plasma region remained between two electrodes when the following data were taken.
R[~
3. Results and discussion
Fig. 1. Experimelatal apparatus for rf optogalvanic spectroscopy.
fig. 1. A cw dye laser (Coherent, CR 590) is operated at a bandwidth of 20 GHz over the tuning range of rhodamine 6G pumped with a krypton-ion laser (CR 3000K). The spectra shown are not corrected for the varying laser output which has a maximum power of 0.25 W around 600 nm. The laser beam is mechanically chopped at a frequency of 218 Hz and directed into the discharge with a beam diameter of 5 mm. An rf electrical field at 13.56 MHz is applied to the ionization region of discharge by two cupper rings wound outside the tube. A pick-up coil (10 nil) wound around the tube and a capacitor forms a tank circuit'~vhich is adjusted to be resonant with the rf of 13.56 MHz. Signals picked up by the coil are rectified and passed through an amplifier tuned to the modulation frequency of 218 Hz. The output is monitored by an oscilloscope and measured with a lock-in amplifier at an integration time of 0.3 s. The laser wavelength is scanned at an approximate rate of 0.03 nm/s with a gear-coupled stepping motor which rotates the micrometer drive of the birefringent fdter. The wavelength of distinct lines is measured by a monochromator (spex, 1701). The gas is continuously pumped through the discharge, where the pressure measured by a pirani gauge at the outlet of the tube is between 0.01 and 0.4 Torr. The discharge condition depends on gas pressure, tube diameter, electrode spacing, rf power and molecular species. The best combination of these parameters which gives the maximum signal is different for each molecule. We have used pyrex tubes of 40 cm length with diameters between 10 and 30 mm. The discharge is produced by connecting an rf transmitter having an output power of 4 to 45 W to two electrodes separated
Fig. 2 shows a typical optogalvanic spectrum of N 2 discharge at a pressure of 0.1 Torr and an rf power of 20 W. A discharge tube of 10 mm O.D. is used. The prominent band seen in fig. 2 can be identified as one of bands detected and assigned by Ledbetter [6]. The lines of this band is assigned as shown in fig. 2 representing transitions between the Rydberg states c 4 1 II u ~- a" I y,g+ . This band was also observed by dc OGS [2] at low pressures below 1 Torr and high discharge voltages with some disturbance by interfering lines of the first positive system. In our experimental condition the first positive system expected in this wavelength region [7] does not appear. The identification of the Ledbetter system is based on shifts of the P(10) and R(8) lines caused by a perturbation in the upper state at J ' = 9. This perturbation also appears in rf
--, Q
v v v
c IIT ... a,,1F•
WAVELENGTH~=~
o:~u
ou,~lnm)
Fig. 2. Optogalvanic spectrum of N2 discharge. Assignments by Ledbetter [6] are indicated for P-and R-branches for c4 ] Flu ,-- a" 1: ~ transition. The identification of c'51E+ +- a" 1E~ is based on the comparison with the absorption data in the vacuum ultraviolet region by Carroll and Yoshino [8]. This gives the assignment shown, following the similarity of spectrum profile and the coincidence of wavelength for transitions indicated by solid lines. 365
Volume 38, number 5,6
OPTICS COMMUNICATIONS
OGS spectrum with opposite sign of P(10) and R(8) lines. The P(11), P(12), R(9) and R(10) lines in fig. 2 are seen to be also perturbed. In the shorter wavelength region than the Ledbetter band, there appear some lines which have not been reported in the literature so far. Comparison with the Rydberg system series of nitrogen in the vacuum ultraviolet region [8] shows that these lines belong to the c~ 1Eu+ ._ a" 12;~ transition. This is confirmed by the following discussions. Carroll and Yoshino [8] have observed two Rydberg series, cn 117 ~ X 1 ~ ; and C ' n l ] ~ + "~- X l y , g+ in the far ultraviolet absorption spectrum. The spectrum profile shown in fig. 2 is very similar to the photographic spectrum shown in fig. 1 of ref. [8]. This can be expected when transitions of two bands in fig. 2 originate from the ground Rydberg state (a" 1Z~- ) which has a similar electronic configuration to the molecular ground state (X 12;~-). By comparing fig. 2 with the photographic spectrum in ref. [8], P(8), P(9), R(12) and R(13) lines can be assigned as shown in fig. 2. In our data, the wavelengths of three transitions indicated by solid lines in fig. 2 are measured with a monochromator. The energy difference between X 12;~ and a" 12;~- in ref. [5], and the energy difference between c~ 12;+ and X lye; in ref. [7] are used to calculate the possible transition frequency. This gives a coincidence within -+1 cm -1 with the measured three lines when they are assigned as shown in fig. 2. The other lines may be also assigned as shown by dotted lines when compared with data in ref. [8]. Carroll and Yoshino pointed out the coincidence of energy levels of c4 117u and c~ 12;~ at J = 20 and observed P-branch for J ~< 16. This also appears in our data observed by rf OGS. Further discussions are not practical from our data obtained with a low resolution of 20 GHz. We are currently investigating these bands with a single-frequency dye laser. Fig. 3 is a typical optogalvanic spectrum of NH 3 discharge measured at a pressure of 0.1 Torr and an rf power of 13 W. A discharge tube of 30 mm O.D. is used. The spectrum shows a similar profile observed by Feldmann with dc OGS [2]. The most prominent band is identified as transitions of the 2A 1 - 2 B 1 systim of NH 2 when compared with the data obtained by absorption [9]. The strongest optogalvanic signal obtained with the Q branch of the E vibronic band has a maximum intensity which is comparable with the strongest N 2 signal obtained by the Q branch of Ledbetter band. Noises in fig. 3 is ten times larger 366
1 September 1981
(qg,0)
NH 2
- (O,qO)
0.1 Torr 13 W
P -t (0.9,0) - (0.o.o) (0,10.0)
0¢
- (O,qO)
.V. ee 0
echo
610
590
T (nm) 580
WAVELENGTH
Fig. 3. Optogalvanic spectrum of NH3 discharge. Assignments by Dressier and Ramsay [9] are shown according to their absorption data of NH2. than those shown in fig. 2. The optimum conditions for the NH 3 discharge have not been searched for. Fig. 4 shows a typical optogalvanic spectrum of argon discharge measured at a pressure of 0.4 Torr and an rf power of 10 W. A discharge tube of 30 mm O.D. is used. All atomic lines in this wavelength region listed in [10] can be observed. The strongest signal has a comparable magnitude with the strongest N 2 signal. Optogalvanic spectrum in NO 2 discharge at a pressure of 0.07 Torr and an rf power of 45 W shows structures similar to those observed by Feldmann with dc OGS [2]. The optogalvanic effect in the rf discharge with a simple pick-up circuit is found to be useful. In our experimental system, its sensitivity is limited by the dis-
Ar ~Torr
tO W
$ i
J i L,,t, 63o
620
6;0
~o
s~o 5~oc~
WAVELENGT H
Fig. 4. Optogalvanic spectrum of Ar discharge.
Volume 38, number 5,6
OPTICS COMMUNICATIONS
charge noises. For consideration of possibility for improving the sensitivity, noises in the rf discharge are analyzed with a selective level meter (Anritsu, ML11A), which has an analyzing bandwidth of 8 Hz. A typical noise power spectrum is shown in fig. 5, which is for argon discharge with a tube of 30 mm O.D. and an rf power of 4 W. The argon pressure is 0.4 Torr. Strong components at an interval of 50 Hz are due to residual line components in the dc power supply of the rf transmitter. A large signal at 1.8 kHz originates in a superposed amplitude modulation of 1 0 - 4 in depth to the driving rf field. Noises caused by the discharge have different frequency with different discharge conditions, but have similar profile shown in fig. 5. By comparing noise spectrum for various discharge conditions, we find its tendency as follows. In the low frequency region below 500 Hz, apparant noises besides line components have its power rapidly decaying with frequency. In the region between 1 and 3 kHz, there appear very large noise caused by ion instabilities. Its center frequency becomes higher as the gas pressure is decreased. In the higher frequency region, there appear harmonics of the above noise until about 5 kHz. Above this frequency, there appear no such distinct noises up to the observed 20 kHz. Therefore, modulation above 5 kHz is suitable for minimizing discharge noises. It is found that noise power at 218 Hz is typically 15 dB higher than that at 5 kHz. This implies that noise voltage at 218 Hz is about 5 times larger than that at 5 kHz. The comparison of the magnitude of discharge noise with that of amplifier noise may give an estimation to
I 0.5
1 September 1981
the desired fabrication of amplifier to be used in rf OGS. Discharge noises at the input of the lock-in amplifier can be estimated from the recorder trace to be ~100 ~V at 1 Hz bandwidth in argon experiment. The homemade tuned amplifier has a gain of 40 dB at 218 Hz and a bandwidth of 60 Hz. The intrinsic noise voltage of this amplifier can be estimated by monitoring output noise voltage when there are no rf fields. The output voltage thus obtained is 2.5/aV ~ . This value is a moderate noise voltage for an amplifier and is 40 times smaller than that of discharge noise at 218 Hz. In our experimental system, it is found that discharge noise is still one order of magnitude larger than the amplifier noise even with 5 kHz modulation.
4. Conclusion The simplicity and wide applicability of rf OGS has been shown with preliminary results. It is found that our experimental system is not fully optimized for rf OGS. Nevertheless, the signal-to-noise ratio of larger than 200 is obtained for a molecular transition with a detector bandwidth of 1Hz. Optimization of the discharge tube design and electrical circuit for optogalvanic spectroscopy will lead to improvements in the signal-to-noise ratio. We are currently investigating the nature of this problem further. We have found rf OGS is a powerful method both for studying optical transition gaseous discharges and for investigations of discharges processes. In addition, high resolution laser spectroscopy of Rydberg state of N 2 has become possible.
Acknowledgements
1
k_ 2
FREQUENCY
3 4 5 6 7 ~ 9(kHz)
Fig. 5. Noise power spectrum of discharge in argon taken with a bandwidth of 8 Hz.
The author wishes to thank Professor K. Shimoda for valuable comments. Thanks are also due to Dr. T. Kasuya, Dr. M. Kakimoto and Dr. S. Iwata for helpful discussions, and to H. Kato and Y. Nomiya for technical assistance.
References [1] R.B. Green, R.A. Keller, G.G. Luther, P.K. Schenck and J.C. Travis, Appl. Phys. Lett. 29 (1976) 727. 367
Volume 38, number 5,6
OPTICS COMMUNICATIONS
[2] D. Feldmarm, Optics Comm. 29 (1979) 67. [3] J.E. Lawler, A.I. Ferguson, J.E.M. Goldsmith, D.J. Jackson and A.L. Schawlow, Phys. Rev. Lett. 42 (1979) 1046. [4] J.E.M. Goldsmith and A.V. Smith, Optics Comm. 32 (1980) 403. [5 ] C. Stanciulescu, R.C. Bobulescu, A. Surmeian, D. Popescu, I. Popescu and C.B. Collins, Appl. Phys. Lett. 37 (1980) 888.
368
1 September 1981
[6] J.W. Ledbetter Jr., J. Mol. Spectr. 42 (1972) 100. [7] A. Lofhus and P.H. Krupenie, J. Phys. Chem. Ref. Data 6 (1977) 113. [8] P.K. CarroU and K. Yoshino, J. Phys. B5 (1972) 1614. [9] K. Dressier and D.A. Ramsay, Phil. Trans. Roy. Soc. Lond. A251 (1959) 553. [10] American Institute of Physics Handbook, ed. D.E. Gay (McGraw-Hill, 1972).